THE ROLE OF BOUNDARY-LAYER MASS-TRANSFER IN PARTIAL OXIDATION SELECTIVITY

Simple models predict that mass transfer in the boundary layer over a catalyst surface should have a large influence on reactor selectivity for any fast series reaction, but few experimental cases demonstrate this explicitly. In this paper, we show experimentally that selectivities of CH4 oxidation to CO and H2 over Pt-Rh gauzes and HCN synthesis from CH4, NH3, and air over Pt-coated ceramic monoliths are strongly affected by the gas flow rate and the catalyst geometry. For any series reaction in which the desired product is an intermediate species, such as HCN synthesis, an optimal residence time exists where the production of the desired component is maximized, and the selectivity of formation of the intermediate product improves with increasing rates of mass transfer. For parallel and series-parallel reactions, mass transfer may improve or reduce selectivity of formation of the intermediate product, with the effect of mass transfer depending strongly on the reaction orders of the kinetics. However, typical series-parallel partial oxidation reactions such as the partial oxidation of CH4 to synthesis gas require high rates of mass transfer for maximum reactor selectivity. © 1992.