To elemental sulfur

Orga.nic Carbon. Organic materials interfere with plant operation because these compounds react with sulfuric acid under furnace conditions to form sulfur dioxide. There is a reducing atmosphere in the furnace which may reduce sulfur dioxide to elemental sulfur, which results in sulfur deposits in the gas handling system. 

The Claus process converts hydrogen sulfide to elemental sulfur via a two-step reaction. The first step involves controUed combustion of the feed gas to convert approximately one-third of the hydrogen sulfide to sulfur dioxide (eq. 9) and noncatalytic reaction of unbumed hydrogen sulfide with sulfur dioxide (eq. 10). In the second step, the Claus reaction, the hydrogen sulfide and sulfur dioxide react over a catalyst to produce sulfur and water (eq. 10). The principal reactions are as foUow ... 

Conversion Processes. A number of options exist for handling concentrated sulfur dioxide streams. One option is the sale of a Hquid sulfur dioxide product. Alternatively, the sulfur dioxide can be converted to elemental sulfur or to sulfuric acid. 

Disposal of the spent caustic solution can be a troublesome environmental problem. Depending on the plant location, acid gases are either sent to a fired heater or treated in a Claus unit for conversion of hydrogen sulfide to elemental sulfur. 

By-product processing Hydrogen sulfide Conversion to elemental sulfur or sulfuric acid by liquid absorption, wet oxidation to elemental sulfur, combustion to SO2... 

Ultimately, pollution can only be avoided by complete removal of SO2 from the effluent gases, but this council of perfection is both technologically and economically unattainable. Many processes are available to reduce the SO2 concentration to very low figures, but the vast scale of power generation and domestic heating by coal and oil still results in substantial emission. SO2 can be removed by scrubbing with a slurry of milk of lime , CafOH) . Alternatively, partial reduction to H2S using natural gas (CH4), naphtlia or coal, followed by catalytic conversion to elemental sulfur by the Claus process can be used ... 

C2H5SH + %02 SO2 + 2CO2 + 3H2O Sulfur dioxide is then reduced in the Claus reactor to elemental sulfur. 

An alternative procedure for removing an ion from solution is to change its identity by changing its oxidation state. The metal ions in very insoluble heavy metal sulfide precipitates can be dissolved by oxidizing the sulfide ion to elemental sulfur. For example, copper(II) sulfide, CuS, takes part in the equilibrium... 

However, when nitric acid is added, the sulfide ions are oxidized to elemental sulfur ... 

The latter reaction has been studied numerous times because of its relevance for the autoxidation of hydrogen sulfide in seawater and other aqueous systems [112, 113]. 8ince the polysulfide ions can be further oxidized to elemental sulfur which precipitates from the solution, these reactions are the basis for several industrially important desulfurization processes (e.g., the 8tretford, 8ulfolin, Lo-Cat, 8ulFerox, and Bio-8R processes) [114] ... 

The spontaneous thermal decomposition of S2O on condensation at low temperatures and of some of the homocychc sulfuroxides on storage at 20 °C results in polysulfuroxides (PSO) of composition S 0 with n>3. These products are mixtures of molecules of unknown molecular mass which slowly decompose at 20 °C to elemental sulfur and SO2. Consequently, their composition depends on the preparation and changes with time. At 100 °C this decomposition is quantitative within a few minutes. 

A number of microorganisms have been found to degrade hydrogen sulfide to elemental sulfur or sulfate. Among these are a heterotrophic bacterium of the genus Xanthomonas isolated from dimethyl disulfide-acclimated peat (Cho et al. 1992), heterotrophic fungi (Phae and Shoda 1991), and a marine isopod (Vismann 1991). 

Standard processing combines sorption in alkaline solution, regeneration of the solvent with concurrent release of high-concentration H2S, and finally conversion of the H2S to elemental sulfur through the Claus reaction scheme ... 

Plummer invented a process for the biodesulfurization of hydrocarbons [157], in which organic sulfur compounds contained in liquid hydrocarbons are converted to elemental sulfur. The reaction is carried out in the presence of a biocatalyst and hydrogen, by dissolving completely the liquid hydrocarbons in an organic solvent, such as a nucleophilic and/or electrophilic solvent(s). The nucleophilic solvent should have a pKa greater than 2, and the electrophilic solvent more negative than -2. Recommended nucleophilic solvents include -butylamine, diethylamine, butanediamine, ethylenimine, toluene, pyridine, aniline, and acetophenone. The electrophilic solvents could be methylethylketone, pyrrole, or benzaldehyde. 

Fig. 5.2. The photosynthetic membrane of a green sulfur bacterium. The light-activated bacte-riochlorophyll molecule sends an electron through the electron-transport chain (as in respiration) creating a proton gradient and ATP synthesis. The electron eventually returns to the bacteri-ochlorophyll (cyclic photophosphorylation). If electrons are needed for C02 reduction (via reduction of NADP+), an external electron donor is required (sulfide that is oxidised to elemental sulfur). Note the use of Mg and Fe.

Boliden (1) An obsolete process for reducing sulfur dioxide to elemental sulfur, using hot coke. Operated in Sweden by Boliden from 1933 to 1943. See also Trail, RESOX. 

Cataban A process for removing small amounts of hydrogen sulfide from industrial gas streams by oxidation, in aqueous solution, to elemental sulfur. The oxidant is the ferric ion,... 

Catasulf A catalytic process for converting hydrogen sulfide in gas streams to elemental sulfur. The gas, to which a stoichiometric quantity of air or oxygen has been added, is passed over the hot catalyst. Invented in 1983 by BASF. One plant had been built as of 1990. 

CONOSOX A complex flue-gas desulfurization process using potassium carbonate solution as the wet scrubbing medium. The product potassium bisulfite is converted to potassium thiosulfate and then reduced with carbon monoxide to potassium carbonate for re-use. The sulfur is recovered as hydrogen sulfide, which is converted to elemental sulfur by the Claus process. Developed by the Conoco Coal Development Company and piloted in 1986. 

Double Loop A process for removing hydrogen sulfide from natural gas. In the first loop an organic solvent absorbs the hydrogen sulfide from the gas. In the second, an aqueous solution of an iron chelate converts this to elemental sulfur. Developed by Radian International... 

Konox A process for removing hydrogen sulfide from industrial gases by absorption in aqueous sodium ferrate (Na2Fe04) solution. The ferrate is reduced to ferrite (NaFe02) and the sulfide is oxidized to elemental sulfur. The main reactions are ... 

Mond A process for recovering sulfur from the residues from the Leblanc process. The sulfur is partially oxidized to thiosulfate and converted to elemental sulfur by adding hydrochloric acid. This process recovers only half the sulfur it was supplanted by the Chance process. Invented by L. Mond and operated by the Netham Chemical Company at Bristol from 1868 to 1888. 

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