Reactor Input Manipulation for Developing Models for Catalytic Reactions

Citation

Shanks, Brent Howard (1988) Reactor Input Manipulation for Developing Models for Catalytic Reactions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/pf3v-jt53. https://resolver.caltech.edu/CaltechETD:etd-11092007-141214

Abstract

A variety of phenomenological experimental methods are employed to construct and validate kinetic models for heterogeneous catalytic reaction systems. Both static input and stimulus-response experiments are performed with particular emphasis placed on the novel feedback-induced bifurcation method. These techniques are applied to CO oxidation on Rh/Al₂O₃ and Ag/Al₂O₃ to identify kinetic models that are able to reproduce transient as well as steady-state experimental data.

Static input experiments for CO oxidation on supported Rh, reduced at 170°C, reveal oscillatory responses for some inlet conditions. Two adsorbed CO species, linear and dicarbonyl forms, are observed with the linearly adsorbed CO shown from step-response experiments to be more reactive than CO adsorbed in the dicarbonyl form. The oscillations seem to be driven by surface temperature fluctuations rather than by interconversion between the adsorbed CO species.

Steady-state and step-response experiments at 147°C are used to estimate and to bound parameters in two reaction models that were previously postulated for CO oxidation on supported Ag. Data from feedback-induced steady-state bifurcation experiments are reproduced well by both models but neither reaction model is able to reproduce closed-loop Hopf bifurcation data. Also, long-time-scale transients observed in some step-response experiments are not accurately simulated by either model. A new model is proposed for CO oxidation on Ag/Al₂O₃, which incorporates a slow reversible step that forms an adsorbed oxygen species that blocks reaction sites. This model is able to reproduce the long-time-scale dynamics and a variety of cycled feedstream experiments.

The feedback-induced bifurcation method is also applied to the CO oxidation reaction on supported Rh. After reducing the Rh/Al₂O₃ catalyst at 500°C, only linearly adsorbed CO is observed. Measurements of the fractional coverage of CO and the gas-phase CO are used for feedback control. The closed-loop bifurcation data are used to discriminate between two reaction models for CO oxidation on supported Rh, which are determined from steady-state experiments. Higher order dynamics resulting from closed-loop operation are also observed. The usefulness of various types of experimental feedback-induced bifurcations, as applied to model development, is discussed.

Finally, the feedback-induced bifurcation method is used to generate basis oscillations that are subjected to forced periodic operation. This provides a process in which entrainment behavior can be systematically examined. Experimental and simulated entrainment responses are compared and are found to be in close agreement.

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