Sulfur, Fischer-Tropsch process

The Fischer-Tropsch process has attracted renewed interest as a way to produce high quality, sulfur-free diesel fuel from natural gas and, possibly, an opportunity to utilize natural gas at remote oilfields. The process represents proven technology and is regarded as an alternative for when oil may no longer be widely available, and one has to resort to natural gas and coal. In a really futuristic scenario one may even contemplate the use of GO and H2 produced by photo-catalytic dissociation of GO2 and water. 

The Fischer-Tropsch process requires synthesis gas in which the total sulfur does not exceed 2.0 mg./cubic meter. This purification was done in two stages (1) Removal of hydrogen sulfide and (2) removal of organic sulfur. The removal of hydrogen sulfide is almost universally carried out by the well known iron oxide process. At the Luetzkendorf plant, the so-called Alkazid process had been installed, in which a solution of an alkaline organic compound absorbs the hydrogen sulfide, which is then continuously stripped from the solution by steam. At Luetzkendorf, the evolved hydrogen sulfide was converted into elemental sulfur. 

Indirect coal liquefaction first gasifies the coal with steam to form synthesis gas (syngas—a mixture of hydrogen and carbon monoxide). The sulfur is removed from this gas and the mixture is adjusted according to the desired product. The synthesis gas is then condensed over a catalyst—the Fischer-Tropsch process— to produce high-quality, ultraclean products. 

Other synthetic methods have been investigated but have not become commercial. These include, for example, the hydration of ethylene in the presence of dilute acids (weak sulfuric acid process) the conversion of acetylene to acetaldehyde, followed by hydrogenation of the aldehyde to ethyl alcohol and the Fischer-Tropsch hydrocarbon synthesis. Synthetic fuels research has resulted in a whole new look at processes to make lower molecular weight alcohols from synthesis gas. 

The fact that Fischer-Tropsch fuels contain neither sulfur nor aromatics may become a strong selling point for the process. Less sulfur in the fuel has, of course, a direct effect on the sulfur oxides in the emissions, and the newly developed exhaust purification systems for lean burning engines that can be introduced means that all emissions, including GO2 and NOx, will diminish. Aromatics promote particulate formation in the combustion of diesel fuels and are therefore undesirable. We discuss this further in Ghapter 10. 

The Fischer-Tropsch (FT) catalytic conversion process can be used to synthesize diesel fuels from a variety of feedstocks, including coal, natural gas and biomass. Synthetic diesel fuels can have excellent autoigitition characteristics. The Fischer-Tropsch diesel is composed of only straight-chain hydrocarbons and has no aromatics or sulfur. The synthetic Fischer-Tropsch diesel fuel can provide benefits in terms of both PM and NO, emissions. 

It is well known that all sulfur compounds rapidly deactivate iron, cobalt and nickel Fischer-Tropsch catalysts. However, due to the efficiency of modem gas purification processes such as (he Lurgi Kectisol process, the sulfur level in synthesis gas can be reduced below 0.03 tng/mj. Tiiis level is tolerable and a constant synthesis gas conversion can be achieved [15], Iron catalysis which have been poisoned by sulfur are not readily reactivated. Only very thorouglt reoxidation by which all traces of sulfur are burnt away efficienily. followed by reduction, is effective [15,21J. 

The Rectisol process, developed by Lurgi, is the most widely used physical solvent gas treating process in the world. More than 100 Rectisol units are in operation or under construction worldwide. Its most prevalent application is for deep sulfur removal from syngas that subsequently undergoes catalytic conversion to such products as ammonia, hydrogen, and Fischer-Tropsch liquids. 

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