# Direct Selective Oxidation of Hydrogen Sulfide: Laboratory, Pilot and Industrial Tests
## 3 Claus Process  
The most widely-used procedure for the large-scale reprocessing of highly-concentrated gases is the Claus method [13], which consists of several steps (Figure 2). The feed for the Claus process is acid gases. The Claus process is the dominant technology to produce gas (regenerated) sulfur. It is worth noting that the vast majority (about 94%) of the 8.1 million metric tons of sulfur produced in the United States in 2020 was synthesized using the Claus process [14]. The term \"acid gases\" is used to designate gases obtained after the absorptive treatment of hydrocarbon raw materials. Some typical characteristics of acid gases of various origins are shown in Table 1.  
As a rule, acid gases formed in the process of hydrotreating oil fractions are characterized by rather low flow rates (\(\leq\)1000 nm\({}^{3}\)/hour) and high contents of hydrogen sulfide, the concentration of which, depending on the efficiency of the primary cleaning unit, usually \begin{table}
\begin{tabular}{c c c} \hline
**Process** & **H\({}_{2}\)S Content, Vol.\%** & **Other Gas Components** \\ \hline Purification of gases from oil processing & 90–98 & Carbon dioxide, hydrogen, \\ (MEA treatment) & & methane \\ Purification of natural and oil-associated gas & & Carbon dioxide, \\ (MEA or DEA treatment) & 10–70 & water vapor, hydrocarbons C\({}_{1}\)–C\({}_{6}\) \\ \hline \end{tabular}
\end{table} 
Table 1: Typical characteristics of acid gases of various origins. Figure 2: Schematic of Claus installation (adapted from [13]). exceeds 90%, compared with acid gases produced by a gas processing plant (for example, the power of only one technological line at the Astrakhan GPP is over 15,000 nm\({}^{3}\)/hour). The Thermal Stage of the Claus Process. Process Conditions. Chemical Reactions Proceeding in the System  
The thermal stage of the Claus process largely determines the efficiency of the process as a whole, because at this stage, the main part (up to 70%) of the target product, i.e., elemental sulfur, is produced. Upon mixing the acid gas with air (at the same stoichiometric hydrogen sulfide:oxygen ratio used in Reaction 1), a gas stream containing H\({}_{2}\)S, O\({}_{2}\), N\({}_{2}\), CO\({}_{2}\), H\({}_{2}\)O, sometimes hydrocarbons, and in some cases NH\({}_{3}\), HCN, etc. is formed, which is fed to the Claus furnace. Accordingly, during H\({}_{2}\)S oxidation in the furnace, in addition to the main reactions [15; 16; 17]:  
\[\mathrm{H_{2}S+0.5O_{2}\to 0.5S_{2}+H_{2}O} \tag{1}\]  
\[\mathrm{H_{2}S+O_{2}\to H_{2}+SO_{2}} \tag{2}\]  
\[\mathrm{H_{2}+0.5 SO_{2}\to H_{2}O+0.25S_{2}} \tag{3}\]  
\[\mathrm{H_{2}S\leftrightarrow H_{2}+0.5S_{2}} \tag{4}\]  
\[\mathrm{CH_{4}+2S_{2}\leftrightarrow CS_{2}+2H_{2}S} \tag{5}\]  
\[\mathrm{CO+H_{2}O\leftrightarrow CO_{2}+H_{2}} \tag{6}\]  
\[\mathrm{CO+0.5S_{2}\leftrightarrow COS} \tag{7}\]  
\[\mathrm{CH_{4}+0.5O_{2}\to CO+2H_{2}} \tag{8}\]  
Figure 2:0  
Since the oxidation of hydrogen sulfide is an exothermic reaction, the temperature in the Claus furnace can reach 1200-1500 \({}^{\circ}\)C, while the minimum critical value of the temperature for sustaining a steady flame in the furnace is 1050 \({}^{\circ}\)C. The factor determining the temperature of the flame in the standard implementation of the process is the concentration of hydrogen sulfide in the acid gas. Despite significant progress in developing burner devices for the combustion of hydrogen sulfide-containing mixtures, the optimal conditions for the stable operation of the flame furnace are those with a hydrogen sulfide content in the feed gas of \(\geq\)60 vol.%. Technical approaches for maintaining sustainable operation in Claus furnaces are examined in [18] where, along with the results of calculations carried out using the Gibbs energy minimization method, the experimental data are in good compliance with the results of the calculations.