Induction period of coking in the steam reforming of hydrocarbons

Abstract

Publisher Summary Carbonaceous filaments constitute the main kind of deposits formed in the steam reforming of hydrocarbons, the essential process of obtaining hydrogen or synthesis gas. The formation of the deposit is preceded by a period of the so-called induction time whose length depends on many factors that also influence the catalysts resistance to coking, such as the kind of hydrocarbon, the ratio of reagents, and temperature. After the induction time, coking accelerates and the coking rate reaches a constant value. When steam reforming of saturated hydrocarbons was carried out on nickel catalysts at a high steam—carbon ratio, no carbon deposition was observed after several hours of reaction. It is believed that the initial catalyst coking above 1 wt. percentage of carbon results in a considerable shortening of the induction period and in obtaining a straight-line dependence of the coking rate on time, called a “steady-state rate of deposition.” Earlier studies in nickel catalysts resistance to coking in steam reforming showed that the carbon deposition rate depends not only on such direct factors as nickel dispersion or the support composition, but also on indirect factors, connected with the preparation and pretreatment conditions of the systems; the latter influence the coking rate by causing changes of the direct factors. The chapter studies the effects of the direct factors on the length of the induction period of coking and various ways of catalyst pretreatment on the steady-state rate of coking.