pulse-jet filtration: an effective way to control industrial pollution part ii: process...
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This article was downloaded by: [University of Stellenbosch]On: 02 May 2013, At: 19:27Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office:Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
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Pulse-jet filtration: An effective way tocontrol industrial pollution Part II: Processcharacterization and evaluation of filtermediaArunangshu Mukhopadhyay aa National Institute of Technology, Jalandhar, IndiaPublished online: 12 Apr 2010.
To cite this article: Arunangshu Mukhopadhyay (2010): Pulse-jet filtration: An effective way to controlindustrial pollution Part II: Process characterization and evaluation of filter media, Textile Progress, 42:1,1-97
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Textile ProgressVol. 42, No. 1, March 2010, 1–97
Pulse-jet filtration: An effective way to control industrial pollutionPart II: Process characterization and evaluation of filter media
Arunangshu Mukhopadhyay∗
National Institute of Technology, Jalandhar, India
(Received 22 October 2009; final version received 21 November 2009)
The fundamental concept of design and development of the pulse-jet filter have beendiscussed in Part I of this monograph series [Textile Progress, Vol. 41, No. 4]. Forsuccessful running of the industrial pulse-jet filter, fundamentals of the filtration processand operating principle should be well understood. In view of the above, the monographis intended to develop in-depth understanding of the mechanism and factors governingthe filtration process. Modeling and simulation aspect related to filtration process isalso included, which is helpful to judge process performance for effective processmonitoring, and also to set the process and design parameters at an optimum level. Inview of selection and designing of new filter media, a comprehensive examination ofvarious methods of testing and evaluation of filter media is incorporated.
Keywords: filter media evaluation; mechanism; modeling and simulation; pulse-jetfiltration
1. Introduction
Pulse-jet filtration has become the most preferred choice providing sound technical andcommercially attractive solutions in controlling industrial pollution throughout the world.The fundamentals in the selection and designing of the pulse-jet filter unit have beendiscussed in Part I of this paper [1]. Despite the wide applications of pulse-jet fabric filters,the operating principle remains poorly understood. As a result, the operation condition isusually specified by the manufacturer at the time of commissioning. This is not desirable interms of achieving best possible filtration performance at lower energy cost of the process.Understanding the mechanism and factors governing filtration is very important in view ofdesigning of filter media, construction of filter unit, and setting the operating parameters atthe right level for achieving desirable filtration performance.
Knowledge of the mechanism is also necessary for developing process models. Therequirements in pulse-jet filtration systems are rise in low pressure drop; sufficiently highseparation efficiency; and a low residual pressure drop, which does not increase significantlyafter several cycles of filtration and regeneration (stable conditions). Typically, the optimaloperation and design-parameters are found by a kind of trial and error strategy, where theimportant parameters are varied under the guidance of empirical knowledge. Therefore,the optimal configuration can be evolved after a tedious experimental effort. Modeling andsimulation always helps in understanding the process and to find out suitable operationalparameters for filtration processes. The experimental effort can be drastically reduced byusing a simulation model for the whole filtration process. Initially, the parameters of the
∗Email: [email protected]
ISSN 0040-5167 print/ISSN 1754-2278 onlinec© 2010 The Textile Institute
DOI: 10.1080/00405160903438367http://www.informaworld.com
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model have to be determined from a few experiments with a given type of dust and thenit should be possible to evaluate performance parameters with the model instead of thefiltration apparatus.
A summary of the modeling work before 1985 was given by Leith and Allen [2]. Therecould be many operational parameters, which can be modeled such as emission/particleretained, filter drag, filtration velocity, compressibility in dust cake, unstable operation ofjet-pulsed filters, cake growth, cake detachment, system design, modeling media movement,gas/solid reaction in filter cake, etc. Further, the model is able to simulate filtration behaviorfor several cycles of filtration and regeneration to see the long-term behavior of the filter.Characterization of the filtration process is also inherently linked with functioning of filterfabrics. A well-designed filter fabric has become essential for proper working of a filterunit. It is therefore necessary to evaluate the performance of filter media for its selectionand also for designing new filter material.
2. Mechanism and factors governing filtration
2.1. Mechanism of particle capture/emission
In terms of the way in which a particle is retained by a filtrate, and so removed fromthe air/gas, a number of mechanisms can be distinguished. As a particle-laden gas streamapproaches a fiber, particles suspended in the stream move towards the fiber surface by anumber of forces. Capture of particles by a filter can be explained on the basis of differenttheories, and final effect depends on a combination of several mechanisms/factors classifiedas follows:
� Single fiber theory;� Capture by fibrous assembly;� Capture based on the mode of filtration;� Capture influenced by design of filter unit;� Capture governed by operating parameters.
Bemer et al. [3] explained the mechanism behind particle puffs emitted during the stagesof pulse cleaning as the combination of the following mechanisms:
� Partial disappearance of the cake of particles;� Migration of particles in the medium;� Re-suspension of the upstream particles due to the rupture of the cake;� Re-suspension of the deposited particles on the walls of the duct.
The contributions of these different mechanisms strongly depend on the association ofthe medium/aerosol and, to a lesser extent, on the operating conditions of filtration andunclogging. The nature and size of the particles remain important, the effectiveness ofthe medium remaining decisive at the beginning of the operation of an installation andafter many cycles. However, difficulties persist, including the inability to assess the actualcontribution of the mechanisms, such as the migration of the particles (phenomena ofseepage) or re-suspension of the cake particles when unclogging. It was observed thatthe mechanism mainly responsible for particle puff for alumina particles (<5 µm) isdifferent from wood dust (<10 µm). Regarding the nature of the medium, tests haveshown that the addition of polytetrafluoroethylene (PTFE) membrane on the filter surface
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Figure 1. (a) Diffusion; (b) Inertial impaction; (c) Direct interception; (d) Gravitational; and (e)Electrostatic attraction.
could significantly reduce these puffs [3]. In the foregoing sections, the role of differentmechanisms on particle capture/emission is discussed.
2.1.1. Particle capturing based on single fiber theory
Fabric filter often captures particles much smaller than the fabric pore size, which showsthat the mechanism of capture goes beyond simple sieving. Capture of particles on afiber element follows one or combinations of the following mechanisms: diffusion, inertialimpaction, direct interception, gravitational settling, and electrostatic attraction. A briefdefinition of different mechanisms is given, the details of which can be seen from theliterature [4–12].
1. Diffusion: In case of fine particles below 0.5 µm, capture by diffusion followingBrownian motion (irregular wiggling) (Figure 1a) becomes significant. Aerosolparticles suspended in an isothermal gas are constantly subjected to bombardmentby the molecules of the gas due to their random thermal motion. At each collision,an exchange of momentum occurs. If the particles are very small, these collisionscause Brownian motion. As a consequence of Brownian motion, very small aerosolparticles tend to diffuse through a gas from the regions of higher concentration toward
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the region of lower concentration. During the random movement of particles in thegas, certain particles are captured by the fibers or the dust layer. This phenomenonis significant when flow velocity is low, particles are smaller (collection by diffusiondecreases as the particle diameter increases), and temperature is high.
2. Inertial impaction: When a fluid flows past a stationary cylindrical object (in thiscase a fiber of the fabric), it diverges before the body, flows past, and then againconverges. A suspended particle in the fluid may not be able to move along withfluid if its momentum is high (for particles of larger size/higher mass); higherinertia may be sufficient to break away from air streamlines and impact the fiber(Figure 1b).
3. Interception: This occurs when a particle does not have sufficient inertia to breakaway from the streamline; however, it comes close enough to the fiber so that naturalforces will attach the particle to the fiber (Figure 1c).
4. Gravitational: Large particles simply settle out of the gas stream before interactingwith fiber. Due to inertia, dust in a gas stream has a tendency to move in thedirection of the gas stream. If the gas velocity is reduced substantially, then the dustis acted upon by the gravity and the particles settle down at the bottom (Figure 1d).Although, in this mechanism, there is no interaction between fiber and particle, theoverall filtration efficiency will be affected by this mechanism.
5. Electrostatic attraction: It is based on an electric or electrostatic charge on theparticles and/or fiber that will force the particle to divert from the streamline andattract to the fiber. Collection by electrostatic force becomes prominent with particlesize ranging from 0.1 µm to 1 µm. When charges held by particles and fabric (fiberassembly) are opposite in nature or there is a sufficient difference of potentialswhich can overcome the aerodynamic forces, the particles are deposited on thefiber (Figure 1e) [3]. If particle and fiber possess the charges of the same polarity,an easily removable dust cake is formed on the filter fabric. During filtration, areduction in pressure drop is perceived due to two main effects – mass redistributionand increased porosity, the former being dominant. If a fiber in a filter carries anelectric charge, the associated electric field can act on an airborne particle in twodifferent ways. It can attract an electrically charged particle by means of Coulombforces, or it can attract any particle by means of polarization forces. Four types ofsolid particles (two types of fly ash particles and two types of ceramic particles)captured by a single fiber under three different types of charging pre-treatmentshowed that the deposited particles developed in different ways in dendrites.
In general, the theory considers that the separation of particulate matter from the gasflow by fibrous filters happens by the combination of a number of collection mechanismsand the effect of Brownian diffusion is predominant at the submicrometric size range[13,14]. The effectiveness of the collection mechanism will finally depend on particle sizeand its mass, velocity, density and viscosity of the gas, electrostatic forces, and the filterused. Moreover, the different mechanisms are not independent but operate simultaneously.Figure 2 shows general trend of particle penetration with the increase in particle size. Asthe velocity changes, there is shifting of resultant fractional penetration curve; showingthat, at lower velocity, particle penetration reduces even for the same size of particles.
Wang [15] reviewed both theoretical and experimental studies on the application ofelectrostatic forces in filtration. In the absence of electrostatic forces, movement of asmall particle in gas is governed by thermal forces and particle inertia, and a fibrous filterefficiently captures particles by inertial impaction, interception, and convective Brownian
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Figure 2. Fractional penetrations of particles as a function of particle diameter.
diffusion. The relative contributions of thermal forces and particle inertia to depositionare mainly the functions of particle diameter, gas velocity, and fiber diameter. In general,particle inertia makes a greater contribution for particles larger than 1 µm, while Brownianmotion caused by thermal forces plays a greater role for particles smaller than 0.1 µm.For particles in the size range of 0.05–0.5 µm, both particle inertia and thermal forcesare relatively weak. As a consequence, the collection efficiency of a fibrous filter has aminimum effect in this size range. Application of electrostatic forces can significantlyaugment the collection efficiency of a fibrous filter. The process of aerosol filtration inthe presence of electrostatic forces is complicated and is particularly useful for improvingthe collection of particles in the size range of 0.15–0.5 µm, which are difficult to captureby other mechanisms. The variables that influence the collection efficiency of a filter inthe presence of electrostatic forces include chemical composition of particles and fibers,charges on particles, surface charge density of fibers, and the intensity of the externallyapplied electric field, in addition to those variables that affect the collection efficiency inthe absence of electrostatic forces.
Most of this theory, however, was developed and validated for micron-size particles,and their extension to the nano-size range needs attention, both experimental and theoret-ical. The filtration of nanoparticles in a polyester filter had shown that particle efficiencydecreases with increasing particle diameter and increasing gas superficial velocity. The re-sults were compared to well-known theoretical predictions based on the classical collectionmechanisms: diffusional and direct interception. The comparison of the calculated collec-tor efficiency with the experimental results showed that the predictions underestimate theresults. A correction was proposed to the existing correlation for direct interception wherean ‘effective’ diameter accounting for the Brownian motion of the particle in the vicinityof the collector surface was accounted for. The results showed a considerable improvementin the prediction correlation [16].
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Figure 3. Surface and depth filtrations.
2.1.2. Mechanism governed by structure of filter material
In the case of fibrous assembly, in addition to single fiber theory, size and shape of poresdefined by the fibrous assembly play a vital role in particle filtration. The orientation of fibercan also influence the particle filtration. Surface type media are not perfectly smooth ontheir surfaces nor are their pores perfectly uniform in shape and direction. Broadly, particlescapturing by filter media can be distinguished as surface and depth filtration (Figure 3). Thesmall particles, which are unable to retain over the surface of filter media, penetrate insidethe fibrous assembly and are likely to get trapped due to the tortuosity and confined regionin the pore structure. The phenomena can be distinguished as depth filtration. However, insurface filtration, even the particles which are smaller than pores can be retained over thesurface through the formation of bridges over the pores. Therefore, in surface filtration,two different mechanisms – sieving and bridging filtration (Figure 4) exist, which havebeen explained below. Most of the cake formation occurs by a combination of blocking andbridging.
Figure 4. Mechanism of surface filtration. (a) Complete blocking filtration; (b) Bridging filtration.
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Figure 5. Depth filtration. (a) Depth straining mechanism; (b) Depth retaining mechanism.
1. Sieving: When the pores of the fabric are smaller than the incoming dust, thenthe particle is completely captured through screening process. Sieving is usuallypredominant for particles above 10 µm for felt fabric. Through the enhancement ofsurface filtration by using membrane at the upstream side of the fabric, filtration ofparticle size even below 2.5 µm is possible without the formation of dust/cake layer.
2. Bridging filtration: This mechanism is prominent if the particle concentration isrelatively high for the particles smaller than the pores (down even to about one-eighth of the pore diameter). The bridging of the particles across the entrance to apore forms a base upon which a stable and permeable cake will grow. In reverse-jetsystem, filtration occurs through cake formation. On the other hand, in pulse-jetsystem, both cake and non-cake filtration can be distinguished; the latter being morecommon in industrial practice.
In the depth filtration, the mechanisms of filtration may result in the trapping of farsmaller particles that might be expected from the size of the pores in the medium. In aerosolfiltration, both depth straining and depth retaining are common (Figure 5).
1. Depth straining: For a filter media, particles will travel along the pore until theyreach a point where the pore is confined to a size too small for the particle to go anyfurther so that it becomes trapped. Particles are also trapped while passing throughblind pores.
2. Depth retaining: A particle can also be retained in the depth of the medium, eventhough it is smaller in diameter than the pore at that point. Such behavior involvesa complex mixture of physical mechanisms. In a tortuous pore, the particle loses itsvelocity and becomes attached to the pore wall, or to another particle already held,by means of van der Waals and other surface forces.
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In industrial filtration, as particle concentration is much above 5 mg/m−3, surfacefiltration is the only option where the fabric filter is needed to be regenerated to regainthe effectiveness of the filtration process [5,7]. The actual mechanism, or combination ofmechanisms, pertaining in any specific instance is dependent on the characteristics bothof the medium and the suspension being filtered [17]. In case of depth filtration, the dustparticles are retained at the certain depth of filter medium, which are difficult to clean. All theindustrial filter fabrics are therefore predominantly surface filters, although some amountof depth filtration is inevitable, which results in the blinding of pores. Depth filtration canhave a profound influence on the filtration performance and life of the filter fabric.
The industrial fabric filtration process is conceived of three different stages, i.e. depthfiltration, transition filtration, and surface filtration stages. The depth filtration takes placeon the cleaned surface just after the filter cleaning until the dust load reaches a maximumretention in the filter. Once the dust begins to accumulate on the cleaned surface, which isthe end of the depth filtration stage, the second stage of transition filtration stage begins.During the transition stage, the surface filtration on the uncleaned surface also plays a rolefor dust collection. The local filtration velocity varies over the filter surface because of thenonuniformity in the dust cake distribution. As time passes, the nonuniformity will graduallydisappear because of self-equalizing mechanism for parallel path flow. The difference inlocal filtration velocity will then diminish and this will lead to uniform deposition of filtercake, which is the beginning of surface filtration stage [18]. It is important to note thatdepth filtration can be reduced to a large extent in case of membrane/coated filter media.
In case of ceramic filter media, particles’ migration and fine particles’ penetration werefound to be the main reasons for dust deposit within the structure, especially at the surfaceregion. The migration and penetration of particles become aggravated with cycle number.Structural analysis using scanning electron microscope (SEM) shows that the depositedamount of dust inside the medium decreased linearly in the radial direction away from thesurface. The face velocity and the dust size are the key factors that influence dust depositwithin the filter medium. Dust deposit within the medium is the main reason for the increasein filter resistance and consequently makes the ceramic filter cycle irreversible [19].
2.1.3. Mechanism governed by mode of filtration
2.1.3.1. Direct penetration and seepage theory. Despite the wide-spread and growing useof jet-pulsed filters for cleaning up of dusty gases, the mechanisms leading to particle emis-sion by surface filter media have received less scientific attention than the filter-regenerationprocess and the various causes of its malfunction. Through a series of experiment, [20–22]have established ‘seepage’ as a dominant mechanism of particle penetration in conventionalonline pulse-jet fabric filtration. Dust can penetrate through the fabric filter in two ways: itcan fail to be collected and penetrate straight through the fabric, or it can be collected atfirst but ‘seep’ through later [21]. It is important to note that pulse-jet cleaning is associ-ated with a sudden disruptive increase in the downstream particle concentration. A numberof physical mechanisms govern these so-called particles puffs influencing dust collectorperformance determination [5,7,23,24].
According to straight-through theory, particles should be captured with greater effi-ciency as dust collects upon the fabric. A fabric well conditioned with dust should allowvirtually no subsequent dust to pass straight through it; and if a dust deposit is free of cracksand pinholes, it will be nearly impenetrable [25]. However, it was reported [26–28] thatgradual seepage of collected dust through the fabric into the cleaned gas stream is moreimportant than straight-through penetration in a pulse-jet filter well conditioned with dust.
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Seepage is a failure of the fabric to retain collected dust rather than a failure to collect thatdust in the first place. Seepage implies a mechanism in which particles already capturedby the fabric are subsequently released and eventually re-entrained in the exit gas. Further-more, the primary driving force for this release and re-entrainment mechanism comes fromthe rapid deceleration of the particle-laden fabric as it collapses back on the cage at the endof the cleaning pulse [96].
In addition to straight-through and seepage mechanisms, the dust penetration through afilter medium can also be accomplished by pinhole bypass. Pinholes are small holes, whichmay be formed during cleaning or filtration [26,29,30]. During filtration of hot/pyrophoricparticles and particles of abrasive nature, pinholes can be produced. Also during the needlepunching process on a synthetic felt, small ruptures caused by mechanical stress mightlead to pinholes. The pinhole bypass mechanism is of particular importance when straight-through is not possible any more due to filter cake. Pinholed surface filters show a clearimpact on the separation efficiencies of the test filters in experiments, such as differentparticle-size distributions and much higher integral clean gas side concentrations than forfaultless filters. The observed pinholes in surface filters are required to be rated criti-cally concerning actual emission limit values. Bigger pinholes decrease the collection effi-ciency, and higher filter face velocities increase the collection efficiency of pinholed filtermedia [31].
Considering seepage as a dominating mechanism for emission, [27] presented a modelfor seepage penetration that uses impulse and momentum principles and is based on theassumption that all seepage occurs as the bag strikes its cage. The proposed equation is asfollows:
N = kw2v/t, (1)
where
N = mass outlet flux from the fabric in kg/m2/s,w = the areal density of dust on the fabric in kg/m2,v = superficial filtration velocity in m/s,t = the time in seconds between cleaning pulses, andk = a proportionality constant that was reported to depend on the dust and fabric used.
The above equation shows that outlet flux should be zero with a clean fabric, but shouldincrease as the fabric becomes conditioned and dust areal density increases. If most ofthe dust that penetrates passes straight through the filter, outlet flux from the filter shoulddecrease over time, whereas if most of the dust passes through by seepage, outlet fluxshould increase over time [21]. In support of seepage mechanism, the outlet mass flux froma pulse-jet filter has been found to increase with time. Immediately after installing newbags, this flux was nearly zero. Initially, outlet flux increased rapidly with time, but laterthe increase was much more gradual. This may occur because, over time, more dust worksits way below the fabric surface and is thereby able to penetrate by seepage. These resultssupport earlier supposition that seepage accounts for virtually all the dust that penetrates apulse-jet filter. A series of tests was performed to determine outlet flux for a filter operatedat three filtration velocities, equipped with three types of bags and dusts. The amount ofdust carried by the bags, and the outlet flux from the bags, varied greatly in these tests.However, all outlet flux values, regardless of velocity, fabric, or dust type, correlate wellwith a single parameter, i.e. w2v/t . Fabric type and dust type affect the amount of dust (w)carried by a bag, which in turn largely determines outlet mass flux [21].
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It has been established by the early works cited above that emissions from a pulse-cleaned filter peak immediately after each regeneration pulse [32] and then decrease rapidly,while the filter cake builds up and takes over as a more efficient filtration layer. Such datahave been obtained repeatedly for different types of filter media ranging from rigid ceramics[33] to needle felts. The emissions spike filter immediately after each cleaning pulse andare consistent with the hypothesis that seepage accounts for virtually all dust that passesthrough these filters, as under this hypothesis, emissions occur as each bag strikes itssupporting cage. However, decrease in filtration efficiency with time is contradictory to theearlier findings backed by theoretical proposition. These facts create some suspicion thatthe ‘re-entrainment hypothesis’ may not apply as universally as often presumed, or perhapsonly to the kind of very open and porous media used widely during the 1970s and early1980s. With time, there is a significant improvement in filter media through enhancingsurface filtration amidst many other functional requirements. It has been pointed out thatthe emission processes in surface filter media are difficult for assessment due to their rathertransient nature and the experimental facilities required [34].
Binnig et al. [34] re-examined the contribution of each of the above-mentioned factorsto the emissions from pulse-cleaned needle felt media. The theory applies to two types ofmedia (singular and layered fabric) using two different types of test dust (agglomerating andfree-flowing) and under all aging conditions used during the experiments. Considering theeffect of different dust types and filter aging, experiments were performed on flat circularsamples of calendered needle felts made out of polyphenylene sulphide fibers. At differentstages of aging, it was found that ∼96–99% of recorded emissions were caused by directparticle penetration, which is by far the dominant emission mechanism as compared to re-entrainment. With increasing filter service life, simulated by accelerated aging up to 20,000cycles, the emission level decreases by factors of about 10 to 20 due to the progressionof clogging and a corresponding increase in filter efficiency. The observed behavior is insupport of direct penetration of dust particles and is assumed to be valid for the entireoperating life of a surface filter, except when mechanical failure occurs and disregardingthe effect of holes caused by stitches.
Figure 6 shows typical behavior of filter fabric at different numbers of cycles with andwithout dust load. As and when dust loading is stopped, penetration is solely due to re-entrainment wherein emitted dust mass per cycle was of the order of 10−5 of the mass storedinside the medium. Its impact on overall emission is much lower than the combined impactof direct penetration and re-entrianment. The prevalence of direct penetration is furtherconfirmed by the size distribution of the emitted particles, which is centered narrowlyaround the most penetrating particle size between roughly 0.5 µm and 1 µm. Contrary toexpectations, a significant fraction of re-entrained dust should have lead to a noticeablycoarser size distribution with aging; instead, the emitted particle size tended toward finerparticles with aging due to an increase in filter efficiency. However, the data did not showa clear difference in aging behavior between free-flowing and agglomerating dust in thisregard. Cross-sectional analysis of a few media by electron microprobe indicates thatthe support scrim may act as an effective barrier to particle seepage more than to directpenetration [34].
Re-deposition of parts of a filter cake immediately after the cleaning pulse (particularlycaused by insufficient time between the pressure pulse and poor flow reversal) is shown tohave a very significant effect on emissions. In addition, the well-known increase in emissionlevel with cleaning pulse intensity can be attributed to a slowing down or prevention of theclogging process rather than enhanced re-entrainment of stored dust. Earlier reports ofincreased emissions at higher filter face velocities are probably due to the re-deposition of
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Figure 6. Emissions per cycle (normalized to the mean of the first eight cycles) during regenerationunder full dust load and without dust load (shaded) [34]. Reprinted from J. Binnig, J. Meyer andG. Kasper, Origin and mechanisms of dust emission from pulse-jet cleaned filter media, PowderTechnology 189 (2009), pp. 108–114, with permission of Elsevier B.V.
dust from the filter cake immediately after a cleaning pulse. Seams in the filter medium canhave an increasingly strong effect on emissions and filter service life [34]. It may be notedthat the above hypothesis is based on flat circular samples, and therefore the influence ofdust cake re-deposition of seams or the rebounding of the filter on the supporting cagemay mismatch the prediction in a filter test unit (VDI 3926, Type-1) with small samplecoupons.
2.1.3.2. Cake vs. non-dust cake filtration. Most pulse-jet operations eliminate the need ofprimary dust cake [35] but not all. Failure to make the distinction between these two pulse-jet-operating modes causes confusion, as one group working on the dust cake pulse filtersreports results in conflict with those of the other group dealing with non-dust cake pulsefilters. In non-dust cake filtration, the layer of dust is cleaned before the cake is formed.In the said mode, the fabric plays a more active filtration role and the properties of porousmedia depend on both the dust and the fabric throughout the filtration period. Non-dustcake filtration also implies that the cleaning action itself is adequate to remove sufficientdust before reaching the dust cake threshold and that a steady state will be reached withoutthe formation of a homogeneous dust cake. However, if filtration is allowed for a longertime without cleaning, a surface dust cake forms on the fabrics. In case of cake filtration,once the primary dust cake is formed over the fabric, the role of fabric is secondary in theoutgoing emission of dust particles.
The performance characteristics of these two operating modes differ, as in ideal dustcake filtration the drag characteristics depend only on the dust itself and the dust cake itforms; in non-dust cake filtration, the drag characteristics depend mainly on the interactionof the dust with the fabric substrate. Drag is often linear with areal mass density in dustcake filtration; it is seldom linear in non-dust cake filtration [5]. In the process of dust
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deposition over filter material, the phenomenon of filter clogging and/or particle dendritegrowth has been observed in a number of studies [36–43]. The operational characteristicsduring filtration are strongly influenced by the structure of the dust cake.
Unlike the liquid filtration process in which cakes are built in a two-phase system (solidand liquid phases), dust filter cakes are built sometimes in a three-phase system (solid, liquid,and gaseous phases). Dust cake buildup therefore introduces more influencing parameters.The tendency of dust cake compression depends on nonequilibrium pressure drop across thesystem and the resistance forces of the cake material (interparticulate forces and stiffnessof particles) resisting compression. In dust filter cake, slight change in adhesion forcesbetween particles has much larger implications than the liquid filtration process [44].
In case of cake filtration, the structure of the cake depends, among other factors, onparticle-size distribution, cake compaction, particle charge, adhesive and cohesive prop-erties, and pressure drop during filtration. Further, quantitative knowledge of cake heightdistributions, their evolution, and the factors affecting them are important for better under-standing of the filter behavior. In general, the characterization of dust cakes is difficult. Thecake properties, e.g. porosity can be estimated from semiempirically-derived equations,e.g. Karman–Kozney, Ergun’s, or Rudnick and Happel, describing flow through porousmedia [45]. The operational characteristics of filter apparatus are strongly influenced bythe structure of the dust cake. Dust is found to be deposited nonuniformly on the filtersurface [46,47]. One important term in cake filtration is specific dust cake resistance (K2),which is often used for the assessment of filtration behavior. This term has characteristics ofdust, varies for different dusts, and measures how rapidly pressure drop will build up in thesystem. The factors influencing the specific dust cake resistance coefficient (K2) includethe filtration velocity, the particle shape, and the size distribution of the particles [45,48,49).
The dust cake mass, its flow resistance and pressure drop increase with the filtrationtime. In case of high efficiency particulate air (HEPA) filters, filtration starts with depthfiltration prior to cake buildup over a needle felt filter. The first phase prior to cake buildupis depth filtration, which is manifested by a weak rise of the pressure drop with the timeprior to the cake formation. Later on, filtration process is followed by a stronger rise ofpressure drop once a cake has been formed [50]. Using monodisperse particles as thechallenge aerosols, the loading curves (plot of pressure drop vs. time) of tested needle feltfilters were found to display three regions: an initial region of fast increase, a transitionregion, and a final linear region after the formation point of the dust cake. The initial regionof fast increase is due to the process of filling up the cavity surrounded by the meltingclumps formed during the process of the surface treatments, i.e. heat stabilizing, singeing,and calendering [51]. As is apparent, the pressure-drop pattern with time is different tothe behavior of HEPA filters [50,52]. The practical application of this research [51] is todefine the interaction of monodisperse solid particles with the cavity formed by the surfacetreatment in order to minimize the increase in pressure drop across the surface layer. Thepressure drop across the needle felt filters strongly depends on the characteristics of surfacetreatment. The mass and distribution of the layer of surface treatment is critical to therise in pressure drop during the solid gas separation, which directly relates to the energyconsumption. The thickness and surface area of the lamination/coating layer ideally shouldbe minimized just enough to maintain the desired dust release characteristics with minimumincrease in air resistance. It was found that the increase in pressure drop per unit mass ofparticle loading of a filter in cake filtration is independent on the size of challenge aerosols.At higher-face velocity, a stronger fluid drag force makes the filter cake compact to a greaterextent leading to smaller dust cake porosity and consequently a greater specific dust cakeresistance coefficient [51].
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Earlier, the observation of Schmidt [52] showed that the rise in pressure drop in manycases is not linear but is characterized by sudden sharp increases. These jumps in the pressuredrop result in a higher cleaning frequency, although in most cases there is deterioration infiltration performance. Local compressions of the dust cake are believed to be responsiblefor the progressive increase of pressure loss with time. This theory was confirmed byquantitative analyses of dust cake structures [52]. The nature of particles in dust cake canhave significant influence on pressure drop across the system [53,54] with the result thata pulverized coal-fired boiler operates at higher pressure drop than a stoker-fired boiler.The reason is attributed to coarser and irregular particles from stoker-fired boilers. It wasalso found that the cake porosity in a filter cake formed at constant gas velocity is lowclose to the filter medium and higher toward the surface of the filter cake [44,55,56]. Thisdistribution of cake porosity along the depth of the filter cake indicates cake compaction.Nevertheless, the rise in the pressure drop versus time was reported to be approximatelylinear [57]. It was shown [58] experimentally that the pressure-drop coefficient of the filtercake and the medium cake porosity changed when different constant face velocities ofthe gas through the filter were applied. Consequently, at changing gas velocities, the cakeresistance parameter (which strongly correlates to the cake porosity) depends on the gasvelocity at which the cake was formed. Therefore, at changing gas velocities, the pressure-drop coefficient of the filter cake is intrinsically not constant [45,59]. Further, the dust cakedeposited in an electrostatic field is more fragile than the one deposited on a nonelectrifiedbag, which allows the particles to be more easily dislodged. Fabric filters are also efficientfor the removal of fine particles by using electrical forces [15,60].
A study [61] on a pilot-scale pulse-jet bag filter equipped with a stereo vision-basedoptical system (for in situ cake height distribution measurements on the bag filter surface)showed that the deconvolved cake height distributions (over bag surface) do not changesignificantly shortly after regeneration but grow uniformly later on and become slightlynarrower toward the end of filtration. Further, a steep pressure-drop rise is observed at thestart of the filtration cycle in the absence of reattachment and nonuniform bag cleaning. Inthe process, the gas flow is turned off to ensure no cake is reattached on the bag surfaceduring cleaning. It may be added that steep increase in pressure drop is followed by amoderate and linear rise over the rest of the filtration cycle. This phenomenon is commonlyrelated to incomplete cake detachment or patchy cleaning. The patchy cleaning results in anonuniform cake load distribution on the bag filter and hence affect cake formation and itsdetachment. However, the specific cake resistance remains constant over the linear rise of thepressure-drop curve indicating a noncompressible cake formation. The analysis of residualcake patches shows a large number of small-sized cake patches and a few large-sized cakepatches on the filter surface. The cake patch size increases with the cake formation. Thefractal analysis of patches boundary indicates preferential cake formation at the boundaryof the residual cake patches shortly after regeneration [61]. The extent and nature of cakecan influence the dust-emission characteristics. Further, clogged bag response is differentto new-state bag response as internal clogging of the medium fibrous structure representsa significant factor in filter bag pulse-jet cleaning. With the time, changes in porosity andmedium physical properties greatly modify bag behavior during cleaning [62], which canhave very significant impact on downstream particle emission with time.
2.1.3.3. Filtration/emission at different position of bag height. The extent and nature ofcleaning at different bag heights along with changes in physical properties of fabric withtime can have significant influence on filtration efficiency and emission characteristics
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of fabric filter. Independent of operating conditions, the dust deposit profile along thebag height varies widely [62–64], which affects emission of particles. Simon et al. [62]investigated the mechanism of filtration at different positions of bag height supported byrigid rings. Signals were monitored along bag filters (over their full height at four differentplaces to provide a record of key parameters: pressure drop, filter medium acceleration, faceair velocity, and axial velocity of the compressed air jet). These measurements provided adescription of filter medium behavior at different positions of bag height. In the above study[62], a potential increase in downstream particle emission was observed owing to significantaspiration of dirty air near the top of the bag during pulse-jet cleaning, particularly whenthe compressed air injection nozzle is too close to the mouth of the filter element.
2.1.3.4. Filtration through electrostatic charging. There are a large number of parametersinfluencing the performance of pre-charging, like initial porosity of the dust cake, electricalfield in the dust cake, particle size and charge distribution, and pre-filtering effects in thepre-charger. Electrostatic effects in fabric filters were investigated by several researchers[65–68]. Frederick [66] showed that triboelectric charging is very important in the filtrationprocess and increases filtration efficiency. It was found that the particles are captured in theform of long and thin dendrites on the upstream bag surface [67,69]. Under triboelectriccharging, the dust is deposited on the surface of the fabric rather than between the fibers[70]. The electric forces cause the dust to form ‘bridges’ between the fibers, with free gapsbetween them [71]. In the absence of an electric field, the dendrites are shorter and multi-branched, and form a tight layer. Fabric filters are reported to exhibit reduced pressure dropand increased collection efficiency when operated with an upstream pre-charger [72,73],an external electric field at the filter surface [74–77], or both [78–81].
In an earlier attempt [74] on a pulse-jet baghouse system, electrical stimulation of fabricfiltration was found to be very effective, which results in higher collection efficiency andlower pressure drop. The extent of these improvements was found to be sufficient to justifythe system as commercially profitable. Electrical stimulation of fabric investigated usinga 4.5-kV potential applied between electrodes threaded into the needled polyester fabricresults in a large extent reduction of both penetration and pressure drop. A large reductionin pressure drop would effect significant reduction in energy consumption concomitantwith the considerable increase in capture efficiency.
Based on an experiment on single reverse flow baghouse systems with bottom entryin conjunction with electrical fields generated by electrodes in the bag, Lamb, Jones,and Lee [82] proposed three separate mechanisms for the reduction in pressure drop thataccompanies the establishment of a strong electric field near a filter fabric. One is theformation of a more porous dust cake due to dust capture in the low-packing densityregions of the fabric. A second mechanism is attraction of particles to the bag wall, whichcauses the bag to act like a precipitator. The thickness of the dust cake is then greaternear the entrance than at the end of the bag, resulting in lower pressure drop. The thirdmechanism involves the attraction of particles to the bag electrodes; dust is then depositedin bands with relatively thin deposits inbetween. Measurements and visual inspections ofthe dust deposits indicate that the second and third effects are enhanced when the aerosolis pre-charged. A particle pre-charger appears to be particularly suitable for this purposeand causes major changes in filtration performance.
Donavan, Hovis, Ramsey, and Ensor [83] studied the effects of both external electricfield, applied by electrodes at the fabric surface, and fly ash electrical charge, controlledby an upstream corona pre-charger in a laboratory pulse-jet baghouse. In agreement with
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Textile Progress 15
the previous findings, increasing either the electric field strength or the magnitude of thefly ash charge/mass reduces the rate at which the pressure drop across the bags builtup. Increasing together the field and dust charges produces still further reduction in thepressure-drop built-up rate, but adding charge to the fly ash reduces the quantity of fly ashdeposited on the bag during one filtration cycle, whereas increasing the electric field has nodetectable effect on this quantity. These observations suggest that the mechanism by whichthe external field enhances the performance of the fabric filter is increasing the permeabilityof the collected fly ash layer, while the pre-charger enhancement mechanism is primarilythat of a pre-filter. Further progress in developing techniques for electric stimulation offabric filtration (ESFF) was reported [84]. The original one-bag laboratory baghouse wasdeveloped, which consisted of a bag fitted with alternate live and ground wires spaced aninch apart. An approach to simpler systems had taken the form of a charged axial electrode(or ‘axode’) with grounded bag electrodes.
Experiments were performed [85] to identify collection mechanisms for 0.5-pm di-ameter particles in electret filter media and to determine the effect of particle charge onpenetration. Highly mono-disperse polystyrene particles were charged at various levels,and their penetration through charged and discharged electret filters was measured with anoptical particle counter. Particle penetration through charged filters is significantly lowerthan through discharged filters. Also, in charged filters, large decreases in penetration wereobserved with increasing particle charge, while in discharged filters, much smaller decreaseswere found. Based on these results it is concluded that electrophoresis played a dominantrole in the collection of charged particles, dielectrophoresis was important only at verylow charge levels, and mechanical collection processes were relatively unimportant [85].Many new developments have come up to achieve high collection efficiency for submicronparticles at lower pressure system pressure drop through combining the fabric filtrationmechanism with electrostatic principle.
2.1.4. Influence of filter unit design and operating parameters
2.1.4.1. Design of filter unit. Particle movement inside the filter unit influences particle-capture mechanism and overall performance of the filter unit. Part of the particles fromthe incoming gas stream may settle down before reaching the filter, and not participateon cake formation. Prior knowledge of the settling process was shown to be necessary inorder to predict the filter-cleaning period, particularly when the gas face velocity was lower.Three laboratory filtration experiments, under coal gasification conditions, were used toexemplify the procedure [86].
The increase of pressure drop by the dust layer collected on filters may be reduced whenthe particle concentration is decreased or kept uniform onto the filter surfaces. Subsequently,the cleaning period of the bag filters may decrease and filter lifetime may be lengthened. Asfar as mode of cleaning is concerned, it has been reported [87] that offline cleaning allows ashort dead period following the cleaning pulse during which the removed dust can settle intothe hopper, minimizing re-deposition under the subsequent forward flow. In conventionalbottom entry designs using online cleaning, dust agglomerates removed during cleaningmust overcome the drag forces associated with the forward flow in order to settle intothe hopper. Even with offline cleaning, bottom entry can create turbulent flow, which re-entrains the dust already collected by the hopper. Many traditional pulse-jet designs alreadyincorporate top entry so as to avoid re-entrainment and to promote transport to the hopperduring online cleaning. Effects of tangential or straight can also have significant influenceon filtration performance.
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Further introduction of a baffle plate, diffusor, or damper before the entry of the dustin the filter chamber enhances filtration performance. Dusts impinge/impact on the baffleplate and the gas flow changes the direction. After the baffle plate, volume again expandsand velocity falls drastically. A combination of all helps to settle down the heavier dustparticles, thereby reduces the flow of dust particles on the filter bag. Provision of a baffleplate also enhances the unique distribution of dust particles to the bags in order to reducepatchy deposition [7]. Patchy deposition is a result of patchy cleaning, which means thata patch of a filter cake is completely torn off by the pressure pulse, whereas the restof the cake remains intact on the filter. In an experimental investigation with flat baffleplate in front of the top entry inlet of dust feeding, it was found [88] that emission is notaffected by the baffle plate height. With the increase in baffle plate height, pressure dropdecreases, whereas PM2.5 (based on number distribution) first increases and then decreases,and average particle diameter in the emitted dust shows revere trend to that of PM2.5 [88].It may be added that there could be several geometries of diffusor/baffle plate. Gregg andDavies [89] installed a dispersing plate to reduce the particle transport and/or make theparticle concentration uniform on filter surfaces.
In an experimental investigation [90], the characteristics of an electrostatic cyclone/bagfilter with inlet types (upper and bottom inlet) were studied in order to overcome thelow collection efficiency for submicron particles and high pressure drop, which were themain problems of general fabric bag filters. The experiment was performed to analyze thecollection efficiency and pressure drop of the electrostatic cyclone/bag filter compared withthat of conventional fabric bag filters with various experimental parameters such as the inlettype (upper and bottom), inlet velocity (filtration velocity), and applied voltages. From theresults, the upper inlet type showed a slightly higher pressure-drop reduction of 40–90%than that of bottom inlet. However, in the study [90], it was found that the overall collectionefficiencies of pulse-jet cleaning systems are over 99% for both cases of upper and bottominlets.
In a hybrid-type dust collector (CYBAGFILTERTM) developed by Korea Instituteof Energy Research (KIER), the centrifugal dust removal method is combined with bagfiltration [91]. In contrast to the existing cyclone-fabric filter-combined dust collectors (twoor three stages of cyclone precede the filtration), a centrifugal dust-collecting device wascombined as a single body with a fabric filtration system. After a 12-month operation atthe Clinker Calcination Process, the pressure-drop value (80 mm H2O) of the filter wasfound to be 1.2–2.5 times lower than that of conventional filtration systems along withmore than 99.8% collection efficiency and 5 mg/Sm3 outlet dust concentration. With anoperating pressure drop of 100 mm H20, the cleaning interval was much longer than thatof conventional ones, indicating prolonged filter life [91].
Researchers [92,93] had included a shroud into the hot-gas filter vessel so that particlesare transported by the highly swirling flow from an inlet both upward and downwardthrough a shroud and then enter the main body of the vessel (Figure 7). Zhang and Ahmadi[92] found that a large number of particles deposit in the shroud; however, they did notconsider the effect of a shroud on the particle behavior in the filter vessel. In a later work[93], the effects of the shroud tube on the flow field and filtration behavior inside the vesselwere studied wherein the air mixed with dust particles enters the vessel through a tangentialinlet duct. The shroud tube is divided into six pieces to know the influence of differentshroud tube on the flow field and particle behavior. In the presence of a shroud tube in thevessel, the quantity of particle loading onto the filter surfaces is found to be reduced. Afraction of particles are deposited on the inside wall of the vessel and the surface of theshroud tube. The other fraction is collected on the filter surface or passed through it. The
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Figure 7. Geometry of a bag filtration system with a tangential inlet and a shroud tube (the sixpieces of shroud tubes are named as PST1, PST2, PST3, PST4, PST5, and PST6, where PST meansa piece of shroud tube) [93]. Reprinted from S.J. Park, H.K. Choi, Y.O. Park and J.E. Son, Effectsof a shroud tube on flow field and particle behavior inside a bag-filter vessel, Aerosol Science &Technology 37 (2003), pp. 685–693, with permission of Taylor & Francis (Taylor & Francis Ltd,http://www.informaworld.com).
particles deposited on the wall surfaces fall into a hopper by gravity, and those collectedon filters are removed by usual back pulse-jet flow. The particle loading on fabric surfaceincreases as the length of the shroud tube becomes shorter from the down edge because ofthe reduction in both the area of the particle deposition and the blocking effect of particletransport. It was also found that the particle loading is reduced when the upper region of thevessel was not blocked by the shroud tube more than when the vessel was blocked wholly
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with the filters from the upper end wall. However, in the above design, the re-entrainmentof the particles removed from the filters by the pulse cleaning is higher.
In a separate study, particle transport and deposition in the Wilsonville hot-gas filtervessel (accommodates candle filters in two clusters – upper and lower tiers) is observed. Animportant feature in the hot-gas filtration system is that the gas enters the vessel tangentiallyinto the shroud. The results show that the deposition pattern depends on the particle size.Turbulent dispersion plays an important role in the transport and deposition of particles.Lift and gravitational forces affect the motion of large particles, but has no effect on smallparticles [94].
2.1.4.2. Operating parameters. Among different operating parameters, aerosol velocityhas very significant influence on filter efficiency. Inertial impaction mechanism becomesmore prominent with the increase in filtration velocity; on the other hand, efficiency de-creases for the mechanisms like diffusion, electrostatic charging, and gravitational settlingwith the increase in the gas velocity. Normally, overall efficiency of fibrous filters decreaseswith the increase in aerosol velocity [6]. However, in case of smaller particle size, thereduction in efficiency of filter is greater with an increase in gas velocity. But for thelarger particle size the efficiency does not affect much with the change in gas velocity [58](Figure 8).
In a steady filtration operation, the quantity of dust re-deposited on the bags aftercleaning increases sharply at higher filtration velocity. Therefore, an increase in residualpressure drop after cleaning has been observed for high filtration velocities [46,95,96].In an investigation [97], the conditioning behavior and filtration efficiency of Nextel@ceramic fabric filters (woven seamless tubing) under simulated gasification (reducing) andcombustion (oxidizing) conditions for a variety of operating temperatures and gas velocitieswith two different kinds of dusts (3 M char and Highvale coal ash) have been studied. Itwas observed that the pressure drop across the filter bags was primarily affected by the facevelocity. With the increase in face velocity, the baseline pressure drop increased linearly,
Figure 8. Effect of gas velocity on filter efficiency [58].
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Textile Progress 19
whereas the pulse interval decreased exponentially. The filter bags demonstrated highfiltration efficiencies (i.e. 99.95–99.99%) when the face velocity was kept in the range of12–24 mm s−1. At a face velocity of 40 mm s−1, the filtration efficiency was reduced to99.5% [97].
The experimental results indicated that filtration velocity has a more pronounced effecton pressure drop as well as cake properties, cake density, and specific cake resistance[98]. At constant dust concentration, increase in filtration velocity results in higher specificresistance and density of filter cake, and consequently shorter filtration times. In a veryrecent study [88], the effect of different operating parameters and material consolidationof non-woven fabric on the filtration parameters and emitted particulate characteristicshad been investigated under real operating situation in a pulse-jet filter unit. Experimentalinvestigation based on L9 orthogonal design showed that emission is reduced with anincrease in punch density and pulse-cycle time (time interval between the rows of bags),but it increases up to a certain extent with the increase in air-to-cloth ratio (Figure 9).With the increase in air-to-cloth ratio, initially there is increase in emission up to a certainlimit and finally the emission is reduced at higher air-to-cloth ratio. This is contrary to theearlier findings [99], where emission is reported to increase with the increase in air-to-clothratio. This can be explained on the basis of decrease in dust concentration level with theincrease in air-to-cloth ratio (Figure 10). Decrease in dust concentration usually resultsin lower value of emission; but in the present case, it is counterbalanced by the effect ofhigher value of air-to-cloth ratio. Initially, with an increase in air-to-cloth ratio, extent ofemission is predominately affected by air-to-cloth ratio, but beyond a certain level, as thedust concentration decreases substantially, emission value decreases despite the increasein air-to-cloth ratio. Further, the emissions are higher at a lower level of cycle time andthere is drastic decrease in emissions at middle level cycle time, and finally a small changein emissions is observed at higher cycle time. Out of several factors, as mentioned above,cycle time has much greater impact over the emissions, followed by air-to-cloth ratio andpunch density [88].
Experimental observation had shown that pressure drop across the tube sheet increaseswith the material consolidation, air-to-cloth ratio, and pulse-cycle time. PM2.5 (based onnumber distribution) first decreases and then increases with the increase in cycle time. Withthe increase in time of filtration, both emission and pressure drop tend to increase withoutaffecting PM2.5 and average particle diameter based on number volume. The summary ofimpact of different factors on emission and related characteristics during pulse-jet filtrationprocess is shown in Table 1 [88].
2.2. Mechanism of cleaning
2.2.1. General
During filtration of industrial gaseous pollutants, the dust is separated and retained mainlyon the surface of the filter media and forms a dust layer/cake. The forming dust layer/cakeis a source of increasing flow resistance. It is known that a thin layer of filter cake remainson the surface of the filter even after cleaning. The industrial units and hence the bag filtersoperate mostly at constant gas flow; therefore, a fan at the discharge provides the necessaryhead to compensate for increasing pressure drop. A filtration cycle starts at a lower pressuredrop as the dust-laden gas flows through the filter. The pressure drop increases with thecake formation and particle entrapment inside the filter medium. A new cycle begins assoon as the pressure drop reaches a certain pre-defined upper limit (�Pmax) or a pre-definedfiltration time is elapsed. In a very early study [100], the effect of (1) fixing the time interval
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Figure 9. Impact of operational parameter on emission. (a) Effect of punches density and air-to-clothratio on emission; (b) Effect of air-to-cloth ratio and cycle time on emission [88]. Reprinted fromA. Mukhopadhyay and K. Dhawan, An L9 orthogonal design methodology to study the impact ofoperating parameters on particulate emission and related characteristics during pulse-jet filtrationprocess, Powder Technology 195 (2009) pp. 128–134, with permission of Elsevier B.V.
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Figure 10. Effect of air-to-cloth ratio on dust concentration [88]. Reprinted from A. Mukhopadhyayand K. Dhawan, An L9 orthogonal design methodology to study the impact of operating parame-ters on particulate emission and related characteristics during pulse-jet filtration process, PowderTechnology 195 (2009) pp. 128–134, with permission of Elsevier B.V.
between cleaning pulses, or (2) fixing the maximum pressure drop at which cleaning isstarted, on the performance of the fabric filter was investigated. It is found that the systemthat is operated under the mode of fixing a maximum pressure-drop value of 2500 Pa canoffer the following advantages: (1) minimizes the effect of the filter medium resistanceon increasing the pressure drop across the fabric, (2) reduces the energy consumption by
Table 1. Effect of increase in filtration-related factors on filtration efficiency, pressure drop, PM2.5,and diameter based on number volume [88]. Reprinted from A. Mukhopadhyay and K. Dhawan,An L9 orthogonal design methodology to study the impact of operating parameters on particulateemission and related characteristics during pulse-jet filtration process, Powder Technology 195(2009) pp. 128–134, with permission of Elsevier B.V.
Parameters
Emission Pressure drop Average size ofFactors∗ (mg/Nm3) (mm of water) PM2.5 particles (µm)
Punches/cm2
(100-150-200)Decreases Increases No effect No effect
Baffle plate height(mm) (420-840-1260)
No effect Decreases First increasesthen decreases
First decreasesthen increases
Air-to-cloth ratio(m/hr) (77-87-97)
First increasesthen decreases
Increases No effect No effect
Cycle time (s) (2-4-6) Decreases up toa certain limit
Increases First decreasesthen increases
First increasesthen decreases
Effect of time Increases Increases No effect No effect
Note: ∗The values in the parenthesis indicate actual experimental values.
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the filter, and (3) minimizes the dust emission from the fabric filter to the surroundingatmosphere [100].
During cleaning, high-pressure, short-duration gas injections in reverse direction blowthe dust layer off the bag surface, which is collected in the dust hopper located at the bottomof the filter housing. As soon as the dust layer is removed, the pressure drop is reduced andfiltration continues for another cycle. During cleaning, the force developed is insufficientfor the complete removal of dust, in particular particles trapped inside the structure. Dueto the above, there will be small but imperceptible change in pressure drop even understeady state and the pattern will proceed until the critical limit of pressure drop whenreplacement of the bag becomes necessary. The pulse pressure used to detach dust fromthe surface filter is critical for the satisfactory operation of continuously rated fabric dustfilters.
There are many operational/design-related parameters connected with pulse pressurein the dust collector system as mentioned below [101]:
� Tank volume and tank pressure (amount and pressure of air inside the supply tank).� Maximum allowable pressure (system pressure to which the equipment may be
subjected without being damaged).� Opening and closing time of valve (should be as short as possible).� Electrical pulse length (energized time of the valve).� Total pulse duration (time from the moment the valve opens until the valve is fully
closed).� Peak pressure (maximum pressure measured at the end of the blow pipe).� Pressure-drop tank (difference between the tank pressure before and after the pulse).� Performance ratio (ratio between tank pressure and peak pressure multiplied by 100).� Volume per pulse (amount of air at atmospheric pressure passing through the valve
for a given pulse time).� Size of the blow pipe and the number and location of the blow pipe holes.� Location and position of the pressure transducer (distance from the valve and radial
or axial mounted on the air stream).� Internal and external leakage.
The cleaning of filters by back-pulses of compressed gases has been the subject ofa number of experimental studies. These studies have observed a number of qualitativeaspects of the cleaning process: (i) an increase in cleaning efficiency with increased filtercake thickness [102,103], and (ii) an increase in efficiency with increase in pressure dropabove a threshold pressure drop [102,104–106]. Effective detachment depends not only onthe nature and duration of clean operation required but also on the stability (cohesion) ofthe layer itself and the adhesion of the layer on the collector [107,108]. It is important tonote that there is a decrease in cleaning efficiency with the increase in the number of cycles[109], a surface deposit is more easily removed than a dust layer that has imbedded itselfmore deeply within the fabric. Humphries [110] demonstrated that the mass of residualparticles after cleaning is strongly dependent on the nature of the filter. Filters that haveundergone surface antifouling treatment have a lower residual mass after cleaning thanuntreated filters. This study also showed that the specific resistance of the deposit formedduring the filtration phase is independent of the nature of the filter. The effect of dustproperties [111], temperature [103], and chemical composition [112] on the filter cakestrength and adhesion has also been studied.
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Among various cake characteristics, cake adhesion, including cake cohesion effects,is a primary factor in the failure mechanism of fabric filters. The stickiness of the dustdetermines how much energy must be put into the system in terms of cleaning effort, andthis determines the amount of damage done to the filter fabric as a result of the regenerationcycle. The energy expended on removing the cake can also be significant, and the means bywhich energy consumption in fabric filters can be reduced is inextricably linked with cakeadhesion. The properties of dusts are known to be dependent on a variety of environmentalparameters, and the main powder characteristics have been investigated, which includethe surface and bulk properties of the filter fabric, the chemical and structural properties ofthe dust, the electrical properties of the dust and fabrics, and the influence of humidity onthe dust and fabrics [113].
Researchers [104,109,114–118] proposed many different indices for dust cake releaseof bags, such as pulse pressure, pulse overpressure, initial pressure rise rate, peak pulse over-pressure, average pulse overpressure, fabric acceleration, and pressure impulse in the fabricbag. The most commonly used index is pulse overpressure, defined as pulse pressure minusthe bag pressure drop. The pulse overpressure is the driving force for dislodging the dustaccumulated on the bag. In one study [119], cleaning process is investigated by simultane-ously observing the motion of the released dust and measuring the pressure changes insideand outside the filter element during the pulse-jet cleaning event. For initiation of pulse-jetcleaning, the injector nozzle blows a high-velocity primary jet coupled with induced sec-ondary airflow [120] into the bag. The increased pressure causes airflow in the oppositedirection to normal operating flow [62], which is closely related with pulse overpressure.
Klingel and Loffler [117] pointed out that when air pressure impulse (PI) in the fabricbag was greater than 50 Pa s, dust removal efficiency would not increase further. Air pressureimpulse (PI) is defined as the integral of pressure versus time over a pulse duration, definedas PI = ∫ TPd
0 p(t)dt , where Tpd is pulse duration. Ahmadi [94] found that there is aminimum pulse pressure of about 300 Pa in the fabric bag that removed about 60% of thedust cake from the fabric. Increasing the pulse pressure beyond this minimum value resultsin only a slight increase in the amount of dust dislodged. Sievert and Loffler [118] havealso shown that it is necessary to reach a critical static overpressure of 400–500 Pa at alllocations along the length of a bag in order to achieve a good fabric-cleaning efficiency.
It has, however, been claimed that the fabric acceleration is the main cleaning mechanismand reverse airflow plays only a minor role [94,109]; an acceleration force of 200 gf isrequired to achieve a complete dust dislodgment during pulse-jet cleaning [114]. However,a later experiment [120] demonstrated that the pulse-jet fabric system can operate steadilywhen the acceleration force ranges from only 30 gf to 60 gf. Later, Bustard and colleagues’observations [121] indicated that the acceleration force of 100–200 gf is necessary todislodge the dust effectively. The variation in the acceleration force data is presumably dueto the use of different fabric material. Sievert and Loffler [104] showed that the dust removalefficiency increases with fabric acceleration and to dislodge dust effectively from the flexiblepolyester fabric only an acceleration exceeding 30 gf was needed. For an inflexible fabric,the fabric acceleration force must reach 200–500 gf to dislodge the dust effectively [104].
Based on a study [118] on pneumo-pulse regeneration of bag filters, it was shownthat pulse pressure exerts the dominant influence on the regeneration efficiency [122]. Theoverpressure pulse for detaching filter cakes from fabrics in continuously rated fabric dustfilters can be regarded as a random sequence of pulses superimposed on a more slowlyfluctuating pressure continuum [123]. These pulses can have rate rise of 200 Pa µs−1,considerably more intense than the continuum value of approximately 600 Pa ms−1, whichis necessary to detach the filter cakes satisfactorily. By isolating the continuum pressure
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electronically, the energy spectra of these pulses have been studied for a range of pulse-jetassemblies. It has been found that the number and amplitude of these pulses can be reducedsignificantly by suitable design of the pulse-jet assembly. These observations are fully inaccordance with jet mixing theory when applied to reverse-jet assemblies, as indicated inthe previous paper in this series [124].
The operating conditions for experimental tests described in various publicationsvary widely. In summary, different key parametric values for a cleaning operation[25,46,64,104,114–116,118,120,121,125–129,130–134,147], as proposed by different re-searchers, which are as follows:
� 30 to 500 gf for the medium acceleration force;� 300 to 6400 Pa for the bag excess internal pressure (pulse overpressure);� 0.03 to 0.555 m s−1 for the reverse airflow velocity.
Since the value varied widely, it is difficult to judge the impact of pulse-jet injection on thekey parameters and their relative importance.
It was found that a number of mechanisms cause dust cake dislodgement[47,61,62,135,136]. Pulse-jet-cleaning mechanism combines a sudden shock sustained bythe filter medium and the rapid reverse flow through the bag fabric [23,64,128,134,137].These studies have shown that both these mechanisms are important and essential forachieving cleaning efficiency. The total mass of clogged medium and its elasticity andflexibility properties influence the rate at which the bag is accelerated and deceleratedby the pulse force [114,120,125–127,137]. The study done by Dennis et al. [114] showstheoretically that increasing Young’s modulus increases the deceleration of the filter, whichconsequently improves cleaning. Later work [138] had established that filter fabric madeout of fibers of higher rigidity improve cleaning performance.
Generally, a filter cake is removed if the force exceeds the detachment stress of the cake.Due to irregularities in the cake formation, there is no sharp threshold for this detachmentstress. The force exerted by a jet-pulse on a filter cake of certain thickness results fromthe deceleration of the filter medium (when it springs back after being inflated by the jet-pulse) and the reverse flow. For ceramic media, only the force resulting from reverse flowis relevant. The linear dependence of the detachment stress on the cake thickness holdswhen only deceleration of noncompacted cakes governs cake detachment, because theoverpressure in the bag at equal pulse pressure decreases with growing cake load, implyinga less intense cleaning action [126].
In a study done by Tsai et al. [133], the mechanism of dust cake detachment fromboth rigid and flexible filter media is considered, with particular reference to the effect ofcake loading (mass per unit area of filter media). At low cake loading (particularly below300 g/m2), the cake detachment stress measured in an acceleration test increases with theincrease in cake loading, while the opposite is true for cake detachment by reverse flow. Athigher cake loadings (about 1000 g/m2), the results of two methods converge. It is likelythat cake removal by reverse flow is influenced by hinging of cake pitches, which remainsloosely attached to the surface after their apparent detachment stress has been overcome.
During the pulse-jet cleaning process, the medium movement responsible for filtration isoften conceived of three phases [64,114,120,125,126]: (1) acceleration of the filter mediumtoward the outside, (2) deceleration of the filter medium once it reaches a circular shape,and (3) acceleration of the filter medium inwards toward the cage after completion of thecleaning cycle and after the decay of the cleaning pulse in the bag interior. The dust cakeon the surface of the bag is broken by the inflation of the filter element. It should be
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Figure 11. Movement of filter media and dislodgement of dust layer.
noted that along the fabric bag, cleaning mechanisms responsible for dust release may bedifferent. The strong acceleration/deceleration in the upper bag regions was found to beresponsible for cake dislodgment, while in the lower bag regions, the dust removal was dueto reverse airflow [104,118]. However, the cause of movement propagation along the bagremains uncertain among pressure waves transmitted through the bag’s internal air or froma shockwave transmitted mechanically along the bag. Some researchers [46,114,125,126]have stated that a pressure wave could be the cause of initial bag movement. De Ravin et al.[126] observed the movement along the bag propagating at a velocity of approximately1/10th the speed of sound (≈35 ms−1), which slowed down with respect to distance fromthe injection nozzle.
Recently, in a detailed study [62], the behavior of filter bags (supported by rigid rings)under pulse-jet cleaning was revealed. The sudden back-pulse caused by injecting com-pressed air results in initial filter bag movement and is characterized by high accelerationvalues over the full bag height. This initial medium movement corresponds to initial inwardcontraction followed by sudden outward ejection. It leads to maximum acceleration valuesand results in the dislodgement of the dust at the surface (Figure 11). This phenomenonlasted for approximately 0.050 s and could only be observed by a high-speed camera.This approach to understanding medium movement is contrary to previous descriptions[64,114,120,125,126].
Two other phenomena are also found to play a part in filter bag cleaning:
� Shaking action at the bag top due to proximity of compressed air injection;� Reverse airflow action in the bottom half of the bag due to significant overpressure
at the bag bottom.
However, in the filtration process, the medium tends to be clogged internally, which is oftenconsidered ‘irreversible’. The impossibility of dislodging many particles trapped within itsdepth is also confirmed through an experiment. Greater particle penetration into the mediumoccurred after each pulse-jet cleaning until saturation of the medium, corresponding to abalanced residual pressure condition. Thereafter it is highly probable that the surface massprofile would reveal only minor cycle-to-cycle variations [62].
Since neither the entire filter area is exposed to the pulses nor the entire cake is actuallytorn off from the exposed areas, the cake either breaks off completely, leaving only aminute adhesive layer of dust on the filter surface, or it stays unchanged on the cloth[105,126,133,139]. Nonuniform filter cake distribution [140] on the bag surface is a naturaloutcome of patchy cleaning, which gives rise to a nonuniform cake load distribution on the
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bag filter and hence affects cake formation and its detachment. Patchy cleaning possessesmany consequences like different residence time of the cake on the bag surface [141],distribution of gas velocities across the filter surface as a result of the cake height distribution[142], different cake characteristics depending on different formation conditions [56], andas a consequence resulting in more frequent bag cleaning.
2.2.2. Role of venturi
The idea behind the introduction of the venturi is to use the injected air pulse to form aregion of greatly reduced pressure in the venturi throat, thereby aspirating a large quantity ofcleaned gas from the outlet plenum to supplement the directly injected cleaning pulse. Theventuri thus acts as a jet pump, converting the kinetic energy of both the injected primaryand aspirated secondary gas into increased static pressure within the bag [126,143,144].The presence of the venturi improves the pressure available for cleaning, and consequentlythe range of stable operation of the filter design. However, it may not significantly reducethe operating pressure drop of the filter if the properties of the dust or the operating regimeof the filter do not demand maximum performance from the cleaning system. The operatingconditions of a given cleaning system may be adjusted to conserve compressed air costs,but this is at the expense of operational stability, and the degree to which conditions maybe changed depending on the intrinsic properties of the venturi and pulse nozzle [144].
In pulse-jet filtration system, there are three commonly used pulse-cleaning designs,classified by the tank pressures, such as low-pressure, intermediate/medium-pressure, andhigh-pressure cleanings [121]. In the low- and intermediate-pressure configurations, useof venture is not needed. In the high-pressure configuration, a venturi is installed on thebag top to induce the secondary airflow. In different venturi assemblies studied [145],maximal values of the initial attack and peak pressure were obtained over a limited spanof jet-injector separations. Such observations are in harmony with general principles ofjet mixing theory applied to pulsed injectors. Any restriction to the flow of secondaryand induced air results in degraded pulses leading to higher intrinsic turbulence. Thecomparative values for commercial venturi assemblies are found to be in accordance withgeneral design principles.
There are two parameters which must be considered when selecting or characterizinga venturi: its shape, and size, normally described by the throat diameter. Different venturishave different characteristics due to their varying shapes. Increasing the pulse pressureincreases the venturi performance. Larger venturis give less pressure but more flow ratethan smaller ones for a given pulse tube geometry. Some venturis are less resistant tofiltration flow than others, and may be selected for that reason [144,146].
In an experimental study on nonwoven polyester filter elements, Morris [120] reportedthe following:
� Venturi reduces the energy needed for adequate cleaning;� Penetration increases with venturi at higher pulse pressure but gets reduced with
lower pulse pressure;� Venturi appears to increase air volume.
In a later study [144], it was found that the cleaning forces increase in response to dustcollection and the limit is reached when the venturi is no longer able to pressurize thebag. Stability of the filter lies in the ability of the venturi to produce adequate pressureduring the cleaning pulse. The use of poor venturis can be ameliorated by cleaning only at
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suitable pressure drop, and this can best be achieved using pressure-initiated cleaning cyclesrather than simple timer-controlled cycles [146]. Performance of a venturi also dependson the characteristics of the pulse from nozzle, diameter of nozzle, height of nozzle fromventure, etc. [5]. In the absence of a venturi, large nozzle diameter causes more air beingdischarged into the bag resulting in a more effective pulse overpressure. When venturi isinstalled, a larger nozzle also causes larger pulse overpressure. However, since a venturithroat constrains the airflow into a bag, an increase of the nozzle diameter will not increasethe effective pulse overpressure appreciably [147].
The advanced system (without venturi) utilizes much lower levels of compressed air(103–206 kPa) to clean the filter bag in contrast with the traditional system, which utilizes481–619 kPa compressed air. It was found that the advanced system design requires alower cleaning energy for an equivalent cleaning efficiency [148]. In the pulse-jet cleaninginstallations, use of higher volume of cleaning air at lower pressure, which is injecteddirectly without a venturi, renders the cleaning pulse less vulnerable to losses as it propagatesdown the bag. As it is less dependent on the secondary air, it also allows the use of longerbags. However, Morris [120] projected a different view and pointed out that the energyneeded for the stable operation of a pulse-jet baghouse was reduced by about 30% onaverage by the addition of venturis. There are situations where venturis are required toincrease pressure pulse inside the bag, and there are also situations where venturis are notnecessary. The jet pump curve can demonstrate the potential cleaning performance of anozzle venture assembly system. It is claimed that an appropriate nozzle venturi systemnot only reduces the consumption of pulse energy but also increase the bag cleaning effect.In this study, a pilot-scale pulse-jet baghouse is tested for determining whether a venturi isrequired under various operating conditions.
Based on the application of steady-state jet mixing studies [124] to simple venturisand injectors, the form of the output pressure wavefront is found to be sensitive to thegeometry of the assembly; the wavefront will be modified further by the transient natureof the pulse-jet in dust filter applications. In the work using high-speed digital techniques,typical pressure wavefronts at selected points adjacent to the filter fabric have been recordedwith accurate resolution of the initial attack of the pulse, and of peak and trough transientpressures. Optimization of pressure wavefront is possible by simple application of jetmixing principles. A comparison is made between commercial venturi and experimentalinjector combinations, which is likely to have direct applications to pulse-jet filters [124].A later study [130] also indicates better results with venturi as its addition increases theaverage pulse overpressure appreciably, hence increasing the cleaning effect (Figure 12).Notwithstanding, pulse overpressure at the time of cleaning is higher at greater tank pressureand nozzle diameter. In this study, a type-1 venturi (Figure 13) with a small nozzle is shownto be a preferred configuration for an effective bag cleaning. The filtration time increaseswith the increasing initial tank pressure or nozzle diameter. However, a critical value of thetank pressure exists for an effective bag cleaning. In many cases, traditional high pressurepulse using venturi has been replaced by medium pressure pulse without venturi [149]. Inthe latter case pulse timing is enhanced to achieve effective cleaning, which is based onhigh-volume–low-pressure concept.
2.2.3. Effect of pulse injection nozzle
A nozzle can be designed in different shapes such as convergent nozzle, convergent–divergent nozzle, and a straight tube. In an experimental study [150] on the nozzle effectof the pulse cleaning for the ceramic filter candle, a convergent nozzle was found to give a
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Figure 12. The relationship between the average pulse overpressure and initial tank pressure withtype-1 venturi and without venturi conditions under various nozzle diameters [130]. Reprinted fromH.C. Lu and C.J. Tsai, A pilot-scale study of the design and operation parameters of a pulse-jetbaghouse, Aerosol Science & Technology 29 (1998), pp. 510–524, with permission of Taylor &Francis (Taylor & Francis Ltd, http: www.informaworld.com).
higher internal pressure compared to a straight nozzle with the same internal diameter (theconsumption of compressed air remains the same). The main reason for the formation ofhigh overpressure in the convergent nozzle is the entrainment effect due to the developmentof high-pressure at the nozzle tip, which is also true for smaller nozzle diameters [150].
In a number of research works [62,63,150–152], the top area of filter bags/ceramiccandles was found to be venerable as negative pressure can develop during pulse-jet clean-ing. This entails significant aspiration of dirty air near the top of the filter element. Theentrainment effect of pulse-cleaning gas is high when the pulse nozzle diameter is small orwhen the pulse pressure is high. Selection of convergent nozzle is also needed for avertingthe development of a negative pressure at the top of the filter element when the pulsepressure was too high [150]. This negative effect can be improved by appropriate designingof pulse-cleaning elements like pulse nozzle and diffuser, selecting appropriate distancebetween nozzle and filter inlet, and applying the proper operation conditions.
Simon et al. [62] carried out research work with pulse-injection nozzles and its effectover the insertion height. The effect of five pulse-injection nozzle geometries with threepositions was studied. Positions of those five pulse nozzles are shown in Figure 14:
� Position 1: Nozzles with orifice of diameter (db = 5, 10, or 15 mm) positioned 0.08m above the bag mouth.
� Position 2: Nozzle positioned at bag mouth (db = 10 mm).� Position 3: Nozzle inserted to 1/4th of bag depth (db = 10 mm).
The larger nozzles allow higher flow rates and hence higher peak pressures withinthe bag. With a large nozzle diameter, the commercial venturi device itself impedes gasflow when compared to the straight inlet pipe. Larger nozzle diameter exhibits greaterexcess pressure at the bottom of the bag, which is in agreement with earlier findings[62,126,128,130,131,146]. The smallest nozzle (db = 5 mm) does not ensure high reverse
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Figure 13. Configuration of type-1 and type-2 venturis [130]. Reprinted from H.C. Lu and C.J. Tsai,A pilot-scale study of the design and operation parameters of a pulse-jet baghouse, Aerosol Science& Technology 29 (1998), pp. 510–524, with permission of Taylor & Francis (Taylor & Francis Ltd,http: www.informaworld.com).
airflow velocities in the lower part of the bag. However, Lu and Tsai [128] have referred to anoptimal orifice size. Nozzle size greater than optimal size causes the shockwave to weakenand energy to dissipate ineffectively without any improvement in cleaning. Therefore, it isimportant to select correctly the injection nozzle diameter for pulse-jet cleaning. There is anoptimal diameter, which delivers a back-pulse to the bag combining acceptable accelerationvalues over the full bag height with good reverse airflow at the bottom of the bag, and atthe same time decreasing the entrainment of particles at the top of the filter element.
Injection nozzle penetration into the bag neither increases the medium accelerationsnor is there is any improvement in reverse airflow velocities. Due to adverse filtrationperformance, nozzle introduction into the filter bag should be avoided [62,131,132,150].In the case of the candle filter the same conclusions had been reached [153]. Therefore,installation of the injection nozzle should be as far as possible from the filter bag opening.This ensures an overpressure in the bag over the maximum surface and overcomes the
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Figure 14. Pulse nozzles’ positions [62]. Reprinted from X. Simon, S. Chazelet, D. Thomas, D.Bemer and R. Regnier, Experimental study of pulse-jet cleaning of bag filters supported by rigidrings, Powder Technology 172 (2007), pp. 67–81, with permission of Elsevier B.V.
detrimental effects of air intake from upstream at the top of the bag. The effects of thepulse-jet cleaning can be evolved according to the equipment design and the operatingconditions being used [62].
2.2.4. Effect of compressed tank pressure and related factors
In a study [154] on characterization of the filtration and regeneration behavior of rigidceramic barrier filters at high temperature, it was found that increase in the reservoir pressureis much more effective than extending the valve opening time for raising the regenerationefficiency. With a higher reservoir pressure, higher separating forces are applied to thedust cake, whereas these forces are not influenced by an extended valve-opening time[154]. In a study by Simon et al. [62], it was found that higher the compressed air tankpressure, the more intense was the back-pulse, and greater were the accelerations of bagfilters. As the tank pressure rises from 3 to 7 bars, the maximum acceleration value in factincreases by almost 60% at the top of the bag and the excess pressure values double atthe bottom of the bag. At higher tank pressure, the reverse airflow velocity is thereforemuch higher at the bottom of the bag. All this leads to increase in cleaning efficiency withcompressed air tank pressure. This finding is in agreement with earlier research outcomes[46,64,114,125,128,130,131,134,146,155]. However, at an increased tank pressure, highernegative air pressure measured at the top of the bag is undesirable. Furthermore, air jetpower and injected compressed air volume increase when the compressed air tank initialpressure is at a higher level. It was found that a 5-bar pressure appears sufficient todislodge dust cake [62]. Depending on material type and bag dimension, the value willchange. Optimal level of tank pressure offers advantages for both financial savings andprevents the filter medium from sustaining high tensile forces, which inevitably lead toearly aging.
It was also found [156] that particle removal efficiency could be significantly affectedby the frequency of the jet. In particular, for a fixed jet velocity, the efficiency increaseswith frequency, reaches maximum, and then decreases. As regards to the effect of clean-ing time, longer back-pulse increases the time for reverse airflow through the bag, but ithas no effect on either initial medium movement or maximum acceleration values along
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Figure 15. Patchy cleaning illustration.
the filter bag. In a ceramic filter [155,225], the opening time of the blowback valve hasno influence on the cleaning efficiency. The reason for this is that a longer opening timedoes not increase the internal pressure inside the filter candle during the cleaning pro-cess. Only the very rapid increase of the internal ‘peak’ pressure is responsible for cakeremoval.
When the number of bags which are cleaned simultaneously are more, and also forlonger cleaning pulse duration, the drop in compressed air tank pressure is higher. Therefore,an excessive number of bags should not be simultaneously cleaned as the compressed airsystem cannot be repressurized quickly enough to maintain constant cleaning air pressureduring the pulse. Pulse duration values commonly used in air cleaning (50 to 150 ms)should be retained to prevent cleaning power losses.
2.2.5. Dust-release behavior and patchy cleaning
Nonuniform gas flow through the filter alongside inhomogeneities in dust concentrationfrom the manufacturing process, affects the filtration behavior of filter media. The saidbehavior could be the plausible reason causing inhomogeneous dust cake buildup leadingto patchy cleaning (Figure 15), which may adversely influence regeneration, resulting in aninstable operating cycle. The distribution of cake properties (e.g. porosity, thickness, etc.)across the filter surface, possible variations of this distribution over a number of filtrationcycles, and how such variations may be connected with the distribution of residual cakepatches after regeneration are important while characterizing the cleaning behavior of afilter fabric.
A recent study [157] showed the release behavior of the accumulated dust on ceramicfilters under a pulse-jet cleaning system based on images taken by using a high-speedvideo camera. The experiments are performed using quartz dust at high temperature. Tosee the influence of packing density of dust layers, two types of packing conditions as pre-consolidated and nonconsolidated dust were tested. Pre-consolidated means accumulated
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dust layer became denser in structure and it was brought out by flowing clean air at5 m min−1 until pressure difference does not change. The measuring principle is based ona projected fringe technique and reconstruction of the surface is done using shape fromstructured light technique. The technique revealed useful information regarding cake loaddistributions on filter surfaces. It may be noted that this technique cannot be employed forflexible filter media where the hidden filter deforms due to increased pressure drop duringcake formation. Thus, simply reconstructing the surface from structured light techniquewill not account for such deformation.
Concerning the releasing pattern, very clear differences were observed depending onpre-consolidated and nonconsolidated dust. When captured particles were released withoutpre-consolidated conditions, release of dust first took place almost instantaneously at thelower section of the filter, and then release area expands to higher section for all testedcleaning pressures. Shape and size of the released dust flake were mostly large and ofvertical strip shape at the beginning but it got smaller afterwards. Under the cleaningpressure of 100 kPa and 200 kPa on the pre-consolidated dust layer, a small amount ofrelease of short and circular dust pieces was observed at the central part of the filter and thiscontinued for a while forming patchy cleaning patterns. Subsequently, the released areaexpanded to the lower and higher sections. However, at the cleaning pressure of 300 kPa,(at strong releasing pressure), almost the similar release pattern was observed for both theconditions (Figure 16). At the regions where dust flakes are released in small amount andsize, dust is not released effectively resulting in patchy cleaning. Patchy cleaning may occurwhen adhesive force of dust flake is relatively large compared to the impact of cleaning air[157].
The performance of two filter media (polyester and cotton with anticlogging coating)used in industrial air cleaning was studied [136], both at the initial state (new filter)and after a number of filtration cycles. The separation efficiency increases very quicklyfrom the new state with the progression of cycles. After a certain number of cycles, thepressure drop of the medium changes in a more or less constant manner. This work hasfinally highlighted the hypothesis of patchy cleaning for high dust concentration conditions[136]. One consequence of patchy cleaning/misdistributions of the cake load is an unevendistribution of gas velocity over the filter area. This causes a rapid growth of filter cake onthe newly cleaned patches and an increase in the growth rate of the pressure drop, whichshortens the filter cycles [158]. The concept of patchy cleaning [159,133] is adopted inseveral researches. The fraction of cake of certain thickness that is removed by a jet-pulseis determined by a cake-detachment function. The cake-detachment function of the filtercake is assumed to obey a log-normal distribution [126], with the dislodgement stress as avariable.
2.2.6. Cleaning behavior at different positions of bag height
Based on the record of aerodynamics and mechanical behavior over the full height ofthe filter element, it was found that there is intense shaking of the medium at the top ofthe bag and strong permeation of reverse airflow through the fabric at the bottom of thebag [62]. Figure 17 provides a summary of behavior of filter bags at different locationsand states. The top and bottom sections represent the bag areas in which regeneration ismost efficient. Filter medium properties change following gradual internal clogging andextensively modify the reaction of the bag during pulse-jet cleaning. This study highlightsa potential increase in downstream particle emission owing to significant aspiration of dirtyair near the top of the bag during pulse-jet cleaning, particularly when the compressed air
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Figure 16. Release behavior of consolidated and nonconsolidated dust (cleaning pressure at 300 kPa)[157].
injection nozzle is too close to the mouth of the filter element. At the top portion, the filtermedium sustains not only the same sudden initial movement but also intensive shakingaction. These high mechanical stresses produce high dust cake cleaning efficiency despiteaspiration of dust-laden air, which could prevent dust cake dislodgement.
Earlier research findings [46,64,104,109,118,126,127,128,130,131,134,137,160] havealso indicated that the cleaning behavior of a bag is different at different heights; theregeneration of the bag is more prominent at the top and the bottom of the bag filter.Therefore, residual particle quantity is greater in the central part of the bag than at its top orbottom sections (Figure 18). This also varies with the number of pulse-jet injections [62].
Furthermore, whatever the cleaning state of the media might be, there is a large amountof dust in the zone close to the bag snap-ring collar fastening. This indicates that thereis a 2–3-cm long dead zone, in which cleaning never occurs. This localized problem wasearlier observed by many researchers [62–64,150–152] both for bag filters and ceramiccandle filters. It is ascribed due to both the effect of air aspiration at the top of the bagand the relative rigidity of the bag near its fastening point [62]. This negative effect canbe improved by using the effective pulse-cleaning elements like the pulse nozzle and thediffuser or applying the proper operation conditions [151].
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Figure 17. Behavior of filter bag at different bag heights [62]. Reprinted from X. Simon, S. Chazelet,D. Thomas, D. Bemer and R. Regnier, Experimental study of pulse-jet cleaning of bag filters supportedby rigid rings, Powder Technology 172 (2007), pp. 67–81, with permission of Elsevier B.V.
2.2.7. Role of filtration velocity
The effectiveness of filter bag cleaning can become a critical factor during filtration athigh velocities. It was justified that as velocity increases, dust removal becomes moredifficult [161]. The dust deposit’s distribution, total mass, and specific resistance followedthe same pattern with changes in superficial filtration velocity. They remained essentiallyconstant at all lower velocities, but changed drastically at the highest test velocity. Afundamental difference in the dust deposit formation and removal process must exist at thehighest velocity to account for these changes [162]. It was also found [45,163] that with
Figure 18. Dust deposit areal density after cleaning along the bag length [62].
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the increase in superficial velocity of filtration, cake porosity decreases and cake specificresistance increases along with the increase in the mean particle size of the material formingthe cake. The later increase is probably due to the deposition of larger particles on filtersurfaces before gravitational settling. The estimated cake/fabric adhesion force increaseswith the increase in superficial velocity [163]. In another study with fly ash and limestonedusts, increasing filtration velocity was reported to increase the mean particle size of thecake, which was explained on the basis of an altered segregation behavior of the incomingdust. Apart from higher cake/fabric adhesion force and increased level of pressure dropacross the filter, higher filtration velocity also affects the cleaning performance of jet-pulsedfilters [134].
2.2.8. Role of dust/cake characteristics
Cleaning of a dust cake depends on a large number of factors such as particle size andits distribution, cake compaction, particle charge, adhesive and cohesive properties, andpressure drop during filtration. Among various dust characteristics, cake adhesion is aprimary factor in the failure mechanism of flexible and rigid filters. The necessary tensilestress induced by applying pulse-jet of high pressure should overcome either the cohesivestrength of the dust cake or the strength of the adhesive bond between the dust cake andthe medium [133]. On the one hand, the maximum value of the detachment force and thedistribution of the adhesive forces, i.e. the adhesion of the particle layer on the substrate,decisively influence the achievable regeneration efficiency, while on the other hand thecohesion of the particles in the layer modeled as friction forces can noticeably modify theregeneration course [164]. In practice, however, neither the adhesive/cohesive cake strengthnor the applied stress is uniform along the filter length from top to bottom of a long filterelement, which often results in ‘patchy’ cleaning [105,136,158,165].
The stickiness of the dust determines how much energy must be put into the systemin terms of cleaning effort, which determines the amount of damage done to the filterfabric as a result of the regeneration cycle. The energy expended on removing the cake canalso be significant, and the means by which energy consumption in fabric filters can bereduced is inextricably linked with cake adhesion. The cake adhesion property is knownto be dependent on a variety of parameters, such as surface and bulk properties of thefilter fabric, the chemical and structural properties of the dust, the electrical properties ofthe dust and fabrics, and the influence of temperature and humidity on them [103,111–113]. Quantitative knowledge of cake height distributions, their evolution, and the factorsaffecting them are also important for better understanding of the filter behavior.
Based on research findings, Morris and Allen [113] have demonstrated the following:
� PVC is the most cohesive dust followed by carbon black, chalk, and glass fibers;� The fabric structure influences cake adhesion. Electrostatics is found to increase cake
adhesion when PTFE-coated material was used;� Water in the liquid phase increases adhesion, although high gas humidity can reduce
it.
Regarding the influence of cake dimension, it is found [133] that thicker cakes suffergreater stresses when pulsed and should therefore be detached to a greater extent. This isattributed to cake compaction, entailing a firmer cake, especially at the interface of the cakewith the filter medium, where the cake actually breaks off [104]. It is also reported thatlarger patches are removed when the filter cake thickness grows [104,133], although it was
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36 A. Mukhopadhyay
found that at times the thicker cakes can be even harder to remove than the thinner ones[102,165].
Dust deposit retention had a significant effect on equilibrium pressure drop at all thetested filtration velocities. The measured values of the mass retention coefficient (γ ) arefound to be in the range of 0.993 to 0.996, indicating that over 99% of the dust deposit wasnot removed by each cleaning pulse. It is therefore important to identify basic changes inthe cleaning process, which increase the cleaning efficiency [162].
2.2.9. Response of rigid filters
Accumulation and release mechanisms of captured dust from a ceramic filter element weretested using five elements made of different materials and structures [166]. The followingconclusions are made:
1. Cleaning efficiency is strongly dependent on the structure of dust cake; the efficiencyincreases with an increase in pulse reservoir pressure.
2. Cleaning behavior is strongly dependent on the permeability of the filter and on thefilter material. In case of a high-permeability filter, the cleaning process starts almostimmediately after the injection of cleaning air; whereas for a low-permeability filter,cleaning air is stored before the cleaning process occurs.
In case of ceramic candles used for collecting fly ash, it is noticed that ash bridging on thefilter element breaks at high temperature. In addition, Ahmadi and Smith [167] have founda sharp temperature gradient and a thermal shock exposed to ceramic candles due to thecold air impingement during pulse-cleaning process. Schildermans et al. [155] pointed outthat troubles mostly occur near the open end of the filter element due to the bad selectionor design of the pulse-cleaning system, the nozzle diameter, or the distance between thenozzle and filter inlet. For a comprehensive understanding of the filter surface-regenerationprocess, a lot of research has been done. Measurements of the transient pressure differencesin the filter candles have been identified as an important factor in the analysis of the cleaningprocess.
Measurement of pulse-jet instantaneous flow rate and the radial velocity through thecandle wall [71,168] are very important in characterizing the cleaning process. Previously,the studies reflecting pressure histories for the filter and chamber during surface regener-ation have been carried out experimentally by several researchers [119,150,151,169,170].Furthermore, video observations were also used to investigate the surface quality of filterafter regeneration [171] and dust-releasing patterns from filter candles [157]. The key ele-ment of any pulse-jet cleaning is the discharge of a high-momentum gas jet from inside tooutside the filter element as the formation of strong reverse flow of the direction of filtrationflow. It needs the specific configuration of the nozzle to generate the maximum momentuminside the filter cavity.
Three shapes of nozzle are used in industrial ceramic filter: straight, convergent, andconvergent–divergent [155]. Using a long filter element and convergent nozzle, Ito, Tanaka,and Kawamura [172] studied the propagation and attenuation of cleaning pressure in a filtercavity at the beginning and end of cleaning. Choi et al. [150] discussed the effect of shapeand size of the convergent nozzle for pulse cleaning of ceramic a filter candle. Accordingto these researches, a convergent nozzle gives a higher internal pressure due to strongerentrainment effect. However, it needs careful consideration to avert the development of anegative pressure at the top of the filter element when the pulse pressure was too high.
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In order to optimize the pulse-cleaning system in ceramic filter candles with operatingtemperature from 400 to 600◦C using an oil combustion gas and fly ash, the flow simulationaround the pulse nozzle at steady state was carried out. One method suggested that oneway to determine the optimum size of the pulse nozzle and the diffuser was to measureoverpressure in the filter cavity. There was no clear criterion to determine the optimumnozzle size because the overpressure increases with the nozzle size. The diffuser at its throatsize of 23 mm had its maximum value of the overpressure [173]. In another experimentalstudy [174] the nozzle shape and dimension for the pulse cleaning of a ceramic filter candlewas optimized. A bench scale unit of ceramic filter consisting of four commercial filterelements was used to measure the traces of the transient pressure around the nozzle and theoverpressure in the filter cavity during the pulse-jet injection of pulse gas. Overpressure inthe filter cavity is related to the pulse-cleaning force wherein changes in nozzle design areintended to increase the overpressure at the open end of the filter element, and to minimizethe consumption of pulse gas. The following conclusions are drawn.
The convergent nozzle (Figure 19) displays a better cleaning performance in comparisonwith the straight nozzle due to its strong sucking effect, which can entrain more surroundinggases into the candle cavity. It is the outlet diameter that seriously influences the pulse-cleaning potential. The optimum outlet diameter of the convergent nozzle, which representsthe optimal combination of the primary gas flow and the entrained gas, increases with thepulse pressure. The convergent nozzle of outlet diameter of 12 mm was suitable for obtainingthe optimum pulse-cleaning condition. The small convergent angle should be recommendedto reduce the resistance to gas flow.
The maximum overpressure is obtained at the bottom of the filter element and increasesalmost linearly with the pulse pressure with the high consumption of the pulse gas. On
Figure 19. Schematic view of convergent nozzle and a diffuser.
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the economical evaluation of the pulse cleaning, the criterion for choosing the optimalcondition can be determined at the point where overpressure is maximized with the lowestconsumption of pulse gas, which depends on the nozzle size, configuration of pulse-cleaning system, and the filter element properties. The pulse pressure was determinedsuitably between 500 kPa and 700 kPa in 12-mm nozzle diameter.
Pulse gas distribution along the candle length is not uniform. A negative pressuremay appear at the upper region of the filter element due to bad selection of the operationcondition and/or the awful design of the pulse-cleaning system. Appropriate enlarging ofthe gap between nozzle and diffuser can be used to improve the pulse gas distribution in thecandle cavity and to enhance the cleaning potential. The suitable distance between nozzleand diffuser was determined to be around 60 mm in the case of nozzle diameter of 12 mmoperated between 500 kPa and 700 kPa.
2.3. Mechanism governed by characteristics of aerosol
The change in physical and chemical properties of gas and dust can significantly affectthe filtration performance. The common properties of gas which can affect the filtrationperformance are temperature, moisture, dust concentration, particle type, size, and shape,density, electric charge, chemical reactivity, and the surface morphology [45,175]. Apartfrom the above-mentioned factors, chemical composition of gas greatly affects the filtrationperformance. For example, during filtration under chemical environment, the filter absorbs‘fogs’ of sulfuric acid (H2SO4), oil, or solutions of various substances. Absorption of fogsgreatly increases its resistance to the airflow. Carr and Smith [176] observed that, in thepresence of H2SO4 (a product of combustion), the force of adhesion and cohesion betweenash and fabric is changed, which in turn affects the cleaning of the fabric. In the foregoingsections, the effects of few major parameters are discussed.
2.3.1. Effect of temperature
Temperature plays a very important role in case of particle-capturing mechanism as wellas performance of filter material. The settling rate, which contributes to the efficiency, isgreatly influenced by the temperature [13].
Settling Rate = d2ρpg/18 µ,
where d = particle diameter, ρp = particle density, g = acceleration due to gravity, µ =dynamic viscosity of gas.
As the dynamic viscosity increases with increase in temperature, the settling rate de-creases at high temperature, which in turn affects filtration performance. It may be addedthat density of gas decreases with the increase in gas temperature. In an investigation withinan elevated temperature range (RT = 673◦K) during coal gasification, the pressure dropacross the ash cake on a ceramic filter was found to increase with an increase in the filtrationtemperature. This temperature effect was found to be deeply related with the fluid viscosity,which also increases with temperature. The temperature dependency of the air viscosity canbe presented by the following equation: µ = µo (T /293)0.75. Consequently, the increasedpressure drop at high temperature results in an increase in the residual pressure drop rate,as well as the decrease in the cleaning efficiency of the filter (as a high pressure-dropleads to a high compaction of the ash cake) [177]. At higher temperature, re-entrainmentof dust suspension of fine particulates (due to change in dynamic viscosity/settling rate)
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Textile Progress 39
also becomes higher, which further increases the pressure drop. Furthermore for a pulse-jetfilter, pulses are generally less effective in low-density air as a result of higher temperature.
Adequate temperature is essential for proper functioning and longer life of a filterunit. To avoid corrosion, operating temperature must be at least 10–15◦C above the dewpoint temperature of the gas. Possibility of corrosion is increased when the presence ofmoistures is associated with the elevated temperature. It becomes still more acute whencondensed water combines with acid-forming gas component. During the process, presenceof moisture may lead to the blinding of the filter element. Since the condensation oftentakes place, proper care is needed during the startup and shutdown of the filter unit [51].
The increase of pressure drop during dust collection hinders the long-term stable op-eration of coal-fired power generation systems (e.g. PFBC and IGCC). The effect of in-creasing pressure difference in a high-temperature gas cleanup system with a ceramic filterwas studied based on fundamental measurements and analyses using a laboratory-scalefilter apparatus. Dust-detachment behavior of the ash layer on the ceramic filter was alsoevaluated by using both quasistatic and pulse reverse-flow methods. Under conditions ofrelatively low temperatures (below 1050◦K), the pressure drop in the gas permeation testwas almost fixed, and the maximum pressure drop (�pmax) at the first detachment of adust layer by quasistatic reverse flow increased gradually in proportion to temperature. Attemperatures higher than 1123◦K, a time-dependent increase of pressure difference duringgas permeation occurred before dust detachment, and a rapid increase of �pmax during dustdetachment was also observed. The time-dependent densification of ash layers and a rapidincrease of adhesion between ash particles were observed by thermomechanical analysis(TMA) and a diametric compression test of ash powder pellets. The increase in pressuredifference during dust collection is justified in terms of the decrease in pore volume andconsequent increase in fracture strength within the ash layer [178].
2.3.2. Effect of moisture
Generally, there are three types of adhesion forces between dust particles, electrostaticforce, van der Waals force, and forces from adsorption layers on particles or liquid bridgesresulting from sorption and condensation mechanisms due to the existence of humid gas.Humidity can also influence electrostatic and van der Waal forces, and is considered as amajor parameter influencing particle adhesion forces and compressibility of dust cake. Athigh relative humidity, the resulting cake resistance of the coarse dust is high and can evenreach values above those of the fine dust [44].
Due to moisture or condensed humidity to the system, the filter bag may have a dustcake that is difficult/impossible to clean and ultimately results in the formation of patcheson the bag surface. Once these patches are formed on the bag surface, removal of thesepatches become very difficult. This reduces effective bag area for gas flow, and consequentlydowntime of the unit increases. Moisture aside, the primary dust cake becomes dense overa period of time, decreasing the airflow permeability of the bag [35]. Further, water vaporsin the gas stream may result in water droplets to condensate in the filter. This condensedwater will reduce the available pore openings for gas flow, thus increasing the pressuredrop across the filter [57]. The presence of water vapor can also change the air density andviscosity, which subsequently effect filtration performance. Certain dust, like coal tar, isinherently glutinous; therefore, with the presence of moisture firm cake is built on the filtersurface, which is difficult to remove. The presence of moist heat can also cause a hydrolyticreaction with the filter material, which decreases the lifetime of filter. The polyester materialhas a poor resistance to moist heat, and loses its strength from prolonged exposure. The
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material strength reduction occurs up to 12% with 10% relative humidity and an operatingtemperature of 140◦C [179].
2.3.3. Effect of particle type, size, and shape
The particulate type, shape, and size play an important role in determining flow behaviorof gas through the fabric, particle capturing, re-entrainment, and cleaning. Leith and El-lenbecker [21] and Ellenbecker and Leith [180] had shown that the flux of dust throughpulse-jet-cleaned bags was predominantly affected by the dusts with three size distributions(granite dust, limestone dust, and fly ash), using polyester felt fabrics with three surfacetreatments (untreated surface, singed surface, and surface with microporous PTFE laminate)at three superficial filtration velocities (50, 75, and 100 mm/s) (Figure 20). PTFE-laminatedfabric showed the highest fabric and system filtration efficiencies (i.e. lowest outlet flux),whereas limestone and granite dust deposited on untreated and singed fabrics had the low-est fabric and system filtration efficiency. Interestingly, fly ash deposited on untreated andsinged fabrics had fairly high fabric filtration efficiency, but the lowest system filtrationefficiency. This poor system filtration efficiency is primarily due to inefficient fallout ofthese dusts to the hopper. At the time of filtration, fly ash as agglomerates is too small tosettle efficiently to the hopper before re-depositing on the fabric, which results in lowerfallout efficiency. These experiments also indicate that dust re-deposition is a significantcontributor to dust buildup on nonwoven fabrics. However, all outlet flux values, regardlessof velocity, fabric, or dust type, correlate well with a single parameter, w2v/t (see Section2.1.3.1.). Fabric and dust types affect the amount of dust w carried by a bag; the amount ofdust carried largely determines outlet mass flux.
An experimental study [181] showed the dependency of the dust cake structure on theadhesion characteristics of dust particles. The dust cake composed of sticky particles islikely to have loose structure and consequently high permeability, whereas the dust cake
Figure 20. Outlet mass flux for fly ash, limestone, and granite dusts from polyester filter bag withPTFE-laminated surface (empty bars), singed surface (cross-hatched bars), and untreated surface(filled bars) [21]. Reprinted from D. Leith and M.J. Ellenbecker, Dust emission characteristics ofpulse-jet-cleaned fabric filters, Aerosol Science & Technology 1 (1981), pp. 401–408, with permis-sion of Taylor & Francis (Taylor & Francis Ltd, http:www.informaworld.com).
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composed of less sticky particles is likely to have dense structure and consequently lowpermeability. Some dusts are cohesive at low temperature but become tacky at highertemperature. Hygroscopic or deliquescent dusts attract moisture and agglomerates, whichaffects the effectiveness of filtration. However, not all the dust material plays a role in theefficiency. From a research study done by [182], it was found that type of material (Portlandcement, gypsum, coal, flour, and bran) has no significant effect on collection efficiency.
With the variation of dust particles and under the influence of high temperatures, cakesformed over rigid ceramic barrier filters are found to have considerable influence on thepermeability of the dust cake and the cake removal results. For all types of dusts, increasingtemperature leads to the formation of a more porous dust cake so that the pressure drop overthe dust cake is not influenced even though viscosity increases with rising temperature. Thedifferences in cake structure can be explained with two opposing mechanisms. On the onehand, the separation process of single particles is influenced by temperature via viscosityand smaller repulsive forces due to softening of the particle surface. On the other hand,stronger interparticle forces result in a more stable cake so that it does not collapse if highpressure differences come into effect. However, the resultant effect depends on the chemicalcomposition of the ashes. It was found that the cohesive properties of hard coal fly ashchange more with rising temperature than those of brown coal fly ash. It was clearly shownthat brown coal fly ash cakes can be removed more easily than hard coal fly ash cakes evenat 850◦C [154].
Regarding the impact of particle size, with the increase in the said parameter (in a widerrange), efficiency first decreases and then increases. In the surface filtration process, finersubmicron particles have the possibility to pass through the pores leading to greater pene-tration. The most penetrating particle size was identified as 0.1 and 0.4 µm, which dependson the aerosol velocity and charge [6]. With decrease in the particle size, pressure dropacross the filter increases. Theoretically, several equations [56,175,183,184] are developedto predict the pressure drop with the changes of particle size. Particle polydispersity andshape are also dominant factors of filtration drag [183,184]. Choi et al. [185] proposedequations which indicate that the dust cake of high porosity leads to the low value of pres-sure drop. However, pore size is an important factor affecting the filtration drag as in thecase where small particles form a dust cake of high porosity, these reveal a higher pressuredrop than that of large ones [185].
In order to investigate the filtration properties of fly ash from a conventional coal powerplant, the filtration drag across the dust cake over an absolute fiber-glass filter elementwas measured. A fluidized bed column was utilized to obtain a well-characterized particlestream. The cake resistance coefficient was analyzed by the equation proposed by Endoet al. [184] in order to observe the effect of particle size and polydispersity. The filtrationdrag was measured for three types of particle streams having the geometric mean particlesize of 3.15, 6.07, and 7.83 µm for the field applications of face velocity of 0.03–0.06 m/s−1
and area dust load of up to 0.2 kg m−2. A dust cake of smaller particle size showed largerpressure drop even though the porosity was higher and presented high compressibilityaccording to the face velocity. The particle polydispersity was also a dominant factoraffecting the compressibility of the dust cake. The trend of the compression presented bythe cake porosity showed that the porosity of the dust cake decreases almost exponentiallyaccording to the face velocity for a dust cake of small thickness [185].
Shape factor is an important parameter governing the dust behavior during filtration. Ina study [186] to find out the effect of particle shape on the filtration performance, pressuredrop across the dust cake of fly ashes from a conventional power plant (PC), fluidized bedcombustion (FBC), and paint incinerator (FI) were measured over a metal filter element. The
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Figure 21. Effect of particle type differing in shape on filter drag [186]. Reprinted from J.-H. Choi,S.-J. Ha and Y.-O. Park, The effect of particle shape on the pressure drop across the dust cake, KoreanJournal of Chemical Engineering 19 (2002), pp. 711–717, with permission of Springer Science andBusiness Media.
shape factors of PC, FBC, and FI ash were estimated as 0.91, 0.76, and 0.65, respectively,by the Ergun equation. FI ash with lowest shape factor and composed of aggregates of veryfine carbon particles presented the highest pressure drop among the fly ashes tested. On theother hand, PC ash is mostly composed of spherical particles and presented lower filter drageven though its particle size was smaller (Figure 21). The results implied that the irregularparticle tends to form a higher pressure drop and to be more compressible than a sphericalone. Further, the fine particles of FI ash have a tendency to be agglomerated at low transportvelocity. The aggregates are broken at high velocity of more than 21 cm sec−1. FBC ash,composed of jagged-type particles and containing high concentration of unburned carbon,showed higher pressure drop than that of PC ash composed mostly of spherical particles.
2.3.4. Effect of dust density
The effect of dust concentration and filtration velocity on filtration time, specific cakeresistance, and mean cake density was investigated in a pilot-scale jet-pulsed bag filter[98]. The experimental results indicated that the dust concentration has small influence onfilter cake density and specific resistance at constant filtration velocity, besides the obviousand proportional filtration time reduction with increasing dust concentration. Cake densityis also affected by dust concentration; a denser cake evolves at low dust concentration [98].
3. Modeling and simulation
In the foregoing sections, a brief summary of different modeling and simulation on differentaspects of pulse-jet filtration are presented.
3.1. Modeling on emission
There have been several attempts to theoretically correlate emission with different param-eters of a fabric filter. This has been difficult because of very complex behavior of fabricfilter. However, Viner, Donovan, Ensor, and Hovis [187] presented an empirical equationfor penetration, which is as follows:
Pn = Pns + (Pno − Pns)e−aw + Pnseep, (2)
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where
Pn = penetration,Pns = constant low-level penetration characteristics of a well-established cake,Pno = penetration through a portion of fabric from which the cake has just been re-
moved,Pnseep = penetration due to seepage of dust through the fabric,a = cake penetration decay rate, andw = amount of dust per unit area added to the cake over the filtration period,
From the above equation it is clear that penetration decreases from Pno just after cleaningto the constant value of Pns , which is a function of filtering velocity. The parameter Pns
increases with increasing filtering velocity. Pno can be considered as 0.01 since it is assumedthat clean fabric is 90% efficient. Low-level penetration Pns is the result through pinholes,i.e. when the cake is stabilized.
A fundamental relationship for emitted mass flux (N ), proposed by Leith and Ellen-becker [21,22], is modified to allow for an exponent, n, and was linearized by taking logsso that the constant k and exponent n could be determined by linear regression [188]
N = k
[W 2Vf
t
]n
, (3)
where
N = mass outlet flux from the fabric in kg m−2 s−1,w = the areal density of dust on the fabric in kg m−2,v = superficial filtration velocity in m s−1, andt = the time in seconds between cleaning pulses.
Analysis of the data showed that fabric surface treatment has an important effect onoutlet flux from a pulse-jet filter; hence surface treatment must be considered in a modelto predict outlet flux. However, the model’s predictability was found independently forcleaning mode or for time between cleaning pulses. Predictability for the developed modelwas found to be reasonably good.
3.2. On filter drag
The term ‘filter drag’ or ‘filter resistance’ is often used in the analysis of pressure drop.Apart from fabric, flow resistance is largely dependent on the cake porosity (dependent onparticle size and shape distribution) or density, permeability of the cake, and density of thesolid particles. However, in spite of many studies on the gas/solid filtration, the accurateprediction of the pressure drop for a special system is not easy because filtration behavioris multi-dependent on many factors from particle (shape, size, and density), gas (density,viscosity, and humidity), and the filtration conditions (face velocity, and cleaning methods)[45,48,56,163,185,186]. Over time, the filter resistance increases to as much as 10 timesthat of a clean bag [131].
Flow resistance increases as the surface area of particles increases, i.e. as the size of theparticle decreases or particles become rougher. Resistance also increases as porosity of dustmass decreases and as degree of cohesion between the dust and the fabric increases. Highdegree of cohesion also results in thicker dust [53], which further increases the filtrationdrag. There are several other factors, such as compressibility, nature of cake growth, and
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cake detachment which have a very significant influence on filter drag. However, in thissection only simplified modeled expressions are discussed.
The pressure drop across the filter fabric without considering dust layer/cake formationcan be derived from the following expression [189]:
�P = ηLVf (α)
d2f
, (4)
where
η = fluid viscosity,L = thickness of filter,V = face velocity/aerosol velocity,f (α) = function of filter solidity, anddf = fiber diameter.
The resistance to the flow of air by a fabric is dependent on fiber fineness, fabricporosity, weight, and thickness. An empirical relationship based on step-wise multiplelinear regression was built to find out air resistance of filter fabric (without dust) [190].
r = 15.73 + 141.1 m − 0.012h3
(1 − h)2 d+ 29034
t
d, (5)
where
r = air resistance (Ns m−3),m = weight per unit area of fabric (kg m−2),h = porosity of fabric,d = fiber fineness (dtex), andt = thickness of fabric (m).
Prediction of initial pressure drop of fibrous filter media can also be made using anumerical modeling (CFD) approach [191]. Although applicable for HVAC filters, usingstochastic simulation, Wang, Kim, Lee, & Kim [192] found that the pressure drop has alinear relationship with filter thickness and exponential relationship with filter porosity andfiber diameter.
In the filtration process, the dust accumulates on the fabric to form a dust cake, althoughnon-dust cake filtration is more common. Mostly a one-dimensional flow and uniformdistribution of cake area load (w), mass of filter cake per unit of filter surface area, isassumed for modeling overall filter pressure drop. Koehler and Leith [116] proposed theempirical relationship to express the fabric contribution to pulse-jet pressure drop underthe conditions of non-dust cake filtration.
�P = PS + K1Vf − √(PS − K1Vf )2 − 4 WOVf K2/K3
2+ KV V 2
f , (6)
where
�P = the equilibrium pressure drop across the dust loaded bag (Pa),PS = the maximum static pressure developed inside the bag during cleaning (Pa),K1 = the clean fabric resistance (Pa s m−1),
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K2 = differential dust deposit specific resistance (s−1),K3 = a coefficient relating dust removal efficiency to bag pulse properties,Vf = face velocity (m s−1),WO = dust areal density added during one filtration cycle, kg m−2, andKV = venturi nozzle resistance (Pa m−2 s−2).
Bush et al. [53] suggested an empirical formula for pressure drop after an extensivestudy through laboratory research of dust cake ash,
�P = 9958(µUW/ρD2p)[{(1 − ε)/ε}{30.0 + 36.2(1 − ε) − 143(1 − ε)2
+ 2240(1 − ε)3}], (7)
where
�P = pressure drop (inch WG),µ = gas viscosity (poise),U = face velocity of gas (ft min−1),W = areal density of ash (�b ft−2),p = average particle density (g cm−3),Dp = drag equivalent diameter of ash (µm), andε = porosity of the porous bed.
Various theoretical and experimental studies are reported in literature to provide under-standing of the underlying processes of cake formation and detachment, which affect thepressure differential (�P ). Mostly, the �P within the filter media (�Pm) and the �P
within the cake (�Pc), are taken additives based on the assumption that the resistancescontributed by the filter media and the cake act in series [141]. It is also important tonote that most of the studies on cake filtration are based on an approximate theory of flowthrough compactable, porous media, which consists of Darcian-type equations that give theflow rate a function of the gas pressure gradient. The application of the law of D’Arcy withconstant specific filter cake resistance to filter modeling at constant gas velocity is valid ifthe rise of the pressure drop versus time is linear. In one approach [5], the overall pressuredrop has been conceived of two components: pressure drop across the conditioned filter(a filter that has been exposed to dust for some time), and pressure drop across the filtercake. The former is a function of filter permeability, filtration velocity, and fluid viscosity.The latter depends on cake area load, dust concentration, filtration velocity, and a constant,which accounts for cake properties, i.e. specific cake resistance. The equation for drag issemiempirical and can be described by the following basic filtration equations [5]:
�p
vf
= Sf = SE + K2 (w − wR) = SE + K2w0, (8)
�p
vf
= (SE + K2w0) vf = Rf vf , (9)
where
�p = pressure drop across the filter bag,vf = filtration velocity,K2 = specific resistance coefficient of dust cake,Sf = filter drag,
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SE = effective drag, w is mass areal density of the dust cake,wR = residual dust areal mass density,w0 = dust mass areal density added during filtration cycle rather than the total mass
areal density, andRf = filter’s final resistance coefficient.
The constants for the equations can be determined by curve fitting these pre-determinedfunctional forms to data, which may or may not realistically describe the general filtrationbehavior. Major significant variables, such as time, electrical properties, dust chemicalcomposition, and others, remain unincorporated. Dennis and Klemm [25] have proposed amodified equation for the filter drag by adding a new term (K2)c wc, as described by thefollowing relationship:
Sf + SE + (K2)c wc + K2w0, (10)where
(K2)c = specific resistance coefficient for the cycling fraction of the total dust massareal density that is alternately dislodged and re-deposited on the fabric,
wc = cycling portion of the dislodgeable dust mass areal density, and(K2)cwc = drag contribution of the cycling portion of the dust mass areal density on the
fabric.Dennis et al. [114] had rewritten the above equation as
�p = (pE)�w + K2w0vf = (pE)�w + CK2v2f �t = Rf vf , (11)
where the fresh added dust areal density, w0, during the filtration interval, �t , is expressed asCvf �t , with C as the dust inlet concentration. The variable, (pE)�w, herein is defined as theeffective residual pressure loss. The (pE)�w can be obtained from the intercept of the linearextrapolation of the pressure–time curve with the vertical axis at the resumption of filtration.The slope of the pressure–time curve equals CK2v
2f , where K2 is easily obtained from the
slope when the inlet dust concentration and filtration velocity are constant. The similaritybetween Equation (11) and the basic filtration equation (Equation (9)) is obvious, and, inlimit of negligible wc, Equation (11) becomes identical to the basic filtration equation.However, Equation (11) can be applied to the online pulse-jet filtration. When the filtrationprocess reaches steady state, the effective residual pressure loss and the specific resistancecoefficient of dust cake will be constant [193]. Equation (11) also shows that an increase offilter face velocity influences the pressure drop disproportionally – theoretically with thesquared speed, and in practice with compressing filter cakes coupled with other factors, theeffect is much stronger. A change in filter face velocity by 5% can already be decisive forstable or unstable filtration operation [194]. For a constant filtration velocity and inlet dustconcentration, lower effective residual pressure loss represents a longer filtration time and abetter cleaning effect. The magnitude of (pE)�w is related to the cleaning energy. The effec-tive residual pressure loss can be used as an index to evaluate the bag-cleaning effect [193].
A widely used relation to predict pressure drop has been suggested by Viner et al.[187,195], Shilling et al. [196], Pontius et al. [197], and Crawford [198] similar to the onegiven by Dennis et al. [114], which is of the form as shown below:
�P = �PResidual + �PVariable
= (Vf RR + RSCiV
2f t
)(12)
= Vf (RR + RSmdust), (13)
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where
�P = pressure drop (kPa),RR = residual resistance or initial drag (kPa.min/m or N.min/m3),RS = specific resistance factor or specific cake resistance (kPa/(mm/s)/(g/m2) or
N.min/g.m),mdust = mass of dust cake or arial density (kg/m2 or g/m2) = CiVf t ,Vf = filtering velocity (mm/s),Ci = dust concentration at inlet (kg/m3 or g/m3), andt = time at any instant (s).
Pressure drop due to residual resistance (�PResidual) is caused by fabric under a steady state(with changed flow characteristics due to trapped finer particles), whereas variable pressuredrops (�PVariable) is the pressure drop due to gradual dust deposit and increases with thethickness of dust cake. This pressure drop depends on the specific cake resistance.
Residual resistance is at a minimum when filtration process starts with a new fabricbut reaches a stable value as the fabric is stabilized. Initial drag depends on various factorssuch as cleaning intensity, number of pulses, particle-size distribution, and fabric slack.The penetration or accumulation of particles in the fabric material is assumed to take placewhen the fabric is least protected, i.e. exposed to high-impact velocity, which takes placejust after the cleaning. Initial drag increases with the number of pulses until the drag isstabilized after a number of cleaning cycles (Nc) is reached. The dependence of residualresistance on particle size is also found to be related to particle diameter of D10 (10%particle is smaller than the given size). Usually coarser dusts give rapid stabilization forwoven fabric with larger pores, while fabric with fine pores handling fine dust particlesrequires a longer time to stabilize. The specific cake resistance (RS) varies with the inverseof particle mass mean diameter. Since it is progressively increasing with time, it is logicalto consider a mean value over the cycle time tc [7].
Another way of representing D’Arcy’s equation is as follows [158]:
�p = η
A
(αm
m
A+ RT
) dV
dt, (14)
where
�p = pressure drop (Pa),η = dynamic viscosity of the gas (Pas),αm = mass-related specific filter cake resistance (m kg−1),m = number of rows/compartments of filter bags,A = total filter area (m2),RT = total cloth resistance (m−1),V = filtrate volume (m3), andt = time (s).
The simple D’Arcy equation with its linear dependence of pressure drop on the cakemass does not consider cake compaction or depth filtration shortly after filter cleaning. TheD’Arcy’s law with constant specific filter cake resistance (α) is applicable to filter modelingat constant gas velocity if the rise of �P versus time is linear. At changing velocity, the�P coefficient of filter cake is intrinsically not constant [45,59]. A similar approach tothe above was adopted by Sasatu, Misawa, Shimizu, and Abe [199], wherein pressuredrop across a hot-gas filter (ceramic tube filter) is considered to consist of irreversible and
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reversible pressure drops. In the study, two different configurations of cyclones and ceramictube filters were examined in a commercial size PFBC system, where several coals anddomestic limestone as absorbents were used. Dust concentration to the filter was foundto improve the predictability of the irreversible pressure drop across the system. It wasproposed that the particle size of ashes on the filter must be considered in order to obtainbetter prediction of maximum pressure drop in the future.
A two-dimensional quasi-stationary flow model [159] is used to predict pressure dropsas a function of regeneration efficiencies and regeneration patterns of ceramic filters, takinginto account the finite thickness and flow resistance of the medium itself. The effect ofnonuniform cake buildup on the pressure rise during a filter cycle is also modeled fora partially regenerated filter. The calculations prove that the pressure-drop rises fasterfor lower-regeneration efficiencies and that cycle time also becomes briefer with lower-regeneration efficiencies. It can also be shown that the regeneration pattern only influencesthe pressure-drop curve at the very beginning of the filtration cycle but does not influencethe filtration cycle times [159]. In order to predict the pressure drop across the cake of coalgasification ash formed on a ceramic filter, an empirical equation was developed, takinginto account several factors, such as the face velocity, ash load, shape factor and size ofparticles, and especially the operating temperature [177].
Different imperfections in dust cake manifest themselves most obviously in the curveof the pressure drop versus time. A convex pressure-drop curve indicates cake com-paction. Compressibility of dust cake is an important consideration during cake growth[45,56,200,201]. However, jet-pulsed filters frequently show a concave rise of the pressure-drop curve. This phenomenon is due to a strongly nonuniform cake area load on the filterand is generally attributed to incomplete cake removal. Incomplete cake removal takesplace when only a fraction of the total filter area is cleaned at the end of a filter cycle orwhen patchy cleaning prevails. Patchy cleaning means that a jet-pulse removes the entirefilter cake of only a fraction of the exposed filter area except for a thin adhesive dust layer[141].
A procedure was developed [86] to calculate the pressure drop during cake buildup,taking into account the adjustment of the cake properties to the compressive stress througha constitutive equation. The rise of pressure drop with time in a cycle was determinedassuming that the gas velocity was constant and that the layers were formed in equal timeintervals. Most studies on cake filtration are based on an approximate theory of flow throughcompactable, porous media, which consists of Darcian-type equations that give the flow rateas a function of the gas pressure gradient, and of constitutive equations that relate the cakeproperties to the compressive loads due to frictional drag [202]. D’Arcy’s law, the Happelcell model, and the Carman/Kozeny equation were tested by calculating the pressure dropthrough each layer of filter cake [86]. The introduction of an empirical settling factor in themethod brought the results closer to the experimental values in all three cases. However,D’Arcy’s law, which is a pressure-drop equation that does not depend directly neither onthe knowledge of the particle diameter and sphericity, nor on cake’s porosity, provided thebest agreement with experimental data [86].
3.3. On cake growth
The formation and characterization of dust layers are of great significance in filtrationprocess engineering. Cake growth depends on the efficacy of cake detachment and alsoinherently linked with cake compaction. Filter cleaning occurs if the dislodgement forceinduced by air jet exceeds the cake-fabric adhesive force. As adhesive force of a given
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system is often not a fixed value but covers a range of values, filter cleaning may beincomplete. Repeated filtration and filter cleaning lead to the presence of dust deposits ofdifferent thicknesses over bag surface. This is a common feature of fabric filtration. Cakegrowth also depends on the solid mass stream, which gets retained at the filter cloth. Thisstream may vary with time, e.g. it may be enhanced shortly after each jet-pulse during acleaning procedure, when the solid concentration in the gas is increased due to shatteringof removed cakes or hampered sedimentation of the newly arriving solid stream. Further,nonuniform distribution of residual cake is a result of incomplete cake detachment, butmay also be due to the reattachment of detached cake on the neighboring or the same bag.The comparison of the modeling results to experiment on the above aspect provides insightinto the reasons for patchy cleaning and shows the influences of cohesive and adhesivebonds as well as of the regeneration parameters on the patchy cleaning patterns on thelocal frequency of regeneration and on the overall regeneration efficiency over a seriesof filtration cycles. The cake height distribution affects the filter medium as well as thefilter cake resistance parameters. The increased resistance is responsible for shortening offiltration time, which may lead to unstable operation in extreme cases.
Schmidt [203] presented a computer simulation with which the creation and growth ofa dust cake can be modeled during the filtration via particle trajectory calculations. Theinfluence on the gas flow field exerted by those particles which have already been depositedis taken into account in a macroscopic way. Simulation is based on the input parameterssuch as the size distribution of the raw-gas particles, the separation characteristics of theclean filter medium, and the filter face velocity. The output parameters include the localdust cake porosities, the time-dependent pressure loss, and fractional separation efficiencytrends, together with the clean-gas particle concentration [203].
In the work of Hoflinger, Stocklmayer, and Hackl [204], the load force acting on theparticles and the shear force between particles were estimated when spherical particleswere collected on a filter. By comparing both forces, the deposition state of particles wasdetermined. Upon adding particles continuously, a dust cake was formed. The approachcould only provide qualitative explanation of dust cake formation because of the uncertaintyin estimating the shear force between particles in the dust cake. In one approach, thesimulation of the buildup of a filter cake was based on the division of a dust cake layerinto many tiny thin layers [205]. For each layer, a different porosity could be given and thepressure drop of each layer was then calculated. As a result, the relationship between thetotal cake thickness and the total pressure drop was determined. A fitting parameter, thevolume-loading data, was required for the general application [184].
Dust cake formation and structure on the surface of air filters were studied theoreticallyand experimentally. The principal results are as follows [184]:
1. Dust cake loading parameters have been obtained, which incorporate the effectsof particle polydispersity and nonspherical shape. In the derivation, the dust cakestructure (porosity) has been given as a function of operation conditions (pressuredrop and filtration flow rate), particle characteristics (panicle-size distribution andparticle shape), and cake layer thickness.
2. The correlation suggests that the effects of polydispersity and irregular shape of dustparticles are significant for dust cake loading.
3. In the experiments, dust cake height, pressure drop, and final cake mass weremeasured for Arizona Road Dust, alumina particles, and talc particles. Experimentalresults for all dust particles are shown to correlate well using the theoretical equation.
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The equation may be used for designing of filtration devices operating in the dustcake filtration regime.
4. When the cake height reached several hundred micrometers thick, a compressionof dust cake layer occurred. The result of dust cake compression can be explainedtheoretically by comparing the strength of cake layer and the drag force on the cakelayer.
It is possible to simulate the buildup of a filter cake with the so-called ‘layer-by-layer’model. The calculated pressure change during the buildup was found to be in agreementwith the measured one. The calculated specific filtration resistance, however, showed somedeviation compared to the measured data [205].
A filter model is proposed [141] in which the different classes of cake thicknesses areunderstood to result from different cake generations. A cake becomes one generation olderwhen it survives the jet-pulse cleaning at the end of a filtration cycle, although the area thatis occupied by the cake on the filter medium is diminished by the jet-pulse. The generationof filter model involves the distribution of age, thickness, and gas velocities in the cake fromsteady-state operational data. The filter model simulates the transient growth of the cakewithin one filtration cycle in a steady-state, periodic run of the filter. The model based onthe assumption of rectilinear, parallel gas flow is validated with macroscopic experimentaldata measured in a pilot plant of a flue gas-cleaning process. The transitional condition isfulfilled in the model, i.e. the initial distribution of the cake thickness over the filter area atthe start of a cycle is restored at the end of the cycle by the jet-pulse cleaning. The filtermodel allows us to evaluate the distribution of the cake thickness and gas velocity over thefilter area as well as the distribution of the age of the solid in the cake to be determined atany time during the filtration cycle. It is demonstrated that the open-model parameters areconfined to narrow limits for good agreement between model and experiment. The behaviorof the filter can therefore be described unambiguously, using periodic, steady-state data ofa filtration process alone [141].
Consideration of cake compaction for the modeling of patchily regenerated filterscould be made when a complex cake formation model [206,207] is combined with a modeldescribing the maldistribution of solid over the filter area. Based on the above, a filter model[158] was developed to decide the distribution of the cake thickness and gas velocity overthe filter area as well as the distribution of the age of the solid (dust) in each layer of thefilter cake. The model provides a comprehensive basis for the simulation of the filter cake inits capacity as a fixed bed reactor and also the distribution of the residence time of the solidin the filter. A by-product of the model application is an estimate of the total solid holdupon the filter cloth at the modeled filter-operating point. The model considers an imperfectlycleaned jet filter (as a result of patchy cleaning) with constant gas stream. It divides the filterin discrete areas where each bears a filter cake of uniform thickness, and each model area isrepresented by one D’Arcy equation. The D’Arcy equation can be applied simultaneouslyto any number of model areas with uniform pressure drop, when rectilinear and parallel gasflow through each model area is assumed. In the model, the compartments that are cleanedand those that are not cleaned are distinguished. It was explained that this separate treatmentof cleaned and not cleaned areas of filter is vital when the model yields the genuine functionof the fraction of detached cake dependent on cake thickness [158].
The solid hold up of the filter was predicted by the model application on the basis oflimited macroscopic experimental data and it was reported to agree excellently with the ex-perimental values. Besides, it was demonstrated that the distribution of the solid load versusthe complete filter area is equal at any one start of periodic, steady-state filter operation,
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provided there is a set cyclical pattern of rows of filter bags to be cleaned. However, inreal practice, under each cleaning procedure, different rows of filter bags undergo jet-pulsecleaning and usually not all rows are cleaned in one cleaning procedure. The filter-operatingpoint at the higher pressure-drop level revealed a stronger maldistribution of the solid loadover the filter area, which was proven by an objective criterion. One consequence of suchmaldistributions of the cake load is an uneven distribution of the gas velocity over the filterarea. This causes a rapid growth of the filter cake on the newly cleaned patches and anincrease in the growth rate of the pressure drop, which shortens the filter cycles [158]. In afurther study, the effect of dust concentration and filtration velocity on filtration time, spe-cific cake resistance, and mean cake density was investigated [98] in a pilot-scale jet-pulsedbag filter. An in situ optical system was used to measure cake thickness distributions onthe filter surface. Additionally, the operation was simulated using one-dimensional model[158] and results were compared with experiments.
During the periodical regeneration of filter media, the dust cake may be detached fromparts of the filter surface while other regions remain intact (referred as patchy cleaning).The filtration process depends on how these patterns of incomplete regeneration evolve overa number of cycles, how they change the buildup of the new cake, and how they affect thepressure drop. Nonuniform distributions of the cake load over the filter area as a result ofpatchy cleaning cause severely uneven distributions of the filtrate stream over the filter area.A one-dimensional model of the imperfectly cleaned pulse-jet candle filter was proposed[165,208] based on the simulation of the filtration process from its startup, where a uniformcake is formed on the clean filter medium during the first filter cycle. In the beginning ofthe next cycle, two areas are considered, one of these bears no cake at all and the otherbears the cake remaining from the first cycle. The tendencies of the cakes to be torn offthe filter depending on the number of cycles they survived are the free model parameters.Consequently, in the third cycle, three areas are to be treated and in the nth cycle there aren different areas. The developed probabilistic model describes the area distribution of cakepatches with different ages, the thickness distribution, the velocity distribution, and thepressure-drop history. A generalized probability distribution had been derived. Differentmodes of patchy cleaning were proposed and could be simulated by changing the value ofa weighting factor contained in the generalized distribution. Incorporated into a fixed-bedreaction model, this model makes it possible to examine the effect of patchy cleaning ondry scrubbing of acid gases in filter cakes [165].
In an extension to the earlier study, it has been found [208] that patchy cleaning of thefilter was identified as the major cause of inefficiency of filter cleaning indicated by themeasured residual pressure drops. A simple correlation had been proposed to determinethe cleaned fraction from pressure differences as a measure of cleaning efficiency. Theconditioning curves had been simulated using the earlier probabilistic model and it had beenshown that the model could also be applied to simulate the experimental results obtainedin a pilot plant operating at high temperatures. The modeling results were consistent withthe experimental observation that patchy cleaning with a thin residual dust layer in thecleaned areas could explain the conditioning behavior. The experimental and modelingresults suggested that the mode of patchy cleaning varies with operational conditions. Cakecompression occurring at higher velocities, higher cake loadings, and higher temperaturescan influence the cleaned fraction and the mode of patchy cleaning [208]. However, theabove model does not determine the intrinsic detachment property of the filter cake, whichis the fraction of a filter cake of certain thickness that is removed when exposed to jet-pulsecleaning. Moreover, the removed fraction of filter cake is related to the number of filtercycles it has undergone instead of the more practicable cake thickness.
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Dittler and Kasper [159] performed a numerical study of the two-dimensional flow fieldto simulate the transient growth of the filter cake in which neighboring patches with andwithout filter cake interacted and used the same model to simulate the transient growthof the filter cake. The size and cake thickness of single patches in conjunction with thelength of the boundary to the neighboring patches are decisive for the plausibility of theassumption of rectilinear, parallel gas flow. If the cleaned patches are small, then thepressure drop is also smaller. In the fringe zone between thick and thin cakes gas enters thethick cakes sideways, thus increasing the permeability of the cake. However, it could beuseful if the actual sizes of cleaned patches in filtration systems allowed for a simplifyingone-dimensional model approach.
Ahmadi and Smith [209] developed a computer simulation model for gas flow andparticle transport and deposition in the Integrated Gasification and Cleanup Facility (IGCF)filter vessel with ceramic candle filters. The IGCF, which is an experimental pilot plant fortesting performance of ceramic candle filters for hot-gas cleaning, has been operational atthe Federal Energy Technology Center (FETC) in Morgantown, West Virginia, for severalyears. The stress transport model of FLUENTTM code is used for evaluating the gas meanvelocity and the root mean-square fluctuation velocity fields in the IGCF filter vessel. Theinstantaneous fluctuation velocity vector field is simulated by a filtered Gaussian white-noise model. Ensembles of particle trajectories are evaluated using the recently developedPARTICLE code. The model equations of the code include the effects of lift and Brownianmotion in addition to gravity. The particle deposition patterns on the ceramic filters areevaluated, and the effect of particle size is studied. The results show that, for a cleanfilter (just after the backpulse), the initial deposition rate of particles on the candle filtersis highly nonuniform. Furthermore, particles of different sizes have somewhat differentdeposition patterns, which could lead to nonuniform cake compositions and thicknessesalong the candle filters. The effects of variations in filter permeability on the vessel gas flowpatterns and the pressure drop, as well as on particle transport patterns, are also studied[209].
The filtration and the reversed flow process at ambient conditions were investigatedby Chuaha, Withers, and Seville [210] using a cylindrical and tapered ceramic filter. Theymodeled the gas flow along the ceramic candle filter by assuming one-dimensional steadystate flow. For the tapered filter, the model’s prediction was shown to have good agreementwith their experimental data. Tanthapanichakoon et al. [211] investigated the removalof particulate matter smaller than 2.5 µm (PM2.5) from high-temperature exhaust gasusing a twin ceramic candle filter. Computational fluid dynamics (CFD) was used tosimulate gas flow behavior since the technique could provide crucial support to experimentalinvestigation, especially for harmful conditions, as well as yield cost savings and fast results.During the study of dynamic behavior of gas flow in a prototype twin-candle ceramic filter,cleaning effect on an adjacent candle filter due to the pulsed cleaning of another hadbeen observed. It was useful to investigate the face velocity distribution on the candleas it illustrates how the filter cake is formed on the filter surface and hence leads toimprovement in the dust cleaning. The obtained simulation results were validated withavailable experimental data.
3.4. On cake compaction
Gas flow through the cake formed over the surface of a filter generates an aerodynamic dragthat increases mechanical compression on the layers closer to the filter surface, reducingtheir thickness and permeability [86]. Cake compaction at a constant gas stream can be
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Figure 22. Three-layer structure of a dust filter cake [200] Reprinted from Ch. Stocklmayer andW. Hoflinger, Simulation of the filtration behaviour of dust filters, Simulation Practice & Theory 6(1998), pp. 281–296, with permission of Elsevier B.V.
indicated by a progressive shape of the pressure-drop curve [158]. The compression behav-ior of a dust filter cake during cake buildup was experimentally and theoretically analyzedby several researchers [44,86,200,204]. The cake was found in general to consist of at leastthree layers of different structures (Figure 22). There exists a loose upper layer, a strongbut compressible middle layer, and a compressed underlayer, which cannot be further com-pressed without destroying the particles. At the bottom of the layer, interparticle forces arehigh and particles move in a few compressible layers whereas in the middle layer, there isa possibility of random movement of particles in many partial layers. Interparticle force islowest at the top of the filter cake and the particles can move immediately after deposition[44].
In a computer model, the compression behavior was described by the friction angleand the maximum adhesion force between particles. The thickness of each layer in thecake or whether the cake consists of only one or two layers depends on the values ofthe friction angle (measure of shear behavior) and the maximum adhesion force, andthe whole cake thickness. The filtered particles initially form a loose layer on the filtermedium, with a low compression pressure �pk,j ,so that the strength parameter fj ,inthe layer was not exceeded. With cake building, this layer gradually becomes stiff andincompressible. At first, the strength increases very slightly because only a few particlesstart to move. With increasing the compression pressure, more and more particles areforced to move, in which particles can change partial layers at the compressible layer, andhence strength can reach a value which lies above the compression pressures. Later thispartial cake layer will appear incompressible in comparison with the compressible layer[44,200].
A computer simulation program was developed [204] to simulate the dynamics ofthe compression behavior inside the cake. The simulation reveals a correlation betweeninterparticulate forces and the resulting pressure drop in the cake. The simulation resultstheoretically demonstrate the internal structure of a filter cake, and give an explanationfor the layered structure of the filter cake. In their simulation method [204], they used themaximum adhesion force and friction angle of dust particles as simulation parameters tocalculate the resistance force on the compression of dust cakes through fluid drag force.By comparing the magnitude of the compression force with that of the resistance force on
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compression, the possibility of dust cake compression was determined. It was shown [212]that these parameters can be determined from a single-pressure-drop curve of one filtrationcycle.
Theoretically, filtration drag is expressed linearly with the area dust load at constantface velocity. However, several papers [52,56,204,213] have reported its curvature depen-dences, which show the compression effect with the increase of the dust cake. Aguir andCoury [56] had found that the cake porosity of limestone particles in a filter cake formedat constant gas velocity decreases towards the surface of the filter medium, although therise of the pressure drop versus time was approximately linear in their experiments. Thisprofile of the cake porosity along the depth of the filter cake indicates cake compaction.Cheung and Tsai [45] have shown experimentally that the pressure-drop coefficient ofthe filter cake and the average cake porosity changed when different constant face ve-locities of the gas through the filter were applied. Consequently, at changing face veloci-ties, the pressure-drop coefficient of the filter cake (which strongly correlates to the cakeporosity) is intrinsically not constant [45], and the nature of the filter cake will also bedifferent.
Face velocity is a dominant factor on the compression effect of dust cake [163]. Cheungand Tsai [45] correlated the specific cake resistance coefficient (K2) and the face velocity(Vf ) in equation K2 = fVf
n, where f and n are constants. They reported that several flyashes showed a wide range of compressibility constant (n) values from about 0.5 to 0.75.Later, Choi et al. [185] found that n had a value of 0.4–0.5, which is slightly lower thanthat reported earlier. This discrepancy in the n value implies that the geometric standarddeviation representing the polydispersity of the particle distribution was also one of thedominant factors affecting the compressibility of the dust cake.
In a simulation model [200] regarding dust cake buildup on needle felts, the programcalculates the growing of a dust filter cake and the cake compression for different operationand material parameters. Analyzing the resulting pressure drop curves and especially thestructure of the simulated filter cakes improves the understanding of the mechanisms causingcompression. The simulation program is capable of describing the particle penetrationleading to clogging of the filter medium and the growing of the dust cake on the filtermedium, whereby the corresponding pressure drop and the number of particles in theclean gas as a function of filtration time show good qualitative agreement with knownexperimental data [200]. It was found that the compressibility of the dust has a majorinfluence on the operation costs. The more compressible the dust, the sooner the filter hasto be replaced due to particles inside the filter medium, which cannot be removed anymore[201].
In one study [214], a simulation method was developed to get some insight aboutthe compression mechanism of dust cakes. The effect of particle size on the compressionbehavior of dust cakes was also investigated in this study aiming at extending the simulationmethod to the more general but complex case of cake filtration, which uses polydisperseparticles. This study considered the compression behavior of the dust cakes composedof monodisperse particles as a starting point followed by a complete understanding ofthe compression behavior of the dust cakes composed of polydisperse particles. In thesimulation method, the adhesion force between two contacting particles on the compressionof dust cakes was calculated by using the classical van der Waals theory, and the frictionforce resisting the compression force was calculated by introducing the friction coefficientbetween two contacting particles. Simulation results clearly show that adhesion, friction,and compression forces play a major role in determining the deposition morphology ofdust cakes depending upon their relative magnitude to each other. It was also shown from
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the simulation results that at the same extent of mass areal density of particle deposition,smaller particles tend to result in larger pressure drop through the dust cakes, as observed inthe experimental works of other investigators, as well as the more compact or compressedstructure of dust cakes [214].
While working on ceramic filter, the compression phenomena of ash cake have beenreported by several investigators [185,215]. Neiva et al. [175] proposed a cake buildup modeland reported that the compression of a given ash layer was related with the drag forces of itsupper layers. Compression of the ash cake leads to the compaction of the ash layer, whichresults in a reduction of cake porosity as well as an increase in pressure drop across the cake.Some computer programs [175,204] have been developed to calculate the pressure dropacross ash cake (�Pc), based on the above-mentioned theories, which have shown goodagreement with experimental results in limited cases or valid by specific parameters. Endoet al. [184] derived an explicit equation for predicting the �Pc of polydispersed particlestaking into account the geometric mean diameter, the dynamic shape factor of particles,and the particle size. However, the assumption of uniform porosity throughout the entireash cake restricts its application. Choi, Bak, Jang, Hyung Kim, & Hyun Kim [216] andChoi et al. [215] developed a more general modified Endo equation by adopting an extratwo empirical equations with respect to (1) the ash cake porosity depending on the particlesize, the face velocity, and the ash load, and (2) the void function depending on the ash cakeporosity and particle size. The predicted values obtained using the modified Endo equationshowed good agreement with the experimental data found for coal gasification fly ash atroom temperature [216]. Since, at high temperature, gas viscosity increases, it eventuallyleads to high compaction of the ash cake [177].
3.5. Modeling media movement and cake detachment
The dynamics of the fabric motion during cleaning pulse are modeled and comparedwith experimental measurements performed on an industrial-scale pilot plant filter [128].Building upon earlier work, a picture of how the cake is removed from the fabric during thecleaning process was presented. Equations describing the propagation of the cleaning pulsedown the filter and the acceleration that it produces are presented. The pulse is transmittedin a wavelike manner causing the fabric to expand, which controls the cleaning process.The model is extended to consider cage design and to allow for the slackness of fit betweenthe bag and the supporting cage. Good agreement was demonstrated between theoreticalprediction and experimental results [128].
De Ravin et al. [126] presented a detailed study on particle adhesion on filter surfaces andparticle removal by pulse air jets. They obtained an empirical relationship that can be usedin conjunction with jet jump characteristic curve to estimate the extent of filter cleaningas a function of cake thickness and the operating pressure. Modeling and experimentalinvestigations [16,40,102,103,105,111,139,217,218,219] demonstrated that stress buildupat defects in dust cake significantly reduces the strength of the real material well belowthe strength of an average bond in the material. Therefore, the stress needed to remove asizeable percentage of layer of filter cake can be as low as one-half the average bond strengthin the material. However, besides dust cake adhesion to the filter surface [102], the cohesivestrength of the dust cake [217,219], and operating conditions (e.g. the humidity) [220], thestructure of the filter medium itself affects cake detachment and hence the regenerationbehavior of the filter medium.
Ju, Chiu, and Tien [221] used the cake-detachment function of De Ravin et al. [126],which gives the fraction of cake removed when jet-pulsed as a function of cake thickness
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in order to predict an arbitrary filter-operating point. For the determination of this cake-detachment function, De Ravin generated a uniform filter cake on a small coupon filterand subjected it to deceleration tests. However, it is required to validate whether such filtercakes used to determine detachment functions reveal basically the same properties as filtercakes formed under real conditions in imperfectly cleaned filter installations.
Calle, Contal, Thomas, Bemer, and Leclerc [222] have developed a model describingthe behavior of filter media during clogging and cleaning cycles. Based on experimentalvalues of the residual pressure drop of a filter medium after each cleaning, its physicalproperties, and filtration characteristics, the model allows the determination of cleanedfraction at each cycle with the assumption of patchy cleaning. This characterization allowsus to determine the pressure-drop pattern of the filter medium over a number of cycles.Applying the model to three filter media with very different physical properties showedthat the assumption of patchy cleaning could be applied to these filtration conditions andgave satisfactory results regarding the cleaned fraction as the cycles progress. However, themodel does not take into consideration any treatment undergone by the medium to increaseits regeneration capabilities. Furthermore, it is based on the assumption of ideal patchycleaning, presenting either totally cleaned or uncleaned areas, which is far from reality.Nevertheless, this modeling has provided an understanding of the phenomena linked tocleaning and offers an approach which is coherent with reality.
The cake formation on nonuniformly covered filters may influence the cake detachmentdue to highly nonuniform gas velocities, though the overall gas flow remains constant.Therefore, ignoring the cake formation may lead to erroneous results. Other models de-scribing filter cake additionally consider cohesive stresses between single patches of filtercakes. However, the three-dimensional structural information of the filter cake needed bythese sophisticated approaches is not easily available [201,217,223,224].
It was shown [158] that cake detachment function strongly correlates to the operatingpoint of the filter. At an operating point with a higher pressure-drop level, a smaller fractionof cake of equal thickness is removed than at lower pressure drops; this was found foridentical pulse-cleaning conditions. Therefore, the prediction of filter behavior through themeasurement of such a cake-detachment function (e.g. laboratory-scale deceleration tests)is quite problematic. Good agreement between experiments in a pilot plant-scale cloth filterand simulation could only be achieved, when during the jet pulse cleaning the solid massstream to the filter was increased above its normal level (caused by the disintegration ofremoved filter cake) [158].
Zhang and Schmidt [164] introduced computation results of the model considering theadhesion of the layer on the substrate and the cohesion of the particles in the layer asimportant governing factors. The course of changing detachment force in modifying theregeneration course is not only time-related but also location-related. It is noted that, whilethe pre-assigned detachment forces can be partly changed (degraded) due to the detachmentof some cake elements during the course of regeneration, the influence on the achievableregeneration efficiency can be prognosticated to some extent. Since, in reality, detachmentis extremely nonstationary concerning place and time primarily due to changing conditionsof the detaching forces on the one hand and changes in the particle layer morphology on theother, setting the parameters for modification calls for great care. More computations areneeded to further test the reliability of the model and to explore its full potential: the size ofthe cake elements and the layer thickness could be varied; and combined modification oflocation-related gradients of the time constant and the limit of the detachment force wouldalso be worth trying. Furthermore, it is conceivable that the values of the adhesive forcescoming from some distribution can also be located according to some rules possibly due
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to surface characteristics of the substrate. So it is meaningful to specifically modify thelocation of the adhesive forces [164].
Schildermans et al. [155] discussed previously published models to predict the internalpressure and axial velocity profiles during pulse cleaning of rigid filters. All of these modelswere very complex, generally difficult to apply. In an experimental setup of pulse-jet-generating system of industrial ceramic filter, the instantaneous velocity of pulse-jet fromnozzle was measured and the transient flow rate of the pulse-jet nozzle was determined[225]. The influences of reservoir pressure and pulse duration were discussed. The resultsshow that the pulse-jet flow rate increases with the reservoir pressure. The long pulseduration results in an increase in the duration of the pulse-jet and has no influence on themaximum mass flow rate of the pulse-jet. The gas flow from the reservoir was treated as anadiabatic process to calculate the mass consumption of compressed gas per pulse. Basedon flow transients theory and thermodynamic relations, a dynamic model was presentedfor the pulse-generating system consisting of pulse gas reservoir, nozzle, solenoid valve,and connecting pipelines. The model was used to simulate the variations of the pulse-jetconsumption and the mass flow rate versus time. The modeling results of the mass flow rateagree well with the experimental data. The effects of pulse nozzle diameter, compressedgas reservoir volume, and interconnecting pipeline length on mass flow rate of the pulse-jet-cleaning system were analyzed. The results show that the dynamic model can be appliedto optimize the design of the pulse-cleaning system of industrial ceramic filters [225].
Li, Ji, Wu, and Choi [226] carried out computer simulation to analyze the steady-state filtration as well as the transient gas flow in ceramic filters during the pulse-cleaningprocess for the cleaning of a candle filter. The flow field is simulated for a ceramic filtervessel containing three candle filters, which are arranged in the form of an equilateraltriangle. The Reynolds stress transport model in the FLUENT code is used to simulate theuncompressible flow field in the normal filtration and the compressible flow field duringpulse cleaning. To study the pulse-cleaning process, a pressure boundary condition, whichvaries rapidly with time, is specified to one of the three nozzles. The simulated results showqualitative agreement with the experimental field observations with the filter vessel.
During normal filtration, the velocity vector distributions differ widely outside thecandle filter in the filter vessel. The radial velocity in the porous wall of the filter beingcleaned is uniformly distributed along both the circumferential and the z-axis directions,while the distribution of the radial velocity around the filter being cleaned is nonuniform.The variation of the radial velocity in the porous wall of the normal working candle filtersis quite small along the circumferential direction, whereas the variation along the z-axisdirection is significant. The calculation model applied in the study successfully shows thechange in transition flow during pulse cleaning [226].
The temperature profiles in the ceramic candle filter cavity during the pulse-cleaningprocess are also analyzed under different operating conditions and for different lengthsof candle filter. The temperature in the cavity of the filter being cleaned increases in theinitial stage of the pulse-cleaning process and then decreases as the cold pulse gas entersthe filter cavity. Sharp temperature gradients occur at the top of the candle filter in theinitial and final stages of pulse cleaning, so these places are susceptible to thermal shock.The maximum temperature increase occurs near the closed end of the candle filter withthe compression effect caused by the pulse gas. This compression effect is dominant if thepulse gas pressure is high and the length of the filter element is large. A higher temperatureof the pulse-cleaning gas can significantly reduce the thermal shock on ceramic candlefilters. For pulse gas with higher pressure, the change in gas temperature in the filters isgreater [226].
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In order to investigate the regeneration behavior and the operational performance of par-tially regenerated, rigid gas-cleaning filter media over many filtration cycles, experimentswere performed in a filter test rig [177]. The regeneration behavior of the filter sample wascharacterized by the overall regeneration efficiency, the local frequency of regeneration,and the number and size of regenerated filter areas. Favorable comparison of the regener-ation behavior between modeling and experiment was achieved with the assumption thatcohesive and adhesive bonds, which were broken during filter regeneration, did not repairduring the next filtration cycle. Assuming otherwise would cause (i) the dust cake to beremoved at the same positions during every regeneration, and (ii) the patch size to increasefrom cycle to cycle instead of decreasing as seen in the experiment. Therefore, the modelingwas found to be effective and realistic. Both filter conditioning and dust cake compressionsignificantly influenced the operational performance of partially regenerated filter media.The effects of dust properties [111], temperature [103], and chemical composition [209]on the filter cake strength and adhesion have been studied for ceramic filter [177]. Theseparameters have significant influence on cake detachment behavior.
During the operation of a high-temperature gas filter, varying degrees of incompletefilter regeneration lead to a progressive shortening of filtration cycles, which was bothmeasured and modeled as a function of time interval between cleaning pulses. Experimentswere performed with pressure-pulse-cleaned ceramic filter elements typically used in high-temperature gas filtration. It was found that a thin incremental layer of cake was left behindafter each regeneration pulse, which drove the loss of cycle length. The thickness of thisresidual cake layer was related to the solidification rate in the cake. For cake residencetimes below a critical value, the degree of solidification is too low to affect the stability.The regeneration efficiency is then controlled by other factors such as filtration velocityand regeneration intensity (cake thickness), as in any typical baghouse filter [227].
3.6. Steady state/unstable operation of jet-pulsed filters
Ju, Chiu, and Tien [228] presented a pulse-jet fabric filtration model that predicts filter per-formance from cycle to cycle and accounts for the nonuniformity of dust deposit thickness.It was shown from model simulations that, as the cycle of operation increases, a steadystate might be established. Explicit expressions of steady-state filter performance were alsoobtained. In another work, Ju et al. [221] tried to establish the applicability and limitationsof the simplified procedure by comparing the results from the simplified procedure withthose based on a more complete model. Based on their study, the following conclusions aredrawn:
1. In its basic form, a steady-state model has only limited predictive capability.2. The developed model on the dynamics of pulse-jet fabric filtration is valid for both
transient and steady states.3. Based on the comparison of results from these two models, an approximate ex-
pression that estimates the average filtration velocity over a cycle time from theinstantaneous filtration velocity at the beginning and end of filtration was estab-lished. With this approximate expression, all estimates of the essential features offabric filtration, such as cake dislodgement force, cake filtration resistance, fabricmedium resistance, residue case filtration resistance, instantaneous and average fil-tration velocity, mass of cake deposit, amount of particles retained per cycle, etc.can be evaluated in a simple and straightforward manner.
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Figure 23. Unstable filter operation at intermediate stage. Model simulations: deficient cleaning(top), filter media blinding (middle), increase of cake resistance (bottom) [229]. Reprinted from A.Kavouras and G. Krammer, A model analysis on the reasons for unstable operation of jet-pulsedfilters, Powder Technology 154 (2005), pp. 24–32, with permission of Elsevier B.V.
In the unstable filter operation, there could be steady increase in pressure level acrossthe filter and at times it can increase far beyond the set upper pressure drop (e.g. 1200Pa). Through pulse cleaning, both the pressure drop and the gas flow may be shortlyrestored at the old set level but immediately afterwards, unstable operation took placeagain. Unstable filter operation can be defined by a continuous or periodic reduction offiltration time per cleaning pulse. In a study done by Kavouras and Krammer [229], threedifferent mechanisms were investigated separately that, under certain circumstances, canlead to an unstable filter operation performance: cake detachment where a cake cannotbe removed once it has survived for a sufficiently long time, an increasing filter mediumresistance (entrainment of fine dust particles and more generally filter media aging), andan increasing filter cake resistance. While an increase in filter medium resistance and filtercake resistance leads to a more uniform cake distribution, deficient cleaning results in anincreasingly uneven cake distribution.
In the work done by Kavouras and Krammer [229], transient pressure difference patternswere simulated considering the above three different mechanisms separately at differentstages. During the first stage, the pressure-drop patterns are the same irrespective of themechanism that is taken into account. The following intermediate stage of unstable filteroperation is characterized by a distinct pressure difference pattern. Finally, the last stagebegins when permanent cleaning occurs and the patterns become similar. Figure 23 showsthe unstable filter operation at an intermediate stage at which the number of pulses requiredto reach the minimum pressure drop increases significantly (between 15 and 25 pulses)and the actual filtration period is down to 10–20 s. A comparison among different pressuredifference patterns of the intermediate stage reveals that the duration of a filtration stepincreases when the filter medium resistance increases, while the deficient cake detachmentand increasing cake resistance result in the reduction of the duration of the filtration step.Hence the pressure difference pattern is suitable to evaluate whether the filter medium
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Figure 24. Regular filter operation after increase of overpressure of gas cleaning [229]. Reprintedfrom A. Kavouras and G. Krammer, A model analysis on the reasons for unstable operation ofjet-pulsed filters, Powder Technology 154 (2005), pp. 24–32, with permission of Elsevier B.V.
resistance is the predominant mechanism for unstable filter operation. Since deficient cakedetachment and an increasing filter cake resistance cannot be clearly distinguished by theevolution of their pressure-drop pattern, the information about the dust holdup can be used.While both the increasing cake resistance and filter medium resistance cause reduction ofthe dust holdup, a deficient cake detachment leads to an increasing dust holdup. Therefore,the information of the pressure-drop pattern and the solid holdup allows the determina-tion of the governing mechanism. Unfortunately, the information of the solid holdup isoften not accessible. The number of cleaning pulses always increases as operation tendstoward permanent cleaning. Yet a comparison between deficient cleaning and increasingcake resistance with respect to the number of pulses required to reach the lower pressuredifference showed that substantially more pulses are required when deficient cleaning dom-inates. However, since the filter is intrinsically loaded unevenly, the filtration velocity isalso locally different. The velocity that prevails during cake formation may also influencethe cake resistance [229].
It has been observed [229] that filter operation remained sustainably stable after havingincreased the pulse reservoir by 1 bar (100 kPa) (Figure 24). A higher pulse reservoirpressure results in a more intense cleaning and a different filter cake buildup. Hencethe filter cake resistance remains within the acceptable limits and the shape of the cakedetachment function may change in such a way that the formation of less cleanable patchesdoes not arise [229].
3.7. On system and filter media design
It is an important task to develop a control scheme to maintain optimal operating conditionsin the presence of process disturbance and/or fluctuation in filtration demand. A numericalmodel was developed to simulate the unsteady, pulse-jet cleaning process for a fabricfiltration system and subsequently find the best design and operating conditions that providemore cleaning force for bag cleaning [131]. The simulated results of air pressure and flowdistributions in the system agree reasonably well with the experimental data. The studyshows that tank volume, initial tank pressure, nozzle diameter, distance between nozzle
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and bag top, and pulse duration are the major parameters influencing the pressure impulsein the fabric bag. For the investigated system, the optimized nozzle diameter is 30 mm,pulse duration is from 300 to 600 ms, distance between nozzle and bag top is 60 cm, andtank volume is 0.3−0.5 m3. Keeping all other conditions fixed, increasing the tank pressureappears to be the most convenient way to achieve a higher bag-cleaning efficiency [131].
In pulse-jet filtration technology, if mathematical design procedures are not availableand predictive techniques are not yet sufficiently accurate to allow more simple processcontrol algorithms to be applied, then other types of control strategies are required. In thisregard, an expert system can be devised to assist in the process control and to ultimatelyoptimize the plant. Due to the complex characteristics inherent in the filtration and cleaningprocess, the conventional proportional integral (PI) controllers may not provide adequatecontrol of fabric filtration systems. Earlier, Morris [146] discussed some preliminary resultson the expert control of fabric filter operation. The use of expert system controllers dependson several factors: firstly the gathering of critical data, secondly the application of rules, andthirdly the action taken when required by the data that is received. The parallel within theprocesses used by experts is direct and the aim of successful expert system design is to allownonexperts to interact with, modify, utilize, and interrogate the system in a user-friendlymanner.
A modeling and control strategy based on the local model network (LMN) was proposedto handle the nonlinear dynamics of pulse-jet fabric filters [230]. In the absence of priorknowledge for the determination of LMN structure and the weighing functions, an extendedself-organizing map (ESOM) network could be useful. The ESOM was developed [230]to construct the LMN model of the filtration process using the filter’s input–output data.These ESOM local models are incorporated into the design of local generalized predictivecontrollers (GPC), and the proposed controller design is obtained as the weighted sum ofthese local controllers. Using the model parameters of Ravin and Humphries [126], thesimulation results showed that the proposed controller design yields a better performancethan both conventional GPC and PI controllers [230].
In one study [231], a model predictive control (MPC) technique is employed owing toits distinct advantages. Unlike its conventional counterpart, the proposed MPC algorithmtakes the economic factor into consideration, which is formulated in a multiple-objectiveoptimization framework. Since the nonlinearity is inherent in the studied filtration process,the resulting problem is a nonconvex problem. Thus the goal attainment is advantageousover the simple weighted sum method. Due to the limitation of the conventional opti-mization solvers, which often yield a local optimum, the global optimization techniqueis incorporated into the proposed MPC design. Simulation results show that the proposedmultiple-objective optimization-based MPC design method is especially suitable for thepulse-jet fabric filtration process, where the set point changes and process disturbanceoccurs frequently [231].
Based on flow transients theory and thermodynamic relations, a dynamic model waspresented for the pulse-generating system consisting of pulse gas reservoir, nozzle, solenoidvalve, and connecting pipelines. In the model, the gas flow from the reservoir is treatedas an adiabatic process to calculate the mass consumption of compressed gas per pulse.The model is used to simulate the variations of the pulse-jet consumption and the massflow rate versus time. The modeling results of the mass flow rate agree well with theexperimental data. The effects of pulse nozzle diameter, compressed gas reservoir volume,and interconnecting pipeline length on mass flow rate of the pulse-jet-cleaning system areanalyzed. The results show that the dynamic model can be applied to optimize the designof the pulse-cleaning system of industrial ceramic filters [225].
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Computational simulation [232] was performed to examine characteristics on a flowfield and particle behavior in a bag filter that combines centrifugal force in existing bagfilter system. The fluid coming in from the tangential direction forms a strong vortex inthe space between the inner and outer tube and the flow goes down along the hopper wall.Partially, this flow merges with the up-flow from the inner tube to the upper tube wall. Theup-flow changes its direction at the shell plate where the bag filters are mounted and it startsto go down with a vortex along the central axis. It may be added that most of the bag filtersare located in the down-flow region, which plays an important part in the particle fallingdown to the hopper more easily when dust cake is removed by cleaning [189].
To optimize the filter design, Genetic Algorithm (GA) can be used. GA is used in searchand optimization, such as finding the maximum of a function over some domain space [233].GA is robust in selecting optimum size and amounts of nanofiber microfiber, and otherdesign parameters for composite filter media used for depth filtration [234]. The goal is tofind an optimum solution, given the range, for a set of design parameters: thickness of themedia, diameters of microfiber and nanofiber, surface area ratio of nanofiber to microfiber,and mass of microfiber. The objective of the work was to develop a software programthat can be used as an aid to accelerate filter design and reduce the number of laboratoryexperiments. This program applies GA to search for an optimum filter media design basedon quality factor, which quantifies the filter performance. The program provides a startingpoint for constructing filter media for the testing and design of particular applications [234].A similar approach (GA) can be adopted for design optimization of industrial surface filters.
3.8. Modeling on gas/solid reaction in filter cake
In the process of dry flue gas cleaning, a prior attempt to model SO2 removal in the jet-pulsedfilter was made by Chisholm and Rochelle [235], who examined the reactions of SO2 andHCl in a growing filter cake starting from the perfectly clean filter cloth. Marques, Herrera,Garea, and Irabien [236] also simulated SO2 removal in a growing filter cake. However,in both studies, the maldistribution of the solid load over the filter area and its residencetime distribution has not been taken into account. In recent decades, a large number ofkinetic studies on the dry reactions between SO2 and Ca(OH)2 have been reported. Basedon laboratory experiments, different effects were deemed to be crucial for the sorbentreactivity with SO2 and the maximum sorbent utilization: fly ash addition to the sorbent[237], activation with water [238], competitive carbonation and sulfation [239], and thepresence of HCl and NOx in the flue gas [235]. As a consequence, while comparing theresults of Chisholm and Rochelle [235] Garea et al. [237] and Krammer, Brunner, Khinast,and Staudinger [240], extreme discrepancies were also found in the reaction rates and finalsolid conversions of the Ca(OH)2 sorbent with gaseous SO2 in laboratory experiments,depending on the sorbent treatment and the experimental gas atmosphere. The fixed-bedreaction algorithm is based on the mass balance equations for SO2 of the solid and gasphases in the filter cake. This algorithm considers a moving boundary due to the growthof the filter cake. The source/sink term in the mass balance equations is formed by anempirical kinetic equation [241], which was derived using a solid sample from the pilotplant experiment and also modeled in the study by Kavouras et al. [242]. The concentrationof SO2 in the flue gas upstream and downstream of the filter was measured at the modeledoperational point of the pilot plant to evaluate the combined filter and reaction model.
One important phenomenon concerning the formation of the filter cake on jet-pulsedfilters is imperfect cake removal. Here a jet-pulse tears off the entire filter cake from only afraction of the exposed filter area, and only a part of the total filter area is subjected to the
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jet-pulse cleaning. This property of jet-pulsed filters has a great influence on the chemicalreaction simulation between gas and solid in the filter cake because the gas velocity throughthe cake, the cake thickness, and the residence time distribution of the solid forming the cakediffer widely over the entire filter area. The developed filter model [242], in which differentclasses of cake thicknesses are understood to result from different cake generations, is usedto determine the distributions of cake thickness, gas velocity, and residence time of the solidover the filter area. With the combination of the filter model and a fixed-bed reaction modelusing an empirical kinetic equation, the SO2 removal in the fixed bed of the filter cake canbe simulated. The combined filter and reaction model was successfully validated with anexperiment from a pilot plant for dry flue gas cleaning, where solid Ca(OH)2 was used asa sorbent. A sample of the partially reacted sorbent from the pilot plant had been used toderive the empirical kinetic equation for SO2 sorption in fixed-bed laboratory experiments.The application of the combined filter and reaction model revealed that SO2 removal inthe filter cake is dominated by the newly cleaned patches of a filter-cleaning procedure.The flue gas pervading the older cake generations is cleaned of SO2 almost completely,even at the start of the filter cycle, within a tiny fraction of the total length of these cakes[242].
Combining the predictive filter model and a model for the chemical reactions in thefixed bed of the filter cake [141,242], the filter performance as a gas/solid reactor canbe simulated in a predictive way provided a kinetic equation for the description of thereactivity of the sorbent with the reactive gas compounds is available. The model allowsone to predict the cycle duration, the shape of the pressure-drop curve, the solid holdup onthe filter medium, and the distribution of the cake load over the filter area. The necessarydata for such a prediction are the solid mass stream caught at the filter cloth, the cakeresistance parameter, the gas stream, and filter design data. However, filter behavior interms of the fraction of the filter area cleaned when jet-pulse cannot be predicted is stillwithout experimental data. This is due to the variable cleaning properties of the cake, whichare dependent on the filter-operating parameters [243].
Based on the model results, it is explained that the filter as a gas/solid reactor respondsweakly to a variation of the upper and lower pressure drops; hence mere increase of the solidmaterial (expressed by a higher pressure level) does not necessarily lead to a distinctivelyhigher conversion. This is due to compensating effects: a higher solid load results inan augmented solid residence time and thus reduced reactivity. Moreover, a pronouncedmaldistribution of the filter cake has a wide gas velocity distribution as a consequence, i.e.a fraction of gas passes for short duration through the filter almost unreacted. The modeland its simulation results are suitable to capture these qualitative features quantitatively.For the optimization of the filter as a gas/solid reactor through the selective recycle of thegas stream that passes through the newly cleaned patches, a modified optimization policyhas been proposed that involves a variation of the solid mass stream [243]. It is also usefulto apply to the filter/reaction model these optimization policies on the associated operatingcosts.
Models of mercury (Hg) removal during activated carbon-injection upstream of a bag-house during Hg emissions from coal-fired utilities were developed to help in understand-ing the fundamental process parameters that impact removal efficiency [244–247]. In thesemodels, a constant velocity through the sorbent bed growing on the baghouse filter is as-sumed. However, in pulse-jet fabric filters, a fraction of the filter is periodically cleaned torelieve the pressure drop across the baghouse. The cleaned section of the filter would haveless hydraulic resistance, resulting in a larger fraction of the flow diverted to this section.There would be a dynamic redistribution of flow as the cake grows on the filter bed.
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The effect of dynamic redistribution of flow on the removal of Hg using activated carboninjection in a baghouse filter system is evaluated [248]. Because the primary parameter gov-erning the operation and cleaning of filters used for particulate removal bed is the pressuredrop, equations describing fluid and solid mass balances, force balances (shear, pressure, andadhesion), and particle sizes were accounted for in the various models. However, becauseof uncertainties associated with the parameterization of the more complex pressure-dropmodels, a simpler approach was taken by Flora et al. [248] using D’Arcy’s law to describethe pressure drop and flow across a baghouse filter. Higher flow rates correspond to newlycleaned sections of the baghouse. The permeability and filter resistance were estimatedusing transient head loss data. The model was coupled to a two-stage model describingin-flight Hg removal in a duct during sorbent injection with subsequent Hg removal in thebaghouse. The flow redistribution results in a lower average Hg-removal efficiency becauseof the high mass flux of Hg exiting the filter in the newly cleaned sections [248].
The calculated average Hg-removal efficiency is affected by the permeability, filterresistance, the fraction of the baghouse cleaned, and the cleaning interval. However, themagnitude of this impact is small compared with the potential impact caused by uncer-tainties in the isotherm and mass transfer parameters. Although models can be developedthat more accurately describe the dynamics of the pressure drop in the baghouse and theresulting flow redistribution across the filter sections, it is unlikely that such models wouldsignificantly impact process models describing contaminant removal using sorption andsubsequent filtration in a growing bed. For the case of Hg removal in a baghouse under theconditions of this study, one could reasonably assume that the effects of flow redistributionon Hg removal are negligible [248].
Using recently developed models of Hg adsorption within an electrostatic precipitationand within a growing sorbent bed in a fabric filter, parallel analyses of elemental mercury(Hg0) uptake have been conducted. The results show little difference between an electro-static precipitation and a fabric filter in absolute Hg removal for a low-capacity sorbent,with a high-capacity sorbent achieving better performance in the fabric filter. Comparisonsof fractional mercury uptake per-unit pressure drop provide a means for incorporatingand comparing the impact of much greater pressure drop of a fabric filter as comparedto an electrostatic precipitation. On a per-unit-pressure-drop basis, Hg uptake within anelectrostatic precipitation exhibited better performance, particularly for the low-capacitysorbent and high-mass loadings of both sorbents. Thus, an optimal configuration for Hgadsorption for existing sources would have to balance the costs of higher sorbent injectionrates with an electrostatic precipitation against the costs of sustaining the pressure dropacross a retrofitted fabric filter [249].
4. Testing and evaluation
4.1. Performance parameters
Among various parameters, emission of particulates/gaseous matter through filtration is oneof the major governing parameters for the design and development of a filtration system.Evaluation of the said parameter is also necessary for the purpose of selection of controlequipment or for compliance of statutory obligations, or to evaluate the performance ofcontrol equipment. There are different ways to analyze the pollutants [250]. For meetingthe regulatory norms, stack monitoring is done either with batch measurement at regularintervals or by providing continuous monitoring systems installed in the stack. Generally,measurements are carried out in stack and analysis in the laboratory. A comprehensiveoverview of the fundamentals of environmental sampling and analysis can be obtained
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from recent publication [251]. For performance analysis of control equipment, filtrationparameters both at inlet and outlet of equipment are necessary. Parameters such as gasflow, temperature, pressure, dust concentration, moisture content, particle-size distribution,gaseous concentration, and physical and chemical characteristics of gas and dust are oftenmeasured. At the outlet, most of these parameters are obtained from stack sampling.Sampling may also be done in the duct if proper measurement requirements are met [7].For design and development of fabric and also for comparative evaluation of filter fabric,assessment can be done using test apparatus or at the pilot plant. In this regard, particle-size-dependent filtration efficiency, differential pressure drop, dust loading capacity, andlife of the filter media are the most required parameters for filter material characterization.
The particle size and its distribution will also be of great importance to the mediamanufacturer since these will determine the construction of the fabric. If the particles areextremely fine, this could lead to penetration, plugging of the fabric pores, ineffective clean-ing, and a premature high pressure drop. For the evaluation of fabric filtration performance,understanding particles and fabric pore construction are of utmost importance for the de-sign and development of fabric. Apart from fabric filtration, fabric mechanical propertiesare important for judging its performance under pulse-jet filtration. General quality controltests include area density, thickness, air permeability, tensile properties, and fabric stiffness[252]. Regarding tensile properties, resistance to stretch at relatively low loads (e.g. lessthan 100 N per 5 cm) is of particular importance from a control point of view. Furthermore,since this phenomenon is temperature-related, the ability to carry out such measurementsat elevated temperatures is also useful. Measurement of the fabric’s free shrinkage in anair circulating oven is the standard practice, the time of exposure and temperature variesaccording to the specific test procedure [253]. Further, a fabric’s dimensional stability underrepeated loading is also very important for proper functioning of filter bags.
Chemical, thermal performances, flammability, water repellency, and static chargepropensity of fabric are also assessed depending on the application. The particles mayalso present a challenge according to their abrasive nature, this giving rise to internalabrasion that will be further aggravated by the flexing actions to which the sleeve will besubjected. Conventional textile abrasion test methods will be of marginal value in predict-ing performance unless a mechanism for introducing the actual dust being processed canbe introduced [253]. Assessment of fabric’s antistatic properties can be made relativelyeasily by the measurement of surface resistivity (W ) between two concentric rings placedon the surface of the fabric, each carrying a potential difference of 500 V (BS6524: 1984,Method for determination of the surface resistivity of a textile fabric, BS Handbook, 1986,London, BSI). However, based on the mechanical performance, prediction of bag life is noteasy. Traditional textile measurements of fabric strength, such as MIT flex (ASTM D2176),elongation at break (ASTM D1682), tear strength (ASTM D3822), or Mullen burst (FTM– 191-5127.2), fail to correlate with actual field tests in any consistent relationship [5].
Occasionally, it happens that the selected bag material did not perform as expected, orsome unfavorable plant conditions ended its bag life prematurely. Typical failure patternsare clogging and blinding caused by dew point crossings, chemical deterioration of the basefiber material by acid or hydrolytic attack (from H2SO4 and moisture being present in the fluegas or moist pulsing air), thermal overstressing, and not too seldom mechanical damage byincorrect pulsing cycles, pulse air pressure, and bent cages. Failure indications are typicallyeither increased dust emissions, which is a sign of broken bags, or a differential pressureincrease caused by blinding and chicken effects (dust deposition behind the membrane).The degree of dust penetration can be assessed through cross-section microscopic analysis,which can be followed by air permeability tests in all three fabric stages, i.e. dust-laden,
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pulsed, and washed-out. Through chemical or thermal analysis, it is possible to assess thedegree of degradation in the filter media. The remaining solubility of the polymer indicatesthermal stresses. For in-depth analysis, it may be required to measure the inherent viscosity(indicate the chain length of the polymer) followed by Fourier Transmission Infrared(FTIR) analysis and hydrazinolysis via high pressure liquid chromatography (HPLC). Incase of woven glass bags with PTFE membranes, the main problem lies in the mechanicalsensibility due to brittleness of the glass-backing fabric and the extremely thin e-PTFElayer on the filtration side. Delamination and particular membrane damage on the surfaceallow such problems to arise [254]. In the following sections, the testing and evaluation ofonly filtration-related parameters are discussed.
4.2. Particulate solids
According to physical properties, the dust can be categorized as hygroscopic dust, abrasivedust, sticky dust, light and fluffy dust, dust with buildup characteristics, agglomerating dust,and explosive dust. All the above dust characteristics, except agglomerating, usually causetrouble during filtration operation. Agglomerating characteristics can be resulted with orwithout any agent (like ammonia gas helps in agglomerating fly ash). This property of dusthas a positive effect on the efficiency of the collector.
Various classifications and terminologies have been used to define particle size ranges.
Ultramicroscopic: 0.001–0.1 µ
Microscopic: 0.2–10 µ
Visible with naked eye: ≥10 µ
Injurious to lungs: ≤2.5 µ
Fine particles are conceived to be smaller than 2.5 µm, whereas ultrafine particles havebeen defined as those smaller than 0.1 µm. Another classification is into submicrometerparticles, which are smaller than 1 µm, and supermicrometer particles, which are largerthan 1 µm. Most often PM2.5 and PM10 fractions have been used to express ambient airquality standards, and to characterize indoor and outdoor particle mass concentrations.
Most particulate systems consist of particles of a wide range of sizes and it is necessaryto be able to give a quantitative indication of the mean size and its distribution. Further,composition (determines its density and conduction), particle shape (influences its abrasivenature and difficult flow property), and its state (such as hot particles, particles possessingstatic charge) are important factors in the designing processes of the equipment for dealingwith streams containing such solids. The three most common ways to characterize particlesare its composition, shape, and size [255]. Regarding particle shape, it may be regular, suchas spherical or cubic, or it may be irregular as a piece of broken glass. Only regular shapesare definable by equations. Despite the fact that it is a common practice to refer to the sizeof particles by a single linear dimension, with rare exceptions, this is an approximation thatcan be slightly or greatly misleading [9]. It implies that the particles are spherical, whichmay be true for some metal shot and fly ash (although the latter often comprises a mixtureof hollow spheres and fragments of shattered spheres). But, in general, particles are morelikely to be of almost any shape other than spherical, ranging from plates and deformedblocks to needles. The irregular shapes observed tend to fall into one of the following threegeneral classes [256]:
� Isometric (granular, modular), in which all three overall dimensions of the particleare roughly of the same magnitude.
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� Flat (flaky), in which there is much greater length in two dimensions than in the third,e.g. platelets, scales, leaves, etc.
� Needle like (acicular, fibrous), in which there is a much greater length in one dimen-sion than in the other two, as for instance in prisms or fibers.
The irregular-shaped particle can determine the abrasive nature of the dust particles.Further, capturing of particles, cleanability of filters, and above all the life of a filter
bag can be affected largely by particulate shape. A measure of the extent to which particlesdepart from the ideal sphere is given by the magnitude of their shape coefficients, Ka andKv . Using these coefficients, the surface area and volume of a particle are related to its’average’ diameter, dav , which is as follows:
surface area = Kad2av
volume = Kvd3av.
For spherical particles, Ka = π (= 3.412) and Kv = π /6 (= 0.502). In general, largevariation can occur with industrial particles [9].
Most descriptions of the particle diameter involve some aspect of the particle to aparticle having an equivalent spherical diameter. Particle size is an important parameter forcharacterizing the behavior of aerosols and is often expressed in terms of a single dimensionfor the ease of analysis. Since not all particles are spherical, the size of nonspherical particlesis usually transformed into other terms based on their physical behavior. Figure 25 showsthe concept of expressing the size of particles on the basis of equivalent sphere [257].Following are some of the commonly used definitions of particle size [258,259]:
1. Projected area diameter: The diameter of a circle having the same area as the image ofthe particle projected parallel to the plane of the microscopic view. Equivalent diameter
Figure 25. Different concepts of equivalent diameters.
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68 A. Mukhopadhyay
is defined as the diameter of a circle of equal area to the object:
Equivalent diameter = 2
√Area
π.
2. Equivalent volume diameter: The diameter of a sphere having a volume equal to that ofthe particle:
Equivalent volume diameter =(
6Vm
π
) 13
,
where Vm = measured volume of the particle.3. Sedimentation diameter (free falling diameter): The diameter of a sphere of equal
density, having the same settling velocity as the particle in a specified fluid.4. Aerodynamic diameter: The diameter of a sphere of unit density (1 gm·cm−3) having
the same settling velocity in air as the particle.
A wide variety of particle-size measurement apparatus are available based on differentprinciples exhibiting different concepts of equivalent particle diameter. For example, incase of particle measurement based on time of transition technique using laser, there is aprovision to obtain equivalent diameter on the basis of area. On the other hand, in the case ofthe laser ray diffraction technique, equivalent diameter is derived on the basis of a sphere ofthe same volume. Taylor [260] reviewed the previous efforts to define and quantify particleshape and size and highlighted their deficiencies. In his paper, new quantitative measuresof size and shape are proposed based on measurements made possible by image analysistechnique. However, it may be pointed out that no system is perfect for the evaluation ofparticle size and shape.
For the measurement of average particle size and its distribution, out of several tech-niques [261–265], laser-based techniques for particle-size measurement have become in-creasingly important in combustion research and pollution-controlling industries. This hasoccurred because a laser’s distinctive properties of nearly planar waves, like monochromaticnature, coherence, and high spectral power make it an extremely useful tool for particlesizing. Use of lasers allows detailed in situ measurements to be made in environments notaccessible to other types of particle-size systems due to high temperature, hazardous natureof the environment, or risk of contamination. Instruments are continually being developedand improved to meet the demanding geometric accuracy and other requirements asso-ciated with current research and industrial applications. Two distinct classes of methodsare identified: amplitude dependent and amplitude independent. A large variation in themeasuring techniques exists even under the same classes of particle sizing [257].
One of the popular techniques of particle-size measurement using laser diffractiontechnique is amplitude-dependent and is more accurately called Low Angle Laser LightScattering (LALLS) (Figure 26). The method relies on the fact that diffraction angleis inversely proportional to particle size [266,267]. The technique (Time of TransitionTheory) adopted in Ankersmid’s EyeTech Particle Size and Shape Analyzer is amplitude-independent [268,269]. In this case, the time taken by a laser beam moving at a fixedvelocity to interact with a particle and cause a shadow on the detector is directly dependenton the particle diameter. A rotating laser beam scans individual particles in the samplezone. As the particles are encountered, the laser beam is obscured and interaction signalsare detected by a photodiode. Since the laser beam rotates with a constant speed, theduration of the obscuration provides a direct size measurement of each particle. Hence, the
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Figure 26. Schematic diagram of components in a typical laser diffraction instrument.
size is not determined by the pulse height, rather it is determined by the pulse width [270].During obscuration of a laser beam, a voltage drop on the detector is seen and used by thesoftware to determine the diameter of the particle (Figure 27). Results are independent ofphysical or optical properties of the particle or medium. Through combined laser and videochannel, dynamic shape analysis is also possible.
Most particulate systems consist of particles of a wide range of sizes and it is necessaryto be able to give a quantitative indication of the mean size and the spread of sizes. Theresults of size analysis can be represented by means of a cumulative mass fraction curve,in which the proportion of particles smaller than a certain size is plotted against that size.A typical curve for size distribution on cumulative basis is shown in Figure 28.
Various parameters like D10, D50, D90, D97, number length, number area, numbervolume, length area, length volume, area volume, volume moment, and spread of the sizes(span) can be evaluated through a particle-size analyzer. D10, D50, D90, and D97 indicate thesize of a particle such that a definite fraction (10, 50, 90 and 97%, respectively) of the totalquantity of particle is below the specific size in respective cases. Span ((D90–D10)/D50)
Figure 27. Principle of time of transition theory. Figure reproduced with permission of AntersmidPSA, Netherlands (www.ankersmid.com).
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Figure 28. Particle-size distributions (cumulative number).
can be derived from the above parameters. The concept of various mean diameters alongwith their description is presented in Table 2 [257,271]. If the particle under considerationis spherical, then all the mean diameters become equal. The wide variation in particle shapeleads to terms such as the volume diameter, the surface diameter, etc. Each of these diameterdefinitions stems from the application of one or more methods of size analysis [257].
Each of the parameters is important depending on the specific field of application[272]. During particle-size measurement, there should be proper understanding about the
Table 2. Definitions and descriptions of mean particle diameters.
Name Numeric expression Description
Arithmetic mean diameter D [1,0] =∑
xdN∑dN
Normal average particle diameterof size distribution
Surface mean diameter D [2,0] =∑
x2dN∑dN
Diameter of a sphere with theaverage surface area of theparticles in size distribution
Volume mean diameter D [3,0] = 3√∑
x3dN∑dN
Diameter of a sphere with theaverage volume of the particlesin size distribution
Surfaced diameter D [2,1] =∑
x2dN∑xdN
Diameter of a sphere having thesurface area of the averageparticle size in distribution
Volume diameter D [3,1] =√∑
x3dN∑xdN
Diameter of a sphere having thevolume of the average particlesize in distribution
Surface area moment mean-sautermean diameter
D [3,2] =∑
x3dN∑x2dN
Diameter of a sphere with theequivalent surface to volumeratio as all particles in sizedistribution
Weight mean diameter (DeBrouckere mean diameter)
D [4,3] =∑
x4dN∑x3dN
Diameter of a sphere having theaverage weight of all particles insize distribution
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inherent limitations of the particle measuring systems and physical significance of theoutput parameters. During the measurement of offline particle-size distribution, one ofthe important precautionary measures is that the dust particle should be well dispersedduring the measurement. Dry powder dispersers have been widely used and are requiredto deagglomerate the powder in order for commercial instruments to measure particle-sizedistributions based on laser diffraction [273] and time-of-flight techniques [274]. It hasbeen recognized that complete dispersion of dry particulate solids, especially in the sizerange of less than 20 µm, is difficult to achieve due to strong cohesive forces, namely vander Waals, magnetic, electrostatic, and forces due to solid or liquid bridges. In general, thestrength of these forces increases with decreasing particle size [275].
Deagglomeration in the dry powder feeder associated with Malvern Mastersizer 2000(Malvern, Worcs, UK) laser diffraction instruments is achieved by accelerating particlesclose to sonic speed along a tapered transport tube. This mechanism provides the shearforces and collisions between the particles and the feeder’s walls in order to break downthe aggregates. In another design, previous versions of dry powder dispersers associatedwith time-of-flight equipment, such as the AerosizerTMLD (Amherst Process Instruments,Hadley, MA) are based on the application of a fluidized pulsed jet, impaction with adisperser pin, and acceleration of the agglomerated powder particles through a sonic nozzle[276,277]. There was also a report about a simple and cost-effective powder disperser,which has been designed to disperse Mannitol and BSA powders. The disperser has beenshown to disperse the powder as efficiently as the more expensive commercial devices[278].
In contrast to the offline particle-size measurement technique, in many cases, onlinemeasurement is far more desirable. The development of a reliable, cost-effective instru-ment for online particle sizing remains a challenging area. Online particle sizing plays animportant part in many areas of industry. For instance, the size of pulverised fuel used ina coal-fired power plant is an important parameter that affects both combustion efficiencyand atmospheric emissions of the plant. Various online systems are available for moni-toring the dust emission. Scanning Mobility Particle like SizerTM Spectrometer (Model3034) can be used for continuous emission monitoring. It measures aerosol particles inthe size range from 10 to 487 nm. It uses a continuous, fast-scanning technique to avoidgaps in the particle-size distribution data and measures aerosol particles [279]. There isalso a report about a cost-effective dust-emission-monitoring system for baghouses, in-corporating network and advanced graphics capability [280]. The system comprises up to32 networked sensors, a central control unit, data acquisition, and optional reporting soft-ware. Its minimum detection level of 0.1 mg/m3 easily meets the Environmental PollutionAgency (EPA) requirements under the Maximum Achievable Control Technology (MACT)standards. Consistent with EPA Guidance, the instrument has two options to set alarms:
� Using dust peak height as an early indication of filter failure;� Using average dust level to identify significant filter failure. Alarm delay is used to
discriminate filter failure from dust pulses arising from bag cleaning.
Horne [281] discussed optical and other methods for continuous monitoring of par-ticulate emission from a point source. However, it was found that the optical probes oflaser-based and imaging-based techniques are unsuitable for installations in a harsh envi-ronment, such as on a pneumatic conveyor, where the optical entry and exit windows areoften contaminated due to accumulation of fine particles on the windows [282]. Zhang andYan [283] reported the design and evaluation of an electrostatic sensor combined with a
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novel digital signal-processing algorithm for the online continuous measurement of massmedian size of particles in a dilute-phase pneumatic suspension. Carter, Yana, and Cameron[284] developed a novel instrumentation system that uses a combination of electrostaticand digital imaging sensors for online measurement of particle-size distribution and massflow rate of particles in a pneumatic suspension. An inferential approach was adopted forthe mass flow measurement of particles, velocity and volumetric concentration of particlesbeing measured independently. The velocity of particles is determined by cross correlatingtwo signals derived from a pair of electrostatic sensors. The volumetric concentration ofparticles is obtained using a novel digital imaging sensor, which also provides particlesize distribution data. Particle size measurements were found to be comparable with thoserecorded offline with an ISO13320 compatible laser diffraction instrument.
In a recent report [285], the combination of laser diffraction with upstream samplinghas been stated as a breakthrough for the in- and online particle size analyses in industrialapplications. The combination of representative sampling, dry dispersion, particle sizeanalysis by laser diffraction, and integrated feedback of the sample is well accepted inmany industrial applications. The system does not need user interaction. Currently, particlesize ranging from 0.25 to 3,500 µm and pipe diameter ranging from 50 to 860 mm arecovered. Applications at high temperatures, for high-particle velocities, low concentrations,abrasive or sticky particles in standard or hazardous areas (also following the new ATEX20/200 standard) have been successfully implemented. The new software allows for theparallel operation of many particle size analyzers on the same database. It can handle verylarge volumes of data, as acquired over years [285]. However, one has to recognize that auniform solution to the problems of online particle size analysis is difficult. The boundaryconditions vary to such an extent that similar methods or combinations of methods canrarely be used in different cases. Since instrument manufacturers will, in most cases, notbe able to adopt their instruments to different industrial applications, specialists will haveto solve the problem.
4.3. Characterization of dust layer/cake
In general, the characterization of dust cakes is difficult. Cake solidosity and permeability,which describe and define cake structure and its resistance to filtrate flow, is crucial infor-mation to have for the analysis and simulation of cake filtration, and also to design and scaleup of the filtration system. The methodology for the determination of various cake proper-ties, such as cake porosity, specific cake resistance, can be obtained from literature [286].It may be noted that the cake properties, e.g. porosity can be estimated from semiempir-ically derived equations, e.g. Karman–Kozney, Ergun’s/Rudnick, and Happel, describingflow through porous media [45]. Naturally, the derivations of these equations are basedon assumptions and involve empirical constants. Alternatively, the cake samples can becharacterized by direct microscopic examination [52,287]. A large number of observationsand statistical methods provide a good estimate of cake properties.
The dust layer formed over the fabric can be schematically represented, as in Figure 29.Schmidt [218] presented a technique with which surface-adhering dust layers may be cre-ated under defined conditions for characterization. The procedure was found to be suitablefor the quantitative evaluation of the effects of different influential parameters such as theparticle size, porosity, and surface roughness on the stability of particle layers. The layerstructure can also be characterized using a preparation and evaluation technique adapted tothe specific conditions [108,287,288]. Following the pre-solidification of the particle sys-tem using a cyanoacrylate-based adhesive vapor, the layers were embedded together with
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Figure 29. Schematic view of bag with filter cake.
the discs in a low-viscous synthetic resin. Upon solidification, the samples were dissected,ground, polished, and etched. Following this, images of the sections perpendicular to thedisc surface were made using a scanning electron microscope [108]. Earlier, Schmidt andLoffeler [287,289] developed a method to freeze the dust cake structure for subsequent mi-croscopic examination. A procedure can also be developed [34] to measure the distributionof stored dust inside the medium after stabilizing the internal cake structure in a polymermatrix followed by a section cut.
Three-dimensional characteristics of surface-adhering dust layer is mostly inferred fromtwo-dimensional examinations. The cake undergoes extensive preparation steps before it isexamined under the microscope. Therefore, uncertainties are also there. Fractal dimension(FD) is a property of self-similar objects and is used to characterize complex systems,e.g. dust powders or cake structure. The fractal analysis of dust cake structures frommicrographs of a cake cross-section of prepared dust cake samples was reported [52]. Thelimestone dust (mass mean diameter of 3.5 µm) cakes prepared at 500, 1000, and 2000Pa are analyzed using Box Counting Method. The FD is scattered in the range 1.62–1.66and no dependency of cake structure on the studied parameters was reported. The invasivetechniques for characterizing filter cake were found to be superior since the cake is examinedin situ on the filter surface with minimum disturbances, if any [61].
Some in situ measurement techniques for determining cake height/mass distributionon gas filters were also reported in the literature. A radiometric method based on beta rayabsorption and laser light reflection is illustrated [46]. Three aerial dust density profiles onpolyester bag filter of 2.5 m length using X-ray absorption were measured. Nonuniformdust cake profiles along the length of the bag and a higher dust load near the bottom ofthe filter are observed. In another method, beta mass probe was used for investigating thecake formation and release on a flat filter felt [47]. The β-ray source and the detector aremoved together on opposite sides of the dust-laden bag for scanning the bag surface. Thefilter bag was supported on a rectangular frame of 6 in × 11 in, divided in 1 in2. Dueto the interference of metal wires of the frame on radiation, a continuous measurementof the cake load along the length or width of the filter is not possible and therefore onlypoint-wise measurements could be obtained. The system required calibration under exactlythe same condition as expected in operation using sheets of known mass per unit area.Dust and gas flow were stopped during scanning and cleaning were offline. The dust wasfound depositing nonuniformly on the filter surface. The reported detachment mechanismis acceleration and deceleration [47].
In a study [184], the dust cake layer thickness was measured through a clear plastic pipeof the filter holder with a TV-CCD camera equipped with a zoom lens of 40×. The CCDcamera angle was inclined at 10◦ from the filter surface so that the boundary between filter
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media and dust cake could be clearly observed. The resolution was approximately 25 µmin measuring the cake height. During the test, uniformity of the cake height increase waschecked regularly. The final cake height after the test was observed from every direction toconfirm the uniformity of the height. Almost all cake layers grew uniformly. Dittler et al.[140] demonstrated that cake height distributions on rigid filter media can be determinedby an optical sensing technique. During reconstruction of the height profile from the phasedifference, phase stepping technique offers slightly better resolution and is more robust withrespect to the ability to handle complex, fragmented surface textures typical of filter cake.With the system, measurement can be made in situ during the operation of the filtrationunit with a time resolution limited only by the read-out frequency of the CCD camera. Thetopographical resolution of 50 µm was found to be sufficient for typical overall cake heightsin the order of 1 mm to resolve features of interest for the investigation of filter performanceand regeneration behavior [140]. However, this technique cannot be employed for flexiblefilter media where the hidden filter deforms due to increased pressure drop during cakeformation. Thus, simply reconstructing the surface from structured light technique will notaccount for such deformation.
A laser displacement system is reported [45,163] to measure the dust cake thicknessand the amount of dust cake compaction. Higher filtration velocity is found responsible forirreversible dust cake compaction. The laser displacement system works on the principle ofreflection, thus it requires a rigid surface. The invent of high resolution digital cameras andimproved machine vision techniques has led to wide-spread applications of digital camerasfor remote sensing and monitoring of objects and processes. The approach is nonhazardousand provides in situ measurements of a large surface from relatively long distances. Useof CCD-color vision camera for characterizing the surface treatment of needle felts wasalso reported [290]. The images are processed to compute the surface porosity and sizedistribution of the pores.
Saleem and Krammer [61,98] presented an optical system for measuring the cakeheight distributions in the course of cake formation and after regeneration on flexible filtermedia. The optical thickness measurement technique is based on the principle of StereoVision, i.e. two images of the same object from different views are used to generate athree-dimensional surface of cake. The clean bag surface is aligned after deformation to thebag behind the dust cake, which allows the relative height measurements. The stereo setupconsists of two industrial complementary metal-oxide semiconductor (CMOS) cameras.The whole setup was placed outside of the filter unit from where filter bags were visiblethrough optical glass window. The movement of the stereo setup and image acquisitionwas automated through graphical user interface in Matlab
©R environment. The details ofthe technique and its boundary conditions are available elsewhere [291]. The residual cakeheight measurements were used for estimating the residual cake patch size distribution.
Tien [292] had presented an outline of the directions and topics of future cake filtra-tion research. The proposed topics include the formulation of more accurate and efficientprocedures for determining filter cake characteristics from experimental data, more com-plete analysis of cake formation and growth, and the effect of accurate and nonintrusivemeasurements of evolution of cake thickness histories. The selection is based on practicalneeds, intrinsic significance, or both. The relationships between these proposed topics andsome current and past research were also discussed.
4.4. Characterization of pore
Both porosity and pore dimension are important for filtration application. Depending onthe product design and processing technologies, pore size distribution varies. Pore structure
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Figure 30. Schematic presentation of different types of pores.
of nonwoven filtration media is quite complex as the pores are interconnected and haveirregular cross-sections that vary along pore path. Further, the pore structure is usually notisotropic. The structure in the z-direction (thickness direction) can be considerably differentfrom those in the x–y plane (plane perpendicular to the thickness direction). For filtrationapplication, pore construction along the z-direction is important. Characterization of sucha complex pore structure requires specification of a number of characteristics. The throughpore along the z-direction varies in dimension across the fabric. The different types of poresare schematically presented in Figure 30.
Pore size for nonwoven filter varies in a wider range of 12 to 66 µm. However, newnonwovens in filtration require a media designed to particular pore size specifications in the3- to 20-micron range [293]. The size of particles that cannot pass through filtration mediais determined by the size of pores at their most constricted parts. Therefore, the largest, themean and the range of the most constricted through pore sizes and the pore distributions areimportant characteristics that influence barrier properties of filtration media. The throughpores extend from one side of filtration media to the other and permit airflow; whereas blindpores terminate inside the filtration media. Blind pores do not permit fluid flow, but blindpore surface area can adsorb gases, capture small particles, and participate in reactions(in case of catalytic fabric filter). The closed pores are not accessible. The parameterscharacterizing pores are discussed in the following section.
4.4.1. Porosity
In order to understand the mechanism and efficacy of airflow during aerosol filtrationthrough a porous media, it is necessary to know the nature of its porosity. Porosity (ε) isdefined as the ratio of total pore volume (Vp) to bulk volume (Vf ) [6,294].
ε = Vp
Vf
.
In terms of textile properties, it can be expressed as
ε = 1 − m
pt,
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where m is fabric mass per unit area (g/m2), t is the nominal thickness (m), and p is the bulkdensity of the fiber (g/m3) from which it is produced. The range of porosity of nonwovenfabric used in industrial filtration could be 45 to 80% [295]. The average porosity can givelittle insight about the structure of network. Entirely different fibrous materials can have thesame porosity values. Thus, the dimension and orientation of the pore is more important.
4.4.2. Pore dimensions
Pore-size distribution in nonwoven is typically unimodal and relatively broad, with a rangeof values that may cover several orders of magnitude. Nonwovens with bimodal and trimodaldistributions of pores are also possible. While average values of geometric quantitiesdescribing pore dimensions provide valuable information about network structure, they areonly a little more informative than average porosity values. Entrance and constricted exitpore dimensions would be crucial in predicting the probability of particulate penetration andbeing retained by the interior of a structure. Other than diameter of pore, pore dimensionscan be described in terms of volume and surface area, each of these could be critical incontrolling a specific kind of behavior (Figure 31). Pore size and shape resulting from fiberarrangements in textile structure depend on fiber parameters, compression, and orientation[296–299]. Pore size and shape in a real textile structure are generally random. For thisreason, some researchers [296,300] have used stochastic, fuzzy, and neural network modelsto describe and estimate pore characteristics. There are also several theoretical approachesfor characterizing pores [293,294,299,301].
Figure 31. Schematic of pores in fibrous network (d1 = constricted pore diameter, d2 = average porediameter, d3 = surface pore diameter, d4 = exit pore diameter, S = pore surface, V = pore volume).
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Figure 32. Pore structure in nonwoven material.
In addition to the above, concepts of conduits and tortuosity [294] in relation to porestructure are important in describing pore geometry. The pores of elementary thin filmare described as cylinders with axis perpendicular to the longitudinal plane of sheet, witha circular cross-section of diameter distributed within the fabric. These cylinders will becalled elementary conduits, and the succession of elementary cylinders of total length iscalled a conduit (Figure 32). Tortuosity, which also characterizes the structure, is definedas the ratio of actual length followed by a stream line to the thickness of the filter bed [6].
There are several experimental approaches in characterizing the pore size and its distri-bution. However, pore throat dimension (constricted pore) of nonwoven fabric as measuredby critical bubble pressure method [302,303] would be most important in predicting theprobability of particulate penetration and being retained by the interior of the structure.Following British Standard 7591 or ASTM F316-86 [252], the pore throat characteristicsrequired for the evaluation of filtration performance can be determined. The basic prin-ciple of pore size measurements is that liquid-filled pores will become gas permeable ata certain pressure, because the liquid has first to be displaced by the gas. This pressure,the so-called bubble point, depends on the surface tension of the liquid and the pore sizediameter. As real materials almost always show a distribution of pore sizes, the pressureat which the initially liquid-filled pores become gas permeable corresponds to the openingpressure of the biggest pores. By further increasing the pressure drop across the materialunder test and measuring the volumetric flow rate, it is possible to obtain data from whichthe pore-size distribution can be derived. The pore size is calculated based on the followingrelationship:
r = 2T × 105
σPg,
where r is the pore radius (mm), T is the surface tension of the fluid (m·Nm−1), σ is thedensity of water at the temperature of test (g/cc), P is the bubble pressure (mm H2O),and g = 981 cm·s−2. However, membranes made by irradiation, such as Nuclepore, are anobvious exception in which the above method cannot be applied.
There are several instruments such as PMI capillary flow porometry, Coulter Porometer,TOPAS pore size meter [304–306], etc., which can provide several parameters importantfor nonwoven filters as listed below:
� Most constricted pore diameter of through pore;� Largest through pore throat diameter (pore size corresponding to pressure drop at
which the wetted sample is starting to become gas permeable);
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� Mean flow pore diameter (pore size diameter corresponding to the pressure dropwhere the wet flow value is half (50%) of the dry flow);
� Pore size distribution (calculated from wet flow curve (pressure drop vs. volumetricflow rate of the wet sample) and dry flow curve (similar to wet flow, but obtained ona dry sample));
� Through pore surface area.
A method was proposed by which the particle penetration and the particle-holdingcapacity of different surface-treated filter media can be comparatively characterized. Forsurface-treated filter, media porosity properties at and just below the filter surface areessential for the particle penetration and particle-clogging behavior. In order to comparethe porosities of different surface-treated filter media, a determination method, which workswith a transmitted light microscope and an analysis software, was developed to determinethe two-dimensional top-layer porosity (surface porosity) and a pore depth distributionbelow the outer surface of the filter medium. Based on filtration test on VDI 3926 (type-2test) equipment, a linear relationship between the clean gas concentration and the surfaceporosities of the filter media is found. The effective surface porosity is determined by apore-blocking diameter, all pores below remain blocked and above remain open duringthe stationary cycle filtration procedure. This value of pore-blocking diameter appearedto be constant for different surface-treated filter media, presuming the tests have beenexecuted at the same test equipment with the same test dust. Further, the so-called porevolume equivalent can be determined by the microscopic measurement, which can be alsolinearly related to the deposited dust mass inside the filter media. With these two linearrelationships, the particle penetration and the dust-holding capacity can be estimated fordifferent surface-treated filter media [307].
4.5. Fabric filtration performance
It is often difficult to study the filtration performance primarily because the emissionprocesses in surface filter media are transient in nature. Further, there are difficulties indeveloping standard test apparatus simulating the practical situation as design of filter unit,and aerosol characteristics vary widely even in the same application at different places.The standard testing of filters and filter media is important for the design and development,manufacturing and selection of filter media, as well as for quality assurance during theproduction process. Particle size-dependent filtration efficiency, differential pressure drop,dust loading capacity, and life of the filter media are the most required parameters for filtermaterial characterization. There are few more filtration indices possible in quantifying theperformance of filter fabrics in solid/gas-separation processes [308].
The search to reveal the mysteries behind the baghouse aerosol filtration process andto find the suitable material for the end application is still under pursuit. It is worthmentioning that the research work on pulse-jet filtration started way back to the 1980s andthe investigation was carried out either in a designed apparatus or in a pilot plant in thelaboratory. All aid apparatuses were developed based on the focused attention on a veryspecific research area. It is quite obvious that the impact of the number of bags cleaned andpotential interactions within a dust collector featuring many filter elements can influencethe movement of aerosol, dust re-deposition, and overall performance of a filter unit. Inspite of the above, in many cases experimental investigation was carried out on a flatsurface [136,309–311] due to simplicity, ease of instrumentation for data recording, andreproducible results. Further, thermal and chemical conditions, condition of the cages inside
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the filter, and as a whole filtration operation in many cases are difficult to reproduce in thelaboratory, and therefore the results may well eclipse laboratory predictions. Further, resultsoriginate exclusively from pilot test equipment incorporating few bags (often only one),which may provide very different operating characteristics to real industrial dust collectors.The performance of pulse-jet filtration apparatus featuring a single bag [47,135,288] willbe different from the system consisting of multiple bags [61,62,146]. The apparatus arealso designed for testing of rigid ceramic filters [166].
Filtration efficiency is usually stated in terms of the percentage of particles of certainsize that would be stopped and retained by a filter medium and can be derived either fromnumber or mass concentration in upstream and downstream aerosol [6]. Efficiency basedon number can be expressed as
E(%) = Nin − Nout
Nin× 100,
whereNin = number of concentration of particles entering the filter.Nout = number of concentration of particles leaving the filter.
Efficiency based on mass can be given by
Em(%) = Cin − Cout
Cin× 100,
whereCin = mass concentration of particles entering the filter.Cout = mass concentration of particles leaving the filter.Efficiency can also be defined by penetration of particles:
Pm = Cout
Cin.
As in the case of industrial filtration, incoming dust concentration is very high and num-ber concentration of particles entering the filter is difficult to measure; therefore efficiencyis derived based on mass concentration. However, level of filtration efficiency in improvedfilter media has gone above 99.99%, and a small difference in emission is less likely to becaptured in filtration efficiency data. Therefore, direct use of emission in terms of numberor mass concentration would be a better indicator.
Much earlier, ASTM F1215-89 (Test Method for Determining the Initial Efficiencyof a Flatsheet Filter Medium in an Airflow Using Latex Spheres) specified a standard testmethod for determining the initial efficiency of flat sheet filter media in an airflow using latexspheres. However, in the method, aerosol is generated by using a nebulizer and the efficiencyis calculated by measuring the upstream and downstream aerosol concentration [312]. Thetest method was withdrawn in 1998. It was likely that the earlier standardization for testingfilter media becomes invalid as, with the change of time along with the growing popularityof pulse-jet system, the exact characterization of filter media become indispensable. Testingsystems should also be fast, feasible, and reproducible. In the last decade, a number of testprocedures have been developed based on filtration following pulse-jet cleaning. Varioustest procedures based on available national standards on the testing method for cleanablefabric filters are enlisted below:
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1. VDI/DIN 3926: ‘Testing of cleanable filter media’ in Germany [313].2. ASTM D 6830-02: ‘Standard test method for characterizing the pressure drop and
filtration performance of cleanable filter media’ in the US [314].3. JIS Z 8909-1: ‘Testing methods of filter media for dust collection’ in Japan [315].4. GB 12625: ‘Technical requirements of fabric and bag for bag filter’ in China [316].5. ISO 11057: ‘Air quality – Standard test method for filtration characterization of
cleanable filter media’, International Organization of Standardization [317].
The VDl/DIN guideline 3926 - Testing of cleanable filter media is an important steptoward improving the characterization and assessment of cleanable filter media. Since itsintroduction in December 1994 and the amendment in October 2004, both the testingmethod as well as the test equipment are widely used throughout Europe and meanwhilealso applied in Japan, China and the USA. Especially in the USA, the guideline was largelyintegrated into the ASTM 06830-02. At present, an ISO standard (ISO 11057) is beingdeveloped, which largely orients itself on VDI/DIN 3926 (type-1) [194,317].
The VDl/DIN guideline 3926 offers the possibility of comparative testing of cleanablefilter media under exactly defined and controlled laboratory conditions. However, exami-nations concerning the suitability of a medium for a specific task or obtaining data for thedesign or optimization of a filter system are expressively not part of the guideline. Theguideline does not expressively cover this either. When using the final results, one shouldalways be aware that, although they allow a comparative characterization and evaluation ofcleanable filter media, it is not possible to transfer the laboratory results directly to practicalapplications. However, in most application cases, it was possible to transfer the trends mea-sured in the laboratory to practical applications so that the data situation has significantlyimproved with respect to decision-making [194,313]. The apparatus is composed of threeparts, i.e. the dust feeding and dispersing part, the dust collecting and cleaning part, andthe monitoring and controlling part. Two different types of test installations, which arepresented in the guideline (types-1 and 2), are shown in Figures 33 and 34. In the new VDI3926, the basics are described – necessary to understand the testing of cleanable surfaces,as well as a further innovation in the form of a comparison of filter media by aging. Anotherpopular test rig is JIS rig, which is to some extent similar to that of VDI type-1 (Figure 35).
In ASTM standard for performance analysis of cleanable filter media, the apparatussetup is similar to that of VDI type-1. During the performance test period, the test specimenis subjected to normal filtration cycle pulsing and the parameters like outlet particle concen-tration at standard conditions, total inlet mass concentration in g/dscm, PM2.5 (optional) ing/dscm, average residual pressure drop in cm w.g., initial residual pressure drop in cm w.g.,increase in residual pressure drop in cm w.g., filtration cycle time in seconds, mass gain oftest sample filter in gram, and number of cleaning cycles are measured and recorded [314].
It is important to note that the residual pressure drop can be measured automatically bymeans of measuring value recording without interrupting the measuring operation, providedthe waiting time between the cleaning impulse and the measuring operation was selectedcorrectly. The waiting time must be determined in experiments, because the pressure-dropcurves back-swing with varying intensity depending on the permeability of the filter. Theback-swinging and thus the measuring error become larger the more the filter is clogged orthe more convex the pressure-drop curve rises. The typical waiting time could be 3–4 s ina standard test apparatus [194].
Testing is performed following different stages with standard test dust. During all stages,test conditions for dust flow rate, raw gas flow rate, and filtration velocity must be met inaccordance with standard. The steps are mentioned below [194,313–315].
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Figure 33. Schematic diagram of the filter test rig (VDI 3926, type-1). Flow directions are indicatedby arrows. The optical particle counter (OPC) can be used to record emitted particles in real time.
Step 1 (Virgin performance test period): 30 filtration–cleaning cycles with a prescribedpressure drop during cleaning (1000 Pa) is adapted. In VDI guideline, Plural NF dust isrecommended along with filter face velocity of 2 m/min while keeping the target valueof dust concentration at 5 g/m3 near the filter. The testing results are used to evaluatethe filter performance at the initial stage of filtration. In ASTM standard, no such test isrecommended.
Step 2 (Aging period/conditioning): A large number of rapid cleaning pulses (5000cycles for VDI and 10,000 for JIS and ASTM) at 3–5 s between pulses to obtain a seasonedfilter in a short period of time. In new ISO 11057, cleaning cycles of 2500 at an interval of20 s each are recommended.
Step 3 (Stabilizing/recovery period): 10 filtration-cleaning cycles (30 cycles for ASTM)with a prescribed pressure drop during cleaning (1000 Pa) are adapted. This is performed
Figure 34. Test filter equipment according to VDI 3926, type-2.
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Figure 35. Schematic diagram of JIS rig.
since after aging the dust load on the test filter is different from that under the testingcondition; therefore, recovery of material is essential for performance test.
Step 4 (Performance test period): Normally 2 hr filtration–cleaning cycles with a clean-ing set point of 1000 Pa is adapted. The test results are used to evaluate the filtrationperformance.
Step 5 (Performance at higher cleaning pressure – optional): Normally a 2 hr filtration–cleaning cycle with a cleaning set point of 1000 Pa is adapted in ISO. This step is notincluded in other standards.
Due to the differences in filtration apparatus and also in test conditions, the differencein output parameter is quite obvious. In one study, the filter cleaning performances usingVDI type-1 rig and JIS rigs were compared in order to characterize the two testing methods.The JIS rig is small in dimension compared to VDI type-1 rig. Fabric sample is rectangularin JIS, whereas it is circular in VDI. Moreover, flow of air during pulse cleaning is paralleland perpendicular to the fabric plane in JIS and VDI, respectively. Since all the dust-laden gas passes through the test filter in JIS rig, it is called total-flow type. On the otherhand, in VDI type-1 rig, only a small fraction of gas passes through the test filter anda larger fraction of gas leaves from the rig at the bottom of the vertical duct. Therefore,VDI type-1 rig is called part-flow-type rig. The filter performance tests under similarconditions showed that the filter-cleaning efficiency measured with VDI type-1 rig ishigher than that with JIS rig [18,318]. The difference in cleaning efficiency led to thedifferent residual pressure drop after the aging of the filter. During pulse-jet cleaning,JIS rig gave a higher peak pressure and a shorter time period of pulse-jet compared toVDI type-1 rig. The difference in filter performance measured by the two rigs can be
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minimized by modifying the aging condition, i.e. either reducing the number of filtrationcycles during the aging with the JIS rig or increasing it with the VDI type-1 rig so as togive the same residual pressure drop after aging. Furthermore, it is found that there is anoptimum filtration cycle time to grow the residual pressure drop during the filter aging inorder to minimize the time period of filter aging. It is proposed that the difference in filter-cleaning performance measured by VDI type-1 and JIS rigs can be expressed in terms of twoparameters, i.e. the surface cleaning fraction, which is the ratio of cleaned surface area to thetotal surface area of a filter, and the residual dust load on uncleaned surface after cleaning[18,318].
It is important to note that the emissions of modern needle felts are very low; andtherefore it is difficult to accumulate enough mass over a few filter cycles for a meaningfulmeasurement by traditional gravimetric methods. It is more meaningful to evaluate emis-sion behavior from cycle to cycle. Alternatives with higher sensitivity in the size rangeof above 1 µm include the Aerodynamic Particle Size (APS) and Tapered Element Os-cillating Microbalance (TEOM). The APS would also require an additional, dust-specificcalibration to convert particle number to particle mass, and it has a rather limited oper-ation range in terms of concentration. On the other hand, TEOM can detect total masswith good sensitivity but in itself has no capability to resolve the particle size. Conse-quently, there are many arguments in favor of light scattering optical particle counters(OPC) to do mass-based emission characterization, including very good size and timeresolution in the particle size range of interest. Aside from sensitivity, time resolution isespecially interesting for testing pulse-cleaned filter media, because particles are emit-ted in bursts immediately following regeneration, and because the aging process can beresolved [319].
Despite this potential, there is a significant drawback with OPCs due to the fact thatoptical sizes and number concentrations are not easily converted into aerodynamic diametersand particle mass required for most standards. However, since filter fabrics are typicallytested with standardized inorganic test dusts of uniform composition, it is possible for achemically and physically well-defined particle material, in principle, to calibrate an OPCto read the mass directly [320]. It may be added that some OPCs are less suitable forconverting number concentrations into mass, because they cannot resolve the size rangearound 1 µm (near the most-penetrating particle size) due to the shape of their responsecurve.
Binnig et al. [319] integrated an OPC into a test setup for pulse-cleaned filter media tomonitor particle emissions in terms of particle mass versus aerodynamic diameter, and interms of total PM2.5 dust mass emitted. The detailed approach taken to convert OPC sizespectra in the size range of typically 0.3 to 10 µm into PM2.5 mass for a known particlematerial is discussed in the literature. In the later work, Binning, Meyer, and Kasper [321]recalibrated light scattering OPC for a well-defined particle material, to convert opticalequivalent size both into aerodynamic and volume-equivalent diameters. For specific testdusts, the calibrated OPC provides a direct read-out dust mass versus aerodynamic diameterand total PM2.5 mass. The time resolution is in the order of 1 s. In a recent study [34],optical particle counter is used in VDI (type I) test system wherein an emission is expressedas total number of particles per cycle. Size distribution information is also evaluated.
There are now an increasing number of companies providing specialist test rigs, whichmeet the standard guidelines and at the same time are compact and versatile enough toallow testing to be undertaken in the factory itself, as opposed to a laboratory [322]. Palasfrom Karlsruhe, manufactures the filter test rig according to VDI 3926 Types-1 and 2for cleanable surface filters (MMTC 2000 and MMTC 3000) [323]. Pallas multi-modular
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test system MMTC-2000 upgraded with test concept and through controlling humidityand temperature enables the new requirements to be realized according to VDI 3926type-2. The dust feeder RBG-2000 according to VDI 3491 sheet 9, is used as a standarddosing system for almost even particle size distribution on the filter to be tested. Properclamping, installation of the isokinetic sampling probe, and in situ measurement of particlecharacteristics are some of the major features in the filtration equipment.
However, practice relevant tests, in particular regarding the lifetime, cannot be accom-plished with the described procedures [313]. For practical evaluation, filter media withdifferent parameters should be tested, taking into account of the following factors:
� Different types of dust in practice;� Practical fixing of the medium (garland effect);� Time-resolved emission measurement in clean gas;� Practical adjustment of temperature and relative humidity;� In situ measurement;� Aging.
In order to be able to judge the long-term behavior of a filter medium, the filter media canbe artificially aged before the filter testing. For aging, as 10,000 cycles are recommended,one has to reckon on testing times of 100 hours and more. A test series including aging andrepetition measurements would take several weeks. Therefore, a practice-oriented agingchamber was developed. Depending upon the type, up to four filter media or complete minifilters can be aged homogeneously at the same time under the same conditions (Figure 36).In the aging chamber, use of different types of test dust is possible and can also be equippedwith humidity and temperature controller. During the laboratory operation, the test dustcan be re-fed in order to work more economically. For this purpose, the used dust, whichwas emitted by the filter, is accumulated underneath the test chamber, to be transportedback to the aerosol generator and dispersed again. Since the functions of aging chamber
Figure 36. Setup of separate aging chamber [325]. Reprinted from Practical filter testing with Palasaging chamber, Palas GmbH, Palas Particular: Information brochure, No. 1, and from Taking filtertesting out of the lab, Feature article, Palas GmbH, Filtration & Separation 42 (2005), pp. 32–35,with permission of Elsevier B.V.
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Figure 37. Development of residual pressure drop before and after aging. Figure reproduced withpermission of Fil T Eq – Filtration Testing Equipment & Services GmbH.
is completely independent, the filter media (after aging) can be tested with both versionstype-1 and type-2 VDI 3926, or with other filter test rigs for these filter media, for examplethe new test rig TC-2000, a combination of type-1 and type-2 [324,325].
Topas has developed a series of AFC test rigs, which facilitate filter tests in accordancewith VDI 3926-II for filters with a wide variety of test aerosols (atmospheric, dust, ordroplets) [326]. FilTEq has reported different modes of filter testing such as (a) testingaccording to VDI 3926 at ambient or enhanced temperature conditions (maximum 200◦C),(b) filter testing with aging-phase as proposed in update of VDI/DIN 3926 or in ASTMD6830-02, (c) application testing in the former two cases using different dusts and opera-tional conditions. Several probes for simultaneous measurements of flow velocity, pressurepulse development, and temperature are also feasible. Figures 37–39 represent the filtrationtesting results of fabric filter under different conditions [327]. Figures 37 and 38 showthe residual pressure drop and cycle time before and after aging. After aging, the residualpressure drop is much higher, whereas cleaning time is lower by a large extent than theinitial phase of test. Figures also show the nature of behavior of the above two parametersafter aging with cleaning at higher cleaning pressure (1800 kPa). Figure 39 illustrates thetypical development of filtration behavior of a filter sample based on the initially concave
Figure 38. Development of cycle times before and after aging. Figure reproduced with permissionof Fil T Eq – Filtration Testing Equipment & Services GmbH.
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Figure 39. Pressure-drop progression of selected cycles before and after aging. Figure reproducedwith permission of Fil T Eq – Filtration Testing Equipment & Services GmbH.
progression of the pressure-drop curve (with particle deposits in the depth of the medium)up to a pronounced convex curve progression after aging, which inevitably leads to a re-duction of cycle times. This behavior is more or less dominant with the different availablefilter media [328].
In a further step, a mobile filter probe for the performance of field tests was developedas a derivation of the technology applied in the laboratory (Figure 40). With this technology,it is possible to obtain data required to assess the suitability of a medium for a specificapplication and to improve the design of a filter system or optimize plant operation [328].
However, results obtained by the above-mentioned test method should not be used topredict absolute performance on full-scale fabric filter (baghouse) facilities. Further, differ-ent filter media made from different materials react differently in different environments,
Figure 40. Setup for a mobile filter probe for field measurements in filter plants [328]. Reprintedfrom P. Gang, Testing and selection of filter media for dedusting, Part 2: Field measurements witha mobile filter probe derived from VDI/DIN 3926, Filtration & Separation (International Edition), 9(2009) pp. 17–23, with permission of Fil T Eq – Filtration Testing Equipment & Services GmbH.
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and this is difficult to replicate in the current standards. Dust types and concentration varygreatly; therefore, the results obtained using the standard dust cannot be extrapolated toother dust types at different concentration levels (dust concentration level in the above-mentioned methods is much lower as compared to practical system). It may be added thatas particle size differs in dimension, fractional efficiency/particle removal in different sizesof dust level becomes a more meaningful parameter instead of cumulative data. Neverthe-less, the results obtained from current standard are useful in the selection of proper filtermedia and identification of recommended operating parameters for these full-scale fabricfilter facilities.
5. Conclusions
In pulse-jet filtration, there are large numbers of governing parameters such as dust prop-erties, fabric properties, aerosol characteristics, dust cake properties, aerosol variables, andconstruction of filter unit, which affect filtration performance. Studies are often conductedtaking into account few parameters keeping other variables at fixed levels. Therefore, thesituation changes if the level of control parameters is changed at different levels. It is alsovery likely that potential interaction can exist among the variables. Filter performance alsochanges with time as with the number of cleaning cycles, internal structure of medium, andalso surface deposition changes. The above matter becomes complicated depending on thenature of the aerosol. In the case of filtration of a nonvolatile liquid aerosol on the flat filter,the penetration obtained will be different from those obtained for the filtration of solidaerosols. The study of filtration of mixed solid and liquid aerosols is more logical than thestudy of solid and liquid aerosols alone. In reality, very few studies and publications existon this subject. This is all the more damaging as this case is frequently encountered in theindustry. A greater emphasis is also required on the understanding of filtration behavior ofnanoparticles by the fabric filter media. In all these, it is indeed essential to quantify thecontribution of various filtration parameters on filtration performance considering inherentprocess variation during filtration.
Regarding the development of model, it is often very complex as there are large numbersof parameters affecting filtration with the change in time domain. Further, the operationis often subjected to substantial external disturbances and/or fluctuations in filtration de-mand, which often occur in the fabric filtration process. Whilst significant work has beenperformed on this subject, it is often difficult when attempting to integrate many researchcontributions. Under normal operational conditions, there are a number of design and oper-ational problems. First, the design process is such that the primary operating variable, thepressure drop at steady state, is not predictable from conventional theory and is known, andthe requirement for cleaning is therefore unknown. Normally design is accomplished on thebasis of operating experience in order to select an appropriate filter type, filtration velocity,cleaning frequency, pulse-jet pressure, and so on. Secondly, the operating conditions of thefilter are normally fixed by the supplier at the time of commissioning, and changing processconditions are not taken into account. Thus, it is an important task to develop a controlscheme to maintain an optimal operating condition, which can be applied throughout thelife of the filter in order to increase plant performance either in terms of energy efficiency,security of operation and tolerance to process disturbance, or reduced effluent concentra-tion. In general, for process optimization, there are several experimental approaches such asdesign of experiment, multi-variate analysis, ANN approaches, and expert systems, whichmight prove to be effective.
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The present situation is that cleanable filters have achieved a high stage of developmentwith respect to filter technology, and can be applied for sophisticated separation tasks. Theselection of the filter medium, however, takes up a special position in system planning,and is often still based on empirical criteria these days. Although in the meantime, thecharacterization of cleanable filter media and possibility of their evaluation were improved,this is still not sufficient. At present, a model-assisted design of filters and above all ofthe cleaning properties, which consider the substance-specific data of the dusts (e.g. theircohesive and adhesive properties), is not yet possible. The material-specific or textile dataas well as data concerning particle separation usually communicated by the manufacturers,and data on the pressure drop of the new filter media do not provide enough informationabout their long-term filtration behavior. It is also of specific importance to evaluate filterperformance in case of nanoparticles (100 nm or less). The reason why there is a lotof interest around them is due to the fact that at the nanoscale, particles show differentproperties of the bulk material they are made of. As of today, there are some concernsabout human-made nanoparticles escaping into the environment. In fact there are somesafety issues that are currently not fully understood. Although fibrous air filters can bevery effective in removing nanoparticles from air streams, no standardized test method isavailable for measuring the filter performance. Therefore it has become difficult to draft anyregulation for safe handling of nanoparticles [329]. Therefore, it is still necessary to improveboth the characterization and evaluation of cleanable filter media. In general, testing systemrequires more refinement and predictability for assessing long-term operating behavior offilter media. It would also be very useful if any scheme can be developed to predict thelifetime of filter media.
Acknowledgements
The author and publisher acknowledge the contribution of various publishers such asElsevier B.V.; Springer Science and Business Media; Eckhard von der Luhe, Germany;and organizations such as Fil T Eq - Filtration Testing Equipment and Services GmbH;Ankersmith PSA, Netherland; for giving kind permission to reproduce figures and tablesthat originally appeared in their respective journals/websites. The author wishes to put inrecord the extensive use of ScienceDirect (Elsevier B.V.) and consulting website materialsfrom the Department of Energy (DOE), USA; ASCO Numetics, France; PALAS, GmbHand many others. The authors of various publications consulted in the preparation of thismonograph are gratefully acknowledged.
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[8] W. Licht, Air Pollution Control Engineering, Basic Calculations for Particulate Collection,2nd ed., Marcel Dekker, New York, 1988.
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