j o u r n a l of MEMBRANE SCIENCE E LS E V I E R
Journal of Membrane Science 105 (1995) 287-29 !
Rapid communication
Newly developed ceramic membranes for dehydration and separation of organic mixtures by pervaporation Robert W. van Gemert *, F. Petrus Cuperus Agrotechnological Research Institute (ATO-DLO), P.O. Box 17, NL-6700 AA Wageningen, Netherlands
Received 1 February 1995; accepted 5 April 1995
Abstract Polymeric pervaporation membranes sometimes show great variety in performance when they are alternately used for different solvent mixtures. In addition, membrane stability in time is a problem in case of some solvents. Therefore, newly developed ceramic silica membranes with a "dense" top layer were tested for pervaporation. In dehydration of the lower alcohols methanol, ethanol and 2-propanol selectivities of 400, 200 and 600, respectively, were found at water concentrations of about 2 w/w% in the feed. Fluxes were 50, 150 and 160 g/m2/h, respectively. At higher water concentrations in the feed selectivities decreased and fluxes increased. Especially methanol selectivity decreased drastically at water concentrations above 5 w/w%. Fluxes increased significantly with water concentrations above 5 w/w% in the 2-propanol/water case. The overall performances were comparable with those found with commercially available polymeric membranes. The performance of the ceramic membrane did not decrease noticeably during an experimental period of three months, although a wide range of alcohols were tested with changing water concentrations. Also periodic changes to the system methanol/MTBE did not decrease the performance for dehydration. Methanol could be recovered from MTBE with selectivities of about 19 and fluxes of 41 g/m2/h with 9 w/w% methanol in the feed. Activated diffusive transport through micropores is the suggested separation mechanism for this membrane. Keywords: Pervaporation; DehydratiOn;Organic-organic separation; Ceramic membranes; Activated transport
1. Introduction The use of pervaporation as a separation technique in multi-purpose equipment seems very attractive. The broad applicability of the membrane, e.g. dehydration o f various solvents, is a main criterium in such case. Extensive research has been done in finding the optimal membrane material that has special interactions with a specific component to maximize performance like selectivity, flux and stability [ 1,2]. Until now the best results were obtained using polymers as dense top layers. However, the performances o f these membranes * Corresponding author. 0376-7388/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10376-7388 ( 95 ) 00098-4
can be largely influenced by changes in process conditions like concentration and temperature. Also different organic media sometimes highly reduces the performance o f the membranes. This is probably due to anomalous swelling of the polymer materials by water or solvents, or to cracks that are formed in the membrane top layer. In this perspective a stable multipurpose membrane, made o f ceramics, could be a major improvement. However, until now no satisfying dense ceramic membrane has been developed for pervaporation. Most membranes are not defect free and have mesopores, which yield an unsatisfactory selectivity. W h e n the top layer consists exclusively of micropores a somewhat different separation mechanism is sug-
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gested [3-5]: the so-called activated diffusion. Also here specific interaction of solvent and membrane material are important. An additional effect is contributed to an activated transition state when pores and molecules have a certain ratio. In the activated state, diffusion of one component can be relatively enhanced. In this paper we introduce a newly developed ceramic silica membrane that shows good performance for dehydration of organics and for organic-organic separation.
2. Experimental 2.1. Materials The solvents methanol, ethanol, 2-propanol and methyl tert-butyl ether (MTBE) were obtained from Merck (Amsterdam, The Netherlands).
2.2. Membrane module A stainless steel membrane module has been developed which is applicable up to temperatures of 200°C and pressures of 2000 kPa [5-7]. Sealing for the vacuum compartment was a Viton A O-ring and for the feed stream compartment a flat graphite gasket was used. The space above the membrane is 2 mm which assures low residence times.
2.3. Membrane preparation y-Alumina has been used as membrane material on which silica is deposited by the sol-gel technique [4,5]. Special dip equipment has been developed by Velterop B.V. to apply a consistent coating on a flat membrane from a suspension or a sol. The ceramic membrane is sealed defect free with an inorganic adhesive to a dense ceramic ring. This makes it possible to seal the membrane in a module with O-rings or graphite gaskets. For laboratory experiments a membrane is manufactured with a diameter of 39 mm sealed on a dense ceramic ring with outer diameter 60 mm. The effective membrane surface area is 1.2 × 1 0 - 3 m 2
2.4. Laboratory set-up On the feed side of the membrane a recycle stream was applied in connection with a stainless steel 500 c m 3
storage vessel. Stainless steel tubing was used with 6 mm i.d. and Swagelock connections. Temperature of the feed stream was controlled by a temperature sensor close to the membrane surface in connection with a controllable heating plate attached to the storage vessel. The temperature at the membrane surface was controlled within 0.5°C. An Ismatic gear pump was used with a flow rate of 100 ml/min corresponding to a flow velocity of 0.03 m s - 1 along the membrane. The complete set-up was applicable up to temperatures of 200°C and pressures of 20 bar. On the permeate side vacuum was kept below 2 mbar with an Edwards E2M5 double stage vacuum pump. The vacuum was measured with an Edwards PiraniGauge PRM 10 and displayed with an Pirani controller 1101. A cold trap was used just below the membrane to collect the permeate for further analysis.
2.5. Selectivity measurement The permeate composition was measured using the refractive index of the liquid. An EUROMEX Abbe refractometer RF490 was used kept at 20°C with a thermostat bath. Selectivity is defined as the quotient of the concentration ratio of the components in the permeate and in the feed mixture.
3. Results and discussion 3.1. Dehydration To determine the applicability of the newly developed ceramic membranes for dehydration with pervaporation, first lower alcohols were tested to compare with wide-spread published results obtained with polymeric pervaporation membranes.
3.2. Methanol Several feed mixtures with methanol and varying water concentrations between 2 and 10 w/w% were used at 60°C. At high water concentrations (9 w/w%) fluxes were about 200 g/m2/h which is quite promising with a selectivity of 10-15. At low water concentrations (2 w/w%) fluxes decrease until 60 g/m2/h but selectivity increases to 200. The flux increases almost linearly with the water concentration in the feed (Fig. 1).
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R. W. van Gemert, F.P. Cuperus / Journal of Membrane Science 105 (1995) 287-291 800
700
600
500 ¢q
g
400
300
200
100
0
I
I
I
I
2
4
6
$
Concentrationwaterin feed (w/w%) --m-- methanol
- . o - ethanol
~
10
2-propanol
Fig. 1. The total flux for dehydration of lower alcohols using a silica membrane with various water concentrations in the feed.
However, the selectivity increased rapidly at water concentrations lower than 5 w / w % (Fig. 2).
3.5. Organic-organic separation
3.3. Ethanol
Methanol/MTBE
Dehydration of ethanol was performed at 70°C with water concentrations between 2 and 9 w/w%. The fluxes were between 150 and 350 g/m2/h with selectivities between 160 and 50 respectively. Apparently, the selectivities were lower for ethanol than for methanol dehydration below feed concentrations of 3 w/ w% water (Fig. 2). The fluxes, however, were over the whole concentration range more than 100 g/m2/h higher (Fig. 1).
3.4. Iso-propylalcohol Good performances were found for the dehydration of 2-propanol at 70°C. Average selectivities were about 3 to 6 times higher than for methanol and ethanol. Significant higher fluxes, however, could only be measured when more than 5 w / w % water was in the liquid feed mixture. Fluxes of 250 g/m2/h with selectivities above 500 are comparable with most polymeric membranes [ 2].
For the organic-organic separation the selectivity of the ceramic membrane for methanol and MTBE is tested. The production of methyl tert-butyl ether (MTBE) is one of the largest growing chemicals in the last 10 years and will probably be in the next 10 years. MTBE is produced by the reaction of methanol with tert-butanol in which an excess of methanol of about 20 w / w % is used to reach high product yields. This excess can be removed by distillation which has the problem of going through an azeotrope. This problem could be avoided using pervaporation to reduce methanol concentrations below the azeotrope composition of about 13% methanol. Pervaporation experiments with the ceramic membrane for the removal of methanol from MTBE did not show a major break through, but selectivities measured were about 19 with fluxes of about 41 g/m2/ h (Table 1 ). From solubility parameters it can be seen that major differences exist in the polar and hydrogen bond forces parameters. This could probably be the main influences that contribute to the selectivities measured.
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R. 144 van Gemert, F.P. Cuperus / Journal of Membrane Science 105 (1995) 287-291 600
500
< 400
'~ 300
,<
200
\
100
2
4
6
S
10
Concentration water in feed (w/w%) methanol
+
ethanol
~-
2-propanol
Fig. 2. Selectivity for dehydration of lower alcohols using a silica membrane with various water concentrations in the feed. Table 1 Flux and selectivity for methanol removal from MTBE at different methanol concentrations in the feed at 35°C Methanol ( w / w % )
Flux (g/mZ/h)
6.0 9.0 15.5 16.6
59.8 40.8 37.5 12.7
Selectivity ( - ) 2.O 18.7 5.9 3.8
3.6. Stability and performance In comparison with polymeric membranes, ceramic membranes should be able to withstand higher temperatures and different solvents without serious degradation of the material. The main problem with ceramic dense membranes is to produce and keep a defect free top layer. It is possible that some defects occur during operation and therefore the membrane loses selectivity. All experiments presented in this paper are performed with the same membrane over a period of three months at temperatures below 70°C. No significant decrease in performance could be noticed and also after many experiments the same performance was measured for every type of mixture again.
4. Discussion It is known that separation of methanol/water is quite difficult using pervaporation (or distillation). This is due to the very low thermodynamic activity coefficient of methanol in water. Therefore, especially the high selectivity for methanol found in this work is remarkable. The reason for this selectivity could be the occurrence of activated diffusive transport. The gas selectivity of the same silica membranes has been studied extensively [ 5 ]. For H2/CH4 gas mixtures at temperatures between 100 and 260°C, selectivities between 20 and 60 were measured. This selective was attributed to activated transport. Activated diffusive transport is suggested as the main diffusion process in micropores which is based on the enhanced adsorption energy of molecules when pore size and kinetic molecular diameter have a certain ratio [3]. When two different types of molecules with different kinetic diameter enter the pore, one molecule will have a higher enhancement of the potential minimum and therefore a higher diffusion rate. When the pore diameter is smaller than the largest molecule this molecule is excluded and very high selectivities are measured. At larger pore sizes than about 1 nm also Knudsen diffusion could contribute to the diffusion
R. W. van Gemert, F.P. Cuperus / Journal of Membrane Science 105 (1995) 287-291
process and activated transport can even become negligible. From characterization experiments average pore sizes in the membranes between 3 and 3.5 ,A were found [4-6]. Water has a kinetic diameter of 2.65 ,~ which gives a pore to molecule ratio between 1.13 and 1.32 with average pore sizes between 3 and 3.5 ,~. This is indeed in the range where activated transport is suggested. In case of a zeolite, with a very narrow pore size distribution, activated transport would lead to the highest separation. In our case the membrane is only zeolite-like, and larger pores are probable. Inevitably, this leads to lower separation factors for mixtures that contain molecules with a size close the pore size. In our experiments increasing water concentration in the feed gave increasing fluxes because the fraction of water molecules which enter the micropores directly, without being adsorbed first, is higher. Selectivities, however, drop significantly at high water concentrations. Obviously, this is not in accordance with the principle of activated transport, and can not be explained by the activated diffusion model. This effect is also found for polymeric membranes and is often attributed to 'drag': alcohols molecules are dragged by water through the membrane. For methanol-MTBE separation fluxes were low which is probably because of the exclusion of both molecules by the micropores. These compounds will diffuse through the larger pores which are less prominent. In some pores the diameter will be close to the kinetic diameter of methanol and therefore enhanced methanol diffusion occurs.
5. Conclusions The newly developed ceramic membranes yielded very promising results for the dehydration of lower alcohols. Selectivities and fluxes are comparable with commercially available polymeric membranes and even higher for 2-propanol/water mixtures. Because of the successful removal of water from organics and methanol from MTBE at low temperatures it can be concluded that polar forces play an important role. However, since the top layer merely consists of micro-
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pores the selectivity can be due to activated transport of one of the components. Higher fluxes, with similar selectivities, should be achievable at higher temperatures. During experiments the performance did not decrease noticeably even with changing solvents and temperatures. Therefore, it can be concluded that no defects are formed in the membrane top layer and the sealing method for the membrane on a dense ceramic ring has proven to be defect free. These newly developed ceramic membranes appear to have high standard performances and could be very interesting for industrial applications when produced at larger scale.
Acknowledgements The authors gratefully acknowledge Velterop B.V. for the supply of membranes, and Mr. Frans M. Velterop for helpful discussions.
References [1] T.M. Aminabhavi, R.S. Khinnavar, S.B. Harogoppad, U.S. Aithal, Q.T. Nguyen and K.C. Hansen, Pervaporation separation of organic-aqueous and organic--organic binary mixtures, J. Macromol. Sci., Part C, Rev. Macromol. Chem., 34(2) (1994) 139-204. [ 21 R. Rautenbach, S. Klatt and J. Vier, State of the art pervaporation -10 years of industrial PV, in Bakish (Ed.), Proc. 6th Int. Conf. Pervaporation Chem. Ind., Englewood, NJ, USA, 1992, p. 2. [3] D.H. Everett and J.C. Powl, Adsorption in slit-like and cylindrical micropores in the Henry's law region, J. Chem. Soc., Faraday Trans. 1, 72 (1976) 619-636. [4] R.S.A. de Lange, Microporous sol-gel derived ceramic membranes for gas separation: Synthesis, gas transport and separation properties, Thesis, Enschede, 1993. [ 5 ] R.S.A. de Lange, J.H.A. Hekkink, K. Keizer and A.J. Burggraaf0 Preparation and characterization of microporous sol-gel derived membranes for gas separation applications, in M.J. HampdenSmith, W.G. Klemperer and C.J. Brinker, Better Ceramics Through Chemistry V, Mater. Res. Soc., Pittsburgh, USA, Mater. Res. Symp. Proc., 271 (1992) 505-510. [6] J.C.S. Wu, H. Sabol, G.W. Smith, D.L. Flowers and P.K.T. Liu, Characterization of hydrogen-permselective microporous ceramic membranes, J. Membrane Sci., 96 (1994) 275-287. [7] F.M. Velterop and K. Keizer, Development of a high temperature resistant module for ceramic membranes, Key Eng. Mater., 61 ( 1991 ) 391-394.