Fouling in crossflow ultra- and micro-filtration

Feature l e s s e n s u n t i l a i lear e q u i l i b r i u m filtration rate: is o b t a i n e d . Here the s h e a r i n g a c l i o n g e n e r a t e d b y the crossflow s t r e a m p r e v e n t s f u r t h e r significant d e p o s i t i o n at the fouling layer or m e m b r a n e surfaces.

Fouling in crossflow ultra- and rmcrofiltration Here P r o f e s s o r R i c h a r d W a k e m a n of t h e U n i v e r s i t y of L o u g h b o r o u g h , UK g i v e s an o v e r v i e w of c u r r e n t t h i n k i n g on t h e s u b j e c t of fouling.

With s o m e ])rocess s t r e a m s the p e r m e a t e flux c a n be s t a b l e for m o n t h s or even years. In most application,~, however, t h e r e is a g r a d u a l llux d e c l i n e with o p e r a t i n g lime, a g a i n s t w h i c h p r e v e n l i v e ~ e a s u r e s are often taken. Prefilters or s c r e e n s are u s e d to remow~ large p a r t i c l e s which might o t h e r w i s e block t h i n c h a n n e l s or a c c u m u l a t e in s l a g n a n l a r e a s of the m o d u l e ; all:hough p r o t e c t i o n of the m o d u l e is prox-ided, l b u l m g or cake f o r m a t i o n on the m e m b r a n e c a u s e d by the fine particles in the feed is not p r e v e n t e d . High crossflow velocities t e n d Io r e d u c e d e p o s i t i o n , a n d low pz-essures help to avoid c o m p a c t i o n ol gels at the m e m b r a n e :surl;ace. S o m e p o l y m e r m e m b r a n e s have a higher s u s c e p t i b i l i l y lo louling, a n d c h e m i c a l m o d i f i c a t i o n of a n ullrafiltration m e m b r a n e surface c a n have a great effect on its p r o p e n s i t y To lou]. If fouling does occur, the n l e m b r a n e c a n s o m e l i m e s be c l e a n e d b y aggressive c l e a n i n g a g e n t s . However, even with r e p e a t e d or periodic c l e a n i n g the flux c a n n o l a l w a y s be r<~stored to its initial value.

The precise m e c h a n i s m s of fouling are several, b u t c a n be related to the type of dispersoid in the feed (table 1). There is n o distinct b o u n d a r y that s e p a r a t e s one dispersoid from its n e i g h b o u r s in the table, a n d it follows t h a t the m e c h a n i s m of t o u l i n g is n o t u n i q u e to a n y one fluid lype n o r to a n y one dispersoid, For example, s u r f a c t a n t d i s p e r s i o n s at h i g h e r c o n c e n t r a t i o n s m a y well c o n t a i n b o l h micelles a n d m o n o m e r : the mieelles m a y b e h a v e as colloids or in s o m e c a s e s as quite large particles a n d be s e p a r a t e d p r i m a r i l y by a sieving process, w h e r e a s the m o n o m e r will have m o l e c u l a r c h a r a c t e r i s t i c s a n d be s e p a r a t e d by a d s o r p t i v e processes. At the s t a r t of filtration t h e r e is a s h a r p fall in p e r m e a t e flux from the "clean water" value. Rapid fouling of the m e m b r a n e d u r i n g this period is a t t r i b u t a b l e to the a c c u m u l a t i o n of solute or particles at or n e a r the filtering s u r f a c e (i.e. d u e to c o n c e n t r a t i o n p o l a r i s a t i o n effects, or to cake formation), the p a r t i c u l a t e s b e i n g d r a w n toward the filtering s u r f a c e by the convective flow of p e r m e a t e t h r o u g h the s e p t u m . After the rapid initial decay the rale of flux decline progressively

Feed type Solutions~ Dispersions~ II SuspensionsI

Dispersoid Molecules Macromolecules Colloids Particles

Filtering solutions and dispersions To filter s o l u t i o n s a n d d i s p e r s i o n s UF is a f r e q u e n l choice of m e m r a n e s e p a r a t i o n process. In t h e s e o p e r a t i o n s high crossflow velocilies g e n e r a l l y r e d u c e fouling effects. T h e s e m a n i f e s t t h e m e l v e s a s a n i n c r e a s e in p e r m e a t e flUX, b u t enerKv c o n s u m p t i o n increases simultaneously (suggesting tha~ costs r e q u i r e m i n i m i s i n g as the crossflow velocity (or feed flow rate) is increased). A h i g h e r feed s o l u t e c o n c e n t r a t i o n c a u s e s the p e r m e a t e flux to decline m o r e rapidly a n d often to a lower s t e a d y state fll_lX. O p e r a t i n g a UF or MF s y s t e m with a feed sohlte c o n c e n t r a t i o n above tile so-called gel c o n c e n t r a t i o n will n o r m a l l y r e s u l t in a p e r m e a t e flux b e i n g o b t a i n e d , b u t its v a l u e m a y be u n e c o n omically low. At p r e s s u r e s above a critical value the flux i n UF p r o c e s s e s m a y n o t i n c r e a s e l i n e a r l y with p r e s s u r e (Fi~ure 1). T h e flux of even m o d e r a t e l y c o n c e n t r a t e d s o l u t i o n s (e.~. !.% dextran} is far lower t h a n that of a p u r e solvent. The p e r m e a t e flux of a p u r e solvent i n c r e a s e s l i n e a r l y with p r e s s u r e w h e n t h e r e is n o solvent-membrane interaction that causes structural changes s u c h as swelling, in the m e m b r a n e . In lhe case of a s o l u t i o n at the s t a r t of filtration the p e r m e a t e flux i n c r e a s e s l i n e a r l y with i n c r e a s i n g p r e s s u r e (OA on Figure I). After a c e r t a i n

General fouling mechanisl Adsorptive t Sieving ~~

Table 1: The main fouling mechanism related to the type of dispersoid.

M e m b r a n e T e c h n o l o g y No. 70

5

Feature

p r e s s u r e , it is possible t h a t there will be no further i n c r e a s e in flux (BC), d u e to either c o n c e n t r a t i o n polarization (a fluid p h a s e m a s s t r a n s f e r effect t h a t c a u s e s build up of rejected species close to the m e m b r a n e surface) or gel formation at the m e m b r a n e surface. The effective driving force is actually the applied p r e s s u r e less the osmotic p r e s s u r e d u e to p r e s e n c e of the solute species. a n d w h e n gel (or cake - noting t h a t a cake is not s y n o n y m o u s with a gel) formation occurs there is a d e c r e a s e in the h y d r a u l i c permeability. In extreme cases the gel layer c a n be c o m p r e s s e d by the i n c r e a s i n g p r e s s u r e , leading to a r e d u c t i o n of p e r m e a t e flux (ODE on Figure 1). Adsorptive effects c a n be i m p o r t a n t in UF. Of the c o n t r i b u t i n g factors, charged a n d polar i n t e r a c t i o n s s h o u l d be c o n s i d e r e d at the f u n d a m e n t a l level. At the p r a c t i c a l level, the m o s t i m p o r t a n t factors are membrane hydrophobicity and hydrophilicity a n d solution pH. Solution pH affects the charge a n d solubility of m a n y solutes. At the isoelectric p o i n t (IEP) s o l u t e - s o l u t e r e p u l s i o n is zero, aggregation occurs, a n d deposition of solute at the m e m b r a n e s u r f a c e b e c o m e s more d e p e n d e n t on m o d u l e hydrodynamics, a l t h o u g h s t r u c t u r a l

Table 2: Influence of parameters during filtration of solutions and dispersions. Property

Comment

Crossflow velocity

An increase in crossflow velocity usually causes an increase in permeate flux, but within the normal range of crossflow velocities its effect on rejection is minimal.

Solute concentration in feed

Increasing the solute concentration in feed causes more rapid flux decline, and lower steady state fluxes.

Filtration pressure

Effect of increasing transmembrane pressure (TMP) depends on properties of any fouling layer or gel that is present: the gel is usually compressible and any increase in flux is not in proportion to the increase in TMP; if gel is highly compressible to flux may be reduced to zero.

pH

More fouling occurs, and hence a lower flux, when the feed is close to the isoelectric point. Structural as well as electrostatic changes may occur in the feed as the pH is varied, which are likely to be system specific.

r e a r r a n g e m e n t s of the molecule m a y also occur. The effects of the m a i n p a r a m e t e r s d u r i n g the crossflow filtration of solutions a n d d i s p e r s i o n s are s u m m a r i s e d in Table 2. The m a g n i t u d e of the change of p e r m e a t e flux or solute rejection b r o u g h t a b o u t by altering a n y one of the p a r a m e t e r s is d e p e n d e n t on a n d specific to the feedm e m b r a n e system.

Solvent

B

High flow rate of feed -C Low flow rate of feed

Gel compaction E 0 Applied Pressure Figure 1 Permeate flux as a function of applied pressure, showing typical ultrafiltration behaviour compared with a pure solvent.

6

Filtering dispersions and suspensions The filtration of s u s p e n s i o n s suggests t h a t p r i m a r i l y p a r t i c u l a t e m a t t e r are to be removed from the liquid; the m a i n process to effect the s e p a r a t i o n would be microfiltration (MF). There is not a clear distinction between UF a n d MF a n d the types of m e m b r a n e s u s e d are b r o a d l y similar. The extent of p a r t i c u l a t e fouling in crossflow microfiltration is related to a m a t r i x of feed stream, m e m b r a n e a n d process p a r a m e t e r s . Among the more i m p o r t a n t of these are the particle size d i s t r i b u t i o n of the feed a n d the m e m b r a n e pore size. The selection of a m e m b r a n e for a given d u t y m u s t be p e r f o r m e d with care; it is p r u d e n t to a t t e m p t to exclude particles from the i n t e r n a l pores of the m e m b r a n e to e n h a n c e flux p e r f o r m a n c e . The general effects of particle a n d pore size are given in Table 3. When filtering a relatively "large" particle size feed s u s p e n s i o n an i n c r e a s e d TMP r e s u l t s in an improved filtration rate. In crossflow filtration the p e r m e a t e flux is rarely p r o p o r t i o n a l to the applied h y d r a u l i c p r e s s u r e gradient; frequently only small i n c r e a s e s in flux are observed for quite

M e m b r a n e T e c h n o l o g y No. 70

Feature M a n y of t h e effects ol c h a n g i n g t h e cr o ssf l o w velocity on filtration flux are directly attribut:able to the particle size a n d size d i s t r i b u t i o n of !he d i s p e r s e d p h a s e V~rhen Iiner s u s p e n s i o n s ar e filtered an e q u i l i b r i u m flux is often e s t a b l i s h e d m o r e rapidly al lower cr o ssf l o w velocities. If t he cr o ssf l o w velocity is r a i s e d the filtration flux c a n i m d e r s o m e p r o c e s s c o n d i t i o n s be s e e n to c o n t i n u a l l y d ecl i n e over a very long period. At lower cr o s s f l ow velocities t h e shq'ar c a u s e d by the c r o s s f l o w i n g s t r e a m is insufficient te o v e r c o m e t h e [orces w h i c h c a u s e p a r t i c l e s to a c c u m u l a t e at the m e m b r a n e . Th e i n c r e a s e d p e r m e a t i o n throu~{h the m e m b r a n e at hit{her c r o s s l l o w s (lor finer d i s p e r s i o n s ) t e n d s to i n f l u e n c e t h e p a r t i c u l a t e s in the flowinf{ s u s p e n s i o n to a g r e a t e r e x t e n t t h a n at low crossflows, a l t h o u g h o l h e r factors s u c h as different c o m b i n a t i o n s of pore plugging/blocking ineehanisms m a y affect the [oulin~ p r o c e s s e s . In UF it is m o r e c o m m o n to find m o l e c u l a r s p e c i e s in s o l u t i o n , a n d t h e rate a n d e x t e n t of tbulin~ is t h e n p r o b a b l y d e t e r m i n e d by m o l e c u l a r art r a c t i o n s b e t w e e n t he m e m b r a n e and the loulant rather t h a n by physiical m e c h a n i s m s . Th e g e n e r a l efleet of i n c r e a s i n ~ t h e solids c o u c e n [ration of the I~ed s u s p e n s i o n is to lower t h e p e r m e a t e flux, a n d

Tcd)lc 3: SI1171,ll(lrl I ,q/ the iqlluence qf particle/pore size Property

Comment At s m a l l e r p a r t i c l e s i z e s filtrate fluxes are lower and an

Particle size

equilibrium permeate flux is established more rapidly. The presence of a small percentage of fines significantly lowers fluxes. At higher crossflow velocities and longer filtration times similar fluxes are often observed for suspensions with differing median sizes.

Size distribution

Influence most pronounced at low crossflow velocity and low concentration where feeds containing the greater proportion of fines give lower filtration rates. At higher crossfiows and concentrations, where the number of particles challenging each pore in unit time is increased, the effects on flux performance are negligible.

Membrane size

Little influence on flux or rejection when the majority of the particles in the feed are significantly larger than the pores in membrane. Filtrate qualilty and flux level are often worse when a significant proportion of the particles in the feed are close to or smaller than the membrane pore size. If the pore sizes in the membrane are much larger than the particles in the feed, fluxes are better but solids rejection may be poor.

pore

slfl)slanliat u l c r e a s e s ill p r e s s u r e , p a r t i c u l a r l y w h e n leeds c o n t a i n hiKher p r o p t , r l i o n s of particle fines. W h e n the p a r t i c l e size of the s u s p e n s i o n is r e d u c e d , t h e r e is a tteneral t e n d e n c y lor the e q l u l i b r i u m Ilux to be e s t a b l i s h e d m o r e rapidly at lower filtration p r e s s u r e s . V~llen tile s u s p e n s i o n pH is s u c h that the zeta p o t e n t i a l a r o u n d t h e p a r l i c l e s is high (i.e. l he s u s p e n s i o n is well d is p e r s e d ) . an i n c r e a s e d TMP often p r o d u c e s a r e d u c e d flux: t h e r e d u c t i o n c a n be Io s u c h an e x t e n t t h a t filtration is e s s e n t i a l l y s l o p p e d afler a short period, a n d r a i s i n g l hc p r e s s u r e f u r t h e r will not re a li s e a llu.x. \Mhen t h e p a r t i c l e s h a v e a platelet or flake s h a p e , for e x n m p l e c h i n a clay, a n i n c r e a s e d p r e s s u r e does n o t a l w a y s i n c r e a s e t h e filtration rate. In c o n t r a s t , for p a r t i c l e s w i t h a 'tmlky' s h a p e an i n c r e a s e in flux level is likely to be o b t a i n e d by ra i sin g t h e fillralion p r e s s u r e . W h e n filtering line p a r t i c l e s u s p e n s i o n s an i n c r e a s e d c r o s s l l o w veIociiy p r o d u c e s a hiKher l]ltralion flux. However, w h e n t h e teed s t r e a m c o n t a i n s a [{rearer p r o p o r t i o n of l a r g e r p a r t i c l e s the filtration r a t e

M e m b r a n e T e c h n o l o g y No. 70

g e n e r a l l y falls with increasing{ crossflow velocity, d e s p i t e a s u b s l a n t i a l t h i n n i n g of t h e fouling layer at t h e h i g h e r crossFlows. For a given s u s p e n s i o n t h e r e m a y exist a critical particle size (and size d is t r ib ut i o n ) w h e r e crossflow velocity h a s little or no eflect on the flux d ecl i n e curve.

105 #

I

--"

[

a~ ,

E

i

,

t.O g litre -I Arosurf TA-IO0 0.2 f.m pore r a t i n g Pressure 2.04 b a r (30 psi)

104

Temperature 60°C -L Crossflow velocity 4.0 m s

10 -~ f

N y l o n 6 6 membrane

.b M

= ~~

..........................

-::- ........

- .....

¢J

102 0

t 500

i 1000

i 1500 Time

.... 2000

i 2500

3000

(seconds)

Figure 2: Fltuc "decline" curves using hydrophilic (Nylon) aad hydrophobic (PTFE) membranes.

7

Feature Table 4: S u m m a r y of the influence of process parameters on MF. Property

Comment

Suspension pH

When the particle sizes in the feed are sufficiently large hydrodynamic forces dominate and suspension pH/ionic strength has negligible effect on flux decline. With smaller particles surface forces are more dominant, and at high zeta potentials fluxes are lower than those at or near the IEP. Differences in flux levels are accentuated at higher feed concentrations. Filtrate quality (and solids retention) are generally improved near the isoelectric pH.

Suspension Concentration

At greater suspension concentrations fluxes are often lower and equilibrium is established more rapidly. The fluxes at longer times are often similar over a range of concentrations, particularly when the feed sizes are smaller and when the feed particle shape is plate-like.

Crossflow velocity

When the proportion of particle fines in the feed is high, an increased crossflow leads to thinner cakes and higher overall fluxes. Flux improvements are greater near the isoelectric pH of the feed, and less at pH's closer to the maximum particle surface charge. A steady state flux is established more rapidly at lower crossflows; filtrate clarity generally improves at higher crossflows. With reduced proportions of fines and larger particle size feeds an increased crossflow produces more fouling and lower flux levels. The size transition between flux "increase or decrease" with crossflow velocity depends on feed concentration.

Filtration pressure

For larger particle size suspensions there is a significant improvement in flux with increased pressure, particularly at lower suspension concentrations. The influence of pressure on flux levels is reduced for feeds with smaller median sizes and higher concentrations and some feeds containing particles of irregular shape.

Particle shape

The influences of irregular particle shape are difficult to quantify or predict, however significant effects on flux decline can result with an adverse shape.

increase the speed of e s t a b l i s h m e n t of a n equilibrium flux; the latter effect is more p r o n o u n c e d at s m a l l e r particle sizes. When the feed s t r e a m is more c o n c e n t r a t e d there is a preference for filtration to occur with particles bridging m e m b r a n e pores r a t h e r t h a n plugging them. If a more dilute s u s p e n s i o n is considered, however, there is a greater likelihood of pore plugging occurring. Crossflow m e m b r a n e filtration s h o u l d be p e r f o r m e d with the p a r t i c l e s in the feed s u s p e n s i o n at or n e a r the point of lowest s u r f a c e charge. S u c h an effect c a n be p r o d u c e d by altering the pH a n d / o r ionic s t r e n g t h of the solution e n v i r o n m e n t s u r r o u n d i n g the particles. The m a i n effects of the process p a r a m e t e r s a n d m e m b r a n e p r o p e r t i e s are s u m m a r i s e d in Tables 4 a n d 5. During the microfiltration of a q u e o u s d i s p e r s i o n s of suspensions using hydrophobic m e m b r a n e s the p e r m e a t e flux rises; w h e n all the m e m b r a n e s u r f a c e s have been completely wetted, flux p e r f o r m a n c e follows the t r e n d s observed with hydrophilic m e m b r a n e s . For particulate suspensions, and at longer filtration times, flux levels

8

Table 5: S u m m a r y of the effects of membrane properties on MF. Property

Comment

Membrane morphology

With small fractions of fines present in a low concentration suspension containing a majority of particles larger than the membrane pore size, significant differences in flux can be observed with different membranes. At higher crossflows and concentrations and smaller particle sizes there is a reduced influence of membrane type on flux. When the majority of particles in the feed have sizes close to the pore sizes in the membrane, little influence of morphology on flux levels is observed.

Membrane wettability

Membranes exhibiting a higher contact angle and greater hydrophobicity produce rising fluxes during the initial periods of microfiltration. At longer filtration times fluxes are similar to those for more hydrophilic membranes.

Membrane surface charge

Only minor differences in flux performance for suspension filtration.

Membrane pore size

Little influence on flux or rejection when the majority of the particles in the feed are significantly larger than the pores in membrane. Filtrate quality and flux levels often worse when a significantly proportion of the particles in the feed are close to or smaller than the membrane pores. If the pore sizes in the membrane are much larger than the particles in the feed stream, fluxes improve to higher levels although solids rejection is poor. Refer to Table 3 for other pore/particle size effects.

are not be adversely affected by m e m b r a n e wettability. MF of a cationic s u r f a c t a n t s u s p e n s i o n s with h y d r o p h o b i c PTFE

m e m b r a n e s shows a rising flux (Figure 2), followed by the flux decline observed with a hydrophilic Nylon 66 m e m b r a n e .

M e m b r a n e T e c h n o l o g y No. 70