Physical Implications of Periodic Solutions in a Fixed Bed Reactor with Recycle

Abstract

Recycling is often used in industrial processes to reduce cost of raw material and energy supply. The nonlinear effects of introducing recycling on a fixed bed reactor are investigated in the present work. A first order exothermic reaction is chosen as a model reaction. The three cases of recycling investigated are recycling of unconverted reactant, external heat exchange and combined mass and energy. The findings are that bifurcations occur as the operational variables are changed. Investigating the periodic solutions show, that the limit cycle periods in all the investigated cases of recycling, are simple fractions(1, ½, ⅓) of the thermal residence time. Consequently three periodic solutions appear in various sections of the operational space. A physical interpretation of the periodic solutions is that they correspond to one, two and three ignition points respectively in the reactor. These periodic solutions are further investigated and compared to see if it is the same dynamic effect that causes them to appear in the three recycle cases. The implications of the periodic solutions upon the control structure selection for mass recycle will also be considered.