Dehydration of acetic acid by pervaporation through an asymmetric polycarbonate membrane

Dehydration of acetic acid by pervaporation through anasymmetric polycarbonate membrane

James Huang a, Meng-Ling Tu c, Yi-Chieh Wang b, Chi-Lan Li b,Kueir-Rarn Lee b,*, Juin-Yih Lai c

a Department of Chemistry, Chung Yuan University, Chung Li 32023, Taiwan, ROCb Department of Chemical Engineering, Nanya Institute of Technology, Chung Li 32034, Taiwan, ROC

c Membrane Research Laboratory, Department of Chemical Engineering, Chung Yuan University, Chung Li 32023, Taiwan, ROC

Received 22 November 1999; received in revised form 11 April 2000; accepted 8 June 2000

Abstract

Dehydration of acetic acid by pervaporation through an asymmetric polycarbonate membrane was investigated.

Investigations focused on the e�ects of feed composition, degree of swelling, and the molar volume of non-solvent

additive on the pervaporation performances. The membrane formation systems are discussed in terms of the presence of

a nonsolvent in the casting solution, the kinds of coagulation media, and the polymer concentration. The rate of liquid±

liquid demixing decreases with increasing alcohol molar volume of the coagulation medium. The durability of the

asymmetric polycarbonate membranes with n-hexanol additive was tested for 3 wt.% aqueous acetic acid solution at

25°C for two months and it was found to be stable. Ó 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Asymmetric polycarbonate membrane; Coagulation media; Liquid±liquid demixing; Pervaporation

1. Introduction

Recently, there has been increased interest in the use

of the pervaporation membrane separation process for

separation of organic liquid mixtures. The advantages of

low cost, acceptable permeation rate, simplicity, and

high separation factor make it a promising separation

process. Particularly for the ethanol/water mixtures, a

lot of research has been carried out to develop new

membrane materials [1±3]. Compared to the separation

of aqueous ethanol solution, the separation of other

organic liquid mixtures have received relatively little

attention. Aqueous acetic acid solution is used in the

chemical industry in the production of acetic anhydride,

vinyl acetate, and terephthalic acid, etc. Nevertheless,

end-product production is almost always accompanied

by waste and/or recycling streams containing acetic acid/

water mixtures. Therefore, separation of acetic acid so-

lutions is of interest in the chemical industry. Straight

distillation consumes too much energy. Due to the

closeness of water and acetic acid boiling points, a large

number of strays and a high re¯ux ratio are necessary

to obtain glacial acetic acid. From an energy-saving

standpoint, pervaporation can be a useful alternative for

acetic acid/water separation. Some investigators have

recently reported studies with respect to the separation

of acetic acid/water systems [4±6]. For example, Nguyen

et al. used blends of hydrophilic polymer to separate

acetic acid solution [7]. Yoshikawa et al. have studied

the separation of acetic acid solutions by pervaporation

through polymer membranes containing pendant carb-

oxylic acid [8]. In addition, aromatic polyimide mem-

branes are particularly stable to organic carboxylic acids

and possess very high thermal stability. Several studies

on high-performance polymer membranes for pervapo-

ration have been reported. Maeda et al. investigated the

separation of acetic acid/water mixture by pervapora-

tion through aromatic polyamide and polyamideimide

European Polymer Journal 37 (2001) 527±534

* Corresponding author. Fax: +886 4563672.

E-mail address: [email protected] (K.-R. Lee).

0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved.

PII: S0 01 4 -3 05 7 (00 )0 0 13 5 -X

membranes containing anionic groups [9]. However,

one of the disadvantages of the pervaporation separa-

tion process is the low permeation rate, especially with

highly selective membrane materials. Thus, we at-

tempted to prepare asymmetric membrane with higher

permeation rate while the selectivity remained the same

or decreased slightly. Wet phase inversion is the most

widely used technique for preparing asymmetric mem-

branes. The asymmetric membrane is formed by the

exchange of solvent and coagulation medium [10]. Loeb

and Sourirajan were the ®rst to prepare asymmetric

hyper®ltration membrane [11]. Polycarbonate mem-

branes possessing excellent mechanical strength have

been regarded as promising membrane materials for

separation. This paper discusses the preparation of

asymmetric membranes from high polymer concentra-

tion and in the presence of a nonsolvent in a casting

solution. The e�ects of feed composition, degree of

swelling, and the molar volume of nonsolvent additive

on the pervaporation performances of the asymmetric

polycarbonate membrane were studied.

2. Experimental

2.1. Material

Polycarbonate (Uplion S-200) was supplied by

Mitsubishi Gas Chemical Co. Chloroform supplied by

Merck Co. was employed as a casting solvent. Metha-

nol, ethanol, n-propanol, n-butanol and all chemicals

were of reagent grade.

2.2. Asymmetric membrane preparation

The polycarbonate membrane was prepared from a

casting solution of polycarbonate in chloroform. The

asymmetric membranes were prepared from a 10 wt.%

(PC/CHCl3) casting solution with added nonsolvents.

The membrane was formed by casting the solution onto

a glass plate to a predetermined thickness by using a

Gardner knife. The glass plate was immersed in the

coagulation media. Then, the membrane was peeled o�

and dried in vacuum for 24 h. The average membrane

thickness was about 50 lm.

2.3. Pervaporation experiments

A traditional pervaporation apparatus was used in

this study [12]. The e�ective membrane area was 10.17

m2. A vacuum pump maintained the downstream pres-

sure at 3±5 mmHg. The permeation rate was determined

by measuring the weight of the permeate. The compo-

sitions of the feed solutions, permeates, and solutions

absorbed in the membrane were measured by gas chro-

matography (GC; China Chromatography 8700). The

separation factor was calculated from

aH2O=acetic acid � �YH2O=Yacetic acid�=�XH2O=Xacetic acid�;

where YH2O, Yacetic acid and XH2O, Xacetic acid are the weight

fractions of water and acetic acid in the permeate and

feed, respectively. The average results were repeatedly

measured ®ve times for each condition. The standard

deviation was within 10%.

2.4. Sorption measurement

The membranes were immersed in acetic acid±water

mixtures for 24 h at 25°C. They were subsequently

blotted between the tissue paper to remove excess sol-

vent and placed in the left tube of a twin tube set-up.

The system was evacuated while the left tube was heated

with hot water and the right tube was cooled in liquid

nitrogen. The composition of the condensed liquid in the

right tube was determined by GC.

2.5. SEM analysis

The membrane structures were examined by a Hit-

achi (Model S570) scanning electron microscope (SEM).

The membrane samples were fractured in liquid nitrogen

and then coated with gold.

2.6. Determination of the phase diagram

The location of the binodal in the phase diagram was

determined by titration. Various coagulation media were

carefully titrated to di�erent weight percent of polymer

solution at room temperature (25°C) until permanent

turbidity (detected visually) was obtained. This repre-

sented the boundary between the one-phase region and

the two-phase region.

3. Results and discussion

3.1. Preparation of asymmetric pervaporation membrane

To improve the extremely low permeation rate of

membrane, a major membrane technology break-

through was the development of asymmetric mem-

branes, where a very thin selective layer is supported by

a porous sublayer of the same or di�erent material. The

wet or dry±wet phase inversion methods have been

widely used to prepare the above membranes. From the

above viewpoint, an asymmetric polycarbonate mem-

brane was prepared in this article. The e�ect of non-

solvent additive in casting solution and the membrane

preparation methods on the membrane porosity are

listed in Table 1. It shows that the membrane porosity

528 J. Huang et al. / European Polymer Journal 37 (2001) 527±534

does not change signi®cantly for the membrane pre-

pared by using the wet method and the dry±wet method.

Therefore, it is necessary to further investigate the e�ect

of membrane formation procedure on the morphology

of the asymmetric membranes. For example, the di�er-

ent morphologies of asymmetric membranes with non-

solvent (n-butanol) additives are shown in Fig. 1.

Compared with the membrane prepared by wet method,

a defect-tree and thicker top layer are evidently present

in the asymmetric polycarbonate membrane prepared

via a dry±wet phase inversion method. Thus, from the

viewpoint of the prepared asymmetric membrane with

higher permeation rate while the selectivity remained

the same, the dry±wet phase inversion method is sug-

gested as the optimum technology in this article. In

addition, the asymmetric polycarbonate membrane with

n-hexanol additive durability was tested for 3 wt.%

aqueous acetic acid solution at 25°C for two months, as

shown in Fig. 2. It shows that the water concentration of

the permeate approach 100 wt.% while the permeation

rate does not change signi®cantly during the long term

testing procedure. Thus, a high stability asymmetric

polycarbonate membrane was prepared via a dry±wet

phase inversion method.

Table 1

E�ect of nonsolvent additives in casting solution on the membrane porositya

Nonsolvent additive (4 ml) Molar volume (ml/mol) Membrane porosity (%)

Wet method Dry/wet method

n-Butanol 91.5 69.9 70.3

n-Hexanol 124.61 71.4 69.7

n-Octanol 156.36 70.5 71.2

n-Decanol 190.71 70.6 71.4

a Casting solution: 5 g PC/30 ml CHCl3 � 4 ml nonsolvent; coagulation medium: CH3OH and dry/wet phase inversion method:

solvent evaporation 60 s.

Fig. 1. The SEM photographs of PC asymmetric membranes from the system of PC/CHCl3/CH3OH with 4 ml nonsolvent (n-butanol)

additive in casting solution ± wet process: A-1, A-2 and dry/wet process: B-1, B-2.

J. Huang et al. / European Polymer Journal 37 (2001) 527±534 529

3.2. E�ect of coagulation medium on the membrane

formation and the pervaporation performance

The pervaporation performances of 3 wt.% aque-

ous acetic acid solution through various coagulation

medium-prepared polycarbonate membranes are listed

in Table 2. The data shows that ethanol was used as the

coagulation medium, resulting in a lower permeation

rate than that of methanol. These phenomena might be

due to the fact that the binodal curve was located far

from the solvent±polymer axis, resulting in slow liquid±

liquid demixing when ethanol was used as the coagula-

tion medium. The experimentally determined binodal

curves for the above systems are shown in Fig. 3. A

higher alcohol molar volume in the coagulation bath

promoted a delayed mechanism of liquid±liquid phase

inversion and resulted in a thicker membrane skin. Thus,

the permeation rate of the membrane prepared in

methanol coagulation system is higher than that of the

ethanol coagulation system. Similar results were re-

ported by Mulder et al. [13]. Moreover, in order to

further investigate the membrane morphology, the SEM

micrographs of surface and cross-section of the mem-

brane were taken as shown in Fig. 4. These results

con®rm the pervaporation performances indicated in

Table 2.

3.3. E�ect of nonsolvent additives in casting solution on

pervaporation performance

In this study, we attempted to improve the perme-

ation rate of polycarbonate membrane prepared via a

dry±wet phase inversion method for aqueous acetic acid

solution by pervaporation separation. The e�ect of

nonsolvent in the casting solution on pervaporation

performances are listed in Table 3. As the molar volume

of nonsolvent additive increases, the resultant asym-

metric membranes have an increased free volume and a

decreased macromolecular packing density. Therefore,

Table 2

E�ect of coagulation medium on the pervaporation performances

Nonsolvent additive

(ml)

Coagulation medium

CH3OH C2H5OH

P (g/m2 h) a P (g/m2 h) a

n-Butanol 85.5 ±a 70.2 ±a

n-Hexanol 90.2 ±a 85.5 ±a

n-Octanol 237 4.2 109.5 3.1

n-Decanol 385 1.1 114.4 4.5

Casting solution: 5 g PC/30 ml CHCl3 � 4 ml nonsolvent; coagulation medium: CH3OH and C2H5OH; dry/wet phase inversion

method: solvent evaporation 60 s; operation temperature: 25°C and feed composition: 3 wt.% aqueous acetic acid solution.a Water in the permeate 100 wt.%: acetic acid cannot be measured by GC.

Fig. 2. The long term pervaporation test with asymmetric PC

membranes ± casting solution: 5 g PC/30 ml CHCl3 � 4 ml n-

hexanol; coagulation medium: CH3OH; evaporation time: 60 s;

operation temperature: 25°C and 3 wt.% aqueous acetic acid

solution.

Fig. 3. Experimentally determined binodal curves of di�erent

coagulation media: ( ) methanol and ( ) ethanol.


the permeation rate increases and the separation factor

sharply decreases. For example, the permeation rate for

the n-decanol added system was 10 times higher than

that of the unadditive system by dry±wet phase inversion

and it could e�ectively improve the pervaporation per-

formance.

3.4. E�ect of polymer concentration on pervaporation

performance

The e�ect of polycarbonate concentration on perva-

poration performances for the system PC/CHCl3/

CH3OH with 4-ml n-hexanol additive in the casting so-

lution is shown in Fig. 5. It shows that the permeation

rate ®rst decreases but then increases with an increase in

the polymer concentration of the casting solution. These

phenomena might be due to the fact that an increase in

the initial polymer concentration in the casting solution

leads to a much higher polymer concentration at the

surface. Thus, the volume fraction of polymer increases

and consequently, a low porosity is obtained. Never-

theless, another remarkable e�ect, i.e., instantaneous

liquid±liquid demixing, tends to produce porous mem-

brane with macrovoids which appear at high polymer

concentration of the casting solution. Thus, the perme-

ation rate presents the above trend. The membrane

structure was veri®ed by (SEM), as shown in Fig. 6(A)±

(D). This observation agrees with the result shown in

Fig. 5 and clearly supports the hypothesis that two dif-

ferent types of phase separation are responsible for the

Table 3

E�ect of nonsolvent additives in casting solution on the pervaporation performances for asymmetric PC membranes

Nonsolvent additive (4 ml) Permeation rate �g=m2 h� Separation factor �a� PSI

± 40 ±a ±

n-Butanol 85.5 ±a ±

n-Hexnol 90.2 ±a ±

n-Octanol 237 4.2 995

n-Decanol 385 1.1 424

Casting solution: 5 g PC/30 ml CHCl3 � 4 ml nonsolvent; coagulation medium: CH3OH; dry/wet phase inversion method: solvent

evaportation 60 s; operation temperature: 25°C and feed composition: 3 wt.% aqueous acetic acid solution.a Water in the permeate 100 wt.%: acetic acid cannot be measured by GC.

Fig. 4. The SEM microphotographs of PC asymmetric membranes from the system of PC/CHCl3/CH3OH or (C2H5OH) with 4 ml

n-hexanol additive in casting solution ± coagulation medium: (A) methanol and (B) ethanol.


formation of asymmetric membranes: gelation for the

formation of the top layer and liquid±liquid phase sep-

aration followed by gelation of the porous sublayer.

Similar results were obtained by Mulder et al. [13].

3.5. E�ect of acetic acid concentration on the pervapora-

tion performance

Table 4 shows the in¯uence of the feed acetic acid

concentration on the pervaporation performances of

asymmetric polycarbonate membranes with n-hexanol

additive. As the acetic acid concentration in the feed

solution increases, the permeation rate increases while

the separation factor decreases. These results might

be due to the fact that the asymmetric polycarbon-

ate membranes were easily swollen at high acetic acid

concentration of the feed solution. For example, the

di�erence of the solubility parameter between the ace-

tic acid and the asymmetric polycarbonate membrane

�dpc ÿ dAA � 1 �cal=cm3�1=2� is lower than that of the

water and asymmetric polycarbonate membrane

�dpc ÿ dH2O � 13:7 �cal=cm3�1=2�. Hence, the strong af-

®nity between the acetic acid and the membrane plasti-

cizes the membranes, resulting in increase of the

permeation rate. The degree of swelling increases as the

feed acetic acid concentration increases as shown in Fig.

7. These results correspond well with the pervaporation

performance study as indicated in Table 4. Moreover,

the polymer chain becomes more ¯exible resulting in the

Fig. 6. The SEM microphotographs of PC asymmetric membranes from the system of PC/CHCl3/CH3OH with 4 ml n-hexanol ad-

ditive in casting solution ± coagulation medium: CH3OH; evaporation time: 60 s and polymer content: (A) 3 g; (B) 4 g; (C) 5 g; (D) 6 g.

Fig. 5. E�ect of acetic acid concentration in the feed on per-

vaporation with di�erent polymer concentration in casting so-

lution ± casting solution: 5 g PC/30 ml CHCl3 � 4 ml n-hexanol;

coagulation medium: CH3OH and evaporation time: 60 s.


separation factor decreasing with increasing acetic acid

concentration in the feed solution.

3.6. E�ect of feed acetic acid composition on the sorption

properties

The e�ects of feed acetic acid composition on the

sorption properties are discussed in this section.

The sorption experiments were performed to determine

the sorption selectivity of the asymmetric polycarbonate

membranes. The results of the sorption selectivity are

shown in Fig. 8. It shows that all sorption selectivities

are less than 1 at the feed acetic acid concentration lower

than 20 wt.%. In general, the sorption selectivity is

strongly related to the interaction between the permeates

and the membrane. Thus, the high a�nity between the

acetic acid and the asymmetric polycarbonate mem-

brane results in acetic acid concentration in the mem-

branes higher than that of the acetic acid concentration

in the feed. However, the excessive swelling due to water

dissolved into the membrane in spite of its low a�nity

toward the membrane as the feed acetic acid concen-

tration is higher than 20 wt.%. Hence, water molecules

can easily dissolve into the membrane, which results in

an increase in the sorption selectivity.

4. Conclusion

It has been shown in this work that, depending on the

casting solution concentration and coagulation medium

composition, a variety of polycarbonate membranes

with di�erent structural characteristics can be pre-

pared. From the viewpoint of the prepared asymmetric

Fig. 7. E�ect of acetic acid concentration on the degree of

swelling of asymmetric PC membranes ± coagulation medium:

CH3OH; casting solution: 5 g PC/30 ml CHCl3 � 4 ml n-hex-

anol; (r) dry/wet phase inversion method: solvent evaporation

60 s and (d) wet phase inversion method.

Table 4

E�ect of acetic acid concentration on the pervaporation performance for asymmetric PC membranes

Feed concentration (wt.%) Permeation rate (g/m2 h) Separation factor (a) PSI

3 90 ±a ±a

10 125 3.1 387

20 138 5.7 787

30 226 1.7 316

Casting solution: 5 g PC/30 ml CHCl3 � 4 ml n-hexanol; coagulation medium: CH3OH; dry/wet phase inversion method: solvent

evaporation 60 s and operation temperature: 25°C.a Water in permeate 100 wt.%: acetic acid cannot be measured by GC.

Fig. 8. E�ect of feed acetic acid aqueous solution concentration

on the sorption selectivity (acetic acid/water) of membranes ±

casting solution 5 g PC/30 ml CHCl3 � 4 ml Cn±OH; coagula-

tion medium: CH3OH; n � 4; 6; 8; 10; evaporation time: 60 s

and operation temperature: 25°C; (r) C4, (j) C6, (m) C8, (d)

C10.


membrane with higher permeation rate while the selec-

tivity remained the same, the dry±wet phase inversion

method is suggested as the optimum technology in this

article. A higher alcohol molar volume in the coagula-

tion bath promoted a delayed mechanism of liquid±liq-

uid phase inversion and resulted in a thicker membrane

skin. The plasticizing e�ect of acetic acid on the asym-

metric polycarbonate membrane plays an important role

in the pervaporation separation process.

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Dehydration of acetic acid by pervaporation through an asymmetric polycarbonate membrane

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