Synthol unit

Table 1. Product Selectivities for Commercial Fixed-Bed and Synthol Units ...

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. 

The purified raw gas goes to a Synthol (Eischer-Tropsch) unit for catalytic conversion of CO and H2 to Hquid fuels. The tars and oils obtained from quenching the raw gas from the gasifiers go to a Phenosolvan plant to provide tar products for the refinery and ammonia for fertilizer. The Synthol plant has seven reactors, each with 1.9 x 10 m /h (1.6 x 10 ft /d) gas feed. Annual plant production is 1.5 x 10 t motor fuels, 185 x 10 t ethylene,... 

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. 

SASOL [Suid-Afrikaans Sintetiese Olie] Not a process but a large coal gasification complex in South Africa, operated by the South African Oil and Gas Corporation. It first operated in 1955 but took several years to be fully commissioned. A Lurgi fixed-bed gasification unit is used for the primary process. Downstream processes include the following ones, described under their respective names Aige, Fischer-Tropsch, Rectisol, Sulfohn, Synthol. 

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... 

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... 

On September 18,1948, W. E. Hanford assumed the direction of Kellogg s Synthol activities. Most operations of the 4-inch pilot plant during 1948 appear to have been at 5 ft/s velocity in the reaction zone and at a nominal pressure of 250 psig. In August, the unit operated for a while at 350 psig and at 4 to 4.5 ft/s (density in reaction zone was 25 lbs/ft3 in standpipe, 51 lbs/ft3). Heat transfer coefficients approached 70 Btu/hr-ft2-°F in the upper cooler and 62 in the lower cooler. Here, upper and lower refer to the two oil-jacketed sections of 4-inch pipe. The pilot unit was shut down on November 12 (permanently, as near as one can tell from Lobo, 1929-1955). 

In August 1948, Kellogg s Engineering Department undertook a design and cost study for a 6,000-barrel-per-day Synthol plant to be erected on the United States Gulf Coast. A memorandum by Johnson provided the design basis and compared this with results obtained from the 4-inch pilot plant (see table). 

The purified gas is fed into the Synthol and fixed-bed reactors. The products from the reactors are cooied and separated in a water phase, oil phase and tail gas. The + Ca olefinic products from the tail gas are separated in an oil absorption tower and oligomerized over an acidic catalyst to gasoline. Tite remaining tali gas can be treated in a cryogenic unit to provide methane and hydrogen, which is partly used as fuel gas or feedstock for ammonia synthesis. The remainder is steam-reformed over nickel catalysts to give CO/H3. 

Mossgas) unit, also in South Africa, with close to 35,000 bpsd capacity, and the Shell unit at Bintulu in Malaysia originally designed for 12,500 bpsd, but since revamped to about 15,000-16,000 bpsd. The PetroSA unit uses Synthol high-temperature reactors licensed from Sasol, while the Shell unit uses fixed-bed tubular reactors (Fig. 11). " ... 

There are processes in which the total amount of catalyst is entrained by the gas. The reactors then belong to the category of transport reactors. Examples are some of the present Fischer-Tropsch reactors for the production of hydrocarbons from synthesis gas and the modern catalytic cracking units. Fig 10.11 shows the Synthol circulating solids reactor. In the dilute side of the circuit, reactant gases carry suspended catalyst upward, and the fluidized bed and stand-pipe on the other side of the circuit provide the driving force for the smooth circulation of the solid catalyst. For the removal of heat, heat exchangers are positioned in the reactor. 

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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