| Hydrogen sulfide, as a by-product of petroleum refining, coal chemical industry and natural gas upgrading, is abundant and valuable resource. For years, the Claus process has helped recover salable elemental sulfur from gaseous H2S, of which the H element is wasted. The endeavor of sulfur-iodine(S-I) thermochemical cycle of H2S splitting for the production of hydrogen and sulfur is more energy-efficient and always economically and environmentally beneficial. Compared to other H2S decomposition process for hydrogen production, the new cycle is easier to achieve industrialization with mild reaction conditions and much higher H2S decomposition ratio.Reactions between hydrogen sulfide and sulfuric acid are key and initial step of the system. In this paper, Fastsage was used to investigate the thermodynamic properties of H2S oxidation reaction (H2S+H2SO4â†'S+SO2+2H2O) and so-called Wackenroder reaction (side reaction,2H2S+SO2â†'3S+2H2O) under different temperature, pressure, reactant ratio. Results show that the critical temperature under which reaction progressed completely producing S and SO2as the stoichiometric number (1:1) was136℃. The critical temperature was found to decrease with the pressure drop. Essentially, the reaction occurred between H2S and molecular H2SO4in sulfuric acid. All sulfur would be oxidized to SO2when the sulfuric acid concentration exceeded94.2wt%(136℃). Pressure had no effect on the side-reaction, and temperature played a negligible role in controlling the side-reaction. Detailed experimental investigations have been carried out with the continuous tower like bubbling reactor system. Several cognitions were obtained by the mechanism and characteristic study taken under different reaction conditions, such as H2S flow rate, temperature and reactant concentration. Gas-liquid contact area and the amount of resulting SO2increased with the increase of H2S flow rate, which suggested the regime of mass-transfer rate control. The effect of increasing reaction temperature is positive to yielding higher H2S conversion and SO2production. When the temperature is above70℃, concentrated sulfuric acid solution (CH2SO4>88.75wt%) is easier to avoid the occurrence of side effects. The reaction between H2S and sulfuric acid (concentration>90wt%) produced SO2at constant growth, and it is likely to obtain a conversion ratio as high as40%. Reaction between H2S and dilute sulfuric acid (57wt%) is feasible, however, the H2S conversion ratio is low and SO2gas production is unstable. Gases with higher H2S content within20%manifested the higher H2S conversion and SO2production, and the influence is particularly evident early in the reaction. The solid product was proved to be orthogonal sulfur crystal (S8) by XRD analysis results. The SEM techniques were used to analyze the surface morphology of the producing powder and lumpy masses. The former has good sphericity and loose porous surface, while the latter show regional features (denoted as smooth area, stalactite like area, common area) that can be explained by the effects of crystal nucleation and growth.At last, the flowsheet of sulfur-iodine thermochemical cycle of H2S splitting for the production of hydrogen and sulfur was designed and simulated by Aspen Plus. The heat and mass balance were calculated at fixed H2production rate of lmol/s. Thermal efficiency for hydrogen production is40.865%with ideal operating conditions and waste heat recovery. The HI conversion ratio, the molar flow rate of HIX phase and the reflux ratio at HI distillation column are the main factors which influence the thermal efficiency. H2SO4concentration system should be reasonably designed to reduce the complexity of process and equipment settings, as well as an important means to improve the thermal efficiency. |
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