MAXIMUM PER-PASS CONVERSION IN POROUS MEMBRANE DEHYDROGENATION REACTORS

Gas permeation rates through commercial ultrafiltration membranes are relatively high. Therefore, using these membranes as passive walls in a catalytic dehydrogenation reactor involves an important difficulty: loss of feed in the permeate zone. The purpose of this study is to evaluate both operational conditions and feed-type's effect upon the amount of feed that is retained in the residue stream. Permeation simulations are performed for a system consisting of a ceramic tube filled with dehydrogenating catalyst particles. No reaction is assumed to occur, so that the relative amount of feed recovery reveals the maximum per-pass conversion attainable by using this configuration. Modeling is based on Ergun's equation, material balances, and a Knudsen permeation rate equation. The results show that maximum per-pass hydrocarbon dehydrogenation conversion can be limited not only by thermodynamics but also by feed losses. The relative amount of recovered feed in the residue increases sharply as the feed flow increases and the pressure decreases. Porous membrane reactors seem better suited for dehydrogenating high molecular weight hydrocarbons. Modification of commercial ultrafiltration tubes, either by reducing the average pore diameter and/or porosity or by enlarging the inner tube diameter and/or tortuosity, is a desired goal for gas permeation applications.