Ind. Eng. Chem. Res. 1994,33, 849-853
849
Vapor-Permeation-Aided Esterification of Oleic Acid Ken-ichi Okamoto; Masuji Yamamoto, Seiji Noda, Toshio Semoto, Yoshiharu Otoshi, Kazuhiro Tanaka, and Hidetoshi Kita Department of Advanced Materials Science and Engineering, Faculty of Engineering, Yamaguchi University, Ube, Yamaguchi 755, Japan
Vapor-permeation-(VP) aided esterification of oleic acid with ethanol was investigated at the boiling temperatures of the reaction mixtures under atmospheric pressure using a laboratory module of polyimide hollow fiber. The combined process provided almost complete conversion in a short reaction time with a decrease in the initial molar ratio of ethanol to oleic acid. This is attributed mainly to more rapid elimination of water as the result of the higher enrichment of water component in the vapor phase. The membrane module worked stably for the long period, because it was in contact with only the volatile components, namely water and ethanol. The VP-aided esterification was well simulated using the experimental data of both the reaction temperature and the permeation flux for each experimental run.
Introduction
Experimental Section
Recently, the chemical reaction process combined with membrane separation has attracted much attention. The use of a reactor equipped with a membrane permeable to product(s) improves reaction efficiency thanks to a favorable shift of the chemical equilibrium and/or a reduction in the inactivation of catalyst. A number of investigations have concentrated on hydrogen-permeable membrane reactors applied to reversible gas-phase reactions (Itoh, 1987;Kameyama et al., 1981;Shinjyo et al., 1982;Sun and Khang, 1988; Umemiya et al., 1991). On the other hand, some investigations have been done on water-permeable membrane reactors applied to liquid-phase reactions (David et al., 1991a,b;Kita et al.,1987,1988,1991;Okamoto et al., 1991,1993). In a previous paper, we reported that the pervaporation- (PV) aided esterification of oleic acid could be performed more efficiently at temperatures above 360 K under elevated pressures (Okamoto et al., 1993). However, the operation at a high temperature is apt to make more severe demands on the durability and performance of the membranes. Asymmetric polyimide membranes were plasticized by ethyl oleate and oleic acid at 371 K, but not at 348 K (Okamoto et al., 1993). It is not easy to develop membranes having both sufficient durability and excellent permselectivity to the reaction liquid mixture at high temperature. Vapor permeation (VP) is another type of membrane system for separation of volatile liquid mixtures. VP is considered to be applicable to liquid-phase reactions satisfying the following conditions. First, the reaction temperature is high enough for water vapor to be present at sufficiently high partial pressure. Second, one of the reactants is as volatile as water, and therefore distillation is not effective for separation of water. In VP, the membrane is not in direct contact with the reaction liquid mixtures, and therefore it is expected to work more stably than in PV. In a previous paper, we reported on preliminary results of the VP-aided esterification of oleic acid (Kita et al., 1987). In this study, the VP-aided esterification of oleic acid is investigated in detail, using a polyimide hollowfiber membrane module. The influence of operating parameters on the productivity of the reaction is examined and discussed in comparison with that for PV-aided esterification.
The hollow fibers used in this study were made from an aromatic polyimide prepared from 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-oxydianiline, and 4,4'methylenedianiline (Nakagawa et al., 1989). They have asymmetric structure consisting of a thin-skin layer on the outside surface and a porous supporting layer. Their outside and inside diameters are 510 and 310 pm, respectively. A one-end-opened type of membrane module was made of 30 pieces of the hollow fiber; one end of the fiber bundle was sealed with epoxy resin and the other was potted with epoxy resin of 5-cm length. The potted zone is inactive to permeation, and the active length of the fibers is 25 cm. The effective membrane area of the module, S, is 120 cm2. The VP and VP-aided esterification of oleic acid with ethanol were carried out using the apparatus shown in Figure 1. A reaction cell consisted of a four-neck round flask (500 or 1000 cm9, an empty distillation column (3 cm in inner diameter and 50 cm in length), and a condenser. The membrane module was set in the distillation column and connected to a vacuum line. The inside of the membrane fibers was evacuated, and the permeate on that side was condensed in a cold trap of liquid nitrogen. The distillation column was wound with ribbon heaters and kept at a given temperature (ca. 378 K) to prevent vapor from condensing. All reagents were purified by ordinary methods. As catalyst, p-toluenesulfonic acid was used. A reactant mixture was fed into the flask and heated rapidly to ita boiling point with magnetic stirring under atmospheric pressure. Just before vapor came up, a weighed quantity of the catalyst was added to start the reaction. The vapor went through the column, contacted with the membrane module, and then condensed at the condenser. The condensate went back into the reaction flask. The feed velocity of the vapor, u, was maintained larger than 4 cm/s by controlling the temperature of a silicone-oil bath. Product samples (ca. 1g every sampling) were withdrawn periodically, and were analyzed by titration for oleic acid and by gas chromatography for water and ethanol. The experimental conditions are listed in Table 1.
Results and Discussion The membrane performance of the module was measured in cocurrent-flow mode at 378 K for the waterethanol system. The composition of feed vapor at the
0000-5005/94/2633-0849$04.5Q/Q 0 1994 American Chemical Society
850 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994
h
.
b4.2
Y
Figure 1. Schematic diagram of the experimental apparatus: (1) magnetic stirrer, (2) oil bath, (3) heater, (4) reaction flask, (5) thermometer,(6)membranemodule,(7)condenser, (8)thermocouple, and (9) vacuum line.
0
1
2
3
4
5
6
Reaction time (h)
Figure 4. Variation in reaction temperature with reaction time for the experimental runs shows in Figure 3.
'0
o
at
a2 (XW."),"
03 (-1
04
Figure 2. Membrane performance for water-ethanol mixtures of the membrane module operated in cocurrent flow at 378 K. Table 1. Experimental Conditions parameters mo 1.2-4
T [Kl
T v [Kl
conditions
5-28 varying from 355 to 405 K ea. 378
inlet was determined from the material balance, using the observed values of both flux and composition of permeate and recycling condensate. Figure 2a and 2b show plots of permeation fluxes of ethanol and water, QB and QW, respectively, and separation factor, a,versus logarithmic . permeability mean feed composition, ( X W , ~ ) ~Specific coefficients for water and ethanol vapors, P w and PB, respectively, were evaluated from qw,QB, and ( ~ w , ~ )by ln the method described previously, and were substantially the same as those reported there (Tanihara et al., 1992).
Figure 3 shows variation in the conversion, X,with reaction time, t, for the VP-aided esterification. The VPaided reaction led to almost complete conversion as the result of elimination of water from the reaction system. The complete conversion was attained in a shorter time with decreasing the initial molar ratio of ethanol to oleic acid, mo;the reaction time required for the conversion of 98%, tB8, was shorten from 8 h for mo = 3 to 2.8 h for mo = 1.2. The production per reaction time and volume, 0.98N~,O/( Vl,otga),can be used as a measure of productivity of the process. The values of this parameter were 0.84, 0.61,0.39, and 0.23 kmol/(m3*h)for mo of 1.2, 1.5, 2, and 3, respectively; thus, it increased significantly with a decrease in mo. This is quite different from the case for the PV-aided esterification, where it increased rapidly with an increase in mo,reached a maximum around mo = 3, and then decreased gradually for a reactor having a relatively highcapacity of elimination of water (Okamoto et al., 1993). As shown in Figure 4, the reaction temperature was higher for smaller mo,because the VP-aided esterification was carried out at the boiling temperature under atmospheric pressure. As the reaction proceeded, the temperature decreased, held constant for a while, and then increased. The increase in the reaction temperature was significant for mo less than 2. The higher productivity for smaller mo is attributed in part to the higher reaction temperature but not all, as mentioned below. Figures 5 and 6 show variation in the composition of liquid and vapor phases and the cumulative permeation
Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 861 I
I
I
I
I
I
I
I
2.01
21.0
0.5
n
productivity of the VP-aided esterification becomes larger with a decrease in mo. In the previous paper, we proposed the kinetic equations of the esterification of oleic acid with ethanol in the presence of p-toluenesulfonic acid, where the rate reduction effect of water and mo dependence of the rate and equilibrium constants were taken into account (Okamoto et al., 1993). -dCA/dt
kCACB - (k/K,)CECw
(1)
k = k'd(l+ aCw)
(2)
12, = 4.1313 X 1O8rn6/(kmol2.h)
E = 60.66 kJ/mol Figure 5. Variation in composition of liquid and vapor phase and cumulative permeation amount with reaction time for the experimental run with mo = 3 shown in Figure 3. The l i e s are the calculated ones. (1) rwj, (2) NwIG"+ Nw),and (3) (rw,&.
f(mo) = 230.10 - 190.82m0 + 71.69mt - 11.61md + 0.65m,' a = 9924.85 - 103.959T
+ 0.40814p - 7.1175 X 1 0 4 p + 4.65171
X
lO-'p (4)
AHr = 3.36 kJ/mol g(mo) = 4.207 - 0.361m0 + 0.019mt It is important to simulate the time course of the VPaided reaction and to predict the influence of various operating parameters. In the present model, the reaction kinetics (eqs 1-51 are combined with the permeation flux equation. Taking into account both vaporization of the volatile Components and permeation of them through a membrane, variations in Ce and Cw with time are represented by eqs 6 and 7. For practice of the calculation, dCddt = dC,/dt - qBS/Vl- V,l(RT,V,)(dp$dt) Figure 6. Variation in composition of liquid and vapor phases and cumulative permeation amount with reaction time for the experimental run with rno = 1.5 shown in Figure 3. The lines are the calculated ones. (1) n,,~,(2) NwI(NB+ Nw), and (3) (nw&,.
amount, Q, with reaction time for mo of 3 and 1.5, respectively. The composition of the vapor phase was determined from the material balance as mentioned above. Both oleic acid and ethyl oleate were not substantially present in the vapor phase. The composition of the vapor phase depends mainly on the composition of the volatile components, namely water and ethanol,in the liquid phase. Therefore, the fraction of water in the vapor, xw,,, was much larger than that in the liquid phase, XWJ. For mo of 3 and 1.5, xw,1 reached a maximum at ca. 1 h. At this stage, xw,1 was only a little different between mo of 3 and 1.5. However, the fraction of water to the volatile components in the liquid phase, Nw/(NB + Nw),was much larger for smaller mo, resulting in much larger XW,, for smaller mo. As a result, for smaller mo,the permeation flux of water was much larger and xw.1 decreased more rapidly with time. This is another reason that the
dCwldt = -dCA/dt
- qwS/ Vi - V,/(RT,V,)(dpw/dt)
(6) (7)
it is necessary to know the vapor-liquid equilibrium relation. For the complete simulation of the VP-aided reaction, one has to estimate the vapor-liquid equilibrium relation of the quaternary system by a method such as UNIFAC, where some parameters must be determined experimentally. In this study, for preliminary simulation, the experimental data of both the reaction temperature and the permeation flux for each experimental run were used as polynomial expressions of reaction time.
T = T,,,(t)
(8)
qw = dQw/(S dt) = qw,,.,(t)
(9)
qB = de$@ dt)
4B,exp(t)
(10)
The volume change of the reaction liquid mixture during the reaction was taken into consideration. The volume of the reaction liquid mixture, VI, is given by eq 11according to the additivity of volume.
852 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994
VI = N&A/
PA
+ N&B/
PB
+ N&fE/PE + N,M,/ PW (11)
The following values of density at 358 K were used for the calculation; PA 852, PE 827, PB = 741, and pw = 970 kg/m3. Equations 1-11can be numerically solved by the RungeKutta-Gill method. The lines in Figures 3,5, and 6 were thus calculated. They are in good agreement with the experimental data. The agreement between the experimental data and the simulation curves are better for the cases with larger mo and larger V1,o. It was thus confirmed that the present model enables us to simulate the time course of the VP-aided reaction sufficiently well provided that the vapor-liquid equilibrium relation is predicted. In Figure 7, line 1 is the simulation curve of the conversion versus time for V1,o = 297 cm3, no = 1.5, and varying experimental reaction temperature; it is the same as the corresponding line in Figure 3. Line 2 is the simulation curve for a constant reaction temperature of 361 K. The difference between lines 1 and 2 is rather small, indicating that the higher productivity for smaller mo is attributed mainly to the higher enrichment of the water component at the vaporization step and the effect of the increase in the temperature at the latter half of the reaction is rather small. Figure 7 also shows the influence of the reaction volume on the conversion-time curves for no = 1.5. Line 3 is the simulation curve for the ideal VP separation (Pw = m and PB= 0). The experimental conversion curve for V1,o = 149cm3 or the ratio of effective membrane area to reaction volume, S/ Vl,o, of 80 m-1 is very close to line 3, indicating that the water-separating capacity of the membrane module used was high enough to remove the water formed in this case. On the other hand, the conversion curve for V1,o = 564 cm3 (S/ V1,o = 20 m-1) is very slow as compared with line 3. However, the production per reaction time at 98% conversion for this membrane module is 0.14,0.17, and 0.22 mol/h for S/ VI,O= 80,40, and 20 m-l, respectively. This means that the optimum condition of S/Vl,o also depends on other operating factors. The membrane module used worked stably for the full term of the experiment and did not show any decrease in the performance. This is because the module was not in contact with the reaction liquid mixture, but only the volatile components, namely water and ethanol vapors. The present module was not so stable for the pervaporation of the quaternary system a t the high temperature range. It is much easier to get the membrane module having*both an excellent performance and an excellent long-term durability for the VP-aided reaction than for the PV-aided one. This is one of the advantages of the VP-aided reaction. S/Vl,o is an important factor determining a waterseparating capacity of a membrane reactor. In this study, S/V1,0 was in the range of 20-80 m-l. For the VP-aided reaction, it is possible to prepare a large-scale reactor having the same level of S/Vl,o as the laboratory-size, because the membrane module can be designed independently of the reaction vessel. On the other hand, it is not easy to prepare a large-scale reactor with high S/ V1,o for the PV-aided reaction (Okamoto et al., 1993). The ease of scaling-up of the reactor is another advantage of the VP-aided reaction. Complete simulation of the VP-aided reaction is necessary to investigate effects of operating parameters on the productivity. An effective system of recycling the vapor must be also considered. These are under investigation.
0
1
2
t
3 4 (h)
5
6
7
Figure 7. Influence of reaction volume on conversion curves for = 1.5 and CC = 11.3 mol/ms. ( 0 ) VLO= 149 cma (S/VLO = 80 m-l), (0) Vl,o = 297 cms (40 m-l), and (A)VLO= 564 cms (20 m-l). The lines (1)-(4) are the simulation curves of the conversionveraua time: (1)for varying reaction temperature (see Figure 4), (2) for constant temperature of 361 K, (3) for ideal VP separation, and (4) for no VP separation.
Conclusion The VP-aided esterification of oleic acid can be performed more effectively for smaller mo especiallyless than 1.5. The membrane module can work more stably in the vapor phase than in the reaction liquid mixture. These are advantages of the VP-aided esterification as compared with the PV-aided esterification.
Acknowledgment We are grateful to Mr. Y. Kusuki and Mr. K. Nakagawa of Corporate Research and Development, Ube Industries Co. Ltd., for the supply of the membrane module.
Nomenclature a = rate
reduction parameter, m3/kmol
Ci = molar concentration of component i in reaction liquid mixture, kmol/m3 E = activation energy of reaction, kJ/mol AHr = heat of reaction, kJ/mol K , = equilibrium constant, dimensionless K,,o = preexponential factor of equilibrium constant, dimensionless k = apparent forward rate constant, mV(kmo1.h) k’0 = forward rate constant, m3/(kmol.h) ko = frequency factor of forward rate constant, m3/(kmolOh) M = molecular weight, kg/mol m = molar ratio of ethanol to oleic acid, dimensionless Ni = amount of component i in reaction liquid mixture, mol Pi = specific permeability coefficient of component i, mol/ (m2.s.Pa) Qi = cumulative permeation amount of component i, mol qi = permeation flux of component i, mol/(m2.h) R = gas constant, kJ/(mol-K) S = effective membrane area of the module, m2 T = temperature of reaction liquid mixture, K T, = temperature of feed vapor contacting with module, K t = time elapsed, h tBa = time required for conversion of 98%, h VI = volume of reaction liquid mixture, m3 V , = volume of vapor phase, m3 u = feed velocity of vapor, cm/s X = conversion of oleic acid, 5% XWJ = molar fraction of water in liquid phase, dimensionless XW,, = molar fraction of water in vapor phase, dimensionless
(xw,,.)~ = logarithmic mean of X W , ~ dimensionless , yw,,.= molar fraction of water in permeate, dimensionless
Greek Letters u = separation factor of water to ethanol = Y W , ~(1- ( X W , ~ ) ~ ) / p
((1- yw,.,)(xw,,.)h), dimensionless = density, kgIm3
Subscripts A = oleic acid B = ethanol C = catalyst E = ethyl oleate exp = experimental data 1 = A, B, E, or W 1 = reaction liquid mixture
v = vapor W = water 0 = initial
Literature Cited David, M.-0; Gref, R.; Nguyen, T. Q.; Neel, J. PervaporationEsterification Coupling: Part I. Basic Kinetic Model. Trans. Znst. Chem. Eng. 1991a,69,336340. David, M.-0.; Nguyen, T. Q.; Neel, J. Pervaporation-Esterification Coupling: Part 11. Modelling of the Influence of Different Operating Parameters. Trans. Znst. Chem. Eng. 1991b,69,341346. Itoh, N. A Membrane Reactor Using Palladium. AZChE J. 1987,33, 1576-1578. Kameyama, T.; Dokiya, M.; Fujishige, M.; Yokokawa, H.; Fukuda, K. Possibility for EffectiveProduction of Hydrogenfrom Hydrogen Sulfide by means of a Poroue Vycor Glass Membrane. Znd. Eng. Chem. Fundam. 1981,20,97-99. Kita, H.; Tanaka, K.; Okamoto,K.; Yamamoto,M. The Esterification of Oleic Acid with Ethanol Accompaniedby Membrane Separation. Chem. Lett. 1987,2053-2056.
Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 853 Kita, H.; Sasaki, S.; Tanaka, K.; Okamoto, K.; Yamamoto, M. Esterification of Carboxylic Acid with Ethanol Accompanied by Pervaporation. Chem. Lett. 1988,2025-2028. Kita, H.; Sasaki, S.; Tanaka, K.; Okamoto, K.; Yamamoto, M. Separation of Water-Ethanol Mixtures by Pervaporation through Asymmetric Polyimide Membrane and Ita Application to Esterification. Kagaku Kogaku Ronbunshu 1989,15,604-610. Nakagawa, K.; Aaakura, Y.; Nakanishi, S.; Hoahino, H.; Kouda, H.; Kueuki, Y. Separation of Vapor Mixtures of Water and Alcohol by Aromatic PolyimideHollowFibere. KobunshiRonbunshu 1989, 46,405-411. Okamoto, K.; Semoto, T.; Tanaka, K.; Kita, H. Application of Pervaporation to Phenol-AcetoneCondensation Reaction. Chem. Lett. 1991,167-170. Okamoto,K.; Yamamoto, M.; Obihi, Y.; Semoto, T.; Yano, M.; Tanaka, K.; Kita, H. Pervaporation-Aided Esterification of Oleic Acid. J. Chem. Eng. Jpn. 1993,26,475-481. Shinjyo, 0.; Misono, M.; Yoneda, Y. The Dehydrogenation of Cyclohexane by the Use of a Porous-Glass Reactor. Bull. Chem. SOC.Jpn. 1982,56,2760-2764. Sun, Y. M.; Khang, S. J. Catalytic Membrane for Simultaneous Chemical Reaction and Separation Applied to a Dehydrogenation Reaction. Znd. Eng. Chem. Res. 1988,27,1136-1142. Tanihara, N.; Tanaka, K.; Kita, H.; Okamoto, K.; Nakamura, A.; Kusuki, Y.; Nakagawa,K. Vapor-Permeation Separation of WaterEthanol Mixtures by Asymmetric Polyimide Hollow-Fiber Membrane Modules. J. Chem. Eng. Jpn. 1992,25,388-396. Umemiya, S.; Sato, N.; Ando, H.; Kikuchi, E. The Water Gas Shift Reaction Assisted by a Palladium Membrane Reactor. Znd. Eng. Chem. Res. 1991,30,585-589.
Received for review May 25, 1993 Revised manuscript received December 6 , 1993 Accepted December 28, 1993. @
Abstract published in Advance ACS Abstracts, February
15, 1994.
