Stretford process

The Lo-Cat process, Hcensed by US Filter Company, and Dow/Shell s SulFerox process are additional Hquid redox processes. These processes have replaced the vanadium oxidizing agents used in the Stretford process with iron. Organic chelating compounds are used to provide water-soluble organometaHic complexes in the solution. As in the case of Stretford units, the solution is regenerated by contact with air. 

Gas treating is defined here as removal of H2S and CO2. Other sulfur compounds are discussed where applicable. Dehydration and sulfur production are not included, except for discussing sulfur production in the Stretford Process and for selective H2S removal. H2S must be removed from natural gas and process streams for health reasons and prevention of corrosion. Natural gas pipeline specifications require no more than % grain/100 SCF. This is equivalent to 4ppmv or 7ppmw (for a 0.65 specific gravity gas). By comparison, the human nose can detect... 

The Stretford Process sweetens and also produces sulfur. It is good for low feed gas concentrations of H2S. Economically, the Stretford Process is comparable to an amine plant plus a Claus sulfur recovery plant. Usually, the amine/Claus combination is favored over Stretford for large plants. Stretford can selectively remove H2S in the presence of high CO2 concentrations. This is the process used in the coal gasification example in the Introduction. 

An example of a process using O2 to oxidize HiS is the Stretford process, which is licensed by the British Gas Corporation. In this process the gas stream is washed with an aqueous solution of sodium carbonate, sodium vanadate, and anthraquinone disulfonic acid. Figure 7-9 shows a simplified process diagram of the process. 

Cleanair A process for pretreating gas streams before the Claus and Stretford processes for removing sulfur compounds. Developed by the J. F. Pritchard Company. 

Holmes-Stretford A version of the Stretford process, developed by W.C. Holmes Company, Huddersfield, England. 

LO-CAT A process for removing hydrogen sulfide and organic sulfur compounds from petroleum fractions by air oxidation in a cyclic catalytic process similar to the Stretford process. The aqueous solution contains iron, two proprietary chelating agents, a biocide, and a surfactant the formulation is known as ARI-310. The sulfur product is removed as a slurry. Developed in 1972 by Air Resources (now ARI Technologies) and first commercialized in 1976. Over 125 units were operating in 1996. An improved version, LO-CAT II, was announced in 1991. 

SulFerox A process for removing hydrogen sulfide and organic sulfur compounds from hydrocarbons, similar to the Stretford process but using an aqueous solution containing chelated iron and proprietaiy additives. The product is elemental sulfur. The basic reactions are ... 

Takahax A variation of the Stretford process for removing hydrogen sulfide from gas streams, in which naphthaquinone sulfonic acid is used in place of anthraquinone disulfonic acid. Four variants have been devised types A and B use ammonia as the alkali, types C and D use sodium hydroxide or carbonate. Developed by the Tokyo Gas Company and licensed in the United States by Ford Baken and Davis, Dallas, TX. Many plants are operating in Japan. 

Unisulf [Unocal sulfur removal] A process for removing sulfur compounds from petroleum fractions, similar to the Stretford process, but including in the catalytic solution vanadium, a thiocyanate, a carboxylate (usually citrate), and an aromatic sulfonate complexing agent. Developed by the Union Oil Company of California in 1979, commercialized in 1985, and operated in three commercial plants in 1989. 

Absorption-oxidation processes oxidize absorbed H2S directly to elemental sulfur in solution (1). The principal example in current industrial use is the Stretford process (3). The chemistry of the process can be represented by the following idealized equations (ADA represents anthraquinone disulfonic acid) ... 

Process Alternatives. Process alternatives for sulfur recovery are shown schematically in Figure 2. The choice of either elemental sulfur or sulfuric acid will depend on economics and markets related to each plant location. Elemental sulfur may be produced by gas-phase oxidation (the Claus process) or liquid-phase oxidation (e.g., the Stretford process). Stretford units were described in Section 1 and are well discussed in the literature (1, 2> 5) Claus sulfur recovery efficiency is usually less than required by current air emission standards. Therefore, some form of tail-gas treating is required. Sulfuric acid may be produced by the well-known contact process (6). This process is licensed by a number of firms, each of which has its own... 

Use of a transportable treatment unit to oxidize thiosulfate salts in the Stretford solution to allow them to be recycled in the Stretford process ... 

Since both H2S and CO2 are acidic, they tend to come out together as mixed "acid gas" when the gas is washed to extract H2S. Various other compounds, including COS, may be extracted at the same time. Extraction of acid gas may not be needed if the treated gas is to be used only as fuel it may be sufficient to treat the raw gas by the Stretford process, converting H2S directly to sulfur. Such treatment removes virtually all H2S but leaves COS in the fuel gas, which may be unacceptable if air pollution regulations require more than about 95 percent removal of total sulfur. 

The Stretford process has the defect of converting a small part of the input H2S to thiosulfate, leading eventually to a... 

In another case, when the acid gas is very rich in CO2 (i.e. 90 percent or higher) and contains olefins, the author knows no suitable process other than Stretford or Unisulf. An example is Rectisol offgas downstream of a Lurgi gasifier the Stretford process has been used, although with some problems. 

Stretford plants have been in operation for 30 years. There are hundreds of such plants worldwide, used in a variety of sulfur removal operations (Dalrymple 1989). In a Stretford process, the hydrogen sulfide in the feed gas stream is absorbed and oxidized to elemental sulfur in aqueous phase, using pentavalent vanadium which is subsequently reduced from a pentavalent form to a tetravalent form. Later in the process, the vanadium is re-oxidized back again, using anthraquinone disulfonic acid (ADA) as a catalyst, and the elemental sulfur is floated to the surface of the solution and removed. 

J ADA Isomer Selection. Limited attention is often given in refineries to the isomer of ADA used (Lorton 1988). 2,6-ADA is a commonly used isomer, although it has been found inferior to 2,7-ADA in converting vanadium to its pentavalent form. If this conversion is not performed efficiently, elemental sulfur production rate will fall, and thiosulfate formation will increase. More attention to procuring only 2,7-ADA could augment the efficiency of the Stretford process. 

For further information on Stretford process improvements and alternative processes, refer to GRI (1986 and 1988) and Trofe (1987)... 

Trofe, T. W., Dalrymple, D. A., and Scheffel, F. A. 1987. Stretford Process Status and R D Needs. Gas Research Institute, Chicago, II. Topical report, GRI-87/0021. 

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