SUPRA-EQUILIBRIUM CONVERSION IN PALLADIUM MEMBRANE REACTORS - KINETIC SENSITIVITY AND TIME-DEPENDENCE

Catalytic membrane reactors are of particular interest for reactions that are severely thermodynamically limited. A case study is developed comparing and contrasting cyclohexane dehydrogenation with isobutane dehydrogenation in a palladium membrane reactor. The former is an often chosen model dehydrogenation, while the latter is of high, current industrial interest. Based on the literature and modeling it is concluded that cyclohexane dehydrogenation accentuates the effect of the membrane on supra-equilibrium conversion. More favorable thermodynamics, stoichiometry and kinetics for cyclohexane versus isobutane dehydrogenation each contribute. To achieve similar levels of membrane-assisted conversion, the reactor volume required for isobutane dehydrogenation would need to be at least 750 times larger than that required for cyclohexane based upon published kinetics for commercial catalysts. Rate limitations at the catalyst rather than for hydrogen transport through the membrane tend to dominate. The hydrogen deficiency in the reaction zone resulting from the membrane transport also leads to enhanced rates of coke formation resulting in catalyst and membrane deactivation. This study points out that more effort needs to be expended on the development of new dehydrogenation catalysts with high activity per unit volume and high resistance to deactivation.