Modelling of non-catalytic reactions in a gas-solid trickle flow reactor: dry, regenerative flue gas desulphurisation using a silica-supported copper oxide sorbent

Abstract

A one-dimensional, two-phase dispersed plug flow model has been developed to describe the steady-state performance of a relatively new type of reactor, the gas-solid trickle flow reactor (GSTFR). In this reactor, an upward-flowing gas phase is contacted with as downward-flowing dilute solids phase over an inert packing. The model is derived from the separate mass heat balances for both the gas and (porous) solids phases for the case of a non-catalytic gas-solid reaction, which is first-order in the gaseous reactant. The reaction rate may also depend on the solid reactant concentration, but this concentration is assumed to be low and uniform throughout the solids volume. From the model, axial profiles can be calculated numerically for the four independent variables, viz. the gas-phase and solids-phase temperatures and the concentrations of the gaseous and solid reactant. Under isothermal conditions, the model equations can be solved analytically; the resulting expressions for the axial profiles of the gaseous and solid reactant are presented. The model is applied to predict the flue gas desulphurisation performance of a full-scale GSTF absorber in a dry, regenative process for the simultaneous removal of SO x and NO x from flue gases. In this process, to be operated at 350–400°C, the sorbent material consists of a porous silica support (spherical particles, 1.5mm diameter) with 7.5 wt% CuO deposited on this support by an ion-exchange technique. The model calculations are based on experimental findings from previous studies regarding reaction kinetics, hydrodynamics of the two-phase flow, gas-solids mass transfer and testing of the integrated process in a bench-scale plant. It appears that SO 2 removal efficiencies over 95% can be achieved in a GSTF absorber with a length of 15 m. Furthermore, the model predicts a large temperature peak for both phases in the absorber if the heat capacity ratio(defined as the ratio of mass flux times specific heat capacity for both phases) is close to one. This large temperature peak is due to the occurrence of the exothermic reaction of SO 2 with CuO in combination with efficient counter-current gas-solids heat exchange. Several parameters influencing the magnitude and axial position of the maximum gas-phase and solids-phase temperatures are discussed.