Modeling and analysis of local hot spot formation in down-flow adiabatic packed-bed reactors

Abstract

Down-flow packed-bed reactors used in hydrodesulphurization and hydrogenations have been known to form localized temperature hot spots during their operation. Local hot spots may form due to non-uniform packing or activity of the catalyst bed. We examine a mechanism for hot spot formation which may occur even in perfectly uniform catalyst beds. In a down-flow reactor, the increase in temperature due to heat released by the reaction causes a decrease in the fluid density and hence creates a buoyancy force (in the upward direction), which can destabilize the uniform flow. Any spatial non-uniformity in the bed packing or activity is amplified by the buoyancy effect. In this work, we consider the case of an adiabatic down-flow catalytic packed-bed reactor of uniform porosity and activity and develop a 3D two-phase model consisting of the continuity and momentum equations, fluid and solid phase species and energy balances, and use the Boussinesq approximation for the variation of density and viscosity of the fluid with temperature. Unlike previously published research on this subject, our model also accounts for the variation of local interphase heat and mass transport coefficients with local fluid velocity. This eliminates the existence of non-physical solutions obtained at small residence times. The steady-state behavior of the 1D (transversely uniform) solutions is analyzed in detail. It is found that up to five steady-states can exist for Le f ⩾ 1 , while for Le f < 1 , there can be as many as eleven 1D uniform flow solutions. The stability of these 1D solutions with respect to two and three-dimensional transverse perturbations is analyzed. We find that the uniform solution can become unstable to small transverse perturbations and lead to transversely non-uniform velocity, concentration and temperature profiles, corresponding to maldistributed flows and localized hot spots. The size of the parameter region in which transverse non-uniformities exist increases monotonically with increasing interphase transport resistances. Spectral methods are used to follow numerically the bifurcating non-uniform solutions and the corresponding velocity, concentration, and temperature fields. The bifurcation from uniform to non-uniform solutions changes from supercritical to subcritical as the interphase transport resistance increases. Finally, the results are summarized in terms of some guidelines for minimizing the occurrence of maldistributed flows and hot spots in down-flow adiabatic packed-bed reactors.