Performance of ultrafiltration membranes in ethanol–water solutions: effect of membrane...

Journal of Membrane Science 198 (2002) 75–85

Performance of ultrafiltration membranes in ethanol–watersolutions: effect of membrane conditioning

Rishi Shukla1, Munir Cheryan∗Agricultural Bioprocess Laboratory, Department of Food Science, University of Illinois,

1302 West Pennsylvania Avenue, Urbana, IL 61801, USA

Received 29 May 2001; received in revised form 15 August 2001; accepted 20 August 2001

Abstract

Several commercial polymeric ultrafiltration membranes were screened for their performance with aqueous ethanol solu-tions. The method of conditioning the membrane has a major effect on solvent flux, membrane integrity and their pressureratings. Gradual solvent exchange with successively higher concentrations increased in small doses appears to work best withcompletely miscible solvents such as those studied here (ethanol–water mixtures). Rapid solvent exchange between water andhigh concentrations of alcohol disrupts the polymer matrix in many cases. The Darcy model was used to correlate the data andit indicated that viscosity differences of the ethanol solutions could account for part of the variations in solvent flux with somemembranes. Exposure to organic solvents significantly reduces the pressure rating of the membranes. Several membranesthat provided acceptable rejection of ethanol-soluble proteins at low pressures (138 kPa, 20 psi) lose its properties at higherpressures (413 kPa, 60 psi) if conditioned incorrectly, and vice versa. © 2002 Elsevier Science B.V. All rights reserved.

Keywords:Ultrafiltration; Organic separations; Ethanol; Protein

1. Introduction

Organic solvents are being increasingly used inextraction, purification and processing of pharma-ceuticals, food, nutraceuticals and flavor compounds.Membrane technology would be the method of choicefor separating the desired compounds and for recy-cling the solvent. However, almost all current appli-cations of membrane technology are with aqueoussystems. The few nonaqueous applications discussedin the literature usually deal with streams containingorganic compounds with concentrations only up to a

∗ Corresponding author. Fax:+1-217-244-2455.E-mail address:[email protected] (M. Cheryan).

1 Present address: James R. Randall Research Center, ArcherDaniels Midland Co., Decatur, IL, USA.

few thousand of parts per million such as oily wastestreams and cleaning solvents [1]. There are very fewexamples of membrane applications with feeds con-taining 50–100% organic solvents. In addition, muchof the membrane work to date with organic solventshas been with nanofiltration membranes, e.g. in thevegetable oil industry for degumming, deacidificationand solvent recovery [2–4].

There are fewer reports on the use of ultrafiltration(UF) membranes with relatively high concentrations oforganic solvents. In one of the earliest reports on thissubject, Nguyen et al. [5] observed that, in the absenceof solutes, membrane permeability of several com-mercial UF membranes increased with some solvents(ethanol, methanol) and decreased with others (chlo-roform, decane, benzene). Lencki and Williams [6]studied 10,000 and 30,000 molecular weight cut-off

0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0376-7388(01)00638-X

76 R. Shukla, M. Cheryan / Journal of Membrane Science 198 (2002) 75–85

(MWCO) membranes with methanol, ethanol and ace-tonitrile. Solvents with solubility parameters similarto the membrane reportedly led to the greatest changein flow resistance, but solvents with a similar solubil-ity parameter but low hydrogen bonding capabilitiescould disrupt the structure of anisotropic polysulfonemembranes to such an extent that a dramatic drop inflow resistance is observed. Jaffrin and Charrier [7] ob-served an 80% drop in flux in 40% ethanol comparedto water in the manufacture of plasma proteins fromhuman serum. The reduction in flux was explainedby an increase in viscosity when ethanol was addedto water and the formation of a thicker polarizationgel layer due to lower back diffusion of the solutes in40% ethanol. Gupta et al. [8] reported a 25–35% dropin UF flux when ethanol concentration was increasedfrom 20 to 30% with albumin in mineral (Carbosep)membranes.

Most polymers used in manufacturing membranesand/or their supports are first dissolved in organic sol-vents as part of their casting process. Consequently,they could swell or dissolve in the solvent, leading tounacceptable changes in solvent flux and solute sep-aration. For example, of the 15 membranes screenedby Koseoglu et al. [4] for vegetable oil processing,only 5 were found to be stable to hexane. Raman et al.[9,10] tested 13 membranes for deacidification ofsoybean oil of which only 6 were stable in methanol,and only one was compatible with hexane. Kuk et al.[11] reported significant degradation in membraneperformance on exposure to aqueous and anhydrousethanol solutions with cellulose acetate and compos-ite reverse osmosis (RO) membranes with vegetableoil–solvent mixtures. Koike et al. [12] screened 18membranes (including some developed for gas sep-arations) for separation of fatty acids and glyceridesfrom lipase hydrolysates of high oleic sunflower oil.Cellulosic membranes gave good results but sufferedfrom poor long-term stability. Zwijnenberg et al.[13] used prototype polyamide and cellulose-basedmembranes for deacidification of vegetable oils inacetone.

This paper reports on our studies on polymeric UFmembranes with ethanol–water solutions. The ulti-mate goal was to develop a process for the manufac-ture of zein, which is a hydrophobic, ethanol-solubleprotein in corn with a molecular weight of∼22,000.Zein has a variety of industrial uses, from fibers and

adhesives to chewing gums and biodegradable plas-tics [14]. The commercial application of this naturalpolymer is limited by its high manufacturing cost, dueprimarily to the high cost of separating and purifyingthe zein from the ethanol extract and recovering theethanol solvent. Ultrafiltration could be used readily torecover and purify the zein while simultaneously recy-cling the ethanol solvent [15]. The best solvent for ex-tracting zein from corn is 70% (v/v) ethanol [16] andthus our studies were limited to a maximum ethanolconcentration of 70% (v/v). This paper specifically fo-cuses on the effect of conditioning on solvent flux andrejection of zein.

2. Materials and methods

2.1. Membrane screening

Ultrafiltration membranes were obtained from sev-eral manufacturers and are listed in Table 1. Flat sheetswere evaluated in an Amicon dead-end stirred cell(model 502) which used 62 mm membrane discs ofarea 28.7 cm2. Pressure was provided by a nitrogengas cylinder and turbulence was created by a magneticstirrer which was operated at 300 rpm. The hollowfibers were tested as-is in their housings in a recyclesystem. A pump provided pressure and the requiredcross-flow for turbulence. All experiments were con-ducted at room temperature (24± 2◦C).

2.2. Ethanol solutions

Ethanol (anhydrous, 200 proof) was obtained fromMcCormick Distillation Co., Weston, MO. Aqueoussolutions of ethanol (EtOH) were prepared on a vol-ume/volume basis. Deionized water was used for allexperiments. Ethanol and deionized water were micro-filtered through a 0.2�m filter before use. Viscosity ofethanol solutions (with and without protein) were mea-sured using Ubbelohde viscometers (model OC withan instrument coefficient of 0.002752 cSt/s and model1 with a coefficient of 0.00963 cSt/s) obtained fromCannon Instrument Co., State College, PA. Densitiesof the solutions were measured using precalibrated py-cnometers. Viscosity measurements were performedin triplicate while density data are the mean of fiveobservations.

R. Shukla, M. Cheryan / Journal of Membrane Science 198 (2002) 75–85 77

Table 1Ultrafiltration membranes selected for screening studies

Materiala Membrane MWCOb Manufacturerc Configurationd

Cellulose ester Type C 10000 Spectrum-Microgon FSCellulose acetate Cell 10000 Pall Filtron FSRegen. cellulose PLGC 10000 Millipore FSRegen. cellulose YM10 10000 Millipore FSCompositee U20S 20000 Koch FSCompositee G80 5000 Osmonics FSCompositee H051 – Osmonics FSPAN-m MX25 25000 Osmonics FSPAN-based U20T 20000 Koch FSPES-m Alpha 10000 Pall Filtron FSPES-m Omega 10000 Pall Filtron FSPES UFC 10000 Hoechst FSPES PES4H 10000 Hoechst FSPS UFP10 10000 A/G Technology HFPS PS10 10000 Sartorius FSPS PM10 10000 Koch HFPS PM30 30000 Millipore FSPVDF AN09 25000 Osmonics FS

a PAN: polyacrylonitrile, PES: polyethersulfone, PS: polysulfone, PVDF: polyvinylidine fluoride, Regen.: regenerated, m: modified.b Molecular weight cut-off from manufacturers’ specifications.c A/G Technology, Needham, MA; Hoechst, Wiesbaden, Germany (through US Tech., Cincinnati, OH); Koch Membrane Systems,

Wilmington, MA; Millipore, Bedford, MA; Osmonics, Minnetonka, MN; Spectrum-Microgon, Laguna Hills, CA; Pall Filtron, Northborough,MA; Sartorius, Edgewood, NY.

d FS: flat sheet, HF: hollow fiber.e Composition is proprietary.

2.3. Membrane conditioning

Most membranes (except Koch U20S) were re-ceived from the manufacturer preserved with glycerolor a similar humectant. Prior to use, the membraneswere conditioned by gradual exposure to solventsusing the following protocol:

1. In each case, the membrane was thoroughly rinsedwith large amounts of the solvent that the mem-brane first came in contact with (depending on themethod of conditioning as discussed below).

2. The membranes were then soaked in the solventfor a period of at least 6 h (overnight in most cases)and then fitted in to the appropriate membrane testcell. The Amicon cell was then filled with 200 mlof the conditioning solvent (for the hollow fibers,the solvent was placed in a separate feed tank con-nected to the module via a pump).

3. The system was pressurized to 69 kPa (10 psi) in allcases except with the U20S and G80 membraneswhich were pressurized to 138 kPa (20 psi), and

the hollow fibers which were pressurized to 35 kPa(5 psi).

4. At least 25% of the initial solvent volume was al-lowed to permeate through the membrane.

5. Flux was then measured.6. The pressure was then raised to the next higher level

(which varied for each membrane as reported later)and steps 4 and 5 repeated. The pressures testedin each case depended on the pressure rating ofthe membrane. In most cases, flat sheet polymericmembranes were tested up to 413 kPa (60 psi) andhollow fibers up to 138 kPa (20 psi).

7. After the tests, the membrane was removed fromthe cell and stored in a petri dish soaked in theappropriate solvent for at least 6 h at 24◦C and theflux measurements (steps 4–6) repeated.

8. The membrane was then conditioned in the nextconsecutive solvent, which depended on themethod of conditioning, as discussed below.

Four methods of membrane conditioning were eval-uated. The upper ethanol concentration was limited to


70% ethanol in most of our studies since this was theoptimum concentration for the extraction of zein fromcorn [16].

Method 1: Gradual change from 0 to 70% ethanol.The membrane was initially exposed to water andthen conditioned to 70% ethanol in increments of10% ethanol concentration (i.e. experiments were per-formed with water, 10% ethanol, 20, 30, 40, 50, 60and 70% ethanol). At each stage, steps 2–7 describedabove were repeated.

Method 2: Direct change from 0 to 70% ethanol.The membrane was first exposed to water throughsteps 2–7 and then solvent-exchanged directly with70% ethanol without any intermediate ethanol concen-trations.

Method 3: 70% ethanol. The membrane was ex-posed directly to 70% ethanol using steps 2–7.

Method 4: 100 ethanol to 70% ethanol. The mem-brane was initially exposed to 100% ethanol and thenconditioned to 70% ethanol in steps of 10% ethanolconcentration (i.e. experiments were performed for100, 90, 80 and 70% ethanol).

At the end of each conditioning experiment, themembranes were tested for flux and zein rejection witha model protein solution using the method describedbelow. This would provide an indication of the effectof membrane conditioning on membrane integrity andperformance.

Conditioning data were expressed in terms of a con-vective transport model:

J = LPPT

µ(1)

whereJ is the flux,PT the transmembrane pressure,µ

the viscosity of the permeate andLP is the permeabilitycoefficient of the membrane. Membranes not affectedby the solvent should give a linear plot of flux versus1/µ. In cases where the solvent does have an effecton the membrane (e.g. swelling of polymer, dilatingof pores), the plot will not be linear. All polymericmembranes listed in Table 1 were first tested usingmethod 1 for solvent stability. Membranes that gavelinear or nearly linear plots were then evaluated withmethods 2–4.

In order to include pressure effects, the data are alsoplotted in terms of relative resistance (R/Rw), whereRis the membrane resistance in the presence of ethanolsolutions, andRw is the membrane resistance mea-

sured with pure water. From Eq. (1), the membraneresistance (R or Rw) can be calculated as follows:

R = 1

LP= PT

µJ(2)

2.4. Membrane screening with zein solutions

Flux and rejection of an ethanol-soluble protein wasmeasured using a solution of 5 g/l zein (F4000, Free-man Industries, Tuckahoe, NY) made up in 70% (v/v)aqueous ethanol. For each experiment, the precondi-tioned membrane was placed in the Amicon stirred cellwith 250 ml of the zein solution. The cell was pressur-ized and flux measurement started. Flux measurementcontinued at each pressure until at least three con-secutive values were constant. This is reported as thesteady-state flux. Permeate samples (approximately2–3 g each) were collected and analyzed for proteincontent by the Kjeldahl method. Rejection is definedas:

R =(

1 − CP

CR

)× 100 (3)

whereCP andCR are the concentrations of zein in per-meate and retentate, respectively. Zein concentrationsfor the model solutions were measured spectrophoto-metrically by the procedure of Craine et al. [17].

All experiments were repeated within 24 h with afresh membrane disc. The membranes were cleanedafter the zein experiments by soaking in a solution of5 g/l NaOH in 70% ethanol for periods up to 6 h. Themembrane was then thoroughly rinsed with severalvolumes of fresh 70% ethanol.

2.5. Static swelling

Membrane swelling was determined as an increasein weight due to solvent absorption (Sw) (%):

Sw =(

Ww − Wd

Wd

)× 100 (4)

where Wd and Ww are the weights of dry and wetmembrane samples, respectively. The wet weight,Ww,was obtained by incubating membranes at 24◦C forup to 24 h in the appropriate solvent. All swelling ex-periments were replicated with at least five samples ofeach membrane and the mean value is reported.


3. Results and discussion

3.1. Membrane conditioning

Figs. 1–4 show typical flux behavior as a functionof ethanol concentration in the solvent and transmem-brane pressure when membranes were conditioned us-ing method 1. Many membranes showed a minimumin flux at 25–50% (v/v) ethanol (0.12–0.26 M ethanol)and the flux was higher at higher pressures. Due tospace limitations, data for all combinations of mem-branes and pressures are not shown here. The com-plete data set for all membranes studied is availablein Shukla [18].

Each data point in Figs. 1–4 was repeated within24 h with a fresh membrane disc. Reproducibilityof flux data was high (±5%) for membranes thatwere minimally or not affected by ethanol solutions,while reproducibility was poor for those membranesaffected by the solvent. As a group, the composite

Fig. 1. Effect of ethanol concentration and transmembrane pres-sure on flux of a modified PAN and cellulose ester membrane.Membranes were conditioned using method 1.

Fig. 2. Effect of ethanol concentration on flux of polysulfone andpolyethersulfone membranes. Membranes were conditioned usingmethod 1. Pressure was 275 kPa except for PM10 membrane,which was 69 kPa.

membranes (G80, H051 and U20S) shown in Fig. 3displayed lower solvent fluxes of<50 l/m2 h (LMH)than most other membranes. Among the polysul-fone and polyethersulfone family of membranes, thePES4H had much lower flux (5–15 LMH) than others

Fig. 3. Effect of ethanol concentration and transmembrane pressureon flux of composites, polysulfone and polyethersulfone mem-branes. Membranes were conditioned using method 1. Pressurewas 275 kPa except for UFP10 membrane, which was 124 kPa((�) UFC; (�) UFP10; (�) PES4H; () G80; (�) H051; (�)U20S).


Fig. 4. Effect of ethanol concentration on flux of cellulose-basedand PAN-based membranes. Membranes were conditioned usingmethod 1. Pressure was 275 kPa.

(Figs. 2 and 3). This could be due to differences inhydrophobicity of the membrane materials, or dueto differences in density of the membrane top layerwhich results in higher membrane resistance andlower permeability. Surfaced-modified PES mem-branes that are usually cross-linked after membraneformation displayed lower solvent fluxes.

Since the viscosity of ethanol solutions is maximumat 50% ethanol concentration (Fig. 5), the flux shouldshow a corresponding minimum value at this ethanolconcentration, assuming the membrane is not affectedby the solvent in any other manner. This phenomenon

Fig. 5. Effect of ethanol concentration on viscosity and density ofethanol solutions at 24◦C.

Fig. 6. Darcy plot for a modified PAN and a PES membrane.Effect of pressure and ethanol concentration (expressed in termsof viscosity) on flux. Membranes were conditioned using method1. (MX25: (�) 138 kPa, (�) 275 kPa, (�) 413 kPa; PES4H: (�)138 kPa, (�) 275 kPa, () 413 kPa).

is governed by the Darcy or Hagen–Pouseuille lawsshown in Eq. (1). Several membranes do show a fluxminima in the middle range of ethanol concentrations(Figs. 1–4). Thus, a linear correlation should be ob-tained between flux and the reciprocal of viscosity ofthe permeating solvent, according to Eq. (1).

A typical Darcy plot for two membranes at differ-ent pressures is shown in Fig. 6. Of the 18 membranestested, 15 generally followed Darcy’s law in that fluxgenerally decreased with increasing viscosity of theethanol solutions [18]. The three exceptions were thecomposite membranes (G80, U20S and H051). Thisphenomenon could be explained by viscosity consid-erations alone, in a manner similar to that observedby others [6,19,20]. Our results are similar to thoseof Machado et al. [21] but in contrast to those ofIwama and Kazuse [20]. The latter group used bi-nary solvent mixtures that followed the viscosity ad-ditive rule, whereas the solvents used by us (aqueousethanol solutions) and by Machado et al. [21] (aque-ous acetone solutions) do not follow the viscosity ad-ditivity rule. Viscosity of acetone–water mixtures alsogo through a maximum at∼0.15 M acetone. However,acetone–water flux did not go through a minimum atthis acetone–water concentration, indicating that sol-vent transport is also affected by other parameters suchas surface tension and solubility parameter of the per-meating solvent.


Fig. 7. Resistance ratio plots for polysulfone and polyethersulfonemembranes. Membranes were conditioned using method 1. ((�)PM10, (�) Omega, () UFC, (�) PM30, (�) PES4H, (�) Alpha,(�) PS10).

3.2. Membrane resistance

The influence of different water fluxes of the mem-branes was factored out by using the resistance ra-tio (R/Rw) to compare the membranes, as shown inFigs. 7–9. As a group, the more hydrophobic mem-branes, such as PS and PES (Fig. 7) and the com-posite membranes (Fig. 8) show a decrease inR/Rw

Fig. 8. Resistance ratio plots for composite membranes. Mem-branes were conditioned using method 1.

Fig. 9. Resistance ratio plots for hydrophilic polyacrylontrile-basedand cellulose-based membranes. Membranes were conditioned us-ing method 1.

with increase in ethanol concentration. The exceptionsin this group are the PM10 and PES4H membranes:their resistance ratios were about 1 but increased athigher ethanol concentrations (Fig. 7). However, themore hydrophilic membranes shown in Fig. 9 did notbehave as a group. Except for the U20T and PLGC,the others in this group also showed a decrease inmembrane resistance (or the resistance ratio remainedclose to 1) with an increase in ethanol concentration.This suggests that factors other than, or in addition to,hydrophilicity and viscosity (e.g. surface tension, sol-ubility parameter or dielectric constant in the case ofcharged membranes) may be governing solvent flowthrough the membrane.

With the composite membranes (G80, H051 andU20T), resistance was high with water and decreasedwhen exposed to ethanol (Fig. 8). The pores on thesurface of G80 and U20S membranes, which havevery dense barrier layers, apparently dilated lead-ing to lower membrane resistance and higher fluxes.This behavior does not appear to be reported in theliterature. With cross-linked PAN-chitosan mem-branes, Musale and Kumar [22] observed the highestflux with methanol followed by ethanol and iso-propanol. These differences in flux were explainedon the basis of the combined effects of increase in


molecular weight, viscosity, hydrophobicity and di-electric constant of the alcohol. Reddy et al. [23] ob-served lower flux of primary and secondary alcoholswith increase in molecular weight and hydrophobicityof the solvent with a polyamide-polyphenylene sul-fone membrane. There was no correlation with viscos-ity. Only Machado et al. [21] reported an increase influx with increase in solvent (acetone) mole fraction.Their nonlinear exponential increase was thought tobe due to decreasing surface tension at higher acetoneconcentrations.

3.3. Membrane swelling

Swelling of several polymeric membranes is shownin Table 2. Except for two membranes (Alpha andOmega), water resulted in higher degrees of swellingby weight while 100% ethanol resulted in the leastweight-swelling. The polysulfone hollow fiber UFP10showed the greatest degree of swelling in water(211%) and in neat ethanol (94%). In general, thedielectric constant of the solvent and difference insolubility parameters of the polymer and solvent gov-ern swelling behavior. Membrane swelling is higherin polar solvents like water due to its high dielectricconstant (ε = 80), and less with ethanol (ε = 25)and nonpolar solvents like hexane (ε = 1.9). Musaleand Kumar [22] observed over 50% swelling byweight with cross-linked chitosan-PAN membranes

Table 2Weight swelling of polymeric UF membranesa

Material Membrane Water 70%EtOH

100%EtOH

Cellulose Type C 112.5 60.2 45.9Cellulose Cell 25.9 29.9 22.5Composite U20S 83.7 48.4 34.6Composite G80 65.8 61.5 51.3Composite H051 31.0 26.4 24.5PAN-m MX25 71.2 56.3 46.8PAN U20T 85.8 39.1 32.5PES-m Alpha 44.7 59.9 52.8PES-m Omega 51.0 46.5 56.3PES PES4H 26.0 18.0 17.1PS UFP10 211.4 161.4 94.4PVDF AN09 54.3 46.2 34.6PVDF AF5 21.0 9.5 6.1

a Percentage increase in weight after exposure to the solventsfor 24 h.

with ethanol compared to 20% swelling in hexane. Itis interesting that the three membranes manufacturedby Pall Filtron (Alpha, Cell and Omega) showedessentially no change or no trend in swelling, eventhough these membranes are made of two differentmaterials.

Length-swelling measurements have been reportedin the literature and were also done in this study [18].However, they were largely inconclusive because thedimensional changes were so small that measurementswere difficult and imprecise. A typical sample of 5 cmlength experienced a length-swelling of 0.1–0.2 cm.Consequently this experimental method had high er-rors and is not useful.

3.4. Effect of methods of conditioning on permeability

The effect of different methods of conditioningis shown in Table 3 in terms of permeability co-efficients of 70% ethanol. Some membranes (Cell,MX25, Alpha, Omega, PS10) show highLP val-ues (>100× 10−15 m) at all pressures. On the otherhand, the composite membranes (U20S, G80, H051)and modified PES membrane (PES4H) had sig-nificantly lower LP values. This is expected sincecomposites and surface-modified membranes havedenser layers and consequently higher membraneresistance. AN09, which is a PVDF-based highMWCO membrane, displayed much higher fluxeswhen conditioned by methods 2–4. The reason forthis is not clear since PVDF is resistant to ethanol.Protein rejection values for this membrane werepoor when conditioned by methods 2–4 (Table 4),indicating that high solvent fluxes might be dueto pore dilation, which also results in low proteinrejection.

If solvent flux increased in proportion to the pres-sure (i.e. if the value ofLP was unaffected by pres-sure), we can assume the membrane integrity remainsintact. Exposure to water followed by exposure to 70%ethanol (method 2) appears to cause some damage tosome membranes, e.g. type C, Cell, G80 and PES4H,which show an increase inLP at higher pressure. Theorder of magnitude of the fluxes with method 2 arecomparable to those observed with method 1, exceptat the highest pressure in the above cases. However,some membranes (PS10 and AN09) show a decreasein LP at higher pressure.


Table 3Effect of methods of conditioning and pressure (kPa) on membrane permeability (LP) with 70% ethanol solventa

Membrane Method 1 Method 2 Method 3 Method 4

138 275 413 138 275 413 138 275 413 138 275 413

Type C 68 58 58 75 150 256 62 140 489 71 141 370Cell 142 132 240 127 130 618 136 232 412 79 119 1112PLGC 51 42 49 45 53 45 38 40 75 26 101 278U20Sb 23 31 28 50 50 51 22 38 38 21 22 42G80b 38 54 62 25 50 81 15 104 665 19 22 21H051 62 46 36 71 70 84 66 95 145 49 92 428MX25 247 235 220 254 261 271 241 249 337 137 164 179Alpha 424 414 443 411 397 445 362 371 463 356 371 655Omega 642 666 655 554 576 585 739 760 855 101 142 483PES4H 21 16 14 20 20 37 18 49 136 16 18 126PS10 205 150 132 432 303 265 853 726 585 150 334 445AN09 50 35 39 282 214 182 323 242 285 219 209 214

a LP values calculated from Eq. (1) (×10−15 m).b Pressures were 275, 310 and 345 kPa.

Direct exposure to 70% ethanol (method 3) seems tohave the most destructive effect on membranes such astype C, Cell, PLGC, G80, H051, MX25, Omega, PS10and PES4H, as measured by their higherLP valuescompared to method 1 and the increase inLP at higherpressure. Only AN09 and Alpha appear to remain in-tact. Method 4 (direct exposure to 100% ethanol) alsoappears to damage the polymer matrix in all cases ex-cept U20S, G80 and AN09.

Table 4Effect of methods of conditioning and pressure (kPa) on membrane permeability (×10−15 m) and rejection (%) of proteina

Membrane Method 1 Method 2 Method 3 Method 4

LP Rejection LP Rejection LP Rejection LP Rejection

138 275 138 275 138 275 138 275 138 275 138 275 138 275 138 275

Type C 56 46 92 97 21 81 91 94 56 199 96 55 68 89 93 99Cell 63 55 99 94 24 127 92 72 39 32 94 93 76 428 69 9PLGC 42 29 94 95 12 71 97 96 23 25 94 99 59 40 96 98U20Sb N.D.c 9 N.D. 93 91 726 18 11 14 17 78 80 18 16 98 87G80b 8 15 87 96 59 136 31 20 15 14 100 93 5 9 90 79H051 34 34 96 92 6 21 91 96 9 25 72 36 15 166 94 10MX25 41 39 98 99 18 82 95 58 26 24 96 97 51 87 84 52Alpha 70 68 99 98 20 82 93 96 35 83 98 53 97 109 88 26Omega 45 N.D. 78 N.D. 20 73 92 94 32 54 98 56 63 41 87 88PES4H 22 23 88 95 7 40 79 95 10 13 91 90 20 19 84 96PS10 114 123 90 86 23 300 48 4 153 138 25 23 616 539 15 19AN09 63 52 79 51 20 112 80 58 28 397 87 0 74 70 81 62

a The protein was 5 g/l zein in 70% ethanol solvent.LP values were calculated from Eq. (1) (×10−15 m).b Pressures were 275 and 345 kPa.c N.D.: not determined.

3.5. Effect of conditioning methods on protein fluxand rejection

A better indicator of membrane damage is changesin rejection properties. This was studied using modelzein solutions. As expected,LP values are generallylower with zein present (Table 4) than in its absence(Table 3), in some cases by a substantial margin, e.g.compare Alpha, Omega and MX25 in Tables 3 and 4.


Among the methods, membranes conditioned bymethod 1 appear to retain their integrity and providereasonably highLP values and high zein rejections.Membranes conditioned using the other methodsresult in lower rejections in almost all cases. TheU20S membrane gave >75% rejection in all casesexcept with method 2. This is probably becausethe manufacturer ships the U20S membrane in 50%ethanol, and exposing it first to water, followed by70% ethanol, could cause drastic structural changes.This also explains why protein rejection with thismembrane remains high (∼80%) after direct expo-sure to 70% ethanol even though this conditioningmethod 3, damages most of the other membranestested.

Some membranes that have high zein rejections atlow pressures (138 kPa) lose their rejection and be-come permeable to the protein at higher pressures(>275 kPa). Organic solvents tend to lower the glasstransition temperature of polymers by acting as poly-mer plasticizers, which in turn reduces their ability toresist high pressures [6]. For instance, soaking poly-sulfone in an organic solvent with a similar solubil-ity parameter will lead to cracking of the matrix [24].There is evidence that hydrogen bonding solvents areusually much less disruptive to the polymer matrixthan strongly nonpolar solvents like hexane [25]. Con-sequently, on exposure to organic solvents, the pres-sure ratings of the membranes are significantly re-duced in many cases, except if they are conditionedusing method 1.

4. Summary and conclusions

The method of conditioning has a strong effecton the solvent flux, membrane integrity and pres-sure rating of polymeric membranes. Gradual solventexchange with successively higher concentrationsincreased in small doses appears to work best withcompletely miscible solvents such as those studiedhere (ethanol–water mixtures). Rapid solvent ex-change between water and high concentrations ofalcohol disrupts the polymer matrix in many andleads to pore degradation. Exposure to organic sol-vents significantly reduces the pressure rating of themembranes. A membrane that provides acceptable re-jection of protein up to 413 kPa (60 psi) under normal

conditioning may lose its rejection even at 138 kPa(20 psi) if conditioned incorrectly.

Acknowledgements

This research was supported by the Illinois CornMarketing Board, Illinois Department of Commerceand Community Affairs Bureau of Energy and Re-cycling, US Department of Agriculture through theNRICGP program (Award No. 97-35504-4296) andthe Illinois Agricultural Experiment Station. Contribu-tions of membranes by Koch Membrane Systems, Os-monics and Pall Filtron are gratefully acknowledged.Analytical assistance was provided by Amanda D.Popp and Melissa M. Stein. Useful discussions wereheld with D.A. Musale, M. Balakrishnan, H. Yacubow-icz and J. Yacubowicz.

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