Arge reactors

In its present commercial operations Sasol uses two types of reactors. In the fixed bed "low" temperature Arge reactors the gas enters at the top (see Figure 2). The catalyst is packed into the narrow tubes. The FT reaction heat is absorbed by the water surrounding the tubes and steam is generated. The desired reactor temperature is maintained by controlling the steam pressure above the water jacket. The catalyst formulation and the reactor process conditions are set for the maximum production of high quality paraffinic waxes. Only the Sasol One plant utilizes these reactors. 

Sasol recently decided to replace the five old ARGE reactors with two new slurry reactors and hence the production will be increased further. 

The slurry phase reactor provides much better and more flexible temperature control compared to the Arge reactor. It can operate at higher temperatures and with more active catalyst without formation of coke or catalyst breakup. The key operating issues are catalyst/liquid separation and catalyst attrition. Sasol has apparently resolved these design issues successfully. 

At the time Ruhrchemie made some minor improvements by using concentric tubes in the medium-pressure reactors, with catalyst in the annular space and coohng water flowing around the tube and through the inner space. This was still inefficient at low gas space velocity. The ARGE reactors used by Sasol in 1955 were conventional boihng water tubular reactors with gas recycle to limit heat evolutiom A typical wartime reactor contained 1250 tubes, whereas the early Sasol reactor used more than 2000 tubes. 

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. 

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. 

L. Seglin Why has Lurgi selected the hot gas recycle process for methanation rather than the isothermal reactor (ARGE) design which they used for the Fischer-Tropsch plant in SASOL s plant in South Africa ... 

The selection of a fixed bed Co-LTFT process supported the objective to apply the SMDS process for beneficiation of remote gas fields. The Co-LTFT catalyst has a useful lifetime of 5 years and the robustness of fixed bed reactor technology has been proven. For example, the fixed bed Arge Fe-LTFT process has now been in operation for more than 50 years at Sasol 1. 

SSPD [Sasol slurry phase distillate] A process for converting natural gas to diesel fuel, kerosene, and naphtha. Operated by Sasol in South Africa since 1993. Three stages are involved. In the first, natural gas is converted to synthesis gas by reforming. In the second, the synthesis gas is converted to waxy hydrocarbons in a slurry-phase reactor. In the third, the waxes are upgraded to middle distillates. See also Arge. 

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 %. 

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. 

In both models the only model parameter used is the mean residence time tpp and tpsR F gtire 6 shows the reactor dynamics of the PFR and the PSR in the normalised time and frequency domain (dimensionless time 0 = t/T, dimensionless frequency fg = l/2jt9). In the time domain the step response F(9) as well as the impulse response E(0) (which is the RTD) can be discussed. This type of data presentation is normally used in chemical engineering application. But the same data can also be presented in the frequency domain, the so called Bode plot. This type of presentation allows to identify effects which are not visible in the classical used plot in the time domain. The Bode plot consists of the magnitude Gj 030)1 and the phase arg G(jO)0). ... 

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... 

Figure 7-10. Synthesis of Tyr-Arg by CPD-Y and PA selectivity and yield as a function of conversion for continuous operation in an enzyme membrane reactor 54 ...

Kellogg fluid-bed units, which also use gas obtained from refonning the Cl and Cg hydrocarbons produced in the Arge section. There -are five fixed-bed reactors, each with a heat exchanger, cooler, and reqrcle blower. The reactors are almost 10 ft in diameter and approximately 40 ft in height. 

Much of the product of the Arge units is high-bcnling hydrocarbons, while the product from the fluidized reactors is chiefly gasoline. The production of refinery and chemical products is listed in Table 11-6. The research octane niunber of the gasoline is about 85, and the diesel oil has a pour point of —5°C, a flame point of 82°C, and a cetane number of 90. The distribution of the products can be shifted to some extent in accordance with market demand. 

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