Use of ceramic membranes

As an example the use of ceramic membranes for ethane dehydrogenation has been discussed (91). The constmction of a commercial reactor, however, is difficult, and a sweep gas is requited to shift the product composition away from equiUbrium values. The achievable conversion also depends on the permeabihty of the membrane. Figure 7 shows the equiUbrium conversion and the conversion that can be obtained from a membrane reactor by selectively removing 80% of the hydrogen produced. Another way to use membranes is only for separation and not for reaction. In this method, a conventional, multiple, fixed-bed catalytic reactor is used for the dehydrogenation. After each bed, the hydrogen is partially separated using membranes to shift the equihbrium. Since separation is independent of reaction, reaction temperature can be optimized for superior performance. Both concepts have been proven in bench-scale units, but are yet to be demonstrated in commercial reactors. 

The use of ceramic membranes in gas separations is not new. Since 1950, alumina membranes were used in the separation of UF isotopes. However, the separation factor is very low in this case (theoretically 1.004 ). 

Swafford [1987] has studied the use of ceramic membranes for processing the washwater of minced Alaska pollack. The membrane with a nominal MWCO of 10,000 daltons yields a membrane flux about 20 to 30% lower than that of a membrane with a MWCX) of 20,000 daltons. 

Nakajima, M., N. Jimbo, H. Nabetani and A. Watanabe, 1989a, Use of ceramic membrane for enzyme reactors, in Proc. 1st Int Conf. Inorg. Membr.,... 

Examples of the use of ceramic membranes in the production of potable water are quite numerous [42,60-65]. An interesting review is presented by Pouet et al. [60] of some 15 installations working with ceramic membranes for the production of drinking water. Sizes of these installations, installed in France between 1984 and 1990, vary from 5 to 100 m /h. 

Merin and Daufin [44] and Bhave [3] present a comprehensive review of the field, the main use of ceramic membranes being protein concentration by micro-or ultra-filtration and bacteria removal by microfiltration. For the latter the Bactocatch process, as described by Gillot et al. [47], Merin and Daufin [44] and Bhave [3] forms an important example. At an average flux of 7001/m h 99.7% of the bacteria are withheld without retaining the proteins. 

Probably the only possibility is the combination of a high driving force (sweep gas or low permeate pressure) and a very high selective membrane. The use of ceramic membranes in an isothermal reactor concept shows better prospects. This process, in combination with high selective membranes and the necessary membrane boundary conditions are being studied, and the results will be reported in future. 

Technical and economic evaluation study for the use of ceramic membrane reactors for the dehydrogenation of propane to propylene. Confidential NOVEM Report No. 33105/0090, by KTl, ECN and HlC, Jan. 1994. 

M. Schwartz, J.H. White, M.G. Myers, S. Deych, and A.F. Sammels, in "The Use of Ceramic Membrane Reactors for the Partial Oxidation of Methane to Synthesis Gas", Preprints 213th ACS National Meeting, San Francisco, CA, April 13-17, 42, 1997. 

Bishop B, Judd S, Cardin J, Alvarez Vazquez H, Jefferson B, and Goldsmith R. Use of ceramic membranes in airlift membrane bioreactors. Proceedings of the Eight International Conference on Inorganic Membranes, Cincimiati, OH, July 18-22, 2004, pp. 142-145. 

Despite all these studies, a recent economic feasibility study of the membrane-assisted WGSR conducted to quantify the advantage over conventional technology disproved the concept for Pd membrane reactors [62] and supported the use of ceramic membranes [36]. 

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