A kinetic study of the sulfidation-regeneration of zinc oxide sorbent

Abstract

Experimental studies on the sulfidation/regeneration of zinc oxide have been conducted to obtain precise data on the kinetics and structural changes involved, and to formulate appropriate rate expressions. The regeneration of zinc oxide (oxidation of zinc sulfide) particles conducted in a thermogravimetric analyzer (TGA) was found to be fast and complete between 548 and 630°C, with zinc oxide being the only product formed. However, under the influence of external mass transfer, an oxysulfate intermediate which decomposes upon completion of the reaction was observed for the first time (by XRD). The intrinsic kinetic parameters for the oxidation reaction were determined and the surface reaction model (-r = k [O2] /1 + [O2]) with no dissociation of oxygen was found to depict the initial rate data very well up to 609°C. Simple power law models were found to be inadequate. Variations in surface area, pore volume, and pore size distributions caused by sintering with and without reaction were determined for the regeneration of zinc oxide between 575 and 640°C. In the absence of reaction, sintering resulted in a reduction of the sorbent surface area and an increase in average pore size. The reaction of zinc sulfide to zinc oxide in the absence of sintering resulted in an increase in surface area, while under reaction/sintering conditions the surface area increased up to a conversion of 0.75 and then decreased sharply. However, the surface area of the product was still greater than that of the reactant at all temperatures investigated.