Experimental and theoretical studies in the field of fuel gas desulfurization at high temperatures are presented. The performance of different oxides as sorbents for high temperature H2S removal is evaluated. A fixed-bed microreactor was used for this purpose. Basically, streams containing different H2S concentrations were passed through the reactor and the outlet H2S concentration was measured as a function of time. Comparisons between observed and theoretical maximum conversion values are used as a measure of sorbent H2S removal efficiencies.
Most of the work was devoted to study zinc oxide and vanadium oxide, but other sorbents were also investigated. ZnO was studied because of its high equilibrium constant for sulfidation. High surface alumina-supported ZnO and high surface bulk ZnO were tested. Alumina-supported ZnO proved to have a smaller desulfurization capacity than bulk ZnO. In the case of V2O5, chemisorption of H2S on reduced vanadium oxide, rather than bulk sulfide formation, was found to be the mechanism of sulfur removal. Adsorption-desorption measurements were carried out in this case to investigate the behavior of the sorbent.
The regeneration step, which restores the oxide by means of oxidation of the spent sulfide, was also studied in detail. The rate of ZnS oxidation was measured using a thermogravimetric analyzer. For the range of temperatures 650-700°C, the rate of reaction was consistent with Langmuir kinetics. The activation energy and the free energy of oxygen chemisorption were determined. An undesirable side reaction during oxidative regeneration is sulfate formation. The kinetics of this reaction was also investigated by thermogravimetric measurements.
Theoretical studies dealing with reaction and diffusion processes in pellets and fixed-bed reactors were conducted, to a large extent motivated by the need to describe desulfurization processes. One chapter is devoted to a survey of exact and approximate solutions for an isothermal fixed-bed gas-solid reactor. Approximate analytical solutions to a class of models of reaction and diffusion inside sorbent pellets are presented next. The last chapter is devoted to a traveling wave solution describing reaction fronts inside non-isothermal fixed-bed reactors. This provides a good analytical approach for the prediction of conditions for the formation of zinc sulfate during the regeneration of ZnS.
