# Direct Selective Oxidation of Hydrogen Sulfide: Laboratory, Pilot and Industrial Tests
## 9 Modifications of the Claus Process
### Chemism of the Process of Direct Catalytic Oxidation of Hydrogen Sulfide  
In the process of direct H\({}_{2}\)S oxidation, the following reactions can proceed [50]:  
\[\mathrm{H_{2}S+1/2O_{2}\to H_{2}O+1/2S_{2}} \tag{15}\] \[\mathrm{H_{2}S+1/2O_{2}\to H_{2}O+1/6S_{6}}\] (16) \[\mathrm{H_{2}S+1/2O_{2}\to H_{2}O+1/8S_{8}}\] (17) \[\mathrm{H_{2}S+3/2O_{2}\to H_{2}O+SO_{2}}\] (18) \[\mathrm{H_{2}S+1/2 SO_{2}\to H_{2}O+3/4S_{2}}\] (19) \[\mathrm{H_{2}S+1/2 SO_{2}\to H_{2}O+1/4S_{6}}\] (20) \[\mathrm{H_{2}S+1/2 SO_{2}\to H_{2}O+3/16S_{8}} \tag{21}\]  
The allotropic form of sulfur S\({}_{2}\) is stable in the temperature range of 100-900 \({}^{\circ}\)C. The characteristic temperature range for the reaction of direct oxidation of hydrogen sulfide is usually 100-300 \({}^{\circ}\)C; in this range, sulfur is present as S\({}_{6}\) and S\({}_{8}\). The so-called reverse Claus reaction accompanies the oxidation of hydrogen sulfide:  
\[3S+2H_{2}O\to 2H_{2}S+SO_{2} \tag{22}\]  
At reduced temperatures which are typical for direct oxidation, sulfur chains predominantly consisting of six or eight atoms are formed. With an increase in temperature to 800 \({}^{\circ}\)C, hydrogen sulfide oxidation proceeds mainly with the formation of sulfur in the form of S\({}_{2}\). In the temperature range of 25-727 \({}^{\circ}\)C, the equilibrium constant of the hydrogen sulfide oxidation reaction with oxygen to elemental sulfur is, on average, 10 orders of magnitude higher than that of the reaction of oxidation with sulfur dioxide. Consequently, the probability of the formation of elemental sulfur in Equations (15)-(18) is higher than by Equations (19)-(21) [50]. The thermodynamic features of the reaction of the direct oxidation of hydrogen sulfide are presented in the form of temperature dependence (Figure 5) [51]. The reaction of direct H\({}_{2}\)S oxidation can proceed with selectivity achieving 100% target product at low temperatures; with an increase in temperature above 200 \({}^{\circ}\)C, the selectivity significantly decreases. The chemical equilibrium is determined by the Claus reaction, i.e., the only reversible reaction of the system. If a catalyst with high activity for the reaction (15) is selected that is practically unaffecting the Claus reaction rate (21), then a super-equilibrium sulfur yield (100%) can be attained [52]. Therefore, the use of TiO\({}_{2}\)- and Al\({}_{2}\)O\({}_{3}\)-based catalysts in this process is ineffective. An increase in pressure in the system favorably affects the yield of elemental sulfur and increases the selectivity, even at elevated temperatures. **12. Main Types of Catalysts Used in the Process of Direct Heterogeneous Oxidation of Hydrogen Sulfide. Industrial Processes. Brief Description of the Most Common Catalysts for the Hydrogen Sulfide Oxidation Reaction with Oxygen to Elementary Sulfur**  
Specific requirements associated with the particular features of the reaction are imposed on catalysts for the process of the direct oxidation of hydrogen sulfide into elemental sulfur. In terms of selecting a catalyst, the thermodynamics of the process should be taken into account, as well as the possibility of the homogeneous evolution of the process through the radical-chain mechanism at elevated temperatures and the condensation of sulfur in catalyst pores at low temperatures. Catalysts for the direct oxidation of hydrogen sulfide to sulfur are used in the temperature range of 200-350 \({}^{\circ}\)C. The sulfur dew point determines the lower limit of the temperatures. The upper limit is due to the possibility of the reactions of sulfur and hydrogen sulfide oxidation to sulfur dioxide, which leads to a significant drop in the reaction selectivity. Despite considerable efforts devoted to the direct catalytic oxidation of hydrogen sulfide described in the literature, the scope of catalytic systems for this process is somewhat limited. These, first of all, are activated carbons and artificial zeolites [53; 54], as well as natural bauvites, traditionally used as catalysts for this process [55].