Kinetics Study of Reduction / Sulfidation of ZnO and Zinc Titanate Powders in a Drop-Tube Furnace
The reaction kinetics of fine particles of zinc oxide and zinc titanate with H2S were studied using a drop-tube furnace in order to test the potential of the sorbent injection hot-gas desulfurization process. Fine ZnO sorbent particles with diameter smaller than 50 tm were sulfided with H2S and/or reduced with H2 in a laminar flow reactor over the temperature range of 620-910 C. The reacted sorbent particles were separated from gas and size-classified using a cascade impactor and an absolute filter. Sulfidation conversion was measured by thermo-gravimetric analyses and compared for different particle sizes and reaction conditions. Other sorbents such as zinc titanate, metallic zinc, and ZnO with different porosity were also tested and compared. In the experiments of the reduction of ZnO without H2S, it was deduced from the change of size distribution of particles that a significant amount of Zn was formed and vaporized. On the other hand, the presence of H2S suppressed the change of size distribution of particles. This suggests that H2S suppresses the reduction of ZnO by rapid surface sulfidation and/or that gaseous Zn reacts with H2S immediately and forms fine particles of ZnS, most of which deposit onto the original particle surface. Formation and vaporization of elemental Zn from zinc titanate sorbents was slower than from zinc oxide with and without H2S. Surface reaction rate of ZnO and zinc titanate sulfidation was obtained and a reaction model for a single particle was formulated and compared with experimental results in the drop-tube furnace. Thesis Supervisors: Professor Maria Flytzani-Stephanopoulos Professor Adel F. Sarofim
THE REDUCTION OF ZINC TITANATE AND ZINC OXIDE SOLIDS
Abstract The reduction of bulk mixed oxides of zinc and titanium of various compositions and ZnTiO crystalline phases was studied in a thermogravimetric apparatus in H 2 H 2 ON 2 gas mixtures at 550–1050°C. Comparative reduction experiments with ZnO were also performed. In the absence of water vapor, activation energies of 24 and 37 kcal mol −1 were obtained for ZnO and ZnTiO, respectively. The addition of water vapor inhibited reduction and resulted in a change in activation energy to 44 kcal mol −1 for both ZnO and ZnTiO solids. Similar to H 2 O, H 2 S inhibited the initial reduction rate of ZnO and ZnTiO materials. Based on kinetic experiments, a two-site model is proposed for ZnO reduction. One type of sites is characterized by a rapid reduction rate but is poisoned by water vapor as well as by the presence of titanium atoms in the solid. The other type of sites has a lower reduction rate, is not poisoned by H 2 O, and is slowly eliminated by the presence of titanium.
The variable property grain model applied to the zinc oxide-hydrogen sulfide reaction
Abstract The variable property grain model, including the effect of grain diffusion resistance, has been applied to experimental results from the reaction of hydrogen sulfide with zinc oxide. Grain radius is assumed to vary under the combined effects of sintering and extent of reaction. Property variations are correlated by the specific surface area, an easily measured quantity. All model parameters with the exception of the grain diffusion coefficient have been evaluated by literature correlations or independent experimental measurements. Significant improvement in the match with experimental data, as compared to the constant property grain model, has been achieved.