Modeling homogeneous and heterogeneous chemistry in the production of syngas from methane

A high-temperature, short-contact-time catalytic rhodium monolith reactor for the production of synthesis gas from methane is modeled as a plug-flow tubular reactor using detailed heterogeneous and homogeneous chemistries. The surface reactions are modeled with a 19-elementary-step mechanism for methane on rhodium surfaces, while gas-phase reactions are modeled with GRI-Mech 2.11 which includes 227 reversible reactions. Simulations are performed at different pressures, preheat temperatures, compositions, and catalyst pore sizes. The model calculations show that there is significant interplay between homogenous and heterogeneous chemistry. Homogeneous chemistry, which is generally unselective for syngas production, is favored by high pressure, large catalyst pores, and high preheat temperature. Heterogeneous chemistry is favored by low pressure, small pores, and low preheat temperature. The onset of gas-phase chemistry can be avoided by feeding air rather than oxygen into the reactor. At industrially practical pressures, air-fed reactors may be viable because less unselective homogeneous chemistry occurs. Experimental agreement with this model is very good for oxygen and preheated-air feeds at low to moderate pressure. (C) 1999 Elsevier Science Ltd. All rights reserved.