Sulphuric acid decomposition using Cr–Fe2O3 catalyst in a tubular Packed Bed Reactor (PBR): Modeling and experimental studies

Abstract

Catalytic decomposition of sulphur trioxide (SO3), is the rate-controlling and high temperature-endothermic reaction step in three-step decomposition of sulphuric acid. In this work, formulation of one dimensional (1-D) model, detailed transport modeling and experimental studies have been carried out for a non-isothermal, non-adiabatic, tubular-Packed Bed Reactor (PBR). This is to study the effect of heat and mass transfer resistances, in the catalyst bed and also within the catalyst particle, for the decomposition of SO3. 1-D model consists of set of coupled Differential Algebraic Equations (DAEs), which are numerically solved for axial profiles (Temperature, Flow, Pressure) with plug flow reactor assumption. Further, a double-porosity modeling of the reactor has been carried out for the detailed transport study using COMSOL MULTIPHYSICS® simulation software. Here transport equations with the rate law are solved for two different porosities (domains); micro pores inside the catalyst particle (ε) and macro pores (void fraction-φ) in the packing. Models are validated with experimental studies and the molar conversion of SO3 obtained using the models are compared with conversion of an isothermal PBR. Chromium doped iron oxide (Cr–Fe2O3) catalyst; a ‘pellet’ type catalyst is used in this experimental study. Experiments are carried out at different flow (liquid) rates (160–315 ml h−1), wall temperature (973–1243 K) and with two different catalyst bed lengths (catalyst loading of 555 g and 288 g). Double-porosity (COMSOL) model predictions are better close to the experimental data. A multi-scale analysis of the system has been carried out using the double-porosity model, in order to get an insight of concentration drops in the particle (pore diffusion resistance) and fluid film (film resistance) at various locations inside the reactor. Film resistance is found to be negligible when compared with the pore diffusion resistance, in the studied experimental conditions. Modeling of transport (heat and mass transfer) resistances together with kinetics of SO3-decomposition, in two different porous-domains in a PBR, is the novelty of present work. The developed models are useful for maximizing the conversion of SO3 (by tuning the model parameters) in a PBR by minimizing the heat and mass transfer resistances.