Thylox process

There are two main processes in the thioarsenate category. The first, the Thylox process, was developed in the late twenties and used in the U.S. for many years, especially for the purification of coke-oven and other manufactured gases. The second, the Giammarco-Vetro-coke (G-V) process, is a dual H2S/CO2 gas sweetening process introduced in Italy in 1955, which was extensively used in Europe and Asia, especially in applications where the H2S concentration in the feed gas was relatively low. 

The Thylox process is no longer used commercially, while the arsenic-based version of the Giammarco-Vetrocoke process is still supported by the licensor in countries where the use of arsenical solutions is permitted. A non-arsenic, evolutionary modification of the G-V process is currently being offered for CO2 removal, and is claimed to be very competitive. This form of the process can not be classified as a liquid redox process, and is discussed in Chapter 5. 

Although the presence of arsenic salts in the treating solution suggests a strong similarity between the two thioarsenate processes, their chemistry and operating characteristics are fundamentally different. The Giammarco-Vetrocoke proces.s, unlike the Thylox process, relies on the use of relatively inert monothioarsenate salts. This practice limits the extent of undesirable side reactions, and makes arsenic sulfide precipitation problems less likely. In spite of the toxicity of the solution, the combination of these two factors has made the arsenic-based Giammarco-Vetrocoke solution somewhat more environmentally acceptable. 

The early applications of the Thylox process were aimed at about 80 to 90% hydrogen sulfide removal, but essentially near total hydrogen sulfide removal was eventually achieved through subsequent process improvements. One variation of the process, the so-called modified Thylox process, was applicable in cases where complete purification of gases containing small amounts of hydrogen sulfide was required. 

A neutral or slightly alkaline solution, containing sodium or ammonium thioarsenate as the active ingredient, is used in the Thylox process. Hydrogen sulfide is converted to ele-... 

The Thylox process offered some economic advantages over processes previously discussed in this chapter. The consumption of alkali due to thiosulfate formation was reduced markedly, and the sulfur was produced in a much more valuable form. Estimated operating requirements for a plant treating 5 million cu ft/day of refinery gas containing 1,000 grains hydrogen sulfide/100 cu ft were reported by Dunstan (1938), and are shown in Table 9-4. 

A basic flow diagram of the Thylox process is shown in Figure 9-5. The gas enters at the bottom of the absorber and is washed countercurrently with solution entering at the top of the vessel. Essentially all of the hydrogen sulfide and hydrogen cyanide are removed in this... 

Figure 9-5. Basic flow diagram of the Thylox process.

A process used in Russia, which is practically identical with the Thylox process, is reported by Jegorov et al. (1954). This paper contains a rather detailed description of process equipment, design criteria, and a sample calculation for a typical plant for the treatment of coke oven gas. 

An interesting feature of the Russian process is the two-step method employed for the complete recovery of arsenic from solution waste-streams. In the first step, which is similar to the recovery method used in the Thylox process, the solution is heated to 70°C (158 F), and arsenic sulfide is precipitated by the addition of 75% sulfuric acid. The precipitate is separated from the liquid by filtration, dissolved in aqueous sodium carbonate, and returned to the circulating solution-stream. The clear liquid is then passed to the second step where it is made alkaline with sodium carbonate solution and treated with a solution of ferric sulfate. In this operation the small amount of arsenic remaining in the solution after the first step is fixed and precipitated as ferric arsenite and arsenate. The precipitate is finally removed by filtration, and the filtrate, which contains about 10 to 20 ppm of arsenic, is either discarded or processed for recovery of thiosulfate. Wooden tanks lined with acid-resistant materials are used in both steps of the arsenic-recovery operation. Each tank is sized for a solution residence time of 4 hr and provided with a mechanical agitator. 

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