performance of wire mesh mist eliminator

Chemical Engineering and Processing 39 (2000) 129–139

Performance of wire mesh mist eliminator

Hisham T. El-Dessouky a,*, Imad M. Alatiqi a, Hisham M. Ettouney a,Noura S. Al-Deffeeri b

a Chemical Engineering Department, Kuwait Uni6ersity, P.O. Box 5969, Safat, Kuwaitb Ministry of Electricity & Water, Kuwait, Kuwait

Received 10 August 1998; received in revised form 8 April 1999; accepted 9 April 1999

Abstract

An experimental study was made to measure the performance of wire mesh mist eliminator as a function of broad ranges ofoperating and design conditions. The experiments were carried out in an industrial scale layered type demister pad made of 316L stainless steel wires. The demister performance was evaluated by droplet separation efficiency, vapor pressure drop of wetdemister, and flooding and loading velocities. These variables were measured as a function of vapor velocity (0.98–7.5 m/s),packing density (80.317–208.16 kg/m3), pad thickness (100–200 cm), wire diameter (0.2–0.32 mm), and diameter of captureddroplets (1–5 mm). All the measurement results lie in ranges where, in practice, the wire mesh mist eliminator predominates. Theexperimental results indicate that the separation efficiency increases with both the maximum diameter of capture water dropletsand the vapor velocity and with the decrease of wire diameter. The pressure drop for the dry demister is relatively low anddepends only on the vapor velocity. The pressure drop increases linearly up to the loading point, thereafter; the rate of increaseis larger. Beyond the flooding point, the increase rate is significant even for the slightest rise in the vapor velocity. The floodingvelocity diminishes with the beef-up of the packing density and with the decrease of the wire diameter. Three empiricalcorrelations were developed as a function of the design and operating parameters for the separation efficiency, pressure drop forthe wet demister in the loading range, and the flooding and loading velocities. These correlations are sufficiently accurate forpractical calculations and demister design. The temperature depression corresponding to the pressure drop in a wire mesh misteliminator systems installed in a typical multi stage flash desalination plant was estimated from the developed correlation. A goodagreement was obtained between the design values and the correlation predictions. © 2000 Elsevier Science S.A. All rightsreserved.

Keywords: Mist eliminator; Wire mesh; Droplet separation; Evaporation; Thermal desalination processes

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1. Introduction

Wire mesh mist eliminator, in the most general sense,is a simple porous blanket of metal or plastic wireretains liquid droplets entrained by the gas phase. Theseparation process in the wire mesh mist eliminatorincludes three steps; the first, being ‘inertia impaction’of the liquid droplets on the surface of wire. As the gasphase flows past the surface or around wires in themesh pad the streamlines are deflected, but the kineticenergy of the liquid droplets associated with the gas

stream may be too high to follow the streamline of thegas and they impinged on the wires. The second stagein the separation process, is the coalescence of thedroplets impinging on the surface of the wires. In thethird step, droplets detach from the pad. In the vertical-flow installations, the captured liquid drains back in theform of large droplets that drip from the upstream faceof the wire mesh pad. In the horizontal flow systems,collected liquid droplets drain down through the verti-cal axis of the mesh pad in a cross flow fashion.

There are a number of industrial processes whereinthe liquid and the gas phases come into intimate directcontact with each other as a part of the process. As aresult of viscous and aerodynamic forces, liquiddroplets of various sizes are entrained and carried alongwith the moving gas stream. In most instances, it is

* Corresponding author. Tel.: +965-4811-1188 (ext. 5613); fax:+965-483-9498.

E-mail address: [email protected] (H.T. El-Dessouky)

0255-2701/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.

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H.T. El-Dessouky et al. / Chemical Engineering and Processing 39 (2000) 129–139130

desirable or even mandatory that these droplets beremoved from the gas stream for different reasons.Examples of such considerations are recovery of valu-able products, improving emission control, protectionof downstream equipment, and improving product pu-rity. A typical use of mist eliminator takes place in suchoperations as distillation or fractionation, gas scrub-bing, evaporative cooling, evaporation and boiling,trickle equipment for sewage and the like.

There are several devices which are offered to indus-try for separating the entrained liquid droplets and eachof which are effective over their own particular range ofmist size. Mist eliminators can be summarized into thefollowing groups settling tanks, fiber filtering candles,electrostatic precipitators, cyclones, impingement vanseparators, and wire mesh.

In the early days of evaporators, especially in thermaldesalination plants, the solid vane type separators wereused. However, the system suffers from the followingdrawbacks: (1) high pressure drop (which could resultin the total loss of temperature driving force betweenstages) and (2) excessive brine carry over. Today, thewire mesh mist eliminator is widely used in thermaldesalination plants to remove the water droplets, whichmay be entrained with the boiled or/and flashed offvapor. This water droplet must be removed before thevapor condensation over the condenser tubes. If themist eliminator does not separate efficiently the en-trained water droplets, it will cause the reduction ofdistilled water quality and the formation of scale on theouter surface of the condenser tube. The last effect isvery harmful because it reduces the heat transfer coeffi-cient and enhances the corrosion of the tube material.

The main features of wire mesh mist eliminators arelow pressure drop, high separation efficiency, reason-able capital cost, minimum tendency for flooding, highcapacity, and small size. The performance of wire mesheliminators depends on many design variable such assupporting grids, vapor velocity, wire diameter, packingdensity, pad thickness, and material of construction.Because the wire-mesh is not rigid, it must be supportedon suitable grids. To obtain minimum pressure drop,maximum throughput, and maximum efficiency, thesupport grids must have a high percentage of freepassage. To take full advantage of the 98% or so freevolume in the wire-mesh, the free passage through thesupport grids should be greater than 90%. If the freepassage through the support grids is much below 90%,the accumulated liquid is prevented from draining backthrough the support grids, causing premature flooding[1].

Typically, maximum allowable velocity for a misteliminator is limited by the ability of the collectedliquid to drain from the unit. In vertical up flow meshdemister, when the gas velocity increases past designlevels, liquid begins to accumulate in the bottom of the

unit. The liquid buildup results in re-entrain of thedownstream. This is because the inertia of the incominggas prevents the liquid from draining out of the unit. Inhorizontal units, the gas inertia pushes the capturedliquid toward the downstream face [2]. As a rule,smaller diameter wire targets collect smaller liquiddroplets more efficiently. For example, a 10 mm wireremoves smaller droplets than a 200 mm wire. However,a bed of 10 mm wires normally has the tendency toflood and re-entrain at much lower gas velocities than abed of 200 mm wire. This is because the thinner wiresprovide dense packing that can trap the liquid bycapillary action between the wires [3]. Interweaving ofsmall diameter wires with larger diameter wire has beenused often to tackle some of the most difficult mistremoval problems. This design uses the metallic orplastic wires as a support structure to hold the finewires apart. Even with this approach, the throughputcapacity of the unit is limited, compared to that possi-ble with conventional mesh. Special internal mesh ge-ometry modifications are now available that allow thesebi-component (that is, small-fiber and large-diameterwire mesh) configurations to operate at velocities essen-tially the same as conventional mesh designs. Theseultra-high-efficiency designs can be substituted for con-ventional mesh and used, for example, in the dehydra-tion towers of natural gas production plants, whereeven small losses of absorption chemicals, such asethylene glycol, can be a significant operating expense[4].

Construction materials for the wires include metal,fiberglass, plastics or polymers such as polypropyleneor Teflon. Recently, three new alloys have been madeavailable in wire form, which routinely provide three tofive times the service lives of the traditional materials.These are Lewmet 66, SX and Saramet. They can offerimproved service depending on the temperature andacid concentration of the gas stream [1].

The basic concept and main features of the wire meshmist eliminator have been discussed in a limited numberof publications. Examples can be found in the studiesby Holmes and Chen [2], Feord et al. [5], Buerkholz [6],Belden [7], Bhatia [8] and Tennyson [9]. The mainobjective of primordial investigators of wire mesh misteliminator system, is comparing the separation effi-ciency of the wire mesh with that of other mist elimina-tor units. A complete review of the origin and earlyhistory of technical papers dealing with different misteliminator systems are surveyed by Buerkholz [6]. Hereported that, in order to prevent any re-entrain of thewater droplets captured in the wire mesh pad, the gasphase velocity should be limited to 4–5 m/s. Addition-ally, he presented experimental data for the floodingload, the corresponding increase in pressure drop, andthe fractional separation efficiency. In a dimensionlessform the fractional degree of precipitation depends on

H.T. El-Dessouky et al. / Chemical Engineering and Processing 39 (2000) 129–139 131

the Stokes, the Reynolds, and Euler numbers. Experi-mental analysis show that for large Reynolds numbersand in a large range of the Euler number, the inertialprecipitation depends on the dimensionless precipita-tion parameter. Therefore, a simple approximation for-mula is given for the fractional degree of precipitationand the limiting droplet size for all types of separators[10].

Bradie and Dickson [11] discussed the factors govern-ing the wire mesh demister, and in particular with theirapplication to entrainment removal in pool boilingsystems. They derived the separation efficiency of thedemister pad from the collection efficiency for a singlewire. This efficiency is used to determine the minimumvapor velocity as a function of droplet size. Correla-tions are developed for the single and two phase (dryand wet) pressure drop through the demister togetherwith empirical formula relating the flooding point tothe vapor and liquid velocities. They measured thepressure drop and flooding conditions for a series ofdemisters installed in a small 140 mm diameter windtunnel. The tested demisters included the layered andspiral-wound configurations. All the experimental re-sults were taken for air–water at atmospheric pressureand ambient temperature. No attempt has been madeto use fluids with different properties or to operate thesystem at other conditions.

Feord et al. [5] reported that the method used todesign and predict the performance of the mist elimina-tor units are deficient in their inability to predict theconcentration of dispersed liquid in the outlet gasstream. This requirement must be specified in manyindustrial applications, distinctively in gas treatmentprocesses. Accordingly, they proposed a mathematicalmodel to specify the outlet concentration and dropletsize distribution for knitted mesh mist pad. The modelpredicts variations in performance and the decrease inthe separation efficiency with decreasing both thedroplet size in the feed mixture and the gas velocity.Quantitative predictions of the flooding velocity com-pared with the experimental results measured for air–water system. The model offers the prospect ofoptimizing the pad construction to maximize the sepa-ration efficiency at a target pressure drop or designingto a maximum pressure drop. The main drawback ofthe models developed by Feord et al. [5] and Bradie andDickson [11] is the need for complete information onthe entrainment level and the droplet size spectra. Thistype of data is not always available in practical units.Measuring droplet size distribution, especially in thesmall size range, is quite difficult, costly, and prone toinaccuracies. It is usually practical to develop such dataonly in the laboratory or pilot plant. Moreover, inmany realistic cases the droplet spectrum is highlydynamic and undergoes rapid changes due to any dis-turbance in the evaporation process. Also, the models

developed by Feord et al. [5] and Bradie and Dickson[11] are tested only for air–water system.

Robinson and Homblin [12] presented detailed exper-imental work undertaken to demonstrate the collectionof liquid aerosols in a helical coil as a function ofaerosol size, gas velocity, tube diameter and number ofcoils. They compared the performance of this helicalcoils with that of a knitted mesh, a cyclone and apacked bed. They found that the performance of thehelical coil demister is superior to the other systems.Nevertheless, the pressure drop for the knitted meshsystem was to some extent lower than that of any oneof the considered systems. Simple knockout drums,mist-pad knockout drums, impingement separators, cy-clones filter-bed mist collectors, wet scrubbers, andelectrostatic precipitators mist eliminator systems havebeen discussed thoroughly by Capps [13]. The discus-sion was intended to serve as a guide to proper misteliminator selection. Bayley and Davies [14] reviewedthe applications of knitted mesh demisters and pre-sented typical design layouts. They developed anomogram, which allows the process design engineersto quickly and accurately assess size and capacity of therequired demister.

To the best of our knowledge, the only paper avail-able in the open literature that deals with the applica-tion of the wire mesh mist eliminator in multi stageflash (MSF) water desalination units is that of Lerner[4]. He stated that the width of the flashing chambers isusually dictated by mist eliminator area requirements.They introduced two new types of mist eliminators. Thefirst one is the mist master while the second configura-tion is the V-2000. Their capacities respectively, are twoand three times those of conventional mesh pads. Theapplication of these types in MSF units would allow forgreater reduction in chamber size. Also they reportedthat both of mist master and V-2000 have been used inmany field applications. However, these mist elimina-tors have not been used in the desalination area whereother complex design considerations come into play.

In view of the previous discussion the followingconclusions can be drawn:1. We believe that the research on performance evalua-

tion of the wire mesh mist eliminators in operatingconditions similar to that dominating in evapora-tors, specially in thermal desalination systems, is stillin an immature state.

2. The application of wire mesh mist eliminator inthermal desalination units is based on either anexperience-based performance specification or datasupplied by the demister manufacturers.

3. The available theoretical models devoted to theperformance of the wire mesh mist eliminators arenot adequate for implementing to the industrialunits. Therefore, in this work the emphasis is laidmore on the experimental investigation.


In the current study, the performance of wire meshmist eliminator was investigated experimentally. Thedemister used was layered type and made of 316 Lstainless steel wire. The demister performance was eval-uated by the droplet separation efficiency, the wetvapor pressure drop, and velocities for flooding andloading. These variables were measured as function ofvapor velocity, wire diameter, packing density, padthickness, and the considered droplet diameter. All themeasured data lie in a range where, in practice, the wiremesh mist eliminator predominates. While experimentaldata provide clearer understanding of the phenomenaoccurring in mist eliminators, the developed correla-tions offer the most promise of improving designpractice.

2. Experimental apparatus and procedure

The experimental apparatus is schematically sketchedin Fig. 1. The system components include a Plexiglascolumn, a direct contact condenser, water circulatingpump, the wire mesh mist eliminator, and a hot watertank. The required steam is supplied from an electricalboiler with a power capacity of 45 kW. The boilergenerates saturated steam with a maximum pressure of16 bars. In all experiments this pressure is set at 2 bars.A water chiller is used to provide the direct contactcondenser with cooling water.

The Plexiglas column has a 75 mm inside diameterand 1500 mm height. It is fitted with several connec-tions for sampling the up and downstream vapors,measuring the pressure drop across the wire mesh pad,and measuring the absolute pressure inside the column.Another line is used to connect the steam flowing fromthe mist eliminator to the direct contact condenser. Thehot water tank has an inside diameter of 150 mm and aheight of 300 mm. The tank walls are made of 3 mm

thick stainless steel. The Plexiglas column is sealed verywell to the tank by a stocky layer of silicon rubber. Thedirect contact condenser is a glass container with 10liters capacity. Thick rubber stoppers are used to plugthe container thoroughly at the top and the bottom.The pipes for the steam and cooling water are suffi-ciently submerged by water inside the condenser. This isnecessary to insure intimate contact between the twophases and to prevent the escape of steam beforecomplete condensation. The water circulating pump is acentrifugal type with submerged inlet and a power of0.375 kW.

The wire mesh mist eliminator, the heart of theexperimental unit, was formed of wires used in con-structing the wire mesh mist eliminator in typical multistage flash (MSF) desalination units. The wire diameterranges from 0.2 to 0.32 mm and is made from Stainlesssteel 316 L. The wire was combed and flattened to givea double layer. Several of the double layered wires areused to form the demister, where the appropriate num-ber is chosen to give the desired pad thickness. The padis placed on an annular ring and a central supportbeam inside the column. The system was held at therequired height by fixing the bottom support grid to thering and beam. The free passage through the supportgrid was 93.5%. This is necessary to obtain minimumpressure drop, maximum throughput and maximumefficiency. If the free passage through the support gridsis much below 90%, the accumulated liquid is preventedfrom draining back through the support grids, causingpremature flooding [2]. The mesh pad was sized to be 3mm larger than the column inside diameter to providea sung fit and to minimize bypassing of the steam.

Demisters are usually specified by means of theirSpecific area As, packing density, rp, and void fractiono. These parameters are defined as:

As=Surface area of wiresVolume of demisters

(1)

Fig. 1. Schematic of experimental system.


rp=Mass of wire

Volume of demister(2)

and

o=1−Volume occupied by wires

Volume of demister(3)

For industrial demisters As ranges from 140 to 300m2/m3, rp from 80 to 268 kg/m3, and o from 0.979 to0.99 [15]. The characteristics of the demister pad usedin the experiments are selected within the above ranges,where As, rp, and o were varied as follows: 150–294m2/m3, 80–208 kg/m3, and 0.974–0.990, respectively.

The measuring system consists of two water rotame-ters, three mercury glass thermometers, two verticalU-tube manometers, and a water level scale. Water flowrates to and from the direct contact condenser weremeasured by two calibrated water rotameters having anaccuracy of approximately 91%. Rotameters were cal-ibrated by time–weight measurements. The three Mer-cury filled glass thermometers were used to measure thetemperature of water flowing to and from the directcontact condenser and the temperature of condensedvapor inside the direct contact condenser. The resolu-tion of these thermometers is 90.1°C. The verticalU-tube manometer used to measure the pressure dropacross the wire mesh was filled with water while theother manometer which is used to measure the absolutepressure inside the column was filled with Mercury. Theaccuracy of the manometers were 0.01 mmHg. Theaccuracy of the water level scale inside the direct con-tact condenser was 91% of the full scale. Two identi-cal syringes, with a 20 cm3 capacity, were used forsampling the vapor flowing upstream and downstreamthe mist eliminator. The inside diameter of the syringeneedle ranged from 1 to 5 mm. The size of the needlediameter gives a measure for the size of the largestcollected droplet. The sampling processes by the twosyringes were carried out simultaneously. Samples arewithdrawn from the center of the Plexiglas columniso-kinetically, where the vapor velocity in the needleinlet is the same as that in the column. The weights ofthe two syringes were measured before and after sam-pling by sensitive balance with an accuracy of 90.0005% of the applied load. The increase in the massof the syringes represents the mass of water droplets per20 cm3 of vapor. This sampling technique is simple toapply, gives a direct in situ measurements of the maxi-mum diameter of the captured droplets, and gives themass instead of the volume of collected droplets. Thus,there is no need to account for the droplets collectedinside the needle or on the walls of the cylinders andthe syringes. It should be stressed that there is no anyrelationship between the maximum droplet diameterdetermined by the needle inside diameter and the parti-cle size distribution and the mean particle diameter.

All experimental measurements were taken at steady-state conditions. In a typical run, the manometer read-ings were adjusted to zero values. The steam valve wasopened and the steam is admitted into the water accu-mulation tank. The mass flow rate of steam was con-trolled to achieve the steam velocity inside the column,which ranges from 1 to 7.5 m/s. The lower limit isdetermined by the minimum vapor velocity for waterentrainment. On the other hand, the steam boiler ca-pacity and the flooding of the demister pad determinethe upper limit. The flow rate and the temperature ofthe cooling water to the direct contact condenser wasadjusted in order to control the vapor condensationtemperature, consequently setting the pressure insidethe Plexiglas column. The liquid level of water insidethe direct contact condenser was kept constant byregulating the water flow rate from the direct contactcondenser.

Data were recorded for each run after steady stateconditions had been maintained for at least 10 min. Anoverall heat balance for the experimental apparatus wascalculated after each test run. Collected data was dis-carded and the experiment was repeated, if the heatbalance was in error by more than 5%.

The vapor velocity inside the column, V, was calcu-lated from the following relationship:

V=4MS

rvpd i2 (4)

where rv, di, and Ms are the steam density, columninside diameter, and the mass flow rate of condensingvapor, respectively. Two different methods are used toevaluate Ms. The first uses the readings of the tworotameters measuring the flow rate of water flowingfrom and to the direct contact condenser. In the secondmethod, the mass of steam condensed, Ms, is calculatedfrom the following energy balance relationship:

Ms=MoCp(T0−Ti)

Cp v(Ts−Ti)+ls

(5)

where Cp and Cpvare the specific heat at constant

pressure of water and steam respectively, ls is the latentheat of evaporation at steam temperature Ts, Mo is themass flow rate of water flowing from the condenser,and T is the temperature. Subscripts o and i denotes theoutlet and inlet conditions respectively. The aboveequation is developed from the total heat balance overthe direct contact condenser.

Any experimental procedure contains uncertainties,and error analysis is essential to attach significance toresults. To estimate the uncertainties in the resultspresented in this work, the approach described byBarford [16] was applied. The overall uncertainly as-signed to a given measurement is defined as the root-sum-square combination of the fixed error due to theinstrumentation and the random error observed during


measurements. Exercising this procedure, the calcula-tions indicated errors of 2.4% for temperature, 3.15%for flow rate, 2.73% for pressure drop, 2.31% for abso-lute pressure and 1.19% for liquid level. On the basis ofthese errors, the pressure drop of the wet demister,separation efficiency, and velocities for loading andflooding may depart by 4.6, 3.2 and 4.1% from the truevalues.

3. Results and discussions

Data required to evaluate or size a wire mesh misteliminator system include the droplet separation effi-ciency, capacity, and the vapor side pressure drop.These three parameters are interrelated and should beevaluated simultaneously. Efficiency of mist eliminatorcan be presented either as fractional removal or asfractional penetration. The summation of the removaland penetration efficiencies is equal to unity. Separa-tion efficiency is a measure to the fraction of droplets inthe vapor swept out by the wire mesh mist eliminatorand is given by

h=Min−Mout

Min

×100 (6)

where Min and Mout are the mass of entrained waterdroplet by the vapor up and down stream the misteliminator, respectively.

The capacity of a wire mesh mist eliminator is deter-mined by the conditions of loading and flooding. Be-yond the loading point, the liquid holdup is highenough to improve the separation efficiency. Thedemister should operate at a velocity higher than theloading velocity. Flooding occurs when the vapor ve-locity exceeds a critical value. To prevent flooding, themist eliminator must be designed and sized so that thedesign velocity is below the critical flooding velocity.

Fig. 2 shows the measured separation efficiency as afunction of the maximum diameter of captureddroplets. Three sets of data are presented that corre-spond to three different velocities of the vapor. As isshown, all curves have similar trend, where the removalefficiency increases with the increase of maximum di-ameter of collected droplets and the vapor velocity.There are three different mechanisms for capturing theentrained droplets by the wire mesh pad. These arediffusion, interception, and inertial impaction. The dif-fusion mechanism, sometimes called Brownian motions,is significant only for the capture of submicron dropletsat a very low gas velocity. Interception occurs fordroplets with dimensions similar to or higher than thewire diameter. Inertial impaction occurs when the va-por is forced to change its direction around an object.In all experimental measurements, the maximumdroplet size was 5 mm and the wire diameter ranged

Fig. 2. Effect of droplet size on the separation efficiency at differentvalues of vapor velocity.

from 0.2 to 0.32 mm. Thus, all these separation mecha-nisms are effective here; however, inertial impaction isclearly the predominant mechanism for droplet separa-tion. As the vapor and entrained liquid droplets passthrough the mist eliminator, the vapor phase movesfreely, but, the liquid, due its greater inertia, is unableto make the required sharp turns. Therefore, thedroplets are impacted and collected on the surface ofthe mesh wires. The droplet momentum or inertia isproportional to their velocity, mass, and diameter.Droplets with sufficient momentum can break throughthe vapor streamlines and continue to move in astraight line until they impinge on the target. Thesecond stage in the separation process is the coalescenceof the droplets, which impinge on the wire surface.Subsequently, the droplets compound and form streamsor rivulets, which drain back against the vapor flow.

Fig. 3. Effect of vapor velocity on the separation efficiency atdifferent values of droplet size.


Fig. 3 displays the variation of the separation effi-ciency with the vapor velocity at different values ofmaximum size of collected droplets. The separationefficiency increases steadily with the vapor velocity untilit reaches a peak value and thereafter diminishes withfurther increase in vapor velocity. This observation canbe attributed to re-entrainment of the water dropletwith the vapor flowing inside the pad. There are threedifferent forces controlling the movement of the waterdroplets accumulated within the demister pad. Theseare the drag, the gravity, and the surface tension forces.When the gravity force is dominant, the droplets aredetached from the wire and drained by gravity. Re-en-trainment of the collected water droplets with the vaporflowing in the demister occurs because the shearingenergy per unit volume of liquid droplet is sufficient tocreate liquid droplets having a large ratio of surface tovolume. This will increase the drag force exerted by thevapor on the droplets external surface. The drag forceis proportional to the vapor interstitial velocity anddroplets surface area and density. The vapor velocity inthe pad increases at larger liquid hold up. This isbecause the liquid occupies part of the void fraction,thus, decreasing the available area for vapor flow. Theliquid hold up in the pad is related to the rates ofdraining, re-entrainment, and collection. The increasein vapor velocity reduces the rate draining and in-creases the rates of collection and re-entrainment.Moreover, at high vapor velocities, the re-entrainedliquid droplets can re-impact on the surfaces of subse-quent wires. This may lead to the atomization of thewater droplet by the force of the impact. Consequently,a very fine spray of droplets is generated, which isdifficult to recapture by subsequent separators. Thismay explain the decrease in the separation efficiency atlarge vapor velocities.

The effect of the wire mesh diameter on the separa-tion efficiency at different values of vapor velocity isdisplayed in Fig. 4. These results are obtained for amaximum droplet size of 5 mm, a pad thickness of 200mm, and a packing density of 176.35 kg/m3. As dis-played, the droplet separation efficiency improves withthe decrease of the wire diameter. The influence of thewire diameter on the separation efficiency is more pro-nounced at lower vapor velocities. This is caused by thefact that the surface area of the wires at constantpacking density and depth is directly related to the wirediameter. As a result, more droplets with smaller sizecan be trapped for mesh wires with smaller diameters,where number of liquid droplets touching the wire isprimarily determined by the ratio between the wirediameter and the droplet size. Moreover, as the wirediameter is reduced, the surface area is increased.Therefore, a larger amount of entrained droplets arecaught within the demister pad due to capillary action.At high vapor velocity, the shearing energy per unit

Fig. 4. Effect of wire diameter on the separation efficiency at differentvalues of vapor velocity.

volume of the liquid droplets captured in the padprogressively increases as the wire diameter decreases.As a result, the liquid re-entrainment becomes exces-sive. Although, the results show the superior perfor-mance of demisters with smaller diameter wires;however, use of larger diameter wires is necessary tofacilitate demister washing and cleaning. Also, use oflarger diameter wires gives the demister adequate me-chanical strength and operational stability.

The wire surface area for the wire mesh mist elimina-tor is directly related to the demister packing density.The increase of the pad packing density is associatedwith the reduction of the porosity of the pad. As aresult, the number of entrained droplets which ap-proaching the wires and the amount of captureddroplets increases at larger surface area for the wires.This fact can be employed to explain the steady aug-mentation of the separation efficiency with the increase

Fig. 5. Effect of packing density on the separation efficiency atdifferent values of vapor velocity.


of the demister packing density as illustrated in Fig. 5.It is interesting to note that the effect of the pad densityon the separation efficiency is more pronounced at lowvapor velocities. This is mainly due to the increase indroplet re-entrainment and liquid hold up at highervapor velocities.

Fig. 6 elucidates the effect of the pad thickness on theseparation efficiency. As it can be seen, the separationefficiency is insensitive to the increase in the pad thick-ness. The pad thickness is a measure of how manytimes the vapor will consecutively impinge on a wireduring its passage through the mesh pad. It was ob-served visually that a thin layer of water was accumu-lated on the bottom surface of the wires facing thevapor. This thin film acts as an extra droplet collectionmedia. However, this layer does not penetrate throughthe wire mesh pad and is not affected by the padthickness. This explains the insensitivity of the separa-tion efficiency with the increase in the pad thickness.

The variation in the specific pressure drop for dryand wet demisters at different packing densities isshown in Fig. 7. The dry demister refers to the condi-tion at which the pad is free of water droplets. On theother hand, the wet demister assigns the state at whichthe water droplets are retained inside the pad. As it canbe seen, the pressure drop for the dry demister, ingeneral, is relatively low and increases linearly with thevapor velocity. The pressure drop for the dry demisteris a measure of the relative resistance of the fluid flowthrough the pad. This arises from the viscous dragbetween the vapor and the wires forming the demisterpad and also because of kinetic energy loss due to thechanges of the flow direction. The viscous drag forceand the kinetic energy loss are directly related to thevapor velocity and the total surface area of the wires(wetted area). The flow resistance is comparatively lowbecause of the high voidage of the demister. Measure-

Fig. 7. Variation in pressure drop as a function of vapor velocity fordifferent packing density.

ments show that the specific pressure drop for the drydemister is nearly independent of the demister thick-ness, density, and the wire diameter. The profiles forthe specific pressure drop of the wet demister can bedivided into three regions. The first occurs at low vaporvelocities, where the specific pressure drop varies lin-early with the vapor velocity and is parallel to thespecific pressure drop of the dry demister. In the secondregion, the specific pressure drop increases more rapidlywith the increase in the vapor velocity. In the thirdregion, the pressure drop rises very steeply even withthe slight increase of vapor velocity. The specific pres-sure drop in wet demisters is caused by the dry pad anddue to the presence of water droplets. The pressuredrop of the wet demister is complicated and there arethree contributions to this pressure drop. The first termrepresents the frictional pressure drop because of theslip between the vapor phase and the demister pad. Thesecond term represents the pressure drop due to thevapor phase acceleration. The last term accounts forthe gravitational effects, which is smaller than the othertwo terms and can be safely neglected. The frictionaland acceleration pressure drops are strongly dependenton the vapor velocity. The vapor velocity inside thedemister is changed as a result of variations in thesystem operating parameters or due to the holdup ofthe liquid phase. As the liquid holdup progressivelyincreases, the free space area available for the vaporflow decreases and results in rapid increase in the flowresistance. The liquid holdup may be either static ordynamic. Capillary action causes the static holdup andoccurs at high retention of the liquid within the demis-ter pad. The dynamic holdup takes place, as the settlingvelocity of the falling droplets becomes lower than the

Fig. 6. Effect of packing thickness on the separation efficiency atdifferent values of vapor velocity.


upward vapor velocity. The static holdup increases withthe increase of the wire surface area, which is directlyrelated to the packing density.

Fig. 8 depicts variations of the specific pressure dropwith the vapor velocity at different values of wirediameter. The figure illustrates that the pressure drop isinversely related to the wire size, which is caused by theincrease of the wires surface area at smaller wire diame-ters. The static liquid holdup augments with the in-crease in the wires surface area. It is interesting to notethat, the effect of wire diameter on the pressure drop ismore pronounced at higher vapor velocities.

Variations in the specific pressure drop at loadingand flooding conditions are shown in Figs. 7 and 8.Below the loading point the liquid holdup is relativelylow and the vapor velocity does not significantly affectthe pressure drop. Above the loading point, the liquidbegins to accumulate or load the demister progressivelycausing the decrease of the free space for vapor flow. Inthis region, the pressure drop increases more rapidlywith the increase in the vapor velocity. The floodingpoint represents the maximum loading the demister canaccommodate. The flooding and loading points occurat lower vapor velocities upon the increase of thedemister packing density and the decrease of the wirediameter. All these variables increase the surface area ofwires forming the demister pad and consequently theextent of liquid holdup.

It is important to emphasize that the design capacityof most mist eliminators is determined by the phe-nomenon of re-entrainment. However, in thermal de-salination processes, especially the multi stage flash(MSF) plants and the multiple effect evaporation(MEE) with large number of effects, the design capacityof the mist eliminator is obtained as a function of the

flooding points. The temperature difference availableper stage in MSF plants is relatively small particularlywhen the number of stages is high. El-Dessouky andBingulac [17] showed that the temperature depressioncorresponding to the pressure drop in the demister canconsiderably influence the plant thermal performance,specific heat transfer surface area and the specific cool-ing water flow rate. All these variables control the costof desalinated water. It is worth mentioning that theflooding velocity is relatively lower than the re-entrain-ment velocity. Thus limiting the capacity of wire meshmist eliminator by the flooding velocities will protectthe heat transfer surface areas downstream the demis-ter, prevent the diminishing of the product water qual-ity, and minimize the thermodynamic losses. However,the vapor velocity must be higher than the loadingpoint to ensure reasonable liquid holdup, which im-proves the separation efficiency.

3.1. Correlation of experimental results

In terms of the forgoing discussion, the separationefficiency of the wire mesh mist eliminator is affected byvapor velocity (V), wire diameter (dw), droplet size (dp),and packing density (rp). On the other hand, the wetpressure drop is affected by the packing density (rp),wire diameter (dw), and vapor velocity (V). The flood-ing and loading velocities (Vf and Vl) are dependent onthe packing density (rp) and wire diameter (dw).

The least square fitting of the experimental data givesthe following empirical correlations:

h=17.5047(dw)−0.28264(rp)0.099625(V)0.106878(dp)0.383197

(7)

DP=3.88178(rp)0.375798(V)0.81317(dw)−1.56114147 (8)

Vl=192.7189(rp)−0.470865(dw)1.75578 (9)

Vf=128.358356(rp)−0.287031(dw)1.220656 (10)

The ranges of the experimental variables were V(0.98–7.5 m/s), rp (80.317–208.16 kg/m3), L (100–200mm), dw (0.2–0.32 mm), and dp (1–5 mm).

Figs. 9–12 show the comparison between the mea-sured data and the calculated values from the devel-oped correlations. The developed correlation predictthe separation efficiency, h, the specific wet pressuredrop, DP, and velocities of loading and flooding veloc-ities, Vl and Vf, with the following standard deviations0.957, 0.9108, 0.9295 and 0.908, respectively.

3.2. Comparison with design data

The correlation for the specific wet pressure dropobtained in this study is used to predict the pressuredrop and the associated temperature losses in the wiremesh demisters of the MSF plants in the Gulf states.

Fig. 8. Variation in pressure drop as a function of vapor velocity fordifferent wire diameter.


Fig. 9. Measured and calculated pressure drop per unit length.

Fig. 11. Measured and calculated loading velocity.The demister has a thickness of 150 mm, a packingdensity of 180.518 kg/m3, and a wire diameter of 0.28mm. Fig. 13 displays the comparison between the de-sign temperature loss with the predicted values. As isshown, a good agreement is obtained between the de-sign and model predictions. The temperature depressioncorresponding to the pressure drop in the demister iscalculated from the relationship developed by El-Dessouky and Bingulac [17].

4. Conclusions

Research on evaluation of the performance of thewire mesh mist eliminator in operating conditions ofMSF plants is still in an immature state. The availabletheoretical models devoted to simulation of the perfor-mance of the wire mesh pads are not adequate forimplementation to the industrial units. This motivatedexecution of the present study, where experiments are

conducted to evaluate the performance of wire meshdemisters typical of MSF plants. The experimentalmeasurements are made as a function of the vaporvelocity, diameter of the wires, droplet diameter, pack-ing density, and pad thickness. Collected data is used toanalyze the system performance and to develop designcorrelations for the separation efficiency, the specificpressure drop for the wet demister, and the velocitiesfor loading and flooding. Within the experimentalrange used, the following conclusions are made1. The separation efficiency increases with the increase

of the droplet size and the vapor velocity.2. The specific pressure drop for the dry demister is

low and varies linearly with the gas velocity.3. The specific pressure drop for the wet demister

increases linearly up to the loading point, thereafter,

Fig. 12. Measured and calculated flooding velocity.Fig. 10. Measured and calculated separation efficiency.


Fig. 13. Comparison of design and calculated temperature losses indifferent flashing stages.

Ms mass of steam condensed, kg/sti Inlet temperature of water entering the

chiller, °COutlet temperature of water leaving theT0

chiller, °CTs Steam temperature, °CV vapor velocity, m/srv vapor density, kg/m3

packing density, kg/m3rp

void fractiono,latent heat of evaporation at steam tempera-ls

ture, kJ/kgdroplet separation efficiencyh

References

[1] P. Fabian, R. Cusack, P. Hennessey, M. Neuman, Demystifyingthe selection of mist eliminators, Part I: The basics, Chem. Eng.11 (1993) 148–156.

[2] T.L. Holmes, G.K. Chen, Design and selection of spray/mistelimination equipment, Chem. Eng. October (1984) 82–86.

[3] P. Fabian, P. Van Dessel, P. Hennessey, M. Neuman, Demysti-fying the selection of mist eliminators, Part II: The applications,Chem. Eng. December (1993) 106–111.

[4] B.J. Lenrner, High-tech mist elimination in multi-stage evapora-tors, Plant Operations Prog. 5 (1986) 52–56.

[5] D. Feord, E. Wilcock, G.A. Davies, A stochastic model todescribe the operation of knitted mesh mist eliminators, compu-tation of separation efficiency, Trans. IChemE. Res. Design 71(1993) 282–295.

[6] A. Buerkholz, Droplet Separation, VCH, New York, 1989.[7] R.D. Belden, Mist eliminators in evaporates and pans, Proceed-

ings of SIA meeting, Savannah, GA USA, May, 1988, pp.219–229.

[8] M.V. Bhatia, Entrainment separators, process equipment series,VCH, New York, 1989.

[9] R.P. Tennyson, Mist eliminator design and application, Proceed-ings of the 70th annual meeting of the air pollution controlassociation, Toronto, Canada, June, 1977, pp. 20–24.

[10] A. Burkholz, Die Beschreibung der partikelabscheidung durchtragheitskrafte mit hilfe einer dimensionsanalytisch abgeleitetenkennzahl, Chem. Eng. Techol. 58 (1986) 548–556.

[11] I.K. Bradie, A.N. Dickson, Removal of entrainment liquiddroplets by wire mesh demisters, Proc. Inc. Mech. Eng. 184(1969) 195–203.

[12] K.S. Robinson, C. Homblin, Investigating droplet collection inhelices and a comparison with conventional demisters, FiltrationSeparation May/June (1987) 166–171.

[13] W. Capps, Properly specify wire-mesh mist eliminators, Chem.Eng. Prog. (1994) 49–55.

[14] R.D. Bayley, G.A. Davies, Process applications of knitted meshmist eliminators, Chem. Process. May (1973) 33–39.

[15] Anon, Selecting and sizing ACS mesh pad mist eliminators,Bulletin SA-12, ACS Industries, Inc., Houston, 1995.

[16] N.C. Barford, Experimental Measurements: Precision, Error andTruth, 2nd edition, Wiley, New York, 1990.

[17] H.T. El-Dessouky, S. Bingulac, Solving equations simulating thesteady-state behavior of the multi-stage flash desalination pro-cess, Desalination 107 (1996) 171–193.

the rate of increase is higher. Beyond the floodingpoint, significant increase occurs in the specific pres-sure drop, even with the slightest increase in thevapor velocity.

4. The flooding and loading velocities increase at lowerpacking density and larger wire diameters.

Results and data analysis shows the new for furtherinvestigative research. Studies should consider perfor-mance evaluation of various demister designs, whichmay involve use of wires of different diameter withinthe same demister. Attention should be given to theeffect of locating the demister pad with respect to thesurface of the boiling or flashing liquid. Further studiesconcerning the static and dynamic hold up data will behelpful in better understanding of the demisterperformance.

Appendix A. Nomenclature

specific area, m2/m3As

specific heat at constant pressure of water,Cp

J/kg°CCpv

specific heat at constant pressure of steam,J/kg°C

dp maximum diameter of the captured droplets,mm

dw diameter of the wire, mmdi column inside diameter, mL packing thickness, mmM0 mass flow rate of water flowing from the

chiller, kg/sMin mass of entrained water droplet by the up-

stream of the mist eliminator, kg/sMout mass of entrained water droplet by the

downstream of the mist eliminator, kg/s .

performance of wire mesh mist eliminator

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