Catalyst deactivation kinetics from variable space-velocity experiments

Abstract

Reliable estimates of catalyst deactivation parameters are difficult to get from conventional experiments at constant space velocity where conversion declines with time on stream, or when temperature is adjusted to compensate for the loss of catalyst activity. An alternate technique, where space velocity is adjusted to maintain constant conversion, is shown here to give reliable rate information for both the catalyst deactivation and the primary catalytic reactions. This technique, at constant temperature, provides kinetics data on a deactivating catalyst in which the catalyst is exposed to reactant and product concentrations that are relatively constant. In principle, the kinetics of deactivation are obtained directly as the kinetic regime remains unchanged. This technique is used quite successfully for many catalytic processes of industrial significance. In this paper, the variable-space-velocity experimental technique is demonstrated for the methanol to hydrocarbons process on a ZSM-5 catalyst. The deactivation kinetic parameters were determined at several temperatures in a fixed-bed reactor. This rate information was then used to determine the temperature-time policy for maintaining constant reactor outlet conversion at a fixed space-velocity. The predicted temperature-time policies are in good agreement with experimental data on fixed-bed and fluid-bed reactors. Mathematical analysis of interpreting experimental data, as well as predicted temperature-time policies for practical applications, is illustrated.