Ceramic membranes concept

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. 

Kim and Datta [1991] tested the above concept with the homogeneously catalyzed ethylene hydroformylation by hydridocarbonyltris (triphenyl phosphine) Rh (I) catalyst dissolved in dioctyl phthalate solvent. They concluded that for effective separation of the product, the transport resistance of the catalyst layer should be less than that for the membranes by controlling the liquid loading of the catalyst layer. They also pointed out that the organic membranes used can not withstand the aggressive reaction conditions and suggested that ceramic membranes appear to hold promises for practical applications. 

FIGURE 6.29 Concepts of membrane reactor based on gas-liquid-solid ceramic membrane contactors. 

Reif M and Dittmeyer R. Porous, catalytically active ceramic membranes for gas-liquid reactions a comparison between catalytic diffuser and forced through flow concept. Catal. Today 2003 82 3-14. 

Both Catofin and Oleflex use an adiabatic reactor concept. The Oleflex process uses four reactor beds in series, which as such is more suitable for addition of a ceramic membrane separation unit than the Catofin process which uses a parallel reactor system. A comparison between the Oleflex process as a base case and an Oleflex process equipped with ceramic membranes is made for the following cases ... 

An isothermal reactor concept incorporating a ceramic membrane is more attractive compared to an adiabatic reactor concept from a thermodynamic point of view. In this concept we assumed a reactor with reactor tubes located in a direct-fired heater and operated in a cyclic way to remove coke formed on the catalyst. Parallel bed and heaters have been assumed [35-37]. 

It is concluded that a ceramic membrane reactor based on Knudsen diffusion membranes can give improvements in an isothermal reactor concept although the difference in price level between feedstock and product is too small to give an economically viable process. 

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. 

When the membranes are used on an industrial scale, a considerable amoimt of surface area will be necessary to process the gas stream involved. A typical surface area necessary is 1500 m for a 300 MW class power plant. For ceramic membranes this is a rather large surface area. Considering that permselectivity is already good for this application, it seems reasonable to direct research towards enlargement of the permeation or explore module concepts with a high surface area to volume ratio (e.g. monolytic systems) next to selectivity improvement. 

The present concept of implementation of ceramic membranes in the styrene process is not feasible, because ... 

Fig. 9.10 (a) Concept for closed-one-ended ceramic tubes bundled for cermet membrane applications, (b) Bundle of ceramic membrane tubes produced by CoorsTek for an earUer industried separation application (Courtesy R. Kleiner, J. Stephan, and F. Anderson, CoorsTek)... 

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