The Fischer-Tropsch Process

FIGURE 12.1 GTL lubes production from natural gas general process schematic. 

Catalyst technology has shifted from iron as the active metal to cobalt and it has been suggested that all new plants in the immediate future will have cobalt catalysts, preferred because of their inherent stability, excellent activity, compatibility with slurry reactor operations, and ability to make waxy products as well as diesel and gasoline.16 

Gas to liquids operating conditions employ relatively low temperatures (200°C to 250°C) and pressures (20 to 30 bar). 

The transformation of synthesis gas (syngas), a mixture containing hydrogen and carbon monoxide gas, into hydrocarbon products using a catalyst is usually referred to as Fischer-Tropsch synthesis (FTS). The FTS process is well established, and was first reported almost a century ago following the work of Fischer and Tropsch in the 1920s. 

FTS produces a wide distribution of products. Conventional catalysts typically give an Anderson-Schultz-Flory distribution of products, hence they are generally non-selective for specific products. Therefore, the development of highly active and selective catalysts has been a key goal. 

Fischer-Tropsch (FT) processes can operate at either low-temperature (LTFT), 190-260 °C, or high-temperature (HTFT), 300-350 With iron catalysts, HTFT processes are used for the production of petroleum-compatible alkanes and low molecular weight alkenes, while low-temperature (LTFT) processes are typically used to generate longer-chain hydrocarbons that can be refined into diesel and waxes.  

Metallic iron itself has very low FTS activity. Although, under operational conditions the activity of metallic iron gradually increases over time. To improve the FTS activity and tune the product selectivity of iron catalysts, promoters such as alkali metals, transition metals and other additives are incorporated into the catalyst structure. Typical promoters and additives include copper, potassium and silica. Copper acts to enhance the rate of catalyst activation, silica improves the dispersion of catalytically active iron species, while alkali metals aid carbon-monoxide dissociation from surface iron.  

LTFT iron catalysts are commonly prepared by precipitation techniques, with a typical composition of potassium oxide, copper, silica and iron (1 1 5 20 by mass). Before use in the LTFT process, the catalysts are prereduced with either hydrogen or a syngas mixture.HTFT catalysts can be formed from the fusion of magnetite with various promoters, typically potassium oxide and aluminium oxide or magnesium oxide. Similarly, HTFT catalysts require a pre-reduction with hydrogen at ca. 400 °C. 

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The Fischer-Tropsch process can be considered as a one-carbon polymerization reaction of a monomer derived from CO. The polymerization affords a distribution of polymer molecular weights that foUows the Anderson-Shulz-Flory model. The distribution is described by a linear relationship between the logarithm of product yield vs carbon number. The objective of much of the development work on the FT synthesis has been to circumvent the theoretical distribution so as to increase the yields of gasoline range hydrocarbons. 

Secunda discharges no process water effluents. AU. water streams produced are cleaned and reused in the plant. The methane and light hydrocarbons in the product are reformed with steam to generate synthesis gas for recycle (14). Even at this large scale, the cost of producing fuels and chemicals by the Fischer-Tropsch process is dominated by the cost of synthesis gas production. Sasol has estimated that gas production accounts for 58% of total production costs (39). 

In Sasolburg, South Africa, a commercial plant using the Fischer-Tropsch process was completed in 1950 and began producing a variety of Hquid fuels and chemicals. The faciUty has been expanded to produce a considerable portion of South Africa s energy requirements (15,16). 

Fig. 1. Routes to Hquid fuels from natural gas and coal via synthesis gas. F-T is the Fischer-Tropsch process.

Metha.nol-to-Ga.soline, The most significant development in synthetic fuels technology since the discovery of the Fischer-Tropsch process is the Mobil methanol-to-gasoline (MTG) process (47—49). Methanol is efftcientiy transformed into C2—C q hydrocarbons in a reaction catalyzed by the synthetic zeoHte ZSM-5 (50—52). The MTG reaction path is presented in Figure 5 (47). The reaction sequence can be summarized as... 

The second reaction is called the Fischer-Tropsch synthesis of hydrocarbons. Depending on the conditions and catalysts, a wide range of hydrocarbons from very light materials up to heavy waxes can be produced. Catalysts for the Fischer-Tropsch reaction iaclude iron, cobalt, nickel, and mthenium. Reaction temperatures range from about 150 to 350°C reaction pressures range from 0.1 to tens of MPa (1 to several hundred atm) (77). The Fischer-Tropsch process was developed iadustriaHy under the designation of the Synthol process by the M. W. Kellogg Co. from 1940 to 1960 (83). 

Fluidized Catalyst Reactor. Two systems have been proposed, based on large scale operation of the Fischer-Tropsch process (to produce liquid hydrocarbons) at SASOL and at Carthage Hydrocol. The SASOL system was designed by M. W. Kellogg and has been operating for about 20 years (57, 58, 59, 60). 

The internal cooling system was applied to the Fischer-Tropsch process by the U. S. Bureau of Mines (48, 49), the British Fuels Board (54), and Rheinprussen-Koppers (52, 53). The external cooling system was applied to the Fischer-Tropsch process by I. G. Farben (61). 

In the Fischer-Tropsch process, carbon monoxide reacts with hydrogen in the presence of a solid catalyst, with the formation of a mixture of hydrocarbons. The composition of the product varies considerably with the catalyst and the operating conditions. The mixture may include (in addition to hydrocarbons) alcohols, aldehydes, ketones, and acids. 

Farley and Ray (F3) have reported holdup and conversion data for the Fischer-Tropsch process carried out in a pilot-scale reactor. 

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 produces alkanes and alkenes ... 

Describe the potential of the Fischer-Tropsch process as a source of transportation fuels. 

The steps in the hydroformylation reaction are closely related to those that occur in the Fischer-Tropsch process, which is the reductive conversion of carbon monoxide to alkanes and occurs by a repetitive series of carbonylation, migration, and reduction... 

The Fischer-Tropsch process is of considerable economic interest because it is the basis of conversion of carbon monoxide to synthetic hydrocarbon fuels, and extensive work has been done on optimization of catalyst systems. 

Indirect coal liquefaction is a technology in which coal is first gasified to synthesis gas ("syngas," CO + H2), which is used to synthesize liquid fuel by the Fischer—Tropsch process. 

Dry M.E. 2004. Present and future applications of the Fischer-Tropsch process. Appl. Catal. A Gen. 276 1-3. 

Pankina, G. V., Chemavskii, P. A., Lermontov, A. S., and Lunin, V. V. 2002. Study of carbon deposits on the surface of supported cobalt catalysts in the Fischer-Tropsch process. Petrol. Chem. 42 217-20. 

Van Der Laan, G.P. 1999. Kinetics, selectivity and scale up of the Fischer Tropsch process. PhD dissertation, Rijksunivertiteit Groningen. 

A major reason for the difference between the target and current technology is the chemistry chosen and the way the flow sheet is put together. At present, CTL processes produce syngas (CO and as an intermediate by gasification of coal, and then convert the syngas into hydrocarbons by the Fischer-Tropsch process. The target for this route can also be calculated. 

The Fischer-Tropsch process has recently received renewed attention due to the increasing demand and rising costs of fuels. Fischer-Tropsch processes are either coal based or methane based. Coal-based processes face serious issues in terms of the increasing regulations regarding limiting C02 emissions worldwide. There is a firm need for innovative and novel solutions for dealing with C02 emissions from the CTL process. 

Weil, B. H., and Lane, J. C. 1949. The technology of the Fischer-Tropsch process. London Constable. 

Because the synthesis gas produced from coal is generally relatively poor in hydrogen, a typical CO H2 ratio being ca. 1 1, and because, as can be seen from Eqs. (14) and (15), a hydrogen-rich gas is required for the production of hydrocarbons and chemicals, a hydrogen enrichment step is usually necessary for the Fischer-Tropsch process. 

ATR (2) [Autothermal reforming] A process for making nitrogen-diluted syngas, suitable for use in the Fischer Tropsch process. Developed by Syntroleum in 1989. 

Duftschmid A variation of the Fischer-Tropsch process in which synthesis gas and an oil are circulated over a fixed bed of iron catalyst in order to increase the yield of olefins from the gas. 

Iso-Synthesis A version of the Fischer-Tropsch process developed in Germany during World War II. 

Kolbel-Rheinpreussen A process for converting syngas to gasoline. The gas was passed through a suspension of an iron catalyst in an oil. Developed by H. Kolbel at Rheinpreussen, Germany, from 1936 until the 1950s when it was supplanted by the Fischer-Tropsch process. 

Krupp-Kohlechemie A process for making hard paraffin wax from water gas by a variant of the Fischer-Tropsch process. The products were called Ruhrwachse. Developed by Ruhr Chemie and Lurgi Ges. fur Warmetechnie. 

Synol A version of the Fischer-Tropsch process developed in Germany during World War II. It used a different catalyst and produced a larger fraction of alcohols and olefins. 

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