Sulfur removal and recovery

Sulfur (qv) is among the most widely used chemicals and often considered to be one of the four basic raw materials of the chemical iadustry. In 1993, worldwide production of sulfur reached 55 million metric tons (1). Production of sulfuric acid consumes the vast majority (- 90%) of sulfur (2) (see Sulfuric acid and sulfur trioxide). This acid is a steppiag stone ia the production of other sulfur-containing compounds, most notably ammonium sulfate fertilizer which accounts for 60% of the total worldwide sulfur consumption (2) (see Ammonium compounds Fertilizers). 

Sulfur can be produced direcdy via Frasch mining or conventional mining methods, or it can be recovered as a by-product from sulfur removal and recovery processes. Production of recovered sulfur has become more significant as increasingly sour feedstocks are utilized and environmental regulations concerning emissions and waste streams have continued to tighten worldwide. Whereas recovered sulfur represented only 5% of the total sulfur production ia 1950, as of 1996 recovered sulfur represented approximately two-thirds of total sulfur production (1). Recovered sulfur could completely replace native sulfur production ia the twenty-first century (2). 

Other factors which have a significant influence on process selection iaclude absolute quantity of sulfur present, concentration of various sulfur species, the quantity and concentration of other components ia the stream to be treated, quantity and conditions (temperature and pressure) of the stream to be treated, and, the location-specific environmental regulations governing overall sulfur recovery and allowable sulfur dioxide emissions (3). 

Adsorption Processes. The processes based on adsorption of hydrogen sulfide onto a fixed bed of soHd material are among the oldest types of gas treating appHcations (4). Two common sorbent materials for low concentration gas streams are iron oxide and zinc oxide. 

Hydrogen sulfide reacts with iron oxide [1317-61 -9] to form iron sulfide, according to the following chemical reaction  

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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... 

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. 

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. 

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). 

Minor and potential new uses for ammonium thiosulfate include flue-gas desulfurization (76,77), removal of nitrogen oxides and sulfur dioxide from flue gases (78,79), converting sulfur ia hydrocarbons to a water-soluble form (80), and converting cellulose to hydrocarbons (81,82) (see Sulfur REMOVAL AND RECOVERY). 

During World War II German scientists developed a method of hydrogenating soHd fuels to remove the sulfur by using a cobalt catalyst (see Coal CONVERSION processes). Subsequently, various American oil refining companies used the process in the hydrocracking of cmde fuels (see CATALYSIS SuLFUR REMOVAL AND RECOVERY). Cobalt catalysts are also used in the Fisher-Tropsch method of synthesizing Hquid fuels (21—23) (see Fuels, synthetic). 

Scmbbers for removing sulfur dioxide from smelter off-gases have been under development for many years. They are widely used in Japan. The calcium sulfate (gypsum) obtained from this process is suitable feed for waUboard production (see Calcium compound, calcium sulfate Sulfur removal and recovery). 

Steiner, R, Juntgen, H., and Knoblauch, K., in Sulfur Removal and Recovery from Industrial Processes, Pfeiffer, J. B., Ed., American Chemical Society, Washington, D.C., 1975, 185. 

The Claus process, which involves the reaction of sulfur dioxide with hydrogen sulfide to produce sulfur in a furnace, is important in the production of sulfur from sour natural gas or by-product sulfur-containing gases (see Sulfur REMOVAL and recovery). 

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