Fischer-Tropsch process high-temperature

Commercial Fischer-Tropsch processes have been based exclusively on gas-particle operations, mainly in fixed beds (P2). The chemical reactions are highly exothermic, however, and accurate temperature control is therefore difficult to achieve in a fixed bed. Good temperature control is important because of the temperature sensitivity of the chemical reactions taking place, and several attempts have therefore been made to develop processes based on other types of operation. 

The reactions are catalyzed by transition metals (cobalt, iron, and ruthenium) on high-surface-area silica, alumina, or zeolite supports. However, the exact chemical identity of the catalysts is unknown, and their characterization presents challenges as these transformations are carried out under very harsh reaction conditions. Typically, the Fischer-Tropsch process is operated in the temperature range of 150°C-300°C and in the pressure range of one to several tens of atmospheres [66], Thus, the entire process is costly and inefficient and even produces waste [67]. Hence, development of more economical and sustainable strategies for the gas-to-liquid conversion of methane is highly desirable. 

The Fischer-Tropsch process has several variants one of which (a high temperature process) produces significant volumes of olefins. This particular variant, known as the Synthol Process, is used to produce fuels from both coal and natural gas in South Africa. A recent development of this process (The Sasol High Temperature Process ) has replaced the original entrained-bed reactors with fluid-bed reactors. The product breakdown is shown in Table 11.3. 

This process will allow the recycling of solid waste to produce a useful product. High pressure and temperature combined with hydrogen can convert most types of domestic and industrial wastes back into products that are currently obtained from fossil coal and oil. No volatile polluting chemicals will be vented into the atmosphere. The metals can be recovered for further use and the ceramic materials will be converted into a product difficult to distinguish from natural rocks. This type of process will not solve all the solid waste disposal problems, but will provide a potential method for recovery of valuable products from waste. When implemented, it will dramatically reduce the amount of solid waste placed in landfills. This process also has the potential to reduce the amount of oil and coal mined to provide the carbon compounds needed to manufacture all petrochemical derived materials. This waste reduction process is a variation on the Fischer-Tropsch process, mentioned on page 101, in use commercially to produce hydrocarbon materials from coal. 

Statoil has been involved in Fischer-Tropsch based GTL technology development since the mid 1980 s (Rytter et al.,1990). In order to maximize distillate production, a low temperature, cobalt catalyst based Fischer-Tropsch technology has been selected. A slurry bubble column reactor offers the best performance in terms of economy of scale, throughput and yield, but presents several technical challenges. A highly active and selective cobalt catalyst is needed and must be adapted to suit the requirements of the slurry reactor. Separation of wax from the slurry is another critical aspect of this technology. Statoil has developed a supported cobalt catalyst and a continuous filtration technique that forms the heart of the Fischer-Tropsch process. 

Fig. 5. Relation between the selectivities of the hydrocarbon cuts for the high temperature Fischer-Tropsch process. The selectivities are on a C atom % basis.

Another synthesis apphcation of syngas is the already mentioned Fischer-Tropsch process. This process combines CO and H2 at high temperatures at varying ratios to make hydrocarbons, which are gasoline-like compounds ... 

It is possible to produce oil from coal either by direct hydrogenation at high temperatures and high pressures or by the syngas route followed by the Fischer-Tropsch process. Though these processes can make an oil which is more expensive than wellhead crude, in exceptional strategic circumstances (war, oil embargo, etc.), it has been produced and used as a viable substitute. 

Generally, the Fischer—Tropsch process is operated in the temperature range of 150—300°C to avoid high methane byproduct formation. Increased pressure leads to higher conversion rates and also favors formation of desired long-chain alkanes. Typical pressures are in the range of one to several tens of atmospheres. The FT hydrogenation reaction is catalyzed mainly by Fe and Co catalysts, while the size and... 

The Fischer-Tropsch reaction is essentially that of Eq. XVIII-54 and is of great importance partly by itself and also as part of a coupled set of processes whereby steam or oxygen plus coal or coke is transformed into methane, olefins, alcohols, and gasolines. The first step is to produce a mixture of CO and H2 (called water-gas or synthesis gas ) by the high-temperature treatment of coal or coke with steam. The water-gas shift reaction CO + H2O = CO2 + H2 is then used to adjust the CO/H2 ratio for the feed to the Fischer-Tropsch or synthesis reactor. This last process was disclosed in 1913 and was extensively developed around 1925 by Fischer and Tropsch [268]. 

The induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

The most difficult problem to solve in the design of a Fischer-Tropsch reactor is its very high exothermicity combined with a high sensitivity of product selectivity to temperature. On an industrial scale, multitubular and bubble column reactors have been widely accepted for this highly exothermic reaction.6 In case of a fixed bed reactor, it is desirable that the catalyst particles are in the millimeter size range to avoid excessive pressure drops. During Fischer-Tropsch synthesis the catalyst pores are filled with liquid FT products (mainly waxes) that may result in a fundamental decrease of the reaction rate caused by pore diffusion processes. Post et al. showed that for catalyst particle diameters in excess of only about 1 mm, the catalyst activity is seriously limited by intraparticle diffusion in both iron and cobalt catalysts.1... 

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