FILTRATION EQUIPMENT
THE SELECTION OF FILTER FABRICS RE-EXAMINED XE
The steady increase in filter f a b r i c technology calls f o r a n u p - t o - d a t e r o u n d - u p o n w h a t is available.
Here’s a
h a n d y check list f o r selecting the best filter m e d i a
by SPERRY EHLERS, Filtration Fabrics Diuision, American Machine and Metals, Inc , East Moline, Ill.
ROLE of filter fabrics in the performance of liquid-solid separation equipment in the process industries has been subjected to close examination in recent years by chemical and mechanical engineers. Many have urged more basic research work to develop empirical or theoretical formulas to predict the performance of a filter medium. Others have upheld pilot scale trial and error as the only sure method of knowing a filter medium’s performance. Between these two viewpoints lies some logical assessment acceptable for the present stage of knowledge. Filter medium is defined as a porous mass through which fluid is passed to separate from it matter held in suspension. The fluid may be either a gas or a liquid. The porous mass may be:
Fibrous ,
I
1
NATURAL FIBERS
Linen
Wool
552
Cotton Hemp Sisal
INDUSTRIAL AND ENGINEERING CHEMISTRY
fabrics made from textile fibers listed in “The Textile Fiber Family Tree”
MAN-MADE FIBERS“
I Asbestos
Fur Hair Silk
. . Tl‘ovan
Acetate Acrylic Modacrylic Nylon Nytril Olefin Polyester Rubber Saran Spandex Vinal Vinyon
I
I
U Cellulose Rayon
Rubber
Glass Metallic
Azlon
a Generic names for mon-mode fibers by the Textile Fiber Products Identification Act approved September 2, J958, made effectiveby Federol Trade Commiss;on March 3, 7 960.
. . . Nonwoven
.
felted materials such as paper and felt made from textile fibers . . Mats such as sponge o r paper pulp or made from textile fibers
Generic Name, Definition, and Trade Names of Man-Made Fibers Acele12 Celanese7 Estron13 Also made and sold under generic name by 4, 5, 6, 9, 21 & 25
Acrylic
-a manufactured fiber in which the fiberforming substance i s any long chain synthetic polymer composed of a t least 85% by weight of acrylonitrile units (-CH2-CH-l.
Acrilans Creslan3 Orlon12 Zefranl'
. . . Carbon, clay, diatomaceous earth, gravel sand, etc.
. . . Ceramics,
natural stone, perforated metal or plastic sheets, sintered metals, or plastics
The selection of a filter medium is a problem frequently encountered in line with present-day demands for increased production rate, improved product, greater recovery of values, lower operating and maintenance costs, and waste disposal. The variety of types of filter equipment and filter media available today makes the problem of selection difficult. A general treatment of the filter equipment and filter media is beyond the scope of this paper, which concerns itself specifically with woven textile fiber filter media. Prior to 1945, the range of filter applications was usually governed by conditions that organic textile fiber filter media could withstand. Separation problems outside this range were handled by other processes. In recent years. the man-made textile fiber filter media have been replacing the traditional natural textile fiber products in spite of the generally higher original costs of the synthetics. This trend to the man-made fibers is due to their better chemical, physical, and thermal resistance which lowers production and service costs. ,First problem of filter medium selection : decide if, in your operation, using existing equipment, you wish to Clarify the fluid, the solids are discarded Separate fluid from solids, retaining both Recover solids, fluid is discarded
,Second problem: character of the solids Size of particles Range of size Shape of particles Specific gravity of solids ,Third problem: character of the fluid. Chemistry-acid, alkali, or neutral Temperature Viscosity Specific gravity
,Fourth problem: character of the combined fluid and solids, called the concentrate, feed, sludge, or slurry Ratio of solids to fluid Particle flocculation Viscosity of slurry
Trade a o m c
-a manufactured fiber in which the fiberforming substance is cellulose acetate. Where not less than 92% of the hydroxyl groups are acetylated, the term triacetate may be used as a generic description of the fiber.
Granular o r Powdered
Porous
1. TC Dsjnition
Generic Name
Acetate
I CN Azlon
-a manufactured fiber in which the fiberforming substance is composed of any regenerated naturally occurring proteins.
N o current commercial domestic production
Glass
-a manufactured fiber in which the fiberforming substance is glass.
Fiberglas2'j Also made and sold under generic name by 14, 22 &
Metallic
-a
Modacrylic
-a manufactured fiber in which the fiberforming substance is any long chain synthetic polymer composed of less than 85% but a t least %yoby weight of acr;ylonitrile units (-CH2-CH-I.
27 manufactured fiber composed of metal, plastic-coated metal, metal-coated plastic, or a core completely covered by metal.
Fi Id o r l7 t u r ex lo MetlonZ3RaymetZ9
I
CN Nylon
-a manufactured fiber in which the fiberforming substance is any long chain synthetic polyamide having recurring amide groups (-C-"-1 as an integral part
Made and sold under name by 2, 4, 8, 12 & 21
ll
0 of the polymer chain. Nytril
-a
manufactured fiber containing a t least
Darvan19
85% of a long chain polymer of vinylidene dinitrile (-CH2-C (CN)2-I where the vinylidene dinitrile content is no less than every other unit in the polymer chain. Olefin
-a manufactured fiber in which the fiberforming substance is any long chain synthetic polymer composed of a t least 85% by weight of ethylene, propylene, or other olefin units.
Polyester
-a manufactured fiber in which the fiber- Docron12 Kodel'3 forming substance is any long chain syn- Teron15 Vycron6 thetic polymer composed of a t least My0 by weight o f an ester of a dihydric alcohol and terephthalic acid (p-HOOC-C8H4-COOHI.
Rayon
-a manufactured fiber composed of regenerated cellulose, as well as manufactured fibers composed of regenerated
Aviscope5 ReevonZ8 R o y ~ l e n eWyneneZ4 ~~ Also made and sold under generic name by 20
Bemberg'j Cordura12
Co rva ID Fortisa n, Tyrex
(Continued on next page)
VOL. 53, NO. 7
JULY 1961
553
FTC D e f i n i t i o n
Trade Name
cellulose in which substituents have replaced not more than 1.5% of the hydrogens of the hydroxyl groups.
Also made and sold under generic nome by 4, 5, 7, 21. 25 & 30
Generic N a m e
Rubber
Saran
Spandex
Vinal
Vinyon
1 2 3 4
5 6
7 8 9 10 It
-a manufactured fiber in which the fiberforming substance is comprised of natural or synthetic rubber. -a manufactured fiber in which the fiber- Made and sold under forming substance is any long chain syn- generic name by 1 1 , 16, thetic polymer composed of a t least 8070 18 & 24 by weight of vinylidene chloride units (-CH2-CCI2--I. -a manufactured fiber in which the fiber- Lycralz Vyrene3? forming substance is a long chain synthetic polymer comprised of a t least 85y0 of a segmented polyurethane. -a manufactured fiber in which the fiber- Limited production by 1 forming substance is any long chain synthetic polymer composed of a t least 50% by weight of vinyl alcohol units (-CH2CHOH--I, and in which the total of the vinyl alcohol units and any one or more of the various acetal units is at least 85y0 by weight of the fiber. -a manufactured fiber in which the fiber- Made and sold u n d e r forming substance is any long chain syn- name by 5 & 31 thetic polymer composed of at least 85y0 by weight of vinyl chloride units (-CHzCHCI--I.
Air Reduction Chemical Co. Allied Chemical Corp. American Cyanamid Co. American Enka Corp. American Viscose Corp. Bpaunit Mills, Inc. Celanese Corp. of America Chemstrand Corp. Courtaulds (Alabama) Inc. Dobeckmun Co. Dow Chemical Co.
12 E. I. du Pont de Nemours & Co.,Inc. 13 Eastman Chemical Products, Inc. 14 Ferro Corp. 15 Fiber Indusrries, Inc. 16 Firestone Plastics Co. 17 Freydberg Bros. Strauss 18 General Tire & Rubber Co. 1 9 B. F. Goodrich Chemical Co. 20 Hercules Powder Co., Inc. 21 Industrial Ravon Corp. 22 Johns-Manvilie Fiber G ! n s s , Inc.
23 Metlon Corp. 24 National Plastic Products Co. 25 North American Rayon Corp. 26 Owens-Corning Fiberqlas Corp. 27 Pittsburgh Plate Glass Co. 28 ReeQesBrothers, Inc. 29 Reynolds Brothers, Inc. 30 Skenandoa Rayon Corp. 31 Union Carbide Chemical Co. 32 U . S . Rubber Co.
,Fifth problem: character of the solids cake, formed during filtration Rate of cake build-up Compressibility of cake Resistance of cake to fluid flolv Cake nature Clean, crystalline and friable Plastic, sticky Slimy ,Sixth problem: quantity of material This in turn will help determine the activating force of flow of the slurry and filter media area required Gravity Osmotic, dialytic Vacuum Pressure Centrifugal to be processed.
,Seventh problem: and most important realization that selection of a filter medium is a compromise of the diametrically opposed desirable filtering characteristics. These characteristics are the points of a large star while the performance ability of any filter medium is like a smaller circle. Moving the circle of ability to gain a maximum characteristic peak, causes a loss in all other characteristics. Variable Details of Filter Fabric Construction
Variation in details of fabric construction should not add to the process variables, but should be used to help solve rhe filtration problems. Here are the construction detail variables:
Generic textile fiber Star of desirable filtering characteristic vs. circle of filter media performancesome characteristics may suffer in order to enhance others
. . . Crass section shape of fiber . . . Crimp or length texture Yarn-type made from fiber
4 ,
Minimum Blinding
Identification of yarns (enlarged) Top. Spun staple. Middle. Continuous monofilament. Bottom. Continuous multifilament
v
. . Monofilament
Plied yarns. Plied yarns of the same weight have some five times the abrasion resistance of single yarns Single yarns. Single yarns are less costly than plied yarns . , Multifilament Plied yarns Single yarns . . . Spun-staple Plied yarns Single yarns
.
Size and weight of yarns Twist per inch in yarn and plies. Plies are twisted opposite the yarn . . . None (spun-staple yarns must be twisted to hold the fiber together) . . . Normal ( 3 to 12 turns pcr inch) , , . High
554
INDUSTRIAL AND ENGINEERING CHEMISTRY
FE / FILTER FABRICS
Plain weave
Twill weave
Satin weave
Enlarged view of weaves
T h r e a d or yarn count p e r inch in t h e fabric Weave pattern
tive fabrics. I n most instances the fiber in question is offered in a maximum strength and resistance form in only one cross section and no special texture. b Second selection factor-the yarn type level. Here it is that diametrically opposed interests become evident. Monofilament fabrics exhibit little “filtration
Selecting the Filter Fabric
,First selection factor-the textile fiber level. Choose a fiber that withstands the chemical, physical, and thermal conditions of the process. Some textile fibers are available in a variety of cross sections and textures for decora-
. . . Plain
. . . Twill . . . Satin Fabric finish.
First Selection Factor-Textile Fiber Type Properties listed in descending order Rerisfonce to Abrasion tY Flex
Nylon
fiesirtance to Acid
Rrsistancc fa Alkali
Fluorocarbon Olefin
Fluorocarbon Nylon
Wool
Ac ry Ii c Modacrylic Saran Polyester Vinyon Nytril Glass
Rayon Acetate
Nylon
Olefin Saran Vinyon Modacrylic Glass Cotton Rayon Nytril Polyester
Cotton Acetate Rayon
Vinyon Saran Modacrylic Polyester Acrylic Olefin Nytril
Cotton Fluorocarbon Glass
Wool
Resistance lo Heat
Resistance t o Organic
Tensile
Water Absorbency
Sbccihc Crmity
Sfrmgfh
Solvent
Glass
Glass
Glass
Glass
Wool
Fluorocarbon Acrylic Cotton Nylon Nytril Polyester Acetate Royon
Fluorocarbon Acrylic Polyester Nylon Cotton Rayon
Nylon
Fluorocarbon Saran Cotton Rayon Modacrylic Vinyon Polyester Acetate
Rayon
Wool
Olefin
Modacrylic Vinyon Saran
Acrylic Acetate
Modacrylic Saran
Nytril Olefin
Wool
Vinyon
Ace tu te
Wool
Polyester Cotton Olefin Modacrylic Acrylic Rayon Nytril Saran Fluorocarbon Acetate
Wool Nytril Acrylic Nylon
Wool
.
Vinyon
Cofton Acetate Nylon Nytril Acrylic Polyester Modacrylic Vinyon Saran Olefin Fluorocarbon Glass
Olefin
Second Selection Factor-Yarn Type Effect on performance listed in descending order Maximum Production
Minimum Cnke Moisture
Maximum Cake Discharge
Spun-staple
Monofilament
Monofilament
Monofilament
Multifilament Monofilament
Multifilament
Multifilament
Spun-staple
Spun-staple
Maxtmum Ret ent t o n
Fiber Form
MaximuLife
Minimum Bltnding
Spun-staple
Monofilament
Multifilament
Multifilament
Multifilament
Spun-staple
Monofilament
Spun-staple
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JULY 1961
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The physical act of solids refention o f a woven filter medium proceeds through three transition phases:
,Particles larger than the openings in the medium bridge or block the openings, and a cake of solids begins to form on the filter medium in a straight-forward screening action. This accounts for the major portion of cake formation. O n this assumption, it: is reasonable to let thread count per inch be the major factor in fabric construction to control the particle size retention ability. The higher the thread count, more interstices per square inch and the smaller the openings. ,Particles smaller than the openings in the medium are retained by attraction or impingement on fibers along or within the openings. These attachments are of such tenacity that the particles remain in place, As small particles are retained on the walls of the openings, the effective size of these openings decreases. The medium, thus retains smaller particles than would be removed by direct screening or than could be predicted by measurement of the opening size. This is an explanation for the concept of “filtration in depth.” This action also accounts for the ultimate blinding to flow of the filter medium. ,Cake filtration is the action effected by previously retained particles. This action is differentiated from direct screening and is independent of the filter medium or its structure. The major portion of most filtering cycles is accomplished by this action. This importance of the cake in a filtering operation accounts for the various cake treatments and additives designed to increase the effective filtering action of the cake.
in depth” action. This results in little tendency of such filter media to become blind to flow. It also means that such fabrics have less small particle retention ability than multifilament fabrics or spun-staple fabrics. Fabrics woven from single yarns reflect the inherent filtering characteristics of the three yarn forms more clearly than do fabrics woven from plied yarns. In effect, plying monofilament yarns before weaving creates a multifilament yarn. ,Third selection factors-the three properties listed are all inter-related and limited to the occupied space and the geometry of woven fabrics.
,Fourth selection factor-the weave pattern level. ,Fifth selection factor-the fabric finish level. Other than raising a nap on a fabric or singeing a natural nap to remove it, most fabric finishes are too transient to be of great value in fabrics used in filter processes. Size stabilizing by heat-setting or shrinking the fabric is of value. Scouring is of value in removing lubricating oils and sizing added to the yarns enhances their weavability. Raising a nap on a fabric increases the filtration depth action of the fabric but it also increases the difficulty of cake removal.
Third Selection Factor -Fabric Geometry Effect on performance listed in descending order Minimum Maximrim Retention
.tfaximum Production
Yarn size
large Medium Small
Small Medium Large
Twist per inch
!ow Medium High
Thread count per inch
High Medium tow
.Waximum Cuke Discharge
.\inximum
Life
.Mi.iimum Blinding
Small Medium Large
Small Medium Large
large Medium Small
Small Medium Large
High Medium tow
High Medium tow
High Medium !ow
Medium Low High
High Medium Low
tow Medium High
tow Medium High
High Medium low
Medium High tow
tow Medium High
Cuke
Moiiture
Fourth Selection Factor -Weave Pattern Effect on performance listed in descending order
Weave pattern
Maximum Retention
iMaximvm Production
Plain
Satin Twill Plain
Twill Satin
556
Mirrimum
Muximum
Cake
Cake
Adoisture
Diirhorge
Maximum Lije
Satin Twiil Plain
Satin Twill Plain
Twill Plain Satin
INDUSTRIAL AND ENGINEERING CHEMISTRY
.Winirnum Blindin8
Satin
Twill Plain
Relative Original Cost of Fabric Construction Details in Descending Order Fiber
Fluorocarbon Acrylic Nytril Polyester
Wool Nylon Modacrylic Vinyon Saran Olefin Acetate Rayon Glass Cotton Fiber form
Monofilament Multifilament Spun-Staple
Thread count per inch
High Medium tow
T w i s t per inch
High Medium tow
Yarn size
High Medium tow
Weave
Satin Twiil Plain
After tabulating the desired objectives against the process requirements and these against the effects of fabric construction details on filter media performance, a general description of a filter fabric to meet these objectives is obtained. By comparing this general description to specification lists of existing fabrics, published by many suppliers, it is possible to select a filter fabric with success. ,Final selection of a filter fabricverification by means of a test leaf. Many equipment manufacturers have such test leaves and well-written instructions available. The economics of filter medium selection should not be considered until performance of the medium is fully evaluated. At the textile fiber level, fluorocarbon fiber costs about $17.50 per pound, nylon fiber about $1.50 per pound, and cotton fiber about $0.35 per pound. RECEIVED for review May 8, 1961 ACCEPTED May 10, 1961 Editor’s Note: An article by Sperry Ehlers outlining a method of verifying filter media performance by the use of a test leaf will be published by I/EC later this year.
