Performance and aging of catalysts for the selective hydrogenation of acetylene: a micropilot-plant study

Abstract

Some catalysts for the “tail-end” selective hydrogenation of acetylene were tested in a micropilot plant. This was operated under conditions similar to those of industrial plants. The influence of temperature, carbon monoxide concentration and H 2/C 2H 2 ratio on the activity and selectivity were investigated. Three possible definitions for the calculation of the selectivity to ethylene were used and compared. Great differences in the physico-chemical properties and in catalytic performance were found among the catalysts examined. A decrease in the conversion of acetylene was found at high temperatures. In spite of the short duration of the tests (⩽350 h), aging phenomena were frequently observed, due to the formation of oligomers on the catalysts. Large amounts of carbon were measured in the unloaded catalysts. The formation of oligomers was favoured by higher acetylene concentrations and occurred only in the presence of the hydrogenation reaction. Deactivation was also correlated with 1) the concentration of carbon monoxide, 2) the hydrogen/acetylene ratio, 3) the use of constant or variable reaction conditions during the test and 4) the linear velocity. It was suggested that carbon monoxide is involved in the formation of the organic compounds that deactivate the catalysts. Aging was faster for fresh catalysts. It caused a loss of total surface area, of activity in acetylene hydrogenation and, often, of selectivity to ethylene. A set of tests, planned according to statistical criteria, was performed. Results were interpolated by second degree polynomials, yielding an empirical model which accounted for the influence of both time and reaction variables. This model was used to find conditions of high selectivity and to simulate some runs at constant conversion, maximum selectivity and minimum deactivation of the catalysts.