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. 

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. 

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. 

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 %. 

The principal reactors used are fluidized bed reactors, called Synthol reactors, in which the feed gas entrains an iron catalyst powder in a circulating flow. The suspension enters the bottom of the fluidized bed reaction section, where the Fischer-Tropsch and the gas shift reactions proceed at a temperature of from 315 to 330°C. These reactions are highly exothermic, as described previously, and the large quantity of heat released must be removed. The products in gaseous form together with the catalyst are taken off from the top of the reactor. By decreasing the gas velocity in another section, the catalyst settles out and is returned for reuse. The product gases are then condensed to the liquid products. 

The Fischer-Tropsch synthesis of hydrocarbons is used on a large scale for fuel production in South Africa [78, 79]. Synthesis gas generated from coal in Lurgi fixed-bed gasifiers enters the Synthol reactor (Fig 18), where it is reacted over an iron catalyst at 340°C. The reactor works on the principle of the circulating fluidized bed. The mean porosity in the riser is 85%, and the gas velocity varies between 3 and 12ms1 [2]. Reaction heat is removed by way of heat-exchanger tube bundles placed inside the riser. 

Figure 18. Fischer Tropsch synthesis in the Synthol reactor [2, 78] (a) hopper, (b) standpipe, (c) riser, (d) cooler (coil), (e) reactor, (f) gooseneck...

The SasoJ II plant ua S only Synthol reactors and the processing of the hydrocarbon products is somewhat different. It comprises catalytic reforming over Pt/Al2 03, hydrofining, isomerizaiion and selective hydrodewaxing. From the olefins only ethylene is recovered as such and the olefins are oligomerized... 

Duvenhage, D.J. Shingles, T. Synthol reactor technology development. Catal. Today 2002, 71, 301-305. 

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. 

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. 

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