Fischer-Tropsch technology

Much of the more recent development of F-T technology has come from the work of scientists at Sasol, but with important contributions from workers at Shell and others. 

The HTFT process operates with an iron based catalyst at about 350°C with the syngas passing through a fluidized bed of finely divided catalyst. Low temperatures cannot be used as the two phase system (gas and catalyst) would become defluidized by the formation of liquid waxes. At the higher temperature of the HTFT process, the catalyst is much more active than it is in the LTFT process and the hydrocarbon production rate is much higher. 

The hydrocarbons produced in the FT process are predominantly linear, and the alkenes are predominantly 1-alkenes. The high 1-alkene content makes it advantageous to use F-T hydrocarbons as feedstocks for the production of higher value added chemicals. 

An interesting variation on this is practised by Shell. In recent years, probably also helped by the sharp rise in the world oil price, there has been renewed interest in the F-T route to high quality diesel fuel. The Shell F-T plant in Malaysia which came on stream in 1993 uses multitubular fixed bed reactors to produce long chain hydrocarbon waxes by a LTFT process (over a promoted cobalt catalyst). These waxes are then selectively cracked over zeolites to give the desired shorter chain molecules suitable as diesel fuel. Since F-T hydrocarbons are predominantly linear they are not suitable for petrol engines, but are ideal for diesel. 

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SASOL. SASOL, South Africa, has constmcted a plant to recover 50,000 tons each of 1-pentene and 1-hexene by extractive distillation from Fischer-Tropsch hydrocarbons produced from coal-based synthesis gas. The company is marketing both products primarily as comonomers for LLDPE and HDPE (see Olefin polymers). Although there is still no developed market for 1-pentene in the mid-1990s, the 1-hexene market is well estabhshed. The Fischer-Tropsch technology produces a geometric carbon-number distribution of various odd and even, linear, branched, and alpha and internal olefins however, with additional investment, other odd and even carbon numbers can also be recovered. The Fischer-Tropsch plants were originally constmcted to produce gasoline and other hydrocarbon fuels to fill the lack of petroleum resources in South Africa. 

Synthesis gas is an important intermediate. The mixture of carbon monoxide and hydrogen is used for producing methanol. It is also used to synthesize a wide variety of hydrocarbons ranging from gases to naphtha to gas oil using Fischer Tropsch technology. This process may offer an alternative future route for obtaining olefins and chemicals. The hydroformylation reaction (Oxo synthesis) is based on the reaction of synthesis gas with olefins for the production of Oxo aldehydes and alcohols (Chapters 5, 7, and 8). 

Fischer Tropsch technology is best exemplified by the SASOL projects in South Africa. After coal is gasified to a synthesis gas mixture, it is purified in a rectisol unit. The purified gas mixture is reacted in a synthol unit over an iron-based catalyst. The main products are gasoline, diesel fuel, and jet fuels. By-products are ethylene, propylene, alpha olefins, sulfur, phenol, and ammonia which are used for the production of downstream chemicals. 

Many chemicals are produced from synthesis gas. This is a consequence of the high reactivity associated with hydrogen and carhon monoxide gases, the two constituents of synthesis gas. The reactivity of this mixture was demonstrated during World War II, when it was used to produce alternative hydrocarbon fuels using Fischer Tropsch technology. The synthesis gas mixture was produced then hy gasifying coal. Fischer Tropsch synthesis of hydrocarbons is discussed in Chapter 4. 

Aasberg-Petersen, K., Synthesis gas production for FT synthesis, in Fischer-Tropsch Technology, Chap. 4, Steynberg, A. and Dry, M., Eds., Elsevier, Amsterdam, 2004. 

Steynberg, A. P. 2004. Introduction to Fischer-Tropsch technology. Stud. Surf. Sci. Catal. 152 1-63. 

The primary product from Fischer-Tropsch synthesis is a complex multiphase mixture of hydrocarbons, oxygenates, and water. The composition of this mixture is dependent on the Fischer-Tropsch technology and considerable variation in carbon number distribution, as well as the relative abundance of different compound classes is possible. The primary Fischer-Tropsch product has to be refined to produce final products, and in this respect, it is comparable to crude oil. The primary product from Fischer-Tropsch synthesis can therefore be seen as a synthetic crude oil (syncrude). There are nevertheless significant differences between crude oil and Fischer-Tropsch syncrude, thus requiring a different refining approach.1... 

Despite the limited number of commercial Fischer-Tropsch facilities that have been constructed to date, a wide variety of Fischer-Tropsch technologies have been employed (Table 18.1). 

Within each syncrude type some variation is introduced by the operating conditions of Fischer-Tropsch synthesis, such as pressure and H2 CO ratio, as well as by the Fischer-Tropsch reactor type. These variations cannot be ignored, and ultimately they have an impact on the refinery design. During the subsequent discussion it will become apparent that the selection of the Fischer-Tropsch technology influences not only the refinery design, but also the efficiency with which different products can be produced. 

The advent of cheap oil in the 1950 s, coupled with escalating coal costs, finally diverted interest from the Fischer-Tropsch synthesis as a commercial proposition in western Europe and America. Until recently, the only noncommunist country which has actively pursued Fischer-Tropsch technology has been South Africa. 

Gas-to-liquids (GTL) is the chemical conversion of natural gas into petroleum products. Gas-to-liquid plants use Fischer-Tropsch technology, which first converts natural gas into a synthesis gas, which is then fed into the Fischer-Tropsch reactor in the presence of a catalyst, producing a paraffin wax that is hydro-cracked to products (see also Chapter 7). Distillate is the primary product, ranging from 50% to 70% of the total yield. 

The first step toward making liquid fuels from coal involves the manufacture of synthesis gas (CO and H ) from coal. In 1925, Franz Fischer and Hans Tropsch developed a catalyst that converted CO and at 1 atm and 250 to 300°C into liquid hydrocarbons. By 1941, Fischer-Tropsch plants produced 740 000 tons of petroleum products per year in Germany (Dry, 1999). Fischer-Tropsch technology is based on a complex series of reactions that use to reduce CO to CH groups linked to form long-chain hydrocarbons (Schulz, 1999) ... 

At the end of World War II, Fischer-Tropsch technology was under study in most industrial nations. Coal can be gasified to produce synthesis gas (syngas), which can be converted to paraffinic liquid fuels and chemicals by the Fischer-Tropsch synthesis. Liquid product mainly contains benzene, toluene, xylene (BTX), phenols, alkylphenols and cresol. The low cost and high availability of crude oil, however, led to a decline in interest in liquid fuels made from coal. 

Fischer-Tropsch Synthesis. Fischer-Tropsch synthesis still receives great interest. Special sections are devoted to the topic at regular symposia on natural-gas conversion,487-489 a journal special issues have been published,490 and there are papers on Fischer-Tropsch technologies,490"495 and an analysis of present trends.496 Two reviews about mechanistic studies497 and the kinetics and the selectivities of Fischer-Tropsch synthesis498 are available. A few significant new findings are collected here. 

The Fischer-Tropsch technology produces a wide variety of products which can be narrowed to gasoline, diesel fuel, boiler fuel, distillate oil, and synthetic natural gas. 

Moreover, lignocellulose is not edible and could theoretically be utilized without any impact on food production. The cellulose and hemicellulose fraction of lignocellulose may serve for the production of cellulosic ethanol, which could be produced via acid or enzymatic catalyzed hydrolysis of cellulose, followed by further fermentation to yield ethanol. Alternatively, the whole plant can be gasified to yield syngas, followed by methanol or dimethyl ether synthesis or Fischer-Tropsch technology that produces hydrocarbon fuels. Furthermore, controlled (bio-)chemical transformations to novel fuel compounds based on cellulose, hemicellulose, or lignin are possible, and numerous recent publications emphasize intense research in this direction. 

Fischer-Tropsch Technology FTS can be carried out in several different reactor types fixed bed, fluidized bed, or slurry phase and at different temperatures. The high-temperature Fischer-Tropsch (HTFT) synthesis runs at 320°C-350°C, at which temperatures typically all products are in the gas phase [22], HTFT is operated in fluidized-bed reactors, with iron catalysts. Selectivities correspond to chain-growth probabilities in the range of 0.70-0.75 and are ideal for gasoline production, but olefins and oxygenates are formed as well and are used as chemicals. 

Figure 5.4.5 Overview of Fischer-Tropsch technologies and reactor types used. Capacities are indicated in barrels per day (b/d).

Applications of Fischer-Tropsch technology in the utilization of biomass are explored as well [40-42], Here the challenge is to cope with higher impurity levels of biomass-derived syngas. Nevertheless, demonstration plants have been built, and it is expected that applications will grow in the future. 

Steynberg AP, Dry ME. Fischer-Tropsch technology. Amsterdam Elsevier 2004. (Studies in surface science and catalysis, volume 152.)... 

Geerlings JJC, Wilson JH, Kramer GJ, Kuipers H, Hoek A, Huisman HM. Fischer-Tropsch technology - from active site to commercial process. Appl Catal A Gen. 1999 186(l-2) 27-40. 

Fischer-Tropsch Synthesis, Elsevier, Amsterdam (1999). b. Dry, M E. and Steynberg, A.P. (Eds.), Studies in Surface Science and Catalysis, Vol. 152, Fischer-Tropsch Technology, Elsevier, Amsterdam (2004). 

In Germany, before and during the Second World War, a large amount of ethylene was produced from acetylene derived from coke oven gas. Coke ovens also provided essentially all the aromatics in Germany. Coal was also gasified, giving synthesis gas, a mixture of CO and H2. Products such as ammonia and methanol were catalytically produced from this synthesis gas. Fischer-Tropsch technology was also developed to convert synthesis gas to motor fuels. 

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