Activation of catalysts in commercial scale fixed-bed reactors: Dynamic modelling and guidelines for avoiding undesired temperature excursions

Abstract

During the exothermic gas phase activation (e.g. reduction) or passivation (oxidation) of a catalyst in a commercial scale fixed bed, the interaction of the heat wave propagation phenomenon and the (desired) exothermic gas-solid reaction(s) may give rise to local temperature excursions way beyond the adiabatic temperature rise of the reactions. Inspired by a decoking study by Westerterp et al. (1988), a full dynamic model for generic gas-phase catalyst activation and deactivation (reduction/oxidation) in an adiabatic fixed-bed reactor was developed. Counter intuitive effects were elucidated, e.g. for cases where a lowering of the reactant concentration or the presence of a significant reactor wall heat capacity can lead to a further increase of the maximum catalyst temperature. An easy-to-use expression was derived, initially using simplifying assumptions of full rate control by external mass transfer and neglecting heat losses and reactor wall effects. This was subsequently tested on its adequacy for more general cases accounting for (1) slower reaction kinetics, (2) significant heat losses and (3) the reactor wall heat capacity. Only for the latter effect the original expression had to be adjusted. The modified expression proved to be remarkably robust and remained relevant as a simple tool to prevent local temperature excursions also for slower reaction kinetics and significant heat losses.