Ind. Eng. Chem. Res. 1989, 28, 757-763
757
Pervaporation of Methanol-Ethylene Glycol with Cellophane Membranes: Performance of Conditioned Membranes? Indrani Ghosh, Shyamal K. Sanyal, and Ram N. Mukherjea* Chemical Engineering Department, J a d a v p u r University, Calcutta 700032, I n d i a
The performance of solvent-annealed cellophane membranes was investigated for the pervaporation of a mixture of methanol and ethylene glycol. Higher permeabilities and selectivities were exhibited by the glycol-conditioned membrane (CEG-30), a t 30 "C, as compared t o those of the untreated membrane for low methanol concentrations in the feed. The poor performance of membranes conditioned in the stronger plasticizing component, methanol, indicates that only an optimum degree of plasticization is necessary t o improve membrane performance. Studies on sorption, diffusion, permeation, and the polymer-liquid interactions indicate a higher interaction of glycol a t higher methanol concentration in the feed, thus lowering the corresponding selectivities. Our results also indicate that the plasticizing coefficients of permeants may not always account for their plasticizing capabilities alone but may sometimes account for some weak bonding and clustering of permeants in the membrane matrix, thus resulting in an anomalous sorption-diffusion vis-&vis transport behavior. Pervaporation is a membrane separation process utilized for fractionating a mixture of liquids. Until recently, this process had no technological significance in spite of excellent selectivity, due to the invariably low permeabilities of polymer membranes. The technique of membrane conditioning by solvent annealing, which consists of treating a membrane in a suitable solvent, at a particular temperature, for a given period of time, has been found to impart higher permeabilities to polymer membranes. Michaels et al. (1969) investigated the permselective properties of conditioned polypropylene films. They attributed the enhanced flux rates of treated membranes to the changes in the spherulitic textures and to the diminished intercrystalline tie-chain constrainments of the membrane matrix. Sikonia and McCandless (1978) reported improved separation efficiency for the separation of xylene isomers, with modified poly(viny1idene fluoride) membranes, by introducing Werner complexes in them. More recently, Rautenbach and Albrecht (1980) showed the effect of membrane pretreatment and reported improved selectivity on the treatment with cyclohexane, for the separation of benzene-cyclohexane with polyethylene films. Hirotsu (1987) showed that, by the use of composite membranes, plasma graft-polymerizedby methacrylic acid, onto porous polypropylene films, a considerably high and constant permseparation could be obtained for the pervaporation of water-ethanol mixtures. The design and tailoring of membranes for a more efficient separation thus opened up new prospects for the commercial application of pervaporation. In our previous study (Ghosh et al., 1988), on the industrially important separation of methanol and ethylene glycol mixtures by pervaporation using commercially available cellophane films, selectivity as high as 58 was obtained, although the corresponding permeation rates were rather low. The primary objective of the present work was to explore the possibility of improving membrane performance by conditioning the cellophane membrane in the permeating solvents, using the solvent-annealing technique. The performance of such conditioned membranes was studied with regard to the effect of feed composition and temperature and compared with that of the 'Dedicated to Prof. W. Funke, Institut fuer Technische Chemie der Universitaet Stuttgart, West Germany, on the occasion of his 60th birthday.
Table I. Membrane Types Used conditions for membrane designatmembrane conditioning ed as cellophane ethylene glycol at CEG-30 30 "C ethylene glycol at CEG-40 40 "C methanol at 30 "C CM-30 untreated PT-300 membrane
0.002 498
density, g/cm3 1.411
0.002 494
1.414
0.00249 0.0025
1.417 1.41
L , cm
untreated membrane. On the basis of the sorption-diffusion results, liquid-polymer interactions, and morphological changes in the membrane upon membrane conditioning, an interpretation of the underlying transport mechanism has been attempted. The results also explain the anomalous sorption-diffusion of the permeants and its effect on the transport behavior, as reported in our previous study.
Experimental Section Materials. Cellophane film (PT-300 of Kesoram Rayon Ltd., Calcutta, India; thickness, 0.025 mm; density, 1.41 g/cm3) was used. Methanol and ethylene glycol (AR grade, Merck) were used after purification by distillation. Membrane Thickness and Density. Membrane thickness was checked by a micrometer caliper, and the density of the dry membrane was determined from a sample of known weight by measuring the thickness and the surface area. The precision of the values obtained was within 5% of the values reported by Kesoram Rayon Ltd., Calcutta, India (Table I). Product Analysis. Analysis of the permeates was made by checking their refractive indices (Abbey Refractometer, Erma, Japan) against a calibrated curve. X-ray Diffraction of Membranes. The X-ray diffraction of the untreated and treated membranes was made in a PW 1710 diffractometer control (Phillips, Holland), in order to detect any morphological changes in the membranes regarding alignment, order of crystallinity, and amorphous-phase content. Conditioning of Membranes. The conditioning of cellophane membranes (PT-300)was accomplished by the simple procedure of pretreatment of the membrane (Rautenbach and Albrecht, 1980) in pure methanol at 30 "C and in pure ethylene glycol at 30 and 40 O C . Disks of
0 1989 American Chemical Society 08S8-588~/89/2628-0757~~~.50/0
758 Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989
12.5-cm diameter were immersed in pure solvents taken in closed traps, at a constant temperature, for 24 h. The solvent-swollen and -annealed membranes were then blotted free of excess solvent and air-dried between pressed filter sheets for 72 h. The completely dried membranes were then ready for use and stored in a dessicator. Table I lists the types of conditioned membranes used in the present study. Pervaporation. The schematic diagram of the apparatus and the procedure of the pervaporation experiments have been described in our previous work (Ghosh et al., 1988). A membrane of area 0.0038 cm2 was placed on the porous disk of the pervaporation cell, and these were then bolted together. The downstream side of the cell was evacuated a t a pressure of 533.1 Pa for 1 h, while maintaining an upstream pressure of 1.01 X lo5 Pa. The liquid mixture at the upper compartment was kept under continuous stirring at 100 rpm. The feed temperature was controlled by circulating hot water from a t,hermostatic bath through a coil immersed in the feed compartment. Steady-state permeation rates and selectivities were determined by analyzing the permeates that were collected in traps immersed in liquid nitrogen (-195.8 "C). Sorption Measurement. The solvent-swollen membranes, which were equilibrated at 30 "C in various mixture compositions of methanol and ethylene glycol, were desorbed to a constant weight under vacuum. The sorbates, collected in traps immersed in liquid nitrogen, were weighed and analyzed from their refractive indices. Diffusivity Measurement. The diffusivities a t zero concentration, Do,of methanol and ethylene glycol were determined by measuring the desorption as a function of time from a membrane sample equilibrated in the pure liquid component (Rogers et al., 1976). The equipment used for this study was a Cahn 2000 electrobalance (Cahn Instruments, Inc., Cerrites, CA). The integral diffusivities, fi, of methanol and ethylene glycol a t steady state were estimated from eq 1 using the values of equilibrium sorption for their mixtures, Cs, steady-state permeation rates, Ji, and the membrane thickness, L , as reported in our earlier work (Ghosh et al., 1988). Membrane Performance. Membrane performance is characterized by the total permeation rate, Jp,expressed in kg/(m2 h), and the membrane selectivity or separation factor, a , expressed with respect to the preferentially transferred species, methanol.
Results and Discussion Effect of Operating Conditions on Membrane Performance. The results on the effect of feed composition and temperature on the performance of conditioned membranes are shown in Figures 1-4. It is found that, below 32% methanol content in the feed, the CEG-30 membrane exhibited the highest selectivities. However, for all the feed compositions studied, the pervaporation rates of the CEG-30 membrane were the highest. Hence, including the results for the untreated membrane (PT-300) reported earlier, the overall performance of the membranes studied is in the following descending order: CEG-30 > PT-300 > CEG-40 > CM-30 The performance of the individual membranes is detailed below. CEG-30 Membrane. Figure 1 shows the variation of total permeation rates and selectivities with increasing methanol content in the feed, a t 30, 40, and 50 "C. Per-
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Figure 1. CEG-30 membrane. (A, top) Dependence of total permeation rate, Jp,on the methanol concentration in the feed, a t 30, 40, and 50 "C. (B, bottom) Dependence of separation factor, a , on temperature a t (0) 7 % , (A)15%, (x) 23%, (0) 32%, ( 0 )52%, (A) 74%, and (m)86% methanol content in the feed.
Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989 759
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Figure 3. CM-30 membrane. (A, top) Dependence of total permeation rate, Jp,on the methanol concentration in the feed a t 30, 40, and 50 OC. (B, bottom) Dependence of separation factor, a,on temperature a t (0) 7 % , ( 9 ) 15%,(X) 23%, (0) 42%, (A) 62%, and ( 0 )84% methanol content in the feed.
CEG-40 membrane. (A, top) Dependence of total permeation rate, Jp,on the methanol concentration in the feed, a t 30, 40, and 50 OC. (B, bottom) Dependence of separation factor, a,on 7 % , ( 9 ) 15%, ( 0 ) 23%, (A) 329'0, (m) 42%, (X) temperature a t (0) 52%, (0) 62%, and ( 0 )86% methanol content in the feed.
meation rates were found to increase with the increasing methanol concentration in the feed and with increasing temperature. On the other hand, selectivities for feed compositions below 52% methanol content were found to
increase up to 40 "C and then decrease above that temperature, the effect being most pronounced for 7% methanol content (Figure 1B). However, above 52% methanol in the feed, the dependence of the selectivities on temperature decreased. This indicates that an optimum plasticization of the membrane is obtained with 32% methanol content in the feed at 40 "C, thus facilitating the preferential transport of methanol through it. Enhanced plasticization with feed composition having more than 32% methanol content appears to increase the interaction of
760 Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989
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Figure 4. (A, top) Dependence of the total permeation rate, Jp,at CEG30 "C on the methanol concentration in the feed through (0) 30, ( 0 )PT-300, ( X ) CEG-40, and (A)CM-30 membranes. (B, bottom) Dependence of separation factor, a,a t 30 "C on the methanol concentration in the feed of (0) CEG-30, ( X ) PT-300, ( 0 )CEG-40, and (A)CM-30 membranes.
glycol with the membrane, thus reducing the selectivities of the process. This is also corroborated from the estimation of plasticizing coefficients of the permeants at different feed compositions, as discussed in the following sections. CEG-40 Membrane. The effect of feed composition and temperature on the permeation rate and selectivity through the CEG-40 membrane, as shown in Figure 2, varied in the same manner as that of the CEG-30 membrane. Separation characteristics were, however, lower. Selectivities decreased rapidly, above 15 % methanol content in the feed. CM-30 Membrane. Figure 3 shows that the CM-30 membrane has the lowest separation efficiencies as compared to the other membranes.
Effect of Film Conditioning on Membrane Performance. The similar nature of the dependence of the separation factor and permeation rates on feed composition and temperature for all the membrane studied confirms that, the mechanism of transport remaining the same, membrane performance is governed mainly by the effect of film conditioning on membrane morphology, resulting in a change in the diffusive-sorptive capacity and liquidpolymer interactions in the membrane. The results of a comparative study of the performance of treated and untreated membranes are given in Figure 4, and an interpretation of their transport behavior has been made in terms of the above-mentioned parameters. Membrane Morphology. The X-ray diffractions of the treated and untreated membrane samples are shown in Figure 5. The change in orientations and crystallinity due to membrane conditioning is evident from the X-ray analysis. The sharp peaks due to the crystalline components, 002,02l,lOT, and 101, are evident in the patterns of all the treated samples, with virtually no shift in their positions (Mark et al., 1965). In general, the intensity of 002, relative to that of 021, decreases after treatment. This indicates that any treatment carried out disturbs the interplanar parallelism of the 002 planes, with some enhancement of that of the 021 planes. This may be brought about by the slight out-of-plane orientations of the planar molecules, which have been lying in the ab plane of the untreated sample. This mechanism, in general, lowers the ordering in the 002 direction, which results in a lowering of crystallinity. In the treated and untreated samples, amorphous scattering is observed in similar zones extending from 0 = 18 to 26. Relative to its crystalline peaks, amorphous scattering is most prominent in the CEG-30 membrane (Figure 5A), somewhat lower in the PT-300 membrane (Figure 5B) and the CEG-40 membrane (Figure 5C), and least in the CM-30 membrane (Figure 5D). Thus, the minimum crystallinity and maximum amorphous phase content of the CEG-30 membrane may account for the highest permeation rates displayed for all feed compositions studied. The highest selectivities, below 32% methanol content in the feed, may be attributed to the enhanced ordering of the 021 peak. The disordering of this peak, due to plasticization with increasing methanol content in the feed, possibly results in the lowering of selectivities at higher concentrations of methanol. On the other hand, a somewhat similar proportion of the amorphous scatter to crystalline phase content of PT-300 (Figure 5B) and of CEG-40 membranes (Figure 5C) points to their similar separation characteristics. This is corroborated by our experimental results shown in Figure 4. Almost similar selectivity values at higher concentrations of methanol in the feed, with all the membranes except the CM-30 membrane, seem to indicate a more or less similar degree of plasticization. The X-ray diffractogram of the CM-30 membrane (Figure 5D), however, exhibits a very interesting phenomenon. There is evidence of a large restructuring of the membrane matrix, upon conditioning with methanol at 30 "C, to form a highly ordered, crystalline phase, with the most prominent 002, 021, 101, and 101 peaks. The membrane thus exhibits the maximum crystallinity with negligible amorphous phase content. Consequently, the CM-30 membrane is expected to have the lowest permeation rates and highest selectivities. Though our experimental results indicate the lowest permeation rates, the selectivities exhibited are also the least. This phenomenon may be explained based on the assumption that restructuring of the PT-300 membrane by treatment with the plasticizing component, methanol, to form a highly
Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989 761
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Figure 5. Diffractograms of (A, top left) CEG-30 membrane, (B, bottom left) PT-30 membrane, (C, top right) CEG-40 membrane, and bottom right) CM-30 membrane.
(D,
Table 11. Sorption-Diffusion Data 15% membrane CEG-30 PT-300 CEG-40 CM-30
108DoM? cmz/s 0.918 0.104 0.103 0.098
1O8Dmp cmz/s 0.039 0.029 0.025 0.020
CSM, g/cm3 0.0968 0.05 0.09 0.19
CSG,
lo8&,
g/cm3 0.339 0.15 0.4 0.3
cmz/s 10.2 3.3 1.44 1.2
methanol concentration in the feed 52% 86% 108fio, CSM, CSO, lo8&, 108DC, CSM, CSO, lo8&, cmz/s g/cm3 g/cm3 cm2/s cmz/s g/cm3 g/cm3 cmz/s 0.309 0.28 0.424 18 0.82 0.41 0.23 21 0.162 0.27 0.105 0.085 5.52 5.4 0.845 0.44 0.349 0.3 0.29 2.16 0.495 0.36 0.23 2.51 0.24 0.28 0.218 1.6 0.426 0.595 0.18 2.3
crystalline membrane matrix, is extremely unstable. Even a low methanol content in the feed is capable of initiating the destruction of this unstable crystalline order, thus resulting in lower selectivities. The rapid plasticization of the membrane is confirmed from our estimation of the apparent activation energies of pervaporation (Table IV), the values of which were found to fall rapidly with increasing methanol content in the feed. Thus, we may conclude that only an optimum degree of plasticization of the membrane during membrane conditioning is necessary for improved membrane performance.
108DG, cmz/s 0.9 1.69 0.609 0.469
Sorption and Diffusion. Table I1 lists the concentration-dependent integral diffusivities, D, diffusivities a t zero concentration, Do, and sorption, C,, in the treated and untreated membranes at 30 "C. It is found that the sorptive capacity of all the treated membranes has increased. The selective transport of methanol may be attributed to the preferential sorption of methanol from its binary mixtures. However, the diffusivities of only the CEG-30 membranes were found to be higher, while those of the CM-30 and CEG-40 membranes were lower than those of the untreated PT-300 membranes. The highest
762 Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989 Table 111. Plasticizing Coefficients of Methanol (6,) 15-32% membrane CEG-30 CEG-40 CM-30 PT-300
&M, cm3/g
&G, cm3/g
14.3 10.9 10 18.2
2.9 4.1 4.18 16.9
and Ethylene Glycol (hn) methanol concentration in the feed 32-5270 6 ~ cm3/g , 3.43 3.4 7.3 4.8
permeation rates of the CEG-30 membrane may thus be explained. In spite of the higher sorptive capacities of the CEG-40 and CM-30 membranes, relative to PT-300, their lower permeation rates may be attributed to their lower diffusivities. Thus, an estimation of their plasticizing coefficient may indicate the exact mechanism of transport through the membrane. Liquid-Polymer Interactions in the Membrane. Liquid-polymer interactions, represented by the plasticizing coefficients, also evident from the anomalous sorption-permeation behavior through the PT-300 membranes, in our previous study (Ghosh et al., 1988), have been estimated for all the membranes by the trial-and-error solution of eq 2 and 3 (Suzuki and Onozato, 1983). These (2) D M = DOM exp(yMcSM + YGCSG)
DG = DOG ~ X P ( T M ~ S M+ YGCSG)
(3) equations have been obtained from Fick's law, using concentration-dependent diffusion coefficients, which are capable of describing the binary transport behavior in swollen membranes. Table I11 gives the average values of the plasticizing coefficients, yM and YG, for a range of feed compositions studied. The values of yM were found to decrease with increasing methanol content in the feed, while that of YG increased. This suggests that, although the interaction of methanol is initially high, an increase in its concentration in the feed further increases the interaction and plasticizing capability of glycol, promoting the transport of glycol through the membrane, thereby reducing the selectivity of the process. The YG value of the CM-30 membrane at 86% methanol content in the feed is found to be 10, which is equal to that of YM at 15% methanol content in the feed. Correspondingly, the separation factor at 86% methanol content was also found to be about 1. Thus, similar yM and YG values in a given membrane seem to indicate similar interactions of both the permeating components and hence the poor separation. Accordingly, the similarity in the y M and YG values in the range of feed compositions of 32-86% methanol content, for the CEG-30 and CEG-40 membranes, may account for their similar separation efficiencies. However, the abnormally high YG values in the PT-300 membrane may be attributed to the random clustering of ethylene glycol molecules, which have a tendency to associate amongst themselves in the disordered regions of the 021, lOT, and 101, crystalline phases of the membrane matrix. Hence, it may be inferred that the YM and YG values may not always account for their plasticizing capability alone. They may also account for some kind of weak bonding or clustering of the permeants in the membrane matrix, which in turn may result in some anomalous sorption-diffusion of the permeants and its effect on the transport behavior, as found in the present system. Apparent Activation Energies of Pervaporation. Table IV gives the values of the apparent activation energies of pervaporation as estimated from the Arrhenius temperature dependence of permeation rates (Larchet et al., 1983). The activation energies of pervaporation of the CM-30 membrane at 15% methanol content in the feed
6 ~ cm3/g , 6.89 6.9 4.2 21.18
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6 ~ cm3/g , 8.1 7 10 23.02
Table IV. Apparent Activation Energies of Pervaporation, E , (Kilocalories/Mole), for Different Methanol Concentrations in the Feed methanol concn in the feed membrane 15% 23% 42% 74% 84% 100% CEG-30 11.3 11.1 9 6.3 1.8 0.8 5.1 5.0 PT-300 14.7 13.8 7.4 6.2 CEG-40 15.2 14.8 13.4 11.6 7.1 3.43 CM-30 29.6 14.9 10.1 8.08 5.1
could not be measured due to its extremely low permeation rate. Activation energies of pervaporation around 10 and below are indicative of activated transport (Michaels et al., 1969). Thus, the higher values of E, at lower methanol concentration in the feed indicate difficult permeation and hence lower permeation rates, as is evident from our experimental results. The effect of plasticization of methanol on the crystalline structure of the CM-30 membrane is most pronounced, as evident from the E, values, which show a significant, rapid decrease with increasing methanol concentration in the feed. It thus appears that a state of saturated plasticization, like that in the PT-300 membrane, is hardly ever achieved in this membrane.
Conclusions A highly selective separation of methanol and ethylene glycol at 30 "C, with low concentrations of methanol in the feed, was achieved by pervaporation with cellophane membranes that were solvent-annealed in ethylene glycol, a t 30 "C. The lower separation efficiencies of the methanol-conditioned membrane may be attributed to an unstable, crystalline order, resulting from large restructuring of the membrane matrix on treatment with the stronger plasticizing component, methanol. The ethylene glycol conditioned membrane a t 40 "C also showed somewhat lower separation efficiences, almost similar to that of the untreated membrane, thus indicating that only an optimum degree of plasticization of the membrane is necessary for improved membrane performance. Morphological changes in the membrane, upon conditioning, were responsible for changing the sorption, diffusion, permeation, and liquid-polymer interactions in the membrane, thus causing an overall change in the membrane performance. An estimation of the values of the plasticizing coefficients of the permeants indicates a higher interaction of glycol with the membrane, in the presence of increased methanol content in the feed. This is possibly responsible for the corresponding lower selectivities, as obtained with feed compositions with higher methanol content. The abnormally high values of the plasticizing coefficients of glycol in the untreated membrane indicate that the plasticizing coefficients of permeants may not always account for their plasticizing capabilities alone but may sometimes account for some weak bonding or clustering in the disordered membrane matrix, thus resulting in an anomalous sorption-diffusion vis-&vis transport behavior of the permeants, as obtained in the present system. Further studies are, however, necessary to investigate the time dependence of the membrane perform-
763
Ind. Eng. C h e m . R e s . 1989,28, 763-771
ance upon conditioning, to exploit this technique for commercial applications.
Acknowledgment We thank Dr. D. Ganguly and Dr. M. Chatterjee (Central Glass and Ceramic Research Institute, Calcutta) for their cooperation in carrying out the desorption experiments and Dr. S. Dasgupta (Jadavpur University) and Dr. S. Roy (Indian Association for Cultivation of Science, Calcutta) for the X-ray diffraction studies. We are also grateful to S. Datta (Lecturer, Jadavpur University) for some valuable suggestions. Nomenclature Jp = total permeation rate, kg/(h m2) Ji = individual permeation rate, g/(s cm2) a = separation factor Cs = g sorbed/cm3 of dry membrane L = membrane thickness, cm Do= diffusivity at zero concentration, cm2/s D = integral diffusivity, cm2/s y = average plasticizing coefficient, cm3/g E , = apparent activation energy, kcal/mol 0 = angle of scattering Subscripts
M = methanol G = ethylene glycol
Literature Cited Ghosh, I.; Sanyal, S. K.; Mukherjea, R. N. Pervaporation of Methanol-Ethylene Glycol with Cellophane Membrane: Some Mechanistic Aspects. Ind. Eng. Chem. Res. 1988, 27, 1895-1900. Hirotsu, T. Graft Polymerized Membranes of Methacrylic Acid by Plasma for Water-Ethanol Permseparation. Znd. Eng. Chem. Res. 1987,26, 1287-1290. Larchet. C.: Brun. J. P.: Guillou. M. SeDaration of Benzene-n-HeDtane 'Mixture by Pervaporation with Elastomeric Membrane: Performance of Membranes. J. Membrane Sci. 1983,15,81-96. Mark, H. F., Gaylord, N. G., Bikales, N. M., Eds. Cellulose. In Encyclopedia of Polymer Science and Technology; Interscience: New York, 1965; Vol. 111, pp 152-169. Michaels, A. S.; Vieth, W.; Hoffman, A. S.; Alcalay, H. A. Structure-Property Relationships for Liquid Transport in Modified Polypropylene Membranes. J . Appl. Polym. Sci. 1969, 13, 577-598. Rautenbach, R.; Albrecht, R. Separation of Organic Binary Mixtures by Pervaporation. J . Membrane Sci. 1980, 7, 203-223. Rogers, C. E.; Fels, M.; Li, N. N. Separation by Permeation through Polymeric Membranes. In Recent Developments in Separation Science; Li, N. N., Ed.; CRC: Cleveland, OH, 1976; Vol. 11. Sikonia, J. G.; McCandless, F. P. Separation of Isomeric Xylenes by Permeation through Modified Plastic Films. J . Membrane Sci. 1978, 4, 229-241. Suzuki, F.; Onozato, K. Pervaporation of CH30H-H20 Mixture by Poly(methy1 L-glutamate) Membrane and Synergetic Effect of their Mixture on Diffusion Rate. J . Appl. Polym. Sci. 1983, 28, 1949-1956.
Received for review August 4, 1988 Accepted December 28, 1988
Registry No. Methanol, 67-56-1; ethylene glycol, 107-21-1.
PROCESS ENGINEERING AND DESIGN Synthesis of Methanol in a Reactor System with Interstage Product Removal K. Roe1 Westerterp,* Michal Kuczynski,+and Charles H. M. Kamphuis Chemical Reaction Engineering Laboratories, Department of Chemical Engineering, University of Twente,
P.O. Box 217, 7500 A E Enschede, T h e Netherlands
The synthesis of methanol has been carried out in a high-pressure miniplant consisting of two packed tubular reactors in series with a high-temperature interstage product removal. In this way, high per-pass conversions can be achieved, even so high that recycle of nonconverted reactants is not necessary anymore. Methanol was absorbed a t reaction temperatures in a countercurrently operated packed bed absorber with tetraethylene glycol dimethyl ether as the solvent, which proved to be efficient and selective. Solvent vapors have no influence on the activity or durability of the copper catalyst used. A closed absorption-desorption loop has been used for the solvent system. Significant energy savings and raw material savings are expected for large-scale applications of this system. The formation of methanol from carbon monoxide and hydrogen CO + 2Hz = CH30H -AH(298 K) = 91 kJ/mol (1) is a strongly exothermic equilibrium reaction. For reasonable reaction rates, the modern low-pressure copper catalysts require operating pressures and temperatures of
* To whom
correspondence should be addressed. 'Present address: DSM Research B.V., P.O. Box 18,6160 MD Geleen, T h e Netherlands.
at least 5 MPa and 485 K, respectively. At such process conditions, the attainable conversion is strongly limited by the thermodynamic equilibrium. To maintain a high driving force for the reaction, industrial converters are operated at per-pass conversions far below the equilibrium values. Therefore, recycle ratios as high as 5-10 for the synthesis gas are common in industrial practice. Such operating techniques cause high-energy consumptions in the product separation section and in the reactant recycle system (Westerterp and Kuczynski, 1986). The recycle of the reactants affects the energy consumption unfavorably 0 1989 American Chemical Society
