Effect of surface nonuniformity on the kinetics of simultaneous adsorption of SO2-NOx over Na-gamma-Al2O3 sorbent: a coverage-dependent stoichiometry

摘要

The effect of nonunifonnity of the surface of Na-gamma-Al2O3 on the kinetics of surface reactions is investigated for the simultaneous adsorption of NO, O-2 and SO2. 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 SO2* and accounting for the nonuniformity in the adsorption and surface reactions. The SO2* species is produced from the primary adsorption of gas phase SO2 on a free site. The adsorption of NO occurs first on a sulfate species SO2**, resulting from the interaction of SO2* with a free site. Subsequently, a complex formation occurs in multiple series-parallel steps involving many SO2* and 02 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 SO2* coverage. The calculated average number of SO2* 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 SO2* in the complex allows a higher rate of formation of SO2**, leading to a higher NO removal as observed in the riser. There is an optimum inlet SO2/NO ratio of 2.5 at which maximum NO removal in the riser is achieved. (C) 2002 Elsevier Science Ltd. All rights reserved. [References: 21]