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Journal of Membrane Science 276 (2006) 260–271

Pervaporation study of aqueous ethanol solution through zeolite-incorporatedmultilayer poly(vinyl alcohol) membranes: Effect of zeolites

Zhen Huang a,∗, Huai-min Guan b, Wee lee Tan b, Xiang-Yi Qiao b, Santi Kulprathipanja c

a Department of Packaging Engineering, Tianjin University of Commerce, Tianjin 300134, PR Chinab Department of Chemical & Biomolecular Engineering, National University of Singapore, Singapore 119260, Singapore

c UOP LLC, 50 East Algonquin Road, Des Plaines, IL 60017-5016, USA

Received 1 July 2005; received in revised form 26 September 2005; accepted 29 September 2005

Available online 2 November 2005

Abstract

In this study, a series of three-layer zeolite-embedded poly(vinyl alcohol) (PVA) composite membranes have been successfully fabricated with

a casting machine. Zeolites, examined with a loading of 20 wt%, include 3A, 4A, 5A, NaX, NaY, silicalite and beta. These hydrophilic composite

membranes have been evaluated in the dehydration of ethanol aqueous solution by means of pervaporation. The unfilled PVA membrane is observed

to exhibit much higher separation factor than two commercial PERVAP 2210 and PERVAP 2510 membranes. After adding zeolites into the PVA

matrix, higher separation factor and higher fluxes or higher selectivity and higher penetrant permeances are both achieved by these resultant zeolite-

incorporated membranes, indicating that ethanol/water separation has been enhanced with the aid of incorporated zeolites. Through evaluating the

pervaporation performance in terms of water permeance, ethanol permeance and selectivity, we have revealed that the separation performances of

zeolite-filled membranes are strongly related to the zeolite pore dimension, its hydrophilic/hydrophobic nature as well as its crystal framework.

The temperature dependence of the pervaporation behaviors like the penetrant fluxes and permeances has been discussed in detail in terms of

Arrhenius activation energy. The evaluated results have revealed that the permeance and selectivity (i.e., the membrane intrinsic properties) are less

dependent on the operating temperature than the flux and separation factor. Zeolite addition has led to decreased activation energies for water and

ethanol, and more considerable drop of the water activation energy has subsequently resulted in the increased selectivity in ethanol dehydration.

© 2005 Published by Elsevier B.V.

Keywords: Ethanol dehydration; Pervaporation; Zeolite A; Zeolite beta; FAU; PVA multilayer composite membrane

1. Introduction

Pervaporation has gained increasing attention in many chem-

ical processes as an effective and energy-saving membrane tech-

nique for separating azeotropes, close-boiling mixtures, isomers

and thermally sensitive compounds, and purifying species from

highly concentrated streams [1–7]. Pervaporation distinguishes

itself from other membrane processes by a phase transition thattakes place during transporting through the membrane, since the

feed side is liquid mixtures but the permeate side is removed as

a vapor. Compared with other membrane processes like reverse

osmosis and filtrations, the driving force, the pressure differ-

ence across the membrane, for pervaporation is usually higher

because it uses a vacuum on the permeate side of the membrane.

∗ Corresponding author. Tel.: +86 22 8591 3391.

E-mail address: [email protected] (Z. Huang).

The separation is controlled by the differences in diffusivities

and solubilities of the competing components through the mem-

brane. Due to differing permeation rates of the components, one

substance at low concentration in the feed stream can be highly

enriched in the permeate.Thus,the pervaporationperformance is

mainly regulated by the physicochemical structure of the mem-

brane rather than the vapor–liquid equilibria of the system of

interest.So far, many attempts have been made to develop various

membranes (asymmetric, composite or mixed matrix) for per-

vaporation applications [3–26]. Polymeric membranes are the

most commonly used materials since they are inexpensive, eco-

nomically processible and at low operating cost. Significant

progresses and achievements of pervaporation separations with

polymeric membranes have been reviewed in several articles

[3–7]. On the other hand, zeolite membranes have advantages

over polymer membranes, such as better chemical and thermal

stability, and have the great potential to separate the mixtures

0376-7388/$ – see front matter © 2005 Published by Elsevier B.V.

doi:10.1016/j.memsci.2005.09.056



Z. Huang et al. / Journal of Membrane Science 276 (2006) 260–271 26

of molecules through both the adsorption and molecular sieving

because zeolite materials have uniform, molecular-sized pores.

An extensive summary of current advances and potential appli-

cations of pervaporation through zeolite membranes has been

made recently by Bowen et al. [8]. However, zeolite membranes

generallycost considerablymore to fabricateand aremore brittle

than polymer membranes. In this regard, incorporating zeolitic

particles into a continuous polymer phase appears to establish an

alternative way by coupling the easy processability of polymers

with the superior separation properties of rigid adsorptive fillers

to make zeolite–polymer mixed matrix membranes, which are

more flexible and easier to work with than zeolite membranes.

The idea of adding adsorptive fillers to the polymer matrix

to enhance the membrane separation performance was initially

proposed in 1987 by the UOP researchers for gas separations

[9] and The Netherlands scientists for liquid separations [10],

respectively. For mixed matrix membrane materials, the zeo-

lite added does not readily form a continuous phase. Instead,

isolated zeolite particles are surrounded by polymer molecular

chains. Expectedly, zeolites, through molecular sieving, selec-tive sorption and selective diffusion, increase the mobility of the

preferentially permeating species in the polymer but meanwhile

decrease the mobility of the component that is less permeable.

Experimental studies have shown that the incorporation of zeo-

lites usually results in an increase of either separation factor

[11–13] or flux [14–16] f or many liquid separations except a

few systems where both separation factor and flux have risen

[10,17–20]. teHennepeetal.[10] andChenetal. [17] haveincor-

porated hydrophobic silicalite uniformly into polydimethyl-

siloxane (PDMS) membranes to removal ethanol from water,

and found that both the separation factor and flux are promoted

due to the extra sorption capacity to ethanol of added zeolites.Similar observations apply to Y-type zeolite–PDMS membranes

[18,19]. However, Okumus et al. have reported that the incor-

poration of zeolite A and 13X into cellulose acetate (CA) or

polyacrylonitrile (PAN) have caused a decrease rather than an

increase in separation factor dueto porouscave-like structures as

reflected by SEM examination results, and that increased poros-

ity subsequently lead to an increase in the flux [15,16]. Gao et

al. have fabricated hydrophilic composite membranes consist-

ing of A-type and X-type zeolites and PVA [14]. The authors

noted that an enhanced permeation flux could be obtained

with little or no decrease in separation factor by using these

hydrophilic zeolites, and that pervaporation fluxes increased

with increasing zeolite pore size. Their observations are appar-ently due to the permeation of smaller permeants through the

zeolite pores and the poor adhesion between polymer matrix

and zeolite phase, probably related to the membrane preparation

methodology.

Pioneer works on mixed matrix membranes have clarified the

importance of theproper choiceof polymer materialsand zeolite

fillers for different pervaporation requirements [10–26]. For the

separation of ethanol–water mixture, the concentration of feed

solution and molecular size of species should also be taken into

account. For mixtures with low ethanol content, organophilic

or hydrophobic polymers and fillers should be considered so as

to remove ethanol from its dilute aqueous solution [10,17–19];

for highly concentrated ethanol mixtures, hydrophilic polymers

and fillers may be more appropriate for dehydration purposes

[20].

In our studies, hydrophilic PVA is chosen as the polymeric

material since it is the most attractive and economical poly-

mer material for ethanol dehydration [21–25]. More recently

PVA-based multilayer membranes have emerged since the PAN

support layer could suppress the swelling of the PVA layer at

the PVA–PAN interface and thus retain a dense skin [21,26]

As a result, high water permselectivity and durability could

be achieved as demonstrated by recent works on isopropanol

and butanol dehydrations by using commercial PVA-based mul-

tilayer membranes [27–29]. In our proceeding study, nove

multilayer PVA–PAN based membranes by embedding zeolite

KA (obtained from UOP 4A by ion exchange) have been suc-

cessfully developed for removing water from ethanol aqueous

solutions [30]. The pervaporation results show that the penetran

permeances (or fluxes) and the selectivity (or separation factor)

are all promoted, apparently due to the great contribution of

addedzeoliteKA.Theaimofthisstudyistoexaminetheeffectofdifferent zeolites on membrane pervaporation performance and

to better understand the transport mechanism. Seven differen

zeolites (KA (3A), NaA (4A), CaA (5A), NaX, NaY, silicalite-1

and beta zeolite) are selected as adsorptive fillers by considering

their effective pore dimension, hydrophilic/hydrophobic nature

and framework structure. The membrane pervaporation perfor-

mance has been evaluated and compared with unfilled multilayer

composite membranes in terms of flux and separation factor, or

permeance and selectivity.

2. Experimental

2.1. Materials and regents

The materials used for our investigation are summarized in

Table 1. It is noted that polyester non-woven fabric (PET RS21)

was kindly donated by Crane & Co. Inc., USA. Zeolite beta

Table 1

Materials used in pervaporation experiment

Materials Sources

Ethanol (A.R. quality 99.7%, v/v) Hayman Ltd., England

PERVAP 2210 membrane Sulzer Chemtech, Germany

PERVAP 2510 membrane Sulzer Chemtech, GermanyPolyester non-woven fabric (PET RS21) Crane & Co. Inc., USA

Dimethylfluoride (DMF) (99.8% purity) Merck, Germany

Polyacrylonitrile (PAN) Scientific Polymer Products

Inc.

Poly(vinyl alcohol) (PVA) (Airvol 350) Air Products & Chemical

Inc., USA

Fumaric acid (Reagent grade) Aldrich, USA

Zeolite 3A (COM 3A) Aldrich, USA

Zeolite 5A (COM 5A) Aldrich, USA

Zeolite 4A (COM 4A) UOP LLC, USA

Zeolite NaX UOP LLC, USA

Zeolite NaY UOP LLC, USA

Zeolite silicalite-1 UOP LLC, USA

Zeolite beta Our laboratory



262 Z. Huang et al. / Journal of Membrane Science 276 (2006) 260–271

was synthesized by ourselves whereas zeolite 4A, NaX, NaY,

silicalite-1 were sponsored by UOP LLC, USA.

2.2. Preparation of the polymer solution and dope

A 15 wt% PAN clear solution was prepared by dissolving

PAN in DMF overnight at room temperature under agitation.PVA was dissolved in deionized water to yield a 12 wt% concen-

tration. Fumaric acid at 0.05 mol/mol monomeric unit of PVA

was subsequently added, followed by zeolites at a loading of

20 wt% for zeolite incorporations. Fumaric acid, yielding good

results as reported in our proceeding study [30], was selected

as a cross-linking agent. The mixture was rigorously stirred for

6 h in a heated water bath at elevated temperature of 85 ◦C. The

resultant polymeric dopes were degassed and stored in air-tight

containers before casting.

2.3. Membrane casting procedure

The membrane casting procedure used here is different fromthat previously reported [30]. A casting machine (K Control

Coater, Labomat Essor) at a constant rate of 3.0–4.0 mm s−1,

instead of manual casting, was applied for better controlling the

membrane quality. The procedure could be represented graphi-

cally by Fig. 1. The support layer (PAN) was cast with a casting

knife of 150m thickness by pouring the 15 wt% PAN solu-

tion onto the polyester non-woven fabric substrate. It should be

noted that the substrate was secured and firmly pressed onto the

glass plate to prevent air bubbles from forming in the polymeric

solution during casting. After casting, the two-layer sheet was

immediately immersed into cold water for phase inversion to

form a nascent PAN film. Subsequently, it was transferred into

a tub of running water bath to further remove residual DMF

solvent for at least 1 day.

The third selective layer composed of PVA, fumaric acid and

zeolites was cast using the same 150 m casting knife onto the

PAN layer membrane that was properly secured onto a glass

plate to prevent any folding or curling. The cast PVA top layer

was dried in an oven at 40 ◦C overnight before carrying out

cross-linking at 160 ◦C.

The morphologies of the cross-section and surface of the test

membranes were assessed by JEOL JSM-5600LV field emission

scanning electron microscope (FESEM) at an acceleration volt-

age of 5 kV. A small piece of the test membrane was cut in liquid

nitrogen and mounted onto a SEM stub with the use of a carbon

double-sided tape. The sample was coated with platinum using

a JEOLJFC-1300 autofine coater for 40s at 40mA prior to mor-

phology analyses. The composition of zeolite particle samples

considered was characterized by performing elemental disper-

sive spectroscopic (EDS) analysis on SEM JEOL JSM-6700F

with a simultaneous module.

2.4. Pervaporation experiments

The pervaporation experiments were carried out on a

laboratory-scale Sulzer Chemtech pervaporation unit and its

design has been described elsewhere [27]. The experimental

procedure can be described as follows. A circular test mem-

brane with an effective working area of 15.2 cm2 was cut and

placed in the stainless steel permeation cell. The 2.5 l feed solu-

tion was allowed to circulate using a Speck Pumpen pump at a

flow rate of 80 l h−1. The feed solution was heated up to desired

temperatures via a Haake PC30 heating bath. The temperature

of the permeation cell was constantly monitored by a Greisinger

Fig. 1. Schematic diagram of membrane casting procedure.




Electronic GTH 1100/2 DIF digital thermometer. A Western

Pneumatic vacuum pump provided the vacuum to be applied at

the permeate side which maintained the pressure at around 0 Pa

(0 mbar). The vapor permeate was collected in a round-bottom

flask cold trap using liquid nitrogen.

The system was heated up from room temperature to the first

test temperature of 100 ◦C and then allowed it to equilibrate

for at least 2 h. The equilibration permeate was removed and

clean round-bottom flasks were used to collect the permeate

sample at a time interval of 0.5 h with at least twice for each

test temperature. The system temperature was then adjusted to

80 ◦C by loweringthe temperature of theheating bath. Theabove

procedure was repeated for 80 ◦C, followed by for 60 ◦C.

The mass of collected samples was recorded using a Met-

tler Toledo balance (±0.1 mg) and the percentage compositions

of ethanol–water mixtures were obtained by analyzing standard

solutions and collected samples with gas chromatography (GC).

The GC analyzer (Hewlett-Packard GC 6890) consists of an HP-

INNOWAX column packed by using cross-linked polyethylene

glycol) and a thermal conductivity detector (TCD) detector. Thecarrier gas used was helium at a flow rate of 2.0 ml/min. The

temperature of GC oven was set at 150 ◦C. All the analysis data

were recorded by using the hp software (Chemstation Rev A

10.01). Total assay time for each sample was 5 min above the

retention time of either water or ethanol which is less than 2 min

at operational conditions. To minimize the baseline noise prior

to injecting samples to analyze, helium was used to completely

flush the whole system line for 30 min. Every sample was tested

at least twice so as to obtain better than 99% reproducibility.

Each data point is the results of at least two repeated measure-

ments with a 5% deviation or smaller, giving an indication of

the accuracy of the obtained data.The pervaporation flux ( J ) was readily calculated using the

following equation:

J =W

At (1)

where W (g) is the total amount of the permeate during the exper-

imental time interval t (h) at a steady state and A is the effective

membrane area.Separationfactor (α), a very practicaland useful

parameter, was calculated as follows:

α =ywater/yethanol

xwater/xethanol(2)

where y and x are the weight fraction of either water or ethanolin the permeate and feed, respectively.

3. Results and discussion

3.1. Zeolite properties

Zeolites are three-dimensional, microporous, crystalline

solids with well-defined structures that contain aluminum, sili-

con and oxygen in their regular framework [31]. Since they have

pore sizes of several angstroms, zeolites are able to discriminate

components of a mixture on the basis of a difference in molec-

ular size (i.e., molecular sieving effect) [31]. The zeolite pore

Table 2

Physical properties of zeolites used

Zeolite Pore size (A) Atomic compositiona

Si/Al Na/Al K/Al Ca/A

COM 3A <4a 1.0 0.45 0.55 –

UOP 4A 4b 1.0 1.0 – –

COM 5A >4a 1.0 0.34 – 0.33NaX 7.4b 1.3 1.0 – –

NaY 7.4b 2.5 1.0 – –

Beta 7.1× 7.3b 16 1.0 – –

Silicalite-1 5.2× 5.7b 196 1.0 – –

a Based on EDS analysis.b From Ref. [31].

size is mainly decided by its unique crystal structure, but it can

also be affected by zeolitic composition, especially for zeolite

A. Aluminum is trivalent, and thus requires a charge-balanced

cation that is located in the zeolite pore. In the case of zeolite

A, charge-balancing cations occluded in the zeolite cavities are

able to tune the pore size based on the size and number of cationspresent. The sodium form of zeolite A is commonly called zeo-

lite 4A since it has a pore size of 4 A. The ion of sodium can

be exchanged by the other cationic ions. Subsequently, the aper-

ture size of zeolite A has varied due to the molecular size of

the exchanged ion. When the sodium ion is replaced by the cal-

cium cation, the resultant zeolite A will have a size of 5 A; when

the sodium ion is replaced by the potassium cation, the zeo-

lite A will have a size of 3 A. As a consequence, the former is

called zeolite 5A and the latter is called zeolite 3A. Displayed

in Table 2 are characteristic properties of zeolites investigated

in our study. Elemental dispersive spectroscopic analysis results

have shown that commercial 3A and 5A still contain an appreciated content of sodium cations, with the Na/Al ratio of 0.45 and

0.34, respectively. Hence, COM 5A has a pore size of less than

5 A and COM 3A has a pore size of much larger than 3 A. In

our proceeding study [30], ion exchanged UOP 4A with K+

(denoted as UOP 3A here) has a Na:Si:Al:K molar ratio of

0.06:1:1:0.94, likely possessing smaller pore size than COM

3A.

Besides the molecular sieving effect, zeolitic hydrophilic/

hydrophobic nature is also a very important attribute of zeo-

lites for pervaporation. Zeolite hydrophilicity/hydrophobicity

is observed to mainly depend on the Si/Al ratio, i.e., zeolitic

hydrophilicity increases as the aluminum content in the zeolite

framework increases or vice versa [8,31]. The localized elec-trostatic poles between the positively charged cations and the

negatively charged zeolitic framework strongly attract highly

polar molecules, resulting in a hydrophilic structure. Among

these zeolites, A-type zeolites (COM 3A, UOP 4A and COM

5A) have high aluminum content (Si/Al = 1.0), followed by

NaX (Si/Al = 1.3), NaY (Si/Al = 2.5) and beta (Si/Al = 16.0)

Silicalite-1 has the lowest aluminum content (Si/Al = 196)

Hence, this suggests that hydrophilicity of these zeolites

increases in the order of silicalite-1< beta < NaY< NaX< COM

3A≈UOP 4A≈COM 5A (please also refer to Table 2). In this

work, the selection of A-type zeolites may render us to under-

stand the pore size sieving effect while the FAU type zeolites




Fig. 2. FESEM photographs of filled and unfilled multilayer PVA-based membranes: (a–c) unfilled; (d–f) filled with UOP 4A; (a and d) top view; (b and e)

cross-sectional view at low magnification; (c and f) cross-sectional view at high magnification.

(NaX and NaY) chosen may elucidate better the attribute of the

zeolite hydrophilicity for pervaporation.

Hydrophobic/hydrophilic nature of zeolites also appears

to depend on their framework structure [8]. Pure siliceous

zeolite beta has been reported to be much more hydropho-

bic than silicalite-1 and the other siliceous 12-numbered ring

zeolites even though they contain almost no aluminum [32].

In addition, silicalite-1 and beta both possess intricate three-

dimensional channel systems, and may discriminate competing

molecules on the basis of a difference in molecular shape.

In this work, we have selected high-aluminum beta and low-

aluminum silicalite-1 to investigate their effect on the membraneperformance.

3.2. Morphologies of fabricated membranes

Fig. 2 shows the FESEM images of the zeolite-incorporated

and unfilled membranes. From the cross-sectional view of both

membranes (Fig. 2b and e), the multi-layered structure of fabri-

cated membranes can be clearly observed, namely, a very dense

top selective layerof PVA or PVA-zeolite,a porous backing layer

of PAN and a support layer of non-woven fabric (PET RS21).

Fig. 2c and f presents FESEM images at high magnification for

two-layers cast. The top layer is seen to be very dense and thin

with a thickness of less than 10m. The backing layer possessesa cave-like structure and is much thicker (∼70m) than the top

selective layer. It is certain that this highly porous layer only

provides mechanic support to the selective layer and contributes

little to ethanol/water separation. The same can be applied to the

most porous and thickest (∼120m) non-woven fabric layer.

The thickness of the layers can be readily approximated from

the scale bar given at the bottom of the FESEM picture. The total

thickness of the multi-layered composite membrane is approxi-

mately 200m.

The non-porous selective layer is expected to be responsible

for ethanol/water separation. The cross-sectional and top views

(Fig. 2d and f) of the selective layer show that the zeolite parti-

cles are well distributed within the polymeric matrix and form a

good contact with polymer with no visible macroscopic voids.

This suggests that the selective layer is possibly defect-free, and

hence able to be effective in ethanol/water separation. In con-

trast, some literature reports have revealed that the addition of

zeolites caused microporous cave-like structures for cellulose

acetate–zeolite and layered PAN–zeolite composite membranes

which resulted in low separation factor and high permeant flux

[15,16]. This is, however, not the case for our multi-layered

PVA-zeolite composite membrane as confirmed by the FESEM

pictures. Hence, these zeolite-filled three-layer PVA membranes

are expected to achieve good performances in ethanol/water per-vaporation separation.

3.3. Pervaporation results

3.3.1. Comparison with commercial membranes

The pervaporation performance evaluation of the unfilled

multilayer membrane has started from a comparison made with

two commercial PVA/PAN membranes of PERVAP 2210 and

PERVAP 2510 (Sulzer Chemtech, Germany). The pervapora-

tion studies of PERVAP 2510 membrane have been previously

reported for water removal from high concentrated IPA and

butanol systems [27,29]. Their results show that this two-layer

composite membrane has achieved very high separation fac-tor for water. For example, separation factor obtained for water

over IPA has ranged from 300 to 1400 with feed water concen-

tration of 2–15 wt%, pervaporation temperature of 60–100 ◦C

and the downstream pressure of less than 100 Pa (1 mbar). The

separation factors are even much higher for butanol isomer sys-

tems. However, this membrane gives very poor performance for

ethanol system as reflected by an invariant separation factor of

15 throughout the test temperature range (see Table 3). The most

possible reason is that the linear ethanol molecule is (1) much

smaller than IPA and butanol isomers [8] and (2) able to form

stronger interaction with water [33] and thus lead to a consider-

able mutual-dragging effect.




Table 3

A comparison of pervaporation performance of different membranes for 20 wt% of water in the feed

Membrane Temperature (◦C) Ethanol (wt%) F (g/(m2 h)) α

Feed Permeate

PERVAP 2210 100 79.16 5.00 3554 72

80 79.21 1.62 1671 231

60 79.34 1.32 451 287

PERVAP 2510 100 80.21 22.51 4018 14

80 80.29 21.72 2456 15

60 80.35 20.47 1413 16

Unfilled multilayer PVA-based membranea 100 80.59 1.93 1511 211

80 80.60 1.00 771 411

60 80.61 0.53 217 779

a Cast in our study.

PERVAP 2510 has shown the poorest performance amongthe

three non-zeolite multilayer membranes as shown in Table 3. In

the meantime, lowering test temperature from 100 to 80 and

60 ◦C has not led to the better separation for ethanol dehy-

dration. These results may be attributed to the low degree of

cross-linking for this membrane which has been confirmed in

the work of Qiao et al. [29]. Due to the low degree of PVA

polymer chain cross-linking (i.e., looser chain packing), water

and ethanol molecules can readily diffuse through the swollen

membrane with no relevance to the temperature dependence

of polymer chain thermal motions. On the other hand, another

commercial membrane PERVAP 2210 seems to possess higher

degree of cross-linking and tighter chain packing, thus it has

high separation factor in the range of 72–290. The effect of test

temperatures is more pronounced for PERVAP 2210 than PER-

VAP 2510 as increases in the polymer free volume [34] andthe frequency of penetrant diffusion jumps [35] at higher tem-

peratures are able to cause the two species to readily transport

through; thereby the lower separation factor at high temperature

for PERVAP 2210 is expected.

For the three-layer PVA membrane fabricated in our work,

the significantly high separation factor has been obtained as

reflected in Table 3, comparable to that reported previously

[30]. For example, the separation factor of dehydration from

an 80 wt% aqueous ethanol solution can reach 779 at 60 ◦C.

Compared to PERVAP 2210 and PERVAP 2510, the increase of

permeation flux with temperature is less significant for the mem-

branefabricated in our study. Theseresults suggest the fabricatedmultilayer PVA membranes have high degree of cross-linking.

As a consequence, tight chain packing and high selectivity for

water/ethanol can be expected for the fabricated membranes,

thus producing higher degree of separation at low temperatures.

3.3.2. Zeolite-incorporated multilayer PVA membranes

From the experimental results, our multilayer composite

membrane is evidently more superior to commercial mem-

branes in terms of separation factor of water/ethanol, and much

more tolerable to high temperature. In effort to further pro-

mote the pervaporation separation performance of the existing

fabricated membrane, zeolites of several types, at a 20 wt%

loading, were incorporated into the separating PVA layer to

produce multilayer mixed matrix membranes. The experimen-

tal pervaporation results of zeolite-filled membranes, at 80 wt%

feed ethanol concentration, are displayed in Figs. 3 and 4

Fig. 3. Pervaporation fluxes of the zeolite-filled three-layer PVA membranes

with a zeolite loading of 20 wt% for dehydrating ethanol aqueous solution

(20 wt% water).

Fig. 4. Separation factor of the zeolite-filled three-layer PVA membranes with

a zeolite loading of 20 wt% for dehydrating ethanol aqueous solution (20 wt%

water).




It can be seen from Fig. 3 that all these zeolite-incorporated

multilayer PVA membranes, except silicallite-1, have consid-

erably higher total pervaporation fluxes as compared with the

unfilled PVA membrane. In terms of the pervaporation separa-

tion factor, all these the zeolite-incorporated membranes have

higher values than the unfilled one except the zeolite NaY-

filled membrane that exhibits much lower separation factor.

These results indicate that at least some penetrant molecules

transport across the membrane through the zeolite pores and

thus the interaction between the penetrant and zeolite pore sur-

face has an important role in affecting the membrane perfor-

mance. The different performances observed for these zeolites

are believably related to their characteristic features: pore size,

its composition and structure. These properties strongly affect

zeolitic molecular sieve effect and the hydrophilic/hydrophobic

nature.

After incorporating zeolitic molecular sieves into the PVA

polymer matrix, the intrinsic properties of the membrane mate-

rials have varied. As highlighted in recent works [27,29], the

membrane pervaporation flux and separation factor are heavilydependent on the operating conditions, which make a meaning-

ful comparison of data nearly impossible and obscure the effect

of the driving force in the pervaporation process. Therefore, we

have evaluated the pervaporation results of the ethanol–water

mixture in terms of permeance and selectivity for clearly under-

standing the effects of incorporated zeolites on membrane sep-

aration performance.

For the polymer-based pervaporation separation, the

solution-diffusion model may be applied. Thus, the permeation

flux ( J ) can be written as:

J water = Qwater(pfeedwater − p

permeatewater ) (3)

J ethanol = Qethanol(pfeedethanol − p

permeateethanol ) (4)

where p is the partial vapor pressure of each component and Q

is the membrane permeance. The partial vapor pressure of water

and ethanol on the membrane feed side can be calculated by

using the Wilson’s equation, as described in our preceding work

[30]. The membrane selectivity (β) is defined as the ratio of the

water permeance over the ethanol permeance.

β =Qwater

Qethanol(5)

Fig. 5a–c shows the pervaporation performances in terms of per-

meance and selectivity of the A-type zeolite-filled and unfilled

membranes for an ethanol–water mixture containing 20% of

water at various temperatures. Also included in Fig. 5a–c are

those pervaporation data obtained for the UOP 3A-filled mem-

branes [30]. It can be found that the unfilled three-layer mem-brane either manually cast [30] or machine-cast has similar

performance to each other. Similar to the pervaporation flux and

separation factor shown in Figs. 3 and 4, these membranes all

exhibit increased water and ethanol permeances but decreased

selectivities as the temperature arises. The temperature effect

can be explained by the variations of polymer free volume and

mobility of penetrants. At high temperature, the PVA free vol-

ume increases remarkably as a consequence of random thermal

motion of the polymer chains [33]. Furthermore, the theoretical

Fig. 5. Pervaporation performance in terms of (a) water permeance, (b) ethanol permeance and (c) selectivity of the A-type zeolite-filled three-layer PVA membranes

for dehydrating ethanol aqueous solution (20 wt% water). (*) From Ref. [30].




diffusivity of the permeating molecules can increase exponen-

tially as temperature increases [34]. Therefore, more penetrant

molecules can be transported through the membranes at high

temperatures, resulting in enhanced permeances for both water

and ethanol. The temperature effect is more pronounced on the

transport rate of ethanol than water as reflected by more signifi-

cantly increased ethanol permeance (Fig. 5a and b). As a result,

the dehydration selectivity reduces at higher operation temper-

ature (Fig. 5c).

Comparing with unfilled membranes, the A-type zeolite-

incorporated membranes all have higher ethanol dehydration

selectivities. However, Gao et al. and Okumus et al. have

reported that a decrease in separation factor were obtained after

the incorporation of zeolite A, which may be due to their mem-

brane casting techniques as revealed by the formation of porous

cave-like structures [14–16]. The multilayer A-type zeolite-

filled membranes also exhibit higher water permeances and

ethanol permeances except the permeances of UOP 3A-filled

membrane. The higher extent of separation can be explained by

the molecular sieving effect and zeolite hydrophilicity. Both fac-tors tend to increase the water selectivity. These A-type zeolites

all have pore sizes (see Table 2) larger than the kinetic diameter

of water molecule (0.264 nm) but smaller than or close to that of

ethanol molecule (0.430 nm) [8]. This distinction may possibly

induce the molecular sieving effect of the zeolite A-based mem-

branes. The Si/Al ratio of zeolite A is 1.0 which makes it one

of the most hydrophilic zeolites. The hydrophilicity, introduced

by zeolite A along with the polymer itself, can make the pre-

pared membrane able to form more specific interactions, such

as hydrogen bonding between the membrane functional groups

and water molecules, more preferably attract water molecules

and transport through the membrane. This effect has also been

reflected by higher water permeances for these zeolite-filled

membranes as seen from Fig. 5a. Furthermore, zeolite particles

are more resistant to swelling caused by water, and then reduce

possibility of loosening of polymeric chains within the zeolite-

based membrane, thus are able to achieve high selectivity.

Among the A-type zeolite-incorporated membranes, it can be

seen that the pervaporation permeance of either water or ethano

is the highest for COM 5A, then for UOP 4A, COM 3A and

lowest for UOP 3A. But the membrane selectivity follows the

opposite order;this is thesameorderthat their pore size decrease

As described earlier, these zeolites are all hydrophilic due to the

high aluminum content. Ion exchange changes the local polarity

in the pores and therefore the adsorption, but may not affect the

hydrophilic nature. Thus, the pore size variation arising from

the ion exchange treatment seems to be the main reason for the

ethanol dehydration results.To better examine the effect of zeolite hydrophobic

hydrophilic nature on the membrane separation performance

we have chosen two FAU zeolites, i.e., NaX and NaY, to study

their influences in separating water from ethanol–water solu-

tion. These two zeolites have the same zeolitic framework bu

with different aluminum contents. The experimental pervapo-

ration results for zeolite NaX and NaY-filled membranes are

shown in Fig. 6a–c. It can be observed that NaX-filled mem-

Fig. 6. Pervaporation performance in terms of (a) water permeance, (b) ethanol permeance and (c) selectivity of the NaX and NaY zeolite-filled three-layer PVA

membranes for dehydrating ethanol aqueous solution (20 wt% water).




brane shows much higher separation selectivity than the unfilled

membrane, while NaY-filled membrane produces significantly

lower degree of separation performance than the other two.

Zeolite NaX and NaY basically have the same pore size of

7.4 A but the former is more hydrophilic due to a lower Si/Al

ratio of 1.3 than NaY of 2.5. The large pore size and rela-

tively hydrophobic nature of NaY may explain the rather low

separation factor obtained by NaY-incorporated PVA mem-

brane for ethanol/water system. The aperture size of 0.74 nm

is significantly larger than kinetic diameters of both water and

ethanol molecules. Thus, NaY zeolite could not perform any

molecular sieving effect to water and ethanol or exhibit any

preferred attractive forces to water than ethanol, instead, may

provide the preferable passage for ethanol penetrant to trans-

port through the membrane. Subsequently, greater ethanol per-

meance has obtained for the NaY-incorporated membrane as

shown Fig. 6b. The less hydrophilic (or more hydrophobic)

nature of zeolite Y seems to agree with that previously reported,

where Y-type zeolite–PDMS membranes were employed for

removal ethanol from water via pervaporation, and resultedin an increase in both separation factor and flux [18,19].

NaX zeolite, being more hydrophilic, has more trivalent atoms

(e.g., Al) substituted for Si atoms and thus possesses more

charge-balancing cations which are occluded in the zeolitic

framework. The electrostatic forces formed by the negatively

charged framework and positively charged cations have ren-

dered it more hydrophilic and are able to selectively attract water

molecules than ethanol molecules, thereby producing higher

selectivity for water. Therefore, higher separation selectivity has

been obtained after adding NaX zeolite despite the large pore

dimension.

Shown in Fig. 7a–c is pervaporation performance results

for the zeolite silicalite-1 and beta-filled membranes. To our

knowledge, this is the first time to apply zeolite beta to fab-

ricate polymer-based composite membranes for pervapora-

tions. Very interestingly, the beta-incorporated membrane yields

much better separation performance than the unfilled membrane

whereas the silicalite-1-filled membrane produces compara-

ble water selectivities. It must be noted that the water selec-

tivity for zeolite materials can be affected not only by their

hydrophilic/hydrophobic property (due to the Al content) but

also by their surface properties and shape selectivities to water

and ethanol [31]. Since silicalite-1 is the most hydrophobic one

amongst the tested zeolites (which is qualitatively determined

from their Si/Al ratios), its incorporation into PVA membrane

likely results in the decease of the water permeance as compared

to the unfilled membrane. In addition, silicalite-1 has sinusoidal

channels that possibly drag the transport of the penetrants, lead-

ing to the deceased ethanol permeance. However, the presence

of silanol groups on the zeolite surfaces stemming from the

intracrystalline boundaries and defects or the aluminum increasethe local hydrophilicity to water. The unique asymmetrical aper-

ture (5.2× 5.7) and sinuous channels of silicalite-1 may very

likely produce additional shape selectivity to small molecular-

sized water. As a sequence, the membrane selectivity has no

much variation.

Similar to silicalite-1, beta zeolite also possesses sinuous

three-dimensional channel systems and a specified asymmet-

rical aperture (7.1× 7.3). Compared to silicalite-1, beta zeolite

generates better pervaporation results in terms of higher water

selectivity and higher permeances. The Si/Al ratio of zeolite

beta of 16.0 is very much lower than zeolite silicalite-1 of 196,

Fig. 7. Pervaporation performance in terms of (a) water permeance, (b) ethanol permeance and (c) selectivity of the silicalite-1 and beta zeolite-filled three-layer

PVA membranes for dehydrating ethanol aqueous solution (20 wt% water).


http://slidepdf.com/reader/full/copy-of-per-vapor-at-ion-study-of-aqueous-ethanol-solution-thro


hence beta has more charge-balancing cations in the vicinity of

the substituted Al atom and is significantly less hydrophobic

than silicalite-1. This additional feature further augments the

water selective capabilities of zeolite beta, producing greater

extent of separation. The larger dimension of zeolite beta aper-

ture size allows more molecules to transport through, explaining

the higher permeance generated as compared to silicalite-1.

3.3.3. Temperature effect

Temperature is an important process variable affecting the

membrane performances in terms of pervaporation fluxes and

permeances, as shown in Figs. 4–7. Many studies have shown

that the variation of the pervaporation flux with temperature can

be expressed by an Arrhenius Eq. (6):

J = J 0 exp

−EJ

RT

(6)

where J 0, E J , R and T are the pre-exponential factor, apparent

activation energy of the permeation flux, gas constant (kJ/mol)and feed temperature (K), respectively. From the Arrhenius rela-

tionship, the pervaporation activation energy (i.e., the energy

barrier for the species to transport through the membrane) can

be evaluated from Eq. (6). Based on the respective water and

ethanol fluxes obtained at pervaporation temperature of 60, 80

and 100 ◦C, the E J values of water and ethanol through the par-

ticular membrane can be found from the slopes of ln J versus

1/ T plots. The E J values obtained are presented in Table 4.

In terms of the pervaporation permeance, the following rela-

tion can be obtained:

Q = Q0 exp−EQ

RT

(7)

where Q0 and E Q are the pre-exponential factor, apparent acti-

vation energy of the membrane permeance. Eq. (7) can be used

in pervaporation to determine the activation energy E Q from the

slope of ln Q versus 1/ T plot instead of ln J versus 1/ T plot. As

pointed out by Feng and Huang [36], the difference between

E J and E Q is the molar heat of vaporization H v, expressed as

follows.

EQ = EJ −H v (8)

Table 4

Evaluated activation energy data for pervaporation membranes investigated

Membrane E Q (kJ/mol) E J (kJ/mol) H (kJ/mol)

Water Ethanol Water Ethanol Water Ethanol

Unfilled 12.6 47.7 53.6 87.8 41.1 40.1

COM 3A 4.2 49.3 45.3 89.5 41.1 40.1

UOP 4A 2.7 46.0 43.8 86.1 41.1 40.1

COM 5A 6.8 43.1 47.9 83.3 41.1 40.1

NaX 5.4 40.1 46.4 80.2 41.1 40.1

NaY 2.1 44.2 43.1 84.3 41.0 40.2

Beta 5.9 41.2 46.9 81.4 41.0 40.2

Silicalite-1 8.2 34.7 49.2 74.8 41.0 40.2

This equation explicitly shows that the enthalpy change due

to the phase transition in pervaporation affects the permeation

behavior.

Table 4 presents the activation energies estimated for the

zeolite-filled and unfilled membranes. It can be seen that that

for all the membranes the activation energies of ethanol calcu-

lated from either the permeance (i.e., E Q

) or the flux (i.e., E J

)

are much higher than those of water, correspondently. There-

fore, more energy is required for ethanol molecules to transport

acrossthe membrane at the same conditions. This is desirable for

our dehydration purpose from ethanol-rich solutions. The lower

activation energies of water than ethanol are the intrinsic prop-

erties of these hydrophilic membrane materials. On the other

hand, the experimental temperature has significantly stronger

effect on the ethanol permeance than the water permeance since

the activation energy ( E Q) for ethanol is around four times

than that for water. For example, the activation energies ( E Q)

of the unfilled membrane for ethanol and water are 47.7 and

12.6 kJ/mol, respectively.

With the addition of zeolites, E Q of water is observed toconsiderably decrease for all zeolites whereas E Q of ethano

deceases a little, as compared to those of the unfilled mem-

brane. These results may suggest that (1) the energy barrier

for both water and ethanol has decreased and then these two

molecules can more readily transport through the materials

especially for water; (2) the permeances of both water and

ethanol have become less temperature-dependent. Due to the

activation energy deceases for both water and ethanol, the pen-

etrant permeances should increase after adding a zeolite. As

shown in Figs. 5–7, this is true for most zeolites investigated

Since more significant drop of the activation energy for water

than ethanol has resulted, or equivalently, greater amount ofwater molecules than ethanol molecules can transverse across

the membrane per unit time, the water/ethanol selectivity thus

are significantly increased, which is consistent with our perva-

poration results (Figs. 5–7).

By comparing the activation energies ( E Q) obtained from

the permeance with those ( E J ) from the flux, it can be found

that the former are significantly lower than the latter, which

in turn indicates that the fluxes of water and ethanol, and the

separation factor are more strongly dependent on the feed tem-

perature than the permeances of two components and selectivity

These behaviors have resulted from that both permeance and

selectivity only depend on membrane intrinsic properties but

flux and separation factor are also dependent on the experi-mental operating conditions. That is to say, the operating tem-

perature influences both the membrane intrinsic property and

the driving force for penetrant transport through the mem-

branein pervaporation process. Recalling that E J (characterizing

the overall temperature dependence) is the sum of E Q (char

acterizing temperature dependence of membrane permeance)

and H v (the molar heat of vaporization), it is clear that the

driving force is closely related to the phase transition happen-

ing in the pervaporation process, and thus strongly depends

on the operating temperature. As shown in Table 4, the esti-

mated H v values for water or ethanol are almost the same

among the eight different membranes investigated. They are




41.0 and 40.1 kJ/mol, respectively, close to those in literature

[37].

4. Conclusion

In this paper, we have investigated the effect of incorporated

zeolites on the separation performance of the PVA-based mul-

tilayer membranes for the dehydration of highly concentrated

ethanol aqueous solution. These composite membranes were

manufactured by using a casting machine with non-woven fab-

ric polyester RS21 as the supporting layer, PAN as the porous

backing layer and poly(vinyl alcohol) (PVA) as the active sepa-

rating layer.The membrane pervaporation performance has been

evaluated with regard to the flux and separation factor or the per-

meance and selectivity. The following conclusions can be drawn

from this study:

(1) The unfilled PVA-based multilayer membrane is found to

have superior separation performances to the commercial

PERVAP 2210 and PERVAP 2510 membranes, showinggreat potential for industrial applications.

(2) The addition of zeolites into the top PVA selective layer

has increased both separation factor and the overall flux,

suggesting that at least part of the penetrant transport take

place through the zeolite pores and the interaction between

the penetrant and zeolite have an important role in affecting

the membrane performance.

(3) In terms of water permeance, ethanol permeance and selec-

tivity, the separation performance of the A-type zeolite-

filled membranes tend to strongly depend on the zeolite

pore size: smaller pore sized zeolite A canpromote the water

selectivity but decrease the permeance whereas the oppositeapplies to larger pore sized zeolite A.

(4) The zeolite hydrophilic/hydrophobic properties are

observed to significantly influence the separation perfor-

mance of the resultant zeolite-filled membranes. For NaX

and NaY zeolites with large pore-size and the same zeolite

framework, the more hydrophilic NaX has produced higher

water selectivity even though both have led to boosted

permeances for water and ethanol.

(5) Hydrophobic silicalite-1 has rendered the hydrophilic PVA-

based membrane yield much lower permeances for both

penetrants. Less hydrophobic zeolite beta has resulted in

increases of both permeance and selectivity, probably due

to its higher aluminum content and its structural shape pref-erence.

(6) The activation energies obtained for water and ethanol from

their respective fluxes and permeances have revealed that

with the aid of zeolites less energy is required for the pene-

trant molecules to pass through the membrane, and that the

permeances and selectivity are less temperature-dependent

than the fluxes and separation factor.

Acknowledgements

The authors gratefully acknowledge Universal Oil Products

for financial supporting this research. Z. Huang would like to

thank Prof. Neal T.S. Chung for his generous help and valuable

comments. Special thanks must go to Mr. Ralph DiPalma at

Crane and Company Inc. for providing polyester (PET) non-

woven fabric.

Nomenclature

A effective membrane area (m2)

E J apparent activation energy of the permeation flux

from the Arrhenius equation (kJ/mol)

E Q apparent activation energy of the permeance from

the Arrhenius equation (kJ/mol)

H v molar heat of vaporization of either water or

ethanol (kJ/mol)

J pervaporation flux (g/(m2 h))

J 0 pre-exponentialfactor of the permeationflux from

the Arrhenius equation (g/(m2 h))

p partial vapor pressure of either water or ethanol

(kPa)Q membrane permeance of either water or ethanol

(g/(m2 h kPa))

Q0 pre-exponential factor of the permeance from the

Arrhenius equation (g/(m2 h kPa))

R universal gas constant (kJ/mol)

t experimental time interval (h)

T feed temperature (K)

W mass of the permeate during the interval t at a

steady state (g)

x weight fraction of either water or ethanol in the

feed

y weight fraction of either water or ethanol in the

permeate

Greek letters

α separation factor

β membrane selectivity

References

[1] R.Y.M. Huang, Pervaporation Membrane Separation Processes, Elsevier,

New York, 1991.

[2] T. Matsuura, Synthetic Membranes and Membrane Separation Processes,CRC Press, Boca Raton, 1994.

[3] X.S. Feng, R.Y.M. Huang, Liquid separation by membrane pervapora-

tion: a review, Ind. Eng. Chem. Res. 36 (1997) 1048.

[4] A. Jonquieres, R. Clement, P. Lochon, J. Neel, M. Dresch, B. Chretien,

Industrial state-of-the-art of pervaporation and vapour permeation in the

western countries, J. Membr. Sci. 206 (2002) 87.

[5] S.I. Semenova, H. Ohya, K. Soontarapa, Hydrophilic membranes for

pervaporation: an analytical review, Desalination 110 (1997) 251.

[6] H.L. Fleming, Membrane pervaporation–separation of organic aqueous

mixtures, Sep. Sci. Technol. 25 (1990) 1239.

[7] S. Zhang, E. Drioli, Review: pervaporation membranes, Sep. Sci. Tech-

nol. 30 (1995) 1.

[8] T.C. Bowen, R.D. Noble, J.L. Falconer, Fundamentals and applications

of pervaporation through zeolite membranes, J. Membr. Sci. 245 (2004)

1.




[9] S. Kulprathipanja, R.W. Nousil, N.N. Li, Separation of fluids by means

of mixed matrix membranes in gas permeation, US Patent No. 4,740,219

(1988).

[10] H.J.C. te Hennepe, D. Bargeman, M.H.V. Mulder, C.A. Smolders,

Zeolite-filled silicon rubber membranes. Part I. Membrane preparation

and pervaporation results, J. Membr. Sci. 35 (1987) 39.

[11] A. Jonquieres, A. Fane, Filled and unfilled composite GFT PDMS

membranes for the recovery of butanols from dilute aqueous solutions:

influence of alcohol polarity, J. Membr. Sci. 125 (1997) 245.[12] W. Kujawski, R. Roszak, Pervaporative removal of volatile organic com-

pounds from multicomponent aqueous mixtures, Sep. Sci. Technol. 37

(2002) 3559.

[13] I.F.J. Vankelecom, E. Scheppers, R. Heus, J.B. Uytterhoeven, Parameters

influencing zeolite incorporation in PDMS membranes, J. Phys. Chem.

98 (1994) 12390.

[14] Z. Gao, Y. Yue, W. Li, Application of zeolite-filled pervaporation mem-

brane, Zeolites 16 (1996) 70.

[15] E. Okumus, T. Gurkan, L. Yilmaz, Development of a mixed-matrix

membrane for pervaporation, Sep. Sci. Technol. 29 (1994) 2451.

[16] E. Okumus, T. Gurkan, L. Yılmaz, Effect of fabrication and process

parameters on the morphology and performance of a PAN-based zeolite-

filled pervaporation membrane, J. Membr. Sci. 223 (2003) 23.

[17] X. Chen, Z.H. Ping, Y.C. Long, Separation properties of alcohol–water

mixture through silicalite-1-filled silicone rubber membranes by perva-

poration, J. Appl. Polym. Sci. 67 (1998) 629.

[18] H. Yang, Q.T. Nguyen, Z. Ping, Y. Long, Y. Hirata, Desorption and

pervaporation properties of zeolite-filled poly(dimethylsiloxane) mem-

branes, Mater. Res. Innov. 5 (2001) 101.

[19] B. Adnadjevic, J. Jovanovic, S. Gajinov, Effect of different physicochem-

ical properties of hydrophobic zeolites on the pervaporation properties

of PDMS-membranes, J. Membr. Sci. 136 (1997) 173.

[20] X.M. He, W.H. Chan, C.F. Ng, Water–alcohol separation by pervapora-

tion through zeolite-modified poly(amidesulfonamide), J. Appl. Polym.

Sci. 82 (2001) 1323.

[21] H. Ohya, K. Matsumoto, Y. Negishi, T. Hino, H.S. Choi, The separation

of water–alcohol separation by pervaporation with PVA–PAN composite

membranes, J. Membr. Sci. 68 (1992) 141.

[22] Y.S. Kang, S.W. Lee, U.Y. Kim, J.S. Shim, Pervaporation of

water–ethanol mixtures through crosslinked and surface-modified

poly(vinylalcohol) membranes, J. Membr. Sci. 51 (1990) 215.

[23] S. Takegami, H. Yamada, S. Tsujii, Dehydration of water/ethanol mix-

tures by pervaporation using modified poly(vinyl alcohol) membrane,

Polym. J. 24 (1992) 1239.

[24] J.W. Rhim, C.K. Yeom, S.W. Kim, Modification of poly(vinyl alcohol)

membranes using sulfur-succinic acid and its application to pervapo

ration separation of water–alcohol mixtures, J. Appl. Polym. Sci. 68

(1998) 1717.

[25] J.W. Rhim, S.W. Lee, Y.K. Kim, Pervaporation separation o

water–ethanol mixtures using metal-ion-exchanged poly(vinyl alcohol

(PVA)/sulfosuccinic acid (SSA) membranes, J. Appl. Polym. Sci. 85

(2002) 1867.

[26] H. Bruschke, Multi-layer membrane and the use thereof for the separation of liquid mixtures according to the pervaporation process, US

Patent 5,156,740 (1992).

[27] W.F. Guo, T.S. Chung, T. Matsuura, Pervaporation study on the dehydra-

tion of aqueous butanol solutions: a comparison of flux vs. permeance

separation factor vs. selectivity, J. Membr. Sci. 245 (2004) 199–210.

[28] W.F. Guo, T.S. Chung, T. Matsuura, R. Wang, Y. Liu, Pervaporation

study of water and tert -butanol mixtures, J. Appl. Polym. Sci. 91 (2004

4082.

[29] X.Y. Qiao, T.S. Chung, W.F. Guo, T. Matsuura, M.M. Teoh, Dehydration

of isopropanol and its comparison with dehydration of butanol isomers

from thermodynamic and molecular aspects, J. Membr. Sci. 252 (2005)

37–49.

[30] H.M. Guan, T.S. Chung, Z. Huang, M.L. Chng, S. Kulprathipanja

Poly(vinyl alcohol) multilayer mixed matrix membranes for the dehy

dration of ethanol–water mixture, J. Membr. Sci., in press.

[31] D.W. Breck, Zeolite Molecule Sieves, John Wiley, New York

1964.

[32] M. Stelzer, M. Paulus, J. Hunger, Weitkamp, Hydrophobic proper

ties of all-silica zeolite beta, Micropor. Mesopor. Mater. 22 (1998

1–8.

[33] S.S.T. Ting, S.J. Macnaughton, D.L. Tomasko, N.R. Foster, Solubility

of naproxen in supercritical carbon dioxide with and without cosolvents

Ind. Eng. Chem. Res. 32 (1993) 1471–1481.

[34] Z.F. Wang, B. Wang, X.M. Ding, M. Zhang, L.M. Liu, N. Qi, J.L. Hu

Effect of temperature and structure on the free volume and water vapor

permeability in hydrophilic polyurethanes, J. Membr. Sci. 241 (2004

355–361.

[35] S. Glasstone, K.J. Laidler, H. Eyring, The Theory of Rate Process

McGraw-Hill, New York, 1941.

[36] X.S. Feng, R.Y.M. Huang, Estimation of activation energy for perme-

ation in pervaporation process, J. Membr. Sci. 118 (1996) 127–131.

[37] J.M. Smith, H.C. Van Ness, M.M. Abbott, Introduction to Chemi

cal Engineering Thermodynamics, fifth ed., McGraw-Hill, New York

1996.

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