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Membrane Filtration using the SpinTek IIHigh Shear Rotary Filter

David N. SalyerSpinTek Membrane Systems, Inc.

16421 Gothard Street, Unit AHuntington Beach, CA 92647

Membrane Filtration using the SpinTek High Shear Rotary Filter

Table Of Contents

INTRODUCTION 3

CROSS FLOW FILTRATION 4

AFFECTINGPERFORMANCE 4CONCENTRATIONPOLARIZATION 4FOULING 5CLEANING 5

TYPES OF SYSTEMS 5

SPIRALWOUNDSYSTEMS 5ADVANTAGES 5DISADVANTAGES 6

HOLLOWFIBERSYSTEMS 6fiDV.4NTAGES 6DISADVANTAGES 6

TUBULARSYSTEMS 6ADVANTAGES 6DISADVANTAGES 6

PLATEANDFRAME SYSTEMS 6ADVANTAGES 7DNAIWANTAGES 7

SPINTEK H HIGH SHEAR ROTARY FILTER 7

COMPARISONTOCONVENTIONALCROSSFLOW 7A~FECTINGPEREOIMANCE 7

EXPERIMENTAL 7

LATEXCONCENTRATIONTEST 8OBJECTIVE 8RESULTS 8

KAOLIN CLAY CONCENTRATION TEST 8OBJECTIVE 8RESULTS 8

PIGMENTCONCENTRATIONTEST 8OBJECTIVE 8RESULTS 8

CONCLUSION 9

CONVENTIONAL CROSS FLOWDRAWBACKS 9SPINTEKHIGHSHEARROTARYFILTERALNANTAGES 9

REFERENCES 9

c

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IntroductionRecent environmental laws governing wastewaterquality have driven industry to search for alternatemeans of pollution control. A wide variety ofsystems such as biodigesters, centrifuges,evaporators, and settlers can be installed that separatecontaminants with varying degrees of success andcost effectiveness. Membrane filtration is fastbecoming a viable alternative due in part to thedevelopment of numerous types of membranematerials. Membrane materials have been formulatedto be acid and caustic resistant, high temperatureresistant, chlorine and oxidizing agent resistant, andsolvent resistant. Membranes have also been surfaceenhanced to produce a charge or affinity for certaintypes of molecules. Membrane surfaces can bemodified to be hydrophilic or oleophobic to disalloweasy passage to non polar organic substances such asoil and grease. These and other advancements havehelped membrane technology gain industrialacceptance.

Membrane systems also off+ many advantages overother waste treatment systems. Membrane systemseliminate the need for chemical pretreatment, andalso guarantee permeate quality due to a specificparticle rejection based on membrane pore size. Thisguarantees that even in the event of an upset (i.e. asudden change in influent solute concentration), theeffluent quality would not be affected. Membranesystems also allow the concentration of thecontaminants in the waste stream to reduce wasteproduct volume.

Membranes are classified according to the size oftheir pores or by their molecular weight cutoff.Microfiltration membranes are used to removecontaminants in the 0.025 - 2~ range. Althoughmicron-sized particles can be removed by use of non-membrane or depth materials, such as those found infibrous media, only a membrane or screen filter,having a precisely defined pore size, can ensurequalitative retention. Ultrafiltration (UF) membranesare used to separate high molecular weight solutesfrom liquids. The pore sizes are usually given inNominal Molecular Weight Cutoffs (NMWC) ratherthan micron rejections. UF membranes have poresizes that range from I ,OOO,OOO NMWC down to10,000 NMWC. Low molecular weight species (forexample, salts, sugars, and most surf&ants) are ableto permeate through the membrane. Suspendedsolids, colloids, and macromolecules are rejected andconcentrated. Nanofiltration (NF) membranes, with arange of pore sizes from 10,000 NMWC, down to200 NMWC, offer the unique capability of selective

separation of low molecular weight compounds at lowto medium pressures. Organic components such asproteins and sugars are retained as well as a certainpercentage of dissolved sodium chloride, while lowmolecular weight dissolved solids are passed throughas permeate. Reverse Osmosis is a high pressuremembrane process which rejects dissolved salts, butallows pure water to pass through. RO membranesare designed to process those streams with very lowlevels of suspended solids. Many industrialwastewater applications require only a UF or NFsystem, unless high amounts of BOD or dissolvedsolids must be removed prior to discharge. In thiscase, an RO system would be required either alone, orin the case of high suspended solids streams, in serieswith a UF system.

Traditional membrane systems fundamentally consistof dead-end type systems and cross-flow systems.Dead-end flow is the manner in which 100% of thefeed stream flow is through the filter media (seefigure I).

Part&la&s rejected by the membrane surface buildup and create a layer of particles referred to as a cake.This cake initially improves the effectiveness of themembrane and when operated under controlledconditions, could be considered a secondarymembrane. However, when operating in a dead-endconfiguration, the thickness of the layer quickly getsto the point where filtrate tlow is completelyobstructed. Cross flow systems allow only a portionof the feed stream to pass through the membrane (seejigure 2). The remaining feed flow is recirculated ata much greater rate to create a stream of fluid flowparallel to the membrane. The quickly flowingstream creates a fluid shear near the membrane

3


surface, which aids in minimizing the thickness of theparticulate layer. The efficiency of this fluid shear, orsweeping action increases with the velocity of thefluid. Unlike dead-end systems, cross-flow systemscan be operated continuously with a steadythroughput.

Cross Flow Filtration

Cross flow t%ration is widely practiced today. Theability to operate at a continuous filtration rate, orflux is possible in many applications. In order toacheive this, a cross flow system must recirculate theprocess fluid through the membrane modules topermit the sweeping action to take place. Thisrequires the use of recirculation pumps, whichcontinuously feed the material to the membranehousing over and over at a much greater rate than theactual flow of permeate effluent being drawn off.Figure 3 shows a typical flow sheet for a cross flowsystem. In this example, the feed pump fills a holdingtank with process fluid.

MEti-B&NE MODULERECIRCULATION

PUMP

Figure 3 - Typical Flow Sheef

The recirculation pump provides a means ofacheiviug cross flow as well as the driving force forpermeation. A valve located on the tank return line isused to adjust the backpressure, thus controlling thedriving force pressure on the membrane surface.Another valve allows the precise control of

concentrate to be drawn off, allowing control over thesolute concentration in the recirculating fluid.

If the recirculation rate is sufficient to maintain auefficient boundary layer at the membrane surface, asteady state of permeate flow is acheived.

Affecting Performance

Cross flow systems rely on the recirculation flow tomaintain the boundary layer at the membrane. Sincethe pressure losses through the system due to tiictionincrease with an increase in flow, the driving forcepressure becomes dependent on the cross flowvelocity, This severly limits the maximum cross flowvelocity that can be acheived. Typically, cross flowvelocities above 5 meters/second are impractical orimpossible to reach with conventional equipment.

Concentration Polarization

Concentration polarization is a boundary layerphenomenon in which solutes retained by themembrane accumulate at the membrane surlace.During the membrane separation process, solvent andsolutes are transported to the membrane surface. Asthe solvent and permeable solutes pass through themembrane, the concentration of the retained solutesincreases until a critical concentration is reached andsteady state is established. At this point, theconvective transport rate of these solutes to themembrane surface equals the diffusion transport rateof these solutes out of the boundary layer. In crossflow filtration, assume that fluid flowing across themembrane is in turbulent flow and that theconcentration of the solute within the bulk flowregion iS UnifOrIU atCb(See figure 4).

v - - T - -

VELOCITY FORCE CONCENTRATION

Figure 4 - Velocity and Concentration Gradienfs

Further assume that there exists a very thii laminarboundary layer adjacent to the membrane surface.Under the influence of a transmembrane pressuregradient, water and solute will be forced to flowacross this boundary layer in order to pass throughthe membrane. If the membrane, however,completely retains the solute, the solute concentrationadjacent to the membrane will have to increase as a

4


natural consequence of the removal of solvent. This

iiresults in the development of a concentration gradientacross the boundary layer with the maximum soluteconcentration located adjacent to the membranesurface. This concentration gradient is referred to asconcentration polarization, or gel layer formation.

The concentration polarization boundary layer usuallyproduces two adverse effects: a reduction of flux anda change in particle selectivity. The boundary layerresistance to permeation can become much huger tothat of the membrane, and thus significantly reduceflux. As the concentration of retained solutesincreases at the membrane surface, the pressurerequired for permeation of the solvent and permeablesolutes through this layer increases. As a result,membrane system separation capability is adverselyaffected.

Whereas concentration polarization might bedescribed as a dynamic fouling, other types ofmembrane fouling can affect membrane performance,and in some cases, permanently. This kind of foulingmay be composed of materials adsorbed directly onthe membrane, or may accumulate on the surface,where it is difficult to control. In general, fouling is aboundary layer or sub-boundary layer phenomenon,caused or aggravated by concentration polarization,in which solutes deposit on the membrane surface andreduce membrane flux and selectivity. Themechanisms of deposition include chemical reaction,precipitation, electrical attraction, and otherinteractions. Whereas concentration polarization is afluid dynamics phenomenon, fouling is a chemicalphenomenon between solutes and the membrane.Foulants include organic salts, macromolecules,colloids, and microorganisms. Proper selection ofmembrane material is the only defense againstfouling.

Cleaning

Although it is effective at reducing fouling, cross flowalone is often insufficient to eliminate fouling. Iffouling at some level occurs, practically the only wayto reverse its effect is to perform a cleaning on themembrane. Cleaning of membranes typically is in situat an elevated temperature, using either acids,caustics, oxidizing agents, enzymatic detergents, or a

? combination of these. This is done to chemicallyremove the fouiants within the structure of themembrane. In many cases, near completeregeneration of performance can be acheived.However, each cleaning causes a certain amount of

wear and tear on the membrane. A high cleaningfrequency can severly limit membrane life.

Since concentration polarization has an effect onfouling, a system that limited concentrationpolarization would tend to limit cleaning frequency,and thus prolong membrane life.

Types of SystemsThere are several types of cross flow systems in usetoday, the most widely accepted being spiral wound,hollow fiber, tubular, and plate and frame systems.

Spiral Wound Systems

In the spiral wound system, alternate layers ofmembrane sheets. permeate carrier and feed spacermaterial are rolled into a spiral configuration (seeJisure 5).

PERMEATE HDLES / -\FEED CHANNEL SPACER// \ 1

\ ,, / i CONCENTRATE

PERMEATE CARRIER

OliTER COVERING

Figure 5 - Spiral Watnd Membrane Module

The permeate carrier material provides support forthe membrane as well as allowing opencommunication with the permeate tube in the centerof the module. The tube has many holes all along itslength. Modules may be constructed of one or moreindividual wrappings or leaves. Since the permeatestream must flow through the entire length of thepermeate carrier before exiting at the center tube, thelength of each leaf could be so great as to inhibit theflow of permeate. A larger number of leaves mean ashorter length for each leaf, and less permeate sidepressure drop.

The feed spacer material consists of a plastic meshwhich provides a spacing between the membranes.As the feed flow is rapidly circulated through thechannel formed by this spacer, the structure of thespacer causes an increase in turbulence near themembrane which helps to improve sweeping action.

Advantages

Spiral-wound modules utilize membrane materialfound in flat-sheet form, which allows a wide range of

5


membrane materials and pore sizes to select from. Alarge amount of membrane surface area can bepacked into each module, reducing overall systemcost.

Disadvantages

The mesh spacer has a large area of contact with themembrane, reducing effective area. In addition, thepresence of the mesh in the mainstream flow creates atortuous path, causing increases in friction-inducedpressure losses. The structure also restricts the flowof agglomerated or fibrous particles, leading toplugging of the feed channel. Also, membranescannot be visually inspected without destruction ofthe module.

Hollow Fiber Systems

Hollow-fiber membranes are grouped together in tightbundles. The end of the bundle is potted to hold thetiny tubes together and form an end cap which can besealed against. They usually are available only in thevery low NMWC ranges.

Advantages

A large amount of membrane surface area can bepacked into a single hollow-fiber module. Since voidvolume is reduced in both feed and permeatechannels, hollow-fiber systems offer the highestpacking density available.

Disadvantages

An inherent disadvantage owes to the hollow-fibersvery small feed channel. Process streams with anysignificant level of suspended solids should not beconsidered due to the inevitable plugging of the flowchannel. This limits their use to liquid-liquidseparations or certain nanofiltration processes. Due tospecial processing requirements, a limited selection ofmembrane materials is available. Anotherdisadvantage is that membranes may not be visuallyinspected without destruction of the module.

Tubular Systems

In the tubular membrane system (seefipre 6), themembrane is formed into a long tube with one ormore of these tubes fitted into a shell housing. Thefeed flow of the tubular system usually is on theinside of the tube where the membrane surface is.Permeate flows from inside the tube, through the

? membrane on the tube wall, to the outside of the tube.The flow path of the tubular system is much the sameas a tube and shell heat exchanger.

Advantages

Open feed channel permits higher Reynolds numbers,enhancing turbulence. The open channel also permitscirculation of higher viscosity solution than withspiral-wound, hollow-fiber, or plate & frame systems.

Disadvantages

Maximum pressure and temperature are limited dueto the low strength structure created by the membraneand its backing. Also, special manufacturingprocesses limit material availability. Membranescannot be visually inspected without destruction.

PERMEATE

Figure 6 - Tubular Membrane Module

Plate and Frame Systems

In plate and frame systems, membrane packs areconstructed of flat layers of membranes and permeatesupport carriers (see Figure 7). Many different typesof configurations are available from rectangular tocircular as well as elliptical. A stack of membranepacks are spaced by peripheral gaskets and flowdistribution elements and clamped between pressureplates. Since each membrane pack contains its ownpermeate carrier, the exit path can be made veryshort, to minimize permeate side pressure drop.Packing density compares well to spiral-woundsystems. An advantage of the plate and frame systemis that the membrane stack can be disassembled toinspect the membrane surfaces, which is sometimescritical in determining how well the membrane issuited for the application. Peripheral sealing of themembrane packs is critical and varies hornmechanical types to 0-rings as well as ultrasonicwelding of the pack edges.


CONCENTRATE&MEMBRANE PACKS

7

PER&ATE.A 7.

FEED kRMEATE

Figure 7 - Plate and Frame Membrane Housing

I Since the feed flow is usually introduced into arectangular chamber, plate and frame systems mustrely on a flow distribution device that guarantees evenflow across all of the membrane surfaces.

Advantages

Plate & fi-ame systems offi high packing density,nearly that of spiral-wound systems. Virtually any flatsheet membrane may be used. Disassembly of a plate& frame housing allows non-destructive inspection ofa membrane.

Disadvantages

Flow distribution is poorly acheived in most designs.This limits the effectiveness of much of themembrane area. The flow channel typically doesntguarantee an even crossflow velocity across allmembrane surfaces. In systems which do provide formore even flow distribution, devices which contactthe membrane are used, reducing effective area andcausing feed channel plugging.

Spintek II High Shear Rotary Filter

The SpinTek II High Shear Rotary Filter (patentpending) has rotating disks, coated with any availableflat sheet membrane. The disks mount on a commonrotating shaft. The entire stack of membrane disks areenclosed within a pressure vessel.

The feed fluid enters the vessel, flows between disksacross the membrane surface, where permeate flowsthrough the membrane. Concentrate exits the systemat the opposite end. The fluid is recirculated as inconventional crossflow systems, but not at the samehigh rates.

Stationary disks oppose the rotating membrane disksand provide a means for prohibiting fluid rotation andpromoting turbulent flow if desired. This increasesthe shear rate, or the change in surface velocity perdistance from the membrane surface.

The stationary disks may have vane-like protrusionsto enhance fluid flow in and out of the chaonelbetween disks. The vanes can be Tom .06 to .12away f?om the membrane surface.(see Figure 8).

Figure 8 - SpinTek II High Shear Rotary Filter

Comparison to Conventional Cross Flow

The high shear membrane process is basically likethat of conventional membrane systems (see Figure3), with the exception of the rate at which the processfluid is recirculated. Since the cross flow velocity iscontrolled by rotating the membranes, therecirculation flow rate is set based on the amount ofpermeate removed. This way, the dependence ofpressure is decoupled fiorn the feed flow rate,allowing more control over the driving force pressure,and independent control of cross flow velocity.

Affecting Performance

Not only is cross flow velocity directly controlled, itsmagnitude is significantly greater. A conventionalmembrane system may have a cross flow velocity of 8ft/sec. The SpinTek II high shear membrane typicallyoperates at 55 Wsec. This has a dramatic influence onconcentration polarization, enhancing performance,and allowing selectivity to be determined by themembrane instead of by the layer that forms on top ofthe membrane.

Experimental

Comparison testing was performed on severalapplications. In each test, a conventional tubular

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ultrafiltration membrane module was compared to theSpinTek ST-IIL laboratory test unit. Both units usedpolyvinylidiene fluoride (PVDF) membranes with100.000 dalton nominal molecular weight cutoffs. Ineach test, temperature and pressure was held constant.Cross flow velocity in the tubular unit was about 12ft/sec in each test. The SpinTek ST-IIL used had an8 diameter membrane disk rotating at 1750 pm. Inall cases the surface speed at the periphery of theactive membrane area was about 55 Wsec.

Latex Concentration Test

Objective

The Styrene Butadiene (SBD) Latex wastewater is theresult of the internal rinsing of tanker trucks and railcars used to transport the latex emulsions. A filtrationprocess was adopted to separate the latex solids fromthe rinse water, enabling the latex solids to bereblended into fresh latex, and produce a cleanpermeate which can be reused in the tank rinsingprocess. A test was conducted to evaluate the effectsof concentration on flux.

Results

The SpinTek unit outperformed the tubular unit at allconcentrations by a significant margin (see Figure 9).Even at over 40% concentration, the flux was stillaround 30 gfd. The tubular unit could not sustainsignificant flux above 30% concentration.

Styrene Butadiene Latex FiltrationSpinTek ST-IIL vs. Koch Tubular

B o SpinTek 0 Tubulars 140

q 120g l o o

s 803 60IL 40

200

0%-50%

Figure 9 - Latex comparison

In the processes used to produce organic pigments,many unwanted side products are produced and mustbe separated from the pigment product. The sideproducts contain various salts which must be washedhorn the pigment slurry. Once washed free of salts,the pigment slurry must be concentrated and dried.Currently, two processes are being utilized for thiswashing and concentrating. The first, and mostcommon process are filter presses. Filter presses washthe salts from the pigment slurry while creating acake. This process is highly inefficient and can useupwards of 13 lbs. of wash water per pound ofpigment. The second, and much more efficientprocess, membrane filtration is becoming the morepreferred method of washing and concentrating. Aprocess of diafiltration is used to wash the pigmentmore efftciently, using much less water. The last stagein the membrane process concentrates the slurry to besent to the drier. A test was conducted to evaluate theeffects of concentration on flux.Kaolin Clay Concentration Test

Objective Results

Kaolin Clay is mined, washed, refined and dewateredto be used in the paper industry. In the refiningprocess, the clay is classified into various particlesizes each having a different effect in the paper which

Both the SpinTek unit and the tubular unit showedvery high fluxes initially, but the tubular unit wasaffected significantly by the change in concentration.The SpinTek unit was still well above 400 gfd at over

is produced. The filtration process was used as a firststage dewatering of the clay fines. A test wasconducted to evaluate the effects of concentration onflux.

Results

As can be seen from the graph below, the SpinTekunit performed about 400% better than the tubularsystem throughout the concentration run.

Kaolin Clay FiltrationSpinTek ST-IIL vs. AfvlT ?&g-7 Tubular

0

200

l o o 0

010%

1 ~SpinTek ??Tubular 1

00

00

0 0 0 0

15% 20% 25% 30%

Kaolin Soliis Concentratii

Figure 10 - Kaolin clay comparison

Pigment Concentration Test

Objective

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15% solids, while the tubular unit was well below 200gfd at 12%.

Liihd Rubine Pigment FiltrationSpinTek ST-IIL vs. AMT Mag-7 Tubular

800- o SpinTek 0 Tubular700- 00600 - oQoo

????? ?? ?????? ???

??

?300- ??

200 - 0IOO-

0 10% 5% 10% 15% 20% 1

Pigment Solids Concentration

Figure II- Pigment comparison

Conclusion

Membrane separation has gained acceptance in manyindustries due to the use of new membrane materialsand cross flow module designs. What limits theeffectiveness of conventional cross flow systems isthe dependence of cross flow velocity on therecirculation flow rate. This in turn causes adependence of pressure on the recirculation rate aswell, due to friction losses. These limitations keep thepractical limit of cross flow velocity down to levelswhich limit performance and increase cleaningfrequency.

Conventional Cross Flow Drawbacks

? Spiral wound modules cannot handle viscousslurries because of the spacers used in the feedchannel

? Tubular modules require very high recirculationflow rates to acheive needed cross flow velocity

? Plate and frame designs depend on flowdistribution devices which come into contactwith the membrane

? Hollow fiber designs are not recommended forsolutions with suspended solids

0 Reduced selectivity may cause rejection of solidsdesired in the permeate

SpinTek High Shear Rotary Filter Advantages

The SpinTek High Shear Rotary Filter has thefollowing advantages over conventional cross flowsystems:

? Cross flow velocity, flow rate, and pressure areindependent variables

0 Cross flow velocity is at least 4 times greater

? Significantly higher performance

? Higher concentration levels

? Selectivity is defined by the membrane, not bythe boundary layer

The performance advantages of the SpinTek unit canhave further effects on membrane replacement costs.SpinTek units would have less membrane to replace,as well as require fewer cleanings, which are knownto reduce membrane life.

References

Ballew et al, H.W., The Basics of Filtration andSeparation, Nuclepore Corporation, (no date), 23,50,51

Blatt et al, W.F., Ultrqfiltration Membranes andApplications, Cooper, A.R., ed., Plenum Press, NewYork, 1980,141-152

Blatt et al, W.F., Membrane Science and Technology,Flinn, J.E., ed., Plenum Press, New York, 1970,47-97

Murkes, J. and Car&on, C.G., Crossflow Filtration:Theory and Practice, John Wiley and Sons, 1J.K.,1988,2-5

Hickman, Carl E., Membrane Cleaning Techniques,Water Systems Technical Service, Arizona, 1991, l-13

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