Simultaneous production of H2 and sulphur by catalytic oxidative decomposition of H2S

Abstract

The recovery of hydrogen from H2S has recently attracted attention because of the economic and environmental advantages: hydrogen is an energy source, a raw material for chemical industry and in the near future it could become a significant fuel. A great amount of H2S is obtained as a by-product during the H2 reduction processes and the hydrodesulphurization of petroleum and, due to the stringent environmental regulations, H2S has to been removed before its release in the environment. Hydrogen Sulphide is usually removed by the well-known Claus process, where H2S is oxidized to water and elemental Sulphur by two-step reactions. This process is not profitable from an economic point of view because the price of the Sulphur (the primary product) is depressed and the hydrogen is lost as water. Furthermore, due to the thermodynamic limitations, H2S conversion in the catalytic stage is limited to ~95-97%, as a consequence further treatments of the residual tail gas are necessary. So, an attractive alternative could be to produce simultaneously Sulphur and hydrogen by thermal decomposition of H2S (H2S = H2 +1/2 S2). Unfortunately, this reaction is very endothermic and it is thermodynamically favored only for extremely high temperatures, thus requiring large amounts of energy and subsequent separation stages with high fixed and operating costs. A possible solution could be to couple the decomposition reaction with an exothermic reaction, making the system auto-thermal. For this purpose, the reaction of H2S oxidative decomposition at high temperature was studied; the main objective of this work was to find a catalyst active and selective to Sulphur and H2, depressing the possible SO2 formation related to the presence of the oxygen in the reaction system. First of all, this reaction was studied in homogeneous phase, by varying the main operating conditions such as temperature, O2/H2S, residence time. The results have shown an approach of H2S conversion and H2 yield to equilibrium values only at high temperature (1273−1373 K) but with a SO2 selectivity higher than one expected from equilibrium calculations. Different sulphide-based catalysts supported on Al2O3, both monometallic and bimetallic, were prepared, characterized and tested under different operating conditions. MoS2-based catalysts have shown good results at 1273 K, even at very low contact times (20-40 ms) and at high H2S inlet concentration (40 vol%) in terms of H2S conversion, H2 yield and SO2 selectivity, exhibiting values very close to those expected from the thermodynamic equilibrium. Based on the experimental collected data, a kinetic macroscopic model able to describe the main reactions was also developed and compared with the homogeneous phase reaction system. The results have evidenced that MoS2/Al2O3 catalyst (at 10 wt % MoS2), was able to promote the Claus reaction, besides the H2S decomposition reaction and the partial oxidation to Sulphur, determining a decrease of the SO2 formation. The system was also studied in the presence of CH4 in the feed stream. The results showed that the MoS2-based catalysts has favored the formation of undesired by-products as COS and CS2. Different mono and bimetallic catalysts were also tested and compared. The best results in terms of H2S conversion, H2 yield, and SO2, COS and CS2 selectivity were obtained in the case of the Mo-Fe bimetallic samples. As a consequence, the bimetallic 10 wt% MoS2 - 5 wt% FeS2/Al2O3 sample was tested also in presence of ammonia, ensuring a complete NH3 conversion at 1333 K with a contact time of 1 s. Finally, the active phases were supported on cordierite honeycombs monolith, which were tested for 25 hours at T= 1333 K in presence of a feed stream with a defined composition, representative of a typical refinery stream. The catalytic activity of the structured catalyst was also tested for 30 h of TOS at very high temperatures and very stressing conditions in terms of H2S, NH3, CH4 and Oxygen concentrations. The results showed that also in these conditions, an almost total NH3 conversion is possible without any significative apparent deactivation phenomena, making this catalyst a suitable system for H2S oxidative decomposition in presence of other compounds, such as methane and ammonia. [editedby author]