Dehydrogenations with Inorganic Membranes

Large-scale Dehydrogenations with Inorganic Membranes 

Over the years, several processes for the catalytic dehydrogenation of propane to propylene have been developed, which can be divided into processes based on an adiabatic or an isothermal reactor concept, respectively. The processes currently apphed on an industrial scale are based on adiabatic systems, such as the Catofin (Lummus/Air Products) and the Oleflex (UOP) process. As the dehydrogenation of propane to propylene comprises an equihbrium reaction (11), selective removal of hydrogen from the reaction mixture can shift the reaction towards the product side. At high temperatures, thermal cracking may occur. 

Van Veen et al. [126] studied the technical and economic feasibility of the apph-cation of ceramic membranes in different dehydrogenation processes. As the Oleflex process uses four reactor beds in series, this process is more suitable 

The potential application of ceramic membranes for the dehydrogenation of ethylbenzene to styrene (12) has also been evaluated [126]. In the conventional process, two radial reactors in series are used with one preheater and one interstage heater. Steam acts as an energy carrier and as a diluent. 

For the dehydrogenation of ethylbenzene in a packed-bed ceramic membrane reactor, three configurations are possible using a specific sweep gas in combination with a hydrogen or oxygen selective membrane (see Fig. 5.13). 

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Large-scale Dehydrogenations with Inorganic Membranes... 

Pyruvate oxidase. The soluble flavoprotein pyruvate oxidase, which was discussed briefly in Chapter 14 (Fig. 14-2, Eq. 14-22), acts together with a membrane-bound electron transport system to convert pyruvate to acetyl phosphate and C02.319 Thiamin diphosphate is needed by this enzyme but lipoic acid is not. The flavin probably dehydrogenates the thiamin-bound intermediate to 2-acetylthiamin as shown in Eq. 15-34. The electron acceptor is the bound FAD and the reaction may occur in two steps as shown with a thiamin diphosphate radical intermediate.3193 Reaction with inorganic phosphate generates the energy storage metabolite acetyl phosphate. 

Given in Table 8.8 is a summary of experimental data available on dehydrogenation reactions using porous inorganic membrane reactors with a variety of catalysts. Some of those reactions are industrially important and will be discussed separately as follows. 

In the present concept of styrene dehydrogenation implementation of inorganic membranes is not feasible. Application of Knudsen diffusion membranes with a low permselectivity to hydrogen leads to a considerable permeation of ethylbenzene and thus, to lower yields. Microporous and palladium membranes give better results, but worse than a conventional case, because the conversion is limited by reaction kinetics. The ratio of permeation rate to reaction rate is very important in selecting membranes in a membrane reactor process in which equilibrium shift is foreseen. 

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