Synthol-process

Sasol produces synthetic fuels and chemicals from coal-derived synthesis gas. Two significant variations of this technology have been commercialized, and new process variations are continually under development. Sasol One used both the fixed-bed (Arge) process, operated at about 240°C, as weU as a circulating fluidized-bed (Synthol) system operating at 340°C. Each ET reactor type has a characteristic product distribution that includes coproducts isolated for use in the chemical industry. Paraffin wax is one of the principal coproducts of the low temperature Arge process. Alcohols, ketones, and lower paraffins are among the valuable coproducts obtained from the Synthol process. 

A number of chemical products are derived from Sasol s synthetic fuel operations based on the Fischer-Tropsch synthesis including paraffin waxes from the Arge process and several polar and nonpolar hydrocarbon mixtures from the Synthol process. Products suitable for use as hot melt adhesives, PVC lubricants, cormgated cardboard coating emulsions, and poHshes have been developed from Arge waxes. Wax blends containing medium and hard wax fractions are useful for making candles, and over 20,000 t/yr of wax are sold for this appHcation. 

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). 

The use of a fluidized-bed reactor is possible only when the reactants are essentiaUy in the gaseous phase. Eluidized-beds are not suitable for middle distiUate synthesis, where a heavy wax is formed. Eor gasoline synthesis processes like the MobU MTG process and the Synthol process, such reactors are especiaUy suitable when frequent or continuous regeneration of the catalyst is required. Slurry reactors and ebuUiating-bed reactors comprising a three-phase system with very fine catalyst are, in principle, suitable for middle distiUate and wax synthesis, but have not been appHed on a commercial scale. 

Fischer-Tropsch A process for converting synthesis gas (a mixture of carbon monoxide and hydrogen) to liquid fuels. Modified versions were known as the Synol and Synthol processes. The process is operated under pressure at 200 to 350°C, over a catalyst. Several different catalyst systems have been used at different periods, notably iron-zinc oxide, nickel-thoria on kieselgtihr, cobalt-thoria on kieselgiihr, and cemented iron oxide. The main products are C5-Cn aliphatic hydrocarbons the aromatics content can be varied by varying the process conditions. The basic reaction was discovered in 1923 by F. Fischer and... 

As can be seen in Table III the Synthol process produces a large amount of C to C- hydrocarbons which are predominantly olefins. At Sasol tne C stream from the cryogenic separation unit is fed to a standard ethylene plant. The feed is first dried and then fractionated to remove the small amounts of C-. and C products. The C stream is then selectively hydrogenated to remove acetylene... 

A feature of the FT process is that the primary hydrocarbon products are linear alpha olefins. Since chain branching is a temperature sensitive secondary reaction, a higher yield of linear olefins is obtained when the FT process is operated at lower temperatures (eg see the data in Table III comparing the low temperature Arge with the high temperature Synthol process). 

Hydrogenation and dehydrogenation employ catalysts that form unstable surface hydrides. Transition-group and bordering metals such as Ni, Fe, Co, and Pt are suitable, as well as transition group oxides or sulfides. This class of reactions includes the important examples of ammonia and methanol syntheses, the Fischer-Tropsch and oxo and synthol processes and the production of alcohols, aldehydes, ketones, amines, and edible oils. 

Results for the Synthol entrained-bed process (16) are plotted in Figure 8. The available C to C15 data follow the conventional Flory plot with a equal to 0.7. The Synthol process uses a fused Fe catalyst of low surface area and porosity and operates at high temperatures ( 590K). The products in the reactor are mainly gaseous, wax formation is minimal, and the pellet pore structure remains free of liquid products therefore, diffusion-enhanced a-olefin readsorption is much less likely than in the ARGE process. Whereas the product selectivity in the ARGE process is altered by diffusion-enhanced a-olefin readsorption, that in the Synthol process is not. 

Figure 8 Flory plot of SASOL s Synthol process on Fe-based catalysts. Data from Ref. (17). (See legend for Figure 1).

In 1980, Sasol introduced a second generation of its fluidized-bed F-T technology (the Synthol process), and in 1989 a third generation. Each new generation has utilized improved engineering design and has enhanced the utilization of the chemicals... 

The Fischer-Tropsch process has several variants one of which (a high temperature process) produces significant volumes of olefins. This particular variant, known as the Synthol Process, is used to produce fuels from both coal and natural gas in South Africa. A recent development of this process (The Sasol High Temperature Process ) has replaced the original entrained-bed reactors with fluid-bed reactors. The product breakdown is shown in Table 11.3. 

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]. 

Comparative tests carried out by Dry at Sasol with the three types of reactors gave the best results for the fixed-bed reactor, where wax is the desired product, and for the fluidized-bed reactor where lighter hydrocarbons are wanted [15]. The same trend can be drawm from a comparison of the two commercial processes at Sasol I. As can be seen from Table I. the product distributions obtained reflect the different reaction conditions (4. The ARCE process operating at 220- 240 C yields mainly higlier products (>5055 C 2+) iih low olefin content. The Kellogg/Synthol process w ith reaction temperatures of 320 -330 C yields mainly lighter hydrocarbons 95% C] C ) with a high olefin content. 

Sasol advanced synthol process—high-temperature process (approximately 340°C) that uses syngas to produce gasoline and olefins. 

Fischer-Tropsch synthesis. (Synthol process Oxo synthesis). Synthesis of hydrocarbons, aliphatic alcohols, aldehydes, and ketones by the catalytic hydrogenation of carbon monoxide using enriched synthesis gas from passage of steam over heated coke. The ratio of products varies with conditions. The high-pressure Synthol process gives mainly oxygenated products and addition of olefins in the presence of cobalt catalyst (Oxo synthesis) produces aldehydes. Normal-pressure synthesis leads mainly to petroleum-like hydrocarbons. 

Fischer-Tropsch reactions convert synthesis gas into hydrocarbons. The fluidized bed process for this reaction, known as the Synthol process, was developed... 

Storch (7) stressed the preferential formation of alcohols in the synthol process. Here conversion is low and most of the gas is recycled. Since this must have as a consequence that the H2/H2O ratio of the gas is rather low, it would be consistent with the view that oxidic patches are formed on the iron surface and hence, in agreement with Eidus view, the formation of alcohols by terminating the growing chain with oxymethylene groups will be an indirect result of oxidizing the catalyst surface. 

In the high temperature Synthol process as currently operated, about 5% of the reacted carbon ends up as light alcohols and ketones. Extraction of these compounds from the reaction water and their subsequent refining is an expensive process but nevertheless well worthwhile.. The ketones (mainly acetone and MEK) fetch high prices. Those alcohols which are not sold as such are added to the gasoline pool to boost its octane value and so lower the lead requirements. 

One of the key factors controlling the overall product spectrum is the "basicity" of the catalyst. This depends not only on the amount and type of alkali promoter present but also on its dispersion and how it has interacted with other promoters and impurities present (ref. 2). In the case of the fluidized-bed Synthol process the CH selectivity has been progressively lowered over the years from 15 to the current 1%. As the market for fuel gas is limited in South Africa, the excess CH must be catalytically reformed back to CO and (ref. 2). Not only is the production of CH wasteful in that it consumes more and CO than is needed for the formation of olefins but the reforming process itself is inefficient. Cutting back on the CH selectivity has therefore greatly benefited the overall economics of the Synthol FT process. 

The oldest information on the subject is in early patents of BASF on alkahzed Co catalysts, and the patents by Fischer and Tropsch on the so called synthol process. More recently, several papers by the Bureau of Mines Laboratory S have demonstrated that an interesting amount of oxygenates can be produced with standard alkali-promoted Fe catalysts, when proper reaction conditions are chosen and when the catalyst is properly run in . 

Sasol I operates 3 reactors in parallel. The mean catalyst lifetime is 42 days. On the average/ 2.4 reactors are steadily under operation. Each reactor produces 60/000 tns of primary products per year. The product distribution is given in Table 1 and compared with that of the fixed bed process. The Synthol process reveals a maximum in gasoline fuel while the main products of the fixed bed process are higher hydrocarbons like diesel/ heavy oils and wax. 

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