liquid filtration, second edition

Preface ix About the Author xi Chapter 1 An Introduction to Liquid Filtration 1 Introduction 1The Porous Media 2The Filter Media 9Liquid Filtration Classification 10The Formation of Filt

FILTRATION

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This volume is dedicated to the memory of Paul N Cheremisinoff, M.S., P.E., whoauthored more than 300 technical books over his career as a chemical engineer andwas among the pioneers of pollution control and prevention

Preface ix

About the Author xi

Chapter 1 An Introduction to Liquid Filtration 1

Introduction 1The Porous Media 2The Filter Media 9Liquid Filtration Classification 10The Formation of Filter Cake 11Typical Industrial Filtration Conditions 12Washing and Dewatering Operations 12General Considerations for Process Engineers 13The Objectives of Filtration 14Preparation Stages for Filtration 15Equipment Selection Methodology 16Nomenclature 18

Chapter 2 Filter Media and Use of Filter Aids 19

Introduction 19Flexible Filter Media 20Rigid Filter Media 34Filter Media Selection Criteria 43Introduction to the Use of Filter Aids 47Examples of Filter Aids 50Filter Aid Selection 51Suggested Readings 57Nomenclature 58

Chapter 3 Cake Filtration and Filter Media Filtration 59

Introduction 59Dynamics of Cake Filtration 60Constant-Rate Filtration 70

Variable-Rate and -Pressure Filtration 72Constant-Pressure and -Rate Filtration 75Filter-Medium Filtration Formulas 75Constant-Pressure-Drop Filtration 75Filtration Mechanisms 81Constant Rate Filtration 83Suggested Readings 86Nomenclature 87

Chapter 4 Industrial Filtration Equipment 88

Introduction 88Rotary Drum Filters 89Cocurrent Filters 91Cross Mode Filters 98Cartridge Filters 103Diaphragm Filters 110High Pressure, Thin Cake Filters 115Thickeners 117Solids Washing 120Centrifugal Filtration 120Screw Presses 123Ultrafiltration 124Reverse Osmosis 134Closure 141

Chapter 5 Application of Filtration to Wastewater Treatment 142

Introduction 142Granular Media Filtration 142Bed Regeneration 148Flocculation Filtration 149Slow Sand Filtration 151Rapid Sand Filtration 153Chemical Mixing, Flocculation, and Solids Contact Processes 155Suggested Readings 162

Chapter 6 Advanced Membrane Technology for Wastewater Treatment 163

Introduction

Overview of Technology Case Study

Case Study Specifics

Technology Application

Mechanisms of Membrane Separations

Treatment of Hazardous Wastes

Contents vii

Features of the Hyperfiltration System 173Process Economics 184Detailed Process and Technology Description 193Summary of Case Study Analytical Results 202Closure 210

Chapter 7 Sludge Dewatering Operations 211

Introduction 211Overview of Dewatering Technologies 212Use of Drying Beds 217Use of Vacuum Filtration 219Use of Pressure Filtration 222Use of Centrifugation 223Alternative Mechanical Dewatering Techniques 226Suggested Readings 227

Chapter 8 Industrial Wastewater Sources 229

Introduction 229Paper and Allied Products Industry Wastes 230Dairy Products Industry Wastes 232Textile Industry Wastes 237Pharmaceutical Industry Wastes 240Leather Tanning and Finishing Industry Wastes 243Petroleum Refining Industry Wastes 246Food and Meat Packing Industry Wastes 251Beverages Industry Wastes 254Plastics and Synthetic Materials Industry Wastes 258Blast Furnaces, Steel Works, and Rolling and Finishing Wastes 261Organic Chemicals Industry Wastes 265Metal Finishing Industry Wastes 268Closure 271Suggested Readings 271

Chapter 9 Filtration Equipment and Process Flow Sheets 272

Introduction 272Index to Equipment and Flow Sheet Diagrams 272

Index 316

The information presented in this volume is based largely on the author's collectednotes and lectures over the past 15 years The volume is not intended for researches

or equipment developers, but rather for process engineers, plant engineers, andtechnicians who require basic knowledge of this important unit operation Much of thedesign methodology and working equations presented have been tested on pilot plantstudies and applied to commercial and semi-commercial operations with success,however, neither the author nor publisher provide written or implied endorsementsthat these procedures will work in any or all cases As with any piece of equipment

or process, the designer must consult with specific vendors, suppliers andmanufacturers, and further, should field test or at a minimum, conduct pilot tests toensure performance in the intended application Filtration equipment, operationconditions, and the use of filtration aids are highly dependent upon the properties ofthe suspension being filtered Furthermore, overall process constraints and economicscan have major impacts on the selection of equipment, their operating modes andcharacteristics, and efficiency

The author wishes to extend a heartfelt gratitude to Butterworth — Heineinann fortheir fine production of this volume, and to members of the United StatesEnvironmental Protection Agency for their advise and consultation on some of thematerials presented herein

Nicholas P Cheremisinoff, Ph.D

ABOUT THE AUTHOR

Nicholas P Cheremisinoff is Director of the Industrial Waste Management Program

in Ukraine, which is supported by the United States Agency for InternationalDevelopment, Washington D.C He has nearly twenty years of applied research andindustry experience in the petrochemicals, oil and gas, rubber, and steel industries,and is considered a leading authority on waste management and process design Dr.Cheremisinoff provides technical consulting services to both private industry andgovernment agencies and has worked extensively in Republics of the former SovietUnion, South America, Korea, the United States, and Western Europe, He is theauthor, co-author, or editor of over 100 engineering reference books dealing withwaste technologies and process designs, including the multivolume Encyclopedia ofFluid Mechanics by Gulf Publishing Company Dr Cheremisinoff received his B.S.,M.S and Ph.D degrees in chemical engineering from Clarkson College ofTechnology

of the filter medium, the solid particles are retained on the medium's surface or, insome cases, on the walls of the pores, while the fluid, which is referred to as thefiltrate, passes through.

The flow of fluids through a porous medium is of interest not only to the unitoperation of filtration, but to other processes, such as adsorption, chromatography,operations involving the flow of suspensions through packed columns, ion exchange,and various reactor engineering applications In petroleum engineering applications,interest lies in the displacement of oil with gas, water and miscible solvents (includingsolutions of surface-active agents), and in reservoir flow problems In hydrology,interest is in the movement of trace pollutants in water systems, the recovery of waterfor drinking and irrigation, and saltwater encroachment into freshwater reservoirs

In soil physics, interest lies in the movement of water, nutrients and pollutants intoplants In biophysics, the subject of flow through porous media touches upon lifeprocesses such as the flow of fluids in the lungs and the kidney

The physical parameters that relate the porous material to the hydrodynamics of floware porosity, permeability, tortuosity and connectivity This chapter discusses thefundamentals of flow through porous media and relates these principles to theindustrial operations of filtration As indicated in the preface of this volume, thesubject of filtration is discussed from a process engineering viewpoint, and inparticular from that of the chemical engineer Filtration has a long history in thechemical engineering field both from the standpoint of the production of high purityproducts, as well as a technology extensively used in pollution control and prevention

1

The Porous Media

A porous medium may be described as a solid containing many holes and tortuouspassages The number of holes or pores is sufficiently great that a volume average isneeded to estimate pertinent properties Pores that occupy a definite fraction of thebulk volume constitute a complex network of voids The manner in which holes orpores are embedded, the extent of their interconnection, and their location, size and

shape characterize the porous medium The term porosity refers to the fraction of the

medium that contains voids When a fluid is passed over the medium, the fraction of

the medium (i.e., the pores) that contributes to the flow is referred to as the effective porosity.

There are many materials that can be classified as porous media, however, not all ofthem are of interest to the subject of filtration In general, porous media are classified

as either unconsolidated and consolidated and/or as ordered and random Examples

of unconsolidated media are sand, glass beads, catalyst pellets, column packings, soil,gravel and packing such as charcoal Examples of consolidated media are most of thenaturally occurring rocks, such as sandstones and limestones Materials such asconcrete, cement, bricks, paper and cloth are manmade consolidated media Orderedmedia are regular packings of various types of materials, such as spheres, columnpackings and wood Random media have no particular correlating factor

Porous media can be further categorized in terms of geometrical or structuralproperties as they relate to the matrix that affects flow and in terms of the flowproperties that describe the matrix from the standpoint of the contained fluid.Geometrical or structural properties are best represented by average properties, fromwhich these average structural properties are related to flow properties

A microscopic description characterizes the structure of the pores The objective ofpore-structure analysis is to provide a description that relates to the macroscopic orbulk flow properties The major bulk properties that need to be correlated with poredescription or characterization are porosity, permeability, tortuosity and connectivity

In studying different samples of the same medium, it becomes apparent that thenumber of pore sizes, shapes, orientations and interconnections are enormous Due

to this complexity, pore-structure description is most often a statistical distribution ofapparent pore sizes This distribution is apparent because to convert measurements to

pore sizes one must resort to models that provide average or model pore sizes A

common approach to defining a characteristic pore size distribution is to model theporous medium as a bundle of straight cylindrical capillaries The diameters of themodel capillaries are defined on the basis of a convenient distribution function

Pore structure for unconsolidated media is inferred from a particle size distribution,the geometry of the particles and the packing arrangement of particles The theory ofpacking is well established for symmetrical geometries such as spheres Information

on particle size, geometry and the theory of packing allows relationships between poresize distributions and particle size distributions to be established

An Introduction to Liquid Filtration 3

A macroscopic description is based on average or bulk properties at sizes much largerthan a single pore In characterizing a porous medium macroscopically, one must dealwith the scale of description The scale used depends on the manner and size in which

we wish to model the porous medium A simplified, but sometimes accurate, approach

is to assume the medium to be ideal; meaning homogeneous, uniform and isotropic

The term reservoir description is applied to characterizing a homogeneous system as

opposed to heterogeneous A reservoir description defines the reservoir at a levelwhere a property changes sufficiently so that more than a single average must be used

to model the flow In this sense, a reservoir composed of a section of coarse graveland a section of fine sand, where these two materials are separated and havesignificantly different permeabilities, is heterogeneous in nature Defining dimensions,locating areas and establishing average properties of the gravel and sand constitutes

a reservoir description, and is a satisfactory approach for reservoir-level typeproblems Unfortunately, to study the mechanisms of flow, the effects of nonidealmedia require more specific definitions

Any discussion of flow through porous media inevitably touches upon Darcy's lawwhich is a relationship between the volumetric flowrate of a fluid flowing linearlythrough a porous medium and the energy loss of the fluid in motion

Darcy's law is expressed as:

The relation is usually considered valid for creeping flow where the Reynolds number,

as defined for a porous medium, is less than one The Reynolds number in openconduit flow is the ratio of inertial to viscous forces and is defined in terms of acharacteristic length perpendicular to flow for the system Using four times thehydraulic radius to replace the length perpendicular to flow and correcting the velocitywith porosity yields a Reynolds number in the form:

D v p

Darcy's law is considered valid where Re < 1

The hydraulic conductivity K depends on the properties of the fluid and on the porestructure of the medium The hydraulic conductivity is temperature-dependent, sincethe properties of the fluid (density and viscosity) are temperature-dependent.Hydraulic conductivity can be written more specifically in terms of the intrinsicpermeability and the properties of the fluid.

where k is the intrinsic permeability of the porous medium and is a function only ofthe pore structure The intrinsic permeability is not temperature-dependent Indifferential form, Darcy's equation is:

Permeability is normally determined using linear flow in the incompressible orcompressible form, depending on whether a liquid or gas is used as the flowing fluid.The volumetric flowrate Q (or Qm) is determined at several pressure drops Q (or Qm)

is plotted versus the average pressure pm The slope of this line will yield the fluidconductivity K or, if the fluid density and viscosity are known, it provides the intrinsicpermeability k For gases, the fluid conductivity depends on pressure, so that

1+-where b depends on the fluid and the porous medium Under such circumstances astraight line results (as with a liquid), but it does not pass through the origin; instead

it has a slope of bK and intercept K The explanation for this phenomenon is that gases

do not always stick to the walls of the porous medium This slippage shows up as anapparent dependence of the permeability on pressure

Heterogeneity, nonuniformity and anisotropy must be defined in the volume-averagesense These terms may be defined at the level of Darcy's law in terms ofpermeability Permeability is more sensitive to conductance, mixing and capillarypressure than to porosity

Heterogeneity, nonuniformity and anisotropy are defined as follows On amacroscopic basis, they imply averaging over elemental volumes of radius e about apoint in the media, where e is sufficiently large that Darcy's law can be applied forappropriate Reynolds numbers In other words, volumes are large relative to that of

An Introduction to Liquid Filtration 5

a single pore Further, e is the minimum radius that satisfies such a condition If e istoo large, certain nonidealities may be obscured by burying their effects far within theelemental volume

Heterogeneity, nonuniformity and anisotropy are based on the probability densitydistribution of permeability of random macroscopic elemental volumes selected fromthe medium, where the permeability is expressed by the one-dimensional form ofDarcy's law

As noted earlier, the principal properties of nonideal porous media that establish thenature of the fluid flow are porosity, permeability, tortuosity and connectivity In amacroscopic sense, porosity characterizes the effective pore volume of the medium

It is directly related to the size of the pores relative to the matrix When porosity issubstituted, the details of the structure are lost

Permeability is the conductance of the medium and has direct relevance to Darcy'slaw Permeability is related to the pore size distribution, since the distribution of thesizes of entrances, exits and lengths of the pore walls constitutes the primaryresistance to flow This parameter reflects the conductance of a given pore structure.Permeability and porosity are related; if the porosity is zero the permeability is zero.Although a correlation between these two parameters may exist, permeability cannot

be predicted from porosity alone, since additional parameters that contain moreinformation about the pore structure are needed These additional parameters aretortuosity and connectivity Tortuosity is defined as the relative average length of aflow path (i.e., the average length of the flow paths to the length of the medium) It

is a macroscopic measure of both the sinuosity of the flow path and the variation inpore size along the flow path Both porosity and tortuosity correlate with permeability,but neither can be used alone to predict permeability

Connectivity defines the arrangement and number of pore connections For monosizepores, connectivity is the average number of pores per junction The term represents

a macroscopic measure of the number of pores at a junction Connectivity correlateswith permeability, but cannot be used alone to predict permeability except in certainlimiting cases

Difficulties in conceptual simplifications result from replacing the real porous mediumwith macroscopic parameters that are averages and that relate to some idealized model

of the medium Tortuosity and connectivity are different features of the pore structureand are useful to interpret macroscopic flow properties, such as permeability, capillarypressure and dispersion

Porous media is typically characterized as an ensemble of channels of various crosssections of the same length The Navier-Stokes equations for all channels passing across section normal to the flow can be solved to give:

Where parameter c is known as the Kozeny constant, which is essentially a shapefactor that is assigned different values depending on the configuration of the capillary(c = 0.5 for a circular capillary) S is the specific surface area of the channels Forother than circular capillaries, a shape factor is included:

~> ck r" - — (8)

l '

The specific surface for cylindrical pores is:

„ _ n2nrL _ 2

^A J7 ~ nnr L r

* 2 •

To determine the average porosity of a homogeneous but nonuniform medium, thecorrect mean of the distribution of porosity must be evaluated The porosities ofnatural and artificial media usually are normally distributed The average porosity of

a heterogeneous nonuniform medium is the volume-weighted average of the numberaverage:

m

E E",

An Introduction to Liquid Filtration 1

The average nonuniform permeability is spatially dependent For a homogeneous butnonuniform medium, the average permeability is the correct mean (first moment) ofthe permeability distribution function Permeability for a nonuniform medium isusually skewed Most data for nonuniform permeability show permeability to bedistributed log-normally The correct average for a homogeneous, nonuniformpermeability, assuming it is distributed log-normally, is the geometric mean, definedas:

For flow in heterogeneous media, the average permeability depends on thearrangement and geometry of the nonuniform elements, each of which has a different,average permeability Figure 1 conceptually illustrates nonuniform elements, wherethe elements are parallel to the flow

Figure 1 Flow through parallel nonuniform elements of porous media.

Since flow is through parallel elements of different constant area, Darcy's law foreach element, assuming the overall length of each element is equal, is:

(15)

The flowrate through the entire system of elements is Q=QS+Q2+

Combining these expressions we obtain:

(16a)

(I6b)

This means that the average permeability for this heterogeneous medium is thearea-weighted average of the average permeability of each of the elements If thepermeability of each element is log-normally distributed, these are the geometricmeans Reservoirs and soils are usually composed of heterogeneities that arenonuniform layers, so that only the thickness of the layers varies This means that

where n is the number of layers

Permeability is a volume-averaged property for a finite but small volume of a medium.Anisotropy in natural or manmade packed media may result from particle (or grain)orientation, bedding of different sizes of particles or layering of media of differentpermeability A dilemma arises when considering whether to treat a directional effect

as anisotropy or as an oriented heterogeneity

In an oriented porous medium, the resistance to flow differs depending on thedirection Thus, if there is a pressure gradient between two points and a particularfluid particle is followed, unless the pressure gradient is parallel to oriented flowpaths, the fluid particle will not travel from the original point to the point which onewould expect Instead, the particle will drift

Tortuosity and connectivity are difficult to relate to the nonuniformity and anisotropy

of a medium Attempts to predict permeability from a pore structure model requireinformation on tortuosity and connectivity

From an industrial viewpoint, the objective of the unit operation of filtration is theseparation of suspended solid particles from a process fluid stream which isaccomplished by passing the suspension through a porous medium that is referred to

as a filter medium In forcing the fluid through the voids of the filter medium, fluidalone flows, but the solid particles are retained on the surface and in the medium'spores The fluid discharging from the medium is called the filtrate The operation may

An Introduction to Liquid Filtration 9

be performed with either incompressible fluids (liquids) or slightly to highlycompressible fluids (gases) The physical mechanisms controlling filtration, althoughsimilar, vary with the degree of fluid compressibility Although there are markedsimilarities in the particle capture mechanisms between the two fluid types, designmethodologies for filtration equipment vary markedly This reference volumeconcentrates only on process liquid handling (i.e., incompressible fluid processing)

The Filter Media

The filter medium represents the heart of any filtration device Ideally, solids arecollected on the feed side of the plate while filtrate is forced through the plate andcarried away on the leeward side A filter medium is, by nature, inhomogeneous, withpores nonuniform in size, irregular in geometry and unevenly distributed over thesurface Since flow through the medium takes place through the pores only, themicro-rate of liquid flow may result in large differences over the filter surface Thisimplies that the top layers of the generated filter cake are inhomogeneous and,furthermore, are established based on the structure and properties of the filtermedium Since the number of pore passages in the cake is large in comparison to thenumber in the filter medium, the cake's primary structure depends strongly on thestructure of the initial layers This means that the cake and filter medium influenceeach other

Pores with passages extending all the way through the filter medium are capable ofcapturing solid particles that are smaller than the narrowest cross section of thepassage This is generally attributed to particle bridging or, in some cases, physicaladsorption

Depending on the particular filtration technique and intended application, differentfilter media are employed Examples of common media are sand, diatomite, coal,cotton or wool fabrics, metallic wire cloth, porous plates of quartz, chamotte, sinteredglass, metal powder, and powdered ebonite The average pore size and configuration(including tortuosity and connectivity) are established from the size and form ofindividual elements from which the medium is manufactured On the average, poresizes are greater for larger medium elements In addition, pore configuration tends to

be more uniform with more uniform medium elements The fabrication method of thefilter medium also affects average pore size and form For example, porecharacteristics are altered when fibrous media are first pressed Pore characteristicsalso depend on the properties of fibers in woven fabrics, as well as on the exactmethods of sintering glass and metal powders Some filter media, such as cloths(especially fibrous layers), undergo considerable compression when subjected totypical pressures employed in industrial filtration operations Other filter media, such

as ceramic, sintered plates of glass and metal powders, are stable under the sameoperating conditions In addition, pore characteristics are greatly influenced by theseparation process occurring within the pore passages, as this leads to a decrease ineffective pore size and consequently an increase in flow resistance This results fromparticle penetration into the pores of the filter medium

The separation of solid particles from a liquid via filtration is a complicated process.For practical reasons filter medium openings should be larger than the average size

of the particles to be filtered The filter medium chosen should be capable of retainingsolids by adsorption Furthermore, interparticle cohesive forces should be largeenough to induce particle flocculation around the pore openings

Liquid Filtration Classification

There are two major types of filtration: "cake" and "filter-medium" filtration In theformer, solid particulates generate a cake on the surface of the filter medium Infilter-medium filtration (also referred to as clarification), solid particulates becomeentrapped within the complex pore structure of the filter medium The filter mediumfor the latter case consists of cartridges or granular media Examples of granularmaterials are sand or anthracite coal

Process engineers who specify filtration equipment for an intended application mustfirst account for the parameters governing the application and then select the filtrationequipment best suited for the job There are two important parameters that must beconsidered, namely the method to be used for forcing liquid through the medium, andthe material that will constitute the filter medium

When the resistance opposing fluid flow is small, gravity force effects fluid transportthrough a porous filter medium Such a device is simply called a gravity filter Ifgravity is insufficient to instigate flow, the pressure of the atmosphere is allowed toact on one side of the filtering medium, while a negative or suction pressure is applied

on the discharge side This type of filtering device is referred to as a vacuum filter.The application of vacuum filters is typically limited to 15 psi pressure If greaterforce is required, a positive pressure in excess of atmospheric can be applied to thesuspension by a pump This motive force may be in the form of compressed airintroduced in a montejus, or the suspension may be directly forced through a pumpacting against the filter medium (as in the case of a filter press), or centrifugal forcemay be used to drive the suspension through a filter medium as is done in screencentrifuges

Filtration is a hydrodynamic process hi which the fluid's volumetric rate is directlyproportional to the existing pressure gradient across the filter medium, and inverselyproportional to the flow resistance imposed by the connectivity, tortuosity and size ofthe medium's pores, and generated filter cake The pressure gradient constitutes thedriving force responsible for the flow of fluid

Regardless of how the pressure gradient is generated, the driving force increasesproportionally However, in most cases, the rate of filtration increases more slowlythan the rate at which the pressure gradient rises The explanation for this phenomenon

is that as the gradient rises, the pores of filter medium and cake are compressed andconsequently the resistance to flow increases For highly compressible cakes, bothdriving force and resistance increase nearly proportionally and any rise in the pressuredrop has a minor effect on the filtration rate

An Introduction to Liquid Filtration 11

The Formation of Filter Cake

Filtration operations are capable of handling suspensions of varying characteristicsranging from granular, incompressible, free-filtering materials to slimes and colloidalsuspensions in which the cakes are incompressible These latter materials tend tocontaminate or foul the filter medium The interaction between the particles insuspension and the filter medium determines to a large extent the specific mechanismsresponsible for filtration

In practice cake filtration is used more often than filter-medium filtration Uponachieving a certain thickness, the cake is removed from the medium by variousmechanical devices or by reversing the flow of filtrate To prevent the formation ofmuddy filtrate at the beginning of the subsequent filtration cycle, a thin layer ofresidual particles is sometimes deposited onto the filter medium For the same reason,the filtration cycle is initiated with a low, but gradually increasing pressure gradient

at an approximately constant flowrate The process is then operated at a constantpressure gradient while experiencing a gradual decrease in process rate

The structure of the cake formed and, consequently, its resistance to liquid flowdepends on the properties of the solid particles and the liquid phase suspension, as well

as on the conditions of filtration Cake structure is first established by hydrodynamicfactors (cake porosity, mean particle size, size distribution, and particle specificsurface area and sphericity) It is also strongly influenced by some factors that canconditionally be denoted as physicochemical These factors are:

1 the rate of coagulation or peptization of solid particles,

2 the presence of tar and colloidal impurities clogging the pores,

3 the influence of electrokinetic potentials at the interphase in the presence ofions, which decreases the effective pore cross section, and

4 the presence of solvate shells on the solid particles (this action is manifested

at particle contact during cake formation)

Due to the combining effects of hydrodynamic and physicochemical factors, the study

of cake structure and resistance is extremely complex, and any mathematicaldescription based on theoretical considerations is at best only descriptive

The influence of physicochemical factors is closely related to surface phenomena atthe solid-liquid boundary It is especially manifested by the presence of small particles

in the suspension Large particle sizes result in an increase in the relative influence ofhydrodynamic factors, while smaller sizes contribute to a more dramatic influencefrom physicochemical factors No reliable methods exist to predict when the influence

of physicochemical factors may be neglected However, as a general rule, for roughevaluations their influence may be assumed to be most pronounced in the particle sizerange of 15-20 /mi

Typical Industrial Filtration Conditions

Two significant operating parameters influence the process of filtration: the pressuredifferential across the filtering plate, and the temperature of the suspension Mostcakes may be considered compressible and, in general, their rate of compressibilityincreases with decreasing particle size The temperature of the suspension influencesthe liquid-phase viscosity, which subsequently affects the ability of the filtrate to flowthrough the pores of the cake and the filter medium

In addition, the filtration process can be affected by particle inhomogeneity and theability of the particles to undergo deformation when subjected to pressure and settlingcharacteristics due to the influence of gravity Particle size inhomogeneity influencesthe geometry of the cake structure not only at the moment of its formation, but alsoduring the filtration process During filtration, small particles retained on the outerlayers of the cake are often entrained by the liquid flow and transported to layerscloser to the filter medium, or even into the pores themselves This results in anincrease in the resistances across the filter medium and the cake that is formed,

Particles that undergo deformation when subjected to transient or high pressures are

usually responsible for the phenomenon known as pore clogging Fortunately, what

nature has sometimes neglected in the filterability of suspensions, man can correctthrough the addition of coagulating and peptizing agents These are additives whichcan drastically alter the cake properties and, subsequently lower flow resistance andultimately increase the filtration rate and the efficiency of separation Filter aids may

be used to prevent the penetration of fine particles into the pores of a filter plate whenprocessing low concentration suspensions Filter aids build up a porous, permeable,rigid lattice structure that retains solid particles on the filter medium surface, whilepermitting liquid to pass through They are often employed as precoats with theprimary aim of protecting the filter medium They may also be mixed with asuspension of diatomaceous silica type earth (>90% silica content) Cellulose andasbestos fiber pulps were typically employed for many years as well

The discussions of the basic features of filtration given thus far illustrate that the unitoperation involves some rather complicated hydrodynamics that depend strongly onthe physical properties of both fluid and particles, as well as interaction with acomplex porous medium The process is essentially influenced by two different groups

of factors, which can be broadly lumped into macro- and micro-properties factors are related to variables such as the area of a filter medium, pressuredifferences, cake thickness and the viscosity of the liquid phase Such parameters arereadily measured Micro-factors include the influences of the size and configuration

Macro-of pores in the cake and filter medium, the thickness Macro-of the electrical double layer onthe surface of solid particles, and other properties

Washing and Dewatering Operations

When objectionable (i.e., contaminated or polluted), or valuable suspension liquorsare present, it becomes necessary to wash the filter cake to effect clean separation of

An Introduction to Liquid Filtration 13

solids from the mother liquor or to recover the mother liquor from the solids The

operation known as de-watering involves forcing a clean fluid through the cake to

recover residual liquid retained in the pores, directly after filtering or washing If thefluid is gas, then liquid is displaced from the pores Also, by preheating the gas, thehydrodynarnic process is aided by diffusional drying

Dewatering is a complex process on a microscale, because it involves thehydrodynamics of two-phase flow Although washing and dewatering are performed

on a cake with an initially well defined pore structure, the flows become greatlydistorted and complex due to changing cake characteristics The cake structureundergoes compression and disintegration during both operations, thus resulting in adramatic alteration of the pore structure

General Considerations for Process Engineers

In specifying and designing filtration equipment, primary attention is given to optionsthat will minimize high cake resistance This resistance is responsible for losses infiltration capacity One option for achieving a required filtration capacity is the use of

a large number of filter modules Increasing the physical size of equipment is feasibleonly within certain limitations as dictated by design considerations, allowableoperating conditions, and economic constraints

A more flexible option from an operational viewpoint is the implementation ofprocess-oriented enhancements that intensify particle separation This can be achieved

by two different methods In the first method, the suspension to be separated ispretreated to obtain a cake with minimal resistance This involves the addition of filteraids, flocculants or electrolytes to the suspension

In the second method, the period during which suspensions are formed provides theopportunity to alter suspension properties or conditions that are more favorable tolow-resistance cakes For example, employing pure initial substances or performing

a prefiltration operation under milder conditions tends to minimize the formation oftar and colloids Similar results may be achieved through temperature control, bylimiting the duration of certain operations immediately before filtering such ascrystallization, or by controlling the rates and sequence of adding reagents

Filtration equipment selection is often complex and sometimes confusing because of(1) the tremendous variations in suspension properties; (2) the sensitivities ofsuspension and cake properties to different process conditions; and (3) the variety offiltering equipment available Generalities in selection criteria are, therefore, few;however, there are some guidelines applicable to certain classes of filtrationapplications One example is the choice of a filter whose flow orientation is in thesame direction as gravity when handling polydispersed suspensions Such anarrangement is more favorable than an upflow design, since larger particles will tend

to settle first on the filter medium, thus preventing pores from clogging within themedium structure

A further recommendation, depending on the application, is not to increase thepressure difference for the purpose of increasing the filtration rate The cake may, forexample, be highly compressible; thus, increased pressure would result in significantincreases in the specific cake resistance We may generalize the selection process tothe extent of applying three rules to all filtration problems:

1 The objectives of a filtration operation should be defined;

2 Physical and/or chemical pretreatment options should be evaluated for theintended application based on their availability, cost, ease of implementa-tion and ability to provide optimum filterability; and

3 Final filtration equipment selection should be based on the ability to meetall objectives of the application within economic constraints

The Objectives of Filtration

The objectives for performing filtration usually fall into one of the followingcategories:

1 clarification for liquor purification,

2 separation for solids recovery,

3 separation for both liquid and solids recovery, and/or

4 separation aimed at facilitating or improving other plant operations

Clarification involves the removal of relatively small amounts of suspended solidsfrom suspension (typically below 0.15% concentration) A first approach toconsidering any clarification option is to define the required degree of purification.That is, the maximum allowable percentage of solids in the filtrate must beestablished Compared with other filter devices, clarifying filters are of lesserimportance to pure chemical process work They are primarily employed in beveragemanufacturing and water polishing operations, pharmaceutical filtration, fuel/lubricating oil clarification, electroplating solution conditioning, and dry-cleaningsolvent recovery They are also heavily employed in fiber spinning and film extrusion

In filtration for solids recovery, the concentration of solids suspension must be highenough to allow the formation of a sufficiently thick cake for discharge in the form of

a solid mass before the rate of flow is materially reduced However, solidsconcentration alone is not the only criterion for adequate cake formation Forexample, an 0.5% suspension of paper pulp may be readily cake-forming whereas a10% concentration of certain chemicals may require thickening to produce adischargeable cake

Filtration for both solids and liquid recovery differs from filtration for solids recoveryalone in the cake building, washing and drying stages If the filtrate is a valuableliquor, maximum washing is necessary to prevent its loss; but if it is valueless, excesswash liquor can be applied without regard to quality

An Introduction to Liquid Filtration 15

Finally, filtration can be applied to facilitate other plant operations Like other unitoperations, filtration has the most immediate relationship to those operationsimmediately preceding and following it Ahead of filtration, the step is often one ofpreparation These prefiltration steps could include thickening, coagulating, heating,conditioning, pH adjustment or the handling of an unstable flow that must not bebroken by rapid pumping or agitation before filtration Such preparation stages areused to obtain more filterable material This allows a continuous operating mode,smaller filter areas or both Figure 2 schematically summarizes the prefiltration andfinal processing steps

-D E W A T E R I N Q SOLIDS

D I S P O S A L

C H E M I C A L

R E C O V E R Y

Figure 2 Summary of prefiltration and final processing steps in a filtering operation.

Filtration may also serve as the preparatory step for the operation following it Thelatter stages may be dry ing or incineration of solids, concentration or direct use of thefiltrate Filtration equipment must be selected on the basis of their ability to deliverthe best feed material to the next step Dry, thin, porous, flaky cakes are best suitedfor drying where grinding operations are not employed In such cases, the cake willnot ball up, and quick drying can be achieved A clear, concentrated filtrate often aidsdownstream treatment, whereby the filter can be operated to increase the efficiency

of the downstream equipment without affecting its own efficiency

Preparation Stages for Filtration

A number of preparation steps alluded to earlier assist in achieving optimumfilterability The major ones are briefly described below

Use of Precoat and Filter Aids

Where particles of a colloidal nature are encountered in liquor clarification, a precoatand or filter aid are often required to prevent deposited particles from being carried

by strearnflow impact into the pores of the filter medium (or filter cake afterformation), thus reducing capacity.

A precoat serves only as a protective covering over the filter medium to prevent theparticles from reaching the pores, while the filter aid added to the influent assists inparticle separation and cake formation Filter aids serve as obstructions, interveningbetween the particles to prevent their compacting, and producing, under the pressurevelocity impact, a more or less impervious layer on the filter medium, or if a precoat

is used, on it

In some instances, precoats are used, not because of danger to filter cloth clogging,but to permit the use of a coarser filter medium such as metallic cloths This canextend operating life or improve corrosion resistance

Coagulation

This is another means of dealing with colloidal or semicolloidal particles It appliesparticularly to clarification in water and sewage filtration and in the filtration of veryfine solids While flocculation often can be accomplished by agitation, the use ofchemical additives results in alteration of the physical structure of the suspended solids

to the extent of losing their colloidal nature and becoming more or less crystalline.This is usually accompanied by agglomeration Clarification by settling may follow,

if the specific gravity of the particles is sufficient to provide reasonably quicksupernatant clarity Direct filtration may be applied if the filter area is not excessive

or if complete supernatant clarity is needed

Temperature Control

Temperature has a direct impact on viscosity, which in turn affects the flowrate It is

an important factor in filtration, since lower viscosity leads to liquor penetration intosmaller voids and in shorter times Occasionally, temperature plays a role in alteringthe particle form or composition, and this in turn affects the clarification rate

The Control of pH

Proper pH control can result in clarification that might otherwise not be feasible, since

an increase in alkalinity or acidity may change soft, slimy solids into firm,free-filtering ones In some cases precoats are employed, not because of the danger

of filter cloth clogging, but to allow the use of a coarser filter medium, such asmetallic cloth

Equipment Selection Methodology

Equipment selection is seldom based on rigorous equations or elaborate mathematicalmodels Where equations are used, they function as a directional guide in evaluatingdata or process arrangements Projected results are derived most reliably from actual

An Introduction to Liquid Filtration 11

plant operational data and experience where duplication is desired; from standards set

up where there are few variations from plant to plant, so that results can be anticipatedwith an acceptable degree of confidence (as in municipal water filtration); or frompilot or laboratory tests of the actual material to be handled Pilot plant runs aretypically designed for short durations and to closely duplicate actual operations

Proper selection of equipment may be based on experiments performed in themanufacturer's laboratory, although this is not always feasible Sometimes thematerial to be handled cannot readily be shipped; its physical or chemical conditionschange during the time lag between shipping and testing, or special conditions must

be maintained during filtration that cannot be readily duplicated, such as refrigeration,solvent washing and inert gas use A filter manufacturer's laboratory has theadvantage of having numerous types of filters and apparatus available withexperienced filtration engineers to evaluate results during and after test runs

The use of pilot-plant filter assemblies is both common and a classical approach todesign methodology development These combine the filter with pumps, receivers,mixers, etc., in a single compact unit and may be rented at a nominal fee from filtermanufacturers, who supply operating instructions and sometimes an operator.Preliminary tests are often run at the filter manufacturer's laboratory Rough testsindicate what filter type to try in the pilot plant

Comparative calculations of specific capacities of different filters or their specificfilter areas should be made as part of the evaluation Such calculations may beperformed on the basis of experimental data obtained without using basic filtrationequations In designing a new filtration unit after equipment selection, calculationsshould be made to determine the specific capacity or specific filtration area Basicfiltration equations may be used for this purpose, with preliminary experimentalconstants evaluated These constants contain information on the specific cakeresistance and the resistance of the filter medium

The basic equations of filtration cannot always be used without introducingcorresponding corrections This arises from the fact that these equations describe thefiltration process partially for ideal conditions when the influence of distorting factors

is eliminated Among these factors are the instability of the cake resistance duringoperation and the variable resistance of the filter medium, as well as the settlingcharacteristics of solids In these relationships, it is necessary to use statisticallyaveraged values of both resistances and to introduce corrections to account for particlesettling and other factors In selecting filtration methods and evaluating constants inthe process equations, the principles of similarity modeling are relied on heavily

Within the subject of filtration, a distinction is made between micro- andmacromodeling The first one is related to modeling cake formation The cake isassumed to have a well defined structure, in which the hydrodynamic andphysicochemical processes take place Macromodeling presents few difficulties,because the models are process-oriented (i.e., they are specific to the particularoperation or specific equipment) If distorting side effects are not important, thefiltration process may be designed according to existing empirical correlations In

practice, filtration, washing and dewatering often deviate substantially from theory.This occurs because of the distorting influences of filter features and the unaccountedfor properties of the suspension and cake.

Existing statistical methods permit prediction of macroscopic results of the processeswithout complete description of the microscopic phenomena They are helpful inestablishing the hydrodynamic relations of liquid flow through porous bodies, dieevaluation of filtration quality with pore clogging, description of particle distributionsand in obtaining geometrical parameters of random layers of solid particles

Nomenclature

A = area ( m )

b = parameter in slip flow expression for K (sec2-m/kg)

c = shape factor, known as Kozeny constant

K = hydraulic conductivity (m/sec)

L = characteristic macroscopic length (m)

n = number of pore layers

p = pressure (kg/sec -m)

q = seepage velocity (m/sec)

Q = volumetric flowrate (m3/sec)

Qm = volumetric flowrate at average pressure pm (m3/sec)

FILTER MEDIA AND USE OF FILTER AIDS

Introduction

In conventional filter-medium filtration practices, the filter medium may be described

as the workhorse of the process Proper selection is often the most importantconsideration for assuring efficient suspension separation A good filter mediumshould have the following characteristics:

The ability to retain a wide size distribution of solid particles from the suspension,

Offer minimum hydraulic resistance to the filtrate flow,

Allow easy discharge of cake,

High resistance to chemical attack,

Resist swelling when in contact with filtrate and washing liquid,

Display good heat-resistance within the temperature ranges of filtration,

Have sufficient strength to withstand filtering pressure and mechanical wear,

Capable of avoiding wedging of particles into its pores.

There are many filter media from which to choose from; however, the optimum typeoften depends on the properties of the suspension and specific process conditions.Filter media may be classified into several groups, however the two most common

classes are the surface-type and depth-media-type.

Surface-type filter media are distinguished by the fact that the solid particles ofsuspension on separation are mostly retained on the medium's surface That is,particles do not penetrate into the pores Common examples of this type of media arefilter paper, filter cloths, and wire mesh

Depth-type filter media are largely used for liquid clarification They are characterized

by the fact that the solid particles penetrate into the pores where they are retained Thepores of such media are considerably larger than the particles of suspension Thesuspension's concentration is generally not high enough to promote particle bridging

19

inside the pores Particles are retained on the walls of the pores by adsorption, settlingand sticking As a rule, depth-type filter media cannot retain all suspended particles,and their retention capacity is typically between 90-99% Sand and filter aids, forexample, fall into this category.

Some filter media may act as either surface-type or depth-type, depending on the poresize and suspension properties (e.g., particle size, solids concentration and suspensionviscosity)

It is also common practice to classify filter media by their materials of construction.Examples are cotton, wool, linen, glass fiber, porous carbon, metals and rayons Such

a classification is convenient for selection purposes, especially when resistance toaggressive suspensions is a consideration We may also classify media according tostructure, with typical classes being rigid, flexible and semi-rigid or combinationmedia

Filtration aids are employed to enhance filtration characteristics, particularly for to-filter suspensions These are normally applied as an admix to the suspensions Therole of the filter aid is to built up a porous, permeable and rigid lattice structure thatassists in retaining solid particles while allowing liquid to flow through

hard-This chapter provides a working knowledge of the use and selection of filter aids.Further discussions are given in subsequent chapters

Flexible Filter Media

Flexible nonmetallic materials have been widely used as filter media for many years.They are available in the form of fabrics or as preformed unwoven materials, but also

in the form of perforated plates

Fabric filter media are characterized by the characteristics of mesh count, meshopening, yarn size and the type of weave The mesh count or thread count of a fabric

is the number of threads per inch Thread counts in both warp and weft directions arethe same, and are indicated by a single number Warp threads run lengthwise in afabric and are parallel to the selvage edge Weft or filling threads run across the width

of a fabric at right angles to the warp Figure 1 illustrates the important constructionparameters that characterize a fiber-based fabric Note that the space between threads

is the mesh opening It is measured in units of micrometers or inches Different yarnsizes are normally specified as a measurement of diameter in micrometers or mils(thousandths of an inch) Yarn sizes in the warp and weft directions are normally thesame, and are indicated by a single number

Fabrics are available in differing mesh openings, and varying thread diameters Thethread diameter affects the amount of open area in a particular cloth, which in turndetermines the filtration flowrate or throughput

Filter Media and Use of Filter Aids 21

Figure 1 Construction parameters that determine the characteristics of a fiber-based fabric.

A plain weave is the most basic weave, with a weft thread alternately going over one warp thread and then under one warp thread A twill weave produces a diagonal or

twill line across the fabric face These diagonals are caused by moving the yarnintersections one weft thread higher on successive warp yams A twill weave isdesignated 2/1, 2/2, or 3/1 depending on how many weft threads the warp threads go

over and under A satin weave has a smooth surface caused by carrying the warp (or

the weft) on the fabric surface over many weft (or warp) yarns Intersections betweenwarp and weft are kept to a minimum, just sufficient to hold the fabric firmly togetherand still provide a smooth fabric surface The percentage of open area in a textile filterindicates the proportion of total fabric area that is open, and can be determined by thefollowing relationship:

(mesh opening + thread diameter}' x 100 (1)

The following are some examples of different types of common flexible filter media,

Glass Cloths

Glass cloths are manufactured from glass yarns They have high thermal resistance,high corrosion resistance and high tensile strength, and are easily handled; thecomposition and diameter of the fibers can be altered as desired The disadvantages

of glass cloth are the lack of flexibility of individual fibers, causing splits andfractures, and its low resistance to abrasion However, backing glass cloth with a leadplate, rubber mats or other rigid materials provides for longevity Backing with cotton

or rubber provides about 50% greater life than in cases where no backing is used,

Cotton Cloths

Cotton filter cloths are among the most widely used filter media They have a limitedtendency to swell in liquids and are used for the separation of neutral suspensions attemperatures up to 100°C, as well as suspensions containing acids up to 3% or alkalieswith concentrations up to 10% at 15-20°C Hydrochloric acid at 90-100°C destroyscotton fabric in about 1 hour, even at concentrations as low as 1.5% Nitric acid hasthe same effect at concentrations of 2.5%, and sulfuric acid at 5% Phosphoric acid(70%) destroys the cloth in about six days Water and water solutions of aluminumsulfate cause cotton fabrics to undergo shrinkage

Woven cotton filter cloths comprise ducks, twills, chain weaves, canton flannel andunbleached muslins Cotton duck is a fabric weave that is a plain cloth with equal-thickness threads and texture in the "over one and under one" of the warp and woof.The twill weave is over two and under two with the next filling splitting the warpstrands and giving a diagonal rib at 45° if the number of warp and filling threads areequal Canton flannel is a twill weave in which one surface has been brushed up togive a nap finish A muslin cloth is a very thin duck weave, which is unbleached forfiltering In chain weave one filling goes over two warp threads and under two, thenext reversing this; the third is a true twill sequence, and the next repeats the cycle

A duck may be preferable to a twill of higher porosity, because the hard surface ofthe duck permits freer cake discharge Under high increasing pressure a strong,durable cloth (duck) is required, since the first resistance is small as compared withthat during cake building Certain types of filters, such as drum filters, cannot standuneven shrinkage and, in some cases, cloths must be preshrunk to ensure fitting duringthe life of the cloth

Nitro-filter (nitrated cotton cloth) cloths are about the same thickness and texture asordinary cotton filtration cloths, but are distinguished by a harder surface It isclaimed that the cake is easily detached and that clogging is rare Their tensile strength

is 70-80% of that of the specially manufactured cotton cloths from which they areprepared They are resistant to the corrosive action of sulfuric, nitric, mixed nitrationand hydrochloric acids They are recommended for filtering sulfuric acid solutions to40% and at temperatures as high as 90°C, with the advantage of removing finelydivided amorphous particles, which would quickly clog most ceramic media Nitro-filter cloths are composed of cellulose nitrate, which is an ester of cellulose Anychemical compound that will saponify the ester will destroy the cloth Caustic soda orpotash in strengths of 2% at 70°C or over; alkali sulfides, polysuifides andsulfohydrates; or mixtures of ethyl alcohol and ether, ethyl, amyl and butyl acetates,pyridine, ferrous sulfates, and other reducing agents are detrimental to the cloth

Filter Media and Use of Filter Aids 23

Cellulose nitrate is inflammable and explosive when dry, but when soaked in water it

is considered entirely safe if reasonable care is taken in handling For this reason it

is colored red and packed in special containers Users are cautioned to keep the clothswet and to handle them carefully

it does not clog easily Wool cloths are sold by weight, usually ranging 10-22 oz/yd2with the majority at 12 oz/yd2 The clarity through wool cloths is considerably lessthan through cotton cloths

Paper Pulp and Fiber Cloths

Paper pulp and fiber cloths are excellent materials for precoats and filter aids Paperpulp gives a high rate of flow, is easily discharged and shows little tendency to clog.Paper pulp's disadvantage lies in its preparation Soda or sulfate pulp, most commonlyused, must be disintegrated and kept in suspension by agitation before precoating Thisrequires considerable auxiliary equipment Diatomaceous earths, while they should bekept in suspension, are very easy to handle and do not undergo disintegration.Paper pulp compressed into pads is used in pressure filters for beverage clarification.After becoming dirty, as evidenced by decrease in the rate of flow, the paper may berepulped, water-washed and reformed into pads Although this involves considerablework, excellent clarity and high flowrates are obtained The impurities do not form

a cake as such, but penetrate into the pad and can only be removed by repulping andwashing the pad

Pads of a mixture of paper pulp and asbestos fiber are used hi bacteriologicalfiltrations In sheet form it is employed in the laboratory for all kinds of filtration.Filter papers are made in many grades of porosity for use in porcelain and glassfunnels Industrially, paper in the form of sheets is used directly or as a precoat infilter presses

Used directly in lubricating clarification in a "blotter press", it acts much the samemanner as the paper pads, but is much thinner and is not reused As a precoat, paperprotects the filter medium from slimy fines; it may be peeled off and discarded afterclogging, leaving the medium underneath clean

Rubber Media

Rubber media appear as porous, flexible rubber sheets and microporous hard rubber sheets Commercial rubber media have 1100-6400 holes/in.2 with pore diameters of 0,012-0.004 in They are manufactured out of soft rubber, hard rubber, flexible hard rubber and soft neoprene.

The medium is prepared on a master form, consisting of a heavy fabric belt, surfaced

on one side with a layer of rubber filled with small round pits uniformly spaced These pits are 0.020 in deep, and the number per unit area and their surface diameter determine the porosity of the sheet A thin layer of latex is fed to the moving belt by

a spreader bar so that the latex completely covers the pits, yet does not run into them This process traps air in each pit The application of heat to the under-surface of the blanket by a steam plate causes the air to expand, blowing little bubbles in the film of latex When the bubbles burst, small holes are left, corresponding to the pits The blown rubber film, after drying, is cooled and the process repeated until the desired thickness of sheet is obtained The sheet is then stripped off of the master blanket and vulcanized,

Approximately 95% of the pits are reproduced as holes in the rubber sheet The holes are not exactly cylindrical in shape but are reinforced by slight constrictions which contribute to strength and tear resistance This type is referred to as "plain," and can

be made with fabric backing on one or both sides to control stretching characteristics.

If the unvulcanized material is first stretched, and then vulcanized while stretched, it

is called "expanded." Resulting holes are oval and have a higher porosity (sometimes

up to 30%) Special compounds have been formulated for resistance to specific chemicals under high concentrations at elevated temperatures, such as 25% sulfuric acid at 180° F.

The smooth surface allows the removal of thinner cakes than is possible with cotton

or wool fabrics Rubber does not show progressive binding and it can be readily cleaned and used in temperatures up to 180°F On the other hand, because a clear filtrate is difficult to obtain when filtering finely divided solids, a precoat often becomes necessary.

Synthetic Fiber Cloths

Cloths from synthetic fibers are superior to many of the natural cloths thus far considered They do not swell as do natural fibers, are inert in many acid, alkaline and solvent solutions and are resistant to various fungus and bacterial growths (the degree depending on the particular fiber and use) Several synthetic fibers resist relatively high temperatures, and have a smooth surface for easy cleaning and good solids discharge Some of the most widely used synthetic filter media are nylon, Saran, Dacron, Dynel, Vinyon, Orion, and Acrilan Table 1 compares the physical properties

of several synthetic fiber filter media.

Filter Media and Use of Filter Aids 25

Tightly woven, monofilament (single-strand) yarns consist of small-diameterfilaments They tend to lose their tensile strength, because their small diametersreduce their permeability; thus multifilament yarns are normally used Monofilamentyarns in loose weaves provide high flowrates, good solids discharge, easy washing andhigh resistance to blinding, but the turbidity of the filtrate is high and recirculation isusually necessary, initially at least Table 2 provides additional information on varioussynthetic filter fabrics

Flexible Metallic Media

Flexible metallic media are especially suitable for handling corrosive liquors and forhigh-temperature filtration They have good durability and are inert to physicalchanges Metallic media are fabricated in the form of screens, wire windings, orwoven fabrics of steel, copper, bronze, nickel and different alloys

Perforated sheets and screens are used for coarse separation, as supports for filtercloths or as filter aids Metallic cloths are characterized by the method of wire weaves

as well as by the size and form of holes and by the wire thickness Metallic cloths may

be manufactured with more than 50,000 holes/cm2 and with hole sizes less than 20

Solvents

Good Pooi- Good Fail- Good Fail- Good Good Good High Fail-

FiberTensileStrength

High

Low

High High Fail- High High High High Fail-

Low

TemperatureLimit

275750300350200600300275240180300

MetaHic/Nonmetallic Cloth

Combination metallic and nonmetallic cloths consist of metallic wires and weak cloth

or asbestos threads There are some difficulties in weaving when attempting tomaintain uniformity between wires and the cloth, and considerable dissatisfaction hasbeen experienced with such construction While cotton weaves well with the asbestos,the cotton fibers destroy the fabric's resistance to heat and corrosion Its use is,therefore, quite limited, despite its resistance to high temperatures, acids and mildew

Cotton cloths are sometimes treated with metallic salts (copper sulfate) to improvetheir corrosion-resistant qualities Such cloths are in the usual cotton filter clothgrades, and while they are not equivalent to metallic cloths, the treatment doesmaterially prolong the life of the cotton fiber.

Non woven Media

Nonwoven media are fabricated in the form of belts or sheets from cotton, wool,synthetic and asbestos fibers or their mixtures, as well as from paper mass They may

be used in filters of different designs, for example, in filter presses, filters withhorizontal discs and rotary drum vacuum filters for liquid clarification Most of theseapplications handle low suspension concentrations; examples are milk, beverages,lacquers and lubricating oils Individual fibers in nonwoven media are usuallyconnected among them as a result of mechanical treatment A less common approach

is the addition of binding substances Sometimes the media are protected from bothsides by loosely woven cloth Nonwoven media of various materials and weights, and

in several grades of retentiveness per unit weight can be formed, in either absorbent

or nonabsorbent material These filter media retain less dispersed particles (more than

100 jum) on their surface, or close to it, and more dispersed particles within the depths

of the media

Nonwoven filter media are mostly used for filter medium filtration with pore clogging.Because of the relatively low cost of this medium, it is often replaced after poreclogging In some cases, non woven media are used for cake filtration In this case,cake removal is so difficult that it must be removed altogether from the filter medium.Nonwoven filter media can be prepared so that pore sizes decrease in the directionfrom the surface of the filter media contacting suspension to the surface contacting thesupporting device This decreases the hydraulic resistance of filtration and providesretention of relatively large particles of suspension over the outer layer of thenonwoven medium Nonwoven filter media of synthetic, mechanically pressed fibersare manufactured by puncturing the fiber layer with needles (about 160punctures/cm2), and subsequent high temperature treatment with liquid which causesfiber contraction Such filter media are distinguished by sufficient mechanical strengthand low hydraulic resistance, as well as uniform fiber distribution Filter media fromfibers connected by a blinder are manufactured by pressing at 70N/cm2and 150°C.These media have sufficient mechanical strength, low porosity and are corrosionresistant

Filter media may be manufactured by lining a very thin layer of heat-resistant metal(e.g., nickel 360) over a fiber surface of inorganic or organic material Such filtermedia may withstand temperatures of 200°C and higher

Of the flexible filter media described, the synthetic fabrics are perhaps the mostwidely relied on in industrial applications Each filtration process must meet certainrequirements in relation to flowrate, clarity of filtrate, moisture of filter cake, cakerelease and nonbinding characteristics The ability of a filter fabric to help meet thesecriteria, and to resist chemical and physical attack depend on such characteristics as

Thread Dim., Mesh

Weight Threadslia., Warp x Weft opening Air Permeability Thickness

Style No Weave (oz/yd*) Warp x Weft (Pm) ( I 4 (fP/min) bd