Modelling of Complex Dynamics in Fixed Bed Reactors

Sammanfattning: Chemical reactors are a part of modern industry and the catalytic tubular fixed bed reactor examined in this work is an important reactor for chemicals production. In this work two different types of models for the reactor are studied; a pseudohomogeneous model and a heterogeneous model. The goal is to find differences in behaviour between these two types of reactor models and explain these. In a real reactor there exists two phases, a solid catalyst and a fluid reactant. Both these phases are in the pseudohomogeneous model treated as a single phase, a pseudofluid. In the heterogeneous model the two phases are treated separately. When comparing these types of models a few structural differences exist, void fraction, heat exchange between two phases, and heat dispersion in the phases, and all of these will affect the behaviour of the models differently. The models are studied using bifurcation analysis and linear analysis. Bifurcation theory is used to find and track different solutions depending on a certain parameter and to get a good overall picture of a system’s solutions and their type, steady state or sustained oscillation. Linear analysis is used to study linearization around a specific solution and to determine stability and frequency dependency. It is found that the concept of void fraction in the reactor model affects the behaviour only as a time scaling, while the concept of interfacial heat exchange affects the stability. The distribution of heat dispersion between phases has a significant impact on the reaction behaviour. Feedback is determined as the main cause for instabilities and oscillative solutions.