Analysis of thermoflow instabilities in adiabatic packed-bed catalytic reactors

Abstract

Packed-bed catalytic reactors are widely used in industry to carry out many exothermic reactions. Reaction induced flow maldistribution can lead to hot-spot formation through multiple mechanisms in these reactors. This work examines a mechanism due to fluid physical property (density and viscosity) variation and the coupling between momentum, species and energy balances, which is termed thermoflow instability. We show that even in the one dimensional uniform flow case, density and viscosity variations can lead to multiple flow rates with the same pressure drop. We analyze and determine the stability boundary of the uniform one dimensional flow under different operating conditions. We find that uniform velocity distribution at reactor inlet can improve reactor stability compared to fixing only pressures at both inlet and outlet. We examine the impact of catalyst particle size and reactor aspect ratio on the stability boundary of the uniform flow. We also show that thermoflow multiplicity exists in shallow bed reactors whenever thermokinetic multiplicity exists but the converse is not true. Finally, we present velocity, temperature and concentration fields for some typical maldistributed flows.