ARGE catalyst

The results presented in Figures 6.11.17-6.11.21 were calculated with the commercial program Presto-Kinetics (solver for differential equations, www.cit-wulkow. de). The maximum temperature of the ARGE catalyst is about 260 °C as the Fe catalyst then starts to deactivate by sintering, which substantially lowers the internal surface area (Kuntze, 1991). Hence, to be on the safe side, 250 °C was chosen as the maximum allowable temperature. 

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. 

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. 

Like aspartic peptidases, metallopeptidases act by activating a H20 molecule, and they do not form a covalent intermediate with the substrate. Here, the activation of a H20 molecule is mediated by a residue that acts as general base (e.g., Glu, His, Lys, Arg, or Tyr), with a divalent cation (usually Zn2+ but sometimes Co2+ or Mn2+) perhaps also contributing. The major role of the metal cation, however, is to act as an electrophilic catalyst by coordinating the carbonyl (or phosphoryl) O-atom in the substrate and orienting the latter for nucleophilic attack by the HO ion generated from H20 by the general base. 

Cyclo(His-D-Leu) acts as a hydrolytic catalyst. Cyclo(Leu-Gly) blocks the development of (1) physical dependence on morphine, (2) tolerance to the pharmacological effects of /3-endorphin, (3) tolerance to haloperidol-induced catalepsy and hypothermia, and (4) dopaminergic supersensitivity after chronic morphine administration. Cyclo(Tyr-Arg), a synthetic analogue of kyortorphin (an endogenous analgesic peptide), and its A-methyl tyrosine derivatives are more potent than kyotorphin in the mouse tail pressure test. ... 

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. 

The use of suspended substrates proved very useful in the production of arginine-based surfactants. In this two-step procedure, one amino group of a diami-noalkane was acylated with Z-Arg [55]. This step was done in a melted phase constituted by substrates, and quantitative yields were observed without a catalyst present. Subsequently, the second free amino group of the diaminoalkane reacted with an Z.-Arg-alkyl ester (kinetic control). This step was performed in aqueous suspension or organic solvent using trypsin as the catalyst [73]. 

The active site with its catalytic groups is in an extended cleft between the two domains of a subunit. Interestingly, the 6-phosphate lies between two subunits, bound by His-249 and Arg-252 from its own subunit and by Arg-162 and Arg-243 from the neighbor. (The 1-hydroxyl is in a suitable position for nucleophilic attack on the y-phosphoryl of ATP. The carboxylate of Asp-127 is in a likely position to act as a general-base catalyst.)... 

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. 

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]. 

In the ARGE fixed bed process Idgh boiling fractions and waxes are mainly obtained [4J, Under the same curidiliuns, sintered iron catalysts yield mainly hydrocarbons in the gasoline and diesel range. This can be explained by their lower specific surface area, pore volume and specific activity. Of the three catalyst types, (he fused iron catalyst Is characterized by the lowest specific surface area, pore volume and activity and is thus operated at higher temperatures. 

Z-Arg(arylsulfonyl)-OH DOHA (20 mmol) was partitioned between EtOAc and 5% citric acid and, after washing with H2O, the organic layer was dried (Na2S04) and concentrated. The residue was hydrogenated over Pd/C in MeOH. The catalyst was removed by filtration and the filtrate was concentrated. The product was precipitated with Et20 and in some cases recrystallized (H2O) yield 80-95%. 

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