SASOL plants

In 1991, the relatively old and small synthetic fuel production faciHties at Sasol One began a transformation to a higher value chemical production facihty (38). This move came as a result of declining economics for synthetic fuel production from synthesis gas at this location. The new faciHties installed in this conversion will expand production of high value Arge waxes and paraffins to 123,000 t/yr in 1993. Also, a new faciHty for production of 240,00 t/yr of ammonia will be added. The complex will continue to produce ethylene and process feedstock from other Sasol plants to produce alcohols and higher phenols. 

Imperial Chemical Industries (ICI) operated a coal hydrogenation plant at a pressure of 20 MPa (2900 psi) and a temperature of 400—500°C to produce Hquid hydrocarbon fuel from 1935 to the outbreak of World War II. As many as 12 such plants operated in Germany during World War II to make the country less dependent on petroleum from natural sources but the process was discontinued when hostihties ceased (see Coal conversion PROCESSES,liquefaction). Currentiy the Fisher-Tropsch process is being used at the Sasol plants in South Africa to convert synthesis gas into largely ahphatic hydrocarbons at 10—20 MPa and about 400°C to supply 70% of the fuel needed for transportation. 

The first commercial operation of the Lurgi process was in Germany in 1936 using brown coal. The reactor was modified to stir the coal bed to permit utilization of bituminous coal. One plant was built at the Dorsten Works of Steinkohlengas AG, and the Sasol plants were built in South Africa to provide synthesis gas for Hquid fuels. 

Conventional Transportation Fuels. Synthesis gas produced from coal gasification or from natural gas by partial oxidation or steam reforming can be converted into a variety of transportation fuels, such as gasoline, aviation turbine fuel (see Aviation and other gas turbine fuels), and diesel fuel. A widely known process used for this appHcation is the Eischer-Tropsch process which converts synthesis gas into largely aHphatic hydrocarbons over an iron or cobalt catalyst. The process was operated successfully in Germany during World War II and is being used commercially at the Sasol plants in South Africa. 

Consequently, two semicommercial pilot plants have been operated for 1.5 years. One plant, designed and erected by Lurgi and South African Coal, Oil, and Gas Corp. (SASOL), Sasolburg, South Africa, was operated as a sidestream plant to a commercial Fischer-Tropsch synthesis plant. Synthesis gas is produced in a commercial coal pressure gasification plant which includes Rectisol gas purification and shift conversion so the overall process scheme for producing SNG from coal could be demonstrated successfully. The other plant, a joint effort of Lurgi and El Paso Natural Gas Corp., was operated at the same time at Petrochemie Schwechat, near Vienna, Austria. Since the starting material was synthesis gas produced from naphtha, different reaction conditions from those of the SASOL plant have also been operated successfully. 

The findings from two long term test runs in the SASOL plant relevant to catalyst life under design conditions in a commercial methane synthesis plant have already been published (3). This paper reports further test results from both demonstration units concerning the effect of certain reaction parameters which are the basis for flexibility and operability of the Lurgi methanation scheme. 

Finally, it can be stated that variation in the H2/CO ratio will not affect operability of an SNG plant using a recycle system for methana-tion as demonstrated in the SASOL plant. 

The SASOL plant was operated with a surplus of C02 during a long term test of 4000 hrs. Of the C02 in the synthesis gas, 33.4% was metha-nated while the remaining 66.6% left the reaction system unconverted. Product gas from final methanation yielded specification grade SNG containing residual hydrogen of 0.7 vol % and residual CO of less than 0.1 vol %. The heating value was 973 Btu/standard cubic foot (scf) after C02 removal to 0.5 vol % (calc.). 

Catalyst Poisons. It is well known that sulfur, chlorine, etc. are strong poisons for nickel catalyst. Chlorine was not detectable in the synthesis gas downstream of the Rectisol in the SASOL plant. The total sulfur content of this gas—in the form of H2S, COS, and organic sulfur components—averaged 0.08 mg/m3 with maximum values of 0.2 mg total sulfur/m3. 

The effect of sulfur contamination on catalyst activity was examined in a special test run in the SASOL plant. Conversion in 6.3 and 23.8% of the total catalyst bed, taken as an indirect criterion of catalyst activity, is plotted vs. operating time in Figure 3. 

The long term tests in the SASOL plant as well as in the Schwechat plant were run with outlet temperatures of 450°C, but both plants were also operated with higher loads that caused reactor outlet temperatures of 470°C or even higher. In comparison with the test run at 450°C, only a slight increase in deactivation rate was detectable which demonstrates the thermostability of the catalyst. From the aspect of thermostability, outlet temperatures of 450°-470°C are acceptable. Further considerations including the possibility of overload operation, the SNG specification to be achieved in final methanation, end-of-run conditions, and cost of reactor material will affect the selection of optimum outlet temperature. 

Synthol A version of the Fischer-Tropsch process, for making liquid fuels and organic chemicals from syngas. Developed by Pullman Kellogg between 1940 and 1960. First operated at the SASOL plant in South Africa in 1955. The name was used also for the product from the original Fischer-Tropsch process, developed in the 1920s. See also Synol. Hydrocarbon Process., 1963, 42(11), 225. 

The world s largest concentration of Lurgi gasifiers is in South Africa, where Sasol operates three major complexes. The Sasol plants (Sasol I, II, and III) located in Seconda and Sasolburg gasify approximately 30 million ton/year of bituminous coal to synthesis gas, which is converted to fuels and chemicals via the Fisher-Tropsch process. It was recently announced (International Coal... 

The first Sasol plant (Sasol One) came on stream in 1955 and is still in production. The profitability of this operation initially was low because the price of crude oil remained depressed for many years due to the discovery and exploitation of the huge oil deposits in the Middle East. After 1973, however, the price of crude oil rose rapidly and consequently the profitability of the Fischer-Tropsch process in South Africa improved dramatically. This lead to the construction of two much bigger plants (Sasol Two and Sasol Three) which came on stream in 1980 and 1982 respectively. 

In the newer Sasol plants the tailgas from the FT reactors is first water-washed (to extract the remnants of water-soluble oxygenated compounds), then treated in a Benfield unit (to remove all CO ) and then fed to a cryogenic unit where the gas is separated into four streams, a CH -rich, a H -rich, a C and a C /C stream. 

M. . Dry The Fischer-Tropsch Synthesis The fourth chapter concentrates mainly on the development of the Fischer-Tropsch process from the late 1950 s to 1979. During this period the Sasol plant was the only Fischer-Tropsch process in operation and hence a large part of this review deals with the information generated at Sasol. The various typesofreactors are comparedand discussed (198 references). 

Both the ARGE and Kellogg processes were commercialized in 1955 at the Sasol plant in South Africa with a capacity of 240000 t/a hydrocarbons. While the fixed-bed process proved reliable from the beginning, numerous modifications had to be made to the Kellogg process. The major difficulties encountered were concerned with the fluidization of the catalyst. This led to the development of the Sasol Synthol process which is now a highly reliable large-scale industrial operation [15]. 

Packed Bed. The packed-bed reactor used the Sasol plant to cany out Fis-cher-Tropsch syndiesis reaction is shown in Figure El-5.3. Synthesis gas is fed at a rate of 30,000 trf/h (STP) at 240°C twd 27 atm. to the packed-bed reactor. The reactor contains 2050 tubes, each of which is 5.0 cm in diameter and 12 m in length. The iron-based catalyst that fills these tubes usually contains KjO and St02 and has a specific area on the order of 200 in-/g. The reaction products are light hydrocarbons along with a wax that is used in candles and printing inks. Approxirn ly 50% conversion of the reactant is,achieved in the reactor. 

These processes for obtaining synthetic fuels were used by a number of countries during World War II. They are, however, uneconomical in most cases, because hydrogen and carbon monoxide in sufficient quantities must be obtained from coal or petroleum sources. Currently, South Africa, which has large coal reserves, makes the greatest use of Fischer-Tropsch reactions in the synthesis of fuels in its Sasol plants. 

---