microfiltration 1

8/22/2019 microfiltration 1

1/14

6. M crof ra io nby Will iam Eykamp, Universi ty of California, Berkeley

6.1 OVE R VI E WOf the m em bra ne processes includ ed in this s tudy, m icrofi l trat ion is by farthe most widely used, with total sales greater than the combined sales of al l theother membrane processes covered.Fo r al l i ts econo mic size, microfi l trat ion is surpris ingly invisible. I t isubiquitous, with innumerable small applications. A huge fraction of the market

for microfi l trat ion is for disposable devices, primarily for s ter i le f i l t rat ion in thepharmaceutical industry, and for f i l t rat ion in semiconductor fabrication processes.The heart of the microfi l trat ion f ield is s ter i le f i l t rat ion, using microfi l terswi th pores so small that microorganisms cann ot pass through them. The sedisposable f i l ters , typically in the form of pleated cartr idges, are sold to avariety of users , but the major customer is the pharmaceutical industry.Although there is intense competi t ion for new sales in this market , s tablerelat ionships between supp liers and customers are the rule. T he cost of switchingto a new supplier can be high and, thus, there is l i t t le incentive for substi tut ion

of one suppliers product for anothers .The replacement market for s ter i le f i l t rat ion cartr idges is quite large.Micrsf i l t ra t ion car t r idges used as vent air f i l ters may last for months, but thoseused to f i l ter batches of l iquid may have a useful l i fe measured in hours. Thesemembranes may sel l for l i t t le more than f10/f t2 , an order of magni tude belewsome o ther membranes covered in this report . But costs to m an ufa ctu re in thevolumes requ ired by the m arket leave a healthy m argin fo r sel l ing costs , researchand develo pm ent, and profi t . Cash generated by the business, an d the com peti t ionwithin i t , provide a s teady stream of innovation in the industry.A second major application for microfi l ters is in the electronics industry forthe fabrication of semico nduc tors . As semico nduc tor devices shr ink in s ize, thecond uctive p aths on th eir surfaces get closer together . Dirt part icles representpotential sh ort circuits in the sem icondu ctor device. There fore, f i l t rat ion ofvar ious s t reams throughout the manufactur ing process is a vital conce rn. Formicrofi l trat ion companies schooled in the s ter i le f i l t rat ion discipline, theelectronics applications seemed made-to-order. A particularly at tractiveapplication in this industry is final filtration of the water used to rinsesem icondu ctors du rin g fabrication. Most of this water is f irs t t reated by a

reverse osmosis me mb rane. Since this is a much f iner f i l ter than a microf i l ter ,th is water contains only a smal l amount of di r t f rom the piping and equipment .Thus, microfi l ters used in this process have long l ifet imes. An other area in whichmicrofi l trat ion has been applied in the semiconductor industry is in f i l ter ing thegases and liquids used as reactants in making a chip. These chemicals are oftenvery aggressive. an d can no t be prefi l tered by reverse osmosis me mb ranes, so theses t reams are a chal lenge and an oppor tuni ty for microf i lter manufacturers . Th eelectronics industry has proven to be a s trong market for microfi l trat ion, and isnow second only to s ter i l izing f i l trat ion.329


2/14

330 Membrane Separation Systems

In both of the major microfiltration applications, sterile filtration andsemiconductor fabrication, energy considerations are less important than otherissues such as pro du ct quality. Some of the sterile microfiltration applicationsreplace thermal sterilization. In these cases, the re is a direct energy saving inthe process and an indirect saving through avoidance of heat exchange equipment,which has energy-intensive fab ricatio n requirements. Energy is a negligibleconsideration in the electronics applications. Oth er applications formicrofiltration. particularly some of the emerging potential applications, may off ersign ifican t ene rgy advantages ov er alternative methods. Therefore, the emphasisof this report will be less on current dominant applications of microfiltration.where energy is not a significant issue, and more on less developed processapplications.

6.2 DEFINITIONS AND THE ORYMicrofiltration is a process fo r separating material of colloidal size andlarger from true solutions. It is usually practiced using membranes. In thisreport, only microfiltration accomplished by membranes is covered. Microfiltersare typically rated by pore size. and by convention have pore diameters in therange 0.1-10 pm. A photomicrograph of the surface of a typical microfiltrationmem brane is show n in Figure 6-1. A microfiltration membrane is genera lly porousenough to pass molecules which are in true solution even if they are very large.Thus, microfilters can be used to sterilize solutions. because they may be preparedwith pores smaller than 0.3 pm, the diameter of the smallest bacterium,Pseudomonas diminuta.There are several key characteristics necessary for efficient microfiltrationmembranes. These are ( 1 ) pore size uniformity, (2) pore density, and (3 ) thethinness of the active layer or the layer in which the pores are at their minimumdiameter. Th e impact of these parameters on the flow through the mem brane canbe seen by examining the governing equation for flow through the pores of amembrane, Poiseuille's law:

Q/A = [s] nid: (1)where Q/A is the volumetric flow rate per unit membrane area, Ap is the pressuredrop across the membrane, p is the solution viscosity, 6 is the thickness of theactive pore layer, and d. is the diameters of the individual pores in the unitarea A .


3/14

Microfiltration 331

Figure 6- 1. A surface photomicrograph of a typical microfiltration membrane.Membrane shown is Nylon 66 with 0.2 pm pores.


4/14

332 Memb rane Separation Systems

T he im portanc e o f pore s ize uniform ity is eviden t, s ince a mem brane will notrel iably retain anything smal ler than the larges t pore, which determines i t s ra t ing.Smaller pores con tr ib ute far less to f low. Acc ording to Poiseuil le 's law, a pore0.9 t imes as large as the rated pore s ize contr ibutes only two-thirds as muchf low. Pore length may be minimized by making the act ive layer 6 ( in which thepores a re a t the i r min imum diameter ) as thin as possible. T he imp ortance of poredens i ty i s especial ly impor tant in dead-end f i lt ra tion.

Th e most un iform pore sizes are fou nd in membranes made by the t rack-etchprocess, illustrated in Figure 6-2. Track-etched membranes are made by expos inga po lymer shee t to a beam of radiat ion, then select ively etching away the t rackswh ere the polym er was dam aged by the radiation. Photomicrographs of thesemembranes (F igure 6-3) show a uniformity of pore s ize di f f icul t to f ind inmem branes form ed by other techniques . Th e pictures a lso show that the numberof pores per area is low. Track-etched mem branes can not be made wi th highpore densit ies because of the probabil i ty of track intersection, which would resultin pores too large for the rat ing. , Polymer s t rength dictates a minimum f i lmthickness for the membrane which, in the case of the cylindrically shaped poresfou nd in track-etched membranes , governs the pore length.

Membrane pore s ize is rated by, and tested with, latex part icles , bacteria,di rec t microscopic examinat ion, and bubble point. Th e bubble point proceduremeasures the diameter of the larges t pore by forcing ai r through the wet tedme mb rane unti l a bubb le appears . This proced ure is i l lustrated in Figure 6-4.The bubble point i s a funct ion of pore diameter and surface tens ion.Photomicrographs of membranes made by a new technique, the anodicoxidat ion of a luminum, show a membrane s t ructure wi th promise for producingme mb ranes with high densit ies of thin, un iform pores. An example is show n inFigure 6 - 5 . These membranes should be very useful for many low-solids dead-endfil trat ion applications.Microf i l tra t ion mem branes are made in several di f fe rent forms. One of themost common is the pore f i l ter . As shown in Figure 6 - 3 . a photomicrograph ofthis type of membrane looks like a plate with cylindrical holes drilled in it .These f i l ters are usually prepared by the track-etch method, described in moredetai l below. Th ere are , however , many techniques for prepar ing microf i l tra t ionmembranes , and they resul t in phys ical s t ructures qui te di f ferent f rom the porefi l ter . Lik e ultraf i l trat ion and reverse osmosis mem branes, some microfi l trat ionmembranes have conically shaped pores, with the small end of the truncated conefacing the process f luid. In this s tructure, a ny part icle passing through the smallend of the pore encounters a progressively more open path as i t passes throughthe f i l ter . A ll the f i l t rat ion is done at the surface, where the "funnel" isnarrowest . Th is featu re can signif icant ly reduce plugging and enhance masstransfer .Other common ways of preparing microfi l trat ion membranes result instructures tha t resemble porous beds of spheres, s li ts , and f ibrous s tructures. Th ef inal membrane form may be f la t -sheet , ceramic monol i th , tube, capi l lary, or f iber ,and these may be further modified in preparing various forms of modules.


5/14

/ I '\ / I '\-I @TW, I ;II I

I \\ ;;'Tracks" \\ '.J ;Non-conductingmaterial Pores


6/14

334 Memb rane Separation Systems

Figure 6-3. Photomicrograph of a Nuclepore@ membrane made by the track-etch method.


7/14

Microfiltration 335

@ @ER O I N C R EA SI N G BU BBLE@I N 1PRSSURE r u L s s u u c W.PSSURE

Figure 6 - 4 . Procedure used in deter mini ng bubble-point.


8/14

vI


9/14

Microfiltration 337

Microfi l trat ion membranes can be operated in two ways: I ) as a straight-through f i l ter , known as dead-end f i l t ra t ion, or 2) in crossflow mode . In dead -end f i l t ra t ion, a l l of the feed solut ion is forced through the membrane by anap pl ie d pressure. Th is is illustrated in Figure 6-63. Re taine d particles arecollected o n or in the mem brane. Dead-end f i l trat ion requires only the energynecessary to for ce the f luid through the f i l ter . In the s implest applications. alaboratory vacuum or s imple pump provide enough mot ive force to dr ive theap pli ca tio n at an acceptable rate. Th e ideal energy requireme nt. i f rate is notcri t ical . is negligibly low. Th e dead-e nd m icrofi l trat ion me mb rane may be i n oneof m any di ffe re nt forms (f lat-sheet . pleated cartr idge , capillary, tube. etc .)

The second wav to operate microfi l trat ion membranes is in crossflow. I ?this operational mode, shown in Figure 6-6b, the f luid to be f i l tered is pumpedacross the m em brane parallel to i ts surface. Crossflow micro fi l trat ion producestwo solutions; a clear filtrate and a retentate containing most of the retainedpart icles in the solution. By maintaining a high velocity across the membrane.the retain ed ma terial is swep t off the me mb rane surface . Th us. crossflow is usedwhen s ignif icant quant i t ies of material will be retained by the membrane. resultingin plugging and foul ing.

A principal difference in the operation of these two schemes is conversionper pass. o r the am oun t of qolution that passes through the mem bran e. In dea d-end f i l trat ion. essential ly al l of the f luid entering the f i l ter emerges as permeate.so the con vers ion is roughly 100%. all oc cu rrin g in the firs t pass. Fo r acrossf low f i l ter , far more of the feed passes by the membrane than passesthrough i t , and conversion per pass is often less than 20%. Recycle permits theult imate conversion to be much higher, however.

A no the r diff ere nc e between dead-end and crossflow operation is the energyrequired. T he energy requirements of the crossflow method of operation are manytimes higher than those of dead-end f low, because energy is required to pump theflu id across the mem brane su rface. How ever, for high solids applications, and forthose where the solids would normally plug the filter when i t is operating as adead-end f i l ter , crossflow is the method of choice.

6 .3 DESIGN CONSIDERATIONSThe op t imum des ign of a microfi l trat ion membrane system depends o n anumber of parameters and on the characteristics of the feed stream to be treated.Two impor tant des ign cons iderat ions are I ) he choice of operational mode, ei therdead-end or crossflow, and 2) mo dule design. Both of these are discussed below.

6.3.1 Dea d-end vs. Crossflow Op erationOne important characteris t ic of a feed stream is the level of solids thatThe higher the level of solids, the higherust be retained by the microfi l ter .the likelihood that crossflow filtration will be used.Typically, streams containing high loadings of solids ( > O S % ) are processed byme mb rane f i l ters operating in crossflow. Th e operation of microfi l ters incrossflow is s imilar to the operation of ultrafi l ters . Th e ma jor diff ere nc e is in


10/14

338 M e m b r a n e S e p a r a ti o n S y s t e m s

D e a d - e n d f i l t r a t i o nF e e d

P a r t i c l e/ b u i l d - u p onIe*Ab?!&%%4-4 *a6%-@$

Iao ooaqaaooDOe m b r a n e s u r f a c e

P a r t i c l e - f r e e p e r m e a t e

a ) Dead-end f i l t ra t ion

C r o s s - f l o w f i l t r a t i o nI I

F e e d mobo Qa s ooc3Bae e t e n t a t e$Q-s*3Gpsq&gq@&~&Z%&sa@A!3B*9-.P a r t i c l e - f r e e p e r m e a t e8b) Crossflow fi l trat ion

FIgure 6 - 6 Schemat ic representat ions of a ) dead-end and b) crossflowoperation of microfi l trat ion membranes


11/14

Microf i l t ra t ion 339

the behavior of the polarized layer near the membrane. The limit to the rate atwhich a crossflow device produces permeate is paradoxically the rate at whichsolids retained by the membrane can redisperse into the bulk feed flowing pastthe surface. Were it otherwise, the crossflow filter would be acting as a dead-end filter, where the solids simply build up at the filter face. Using the theoryand concepts developed for reverse osmosis and ultrafiltration, the moleculardiffusivity of the retained material is one direct determinate of how fast itdiffuses away from the surface. The colloidal material retained by a microfilterhas even a lower diffusivity than the macrosolutes in ultrafil tration. Theredispersion rate of retained material is thus calculated to be very low. In fact,microfiltration rates are often quite high compared to ultrafiltration, even atlower crossflow velocity. The explanation seems to lie in a shear enhancedparticle diffus ivity which results in dramatic increases in flux.2 The final stateof the solids retained in crossflow filtration differs from conventional dead-endfiltration. Crossflow devices produce a concentrated liquid retentate, not a drycake. As this retentate is recycled, it becomes more concentrated in retainedsolids, the driving force for redispersion of material retained by the membranedeclines, and filtration rates decline. Ultimately, there comes a point where it isnot economical to concentrate further. Depending on the nature and value of thepermeate and the retentate, techniques exist to achieve high recovery of theproducts.

Membrane filters operating on feeds with medium loadings of solids (


12/14

340 Membrane Separation Systems

6.3.2.1 Dead-end fil ter housingsDisk holders represent the simplest membrane filter housing,

and their design has evolved slowly since their introduction in the 1950s. Themembrane is fitted between two plates, a porous one on which the membranefilter is supported, and a feed plate containing a cavity to permit the fluid tocontact the membrane freely. The devices are usually plastic or stainless steel,and the membrane is usually sealed with an O-ring.

Disk holder$

Pleated cart ridees Many membranes are pleated, then formed into acylinder, substantially increasing the membrane area that can be fit into a givenvolume. The devices resemble the familiar automotive air filter. End caps aregenerally attached using curable liquid or melt sealants. Cartridges are thenfitted into housings, either singly or in groups. The housings are simplepressure vessels, although their design may become elaborate.

Dead-end sDiral: Spiral-wound modules are popular crossflow devices, widelyused in reverse osmosis and ultrafiltration. A hybrid crossflow/dead-end fil ter isbeing manufactured for microfiltration using the principle of running a spiral-wound module as a dead-end filter. During initial operation, until significantsolids have built up, most of the feed passes across the membrane, becomingdead-ended only near the outlet of the sealed spiral device. When filled withsolids, the spiral operates totally as a dead-end filter.

Air-Dulsed caDillary A novel system to handle retained solids is employedby Memtec (Australia). Their device, illustrated in Figure 6-7, operates as apulse-cleaned, dead-end and crossflow filter. The feed stream passes along theoutside of microporous capillaries. It quickly builds a layer of retained materialon the surface, acting as a filter-aid formed from retained material. When thelayer has developed enough resistance to impede the filtration unacceptably, thefiltration is stopped, and air is pushed through the inside of the capillaries andthe pores to blow off the filter cake. This backwash frequency is every 10-30minutes, with a duration of 30 seconds. For high solids loadings, Memtec is ableto operate its system in crossflow, since it can run at conversions per pass aslow as 50%.s6.3.2.2 Crossflow devices

For the applications of greatest interest to this study, crossflow devices aredominant. Since they are discussed in more detail in the section onultrafiltration, they are covered only briefly here.

When significant quantities of solids are present, crossflow operation givesthe highest output per unit membrane area. The simplest crossflow device is amembrane formed inside a tube made from a strong, porous material. The feedruns down the inside of the tube, under pressure. Permeate passes through themembrane, then through the porous support.

Another commonly used device is the parallel-plate module, or cassette.Capillaries. membranes spun so that their porous sublayer providesmechanical support against operating pressure, are operated with bore-side feed.By elevating the permeate pressure above the feed pressure periodically, forcing


13/14

Microfiltration 341

OperatingMode

Figure 6-7 . Operating and backwash mode for the Memtec air-pulsed capillarymodule.


14/14

342 Membrane S e p a r a t i o n S y s t e m s

pe r mea t e backwar ds t h r ough t he membr ane , cap i l l a r y membr anes may be c l eanedef fec t ive ly w hi le s t il l runn ing on the p rocess s t ream. By so doing , so l ids bu i l t upon t he mem br ane a r e pus hed back i n t o t he feed . T h i s ope r a t i on is f undamen t a l l yd i f f e r en t than the a i r -purge dead-en d f i l t e r , s ince it re li es on c ross f low to d oalmos t a l l o f the r ed ispers ion of r e ta ined so lids. T he perm eate back-pres su recyc le i s used to r emove smal l quan t i t i es o f fou lan t mater i a l depos i t ed on th em e m b r a n e .

Rever se f low of cap i l l a r i es is a l so usefu l to r emove a par t i a l b lockage of thef low channels . Perm eate be ing forced backw ards in to the cap i ll a ry b ore expandsi t s l ight ly , and also pushes the blocked mater ial back in the di rec t ion f rom whichi t en tered . Rev er se f low is a lso p rac t i ced in cap i ll a ry me mb ranes o n s ome d i r t ys t reams. By revers ing the fee d di rec t ion, mater ia l that accumulates a t or neart he en t r ance to the cap i l l a ry bundle i s swept away f rom the f ace .

As ment ioned under dead-end f low, the a i r -purged cap i l l a ry membraneso f f e r ed by M emt ec may be s e t up t o ope r a t e a t t he l ower end o f c r o s s f l owvelocit ies . Because the fee d is exte rnal to the capi l lar ies , the ef fe ct ive ness oft he hyd r odynami c s weep i ng i s r educed . The use of per iodic ai r -pulse cleaningseems to comp ensa te fo r the r educ ed l eve l o f f low .

6.46.4. I Background

S T A T U S OF T H E M I CR O F I L T R A T IO N I N D U ST R Y

M em brane f i l t e r s can be sa id to have begun wi th Zs igm ondy dur ing Th eGre a t War. Development was very gradua l dur in g the 1920s a nd 1930% an d i tocc urre d pr incipal ly a t Sar tor ius G m b H . At the en d of World War 11, U.S.occupat ion forces in Germany were as s igned to eva lua te German technology and tot r ans fer p romis ing d eve lop men ts to the U.S . Me mb rane t echnology was one of theG e r m a n d e v e l o p m e n t s d e t e r m i n e d to be cri tical , p art icularly fo r i ts usefulness inzssess ing the level of microbial contam inat io n in wa ter suppl ies . A f ter 3 per iodof deve lopment in bo th academic and commerc ia l l abora tor ies , the company tha t i sthe predecessor of Mil l ipore led in the commercial izat ion of microf i l t ra t ionmembranes and suppl ies .6 . 4 . 2 Suppl iers

The microf i l t r a t ion indus t ry f ea tures some la rge companies wi th h igh growthrates , good prof i tabi l i ty , and heal thy balance sheets . Th at general s i tuat io n hasna tura l ly a t t r ac ted a t t en t ion , and newer en t ran t s a re numerous . Tab le 6- 1es t imates the sales a t t r ibutable to microf i l t r a t ion membranes f rom the to ta lr evenues o f these supplie rs . Fo r the es t imate , membranes and mem brane- re la tedha r dwar e have been combi ned . T h e de f i n i ti on of a membrane i s f a i r ly b road ,including polymer ic , ceramic, inorganic o r s in te red meta l dead-end or crossflowdevices tha t make a separa t ion in the 0.1-5 pm range. W ound, s pun - bo nded , andwi re-based f i l t e r s a re exc luded . T he inc lus ion of s in te red meta l , whi le a rb i t r a ry ,does no t in f luence the to ta l numbers s ign i f i can t ly .

Next Page
http://5fc08ad06cc84ee8694201653ba90e1.pdf/http://5fc08ad06cc84ee8694201653ba90e1.pdf/

microfiltration 1

Documents

Transcript of microfiltration 1