the separation of water and ethanol by pervaporation

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Journal ofM embrane Science, 68 (1992) 141-148

Elsevier Science Publishers B.V., Amsterdam

The separation of water and ethanol by pervaporation

141

with PVA-PAN composite membranes

H. Ohya”, K. Matsumoto*, Y . Negishi, T. Hinob and H.S. Choi”

“Department of M aterial Science and Chemical Engineeri ng, Yokohama Nat ional Uni versit y, 156, Tokiw adai, Hodokaya-

ku, Yokohama 230 Japan)

bChiy oda Corporat i on, 12-1, Tsurumi chuo P-chome, Tsurumi- ku, Yokohama 230 Japan)

‘Depart ment of ndustr ial Chemi stry , Kyungpook Sanup U ni versit y, 55, Hy omokdong, Tong-ku, Taegu 701-702 South-

Korea)

(Received May 29,199l; accepted in revised form November 25,199l)

Abstract

Compositemembranes itha thinpoly vinylalcohol)PVA ] layer coated on poly (acrylonitrile ) [PAN]

supportmembranes ereevaluatedor pervaporationeparation f a water-ethanolmixture. he perme-abilityof purewater hrough he PAN supportmembranewas n the range rom0.214 to 4.32 mm3/ ( m2-

set-Pa), and the molecular weight cut-off was in the range of 35 000 to 100 000. The PVA-PAN com-

posite membranes as prepared were water permeable, and the maximum separation factor was 4800.

From the experimental results, a separation model for the composite membrane for pervaporation is

proposed. The function of the PAN support membrane is to restrict physically swelling of the PVA within

the PAN pores at the PVA-PAN interface, thereby maintaining a dense PVA skin and desirable selectivity.

Keywords: composite membrane; pervaporation; ultrafiltration; separation of ethanol-water mixture;

separation factor

1 Introduction

There has been much progress in the re-

search and development of membranes and

their use in pervaporation processes for the

separation of various organic liquid mixtures

and aqueous organic mixtures, especiallyethanol-water solutions. Currently, much at-

tention is being paid to pervaporation as an en-

ergy saving separation method for the replace-

Corr espondence t o: H. Ohya, Department of Material Sci-

ence and Chemical Engineering, Yokohama National Uni-

versity, 156, Tokiwadai, Hodokaya-ku, Yokohama 240

(Japan).

0376-7388/92/ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.

ment of distillation of aqueous alcohol solutions

and azeotropic mixtures [l-4].

The driving force in pervaporation is the

concentration difference across the membrane

resulting from a pressure difference; this offers

substantial energy savings. In addition, i t has

the advantage of simplifying process plants. I talso has the advantage of avoiding the pollu-

tants used in distillation processes to breakup

azeotropic mixtures.

Sander [5] investigated a pilot plant com-

bining pervaporation and extraction-disti lla-

tion. e reported that the operating cost of his

hybrid process was l/3 to l/4 less than that of

a conventional extraction-distillation process.



142 H . Ohya et al./J, M embrane Sci. 68 1992) 141-148

In recent years, priority in pervaporation re-

search has been given to the development of new

polymer membranes which have a high selec-

tivity, acceptable flux rate, and good stability

and/or durabil ity. For example, in the case of

dehydration of a high-concentration ethanol

solution, above 95 wt. , a membrane which

preferrentially allows the passage of water is

needed. The pioneer membrane used ethanol-

water separation by pervaporation is the GFT-

membrane, which was developed by GFT (Ger-

many) at the beginning of the 1980’s. It is a

composite membrane with a poly(viny1 alco-

hol) [PVA] coated on a poly acrylonitrile)

[PAN] ultrafiltration membrane [61.Recently, to improve further the perform-

ance of pervaporation, several researchers have

investigated the performance of membranes

made from other materials. In addition, during

the past several years, pervaporation has been

studied extensively with the aim of industrial

applications. Wesslein et al. [7] studied the

separation of binary mixtures with the GFT-

membrane for 11 kinds of solvents. Huang and

Yeom [B] reported on the effect of the concen-

tration of a crosslinking agent (amic acid) forPVA membranes on membrane performance.

Nobrega et al. [9] investigated the effect of

chemical and thermal treatments on PVA

membrane performance. Spitzen et al. [lO,ll]

reported on the water permselectivity of PVA

and PAN membranes used in pervaporation

processes. Until now, the results of investiga-

tions on conventional PVA membranes indi-

cate that both its water permselectivity and du-

rability are relatively low. Still, the PVA-PAN

composite membrane is now considered to be

one of the best for aqueous-organic pervapor-

ation processes because of its high water perm-

selectivity and durability. However, the char-

acteristics of PAN ultrafiltration membranes

alone are not yet well understood. Therefore,

this study was carried out to determine the ef-

fects of varying the PAN fabrication param-

eters on the membrane’s pure-water permea-

bility and molecular weight cut-off. We also

examined the effect of the PAN dope compo-

sition on the performance of the PVA-PAN

composite membrane. The composite mem-

branes were also evaluated for pervaporation

separation of a water-ethanol mixture.

2. Experimental

2.1.Preparat i on of he membranes

2.1.1. Materials

The polymers used in this study werepoly acrylonitrile ) , supplied by Aldrich Chem-

icals, and poly (vinyl alcohol) (degree of poly-

merization: 1700, degree of hydrolysis:

99.5 ? 0. 1 , supplied by the Kurare Company

(J apan). The solvent used was analytical grade

dimethylformamide (DMF ) from WAKO

Chemicals (J apan). The non-woven fabrics of

polyester used as a base in the fabrication of

PAN membrane were supplied by the Kanai

J uyo Ind. Co. (J apan).

2.1.2. Suppor t membrane PAN UF-

membrane)

The dope solution consisted of 5, 10 and 15

wt. PAN in DMF. First a non-woven polyes-

ter fabric was fixed on a glass plate (with tape),

and the polymer solution was cast onto the fab-

ric. The solvent was then allowed to evaporate

for a specific period of time at room tempera-

ture or at 60 C in an oven. The glass plate was

subsequently immersed in water gelation bath

(ice water at 2’ C ) for a specific period of time.

Table 1 shows the conditions for preparation of

PAN support membrane.

The membrane was rinsed in tap water for

24 hr, and immersed in an ethanol solution for

24 hr at room temperature to remove the resid-

ual solvent. The membrane was then dried un-

der vacuum at room temperature.



H. Ohya et al.j J. M embrane Sci. 68 1992) 141-148 143

TABLE 1

Conditions of preparation of PAN support membranes

Mem- Concen- Predrying Predrying Gelation Thicknessbrane tration of temperature time time of top

dope (“C) (min) (min) IayeP

solution (w)

(wt. )

A 5

B 10

C 5 60

D 10 60

E 5 60

“by SEM observation.

30 20

30 26

5 30 27

5 30 26

5 30 41

2.1.3. PVA-PAN composi te membr ane

PVA-PAN composite membranes were made

by the procedure proposed by Brtischke [14].

The concentration of PVA in the coating solu-

tion was 7 wt. in water, and the PVA was

crosslinked by the addition of maleic acid. The

amount of crosslinking agent was nit of mal-

eic acid per 20 units of vinyl alcohol monomer

base, assuming 100 reaction.

The composite membrane was made by ther-

mally treating PVA dip-coated on the PAN UF-

support-membrane. The thermal treatment

consisted of placing the membrane in an oven

at 150” C for 2 hr. The degree of hydration of

the crosslinked PVA layer was 45 .

2 2 U lt rafi l t rat ion testi ng of the support

membrane

The pure water flux of the PAN membrane

was determined experimentally, and the mo-

lecular weight cut-off of the membrane was de-

termined with the method reported in Ref. [ 121using aqueous dextran solutions instead of

PEG. In order to keep the levels of concentra-

tion polarization of dextran on the PAN mem-

brane surface the same, the trans-membrane

pressure was chosen to give a similar solution

flux, i.e. 2.85 kPa for the loosest membranes A

and C, 4.55 kPa for membranes B and D, and

35 kPa for the tightest membrane E . The dex-

tran solutes used were T-10, T-40, T-70 and

T-500 from Pharmacia Chem.; the solution was

a multi-component mixture with 100 mg/l of

each component. The permeate samples weretaken after the flux had reached steady state.

The determination of the solution concentra-

tion was carried out with a gel-permeation

chromatograph (Shimadzu, model LC-64 sys-

tem). The columns used were OH Pak B803/s,

804/s and 805 from Shodex.

2.3. Pervaporation

The pervaporation separation of water

(component i) and ethanol (component j)

mixtures was carried out with the PVA-PAN

composite membranes. The details of the per-

vaporation equipment are shown in Fig. 1. The

membrane was placed on the porous stainless

steel plate in the cell. The effective area of the

membrane cell was 23.7 cm2. The feed solution

was circulated with a flow rate of 200 ml/min

from the feed reservoir, which was placed in a

thermostatic bath. The operating temperature

of the equipment was controlled at 25, 40, 50,

or 60°C.The concentrations of the feed solution were

20,40,60,80 or 95 wt. ethanol. The permeate

was condensed in a liquid nitrogen trap, and

the flux was calculated directly by measuring

the permeate weight per unit time. The down

FLI

Pervapnatm cell

B Membrane

C Thermocouple

D Feed reservar

E Thermoset balh

F Feed crculalng pump

G Cold trap

” Prani gauge

I Vacuumump

Fig. 1. Apparatus used for the pervaporation experiments.



144 H . Ohy a et al ./J. M embrane Sci. 68 1992) 141-148

stream vacuum pressure was maintained be-

tween 0.01 and 1.5 Torr and measured by a Pir-

ani gauge (Okano, model Pg-25). The feed and

permeate composition were measured by gas-

chromatography. The detectors used were TCD

(Shimadzu, model GC-4C) for high concentra-

tions of ethanol and FID (Shimadzu, model GC-

9A) for low concentrations.

The selectivity of the membrane was deter-

mined by the separation factor, Lyi/j,defined for

binary mixtures as follows:

3. Results and discussion

3.1. Ul trafi l t rat i on testi ng of PAN support

membrane

The characteristics of the PAN support

membrane are shown in Table 2. The permea-

bility of pure water at 25 o C was in the range of

0.214 to 4.32 mm”/ (m2 set Pa), and the relative

molecular weight cut-off was between 35 000

and 100 000.

3.2. Pervaporation

3.2.1. Permseparat i on of w at er- et hanol

mixtures

The measured separation for the composite

membrane of PVA coated on UF-membrane A

TABLE 2

Characteristics of PAN supportmembranes

Membrane Pure water Molecular weight

permeability (at 25°C) cut-off

[mm3/(m2-set-Pa)] (g/mol)

A 4.32 100 000

B 3.28 76 000

C 2.63 42 000

D 1.81 50 000E 0.214 35 000

(listed in Table 2) is shown in Fig. 2 together

with that for the GFT membrane [ 151. The UF-

membrane A is known to be water permselec-

tive and the composite membrane (PVA-PAN-

A) shows better behavior than the GFT mem-

brane for the entire ethanol concentration range

investigated in this study [21.

The dependence of the separation factors and

the permeation fluxes on the temperature for

another PVA-PAN composite membrane is

shown in Fig. 3. The permeation flux increased

with increasing temperature, but the separa-

tion factor did not change. A linear relation-

ship is observed between the permeation flux

and the inverse of the absolute temperature.This suggests that the membrane structure did

not change as a result of temperature changes.

3.2.2. M embrane durabi li ty

Pervaporation experiments were carried out

in which the feed concentration of ethanol was

continuously varied from high to low (H-L

segment in Fig. 4) and low to high (L-H seg-

ment ). The change of the permeation flux and

the separation factor with time for a PVA-PAN

composite membrane with support membraneB is shown in Fig. 4. The numbers in the figure

(data point labels) are the days at which the

data were collected. In the H-L segment, the

separation factor for an initial feed concentra-

tion of 95 wt. was 4200, and 70 for 20 wt. at

EtOHCont. in F eed Cwt./J

Fig. 2. Pervaporation curve of composite membranes and

vapor-liquid equilibria for water-ethanol mixtures.



H. Ohya et al./J. M embrane Sci. 68 1992) 141-148

11 T X103CK- ' II I

60 50 40 25' C

Fig. 3. Dependence of total flux and separation factor on

temperature for a PVA-PAN composite membrane.

(Membrane: PVA-B ) .

v) ' 0 20 40 60 80 100EtOH Cont. in Feed CwtX3

Fig. 4. Permeation flux and separation factor with of time

for a PVA-PAN composite membrane. (Membrane: PVA-

B).

145

the turning point. The concentration of ethanol

in all the permeates was in the range of 0.27-

0.41 wt. for the entire feed concentration

range. Note that hysteresis was observed for

both the separation factor and the flux; the fi-

nal values did not return to their original levels.

The final value of the separation factor in the

L-H segment was approximately l/lOth the

initial value in the H-L segment. The final flux

value was 7.5 times the initial value.

The changes of the permeation fluxes for both

water and ethanol with time and varying feed

concentration are shown in Fig. 5. The differ-

ence in initial value and final value in permea-

tion flux for ethanol was larger than that forwater.

From these results, we believe that the PVA

coating layer of the composite membrane swells

as the ethanol concentration in the feed solu-

tion is decreased, resulting in a more extensive

(more open) polymer network, which remains

open as the ethanol concentration is raised to

its original level.

3.2.3. Compari sons of var i ous supportmembranes

A comparison of the separation factors and

permeate fluxes for PVA-PAN composite

membranes with a different PAN support is

-1 -

k$ 10- l -

so-2 -

;(i l o-3

10-1

' o-5-0

I I I 1 I ,

20 40 60 80 11EtOH Cone. in Feed CwW1

1

Fig. 5. Permeation fluxes for both water and ethanol with

of time for a PVA-PAN composite membrane at 60°C.

(Membrane: PVA-B).



146 H. Ohya et al /J. M embrane Sci . 68 1992) 141-148

shown in Fig. 6 and 7. Also, the relationship

between the separation factor and the pure

water permeability of the PAN support mem-

brane are shown in Fig. 8. It was found that thePVA-PAN composite membranes have higher

separation factors than conventional non com-

posite PVA membranes, and that the water

Feed Cont. CwlXl

A 60

L

- 0 PVA-A

PVA-B

_ , PVA-C

0 20 40 60 80 100

EtOH Cont. in Feed Cwt”1.1

Fig. 6. A comparison of the separation factors for PVA-

PAN composite membranes with different PAN supportmembranes at 60°C.

_ I3 PVA-D

1o-3; 1 I 8 1;O 40 60 60 100

EtOH Cont. in Feed Cwt l

Fig. 7. A comparison of the permeation fluxes for PVA-

PAN composite membranes with different PAN support

membranes at 60 ’ C.

I I , I , , , , ,

10-l 1 IO

Pure Water Permeability ~VII~/ITI~spa3

Fig. 8. Separation factors of PVA-PAN membranes at 6O’C,

as a function of pure water permeability of PAN support

membranes.

permselectivity of PVA increases with the ad-

dition of a support PAN membrane. Unlike

composite membranes, the PVA membrane

swelled and broke up in a feed with a composi-

tion of less than 60 wt. ethanol, as shown in

Fig. 6. Furthermore, it was observed that thepermeation flux decreased as the densification

of the PAN support membrane increased.

A homogeneous PAN membrane has been

reported to be water permselectivity [ 10,ll 1.

Therefore, it should be possible to increase the

separation factor by increasing the densifica-

tion of the PAN support membrane. However,

our experimental results in Fig. 6 show that the

opposite is true, i.e. the separation factor de-

creased with increasing densification of the

PAN support membrane (i.e. from PVA-A to

PVA-E). This difference between PVA-A and

PVA-E is the largest at a high feed concentra-

tion. The separation factor has a maximum

value in the vicinity of 80 wt. ethanol and is

lower for 95 wt. ethanol. This fact suggests

that the relative diffusion rates for ethanol and

water through PVA are reversed between

ethanol concentrations of 80 and 95 wt. [lo].



H. Ohya et al./J. M embrane Sci. 68 1992) 141-148 147

Feed Solution Feed Solution

Permeate

4

Permeate

b)

Fig. 9. Models of PVA-PAN composite membrane during pervaporation.

That is, at 80 wt. ethanol, the ethanol diffu-

sion rate is higher.

From the experimental results of this study,

the model shown in Fig. 9 is proposed. (1) The

feed solution dissolves into the PVA layer, dif-fuses through this layer, and subsequently de-

sorbs, i.e. vaporizes toward the down stream

side through the small pores of the support

membrane. We suppose that a dense, non-

swollen skin forms at the interface between the

PVA coating and the PAN support membrane

[ 131, and that this dense skin increases the se-

lectivity of the composite membrane. (2)

Swell ing of the PVA skin layer is suppressed by

the rigid PAN pore structure in which the PVA

coating is partially imbedded. In the case of a

small pore structure in the support membrane,

the coating is probably not imbedded deeply,

and thus the support only weakly suppresses

swelling (Fig. 9b). This model is consistent with

the selectivities shown in Fig. 8. The increase

in selectivity (separation factor) with respect

to the pure water permeability (pore size) is

relatively large for high feed concentrations, but

small for low feed concentrations. I t may be so

that at the lower ethanol concentrations, moreof the PVA coating layer is swollen, resulting in

a thinner skin.

Furthermore, Figs. 6, 7 and 8 indicate that

highly permeable membranes (PVA-A and

PVA-B ) have relatively high separation factors

at high ethanol concentration, but that their

separation factors drop below those of the less

permeable membranes at middle and low

ethanol concentrations. Possibly, the larger

pore structure of the more permeable mem-

branes does not have sufficient mechanical

strength to suppress the swelling which occurs

at low ethanol concentrations, resulting in the

reduced separation factors discussed above.

4. Conclusions

(1) The permeability of pure water through

a PAN support membrane is in the range from

0.214 to 4.32 mm”/ (m2-set-Pa) and the molec-

ular weight cut-off is in the range from 35 000

to 100 000.

2 ) The PVA-PAN composite membrane as

prepared is water permselective for the ethanol

concentration range of 20 to 95 wt. in the feed,

and the separation factor exhibits a maximum

value of 4800.

(3 ) The dependence of the permeate flux on

temperature is consistent with the Arrhenius

relationship, and the separation factor hardly

varied in the temperature range investigated.

(4) I t is verified that the permselectivity of

the membrane is increased by the addition of

the PAN support membrane, i.e. by making the

PVA-PAN composite membrane. However, the

separation factor decreases with excessively in-creasing densification of the support

membrane.

(5) A function of the PAN support mem-

brane is to suppress the swelling of the PVA

layer at the PVA-PAN interface and to create

a dense skin, thereby improve the selectivity by

allowing PVA to intrude into the micropores of

the PAN support membrane. Therefore, the

support membrane porosity is of great impor-

tance in the selection of a support, capable of



148 H. Ohya et al./J. M embrane Sci. 68 1992) 141-148

forming the dense (non-swollen) skin. The

support membrane having a molecular weight

cut-off of 100 000 (composite membrane A) to

76 000 (B ) might be the best choice. On theother hand a membrane having a lower pure

water permeability (less than 1.8 mm”/ ( m2-sec-

Pa) might not be good because it is not able to

suppress the swelling of the PVA membrane on

top.

8

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13

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