Hydrogen-permeable membrane dehydrogenation reaction

When the membrane performs only a separation function and has no catalytic activity, two membrane properties arc of importance, the permeability and the selectivity which is given by the separation factor. In combination with a given reaction, two process parameters are of importance, the ratio of the permeation rate to the reaction rate for the faster permeating component (c.g. a reaction product such as hydrogen in a dehydrogenation reaction) and the separation factors (permselectivities) of all the other components (in particular those of the reactants) relative to the faster permeating gas. These permselectivities can be expressed as the ratios of the permeation rates of... 

G. Cacciola, Y.I. Aristov, G. Restuccia and V.N. Parmon, Influence of hydrogen-permeable membranes upon the efficiency of the high temperature chemical heat pumps based on cyclohexane dehydrogenation, benzene hydrogenation reactions. Int. J. Hydrogen Energy, 18 (1993) 673. 

The idea of conducting hydrogenation and dehydrogenation reactions simultaneous on opposite surfaces of a membrane, which is selectively permeable to hydrogen, was first presented by Grgaznov et a(6)(7). 

In a separate parametric study, Mohan and Govind(l)(9) analyzed the effect of design parameters, operating variables, physical properties and flow patterns on membrane reactor. They showed that for a membrane which is permeable to both products and reactants, the maximum equilibrium shift possible is limited by the loss of reactants from the reaction zone. For the case of dehydrogenation reaction with a membrane that only permeates hydrogen, conversions comparable to those achieved with lesser permselective membranes can be attained at a substantially lower feed temperature. 

The feasibility of the palladium membrane system with an oxidation reaction on the permeation side and 1-butene dehydrogenation reaction on the reaction side in a membrane reactor has been successfully demonstrated. The palladium and its alloy membrane not only can withstand high temperature but also are selectively permeable to hydrogen... 

As an example the use of ceramic membranes for ethane dehydrogenation has been discussed (91). The construction of a commercial reactor, however, is difficult, and a sweep gas is required to shift the product composition away from equilibrium values. The achievable conversion also depends on the permeability of the membrane. Figure 7 shows the equilibrium 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 equilibrium. 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. 

Important parameters in a catalytic membrane reactor for dehydrogenation are the reaction rate, the permeability (i.e. permeation rate) and the permselectivity for hydrogen. It appears at first sight that good conditions are those where the permeation rate (removal of H2) and the reaction rate (formation of H2) are close to each other, but the role of the permselectivity is also important. 

The extractor mode corresponds to the earher applications of MRs and has been applied to increase the conversion of a number of equilibrium limited reactions, such as alkane dehydrogenation, by selectively extracting the hydrogen produced (Hsieh, 1996). The hydrogen permselectivity of the membrane and its permeability are two important factors controlling the efficiency of the process. 

In porous MRs, the membrane may function as an extractor, a distributor, or an active contactor, as listed in Table 2.6. The extractor mode corresponds to the earlier applications of MRs and has been applied to increase the conversion of a number of equilibrium-limited reactions, such as alkane dehydrogenation, by selectively extracting the hydrogen produced. Other H2-producing reactions - such as water gas shift (WGS), steam reforming of methane, and the decomposition of H2S and HI -have also been investigated successfully with the MR extractor mode. The H2 perm-selectivity of the membrane and its permeability are two important factors controlling the efficiency of the processes [17]. 

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