DESIGN OF MEMBRANAL DEHYDROGENATION REACTORS - THE FAST REACTION ASYMPTOTE

This study presents a systematic analysis of certain design aspects of a packed-bed catalytic membrane reactor for simple and fast dehydroge nation reactions. For sufficiently fast reactions, local chemical equi librium can be assumed everywhere and conversion is transport limited. Under these assumptions, in a plug-flow isothermal reactor with a per mselective membrane, analytical results can be derived showing the con version dependence on reactor length and equilibrium coefficient. Thes e results suggest that significant conversions should be expected in s uch a reactor with a reasonable membrane area-to-feed flow rate ratio (e.g., at 450 degrees C during butane dehydrogenation, with membrane f lux of 1 cm(2)/cm(3)/min). High pressures lead to lower equilibrium co nversions and to higher diffusive fluxes, resulting in a marginal chan ge in overall conversion. Shell-side hydrogen partial pressure affects the conversion significantly, and the shell-to-tube flow rate ratio s hould be sufficiently large (10 to 100). Catalyst loading should be op timized to decrease length and improve selectivity. Ceramic Knudsen-se lective membranes yield poor conversions. A preliminary analysis of an adiabatic reactor in which the diffusing hydrogen is combusted to sup ply the dehydrogenation enthalpy is also presented. These conclusions are contrasted with experimental observations obtained during isobutan e dehydrogenation in a Pd membrane reactor.