Methane-sulfur process

A disadvantage of the hydrocarbon—sulfur process is the formation of one mole of hydrogen sulfide by-product for every two atoms of hydrogen in the hydrocarbon. Technology for efficient recovery of sulfur values in hydrogen sulfide became commercially available at about the same time that the methane—sulfur process was developed. With an efficient Claus sulfur recovery unit, the hydrocarbon—sulfur process is economically attractive. 

These processes are the retort process and the electrothermal process [116]. The third process is the methane-sulfur process. 

Figure 4.7 Carbon disulfide via the methane-sulfur process. Source [121].

Most countries are currently using the methane-sulfur process. Worldwide production of carbon disulfide is about 235,000 ton/yr in 1991 [125]. This is the only process used in the United States [126]. The United States used a total of 108,000 tons in 1990 [127]. A typical methane-sulfur plant tends to produce 50,000-200,000 ton/yr of carbon disulfide as compared to a coal-sulfur plant which produces about 5,000 ton/yr [128]. 

Hydrocarbon—Sulfur Process. The principal commercial hydrocarbon is methane from natural gas, although ethane, and olefins such as propylene (45,46), have also been used. 

The steam methane reforming process is relatively simple. Methane-rich gas enters the unit and is preheated either by reformer flue gas or process synthesis gas, as shown in Figure 3. The preheated feed then enters the desulfurizer reactor to ensure removal of H2S and other sulfur compounds to a specification of 0.1 ppm. Two reactors are used in parallel so that one is in service while the other is on standby [7]. 

In general, the proven technology to upgrade methane is via steam reforming to produce synthesis gas, CO + Such a gas mixture is clean and when converted to Hquids produces fuels substantially free of heteroatoms such as sulfur and nitrogen. Two commercial units utilizing the synthesis gas from natural gas technology in combination with novel downstream conversion processes have been commercialized. 

Steam Reforming Processes. In the steam reforming process, light hydrocarbon feedstocks (qv), such as natural gas, Hquefied petroleum gas, and naphtha, or in some cases heavier distillate oils are purified of sulfur compounds (see Sulfurremoval and recovery). These then react with steam in the presence of a nickel-containing catalyst to produce a mixture of hydrogen, methane, and carbon oxides. Essentially total decomposition of compounds containing more than one carbon atom per molecule is obtained (see Ammonia Hydrogen Petroleum). 

Commercial-scale processes have been developed for the production of hydrogen sulfide from heavy fuel oils and sulfur as well as from methane, water vapor, and sulfur. The latter process can be carried out in two steps reaction of methane with sulfur to form carbon disulfide and hydrogen sulfide followed by hydrolysis of carbon disulfide (116). 

Reduction of sulfur dioxide by methane is the basis of an Allied process for converting by-product sulfur dioxide to sulfur (232). The reaction is carried out in the gas phase over a catalyst. Reduction of sulfur dioxide to sulfur by carbon in the form of coal has been developed as the Resox process (233). The reduction, which is conducted at 550—800°C, appears to be promoted by the simultaneous reaction of the coal with steam. The reduction of sulfur dioxide by carbon monoxide tends to give carbonyl sulfide [463-58-1] rather than sulfur over cobalt molybdate, but special catalysts, eg, lanthanum titanate, have the abiUty to direct the reaction toward producing sulfur (234). 

The earliest method for manufacturiag carbon disulfide involved synthesis from the elements by reaction of sulfur and carbon as hardwood charcoal in externally heated retorts. Safety concerns, short Hves of the retorts, and low production capacities led to the development of an electric furnace process, also based on reaction of sulfur and charcoal. The commercial use of hydrocarbons as the source of carbon was developed in the 1950s, and it was still the predominate process worldwide in 1991. That route, using methane and sulfur as the feedstock, provides high capacity in an economical, continuous unit. Retort and electric furnace processes are stiU used in locations where methane is unavailable or where small plants are economically viable, for example in certain parts of Africa, China, India, Russia, Eastern Europe, South America, and the Middle East. Other technologies for synthesis of carbon disulfide have been advocated, but none has reached commercial significance. 

For equation 26, starting with methane and soHd sulfur at 25°C, and ending with gaseous products at 600°C, the reaction is endothermic and requires 2.95 MJ /kg (705 kcal/kg) of CS2. The reaction of methane and sulfur vapor in the diatomic form is actually exothermic (23,78). Superheating of the sulfur is claimed to be preferable (79), and series operation of reactors offers a means of reducing process temperatures at which the sulfur dissociates (80). 

Raw material usages per ton of carbon disulfide are approximately 310 m of methane, or equivalent volume of other hydrocarbon gas, and 0.86—0.92 ton of sulfur (87,88), which includes typical Claus sulfur recovery efficiency. Fuel usage, as natural gas, is about 180 m /ton carbon disulfide excluding the fuel gas assist for the incinerator or flare. The process is a net generator of steam the amount depends on process design considerations. 

Potential Processes. Sulfur vapor reacts with other hydrocarbon gases, such as acetjiene [74-86-2] (94) or ethylene [74-85-1] (95), to form carbon disulfide. Higher hydrocarbons can produce mercaptan, sulfide, and thiophene intermediates along with carbon disulfide, and the quantity of intermediates increases if insufficient sulfur is added (96). Light gas oil was reported to be successflil on a semiworks scale (97). In the reaction with hydrocarbons or carbon, pyrites can be the sulfur source. With methane and iron pyrite the reaction products are carbon disulfide, hydrogen sulfide, and iron or iron sulfide. Pyrite can be reduced with carbon monoxide to produce carbon disulfide. 

Coal Hquefaction iavolves raising the atomic hydrogen-to-carbon ratio from approximately 0.8/1.0 for a typical bituminous coal, to 2/1 for Hquid transportation fuels or 4/1 for methane (4). In this process, molecular weight reduction and removal of mineral matter and heteroatoms such as sulfur, oxygen, and nitrogen may need to be effected. 

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