Effect of surface nonuniformity on the kinetics of simultaneous adsorption of SO 2–NO x over Na– γ-Al 2O 3 sorbent: a coverage- dependent stoichiometry

Abstract

The effect of nonuniformity of the surface of Na– γ-Al 2O 3 on the kinetics of surface reactions is investigated for the simultaneous adsorption of NO, O 2 and SO 2. A nonuniform kinetic model is developed by considering the variations in the rate coefficients of important surface reactions with species coverages. Experimental data from a transient fixed bed microreactor and a steady-state riser reactor are used for the estimation of model parameters. The model explains the large variation in the NO removal in the above two reactors by the difference in the coverage of the sulfite species SO 2 ∗ and accounting for the nonuniformity in the adsorption and surface reactions. The SO 2 ∗ species is produced from the primary adsorption of gas phase SO 2 on a free site. The adsorption of NO occurs first on a sulfate species SO 2 ∗∗ , resulting from the interaction of SO 2 ∗ with a free site. Subsequently, a complex formation occurs in multiple series-parallel steps involving many SO 2 ∗ and O 2 per mole of NO. In the nonuniform model, the rate of the series-parallel steps and hence the average stoichiometry of the complexes depend strongly on the degree of SO 2 ∗ coverage. The calculated average number of SO 2 ∗ entities in the complex is much smaller at the riser (2–3 moles/mole of NO) than at the fixed bed conditions (9 moles/mole of NO). A lower consumption of SO 2 ∗ in the complex allows a higher rate of formation of SO 2 ∗∗ , leading to a higher NO removal as observed in the riser. There is an optimum inlet SO 2/NO ratio of 2.5 at which maximum NO removal in the riser is achieved.