# Direct Selective Oxidation of Hydrogen Sulfide: Laboratory, Pilot and Industrial Tests
## 19 Description of Modern Industrial Methods Based on the Process of Direct H\({}_{2}\)S Oxidation  
The Catasulf(r) process of the German company BASF (Ludwigshafen, Germany) [127] is based on the reaction of the oxidation of the acid gas containing 5-15% H\({}_{2}\)S (I) in the tubular reactor 1 (Figure 8). The tube space of reactor 1 is filled with a special highly selective catalyst, which is a mixture of aluminum, nickel, and vanadium oxides, and the inter-tube space is cooled with a high-boiling liquid silicon coolant (II), which, circulating, transfers the removed heat to the refrigerator 2. Gases emerging from reactor 1, (III) are cooled in the sulfur condenser 3 and fed into the adiabatic reactor 4, where there is a further interaction of hydrogen sulfide with sulfur dioxide. The resulting sulfur is separated in the second consecutive condenser 5. Removing the sulfur in the first stage is 94%, after the adiabatic reactor up to 97.5%. It is supposed that by increasing the number of stages, it is possible to attain the degree of sulfur extraction of 99.99%. At the oil refining plant, Ludwigshafen (Germany), the only Catasulf(r) industrial installation is operated. The Sulfatereat(r) DO process of the company M-I Swaco [128; 129] is a technology for the purification of associated oil gases from hydrogen sulfide by direct oxidation. The process is carried out at a temperature of 175 \({}^{\circ}\)C, a pressure of up to 65 bar and allows the treatment of gases containing up to 3 vol.% hydrogen sulfide. The catalyst is a mixture of metal oxides of transitional valence, promoted with alkali metals oxides. The results of tests of the experimental installation showed that the H\({}_{2}\)S concentration in the purified gas does not exceed 950 ppmv. The degree of sulfur removal, in this case, was more than 88%, with only a slight conversion of the hydrocarbon part. Other industrial processes based on this reaction are designed exclusively for cleaning tail gases of existing sulfur-producing installations. One of the most common methods is BSR/Selectox(r) of Unocal and Ralph M. Parsons companies. In this process, the exhaust gases of the Claus installation are reduced in a catalytic reactor by synthesis-gas formed in a special generator by a steam reforming of natural gas. Then the resulting gas is cooled, mixed with air, and hydrogen sulfide is subjected to selective oxidation to sulfur at a temperature of 200-230 \({}^{\circ}\)C. The use of a special vanadium oxide catalyst Selectox-67 allows attaining the selectivity of oxidation to sulfur of ca. 100%. The Beavon-Selectox process ensures the degree of sulfur removal of 98.5-99.5% at a relatively low installation cost (about 50-60% of the cost of a Claus installation [130]. The first such installation was launched in 1978 in Germany. Somewhat later, other technologies similar to the Beavon-Selectox process were developed. Among them are the most famous MODOP(r) of Mobil Oil Corp (Dallas, TX, USA) [131]. and SUPERCLAUS(r) of Comprimo BV Company (Dallas, TX, USA) [42]. They differ from the Beavon-Selectox process by using other catalysts (CRS-31 for MODOP and a special highly selective iron oxide catalyst for the SUPERCLAUS(r) process), and by the fact that the oxidation of hydrogen sulfide is carried out in two stages. In addition, in the SUPERCLAUS(r) process, it is possible to use the H\({}_{2}\)S direct oxidation stage without the hydrogenation stage due since in the Claus installation, the process is carried out with an excess of hydrogen sulfide (this allows, among other things, protect the catalyst in the Claus reactors from sulfation). These processes provide the total degree of sulfur removal of 99.3-99.5%. The first two MODOP(r) (Dallas, TX, USA) installations were put into operation  
Figure 8: Schematic of Catasulf® process Catasulf®, perhaps, the only “active” large-capacity technology to obtain sulfur based on the reaction of the direct oxidation of hydrogen sulfide (adapted from [127]).