The role of heat transfer on the feasibility of a packed-bed membrane reactor for propane dehydrogenation

Abstract

The review of published experimental work reveals that per-pass conversion and productivity of membrane reactors for propane dehydrogenation (PDH) with in-situ H2 separation could not improve over conventional commercial technologies. Reported conversion figures were, if at all higher than without hydrogen permeation, achieved with the help of high propane dilution or low space velocity. Slow hydrogen permeation is often blamed for disappointing results in membrane reactor research, but not the fastest hydrogen-permeable membrane have been tested in PDH so far. All reported experimental work deals with lab-scale test reactors with mostly tubular membranes of up to 8 mm diameter, in which heat transfer limitations do not play a dominant role. None of the published general and PDH-specific work did systematically study the combined effect of heat transfer and hydrogen permeation, and the design problem of integrating the catalytic bed with sufficient area for heat transfer and permeation of hydrogen. Parameter studies over a wide range of the dimensionless mass- and heat-Stanton numbers with a packed-bed membrane reactor model show that sufficient heat supply is decisive for exploiting the effect of hydrogen removal on propane conversion. Fast enough membranes are in principle available, but additionally require enough heat transfer area for a feasible membrane reactor design. Higher productivity than in conventional PDH processes cannot be achieved within the kinetic limits of catalytic reaction, hydrogen permeation and heat transfer in scalable membrane reactor designs. Low space velocity and high pressure, both proven to drive the conversion of other H2-producing equilibrium reactions (cyclohexane dehydrogenation and steam reforming) to completion, lead to low selectivity in PDH because of the low thermal stability of the product propylene.