Selective Oxidation of Ethylene to Ethylene Oxide on Silver Catalysts at Industrial Conditions: Reactor Profiles, Kinetics, and Chlorine Inhibition

Selectivity is the key parameter in industrial ethylene oxide (EO) production by oxidation of ethylene with oxygen on Ag/alpha-Al2O3 catalysts. Accurate temperature control in wall-cooled multitubular fixed-bed reactors and chlorination of the silver surface by feeding small chlorinated hydrocarbons such as 1,2-dichloroethane (DCE) are required to fine-tune electrophilicity and surface oxygen coverage for maximum EO selectivity at economic ethylene conversion. Temperature and molar flow rate profiles of C2H4, O-2, EO, CO2, H2O, DCE, and chlorine-containing reaction products vinyl chloride (VC) and ethyl chloride (EC) were measured in a compact profile reactor (CPR) and in a pilot-scale profile reactor (PSPR) to explore the spatial interplay between DCE concentration, temperature, inlet flow rate, and O-2 conversion. Chlorine and oxygen compete for the same active silver sites despite more than 4 orders of magnitude different concentrations (ppm vs vol %). Chlorine coverage increases from inlet to outlet due to the decreasing partial pressure of O-2 along the bed, leading to shutdown of all reactions if all active Ag sites are blocked by chlorine. A kinetic model is derived from a dual-site mechanism taken from the literature. Kinetic parameters are determined from differential initial rate measurements, Arrhenius plots, and by fitting the rate expressions implemented in a plug flow model to the species and temperature profiles in the CPR. A very good agreement is reached. PSPR profiles are modeled by implementing the derived kinetic model into a 2D pseudohomogeneous reactor model. At conversions <10%, the experimental profiles are well captured, but the model fails to accurately reproduce the point of thermal runaway in the catalyst bed of the PSPR caused by a too low reactor temperature and resulting insufficient chlorine coverage of the silver surface. [GRAPHICS]