Mathematical modelling of fixed bed reactors

Abstract

Consideration is given to the solution of the highly exothermic fixed bed catalytic reactor problem taking into account heat and mass transfer resistances inside the catalyst pellets and across the external fluid film as well as radial temperature and oonoentration gradients in the fluid phase. Comparison of the model with the simpler quasi homogeneous repreaenation is made. In the region where the quasi homogeneous case predicts temperature "run-away", the added refinements assume some importance. Very significant; differences in behaviour are predicted. Indeed no temperature "run-away" is apparent. Inolucling simply a film mass and heat transfer resistance is no guarantee that temperature "run-away" will not be predicted. In fact, it is the particle diffusive resistance whioh is the main factor limiting the temperature effects. Since the region of temperature "run-away" is often in the practical range it is essential to use a more detailed model for design such as the one described here, especially if optimal operating conditions are being sought. Even on large digital computers, the computation time is excessively long if the sets of differential equations are solved simultaneously. By examining the intrapartiole equations in detail for a practical range of physical properties and operating conditions, it is shown that they may be reduced, to a lumped parameter form. While still retaining the characteristics of the general problem, the lumped parameter approximation can be solved in a substantially shorter time, thus taking its use in optimization and control studies feasible.