Pervaporation separation and pervaporation-esterification coupling using crosslinked PVA composite catalytic membranes on porous ceramic plate

j o u r n a l of MEMBRANE SCIENCE ELSEVIER

Journal of Membrane Science 138 (1998) 123-134

Pervaporation separation and pervaporation-esterification coupling using crosslinked PVA composite catalytic membranes on porous ceramic plate Yushan Zhu 1'*, Hongfang Chen Department of Chemical Engineering, Tianjin University, Tianjin, 300072, China Received 21 May 1997; received in revised form 27 August 1997; accepted 27 August 1997

Abstract A composite catalytic membrane with a crosslinked PVA dense active layer coated on a porous ceramic plate support was prepared using a novel method and evaluated with a pervaporation setup for the separation of several organic aqueous mixtures. Several key problems occurred during the preparation procedure are discussed. SEM (scanning electron microscopy), IR (infra-red) (ATR) (attenuated total refraction) and XPS (X-ray photoelectron spectrometry) were used to characterize the catalytic membrane natures. N-Butyl alcohol-acetic acid esterification was used as a model system for investigating into the coupling of reaction with pervaporation in a batch reactor. Different reaction parameters, temperatures, catalyst concentrations and initial reactant molar ratios were studied experimentally. © 1998 Elsevier Science B.V.

Keywords: Membrane reactors; Pervaporation; Catalytic membrane; Esterification 1. Introduction The use of membrane in chemical reaction processes is attracting much attention. A number of investigations have been concentrated on the application of hydrogen separation membranes to reversible gas-phase reaction. Other recent investigations have been made on the application of water-permeable membranes to liquid-phase reactions. The potential applications of membrane technology in reaction engineering are being recognized. Since separation membranes permit selective permeation of a compo*Corresponding author. E-mail: [email protected] 1Present address: The Institute of Chemical Metallurgy, The Chinese Academy of Sciences, Beijing, 100080, China. 0376-7388/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. P I I S 0 3 7 6 - 7 3 8 8 ( 9 7 ) 0 0 2 2 1-4

nent from a mixture, membranes can help enhance the conversion of thermodynamically or kinetically limited reactions through controlled removal of one or more reactant or product species from the reaction mixture. The majority of published work on membrane reactor to date is in the field of biotechnology. The membranes used are typically porous, and the function of the membrane is mainly for immobilizing enzymes, eliminating product inhibition, recycling enzymes and other biocatalyst, and manipulating substrates and nutrients. Recently, extensive studies have been carried out on membrane reactors applied to catalytic dehydrogenation, hydrogenation, and decomposition reactions [1-8]. The concept of using pervaporation to remove by-product species from reaction mixtures was proposed in the early stage of

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Y. Zhu, H. Chen/Journal of MembraneScience 138 (1998) 123-134

pervaporation research by Jennings and Binning [9]. However, literature reports regarding studies on pervaporation membrane for liquid-phase reactions are rather limited due to lack of adequate membrane types. During the last decades a number of water permeable membranes with good permeation flux, chemical and thermal stability have been developed. So the interest in pervaporation membrane reactors was rekindled recently when pervaporation has been proved to be a viable separation techniques in the chemical industry. Presently, pervaporation is applied to dehydration of organic solvents. The dehydration membranes normally work best when water content in feed mixture is not high. Thus, reversible reactions that produce by-product water are a nice means of pervaporation for reaction enhancement. Pervaporation membrane reactors have been studied for esterification of oleic acid and ethanol [10,11], propionic acid and propanol [12,13], erutic acid and acetyl alcohol [14], tartaric acid and ethanol [15], oleic acid and butanol [16] and valeric acid and ethanol [17] with various acids or lipases as catalysts. In some cases, the membrane itself can be catalytically active [18,19]. Waldburger et al. [20] studied heterogeneously catalyzed esterification of acetic acid and ethanol, and proposed cascade arrangements of membrane reactors for continuous operation. A pilot plant test was conducted and an industrial plant operation has been designed and scheduled: in both cases the commercial poly(vinyl alcohol)-based membranes were used [21]. Feng and Huang [22] carried out a parametric study on an esterification facilitated by pervaporation in an attempt to provide a fundamental understanding of the behavior of the membrane reactor. In our laboratory, we have started to study pervaporation membrane preparation experiments since 1992 [23,24]. In this paper, the preparation of the composite catalytic membrane was investigated, several key problems have been solved through novel methods. The catalytic characters and separate char-

acters of the composite catalytic membrane have been evaluated in a batch membrane reactor. Recently, the research on the pervaporation-esterification coupling has focused on the development of the mathematical model that correlates the effects of pervaporationesterification coupling, but the most important preparation procedure has been ignored, now the research in this field should be resumed.

2. Membrane preparation experiment and assessment 2.1. Material Three kinds of PVA having a degree of polymerization of 1750 + 50 were obtained from the Tianjin Tianhuo Chemical Reagent Factory (PVA1), Beijing Hongxing Chemical Industry Factory (PVA2) and Tianjin University Chemical Engineering Factory (PVA3), respectively. The porous ceramic plates were obtained from Shandong Zibo ceramic technology company, their physical properties are represented in Table 1. Glutaraldehyde (GA) used was 50 wt% GA aqueous solution. The water was deionized and other chemicals used were A.R. grade and used without further purification. 2.2. Preparation of the poly(vinyl alcohol) (PVA) composite catalytic membrane crosslinked by glutaraldehyde on porous ceramic plates This procedure contains two steps. First, a crosslinked PVA dense active layer was coated on the porous ceramic plate; Second, Zr(SOn)2-4H20, an inorganic solid acid which was used as the esterification catalyst in this experiment, was immobilized on the dense active layer. The aqueous PVA solution was mixed with an aqueous solution of glutaraldehyde as the crosslinking agent, the sulfuric acid as a catalyst and the mixture

Table 1 Properties of the porousplate Plate averagepore radius

Plate thickness

Plate diameter

Pure water permeation rate

Plate void fraction

0.92 ~tm

4 mm

80 mm

334.3 l/m2h

37%

Y. Zhu, 14. Chen/Journal of Membrane Science 138 (1998) 123-134

C

D

A I

F

i

1°°

°IG

Fig. 1. Apparatusfor preparing compositecatalytic membrane: (A) iron-shelf, (B) membrane making solution, (C) porous ceramic plate, (D) support plate, (E) variable-speed motor, (F) adjuster, (G) input of 220 V stationary voltage. Table 2 Dense active layer membrane preparation conditions Concentration of membrane making solution Viscosity of membrane making solution Revolution speed of motor Time of rotation Drying time in membrane making room Heat-curing temperature Heat-curing time

10% PVAwater solution 2.13-2.18 Pa s 750 rpm 30 s 24 h 110°C 30 min

was stirred until homogeneous. The dense active layer was cast on the porous ceramic plate used the preparation apparatus which was designed in our laboratory and is represented in Fig. 1, the conditions of making membrane are stated in Table 2. The excessive amount of the PVA solution after degassing and before gels formed was coated on the porous ceramic plate which was tested and cleansed before using. The porous ceramic plate was set on the support plate of the preparation apparatus, with the change of the speed of motor there would be different thickness thin layers on the porous ceramic plate because of the effect of the centrifugal force. The speed of the motor and the viscosity of the PVA solution were the two important factors that influenced the natures of the membrane, and following in situ crosslinking and dehydration. The samples were annealed at above their Tg of 85°C, with subsequent slow cooling. The amount of glutaraldehyde used was varied in order to determine the density of crosslinking. The thickness of

125

the dry selective separation layer (crosslinked PVA) was about 20-30 ~tm. The esterification catalyst Zr(SO4)2.4H20, which was dissolved in the crosslinked PVA solution and whose amount was determined according to the subsequent experimental need, was immobilized on the dense active layer through effuse procedure, and following in situ crosslinking and dehydration. The samples were annealed at above Tg of 85°C, with subsequent slow cooling. The porous ceramics rather than porous polymer membranes were used as supports for the PVA layer because porous polymer membranes have many defects, such as the weak strength, the weak chemical resistance and the weak pressure resistance. However, the inorganic ultrafiltration membranes have overcome theses shortcomings, these typical characters, such as the strong strength and the strong chemical resistance, have protected the PVA layer from being destroyed when the catalyst was immobilized on the dense active layer. 2.3. Density o f the crosslinked PVA active layer

The density of the crosslinked PVA film was obtained by adding 1,2-dichloroethanol to a membrane sample in 1,1,1-trichloroethane, until the sample was maintained in equilibrium with the liquid mixture, i.e. it did not sink to the bottom or float on the surface. The solvents were chosen because their densities were close to those of the crosslinked PVA samples studied and their non-absorption by the polymer. 2.4. Scanning electron microscopy (SEM)

The cross-section morphologies of the crosslinked PVA composite catalytic membranes were observed with a Hitachi automatic scanning electron microscope. The instrument was operated at an accelerating voltage of 10 keV. Fig. 2 shows that there were two layers which were the catalytic active layer and separate active layer in the cross-section direction of the membrane. The separate active layer was homogeneous, but there were some crystal particles in the catalytic active layer.

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Y. Zhu, H. Chen/Journal of Membrane Science 138 (1998) 123-134

I

z < z I--

[ Fig. 2. Cross-section morphology of the PVA/Zr(SO4) composite catalytic membrane.

l

400o Js'so 3abo z~o 22~ ,7~o 13bo 8s'o

4~o

WAVENUMBERS

2.5. Infra-red spectra (by ATR) or catalytic membrane The polymer material bears the secondary alcohol groups which can be esterified by the acid. In order to check the modification of the membrane dense active layer, the IR spectra (by ATR) of the PVA membrane before and after being in contact with the liquid mixture were compared. The dense active layer of catalytic membrane contacted directly with the catalyst whose concentration was 20.5 g/1 (Fig. 3 I), after three batches of 24 h each with the n-butyl alcohol-acetic acid liquid mixture, a peak at 1750 cm -1 which corresponds to the vibration of ester group ( - C O 0 - ) was found. Any significant esterification of the membrane during the length of several experiments (Fig. 3 II) was not observed, this phenomenon could be explained partly by the fact that there was competition between the hydroxyl groups of the alcohol in the solution and the hydroxyl groups of the PVA which were immobilized in the polymer [ 10], but in our experiments, the main reason that has led into above results differs from this simple reason, because there is no way to prevent the separate capacity of the membrane from being destroyed after being used for long time. Hence, in our experiments the catalytic layer and the dense active separation

Fig. 3. IR spectra main absorbance of composite catalytic membrane. (I) By ATR, after three batches of 24 h each with nbutyl alcohol-acetic acid; contact directly with catalyst, concentration 20.5 g/1. (II) By ATR, after 30 batches of 240 h each with nbutyl alcohol and acetic acid; catalyst immobilized on dense active layer, concentration 20.5 g/1.

layer were divided into two layers in one catalytic composite membrane, this method can prevent the membrane being destroyed through esterification either by catalysts that diffuse from the solution into the active separation layer [10] or by catalysts that contact directly with the active sites in the polymer blend membrane [17].

2.6. Characterization of catalytic membrane through XPS The catalytic natures of the membrane and the interaction between Zr(SO4)2 and PVA were observed with XPS (PHI- 1600ESCA of Perkin-Elmer). Fig. 4(A) shows Ols absorption peaks of different samples, the electron binding energy of Ots in Z r ( S O 4 ) 2 w a s 532.3 eV, in PVA-Zr(IV) catalytic membrane the electron binding energy of O1s was 532.1 eV before reaction as well as after reaction. Fig. 4(B) Zr3d absorption peaks of different samples, the electron binding energy of Zr3d in Z r ( S O 4 ) 2 w a s

Y Zhu, H. Chen/Journal of Membrane Science 138 (1998) 123-134 LQLlC/O1

xl05

2

127

A 1.5

1

0.5

-536

-534

-532

-530

-528

-526

Binding Energy (eV)

4 x104

LOLl C/Zrl

3 x'104

,

LQL1CtS1,

,

/ ::.'. y"\ . . . . . . . . .

Lgo

-188 '

.

-1136 -1k -182 ' Binding Energy (eV~

-180 '

-178 pHI

-164 Bindin8 Energy (eV)

Fig. 4. XPS spectraof Zr(SO4)2and PVA-Zr(IV)catalytic membrane:(a) Zr(SO4)2,(b) PVA-Zr(IV)catalytic membranebefore reaction, (c) PVA-Zr(IV)catalytic membraneafter 240 h reaction, (A) Ols spectra, (B) Zr3dspectra, (C) S2p spectra. 184.0 eV, in PVA-Zr(IV) catalytic membrane, the electron binding energy of Zr3d was 183.4 eV, the shift was 0.6 eV which reflected that there was interaction between PVA and Zr(SO4) 2. Fig. 4(C) shows S2p absorption peaks of different samples, the electron binding energy of S2p in Zr(SO4)a was 169.3 eV, in PVA-Zr(IV) catalytic membrane the electron binding energy of S2p was 169.0 eV, this shift stated that the S must have participated in the interaction between the PVA and the Zr(SO4)2. The variations of the absorption peak intensities of different elements stated that some active components on the catalytic membrane surface run off after 240 h reaction.

2.7. Natures of the PVA-Zr(S04)2 composite catalytic membrane The main feature of the 'bifunctional' membrane in this paper and other literature [18,19] is to combine permselectivity for water with catalytic activity for esterification, in this purpose, an acidic addictive (Zr(SO4)2) is introduced into the PVA (polyvinyl-

alcohol) aqueous dopes used to generate the dense active layer. This method could reduce the separation difficulties caused by the introduction of the catalyst into the reaction mixture, especially the homogeneous catalyst. But in the present these two shortcomings, the small amount and the spatial distribution of the catalyst, have halted the development of the catalytic membrane. The crosslinked PVA is a suitable membrane material for the separation of water and ethanol mixture through the pervaporation if considering both the separation selectivity and the permeation flux. In this paper, water is the preferential component that should be removed, so the crosslinked PVA which is one kind of strong hydrophilic materials is a suitable selection. The membrane structure could be improved through the copolymerization, the blending or the grafting in order to promote the separation selectivity between the water and the acid. In the crosslinked PVA membrane, the PVA has changed into the network structure because of the crosslinking [23,24], this structure could help immo-

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Y Zhu, H. Chen/Journal of Membrane Science 138 (1998) 123-134

bilize the catalyst, which has been stated in the former paragraph.

2.8. Preparation of PVA and poly(acrylic acid) (PAA) blend catalytic membrane on porous ceramic plate The procedure of the preparation of the PVA and the poly(acylic acid) (PAA) blend catalytic membrane on porous ceramic plate was almost the same as that of the preparation of the crosslinked PVA composite catalytic membrane, the only difference between them was that the different membrane making solutions were used.

3. Pervaporation and pervaporationesterification experiment

3.2. Experiment of pervaporation-esterification coupling

3.1. Experiment of separating water-ethanol, wateracetic acid and water-n-butyl acetate by means of pervaporation The schematic pervaporation apparatus is shown in Fig. 5. The permeation cell was made of stainless steel and the composite catalytic membrane with an effective membrane area 34 cm 2 was stick on a porous stainless steel support plate using 704 silicon rubber. Pervaporation experiments were carried out by maintaining on one side of the membrane a mixture of water-ethanol or water-acetic acid or water-n-butyl 4

acetate which was fed batch-wise at atmospheric pressure and on the other (permeate) side a reduced pressure of not more than 5 mmHg. The feed mixture was added on the upper side of the membrane and stirred at the constant temperature. The permeated vapor was collected in liquid nitrogen traps. The feed and permeate compositions were analyzed at 30°C by measuring the refractive indices with a 2WA-J Abbeytype refractometer (Shanghai, China). The flux J and the permeation selectivity c~ are defined in the following paragraph. Ethanol, instead of n-butyl alcohol was used in this experiment because ethanol permeated through the composite catalytic membrane easier than n-butyl alcohol, so it could reflect the characters of the composite catalytic membrane more precisely.

~

The schematic pervaporation-esterification coupling was the same as that of the pervaporation procedure. The feed liquid mixture was acetic acid and n-butyl alcohol. The compositions of the liquid reaction mixture and the permeation liquid were determined by a gas chromatography equipped with a thermal conductivity detector and a 2 m GDX-103 column and operating at 180°C, carrier gas was hydrogen at a flow rate of 30 ml/min. Membrane separation characters were represented by permeation flux J of the component and separation

9

2

10

10 12

13

14

Fig. 5. Diagram of pervaporation and pervaporation-esterification coupling: (1) reaction cell, (2) membrane, (3) porous support steel plate, (4) stirrer, (5) thermometer, (6) input of thermalstated water, (7) output of thermalstated water, (8) three way cock, (9) vent to atmosphere, (10) cold trap, (11) two way cock, (12) barometer of U type, (13) buffer tank, (14) drying column, (15) vacuum pump.

Y Zhu, H. Chen/Journal of Membrane Science 138 (1998) 123-134 100.00

selectivity c~ between components. J was calculated by

80.00

Wi Ji = - st

1

2

(1)

where s is the effective membrane area (m2), t is the permeation time (h), W~ is the permeation mass of component i during permeation time t (kg). The separation selectivity c~0 of component i and j was obtained by

(xij

129

= Yi/Yj xi / x j

(2)

xi and Yi are the mass fractions of the reactant and the permeation liquid, respectively. Component i was water in this experiment, separation effectiveness was characterized by separation selectivities of water to the other three components. The conversion of membrane catalytic reaction was determined by n-butyl alcohol conversion, this calculation method was reasonable because the acid was always excessive and the permeation alcohol was scarce. conversion X% = G - (G1 + G2) 100% G

(3)

where G is the initial mass of alcohol (kg), GI the mass of the surplused alcohol in the reaction liquid mixture (kg), G2 the mass of the permeation alcohol (kg).

60.00

x 4o,0o

20.00

o.oo

,

o.oo

2.00

4.00

6.oo

8.00

t(hr)

lo.oo

Fig. 6. Variation of conversion as a function of time (T-90°C,

R0=1.599, Ccat=10.6 g/l, S/V=22.67 m 1): (1) with pervaporation, (2) no pervaporation.

those of PVA/PAA blend catalytic membrane, and the membrane permeation flux increased as the temperature increased, however, their separation selectivities decreased. The composite catalytic membrane with better separation selectivity was preferential if its permeation flux was not very low. Therefore, the crosslinked PVA composite catalytic membrane was used in the pervaporation-esterification experiments.

4. Results and discussion

4.2. Pervaporation-esterification coupling

4.1. Pervaporation separation characters of the composite catalytic membrane

Pervaporation could remove water that was produced in esterification preferentially, transformed the equilibrium of reaction and drove the reaction to completeness. Figs. 6 - 8 show the results of the esterification with pervaporation; the reaction temperature

Tables 3-5 show that pervaporation selectivities of crosslinked PVA catalytic membrane were higher than Table 3 Experimental results of water-ethanol pervaporation Water content in feed liquid

10% 10% 10%

Temperature

Crosslinked PVA

PVA/PAA blend membrane

Permeation flux (kg/m2 h)

Separation selectivity

Permeation flux (kg/m2 h)

Separation selectivity

(°C) 70 80 90

0.223 0.256 0.282

20.4 19.3 17.0

0.264 0.296 0.328

16.5 14.6 12.2

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Y. Zhu, H. Chen/Journal of Membrane Science 138 (1998) 123-134

Table 4 Experimental results of water-acetic acid pervaporation Water content in feed liquid

10% 10% 10%

Temperature

Crosslinked PVA membrane

PVA/PAA blend membrane

Permeation flux (kg/m2 h)

Separation selectivity

Permeation flux (kg/m2 h)

Separation selectivity

(°C) 70 80 90

0.323 0.352 0.384

10.8 9.9 9.0

0.334 0.380 0.420

7.9 6.7 5.6

Table 5 Experimental results of water-n-butyl acetate pervaporation Water content in feed liquid

10% 10% 10%

Temperature

Crosslinked PVA membrane

PVA/PAA blend membrane

Permeation flux (kg/m2 h)

Separation selectivity

Permeation flux (kg/m2 h)

Separation selectivity

(°C) 70 80 90

0.154 0.172 0.200

441 436 432

0.178 0.202 0.226

315 304 291

1.50

100.00

1.20

80.00

"~'0.90

60.00

I

0 0.6 0

2

40.00

3 0.30

20.00

3

5 0.00

0.00

2.00

4.00

6.00

t(hr)

8.00

10.00

0.00

0.0(

260

460

660

t(hr)

8.60

10.00

Fig. 7. Variation of average permeation flux as a function of time (T--90°C, Ro=1.599, Ccat=20.5 g/l): (1) Jtotal, (2) /water, (3) Jacid, (4) Jalcohol, (5) Jacetate-

Fig. 8. Variation of average separation selectivity as a function of time (T=90°C, Ro=1.599, Ccat=20.5 g/l): (1) water-n-butyl acetate, (2) water-n-butyl alcohol, (3) water-acetic acid.

w a s 90°C; the m o l a r ratio o f the acetic acid to the nb u t y l a l c o h o l Ro w a s 1.599; the c o n c e n t r a t i o n o f the catalyst Cca t was t a k e n as 10.6 g/1 for the c a t a l y s t s i m m o b i l i z e d o n t h e m e m b r a n e b u t n o t i n the r e a c t i o n m i x t u r e . In Fig. 6, the p a r a m e t e r s o f b l a n k esterification w e r e the s a m e as t h o s e c o u p l i n g w i t h p e r v a p o r a -

tion. Fig. 7 s h o w s t h e b l a n k e s t e r i f i c a t i o n r e a c h e d c o m p l e t e e q u i l i b r i u m w h e n t w a s 6 h, t h e e q u i l i b r i u m c o n v e r s i o n w a s l o w e r t h a n 6 5 % , the c o n v e r s i o n o f esterification with pervaporation reaches 95%, almost d r o v e t h e r e a c t i o n to c o m p l e t e n e s s . T h e r e a c t i o n t i m e was only 2 h when reaction conversion with perva-

Y. Zhu, H. Chen/Journal of Membrane Science 138 (1998) 123-134 100.00

100.00

80.00

80.00

60.00

60.00

x

131

x 40.00

40.00

20.00

20.00

0.00 0.00

2.60

4.00

0.60

t(hO

8.00

0.00 t0.00

0.00

2~0

4.~0

6.~0

t(hr)

8.60

~000

Fig. 9. Different temperature influence on conversion (Ro=1.599, Ccat=20.5 g/l, S/V=22.67m l): (1) T=90°C, (2) T=80°C, (3) T-70°C.

Fig. 10. Different initial molar reactant influence on conversion (T=90°C, Ccat=20.5g/l, S/V=22.67m-1): (1) Ro=2.398, (2) Ro=1.599, (3) Ro=1.066.

poration was equal to that of esterification without pervaporation, the reaction time was decreased largely. Fig. 7 shows the permeation flux of water increased at the beginning of reaction then decreased, this was corresponding to the water content in the reactant mixture. The permeation fluxes of acetic acid, n-butyl alcohol and n-butyl acetate were very small, and Jwater>Jacid>Jalcohol>Jacetate . Fig. 8 shows that the separation selectivities were very high, and awateracetate>O~water_alcohol>O/water_acid . The separation selectivity of n-butyl acetate was the best and the acid was the lowest, those results were consistent with their polarities and molecular sizes.

4.2.2. Influence of initial molar reactant ratio (acetic acid~n-butyl alcohol) on reaction coupling

4.2.1. Influence of reaction and pervaporation temperature Fig. 9 shows that an increase in temperature induced not only an acceleration of esterification, but also an acceleration of pervaporation. Furthermore, the water content was changed much faster at higher temperature. The water content in reaction mixture was decreased abruptly when membrane permeation flux was increased, that stimulated the esterification. As a result, the esterification rate increased. Generally, the influences of temperature on esterification and permeation were on the same direction.

Fig. 10 shows that the last reaction conversion was higher when excessive acetic acid was used. It is well known that a sufficient ratio of one reactant to the alcohol leads to a quasi-complete conversion of alcohol even without pervaporation. Obviously, this method would be carried out at the cost of separation difficulties. Decreasing the initial ratio of acid to alcohol when operating with pervaporation may be the optimum performance conditions.

4.2.3. Influence of catalyst concentration on reaction coupling Fig. 11 shows that increasing the catalyst concentration may be an alternative way to accelerate the ester production. Fig. 12 shows that the maximum in water content was higher and appeared earlier when the catalyst concentration increased. At the beginning of the reaction, the rate of the pervaporation was lower than that of esterification. However, increasing catalyst concentration accelerated the production of ester and water. Since, permeation flux was faster at higher water concentration, the extraction rate of water increased, the water maximum content appeared earlier. The decline of water content was

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Y Zhu, H. Chen/Journal of Membrane Science 138 (1998) 123-134 100.00

4.2.4. Comparison of different reaction and pervaporation parameters 1

J

80.00

60.00

x

40.00

20.00

0.00

0.00

2.60

4.60

t(hr)

6.60

8.60

t0.00

Fig. 11. Different catalyst concentration influence on conversion (T-90°C, R0=1.599, S/V--22.67m-1): (1) Ccat:20.5g/l, (2) Ccat=14.8g/l, (3) Ccat:10.6g/l).

30.00

20.00

1 10.00

0.00 0.00

2.6o

4.60

t(hr)

6.60

8.60

10.00

Fig. 12. Variation of water content in reaction mixture as a function of time (T-90°C, Ro=1.599, S/V=22.67m-I): (1) Ccat:20.5 gO, (2) Ccat:14.8 g/l, (3) Ccat:10.6 g/l.

also faster due to the same phenomenon and to a more rapid replacement by the reaction of the water extracted by pervaporation at higher catalyst concentration, which kept up with the level of the water content.

From the experimental results, we could classify three individual factors according to their effectiveness, the temperature had the strongest influence on the performances of the coupling because it acted on both the kinetics of esterification and pervapopration. The second effective factor was the initial reactant molar ratio, it affected only the kinetics of the esterification. The last factor was the catalyst concentration. The conversion curves, indeed, were shifted as the S/V ratio was varied. With the increase of the S/V the reaction mixture could contact with the membrane surface more fully, this could reduce the concentration polarization and benefited the pervaporation.

4.2.5. The working mechanism of the composite catalytic membrane The working mechanism of the composite catalytic membrane has been stated by Kwon et al. [16]. First, the acid and the alcohol have to sorb and diffuse into the catalytic layer toward the catalytic sites situated at different depths, driven by their consumption by the reaction at these sites. The products of the reaction migrates differently from the reaction sites toward the external phases according to the directions of their driving forces. During the first stage of the reaction, the diffusion of the ester from the catalytic layer to the reaction mixture is very large, the water migrates through the separate layer. With the increase of the ester in the reaction mixture, the diffusion of the ester from the catalytic layer to the reaction mixture decreases, the water diffusion from the catalytic layer to the separate layer has become the main push for the reaction. In a general, the process is controlled by the mass transfer of the ester and the water in the first stage, then it is controlled by the water mass transfer only until the reaction end. With the increase of the temperature and the catalyst concentration, the time of the first stage will shorten.

5. Conclusion A novel preparation of the composite catalytic membrane was carried out in this study, several key

Y Zhu, H. Chen/Journal of Membrane Science 138 (1998) 123-134

problems were overcome through the immobilization of catalyst on the dense active layer of membrane. SEM, IR(ATR) and XPS were used to characterize the natures of catalytic membrane. The composite catalytic membrane was evaluated through the pervaporation and a model system of n-butyl alcohol esterification coupling with the pervaporation. The last reaction conversion of n-butyl alcohol reached 95% when crosslinked PVA pervaporation catalytic membrane was used. The order of the permeation fluxes are: Jwater>Jacid>Jalcohol>Jacetate, and Jtotal>0.5 kg/m 2 h during the reaction time; the order of the separation selectivities of membrane are: O~water_acetate>O~water_alcohol>OLwater_acid. The parameters of temperature, initial molar ratio of acid to alcohol or catalyst concentration can be changed in order to attain the optimum of the pervaporation-esterification coupling operating.

6. Nomenclature Jwater Jacid

permeation flux of water, kg/m 2 h permeation flux of acetic acid, kg! m2 h

Jalcohol

permeation flux of n-butyl alcohol, kg/m 2 h permeation flux of n-butyl acetate, kg/m 2 h permeation flux of all components, kg/m 2 h

Jacetate Jtotal R0 Ccat X S T t V

w/ OL O~water-acetate O~water-alcohol O~water-acid

initial reactant molar ratio concentration of catalyst, g/1 reaction conversion effective membrane area, m 2 reaction and permeation temperature, °C reaction time, h volume of the initial reaction mixture, m3 permeation mass of component separation selectivity separation selectivity between and n-butyl acetate separation selectivity between and n-butyl alcohol separation selectivity between and acetic acid

i water water water

PVA PAA GA SEM ATR IR XPS

133

poly(vinyl alcohol) poly(acylic acid) glutaraldehyde scanning electron microscopy attenuated total refraction infra-red spectra X-ray photoelectron spectrometry

Acknowledgements The authors would like to thank Prof. Mooson Kwauk for his help in providing us with the latest journal of Chemical Engineering Science.

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