Mechanisms governing permeate flux and protein rejection in the microfiltration of beer with a...

Journal of Membrane Science, 84 (1993) 37-51 Elsevier Science Publishers B.V., Amsterdam

37

Mechanisms governing permeate flux and protein rejection in the microfiltration of beer with a Cyclopore membrane

P. BlaIrpain”**, J. Hermia” and M. LenoZlb aDepartment of Chemical Engineering, Universitk Catholique de Louvain, 1, Voie Minckelers, B-1348 Louvain-La-Neuve (Belgium) bTEPRAL, Centre de Recherchegroupe BSN, branche biPre, 68 route d’oberhausbergen, 67200 Strasbourg (France)

(Received November 16,1992; accepted in revised form May 7,1993)

Abstract

Mechanisms which determine permeate flux and protein rejection for the microfiltration of a clarified beer using a Cyclopore membrane have been investigated. Experimental results were analysed in terms of existing membrane filtration model equations and membrane resistance. Flux decay follows the standard blocking model (SBM) assuming that pore volume decreases proportionally to filtrate volume. The actual mechanisms involved in membrane fouling are thought to be related both to the internal and external fouling by macrosolutes. Protein-membrane interactions in the form of protein adsorption produce a protein-membrane “composite”. The build-up of this also called “a secondary dynamic membrane” explains the characteristic rejection profiles.

Key words: beer clarification; microfiltration; Cyclopore membrane; protein rejection; protein-membrane interaction

1. Introduction

Membrane separation processes (reverse os- mosis, ultrafiltration and more recently microfiltration ) are increasingly used in food indus- try in order to substitute for conventional concentration, separation and clarification techniques. Crossflow microfiltration seems to be a promising alternative in beer clarification and sterilization for the traditional processes used in brewery. Some authors [l-3], who have studied the feasibility of tank bottoms and wort microfiltration at pilot scale, concluded that

‘To whom correspondence should be addressed.

membrane processes are economically attrac- tive. At the present time industrial membrane development for beer filtration is limited both by low permeate flux and by protein and aroma compound retention. Curiously, although much research has been performed to remedy these undesirable effects, very little work has been published about the local phenomenology of membrane-solute interactions involved in beer microfiltration. However, only knowledge of the mechanisms limiting permeate flux and governing beer protein selectivity would allow elu- cidation of the appropriate technologies re- quired for improvements in the process. The purpose of this article is the study of mem-

38

brane-solute interaction mechanisms involved in beer microfiltration.

2. Materials and methods

The beer was a commercial “Lager” with less than one yeast cell per millilitre. The experimental set-up for filtration is shown in Fig. 1. All filtration experiments were carried out on a dead-end Sartorius filtration cell with a capacity of 200 ml. The cell diameter was 4.7 cm and the effective membrane area was 12.5 cm2. Stir- ring was provided by a magnetic stirring bar suspended 1.5 mm from the membrane surface. The stirring speed could be varied from 0 to 1000 rpm. The operating pressure was supplied by a compressed carbon dioxide line and was

carbon dioxide I pressurkatiin t---w-

Lager beer

Cyclopore

P. Blanpain et al. /J. Membrane Sci. 84 (1993) 37-51

varied from 10 to 100 kPa. Beer temperature was fixed at 0°C by a double casing around the filtration cell and the feed reservoir. An organic Cyclopore membrane was used: this is a hydrophobic PET (polyethylene-terephtalate) membrane with a mean pore size of 0.2 pm, 20 pm thickness. It is a track-etched membrane which has been developed by the IBA Company (Ion Beam Application) and the Laboratoire des Hauts Polymeres in Louvain-La-Neuve and which is currently produced by Cyclopore S.A. This kind of membrane with well calibrated cy- lindric pores is appropriate to this study. The surface porosity of the membrane was 16%. Operating conditions were 10 or 100 kPa and 850 rpm stirring (the corresponding impeller Reynolds number equals 1800, i.e. the flow is in

- Feed reservoir thermostated

--trl

membr

In:i magnetic stirrer

- -

- - FILTRATION CELL

ane

- el 0

Electronic balance

at 0 OC

Fig. 1. Experimental set-up. Filtration cell.

P. Blanpain et al. /J. Membrane Ski. 84 (1993) 37-51

the laminar range). An electronic balance was used for measuring the flow rate of filtrate and the filtrate volume. The signals from the balance were treated using an IBM/PC micro- computer through an analog/digital interface. Experiments were performed at atmospheric pressure with a beer from which CO, had been removed.

Protein retention was determined using the Bradford method [4] involving protein reac- tion with Coomassie Blue. Indeed it was con- firmed, as Lewis et al. [5] showed, that for the measurement of beer and wort protein concentration, only this method is able to follow the variation of the concentration of high molecular weight protein in the filtrate, the others (Kjeldahl, Biuret, Lowry) being subject to in- terferences with other beer compounds like amino acids, peptides and polyphenols. The amounts of protein deposited on the membrane surface after filtration were determined by removal using sodium dodecyl sulphate and then measured by the modified Lowry method [6]. The amounts of protein present in the sample were calculated with a bovine serum albumin (BSA) standardization curve, as reference.

3. Results and discussion

3.1. Filtration models

Theory A first approach to understand the mecha-

nisms limiting permeate flux is to investigate the fouling processes involved in flux decrease. The use of classical filtration models, proposed by Hermia [ 71 in crossflow microfiltration, has the advantage of a non ambiguous interpreta- tion of often complicated phenomena which limit the filtration of a complex solution like beer; the latter contains molecular and colloidal fractions with large particle size distribu- tion. Solutes in beer having a size much lower than that of membrane pores, flux decay may

39

be discussed in terms of the well-known standard blocking model (SBM) and the cake filtration model (CFM) when the pores are clogged by colloids to such an extent that the pores are smaller than the particles and colloids. The standard model consists in a lateral adsorption of solute resulting in a gradual pore volume decrease, proportional to filtrate volume. The permeate flux decrease can be rep- resented by the following relation:

t/V=l/QO +Kt/2 (1)

where t is filtration time, Q0 initial filtrate volume flow, V filtrate volume and K a constant depending on solution and membrane properties.

In the cake filtration model (CFM) particles and colloids deposit over the membrane surface and the decrease in filtrate volume flow can be described as follows:

t/V= l/Q0 +KV/2 (2)

Experimental results These two models were applied to experi-

ments under two different transmembrane pressures P: one is considered as high in microfiltration (P=lOO kPa) and the other as low

i \K_ $. 8b ,bo ,io

P=lO 2: ;i

FILTRATION TIME (min)

Fig. 2. Variation of permeate flux ( Jf) with time (t) under transmembrane pressures (P) of 10 and 100 kPa. Experi- mental conditions: 850 rpm, T= 0 ’ C.

40

11 ok------------------

90 Standard Blocking Model (SBM)

10 MBan dlmneIer 0 2 m,c,on ThlcXness WB 20 m,crons

SO- h

m E

70.

P ;i; 5 60. vz

= 50 SE.

u 40.

30

20 j IO

0 u, 1 0 1 2 3 4 5 6 7 r-h

FILTRATE VOLUME V (lO-s m3)

Fig. 3. (a) Standard blockingmodel (SBM) under a transmembrane pressure of 10 kPa. Experimental conditions: 850 rpm, T=O”C. (b) Cake filtration law (CFM) under a transmembrane pressure of 10 kPa. Experimental conditions: 850 rpm, T=O”C.

(P= 10 kPa). Figure 2 shows the variation of permeate flux (Jr) with time (t) under transmembrane pressures of 10 and 100 kPa. After an important initial decrease, the two permeate fluxes meet after two hours of filtration. It is interesting to note that the flux levels reached at equilibrium are similar under 10 and 100 kPa, which means that in terms of membrane resistance, the membrane is about tenfold more permeable under 10 kPa. Graphical analyses of these experiments using the standard model (SBM) and the cake model (CFM) under transmembrane pressures of 10 and 100 kPa are


T;WC. 850 ‘pm

0 0 1000 2000 3000 4000 5000 6000 7000

FILTRATION TIME t (s)

50 Cake Filtration Model (CFM)

10 PORE membrane pti,

,c*ness 20 m,<mn5

0 0 ‘2E-05 4E-05 6E-05 6E-05 0.0001 0,c

FILTRATE VOLUME V (m3)

012

Fig. 4. (a) Standard blocking model (SBM) under a transmembrane pressure of 100 kPa. Experimental conditions: 850 rpm, T=O”C. (b) Cake filtration law (CFM) under a transmembrane pressure of 100 kPa. Experimental conditions: 850 rpm, T=O”C.

respectively presented in Figs. 3 (a), 3 (b), and

4(a), 4(b).

Discussion It is observed in the latter figures that the

SBM model represents the data much better than the CFM model. The observed initial dif- ference between the measured filtration rates and those predicted by the standard model (see Figs. 3a and 4a) is due to the build-up of pressure and the start of stirring in the initial stages. The calculated mean volume of deposited sol-

P. Blanpain et al. /J. Membrane Sci. 84 (1993) 37-51 41

utes per permeate volume in the standard model, for various experiments, is equal to 1.3 x 10e4 v/v and 2.7 x 10V5 v/v at 100 kPa and 10 kPa, respectively. Such results indicate a better beer filterability under a low transmembrane pressure. It is suggested that progressive pore plugging arises from high molecular weight macromolecules like beta-glucans (MW ranging from 2 x lo6 to 40x 106) and particularly from proteins which, although they have a lower MW (ranging from 5,000 to more than lOO,OOO), are present in higher concentration. Beta-glucan diameters with MW ranging from 2 x lo6 to 40 x lo6 are respectively 0.019 pm and 0.05 pm [8 1. Considering the adsorption of a beta-glucan monolayer on the internal wall of a 0.2 pm diameter pore, it may be calculated that its section decreases from 36 to 75%, and leads therefore to the strong flux reduction experimentally observed.

3.2. Resistance in series analysis

In order to verify more accurately the limiting mechanisms involved in the standard model, measurements of the various membrane resistances in series were carried out to better understand the part played by internal pore plugging in membrane fouling. Indeed when a colloidal suspension like beer, which contains a large variety of molecules and colloids, is fil- tered with a microporous membrane at constant pressure P, the time-dependent permeate flux decrease may be analysed in terms of resistances in series. These resistances add to the initial membrane resistance. Permeate flux can then be described as follows:

Jf=p/wt) (3)

Total filtration resistance Rt can be expressed by adding up several membrane resistance types: - an external and reversible membrane resistance R,, which is due to the concentration po-

larization of macromolecules and colloid deposition on the membrane surface. - an irreversible deposit resistance Rd due to colloid deposition and adsorption on the membrane surface. - the membrane resistance R, including both the clean membrane resistance and the internal plugging due to colloid deposition on the pore walls. Thus Rt can be described as follows:

R, =Ref+Rd +R,

The value of these various resistances can be measured experimentally as follows: - R,,+ Rd+ R, with the permeate flux at the end of the run. - Rd + R, by water flux measurement in situ at the end of the run in dead-end mode. - R, by mechanical removal of the deposit on the membrane surface (by gentle cleaning with a sponge) and water flux measurement.

Experimental results The course of various fouling resistance per-

centages, with respect to total resistance R, with time, is shown in Fig. 5 under a transmembrane pressure of 100 kPa. It is noted that a relatively large part of the resistance is due to external fouling (R,,) which is always higher than 70%. The relative proportion of irreversible deposit resistance Rd on the membrane surface is significant and even exceeds R, after a filtration time longer than 30 min. Figure 6 shows the course of various absolute fouling resistance values with time.

Discussion Stabilization of R, (see Fig. 6) is relevant to

the formation of a macromolecule monolayer on the internal pore wall, the saturation of the internal surface of the membrane ending at a filtration time of about 40 min. The deposit resistance Rd reaches a value which is about


UON TIME

i

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

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . LAGER BEER

. ..____.__ . .._.. CKLOPORE membrane (PET) Mean pore diameter 0.2 micron

__..____ Thickness 20 microns T=O”C, 850 rpm

. . . . ._

. . . --.- ....

Fig. 5. Various fouling resistance percentages with respect to total resistance R, versus time. Experimental conditions: 850 rpm, T=O”C,P=lOO kPa.

threefold higher than the microporous membrane resistance R,. This equilibrium value is probably due to the limitation of the colloidal deposit layer thickness under the effect of shear stress at the membrane wall. The reversible external resistance Ref is dominant and governs the evolution of the total resistance R,. It is dif- ficult to appreciate the actual nature of the external resistance; it may be a polarization concentration formed by macromolecule rejection possibly leading to a gel layer, or it may be formed by a part of the surface deposit which is labile and removable under a simple membrane cleaning with water. These results indicate a disagreement between the resistance in series analysis and the flux expression given by the standard model which is of the type:

4=Pl(P&)

This statement fits with the results obtained by Visvanathan and Ben Aim [9] who measured a low relative internal fouling resistance ( < 10% ) for the microfiltration of a colloidal suspension, although filtration follows the standard model. Resistance analysis shows that solute retention (macromolecules, colloids ) is due to both an internal and an external pore plugging mechanism. The intrinsic properties of colloids present in beer allow them to form a colloidal layer not only inside the microporous membrane, but also over the external surface of the membrane via interfacial phenomena (electrokinetic and adsorption processes ) . These aggregates of colloids can form bridges


E FILTRATION TIME (min) ml

5 d

- RI + Ref *Rd 0 Rm

Fig. 6. Various absolute fouling resistances versus time. Ex- perimental conditions: 850 rpm, T=O”C, P= 100 kPa.

over the pores (for instance by a mechanism of dendrite build-up) and thus reduce the effective pore openings. This bridging of aggregates leads eventually to the formation of a film of colloids on the membrane surface, which is usually called “secondary dynamic membrane”. Some of the results obtained with resistance analysis can be explained by considering the formation of a second membrane: the latter, with its low porosity, reduces the pene- tration and plugging of pores by colloids, and may explain the build-up of a polarization layer resulting from the consecutive rejection of macromolecules. Formation of a second membrane has also been observed by Lee and Mer- son [lo] for the microtiltration of milk proteins on a 0.4 p Nuclepore membrane with a protein concentration similar to that in beer (0.16% ). These authors observed that beta- lactoglobulin and BSA formed an uniform sheet over the membrane surface.

Resistance in series measurements thus sug- gests that the main phenomenon limiting the filtration of beer is the formation of local aggregates of macrosolutes over the pore open-

ings (forming dendrites for instance), which leads to the reduction of the effective passage for the solvent through the pores. Local aggregates over the pore openings would be respon- sible for the extension of the standard model applicability.

3.3. iVembrane-protein interaction

The importance of the membrane surface resistance and the probable existence of a second dynamic membrane enhance the role of phys- ice-chemical membrane-solute interactions in the filtration rate. In beer, the main molecular fraction is composed of proteins whose concentration ranges from 1 to 6 g/l according to the type of beer. Besides, it is known that the protein fraction of the processed suspension is most often the limiting molecular fraction because of its high tendency to interact with microporous media. In order to study the membrane-protein interactions involved in the filtration of beer, the amount of deposited protein was measured under various operating conditions.

Experimental results Figure 7 shows the variation of the quantity

of adsorbed protein (calculated on the basis of the equivalent quantity of BSA) and the membrane resistance R, with time. Adsorption took place under gently stirring the module without pressure so as to reach an adsorption equilibrium. Beer retained inside the microporous structure of the membrane was removed by swilling it with water. It is shown that membrane resistance increases with adsorption time and reaches a limit corresponding to a decrease in permeability of ca. 70% after one hour. Con- currently the adsorbed quantity of protein increases with time, as was reported by some authors [ 11,121 for ultrafiltration membranes. The attainment of an equilibrium resistance, concurrent with an increase in the adsorbed quantity of protein, means that protein also interact with the membrane surface between ad-

44 P. Blanpain et al. /J. Membrane Sci. 84 (1993) 37-51

Fig. 7. Adsorption of proteins: membrane resistance R, and mass of adsorbed protein versus time. Experimental conditions: 850 rpm, T=O”C, P=O kPa.

jacent pores (the solid surface adjacent to pores represents in our case more than 80% of the filter area). Indeed macrosolutes, such as proteins, can bind to polymer surfaces by a variety of mechanisms including electrostatic interaction, hydrophobic effects, charge transfer (e.g. hydrogen bonding and aromatic bonding) or through combinations of these. From Fig. 7 we note that the amounts of deposited protein on a Cyclopore microfiltration membrane are substantial and similar to those observed by Fane et al. [ 131 for the ultrafiltration of a BSA solution (they calculated adsorbed quantities ranging from 20 to 60 &cm”). Assuming proteins have a specific mass of 1.0 g/cm3 and a diameter of about 70 A, the adsorbed layer is up to 0.2 pm thick, which is equivalent to about 30 monolayers. Figure 8 shows the variation of the internal and external deposited amount of proteins under transmembrane pressures of 10 and 100 kPa. The amount of deposited protein over the surface and in the pores of the membrane was measured after swilling of the membrane. The quantity retained in pores was determined after cleaning of the membrane surface with a sponge, which removed the surface deposit without disturbing the internal membrane

s ol ~~~_~ .~ A 0 20 47~ 60 80 -100 120 140 ‘--Go 160

FILTRATION TIME (mid

Fig. 8. Mass of deposited protein at the surface of the membrane (external) and inside the pores of the membrane (internal) versus time. Experimental conditions: 850 rpm, T=O”C.

fouling. The amounts of deposited protein on the surface were calculated by substracting the two values. In Fig. 7, the deposit of proteins results from the combined (simultaneous) influ- ences of adsorption and convection.

Discussion It is shown in Fig. 8 that the amount of de-

posited protein in the pores reach a similar pla- teau under 10 and 100 kPa (about 15 K/cm2), which may be interpreted as the end point in the lateral adsorption of a protein monolayer. Adsorption is lower under low pressure. A lower deposited amount of proteins under low pressure is compatible with a lower value of the deposited solute volume per permeate volume (calculated previously with of the standard law), and supports the results of Visvanathan and Ben Aim [9], who observed that internal fouling by colloids increases with the applied pressure. Using the recorded membrane porosity and assuming proteins have a specific mass of 1.0 g/cm3, it may be calculated that the thickness of the deposited protein layer on the pore walls is equal to 23 A; this value corre- sponds to a protein monolayer deposition. From the Hagen-Poiseuille equation, such a pore area


constriction leads to a membrane permeability decrease of only 8% and hence cannot be re- sponsible for the significant permeate flux decrease; the actual decrease is likely to be due to combined effects of a lateral adsorption of various types of solutes (for instance beta-glucans) and especially to the presence of fouling on the membrane surface. In Fig. 8 it can be seen that the amount of deposited protein on the membrane surface is far higher under high pressure; in this case the surface deposit exceeds the internal deposits collected in the pores beyond a filtration time of two hours. Con- sidering the fact that permeate flux is related to the quantity of protein deposited on the membrane surface, this result explains the enhanced membrane resistance observed under the high applied pressure (see Fig. 2). It follows thus that a high transmembrane pressure con- tributes to the deposition of proteins both inside the pores and on the membrane surface. This phenomenon may be explained by considering the denaturing effect of pore wall shear stresses z,, which are high under transmembrane pressure, on proteins. Assuming that the membrane can be considered as a bundle of cylindrical capillaries, the wall shear stress in the pore is: r,., = 4Mf/ (E/r), where E is the surface porosity of the membrane and r the pore radius.

Indeed for a microfiltration membrane which partially rejects macromolecules, initial permeate flux is high and proteins passing through the pores are subjected to significant shear stresses (typically z, is up to 80 Pa under 100 kPa, and 10 Pa under 10 kPa). As a comparison, the shear force in a concentric viscometer, which is the most commonly used apparatus for generating high shear forces, reaches values up to 25 Pa. Franken et al. [14], who studied the role played by protein conformation on their rejection by a microfiltration Nuclepore membrane, considered that the effect of shear stresses on the protein molecules might be suf-

ficient to expose hydrophobic groups, which have a higher tendency to adsorb onto membranes.

3.4. Role of membrane-solute interactions in protein rejection

It has been seen that during the filtration of beer a protein layer is deposited on the surface of the membrane and on the wall of the pores. This deposited layer is non-labile and can only be removed using a membrane cleaning agent. These observations point to considerable membrane-protein interactions capable of al- tering significantly the membrane selectivity. Indeed Fane et al. [13] reported that membrane-protein interactions are often responsi- ble for an alteration of membrane rejection with time, even if the pores are relatively large. Hence it appeared that it would be interesting to analyse the role played by membrane-protein interactions on membrane selectivity in relation with beer proteins. To this purpose, permeate flux and protein retention measurements were performed with membranes whose “history” is different (e.g. a virgin membrane, an adsorbed membrane, a membrane whose deposit layer was removed by cleaning with a sponge and a membrane whose surface was swilled with water).

Experimental results Figure 9 shows flux (Jf) and protein reten-

tion (R, R= 1 - C,/C,, with C, permeate protein concentration and C, retentate protein concentration) for a new membrane under a transmembrane pressure of 30 kPa. This mod- erate value of P was chosen to restrict concentration polarization phenomena which would add to the selectivity of the microporous media. Figure 10 shows flux and rejection “histories” for a membrane whose deposit at the surface was removed without disturbing internal fouling. With pores whose internal surface area is saturated, rejection at the beginning of the run


0.05

-bO I”0

FILTRATE VOLUME V (ml)

Fig. 9. Permeate flux (Jr) and retention (R) versus filtrate volume (V) for a new membrane. Experimental conditions: 850 rpm, T=O”C, P=30 kPa, initial water flux of the membrane JWi = 4500 l/hr-m’.

0 5 10 15 20 --- 25 30 i5 4,” FILTRATE VOI LIME V (ml)

Fig. 10. Permeate flux (Jr) and retention (R) versus filtrate volume ( V) for a membrane whose surface deposit has been removed. Experimental conditions: 850 rpm, T=O”C, P=30 kPa, initial water flux of the membrane Jti = 630 l/hr-m’.

is lower than with a new membrane (see Fig. 9), flux decay is small and subsequently rejection behaves in a similar manner to that of the new membrane, except that occurrence of steady-state rejection (Rz0.3, see Fig. 10) begins at a lower filtrate volume (the internal surface area of the pores being initially saturated, the second membrane builds up more rapidly). Figures 11 and 12 show flux and re-

jection “histories” for respectively and adsorbed membrane and a membrane whose surface was swilled with water. Swilling enables reversible deposit to be removed without disturbing irreversible internal and external fouling.

T

I

4c

0.4

035

01 g

005

0 /

Fig. 11. Permeate flux (Jf) and retention (R) versus filtrate volume (V) for an adsorbed membrane. Experimen- tal conditions: 850 rpm, T= 0 ’ C, P = 30 kPa, initial water flux of the membrane Jwi= 1500 l/hr-m’.

704

Fig. 12. Permeate flux (Jf) and retention (R) versus filtrate volume (V) for a water-swilled membrane. Experi- mental conditions: 850 rpm, T=O”C, P=30 kPa, initial water flux of the membrane J,.,i = 500 l/hr-m’.


Discussion We see in Fig. 9 that the rejection is initially

high, which is believed to be a manifestation of a significant initial protein adsorption [ 131 on the wall of the pores (a microfiltration membrane has an enormous internal surface area, far much larger than its filter area). In the same filtration time the progressive reduction of pore volume leads to a rapid fall of the permeate flux. Then the internal surface area of the pores be- comes saturated and permeate flux begins to stabilize whereas the retention is at its lower value (R=O.l, see Fig. 9) because of the far larger diameter of the pores in comparison with protein size. Then rejection increases and sta- bilizes at a value close to 0.3. This last step can be explained by the progressive deposition of colloidal components consecutively to pore plugging, thus leading to the formation of a low porosity colloidal film (“second membrane”) which then governs rejection. A membrane it- self with pore diameters one magnitude larger than the protein size cannot produce such high retentions. Only the build-up of a secondary membrane, supported by the existence of a protein layer at the surface of the membrane (see Fig. 8) may actually explain these results. Un- der the effect of shear forces generated by stirring, secondary membrane thickness is re- stricted, this results in the observed rejection stabilization. The absence of a high retention due to initial adsorption inside the pores is clearly shown in Figs. 11 and 12, and the occurrence of a steady-state rejection (R = 0.3) is all the faster as the colloidal film build-up is significant. So, at the early stages of the filtration in Fig. 12, rejection is equal to its steady-state value, where it is achieved for the adsorbed membrane (Fig. 11) at a filtrate volume of 25 ml. It is of interest to compare Figs. 10 and 12: the absence or presence of a second membrane, whilst having only a small proportional effect on the level of permeate flux, does have a strong influence on variations in membrane selectivity.

41

3.5. Transmembrane pressure effect on protein rejection

The foregoing results have demonstrated the importance of physico-chemical interactions between proteins and microporous media in the build-up of a secondary dynamic membrane and the effect of the latter on membrane selectivity. From measurements of the amounts of deposited protein inside the pores and onto the membrane surface (see Fig. 8)) it has been shown that transmembrane pressure played a significant role in the adsorption mechanisms. In this section we will verify the close link between protein retention and applied transmembrane pressure.

Experimental results Figure 13 shows the variation of rejection

with filtrate volume V under transmembrane pressures of 10 and 100 kPa. For a volume less

0.8

0.6

0.4

0.:

0.1

‘A Thickness 20 microns T.0”C. 850 rpm

I / P=lOO kPa

I

a- -P=lO kPa


Fig. 13. Retention (R) versus filtrate volume (V) under transmembrane pressures of 10 kPa and 100 kPa. Experi- mental conditions: 850 rpm, 2’~ 0” C.

48

than 50 ml, rejections are similar, but rejection increases abruptly under 100 kPa at a filtrate volume of 100 ml to reach a steady-state value of 0.7. To confirm the strong influence of transmembrane pressure on protein rejection, two runs were performed with pressure steps during the experiment. Figure 14 shows the variation of rejection consecutively to a pressure step from 10 kPa to 100 kPa and Fig. 15 consecutively to a pressure step from 100 kPa to 10 kPa.

o.5T---^------

0.45 ’

0.4 z I

= 0.35

p 03 . 1 ’

=I L 0.25-

cc z 0.24

F

0

g

0.15 I 0.1

zx--- I 40 50 60 %- --z -iI0


Fig. 14. Retention (R) versus filtrate volume (V) consecutively to a pressure step. Experimental conditions: 850 rpm, T=O”C, pressure step of type 10 ----f 100 kPa.

z 0.6 [r

p 0.5

5 L 0.4 (r.

z if 0.3

B a 0.2

0.1

k 0

0

_J= k- 20 40 60 60 loo 120 140 160 IA0


Fig. 15. Retention (R) versus filtrate volume (V) consecutively to a pressure step. Experimental conditions: 850 rpm, T= 0 o C, pressure step of type 100 -- --) 10 kPa.


The measurements clearly verify the close link between a high applied pressure and enhanced rejection. Figure 14 shows that this mechanism is reversible. This effect may be attributed to the reversible compression-relaxation mechanism with applied pressure, which determines the porosity and hence the selectivity of the second membrane.

Discussion This result supports those obtained by Vis-

vanathan and Ben Aim [9] who observed that retention of colloids increases with the applied pressure in the microfiltration of a colloidal suspension. The achievement of so high a retention cannot be solely due to the intrinsic characteristics of the microporous membrane, but must be closely related to the observed protein deposition enhancement under 100 kPa (see Fig. 8). It is considered that membrane selectivity may be related to the amounts of deposited protein onto the membrane. Thus the observed high rejection is considered to be due to the effect of a second membrane whose selectivity is more pronounced under a high applied pressure (substantial amounts of deposited protein, low porosity of the membrane- protein “composite”, considering its probable compressibility). Indeed it is recognized that a high transmembrane pressure, by increasing the solvent driving force to the membrane, enhances the protein concentration at the surface of the membrane and thus protein adsorption.

3.6. Rejection mechanisms

Theory It has been shown that clarified beer micro-

filtration could be predicted by the standard blocking model (SBM). A pore restriction model presents the advantage of considering the membrane-deposit interaction, which makes it possible to connect the resistance model with retention models. The latter are dependent on


the Ferry equation [ 151 whose physical inter- pretation is clear. It is generally considered in crossflow microfiltration that diffusion can be neglected in comparison with convection as a solute transport mechanism. Solute transfer can be considered as being governed by a sieve mechanism, i.e. only the particles whose characteristic diameter is larger than the pore diameter are rejected, the others passing with the same velocity as the solvent. It is considered that molecules are carried away through the pores by the flow of solvent, but the centre of each molecule with radius a cannot approach the pore wall by less than a length equal to a. The resulting variation, first derived by Ferry [ 151, facilitates the calculation of the steric re-

jection R for a molecule of radius a by a pore of radius r:

R=l-C,/C,= [&S-2)]’ (4)

where &a/r ratio of solute radius to pore radius.

We note that according to eqn. (4) rejection must be independent of operating conditions, the microporous media being considered as in- ert. This equation may be readily applied in our case to the standard law, not considering the single pore diameter but the diameter of the pore minus the thickness of the adsorbed solutes on the pore wall, as reported by Zeman [ 161. For cylindrical pores, irreversible adsorption and laminar liquid flow conforming to the Poiseuille equation, the relative thickness of the adsorbed layer in the pore can be calculated from flux measurements:

Ar/r= 1- (JflJo)“.25 (5)

where Jo is the initial flux of the virgin membrane. It can be stated that

&=u/(r-Ar) (6)

Retention R is thus described as follows:

R=l-C,,/C,= [S’(S’-2)12 (7)

49

Experimental results Experimental and predicted retentions were

compared using a mean protein radius a of 50 A. Pore membrane radius r was fixed to the mean pore radius, i.e. 0.1 pm. Relations (5)) (6) and ( 7 ) were applied for runs performed under transmembrane pressures of 10 and 100 kPa (see respectively Figs. 16 and 17). We see that relations (5), (6) and (7) are able to predict with a good precision experimental retentions under 10 kPa as well as at the start of the run

0.014 I c 0 10 al 30 40 50 60 70 80 90 100 1


0

Fig. 16. Experimental and calculated retention (R) versus filtrate volume ( V). Experimental conditions: 850 rpm, T=O”C. P= 10 kPa.

5 0.4 i I


Fig. 17. Experimental and calculated retention (R) versus filtrate volume ( V). Experimental conditions: 850 rpm, T=O”C, P= 100 kPa.


under 100 kPa. By contrast theoretical retentions do not predict the sudden increase of rejection under 100 kPa for a filtrate volume larger than 100 ml (see Fig. 17 ) .

Discussion Solute radius a was chosen considering the

Stokes-Einstein radius of proteins whose MW ranges from 5,000 to more than 100,000, which is representative of the MW variation of beer proteins. As Le Maire et al. [ 171 reported, the corresponding Stokes-Einstein radius ranges from 30 to 80 A. For filtration under low pressure, the membrane acts simply as a sieve, rejection being governed by a sieve-mechanism, and membrane-solute interactions are limited. Under a high applied pressure, protein rejection can no longer be described considering a simple pore diameter restriction: in such operating conditions, the enhancement of adsorption phenomena and the build-up of a protein layer at the membrane surface likely offer a se- lective barrier to proteins in the shape of a second membrane which then governs rejection properties of the membrane.

4. Conclusions

The standard blocking model that considers an irreversible pore diameter restriction proportional to the filtrate volume fits the fouling of a Cyclopore microfiltration membrane by a Lager beer.

Analysing the various membrane resistances in series, by determining separately plugged membrane resistance, adsorbed surface resistance and external reversible resistance showed that more than 70% of the total resistance was external and reversible.

The measurement of the total adsorbed protein mass showed that a protein layer is deposited both on the surface of the membrane and on the wall of the pores.

Investigating the effect of transmembrane

pressure showed that at 100 kPa the kinetics of build-up of any internal adsorbed protein layer was faster than at 10 kPa, although the final levels were similar. It is stated that a high pressure enhances external protein adsorption processes. This faster adsorption rate may be due to protein denaturation effects under high wall shear stresses in the pores.

Membrane-protein interactions play a significant part in the determination of the flux- rejection characteristics of the membrane. Ini- tial build-up of a protein layer inside the membrane starts to increase rejection slightly. Then a partially rejecting membrane forms a second composite membrane with even higher rejection. These results are explained by a simple rejection theory.

From these studies it emerges that a means to struggle against these undesirable effects (long term loss in throughput capacity due to membrane degradation, alteration of filtrate quality with time) might be to control, at the early stages of the filtration, the operating conditions (mainly a low transmembrane pressure) such that only a limited membrane-protein interaction occurs.

It appears that a promising way to an industrial application of beer processing with membranes is to develop the manufacturing of chemically modified microporous membranes, in order to eliminate or reduce their ability to interact with compounds such as proteins.

Acknowledgements

This work has been supported by TEPBAL Research Centre, BSN. The authors wish to thank the Cyclopore Company who supplied the membranes.

List of symbols

a solute radius (pm )


co, cp D Jf N P 80

;1 Re

protein concentration in feed, in permeate (kg-mT3) impeller diameter (m) permeate flux (1-hr-l-m-‘) rotational speed (set- ’ ) transmembrane pressure (Pa) initial filtrate volume flow ( m3-sec-1) pore radius (pm) ( = 1 -C&T,) protein retention coef- ficient (-) ( = D2Np/p) impeller Reynolds number (-)

Rd

R,f

irreversible external fouling resistance (m-l) reversible external fouling resistance (m-l)

R, Rt t V

6

filter media resistance (m- ’ )

total resistance ( m- ’ ) time (set) filtrate volume ( m3) ( =u/r) ratio of solute radius to pore radius (-) dynamic viscosity of beer (kg-m-‘- set-‘) fluid density ( kg-mM3) shear stress at pore wall (Pa)

References

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