Experimental and modelling investigations of steady-state and dynamic characteristics of ethylene hydrogenation on Pt/Al 2O 3

Abstract

A robust strategy based on fluid-phase measurements is described for the testing and development of kinetic models for heterogeneous catalytic reactions. Steady-state, step-response, feedback-induced Hopf bifurcation and forced concentration cycling experiments were applied to ethylene hydrogenation over 0.05% Pt/Al 2O 3 in a CSTR at 80°C. Two versions of a reaction mechanism that differ in the order for hydrogen adsorption describe the experimental steady-state data. However, only one of these models adequately describes the system responses to step changes in the feed composition. Step-response experiments were used to identify a time scale of 5000 s which is associated with chemisorbed hydrogen. Conversely, feedback-induced Hopf bifurcation data indicate this time scale to be of the order of 1 s in magnitude. In the overall strategy of dynamic modelling, the two techniques are complementary since each inherently focuses on an opposite region in the spectrum of time scales for the reactor system. A detailed method for analysing feedback-induced Hopf bifurcation with a time delay is presented. The dynamic model based upon steady-state, step-response and bifurcation data was found to be inadequate for describing the results from cycled-feedstream experiments. Cycling the feedstream composition resulted in an improvement of the time-average reaction rate for the ethylene hydrogenation reaction compared to steady-state reactor operation. Rate improvement was observed for the entire range of ethylene feed concentrations that were tested, including conditions which give rise to a maximum in the steady-state reaction rate surface.