Sasol Synthol reactor

Sasol uses both fixed-bed reactors and transported fluidized-bed reactors to convert synthesis gas to hydrocarbons. The multitubular, water-cooled fixed-bed reactors were designed by Lurgi and Ruhrchemie, whereas the newer fluidized-bed reactors scaled up from a pilot unit by Kellogg are now known as Sasol Synthol reactors. The two reactor types use different iron-based catalysts and give different product distributions. 

Change occurred in high-temperature Fischer-Tropsch reactor technology. The circulating fluidized bed Sasol Synthol reactors were replaced by fixed fluidized bed Sasol Advanced Synthol (SAS) reactors.44 This did not meaningfully affect the Fe-HTFT syncrude composition, but it reduced the operating cost of HTFT synthesis. 

The fixed bed process worked well. It was not easy, however, to circulate the fluidized dense iron catalyst through the reactor and back through the separator without further development. Good operation was eventually made possible and the process was successful in the Sasol Synthol reactor. Typical product distributions for the two processes are shown in Table 2.16. 

Sasol Fischer-Tropsch Process. 1-Propanol is one of the products from Sasol s Fischer-Tropsch process (7). Coal (qv) is gasified ia Lurgi reactors to produce synthesis gas (H2/CO). After separation from gas Hquids and purification, the synthesis gas is fed iato the Sasol Synthol plant where it is entrained with a powdered iron-based catalyst within the fluid-bed reactors. The exothermic Fischer-Tropsch reaction produces a mixture of hydrocarbons (qv) and oxygenates. The condensation products from the process consist of hydrocarbon Hquids and an aqueous stream that contains a mixture of ketones (qv) and alcohols. The ketones and alcohols are recovered and most of the alcohols are used for the blending of high octane gasoline. Some of the alcohol streams are further purified by distillation to yield pure 1-propanol and ethanol ia a multiunit plant, which has a total capacity of 25,000-30,000 t/yr (see Coal conversion processes, gasification). 

Status of Indirect Liquefaction Technology The only commercial indirect coal liquefaction plants for the production of transportation fuels are operated by SASOL in South Africa. Construction of the original plant was begun in 1950, and operations began in 1955. This plant employs both fixed-bed (Arge) and entrained-bed (Synthol) reactors. Two additional plants were later constructed with start-ups in 1980 and 1983. These latter plants employ dry-ash Lurgi Mark IV coal gasifiers and entrained-bed (Synthol) reactors for synthesis gas conversion. These plants currently produce 45 percent of South Africa s transportation fuel requirements, and, in addition, they produce more than 120 other products from coal. 

SASOL has pursued the development of alternative reactors to overcome specific operational difficulties encountered with the fixed-bed and entrained-bed reactors. After several years of attempts to overcome the high catalyst circulation rates and consequent abrasion in the Synthol reactors, a bubbling fluidized-bed reactor 1 m (3.3 ft) in diameter was constructed in 1983. Following successflil testing, SASOL designed and construc ted a full-scale commercial reac tor 5 m (16.4 ft) in diameter. The reactor was successfully commissioned in 1989 and remains in operation. 

At the big new plants (Sasol Two and Three) only the Synthol reactors are used. Per unit cross-section of the reactors the Synthol reactor has a much higher gas throughput than the Arge... 

The lelectivities t pical y obtained in the fixed bed Arge and fluidized bed Synthol reactors as currently operated by Sasol are shown in Table III. If desired the selectivities can be varied over wide ranges. Thus in Arge the "hard wax" (the material boiling above 500 °C) can be varied from zero to over 50 % while in the Synthol reactors the CH selectivity can be varied from 5 to 80 %. 

Typical equipment Sasol Synthol CFB Sasol Advanced Synthol (SAS) Sasol Arge Shell Middle Distillate Synthesis (SMDS) Sasol Slurry Bed Reactor (SSBR) Exxon mobile demonstration Conoco demonstration... 

The Secunda process scheme was conceived to maximize gasoline production - therefore, it includes hydroprocessing and catalytic reformers similar to those used in petroleum refineries. Due to the scale of operation, it includes facilities for the recovered of ethylene, alcohols, ketones, phenols, ammonia and other chemical products. Its twin plant at the same location, Sasol 3, has a very similar configuration. At present all the original Synthol reactors have been replaced by the more efficient Sasol Advanced Synthol (SAS) reactors, with capacities of up to 20 000 bpd per train. 

The new Sasol II plant is intended to produce mainly gasoline and is therefore based entirely on Synthol technology. The Sasol II reactors have each a capacity of 2 1/2 times that of Sasol I. A simplified process scheme is shown in Fig. 4/ and Table 2 summarizes the production figures of Sasol II (32). The productivity of Sasol II is more than twice as large as that of all plants operated in Germany in 1944 and even considerably larger than the world FT productivity at that time. How-... 

FFB Reactors. The two 5-m i.d. FT reactors in the Brownsville, Texas, plant were FFB units. They, initially, were plagued by low conversion attributed to poor catalyst fluidization. These problems were apparently overcome, but the plant was shut down in the mid 1950s. It took more than 30 years before this type of reactor was again used commercially. Improved versions of the FFB reactors were developed by Sasol R D (see the section The Order of Development of FT Reactors at Sasol R D ) and installed at the Secunda plant. Over the period 1995-1999, 16 second-generation CFB reactors were replaced by 8 FFB reactors, 4 of 8-m i.d. with capacities of 0.47 Mt per year each and 4 of 10.7-m i.d. each with a capacity of 0.85 Mt per year. This increased the Secunda plant s capacity from about 5.1 Mt to about 7.5 Mt per year. These units were named SAS (Sasol Advanced Synthol) reactors. 

Figure 1.16 High-temperature Fischer-Tropsch synthesis reactors, (a) Sasol Synthol circulating fluidized-bed reactor.

Figure 6 Sasol Synthol and FFB reactors. (From Jones, 1991.)...

Following the successful development of the Synthol process, Sasol went ahead and built two larger plants at Secunda, Sasol 2 and Sasol 3, which were based on coal. A further plant using the Synthol reactor but with natural gas as feed was built at Mossul Bay by Mossgas. 

Sasol produces synthesis gas from coal by partial oxidation or from natural gas by steam reforming. The first version of the Synthol process was upgraded to use the advanced Synthol reactor. Both Synthol processes use fluid catalyst beds. A new SSDP has now been introduced and should soon be operating. Distillates and waxes can be produced (25 atm 240°C). 

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. 

Recent advances in Eischer-Tropsch technology at Sasol include the demonstration of the slurry-bed Eischer-Tropsch process and the new generation Sasol Advanced Synthol (SAS) Reactor, which is a classical fluidized-bed reactor design. The slurry-bed reactor is considered a superior alternative to the Arge tubular fixed-bed reactor. Commercial implementation of a slurry-bed design requires development of efficient catalyst separation techniques. Sasol has developed proprietary technology that provides satisfactory separation of wax and soHd catalyst, and a commercial-scale reactor is being commissioned in the first half of 1993. 

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