filtration-131106124031-phpapp01

8/13/2019 filtration-131106124031-phpapp01

http://slidepdf.com/reader/full/filtration-131106124031-phpapp01 1/73

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & MassBalance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance

Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – PetrochemicalsSpecializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries

Web Site: www.GBHEnterprises.com

GBH Enterprises, Ltd.

Process Engineering Guide:GBHE-PEG-SPG-300

FILTRATION

Information contained in this publication or as otherwise supplied to Users isbelieved to be accurate and correct at time of going to press, and is given ingood faith, but it is for the User to satisfy itself of the suitability of the informationfor its own particular purpose. GBHE gives no warranty as to the fitness of thisinformation for any particular purpose and any implied warranty or condition(statutory or otherwise) is excluded except to the extent that exclusion isprevented by law. GBHE accepts no liability for loss or damage resulting fromreliance on this information. Freedom under Patent, Copyright and Designscannot be assumed.






Process Engineering Guide: FILTRATION

CONTENTS

0 INTRODUCTION

1 THE THEORY UNDERLYING FILTRATION PROCESSES

1.1 The Mechanism of Simple Filtration Systems

1.1.2 Cake Filtration1.1.3 Complete Blocking1.1.4 Standard Blocking1.1.5 Intermediate Blocking

1.2 Cake Filtration – Models and Mechanisms

1.2.1 Classical Theory for the Permeability of Porous Cakes and Beds1.2.2 The Rate of Filtration through a Compressible Cake – The

Standard Filtration Equation1.2.3 The Compression or Consolidation of Filter Cakes – Ultimate

degree of dewatering1.2.4 The Rate of Consolidation1.2.5 Useful Semi-Empirical Relations for Constant Pressure and

Constant Rate Cake Filtration1.2.6 Constant Pressure Filtration

1.2.7 Constant Rate Filtration1.2.8 Multiphase Theory of Filtration

1.3 Crossflow Filtration

2 THE RANGE AND SELECTION OF FILTRATION EQUIPMENTTECHNOLOGY

2.1 Scale2.2 Solids Recovery, Liquids Clarification or Feed stream

Concentration2.3 Rate of Sedimentation2.4 Rate of Cake Formation and Drainage2.5 Batch vs Continuous Operation2.6 Solids Loading






2.7 Further Processing2.8 Aseptic or “Hygienic” Operation

2.9 Miscellaneous2.10 Shear versus Compressional Deformation2.11 Pressure versus Vacuum

3 SUSPENSION CONDITIONING PRIOR TO FILTRATION

3.1 Simple Filtration Aids3.2 Mechanical Treatments

4 POST-FILTRATION TREATMENTS AND FURTHER DOWNSTREAMPROCESSING

4.1 Washing4.1.1 Air-Blowing4.1.2 Drying

5 TESTING AND CHARACTERIZATION OF SUSPENSIONS

5.1 Introduction – Suspension

5.2 Properties relevant to Filtration Performance 5.2.1 Pre-Filtration Properties of Suspension5.2.2 Properties of Filter Cake5.2.3 Laboratory Scale Filtration Rigs

5.3 Means of Monitoring Flocculant Dosage

5.4 Filter Cake Testing5.4.1 Strength Testing (See also piston press described earlier)5.4.2 Cake Permeability or Resistance5.4.3 Rate of Cake Formation






6 EXAMPLES OF THE APPLICATION OF THE FORGOING PRINCIPLES

6.1 Dewatering of Calcium Carbonate Slurries

6.2 Dewatering of Organic Products – Procion Dyestuffs6.3 Filtration of Biological Systems – Harvesting a FilamentousOrganism

REFERENCES

TABLES

FIGURES






0 INTRODUCTION

For the purposes of this process engineering guide, filtration will beregarded as the process whereby solids are separated from liquids by the

use of a porous medium. This definition then deliberately excludes thefiltration of gas streams. Filtration together with gravity separation formsthe basis for nearly all unit operations to accomplish the dewatering ofsuspensions [3-5]. It is therefore worthwhile considering the broad factorsthat favor filtration over gravity separation methods for a given system [1].

Perhaps the most important of these involves the use of a porous mediumto effect the operation. The nature of the former may be tailored anddesigned to best suit the requirements of both phases of the suspensionand the sort of dewatering action required. In contrast, all techniquesbased on gravity separation are completely dependent upon the densitydifference, ∆ρ, between solid and liquid phases. Since ∆ρ must beregarded for many systems as an invariant, (It may be slightly perturbedby a change in operating temperature), a small value for the quantityalmost Invariably means that gravity separation will prove difficult. This Isoften the case for biological particles (see Section 3.8). For such cases filtration is then often to be preferred.

The penalty that has to be paid for this versatility of filtration processdesign is usually greater expense and additional complexity whencontinuous or automated operation is desired. It must be stressed that theabove statements are based on broad, generalized principles. For bothfiltration and gravity separation, the ingenuity of solid/liquid separationequipment designers has led to means of at least partly circumventingmany of the disadvantages associated with each [8].

The above definition of filtration encompasses a large number ofpossible operations ranging from the clarification (or even sterilization> ofa very slightly loaded suspension to the removal of a product in the form ofa solid cake. Other variations permit filtration to be used as a thickeningoperation where a feed stream is concentrated in the suspended phasewithout the formation of a cake or the deposition of solids. Since thissection of the manual falls within the dewatering section, most emphasiswill be given to processes where it is the suspended phase that containsthe desired product and Is usually to be further processed. Clarification byfiltration will be discussed briefly but, for greater detail, the reader isreferred to the separate chapter on that subject (GBHE-PEG-SPG-400 -Centrifugation) and to references [1,12]






Finally It is appropriate to set the context for the rest of this guide. InSection 3.5.2 some of the basic, and largely classical, theory of filtrationwill be presented. The range and selection of filtration equipment Is thenbriefly discussed in Section 3.5.3. In this and all parts of this section

emphasis will be placed much more upon relating material properties tothe process design than in providing a comprehensive survey of theavailable technology. To Illustrate process interactions, In Section 3.5.4 a brief consideration is given to those operations most likely to followfiltration in a complete process. In Section 3.5.5, methods for testing andcharacterizing the filtration properties of suspensions are describedtogether with the interpretation of the results in terms of the theorypreviously given. To conclude the section, examples are given of theprocessing of suspensions containing inorganic, organic and “biological”particles. It is hoped that these will illustrate many of the principlespreviously developed.

1 THE THEORY UNDERLYING FILTRATION PROCESSES

The purpose of this part is to provide the necessary theory on which therest of the section is based. It is intended that each of the following topicsshould be self-contained and can therefore be read in isolation. The theorypertaining to washing and dewatering by air-blowing is postponed untilSection 3.5.4.

1.1 The Mechanism of Simple Filtration Systems

Various authors have proposed schemes for classifying the diversemechanisms that may operate during filtration operations. The simplestclassification for solids retaining systems discriminates between caseswhere the solids build up on top of a cake and those where they areretained within the filter medium. A useful classification based on thisapproach was provided a long time ago by Hermans and Bredee [14] whodistinguished four mechanisms, which are sketched schematically inFigure 1:






1.1.2 Cake Filtration (Figure 1(a))

This process involves the removal of solids by the formation of a filtered“cake” on the surface of the medium. It Is the cake itself which effects the

subsequent filtration. Depending upon whether the suspended particlesare smaller or larger than the pores of the medium, this process Is usuallypreceded by a bridging or straining process in order for cake formation toensue. Hermans and Bredee proposed the following equations to describethe time dependence for cake filtration:

where V is the volume of filtrate at time t, Qo. the Initial flow rate

and k an empirical constant.

Alternatively in terms of a mean (cumulative) flow rate, q(t):

Equations (1) and (2) clearly represent a gross oversimplification of thecake filtration of real systems. Modified forms of these are presented Insubsequent sections. Graphical representations of Equations (l)-(2) are

shown In Figure 2(a).

1.1.3 Complete Blocking (Figure 1(b))

This mechanism Involves a straining process either at the medium surfaceor within Its internal structure. This implies that the solids particles arelarger than the. local size of the pores of the medium. This then results incompleting blocking of pores as the filtration proceeds with a lineardecrease in the flow rate with volume:






In surveying a large number of suspensions and filter media, Hermansand Bredee found this type of filtration to be rather rare. It also falls toyield a useful and physically plausible volume-time relationship whenintegrated.

The concept of "complete blocking" as depicted in Figure l(b) still retainssome merit for classification purposes, however.

1.1.4 Standard Blocking (Figure l(c))

This is the mechanism most pertinent to depth filtration where particlesmay pass through the pores of the medium but are retained by eventualadhesion to it. Hermans and Bredee's model ascribed a "fouling" processwhere the internal volume of the pores decreased linearly with V.

Thus they obtained equation (4).

Since the subject of deep bed filtrations falls outside the intended scope ofthis guide, no further discussion of this mechanism or topic will bepresented. More Information may be found in the guide on clarification orIn the books by Svarovsky (Chapter 11 of [1] and Purchas (Chapters 3

and 6 of [12]. In addition, the role of the particle zero-potential has recentlybeen considered by Raistrick amongst others (J H Raistrick in [94]).

The equations (l)-(4) were developed to allow volume, flow rate and timecorrelations to be tested for each mechanism under conditions of constantpressure filtration. A simpler diagnostic means of distinguishing andinterpreting them was provided by the dependence of the rate of changeof total filtration resistance, r , (i.e. medium plus accumulated solids) withfiltration volume, dr/dV, with r. The three mechanisms described so farwere attributed the following dependence:






In order to encompass the sometimes observed relationship, dr/dV a r,Hermans and Bredee's classification allowed for a fourth mechanism:

1.1.5 Intermediate Blocking

Where,

This mechanism may be physically viewed as being intermediate betweenFigures l(b) and l(c).

1.2 Cake Filtration – Models and Mechanisms

In this section of the dewatering chapter, greater prominence will be givento cake filtration than to the other mechanisms just described. In part thisreflects the frequency with which the formation and properties of a cakedominates a separation operation. However, the other reason for thisemphasis derives from the necessity to understand and control theinfluence of the suspension itself as opposed to the hardware; for aproperly conceived cake filtration the suspension properties are dominantand the filtration medium of secondary importance.

Before presenting some of the basic theory for cake filtration it is importantto delineate the factors which relate to the fundamentals of dewateringpresented In Section 3.2. In general for a given cake filtration system onemight want to ask the following two questions:

(i) What degree of dewatering Is attainable for a given suspension In acake filtration rig? How is this ultimate dewaterabillty influenced bychanges in the suspension, filtration conditions and drivingpressure?

(ii) What are the kinetics of the filtration process, ie do they allow theprocess to operate near to or at the ultimate limit as in (1) above.

In general theories describing the permeability of a cake or its rate ofdeposition are very much aimed at addressing the second sort ofquestion.






However, when one is trying to understand, for example, the expression ofliquid in a filter press, it is the ultimate dewaterability that is usually criticalthough kinetics may again be limiting. For a comprehensive understandingof filtration processes It is therefore necessary to have a quantitative

picture of both the kinetics of the process and the maximum degree ofdewatering that can be obtained. Both these Issues will be tackled In thefollowing pieces of theory. Additional aspects of the theory of filter cakesand sediments have been discussed In more detail by Tiller and otherworkers [11].

Other relevant references may be found in the sedimentation section ofthis chapter (3.3).

1.2.1 Classical Theory for the Permeabil ity of Porous Cakes andBeds

From an observation of the rate of flow of liquids through beds of sand,Darcy [15] suggested an empirical correlation between the fluid velocity, u,the pressure drop across the bed, ∆P, and the bed thickness, L. Theresult, Darcy's Law, may be expressed in the simple form:

Some insight into the nature of the constant, K1, is gained by assuming a

result from the Poiseuille [16] Equation for the flow of a liquid through acapillary tube of radius, r, and length, L:

This result enables equation (6) to be modified thus:

where the Influence of the fluid viscosity, Q, has been explicitly Included.K2 has dimensions of (length) 2.






A combination of the equations of Darcy and Polseuille together with theIncorporation of two properties of the bed itself (S A and Ɛ, led to theequation attributed to Kozeny and Carman [17-18]:

In this equation the bed properties S A and Ɛ represent the specific surfacearea (m-1) and the fractional voidage, ie 1 - ɸ, where ɸ is thedimensionless volume fraction of particles In the bed or filter cake.

The Kozeny-Carman Equation, although the precursor for the mostcommonly used filtration equations, suffers from a number of restrictionsand oversimplifications. Basically It Is a sound description for the drainage

rate of viscous flow of a clear liquid through a porous bed of constantpermeability VULCAN VGP systems the permeability at a given point maybe a function of pressure drop, time and the height of that point within thebed. Further discussion of these features is provided later. The constraintof viscous laminar flow is also sometimes not strictly applicable Inpractice. Where turbulence becomes significant a correction to therearranged equation may be made as follows, after Burke and Plummer[20]:

This equation was derived for beds of uniform spherical particles ofdiameter, d; hence S A = 6/d. It can be seen that the leading term for thepressure drop per unit length is the simple, viscous Kozeny-Carmancontribution. The second term is the kinetics energy loss to the pressuredrop through turbulent flow. Whether such a kinetic energy modification isnecessary for a given bed and flow rate may be judged by a plot of theReynolds Number,






against permeability (eg Morgan's work with sintered metal pores [21].Fortunately it Is often the case that the simpler, purely viscous treatmentof permeability Is sufficiently accurate for practical applications.

1.2.2 The Rate of Filtration through a Compressible Cake – TheStandard Filtration Equation

From the somewhat idealized equations for the permeability of porousbeds, a straightforward modification, to allow for medium resistance, Rm,yields a general expression for cake filtration rates:

Rc, the cake resistance (units of m-l) may In turn be related to, w, theweight of solids per unit volume of filtrate (kg rnm3) and a quantity, r, thespecific resistance of the cake (le resistance/weight of solids per unit areaor m kg-1):

As pointed out in (i) for many real systems it is necessary to take accountof the finite compressibility of the filter cake under an applied pressure. A

simple empirical correction to resistance has been very widely used(1,12).






where ro is the specific resistance at zero pressure drop and the exponent“s” is known as the compressibility factor. This factor ranges from 0 for a perfectly incompressible material to unity a highly compressive cake. Thusa general expression for the rate of cake filtration may be written as:

Equation (15) may be Integrated to yield the total filtration volume after aspecified time provided that the functional dependence of the flow rate orthe pressure drop with time is known. It is, however, once again veryimportant to note the simplifications and approximations that are InherentIn this equation.

Firstly, It is very common to assume that the medium resistance, Rm,is a constant in time. Physically this corresponds to no blinding or trappingof solids within the medium, ie a complete absence of Hermans andBredee's mechanisms (ii), (iii) and (iv) of Section 3.5.2(a). In reality themedium resistance is quite likely to change during the initial stages of

cake formation. Once a cake of any substantial thickness has beenformed, the cake Itself will largely prevent any further particulate matterfrom reaching the filter medium or support.

Hence Rm will usually thereafter indeed be a constant unless furthersolids are leached into the medium from the underside of the cake.Thus it is often reasonable to treat Rm as a constant during the cakefiltration but it may prove erroneous to deduce its value from ameasurement of the resistance to "clean" suspension medium alone.

(For an alternative perspective to this Issue, see (b)(v).)

The specific cake resistance, r or r 0 will not be a simple constant for agiven suspended phase and medium. It will depend critically upon thecolloidal properties of the suspension and the consequent structure of thefilter cake. It may also depend upon the mode and rate of cake lay-down.






Factors such as particle size and distribution, particle shape, particlesurface properties, the presence and nature of any flocculating agent, etc,etc, will all strongly influence r . Thus there arises the ability to control thefiltration process by conditioning the suspension by the use of

pretreatments and filtration aids. These will be discussed in more detaillater. One further reservation concerning equations (14) and (15) must beexpressed. It should not be presumed that the simple power lawdependence of r on the pressure drop ΔP will apply over a very extendedrange of pressures.

1.2.3 The Compression or Consolidation of Fil ter Cakes – Ultimatedegree of dewatering

Many of the physical principles pertaining to the consolidation processhave already been derived in previous sections of the manual (3.2 - 3.4).The subject is sufficiently central to many filtration situations, however,that an outline of the theory will be reproduced here. Consolidation of astructured filter cake will occur during its laydown and the filtration processproper. Additionally it may be exploited following cessation of the actualfiltration by the application of pressure to express liquid from the pores ofthe cake. This latter may involve cake collapse, that is consolidation, ordisplacement by gas. Pneumatic dewatering is briefly considered in a latersection (3.5.5(b)).

The ultimate attainable degree of dewatering of a filter cake throughconsolidation is calculated by considering the two opposing forces on thecake. On the one hand, above a certain solids content the cake willpossess a structural resistance to densification which may be quantified Interms of Its uniaxial, compressional yield point, Py(ɸ). Methods formeasuring Py(ɸ) and a description of its application are given later. ThisInternal resistance to densification operates against the externally appliedpressure differential across the filter cake, ΔP.

Consider a filter cake of solids finit undergoing constant pressureconsolidation. Initially, provided that Py(ɸ init) is less than the appliedconsolidating pressure, ΔP, the cake will be compressed and liquidexpelled from it. As this process continues, the concentration of solids inthe cake, ɸ, and hence the function, Py(ɸ), will increase.






The compressional yield point, Py(ɸ), is in fact a very strong function ofsolids content (see Examples 3.5.7) and at some later time it will be ofsufficient magnitude to match the applied pressure and consolidation willcease. This defines that the ultimate degree of dewatering will occur

when:

when this condition Is reached the internal stresses of the cohesive cake are large enough in magnitude to fully resist the applied pressure ΔP.Thus predicting the ultimte dewaterability simply requires a knowledge ofthe function Py($) and this may be measured by a simple laboratory scaledetermination. Examples of this procedure are given later and in Section3.2.6 of the manual.

1.2.4 The Rate of Consol idation

Equation (16) enables the easy estimation of an equilibrium degree ofdewatering for a given filter cake and consolidating pressure. The questionof how fast that ultimate solids content is attained is a more complicatedone involving additional physical factors such as the drag forces exhibitedby the consolidating network on the liquid being expressed from the cake.

A full analytical description of the kinetics of such processes are notpresently available.

The Consolidation Model of Buscall and White, based on the Yield Stress(Py) concept applied to sedimentation, has already been discussed insome detail in Sections 3.2 and 3.3. This model automatically incorporatesthe ultimate dewatering limit of Equation (16) for consolidation. Thisfollows from the choice of constitutive equation relating the time evolutionof the concentration of solids in the cake (the substantive derivative) interms of the yield stress parameter:






A number of attempts have been made to couple equation (17) withcontinuity equations in order to derive scaling relationships for thephysically distinct problem of filtration (cf the results for sedimentation,Sections 3.2 and 3.3 of this chapter). Possible strategies are outlined inreferences [25-273]. Although such an approach has not yet provedentirely successful a number of comments may be made:

(1) As for sedimentation the equation (17) encompasses the notion thatthe driving force for solid/liquid separation is attenuated by the elasticstress in the cake as described by Py(ɸ).

(2) Thus at low driving pressures (ΔP), Py(ɸ)) the rate of cakeconsolidation may be enhanced by manipulation of those factors (Sections3.2.6 and 3.3) that reduce Py(ɸ). Strategies for suspension conditioningmay utilize this type of reasoning and are considered later in Sections

3.5.3 and 3.5.4.

(3) In contrast at relatively higher driving pressures (ΔP >> Py(ɸ)), themain factors controlling consolidation will involve properties of the primaryparticles such as drag coefficients, together with dynamic drag coefficientsfor the network (λ(ɸ) in Equation (17)). The pore structure and cakepermeability will therefore be relevant and hence In this case thecontrolling factors are similar to those affecting the specific cakeresistance as discussed earlier (3.5.2(b)).

Until such a time as a full analysis of filtration in terms of the yield stress

concept has proved possible, the kinetics of consolidation of filter cakeswill remain a largely experimental science with understanding being atbest qualitative. Possible experimental approaches to the problem aregiven In Section 3.5.6 and examples discussed in Section 3.5.7.






1.2.5 Useful Semi-Empi rical Relations for Constant Pressureand Constant Rate Cake Filtration

Constant Pressure Filtration [1,10,12]

As a starting point, the general cake filtration equation, (15), is rearranged in thefollowing form:

Integration of this simple form of the equation allows the relationships betweenfiltrate volume, time and Instantaneous filtration rate to be deduced. The resultsare:






Both the specific resistance and the medium resistance can be evaluatedfrom the general equation, (18), via a plot of reciprocal rate, dt/dV, as afunction of cumulative filtrate volume, V. Such a plot has a slope equal tothe first bracketted term In the equation from which r o may be evaluated

(at a given pressure); the intercept yields the medium resistance,Rm

. It Isquite common with cake filtrations for the extrapolated data to passthrough the origin such that the medium resistance Is negligible comparedwith that of the cake. These simple principles are Illustrated schematicallyin Figure 3. Likewise, for compressible cakes a series of reciprocal rateversus cumulative volume at various pressures yields the relationshipbetween ΔP and specific resistance from which the exponent, S b ofequation (14) can be evaluated (see Figure 3).

Once the specific and medium resistances are known, from laboratory (orplant) measurements of V-l versus V, the relations (19)-(22), and thosethat follow for constant rate filtrations, may be applied In a predictivefashion. It is, however, Important to recall the predictions and restrictionsthat apply to equation (18) and were discussed in Section 3.5.2(b) (II). Thetwo most important caveats in this context involve the scaling up of thequantities r o and Rm. The medium resistance Rm may be an importantparameter and may not hold the same value at plant-scale as measured inthe laboratory. Likewise, r o depends critically upon the mode of cakeformation and so also may vary with scale, Initial filtration rate.

1.2.7 Constant Rate Filtration

For a constant rate filtration, the pressure drop will increase as the cakebuilds up. In practical situations there will be a limit to the magnitude of ΔP that can be applied or tolerated. Hence it is necessary to know thecumulative volume, V*, and time, t*, associated with a given limitingpressure drop, ΔP*. The volume- time relationship is trivial: cumulativevolume is given by the product of the time and constant rate.

The other relations are again derived simply from equation(15):






The equations (19)-(23) have been derived for strict conditions of constantpressure or rate. In reality many dewatering configurations operate inregimes which are intermediate between these two whereupon a moreinvolved numerical integration of equation (15) will be required in order to

derive volume-time relationships. A common configuration involving bothextreme cases utilizes constant rate filtration until the pressure drop hasreached Its maximum attainable (or tolerable) value whereupon thefiltration continues at constant pressure until the rate falls to anunacceptable level.

1.2.8 Multiphase Theory of Filtration

The theory and equations that have been presented in outline here, havelong been accepted as a reliable though somewhat empirical descriptionof the cake filtration process. However, more recently (- 1975 onwards),various workers have re-examined this so-called "two resistance*approach (ie r and ), and contrasted its basis with an alternativedescription, the "multiphase filtration theory" [22,23].

The latter Involves a lengthy derivation of equations based on a continuummechanics approach. This detail will not be presented here but may befound In the references. Rather an outline of some of the results will bediscussed and compared with the "two resistance" formulation.

It is first worthwhile recapitulating on the Interpretation of data analyzedvia the *two resistance" approach. In essence this method attributes theresistance to filtration to two additive contributions: that of the cake whichmay be a function of time and pressure etc and that of the septum ormedium which is rigorously regarded as constant (and often negligible).For this case a linear relationship between the inverse filtration rate V-l andcumulative filtrate volume, V, Is taken to imply the following:

(1) The local porosity and cake resistance are uniform.

(2) The average porosity and cake resistance are constant.






Where non-linear reciprocal rate data is encountered, the followingconclusions are inferred from the apparent compressibility of the filtercake:

(1) lion-uniformity of local porosity and cake resistance.

(2) Time and other dependence for average porosity and cakeresistance.

(3) Dependence of cake resistance on slurry concentration, pressureand septum.

In essence the “multiphase” description is based upon local continuity andmotion equations for both suspending and particulate phases In the cakeand septum. Application of dimensional arguments together with estimatesof the magnitude of the relevant groupings indicate which terms dominateV-l. Briefly it is the pressure driven and drag forces that control the processwhilst inertial and viscous forces may be largely neglected. At the end ofthe analysis the following multiphase cake filtration equation is gained:

where G Is a function of slurry concentration, solid and liquid densities, thefilter area ( A), and the average porosity. For the case where the cakeheight is a linear function of filtrate volume, V, G can be shown to beIndependent of V. Then the reciprocal rate equation depends upon thethree quantities:

K o (V) :- Septum permeabilityJ o (V) :- Septum pressure gradientPo (V) :- Cake pressure drop

and now deviation from a linear relationship between V-l and V for

constant Po, are attributed to changes in the permeability and pressuregradient developed In the septum and its Interface with the filter cake.Willis et al have demonstrated that this theory is plausible by showing thata given cake (formed by the filtration of a Lucite slurry> can be made toundergo a transition from apparent “incompressible” to “compressible”filtration merely by changing the nature of the septum.






In fact, Willis and co-workers cited only one experimental verification oftheir theory and so It Is unwise to speculate on Its generality. Perhaps themost important feature of the analysis Is that the attribution of non- linear

reciprocal rate data to “compressible” cake formation without furthercorroborating evidence may prove fallacious if septum fouling, blinding orcompression Is occurring. Means of obtaining this corroborative evidenceand of quantifying cake compressibility without a septum present areconsidered later in Section 3.5.6.

1.3 Crossflow Filtration [8 -44]

Although a detailed discussion of the various applications and somevariants of crossflow filtration will be provided in Section 3.9 of this manual(“Pressure Driven Membrane Separation Processes”), a brief outline of theprinciples will be given here, The basic objective of a crossflowconfiguration is to achieve the limited dewatering of a suspension (orsolution> using a permeable membrane but without the retention orimmobilization of the solid phase. This is achieved by maintaining atangential flow of the suspension across the surface of the membrane.Thus, Ideally, cake formation is totally avoided and the particulate phaseremains evenly distributed throughout the concentrating suspension as aresult of the convective effects of the flow [28,35,391.

At a simple level the first requirement for an understanding of crossflowdewatering would be a model relating the permeate flux, ie the rate ofconcentration to the primary process variables: temperature, drivingpressure, starting concentration and transmembrane velocity. For aperfectly ideal case, the flux relationship could be simply derived fromtreating the septum as a non-blocking bed of constant permeability. Theflux would then be directly proportional to the driving pressure gradient aspredicted by the Darcy or Kozeny-Carman relationships, equations (6)-(9), In all real cases the behavior of the flux is by no means that simple.

An alternative approach that has been applied, also largelyunsuccessfully, Is to use some sort of modified cake filtration model. Inreality the factors that control the flux of permeate are various, subtle andalmost Inevitably time dependent and hence It Is no surprise that simplemodels are Inappropriate. some useful generalizations may, however, bemade.






In many real examples of crossflow filtration, particularly In ultrafiltration[31-37], the flux rate Is limited by a mechanism called concentrationpolarization. This arises because the layers of suspension closest to themembrane are those which suffer depletion from permeate. There Is

therefore a local increase In concentration (but not necessarily a “cake”)which inevitably leads to a fall in the permeate flux.

Acting In opposition to this mechanism are the effects of diffusion andlaminar or turbulent convective flows. Clearly the diffusion process Is verydependent on the size and nature of the particulate (or dissolved) phase.Some useful relationships have been given to correlate the effects of thevarious factors that may operate when this mechanism is dominant. Thefollowing have been used for dewatering of biological suspensions byultrafiltration [32-33]:

It must be stressed that equations such as (25) and (26) are by no meansapplicable to all crossflow situations.

In addition to the above problems Involving particle concentration gradient,another common source of permeate flux decline is the phenomenonreferred to as "fouling" [35-36].

Fouling encompasses a whole series of processes whereby permeate fluxfalls as a function of time as a result of changes in the membrane Itself.Commonly these changes might Involve deposition of material on thesurface or interior of the membrane often leading to a time-dependentdecrease in Its porosity. Beyond these generalizations lies an enormousnumber of experimental studies and observations but unfortunately thereIs as yet only relatively poor understanding of the phenomenon and how






to avoid It. Current active research Is being carried out in various centersin the world with notable British contributions being made at Bath 1351and at Varren Spring Laboratory - the latter under the auspices of aBIOSEP project [39-41]. This project has utilized various electron

microscopy based investigations to probe the whereabouts of proteinfoulants, identified by staining, during biological separations.

In the future, this sort of experiments should at least assist in theelucidation of the mechanisms of fouling In various cases thereby enablingthe application of collold and surface science to avoid such problems.

Another engineering based strategy for reducing fouling, the deliberateproduction of transmembrane, turbulent vortices, has recently beeninvestigated by Hltchell I951.

Again with respect to potential future developments, it is interesting to notethat developments are being made in new filtration-based dewateringstrategies involving the use of electric fields [38,40]. Examples of theseprocess operations Include electrophoresls, electrodecantation,electroflltratlon, electro-osmosis and others.

Some of these will be discussed in more detail in Section 3.9. However, aparticularly worthwhile technology target In the present context involvesthe concept of harnessing a dielectrophoretic effect to prevent fouling orconcentration polarization during crossflow filtration processes [40].






2 The Range and Selection of Filtration Equipment Technology [1,12]

It is not the purpose of this short section to attempt to provide acomprehensive equipment selection guide for filtration-based solids/liquid

separation operations. There are already established sources for suchInformation; see, for example, Chapter 9 of reference [12] and Chapter 20of reference [1] . Rather it is intended to Indicate how an understanding ofboth the properties of the material and the rest of the envisaged processtrain will facilitate a choice from the available filtration-based options.

The main factors that Influence the choice of technology are:

2.1 Scale

The scale of the operation is not normally too stringent a constraint sincemost devices are available in a range of sizes to handle a variety ofcapacities. In ‘general, however, very small scale separations will notusually command the most expensive filtration plant If thermal drying canfollow the mechanical dewatering stage. For high value feedstreams (e.g.pharmaceuticals etc) other factors may override this option, however.






2.2 Solids Recovery, Liquids Clarification orFeedstream Concentration

As a generalization most solids recovery dewatering operations will

Involve the formation of a filter cake whilst clarification (Chapter 4)procedures will often avoid cake formation in order to maintain a high fluxof liquid. Where feedstream concentration is required two options arise.Either a cake may be formed which Is then reslurried to a higher solidscontent, or a continuous thickening process may be employed. Very oftena crossflow filtration arrangement will be appropriate for such a continuousthickening arrangement.

2.3 Rate of Sedimentation

The rate of sedimentation of a suspension can have various effects on thechoice of filtration plant. For example a bottom fed rotary drum filter maynot be suitable for slurries containing a fraction of very large or very denseparticles since these may settle out to form a "heel" well before they canbe transported to the bottom of the drum. The sedimentation behavior Isalso often critical In determining the structure of a filter cake closest to theseptum. Thus If the initial filtration rate Is properly controlled, the bottom ofthe cake consists of the largest, fastest settling solids which may help totrap the finer end of the particle size distribution and thus reduce blockingand blinding. A third area in which the suspension settling properties areof paramount importance Is where a filtering centrifuge Is beingconsidered as dewatering device. For these machines, the mechanism ofoperation entails a rapid settling of the solids phase In the centrifuge bowlfollowed by flow of the supernatant through the, hopefully, porous bed.The way In which this bed is formed and the properties that result will thusdepend on the settling characteristics of the suspension.






2.4 Rate of Cake Formation and Drainage

The rate at which the height of a filter cake rises can easily be assessedusing simple laboratory filtration tests (see next section). It will depend on

both the solids loading and the porosity and structure of the cake itself.This property has obvious repercussions on the geometry and necessarydimensions of suitable filtration equipment.

2.5 Batch vs Continuous Operation

This is clearly a critical question which must be addressed by looking atthe solids loading and rate of cake build-up, etc.

2.6 Solids Loading

As already explained, this factor will affect (Ill), (iv) and (v) above. Inaddition it will strongly Influence the flow properties and hence the rate atwhich the suspension can be presented to the filter If this proves to belimiting.

2.7 Further Processing

It is necessary to consider the Influence of additional operations whichmay either accompany the filtration or follow it in further downstreamprocessing. Possibilities include washing, air blowing and thermal drying.The physical nature of the final product may also be relevant here (e.g. inre-dispersible systems).

2.8 Aseptic or “ Hygienic” Operation

When handling biological materials for pharmaceutical, food or otherproducts, the necessity to be able to clean and sterilize a filter my imposeparticularly stringent demands. A detailed discussion of the relevantIssues and the suitability of various filters (and other plant) to asepticoperation Is given in the BIOSEP Report SAR 1 1401.






2.9 Miscellaneous

Various other factors are likely to Influence decisions about choice ofdewatering filter device. Of these the economics of the whole process Is

probably the most important. Such considerations will not be consideredhere but are discussed In reference [45]. It is, however, worth pointing outthat process decisions cannot be taken on the basis of economic factors inisolation. Very often physical constraints (e.g. those discussed in Section3.5.2(b)) render an otherwise economically attractive strategy impossible.

In order to illustrate the influence of the factors described in (i) to (ix)above, Table 1 presents an impression of the range of suitability forcommonly available filtration devices.

Having briefly considered the main factors influencing a choice of filtrationtechnology, a short discussion of two related topics is appropriate here.These are the relative merits of dewatering by shear versus compressionand by vacuum versus positive applied pressure filtration.

2.10 Shear versus Compress ional Deformation

During the latter stages of cake filtration, further dewatering is oftenachieved by the application of direct mechanical pressure to the cake itself- this Is the consolidation or expression process described In Section3.5.2(b). Such a densification of the cake, In order to expel furtheroccluded liquid, may be promoted by either an applied shear or uniaxialcompressional deformation. For either case no change In the structure willresult until a critical stress, the yield stress, has been exceeded. Figure 4compares the yield stress for both shearing cay) and uniaxialcompressional (Py) deformations for samples of BaC12-coagulated,polystyrene latex suspensions. The latter provide a convenient modelwhich mimics a typical flocculated cohesive filter cake [46-47]. It can beseen that shearing forces are effective (In the sense of exceeding therelevant yield stress) at much smaller stresses (by some 1-2 orders ofmagnitude); these shearing motions will often enable densification In theirown right via structural rearrangement and the concomitant collapse of thecake structure. The advantages of dewatering by shear or a combinationof shear and compression are already exploited in many filtration rigs, e.g.counter-moving belt filters [49], but there are almost certainly further gainsstill to be made in this area.






2.11 Pressure versus Vacuum

There are a number of hardware-based factors which favor a choice ofpressure over vacuum filtration or vice versa and these are fairly simple to

assess.Thus, in general, positive pressure filtration, being capable ofyielding larger trans-septum driving forces, can yield greater filtration rates

and hence reduce the size of dewatering plant.

On the other hand vacuum filters have the advantage of simpleconstruction and ease of continuous discharge in operation. They are,however, normally limited to total driving pressure drops of - 0.8 bar and,In the normal way, unsuitable for the filtration of suspensions containingvolatile solvents.

The above factors relate to the actual filters. In addition, there are more subtle factors, some of them less well understood, that pertain tosuspension properties. Of these the most important is the cake compressibility. For a perfectly incompressible cake (s = 0) and a constantpressure filtration, equation (20) indicates that the filtration time for a givenslurry volume is inversely proportional to the driving pressure. Thuspotentially large gains in rate may be expected by the use of positivepressure drops greater than a bar compared with the vacuumconfigurations. For compressible cakes (s > 0) the same equation predictsthat the advantage to be gained may be considerably attenuated by thepressure dependence of the cake resistance. An assessment of cakecompressibility, for example by using the methods described later, Istherefore highly desirable if the efficiency of increasing the trans-septumpressure drop is to be predicted.

Finally, to illustrate the subtlety of some of these effects, attention Isdrawn to recent membrane (but not crossflow) filtration studies of Leaverand Bewdick 1421. Studying the filtration of protein (USA) solutions theseworkers have observed twice the permeate flux for vacuum compared withpositive pressure filtration even though the trans-membrane pressuredrops were apparently identical. The reason for this behavior is unclear,but presumably involves some sort of membrane fouling.






3 Suspension Conditioning Prior to Filtration

Suspension conditioning may involve a simple mechanical treatment of

the suspension, the addition of a so-called filtration aid, or a combinationof both. The range of possible treatments may be conveniently divided intothese two categories:

3.1 Simple Filtration Aids

Using the term "filtration aid" in its broadest sense there are three generalclasses of aid. The first class contains those pretreatment chemicalswhich are added to modify the state of flocculation or coagulation of thesuspension prior to filtration [50,51].

Commonly these additives may be inorganic, e.g. Al or Fe salts orpolymeric, e.g. starches, gums, polyelectrolyte’s etc. The conventionalpurpose of such aids is normally to enhance filtration via one of thefollowing:

(i) Production of open aggregates so as to yield a porous filter cake therebyachieving fast filtration rates [50-52].

(ii) To yield strong aggregates so as to prevent wash-off and attrition; blindingand septum fouling is therefore reduced [50-52].

(iii) To improve the suspension rheology (Chapter 7).

(iv) To modify the wetting behavior of the medium on the suspended phase.

It should, however, be borne in mind that if further, mechanical dewateringof the filter cake by compression is ultimately to be sought, then factors (i)and (ii) will later prove deleterious. A compromise must then be struck toenable a structured cake that may be compressed under modest drivingpressures yet retain sufficient porosity during the actual filtration forreasonable flow rates to ensue.

Since the selection and action of flocculants is discussed in detail in aseparate section of this chapter (3.7), no further mention of these will bemade here.






Additionally the reader may wish to refer to Chapter 2 (Sections 2.4 and2.5) for details of flocculation mechanisms and the resulting floestructures.

The other two classes of filter aids are the so-called *pre-coat" and "bodyaid" additives (1, 12). The purpose of the former is obvious and serves toprovide an enhanced filter medium surface on which a cake may be laiddown. It is usually formed by re-circulating a pre-coat slurry through thefilter (typically a rotary vacuum device or similar) prior to the application ofthe suspension of Interest. A Body Feed on the other hand is completelymixed with the suspension requiring filtration before it reaches the filterdevice. It serves to Increase the porosity of the developing filter cake (i.e.Factor (i) above > and hence to lengthen the filter cycle time. An indicationof the efficiency of either pre-coat or body-feed filtration aids may begained by incorporating these additives in a small scale laboratoryfiltration trial such as those described in Section 3.5.6, In the main thefunction of the former may be assessed by its effect on the measuredseptum resistance, The body-feed aid on the other hand should have theeffect of reducing the specific resistance of the filter cake.

The properties of some commonly encountered pre-coat and body-feedfilter aids are presented in Table 2. Further detailed discussion of the useof these is provided in references [1, 12, and 52]. Finally it is worth notingthat surfactants are often employed in order to reduce the ultimatemoisture contents of filter cakes [53].

More information on this aspect of chemical pre-treatments may be foundin a later part of the chapter, Section 3.7.4






3.2 Mechanical Treatments

A large number of options are available for suspension pre-treatments thatdo not necessarily involve inert or chemically active additives. The

following list, plus a few pertinent references, covers some of the mostcommonly used methods:

(i) Shear Treatment - often employed to reduce the apparent viscosity of thesuspension [46-48].

(II) Degassing - more frequently employed prior to gravity separations. It maybe necessary before the filtration of certain biological products, however(see Section 3.8).

(iii) Suspension Ageing - like (i), (iv), (v), this technique is aimed at improvingfiltration performance via a modification of the flocculated structure of thesuspension, e.g. in the manufacture of catalyst supports.

(iv) Heat Treatment/Freeze Thaw I543.

(v) Acoustic Methods - generally used for biological systems (see Section 3.8)[55].

It is important to note the immense potential value of suspensionconditioning to filtration operations. The field of biotechnology coversmany examples where such conditioning has either a profound influenceon the process economics, or is absolutely essential to the Integrity of theproduct. For example, to avoid protein denaturation, degassing may be animperative conditioning step. A full and valuable review of many aspects ofconditioning relevant to bio-separations is provided in BIOSEP SARReport "Primary Solid/Liquid Separation" [40].

Finally in terms of mechanical treatments it Is appropriate here to mentionfor completeness a technology development program being carried out byBatelle into "Combined Fields Separation Processes". The objective of thissort of approach Is to identify combinations of separation means such aselectric and acoustic fields, such that synergistic advantages Indewatering may be achieved.






In the cases of electro-acoustic and ultrasonic-assisted dewatering,Batelle claim highly significant Improvements In the rate and degree offiltration for suspensions containing particles such as coal, biological’s,paper pulp and food materials. Unfortunately technical details are not yet

available though a number of patents have been filed. Although some ofthese treatments do not strictly involve suspension conditioning, It Is clearthat there is considerable potential for the exploitation of filtration-basedprocesses combined with other separation fields in this way.

Post-Filtration Treatments and Further Downstream Processing [56]

An outline of the influence and theory of three typical post-filtrationoperations, the washing of filter cakes. air blowing and thermal drying,serves to illustrate process interaction with the filtration operation.

4.1 Washing [56, 59]

Filter cake washing is usually employed to effect a purification of the cakeby removing entrained soluble’s, or less frequently to recover the motherliquor where the latter is of high value. The two main parameters ofinterest are the quantity of wash liquor required to achieve the requiredlevel of solute removal and the period of time taken for this degree ofwashing to be attained.

Probably the simplest approach to calculating the required quantity ofwash liquor has been provided by Vakeman. He distinguishes betweenfilter cakes still holding filtrate in the voids, i.e., “saturated” cakes, andthose that have been blown dry, the unsaturated cakes. For both casesVakeman has analyzed the various mechanisms influencing the washingprocess and produced charts of the fraction of recovered solute as a function of the wash ratio, (i.e. the volume of wash liquor X the cakevoidage) and one other dimensionless parameter. These then permit avery simple way of calculating the volume of wash liquor from small scalelaboratory tests. Further details are not relevant here but good accounts ofthe use of the tests and theory, together with the charts, are provided inthe references [1,12,56-59].






Once the volume of liquor has been calculated, the washing time is verystraightforwardly estimated from the final filtration rate that was observedfollowing cake build-up. Inasmuch b both the wash time and volumedepend upon cake porosity and tortuosity, it will be appreciated that the

factors that influence the mode of cake lay-down (including the variouspossible pre-treatments) will be very relevant to the washing performance.

4.1.1 “ Air-Blowing”

The use of "air-blowing" as a method of dewatering filter cakes is strictlynot restricted to air alone; other gases or vapors, for example, nitrogen oreven steam may be used. For biological or food suspensions the lattermay provide an additional role for purposes of sterilization (see Section3.5, 7(c)). The gas is propelled through the cake in a fashion appropriateto the filtration mode, hence for vacuum driven systems atmospheric air iscommonly sucked through the cake (deliberately or otherwise) following"breakthrough". With filter presses, pressure nutches etc, compressed airis forced through the pores of the cake in order to displace as muchmoisture as possible. By using heated air or nitrogen some additionaldrying action is available; such techniques are, however, normallyrestricted to small scale or high value products usually having specialproblems of toxicity etc such that normal drying techniques are difficult toapply*

The fundamental guiding principle in "air-blowing" is that the applied gaspressure must be sufficient to overcome the capillary forces tending tohold liquor within the pores of the cake (see Sections 3.2.9 and 3.5.7(a)).Probably the best current model for this process has been provided byVakeman. Unfortunately, in terms of real operating experience, thepredictions that it provides are of limited accuracy even for near-idealsystems containing hard particles of quasi-spherical geometry. Worsethan that, for suspensions of high aspect ratio particles (e.g. needles orplates), or for compressible cakes or those prone to cracking, Wakeman'smethod is of little practical value.

In terms of more empirical approaches, experimental work on a laboratoryor semi-technical scale can be used to make predictions of dewateringtime, final product moisture content and the air flow required. A number ofcautionary points should, however, be noted.






Since the final moisture content can be very sensitive to changes in theparticle size distribution and the way that the cake Is formed, it is veryImportant to use identical material for the laboratory tests and to ensurethat factors such as air flow rate and cake thickness are reproduced as

closely as possible. Even so, as a "rule of thumb", it should be noted thatsmall scale characterization tests tend to yield an optimistic figure for finalmoisture content since effects such as cake compression and crackingtend to be more prevalent on large scale.

Further discussion of most of the above features as well as some morepractical examples are provided in the references [60-65].

4.1.2 Drying [67 -76]

It is not our intention to treat the subject of drying in any detail here.However, a short discussion is included for completeness to highlight theimportance of considering the interaction between the filtration operationand further downstream processes. It is hoped that a future release of theSuspension Processing Manual will contain a more detailed chapter(Chapter 10) based on those aspects of drying that will be alluded to In thepresent context.

A general guiding principle that is invoked for most large scale dewateringtrains is to remove as much water as possible by mechanical means (i.e.the filtration process here). This then minimizes the expenses of theenergy-intensive downstream drying operation. However, it is normally thecase that physical constraints imposed by the mechanical dewatering stepwill intervene before the hypothetical economic optimum is reached (seeSection 3.10 - “Process Synthesis”).

There is a large literature, both Internal and external to the Company,based on drying. A recent report by the FCMO drying team I661 describedthree typical regimes of “paste preparation prior to a drying operation”:

a. Where there is no requirement for pipe flow. An example of thissituation is where a filter cake is discharged at high solids contentand is transported, perhaps by conveyor, to say an agitatedvacuum oven for final drying. It will be typical here to obtain themaximum, physically-possible dewatering during the filtration.

b. Where a filter cake is re-slurried in order to deliver it by pipe flow totypically a spray-drier. Clearly it is pointless in this case to






mechanically dewater to the ultimate physical limit. The filtrationstep should be tailored towards facilitating the re-slurrying o f thefilter cake to a manageable suspension.

c. Where the paste is formed into a chosen, stable, physical shape toaccelerate the subsequent thermal drying. For such cases therequirement of the filtration stage is to provide a paste withrheological properties that allow this shaping process, e.g. byextrusion. This situation is relevant to the formation of catalystsupports and ceramic materials in general.

The sorts of interactions between mechanical and thermal dewateringindicated in (a - c) above are variously discussed in the drying literature[67-76]. It would appear, however, that relatively little Is known of how themorphology of the filter cake influences the rate of thermal drying. Forexample the relationship between, say, filter cake porosity and thenecessary residence time in an oven drier would be a useful one toestablish. Thus such Interactions would usefully be the subject of futureresearch. Finally the subject of drying as part of a solids Isolation processis very critical when a redispersible solid is desired. This latter topic istreated in detail in Chapter 13 of the manual.

5 Testing and Characterization of Suspensions

5.1 Introduct ion – Suspension

5.2 Properties relevant to Filtration Performance

In order to best utilize the principles and theory that have thus far beenpresented, it is necessary to know as much as possible about the"colloidal" properties of the suspension requiring filtration. Both theproperties of the pre-filtration suspension and those of any filter cake thatis formed are of importance. All or any of the following are likely to berelevant:






5.2.1 Pre-Filtration Properties of Suspension

(i) Suspension viscosity including any tendencies towards sheardegradation, thixotropic or any other structural modification

following shear flow 146-481.

(ii) Suspension medium viscosity and wetting characteristics on thesolid (Chapter 2).

(iii) Settling properties of the suspension, particularly the rate ofsedimentation. Relative density of solid phase. Floe size andstructure. (Chapter 2, References [50,5])

(iv) The size distribution of particles and/or aggregates that arepresent (Chapter 2).

(v) The ease with which flocculated structure, and in particular theabove size distribution, may be modified by mechanical treatmentsor inert/chemical additives. Such modifications will, of course, alsoinfluence the other suspension properties above.

5.2.2 Properties of Filter Cake

(i) The mechanical strength of the cake and hence its resistancetowards consolidation and the variation of this property with degreeof consolidation (Section 3.7, References [50-52]).

(ii) The porosity of the cake as a function of voidage, that Is thetortuosity of the path that supernatant must follow through the cake.This property then is correlated with the cake resistances [51].

(iii) The influence of mechanical treatments and additives to thesuspension and the actual filtration conditions, e.g. rate of cake lay-down, on the cake strength and resistance.

Once a representative number of the above suspension properties hasbeen determined so as to enable a good understanding of its "colloidal"behavior, the knowledge may be applied to the following targets:

(i) Identification of the most appropriate plant and scale for thefiltration unit operation or suggestion of a better, alternativedewatering means other than filtration (see Section 3.10).






(ii) Optimization of the way that the operation is carried out. Forfiltration this will include the choice of filtration conditions anddetails of cycle time as well as their Impact on further downstream

processing.

(iii) Prediction of optimal plant operation. Thus, for example, it isessential to know what level of performance in terms of rate anddegree of dewatering can be expected under a given set ofconditions. This Is of paramount Importance for scaleupcalculations (see Section 3.2).

(iv) Removal of process "bottlenecks" and correction of plant operatingproblems. This again relies heavily on (iii) and the identification of"benchmarks" for optimal performance.

(v) To suggest where conditioning techniques 'and/or filtration aidsmay be desirable or appropriate. Whereas the means and optimalextent of pre-treatment should ideally be estimated from small-scale experimentation.

5.2.3 Laboratory Scale Filtration Rigs [77-80]

A number of small-scale rigs exist and these may be applied to themeasurement of filtration rates, filter cake properties and the Influence ofsuspension properties on them. These rigs are commonly used for theInitial derivation of data for scale-up purposes. If there is any doubt, theymay also be applied to the question of identifying the filtration mechanismsof Section 3.5.2, although they are predominantly applied to cake filtrationtests.

Apparatus for measuring filtration rates on a small scale have beendescribed by various workers [77-80]. The rigs of Allen & Stone [77],Gregory [78] and Bridger [80] are representative and of straightforwardconstruction. The Allen & Stone apparatus, is well automated and theirpaper describes its mode of operation in detail. A reproduction from theirpaper is given In Figure 5 from which the basic operating principles areeasily deduced. The original objective of the rig was to obtain data forscale-up purposes. In contrast to this, the equipment of Gregory wasinitially developed in order to assess the value of polymer flocculants asadditives to filtration slurries and to derive optimum polymer dosages byexperiment. The report of Brldger and Tadros uses a test rig to investigate






some fundamental aspects of the Influence of suspension properties onthe mechanistic details of cake filtration.

The "equilibrium" and kinetic aspects of the consolidation process

(Sections 3.2 and 3.5.2) may also be studied on a laboratory scale. Testsmay be carried out on a small-scale variable volume filter such as thePiston press supplied by Triton Electronics, for example. With thesedevices it Is possible to measure the solids content of a consolidating filtercake as a function of pressure and also the rate at which this degree ofconsolidation is approached. In the same vein a gas-pressure drivenpressure filter for laboratory scale tests from 0- "10 bar is now availablefrom Schenk.

5.3 Means of Monitoring Flocculant Dosage

The means of selecting appropriate flocculants and assessing optimaldosages is dealt with more fully in Section 3.7 of this manual. However, a recent addition to the range of portable, small-scale testing methods iswell worth a mention in the present context. The new test method is an on-line monitor for flocculation control [81-82]. Its operating principles arebased on the measurement of turbidity fluctuations In the flowingsuspension of interest. Gregory [82] has shown that the root mean squarefluctuation intensity can be related to the suspended particle sizedistribution via a semiempirical relationship. This conclusion enables theRMS signal to be used as a fast and sensitive Indicator of floe formation.

The device, marketed by Rank Brothers of Bottlsham, is relatively cheap(ca $7.5K at the time of writing) and constructed in a way that makes itideal for portable use and for continuous monitoring. Gregory hasdescribed applications where the device has been tested both inclarification and in achieving flocculation of more concentratedsuspensions such as those requiring filtration. The method may also becomfortably applied to suspensions that tend to foul the sample cell simplyby monitoring the ratio of both the RMS fluctuations and the average lighttransmission. This ratio has been shown to be relatively invariant to thedeposition of modest quantities of material on the surfaces of the samplecell. A recent ad hoc, trial of the Rank Brothers monitor has been made.The device proved a sensitive indicator of flocculation In bacterialsuspensions to which high molecular weight cationic polyacrylamides hadbeen added. The response time was also fast demonstrating the potentialof such instruments as continuous dosage monitors.






5.4 Filter Cake Testing

For the general case of the suspension processing of fine solids the most

common application of filtration involves cake formation and treatment. Itis therefore appropriate to consider the parameters and means by whichthe properties of the filter cake may be characterized. The three principalproperties that define the behavior of the cake are its strength, itspermeability or, conversely, resistance, and the rate at which it is laiddown. Methods for determining these will now be given.

5.4.1 Strength Testing (See also piston press described earlier)

This is relevant both to an understanding of the influence of the pressuredrop on the ordinary compressible cake filtration rate (as described byequation (14)) and to the subject of compression dewatering followingfiltration. Although a number of empirical measures of cake "strength"exist, the most suitable and fundamental parameter to use is the uniaxialmodulus of compression, K [83] or the compressional yield function Py(Ø)described earlier. The former my be defined In terms of the effect ofpressure on a cake volume (V) or concentration (Ø) change:

The modulus, K, is a very strong function, (K ~ Ø3-4 of concentration, Ø,and depends upon the nature, shape and size distribution of the prioryparticles as well as the structure of the cake and the Interparticle forces. KIs related to the function Py(Ø) and is also very similar numerically to theconventional infinitesimal modulus of shear G(Ø). This fact enables itsdetermination by straightforward laboratory techniques. (For further detailssee Section 3.2,4.) Arguably the simplest of these to use is the PulseShearometer Cell (Figure 8 of Section 3.3). This device enables the rapiddetermination of G (~ K) for a small sample of slurry or filter cake bymeasuring the propagation time of a low strain (~ 10-6) shear wave

between two discs mounted on piezo-electric crystals in the cell.Calculation of G requires only the propagation speed of the wave, u (fromthe disc spacing and propagation time), and the density, p, of the cake orslurry:






The shearometry technique is generally restricted to the range 103 < G

< 106

dynes crn-2

but this is not usually a problem. Where cakes of higherstrength need testing an alternative strategy may be adopted bymeasuring the compressional yield point, Py(Ø), in a centrifuge. The filtercake must now be formed in situ from the slurry (to be filtered) in acentrifuge tube.

A measurement of the height of the equilibrium sediment as a function ofgravitational field enables the evaluation of Py(Ø) over a range ofconcentrations (Figure 9 of Section 3.3). The upper bound of Py(Ø)measurable by this technique is constrained mainly by the gravitational

field that the centrifuge is capable of (safely) producing and the density ofthe solid phase.

Measurements of either G or K may then be used to evaluate thepressure, Pt, which must be applied to the cake in order to concentrate itto concentrations, (Ø)*:

This then assumes a long enough contact time such that kinetics will notprove limiting. That Is It represents the equality Ps = Py(Ø), the ultimate orstructural limit. For the centrifuge technique Py(Ø) may, in principle, becalculated from a single experiment. Using the shearometer cell a seriesof determinations at different slurry concentrations must be made. In bothcases equation (29) is solved either by graphical or numerical integration.

An example of the calculation is provided in the next section. Finally it maybe noted for completeness that K may also be measured directly In acompression cell (84) but, for practical purposes, one of the two methodsdescribed above is usually more straightforward and of sufficient

accuracy. For further clarification of the definition, interpretation andmeasurement of G(Ø), K(Ø) and Py(Ø) the reader is referred to Section3.2.






Finally It should be noted that the rheological parameters which quantifycompressional strength, that Is G, K, Py(Ø), may be related to thecompressibility exponent, "s", of equation (14) provided that assumptions are made regarding the dependence of cake resistance on voidage. The

simplest and best known model for the latter relationship is that due toKozeny and Carman. Examples of this approach may be found in the nextsection.

5.4.2 Cake Permeabil ity or Resistance

A number of approaches may be used to quantify the specific orintegrated cake resistance to flow of filtrate. The simplest small scaleapproach utilizes one of the laboratory filtration rigs just describedtogether with the equations for Idealized constant pressure, cake filtration,(18) - (22). A plot of reciprocal filtration rate, dt/dV, as a function ofcumulative filtrate volume, V, should yield a straight line from the slope ofwhich the specific resistance, r , may easily be calculated. The variation ofthe specific resistance with pressure drop may be evaluated from a seriesof experiments at different driving pressures. (A plot of Log (S.Resistance) versus Log ΔP yields the coefficient of variation "s" from themeasured slope.)

Alternatively this variation may be calculated from a knowledge of thecompressional modulus, K(Ø) as will be shown in the next section.However the parameters for cake resistance are determined by laboratorymeasurements, the warnings given in Section 3.5.2(b) (iv) with regard toscale-up must be noted and checked.

In passing It is worth noting that a number of commercial devices exist forrapid, “on-site”, empirical measures of the resistance of a given paste,sludge, or cake to filtration. For example Triton Electronics manufacturesuch an instrument to measure the empirical quantity, the "capillarysuction time" or CST. Although very useful as quick Indicators ofqualitative filtration behavior, such Instruments should In the main bereserved for control monitoring or "trouble-shooting" purposes. In additionto the CST test, many filtration equipment manufacturers have similarquick and simple tests for gaining an immediate feel for the behavior of agiven suspension. Thus, for example, the dipping of an inverted Buchnerfunnel and septum (connected to a vacuum line) briefly into a suspension,enables a good Indication of what thickness of cake would be picked upby a bottom fed rotary vacuum filter.






Despite the usefulness of such as guidelines and the CST approach, It isrecommended that serious design data for scale-up purposes should bederived from more careful and fundamental measurements of the typedescribed above.

Finally, an alternative approach to the study of cake collapse inconsolidation may be gained from an observation of the rate of fall of thesediment zone boundary in a centrifuge. There are problems ininterpreting such experiments to predict filtration behavior since theconsolidating pressure varies down the sediment and the experiment doesnot constitute a true "filtration". However, there is otherwise much tocommend the approach. In particular the network drag parameters used inthe Buscall and White theory of consolidation rates could be estimated bythis means, The experiment to achieve this entails a centrifuge withinwhich the sample tubes are transparent and illuminated by light from astroboscope. The latter is triggered by the centrifuge rotor and thus a"frozen" image results enabling the kinetics of the consolidation process tobe followed. Further research along these lines would be desirable In thefuture.

5.4.3 Rate of Cake Formation

The principal importance of the rate of cake formation in terms of cakeheight is for sizing purposes during equipment selection and scale-upcalculations. Purchas has defined a standard cake formation time, tF, for a1 cm thick cake which IMY be related to the specific cake resistance ro.Hence these quantities may be inserted in the equations for constantpressure or rate filtration given previously. Further details and examples ofsuch calculations are given in Purchas' book, "Solid/Liquid SeparationTechnology" [12].

6 Examples of the Application of the Forgoing Principles

In this final part of the filtration section of the dewatering chapter, someexemplification of the foregoing principles and theory is appropriate. Tothis end, three different examples of processes involving an importantfiltration operation will be presented. These have been selected to providean indication of the variety of suspensions that may be encountered andthe concomitant considerations and difficulties that apply to each.






6.1 Dewatering of Calcium Carbonate Slurr ies

The material for this example has been taken from a recent report, IC01713 C853, by Asher and Stewart - "Prediction of CaC03 Dewatering and

Other Processing Characteristics from “Suspension PropertyMeasurements". The main body of the report involves a comparison oftheoretically-predicted and experimentally-observed moisture contents inpressure-dewatered CaC03 magmas. The shear and uniaxialcompressional moduli, G and K, were measured by Shearometry andcentrifugation respectively as described in Sections 3.2 and 3.5.6. Resultsfor three typical samples are reproduced in Figure 6. It is interesting tocorrelate the different behavior of these samples with their colloidalproperties. Curve (a) shows the greatest resistance to densification whichis in good accord with the behavior expected of a suspension of very smallparticles. For curves (b) and (c) the form of the network modulus, K(Ø), ismore similar. The slightly larger slope for curve (b) may be rationalized interms of a stronger flocculated structure resulting from the MPBD (3%maleinized polybutadiene) coating.

From the data of Figure 6, Asher and Stewart obtained the form of thepressure dewatering curve, Figure 7. This was achieved by the simplenumerical integration, described in Section 3.2 and I251 using equation(29) which Is reproduced below:

Inspection of the figure reveals two interesting features. Firstly there is arapid fall off in the degree of dewatering achieved as the pressure isstepped up - note the logarithmic scale. Secondly the pressure-dewateringrelationship is affected by the same colloidal particle properties as was themodulus curve. Hence for a given applied pressure, the much finersuspension, curve (a), is dewatered to a considerably smaller extent. Theimmense value of using the laboratory-scale modulus measurement inorder to predict plant-scale expectations of dewatering is apparent.

In order to test the predictions arising from the figure, Asher and Stewartmade laboratory measurements of the moisture content remaining in thesamples at various pressures, using a laboratory piston press. The results,expressed in terms of moisture content rather than solids concentration,and calculated by the conventional theory (1), are shown in Figure 8. It






can be seen that there is reasonable agreement between theory andexperiment. There are, however, significant deviations between the two atthe highest consolidating pressures and it was deduced that the origin ofthis discrepancy lay in the derivation of the modulus, K, from the

centrifugation experiments. A careful re-examination of the approximationsinherent in that derivation was made. It was deduced that in general theroutine method of analysis, based on a calculation of the pressure head inthe middle of the sediment (equation for circles in Figure 8), led to anover-estimate (or upper bound) for K at a given solids content. This followsfrom the nature of the solids concentration down the sediment. Fortunatelythe same reasoning led to the conclusion that a calculation of the pressurehead and hence K at the sediment base, where the solids concentrationwas assumed to be [2(H0/H.)-l] Ø0p yielded a lower bound for the modulus.(HO and H. are the initial and subsequent equilibrium heights of thesediment at the various gravitational fields.) As a result of this, thecentrifuge data was re-analyzed using a calculation of K at both the middle(Equation 2 of the Figure) and the base (Equation 2) of the sediment.

The predicted residual moisture was then derived using the mean of thetwo calculations (Equation 3 of Figure 8). The agreement with experimentwas then found to be excellent and this is also illustrated in Figure 8. Asubsequent computer simulation study of the problem demonstrated, thevalidity of the approach of taking the mean value of the modulus from Itsupper and lower bounds. This procedure may be compared with themathematically more rigorous approaches of Buscall and White (Section3.2).

Two other aspects of the CaCO3 work are relevant to this discussion ofsuspension filtration. The first involved a calculation of the pressure atwhich a significant dewatering would occur due to "air blowing". Bycombining the Bartell I861 equation for capillary pressure in a porous plugof powder with a relationship for the effective pore size, the followingrelationship was utilized [87] after the paper of White:






Application of equation (30) yielded the prediction that substantial ‘air blowing" would be expected for the CaCO3 filter cakes at pressures of ~20 atmospheres (typical Winnofil-sized CaCO3) and ~ 60 atmospheres (forultrafine CaC03). These trends parallel those of Figure 6.

The final feature of Interest concerns the rate at which the CaCO3 slurrieswere observed to filter. Asher and Stewart adopted an approach tofiltration rate predictions akin to equations (18) and (20) of Section3.5.2(b). In particular they looked at the variation in rate with appliedpressure drop, ΔP. Increasing ΔP has two main effects on the rate. Firstlyit tends to increase it for obvious rheological reasons, le because of thePoiseuille-like flow rate,

This increase is, however, attenuated by an increase in cake resistancedue to its finite compressibility as per equation (14). The latter was

assessed for CaCO3 cakes from the compressional modulusmeasurements; Figure 9 reproduces the predicted and observed filtrationrate as a function of pressure drop. Although the agreement is far fromperfect it can be seen that the approach yields a very satisfactory estimateof the dependence. Further examples of work in this area are given In thethird set of examples (Example (c)).






6.2 Dewatering of Organic Products – Procion Dyestuffs

An investigation of the dyestuff Procion Blue H-ERD by Ramsay andSadler [88] revealed that three different physical forms for the dyestuff

may be produced following a "salting out" procedure. In the dewateringchain for these dyestuffs the precipitated solids are filtered under pressurein a press (at about 40 psi). The filter cake at about 40% solids is thenmechanically conveyed to a Hy-disperser where it is re-slurried with waterprior to being pumped to a spray drier. This fairly typical example servesto illustrate the influence of interactions between a filtration operation andother operations up and downstream (cf Section 3.5.5(c)).

The main upstream influence arises at the precipitation step where achange in the salting out conditions can cause a transition from theproduction of the favored physical form, essentially a precipitate ofaggregates, to one of two unfavorable forms. The authors of the reportdescribed these unfavorable forms as an amorphous and a quasi-crystalline phase. The morphology of all three forms was deduced from acombination of electron microscopy (TEM, SEW) and X-Ray diffraction (forthe quasi-crystalline phase).

In a simple laboratory filtration test using the apparatus describedpreviously, the three physical forms yielded the following results underidentical experimental conditions:

Thus a control of the precipitation stage can greatly enhance both thefiltration time and also the degree of dewatering obtained. Part of theinfluence of the physical form on the filtration rate may be attributed to thegreatly improved rheological characteristics of slurries of the aggregatedmaterial relative to the other possibilities. Figure 10 is a reproduction of aplot of viscosity as a function of shear rate for all three cases. It can beseen that the apparent viscosity of the aggregate form is an order ofmagnitude lower than the crystalline form.






Finally, this example provides an indication of the sort of considerationthat is needed with a view to further downstream operations such as drying. As mentioned previously, the filter cake requires re-slurrying so as

to provide a pumpable suspension for spray-drying. The present processutilizes a Hy-disperser to achieve this end but this almost inevitably leadsto some breakdown of the aggregates and formation of fines. As a resultof this the rheology of the fluidized paste becomes unfavorable and extrawater must be back added to enable the slurry to be pumped uphill to thespray drier. Clearly a gentler fluidization method would be desirable andthis would be achieved If the filter cake were formed in such a way as topermit facile re-dispersion.

Two other studies of the filtration properties of slurries containing organiccrystalline particles by G Taylor and co-workers Illustrate other aspects ofthe optimization of the suspension processing. In an investigation of plantfiltration problems with the product Pyridone CE [89-90], a number ofanalytical techniques, such as XRD, TGA, DSG and microscopy, wereused to characterize the nature and stability of various crystallinemodifications. In this particular case the major complication arose from thetendency for Pyridone CE to undergo changes of crystal habit during mufiltration. Hence although the feedstock to the plant filters performedadequately in laboratory tests, when scaled-up to plant process times itled to unsatisfactory filtration rates.

By characterizing the stability and means of producing the various crystalhabits for the product, Taylor indicated a strategy (by an elevation ofprocess temperature upstream of the filtration) for producing a favorablecrystal habit which did not undergo polymorphic changes throughout theduration of the plant filtration times and conditions.

Taylor and co-workers similarly studied the filtration properties of 3,5-Dinitro-2,4,6-Trimethyl Benzene Sulphonic Acid [90]. They indicated threekey properties that led to a desirable crystal habit form:

(1) A crystal shape that allows rapid filtration yet yields a cake that Issufficiently well packed so as not to occupy too much volume for a givenamount of feed. Clearly these requirements are to some extent opposingand a compromise in porosity and cake structure must be met.

(II) The cake structure should enable facile washing of the product (seeSection 3.5.5(a)).






(III) The crystal should have a low solubility to enable thorough washingwithout extensive loss of product.

Once again a favorable crystal form, rhombic shaped crystals (~ 100 x 250µm), was identified as that which yielded the most satisfactory filtrations,though the latter performance was sometimes masked by the presence of“fines”. The cake formed from the rhombic crystals was found to besignificantly compressible. Hence it was concluded that a Vacuum filterwould be a more appropriate choice of dewatering strategy than the 40 psiFilter Press then currently employed.






6.3 Filtration of Biological Systems – Harvesting a FilamentousOrganism

This example is included in order to indicate the very different constraints

and requirements of dewatering operations for large-scale bio-technological products. The harvesting of the filamentous organism,Fusarium Graminearum, for human consumption as a foodstuff Is acomplex and highly interactive process. The organism leaves thefermenter in which it is grown as a dilute suspension containing about1.5% of suspended solids by dry weight (Øw) + In order to conserve thegrowth medium and nutrients the ex-fermenter broth will ideally bepre-thickened with a re-cycle of the spent broth. The concentratedsuspension, now at 2-3% dry weight of solids, is then heat treated andfinally dewatered to yield a moist but resilient cake for further processing[91].

Figure 11 shows the uniaxial compressional modulus, K for a typicalsample of this material before and after heat treatment. Concentratinginitially on the pre-reduction material (RNA+), it can be seen that asubstantial modulus exists at very low solids contents. The physicalimplication of this observation is that the material is forming a structuralnetwork of considerable compressive strength at very low dry weightfractions, (Øw). As a result of biological constraints and the fragile natureof the RNA+ hyphae (a hypha Is a single organism), the pre-thickeningstep must avoid both excessive residence times (preferably < 15-20minutes) and high shear fields, The Information from the modulusmeasurements then indicates that a filtration means of pre-thickening islikely to satisfy these two constraints most satisfactorily since the strong,open network of hyphae should filter (or "drain") fast whereas unit gravitysedimentation / flotation will be much too slow. Centrifugation is likewisean unfavorable choice at first sight due to the high shearing forces that areencountered during acceleration and discharge [92].

In contrast the slope and position of the modulus curve for the heat treatedmaterial (RNA-) is quite different. Since most of the hyphae haveundergone only very small changes in their physical dimensions, It isdeduced that the change In colloidal properties is due to a reduction ofcell-internal pressure or turgor. The network of hyphae is now in effectmuch more compressible and this factor Is of key Importance in relation tothe use of a second filtration step in the process to generate thedewatered cake [93].






Some preliminary investigations of the pressure drop profile as a functionof time for RNA- material have been made by A Harrison of AgriculturalDivision. These measurements, carried out at various constant feed rates,were made on a porous, sintered steel candle filter. Using the expression

for the variation of cake resistance with pressure drop given earlier,equation (14),

the experimental data was fitted to an expression analogous to equation(24) with the assumption of negligible medium resistance. Comparison ofthe measured curves and those predicted from a fit to the filtrationequation shows very satisfactory agreement, The fit yielded a value of0.93 for s, the compressibility factor thus confirming the high degree of

cake compressibility. Thus for a constant rate filtration, once the pressuredrop has started to rise, its increase with time is very rapid and this isborne out by Figure 12. Thus factors that control the cake compressibility,eg the mean hyphal length, suspension pH, water hardness, etc, will alsodictate the length of a filtration cycle at constant rate. Conversely for aconstant pressure drop configuration, any advantage in filtration rate to begained by stepping up the driving pressure will be limited by these samefactors. Thus a fundamental study of the cake and suspension propertiesand their variation with these parameters enables the understanding of thelarge-scale filtration behavior. Further details may be found in a recentCCSG report [93].






NB The symbols represent different samples of culture removed from the fermenter over a periodof 4 months. Some of the scatter therefore reflects changes in the morphology of the organismunder varying environments.

filtration-131106124031-phpapp01

Documents