Removal sulfur

The sulfur removed via these fixed-bed metal oxide processes is generally not recovered. Rather the sulfur and sorbent material both undergo disposal. Because the sorbent bed has a limited capacity and the sulfur is not recovered, the appHcation of these processes is limited to gas streams of limited volumetric rate having low concentrations of hydrogen sulfide. 

Table 3. Sulfur Removal from Organic Compounds by Hydrotreating

J. B. Pheiffer, Sulfur Removal and Recovey from Industrial Processes, Hdv. in Chem. Ser. 139s American Chemical Society, Washington, D.C., 1975. 

Cynes, B. L., Chemical Reactions as a Means of Separation—Sulfur Removal, Marcel Dekker, New York, 1977. 

Fig. 14. Coal flotation flow sheet suggested for increased sulfur removal (29). Pyritic sulfur removal from coal makes it imperative to closely control pulp

Fixed-bed desulfuri2ation is impractical and uneconomical if the natural gas contains large amounts of sulfur. In this case, bulk sulfur removal and recovery (qv) in an acid gas absorption—stripping system, followed by fixed-bed residual cleanup is usually employed. 

Because hydrocarbon feeds for steam reforming should be free of sulfur, feed desulfurization is required ahead of the steam reformer (see Sulfur REMOVAL AND RECOVERY). As seen in Figure 1, the first desulfurization step usually consists of passing the sulfur-containing hydrocarbon feed at about 300—400°C over a Co—Mo catalyst in the presence of 2—5% H2 to convert organic sulfur compounds to H2S. As much as 25% H2 may be used if olefins 

Sources of sulfur are called voluntary if sulfur is considered to be the principal, and often the only, product. Sulfur has also been recovered as a by-product from various process operations. Such sulfur is termed involuntary sulfur and accounts for the largest portion of world sulfur production (see Sulfur REMOVAL AND RECOVERY). 

Natural gas contains both organic and inorganic sulfur compounds that must be removed to protect both the reforming and downstream methanol synthesis catalysts. Hydrodesulfurization across a cobalt or nickel molybdenum—zinc oxide fixed-bed sequence is the basis for an effective purification system. For high levels of sulfur, bulk removal in a Hquid absorption—stripping system followed by fixed-bed residual clean-up is more practical (see Sulfur REMOVAL AND RECOVERY). Chlorides and mercury may also be found in natural gas, particularly from offshore reservoirs. These poisons can be removed by activated alumina or carbon beds. 

Ammonia production by partial oxidation of hydrocarbon feeds depends to some degree on the gasification step. The clean raw synthesis gas from a Shell partial oxidation system is first treated for sulfur removal, then passed through shift conversion. A Hquid nitrogen scmbbiag step follows. 

The plant is designed to satisfy NSPS requirements. NO emission control is obtained by fuel-rich combustion in the MHD burner and final oxidation of the gas by secondary combustion in the bottoming heat recovery plant. Sulfur removal from MHD combustion gases is combined with seed recovery and necessary processing of recovered seed before recycling. 

Conversion of carbon in the coal to gas is very high. With low rank coal, such as lignite and subbituminous coal, conversion may border on 100%, and for highly volatile A coals, it is on the order of 90—95%. Unconverted carbon appears mainly in the overhead material. Sulfur removal is faciUtated in the process because typically 90% of it appears in the gas as hydrogen sulfide, H2S, and 10% as carbonyl sulfide, COS carbon disulfide, CS2, and/or methyl thiol, CH SH, are not usually formed. 

Cmde gas leaves from the top of the gasifier at 288—593°C depending on the type of coal used. The composition of gas also depends on the type of coal and is notable for the relatively high methane content when contrasted to gases produced at lower pressures or higher temperatures. These gas products can be used as produced for electric power production or can be treated to remove carbon dioxide and hydrocarbons to provide synthesis gas for ammonia, methanol, and synthetic oil production. The gas is made suitable for methanation, to produce synthetic natural gas, by a partial shift and carbon dioxide and sulfur removal. 

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