# Direct Selective Oxidation of Hydrogen Sulfide: Laboratory, Pilot and Industrial Tests
## 2 Direct Selective Oxidation in the Liquid Phase. RedOx Processes  
One means by which to purify gases from hydrogen sulfide is oxidation to elemental sulfur using oxygen in solutions of complex compounds of metals with wide variation of the pH of the medium. The process proceeds at a rapid rate in a wide range of temperatures at pressures of 5-50 atm and provides a high degree of gas purification from hydrogen sulfide. Especially noteworthy are the processes developed by Wheelabrator Clean Air Systems, Inc (Pittsburgh, PA, USA). (ARI-Lo-Cat I\({}^{\text{\textregistered}}\), ARI-Lo-Cat-II\({}^{\text{\textregistered}}\)), Shell Oil Company (Houston, TX, USA), and Dow Chemical (SulFerox\({}^{\text{\textregistered}}\)), as well as those based on the process of the direct oxidation of hydrogen sulfide in a solution of iron (3+) chelate complexes [9; 10]. In SulFerox\({}^{\text{\textregistered}}\), the reagent used was characterized by increased stability and low capital and operating costs. Reagent costs are 80-100$ per 1 t of hydrogen sulfide. Available data show that in the process of gas purification, up to 50-80% of methyl mercaptan and 30-60% of carbonyl sulfide can be removed from the initial content. The SulFerox(r) process uses a new composition of a complexone, which is similar to EDTA (ethylenediamine-tetraacetate). However, the concentration of iron in the absorbent used is significantly higher (up to 3 wt.%) than in Lo-Cat processes (up to 0.5 wt.%, Figure 1) [11]. The first installation put into operation had a capacity of 120,000 m\({}^{3}\)/day of the gas containing 4.5% hydrogen sulfide and 57% carbon dioxide at a pressure of 20 atm. The largest installation was launched in 1992 in Denver City, Texas. At this unit, 1500 ppmv of hydrogen sulfide in carbon dioxide gas at a pressure of 20 atm was reduced to 20 ppmv. The Sulferox process is currently the object of the greatest amount of research. From 1990 to 1995, Shell designed, built, and constructed more than 20 installations for the cleaning of various technological gases. In the SulFerox(r) process, the concentration of iron compounds is significantly higher than in the ARI-Lo-Cat I(r), ARI-Lo-Cat-II(r) processes. This fact explains the broader introduction of the ARI-Lo-Cat processes in gas cleaning operations. In the literature, information was found on the creation of only a few technological complexes for the purification of gases which simultaneously yield elementary sulfur, as opposed to the conventional procedure of amine absorption coupled with the Claus process. The Volga Research Institute of Hydrocarbon Fuels (JSC VNIIUS, Kazan, Russia) developed the Serox-2 process for cleaning gas flows from hydrogen sulfide with solutions of iron complexes to obtain elemental sulfur. The process is an analog of the \"LO-CAT\" process; its main distinctive feature is the composition of the absorbent with low corrosion activity with respect to carbon steel and high stability under the conditions required for the purification of gases. The process is implemented according to a standard two-step procedure, i.e., sulfur foam filtration and purification of hydrocarbon gas with a residual H\({}_{2}\)S content of no more than 20 mg/m\({}^{3}\) (National Standard 5542-87) [12]. As more than 40 years of field testing experience shows, the process of cleaning gases from hydrogen sulfide with chelate complexes (iron salts of EDTA) has some disadvantages that limit its application for gases with a hydrogen sulfide content of more than 1-2 g/m\({}^{3}\)  
Figure 1: Schematic of the Lo-Cat process (adapted from [11]). Due to the need to use a working solution with a pH no higher than 8.5-9.0, the applicability of this solution with respect to hydrogen sulfide is limited, leading to the need to increase the rate of its circulation through the absorber (i.e., energy consumption for pumping increases). The formation of a side product in the oxidation of hydrogen sulfide-thiosulfate is inevitable, which necessitates the use of a reagent such as EDTA.