Temperature 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.

The second reaction is called the Fischer-Tropsch synthesis of hydrocarbons. Depending on the conditions and catalysts, a wide range of hydrocarbons from very light materials up to heavy waxes can be produced. Catalysts for the Fischer-Tropsch reaction iaclude iron, cobalt, nickel, and mthenium. Reaction temperatures range from about 150 to 350°C reaction pressures range from 0.1 to tens of MPa (1 to several hundred atm) (77). The Fischer-Tropsch process was developed iadustriaHy under the designation of the Synthol process by the M. W. Kellogg Co. from 1940 to 1960 (83). 

Medium Pressure Synthesis. Pressures of 500—2000 kPa (5—20 atm) were typical for the medium pressure Fischer-Tropsch process. Cobalt catalysts similar to those used for the normal pressure synthesis were typically used at temperatures ranging from 170 to 200°C ia tubular "heat exchanger" type reactors. 

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. 

In this chapter a two a selectivity model is proposed that is based on the premise that the total product distribution from an Fe-low-temperature Fischer-Tropsch (LIFT) process is a combination of two separate product spectrums that are produced on two different surfaces of the catalyst. A carbide surface is proposed for the production of hydrocarbons (including n- and iso-paraffins and internal olefins), and an oxide surface is proposed for the production of light hydrocarbons (including n-paraffins, 1-olefins, and oxygenates) and the water-gas shift (WGS) reaction. This model was tested against a number of Fe-catalyzed FT runs with full selectivity data available and with catalyst age up to 1,000 h. In all cases the experimental observations could be justified in terms of the model proposed. 

The use of a Fischer-Tropsch (FT) process to produce long-chain hydrocarbons is well known in industry, and achieving the desired selectivity from the FT reaction is crucial for the process to make economic sense. It is, however, well known that a one-alpha model does not describe the product spectrum well. From either a chemicals or fuels perspective, hydrocarbon selectivity in the FT process needs to be thoroughly understood in order to manipulate process conditions and allow the optimization of the required product yield to maximize the plant profitability. There are many unanswered questions regarding the selectivity of the iron-based low-temperature Fischer-Tropsch (Fe-LTFT) synthesis. 

The first commercial Fischer-Tropsch facility was commissioned in 1935, and by the end of the Second World War a total of fourteen plants had been constructed. Of these, nine were in Germany, one in France, three in Japan, and one in China. Both German normal-pressure and medium-pressure processes (Table 18.1) were employed. The cobalt-based low-temperature Fischer-Tropsch (Co-LTFT) syncrude produced in these two processes differed slightly (Table 18.2), with the product from the medium-pressure process being heavier and less olefinic.11 In addition to the hydrocarbon product, the syncrude also contained oxygenates, mostly alcohols and carboxylic acids. 

After the Second World War a gas-to-liquids facility that employed an iron-based high-temperature Fischer-Tropsch (Fe-HTFT) process was constructed at Brownsville, Texas. The technology was developed by Hydrocarbon Research, Inc.,20 and the commercial facility was operated by the Carthage Hydrocol Company. The Hydrocol plant was in commercial operation during the period 1951-1957, and it was shut down mainly for economic reasons (the oil price was around US 2 per barrel at that time). 

The higher alkene feed (C10-14) for the production of detergent alcohols is either a product from the wax-cracker (terminal and internal alkenes) or the byproduct of the ethene oligomerisation process (internal alkenes). In the near future a feed from high-temperature Fischer-Tropsch may be added to this. The desired aldehyde (or alcohol) product is the linear one and the cobalt catalyst must therefore perform several functions ... 

Finally, catalytic activity was tested and compared on Fischer-Tropsch process (hydrogenation of CO), on doped iron composite (Fe/SC-155) and on doped iron isolated carbon network (Fe/C-155). The reaction parameters were temperature = 270 °C R(H2/CO)= 3 and pressure = latm. 

The properties of these new materials as catalyst support were tested on Fischer-Tropsch process (CO-H2 reaction) in a fixed bed differential reactor. Three materials were tested a) CON, a conventional activated carbon b) SC-155 (G40.60) and c) C-155 (G20.20). All of them were previously iron doped until 5% metallic iron wt/wt was reached. The test conditions were Reaction temperature =270°C H2/CO ratio=3, pressure = latm. The main properties of the tested catalyst supports and their performance in the first hour test are shown in Table 2. SC-155 (G40.60) and C-155 (G20.20) were selected for this test in order to compare materials with near the same specific surface area but with different structural composition, and CON was selected because it is of common use and has very different texture characteristics respect to the other two materials. 

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. 

Iron catalysts have found only limited use in usual hydrogenations, although they play industrially important roles in the ammonia synthesis and Fischer-Tropsch process. Iron catalysts have been reported to be selective for the hydrogenation of alkynes to alkenes at elevated temperatures and pressures. Examples of the use of Raney Fe, Fe from Fe(CO)5, and Urushibara Fe are seen in eqs. 4.27,4.28, and 4.29, respectively. 

Paraffin, Synthetic, occurs as a white wax that is very hard at room temperature. It is synthesized by the Fischer-Tropsch process from carbon monoxide and hydrogen, which are cata-lytically converted to a mixture of paraffin hydrocarbons the lower-molecular-weight fractions are removed by distillation, and the residue is hydrogenated and further treated by percolation through activated charcoal. It is soluble in hot hydrocarbon solvents. 

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. 

An interesting sequel to the Fischer-Tropsch synthesis is the so-called 0X0 reaction. Olefines derived from the Fischer-Tropsch process will react with carbon monoxide and hydrogen in the presence of cobalt carbonyl at temperatures of 110° to 150°C, under pressures of 150 to 200 atmospheres, to yield aldehydes. These can either be reduced to alcohols or oxidized to fatty acids, and they are derived from the petroleum industry... 

Other Syntheses Related to the Fischer-Tropsch Process Comparatively little is yet known of some synthetic reactions which obviously resemble the Fischer-Tropsch process very closely, but they are worth brief mention because they are also likely to be controlled by geometrical factors. The Oxo synthesis (15) of aldehydes by the interaction of ethylene or other olefins with carbon monoxide and hydrogen is carried out in contact with cobalt catalysts at temperatures in the range 110-150°, and under a pressure of 100-200 atmospheres. Cyclic olefins react similarly for example, cyclohexene gives hexahydrobenzaldehyde. There can be little doubt that a two-point adsorption of the hydrocarbon must take place and that the adsorbed molecule then reacts with carbon monoxide and hydrogen the difference between this process and that responsible for the normal hydrocarbon synthesis is that adsorbed carbon monoxide survives as such under the less drastic temperature conditions which are employed. Owing to the fact that a variety of isomeric aldehydes are produced, this system deserves further detailed study on geometrical lines. 

We have studied the synthesis of fatty acids by the closed Fischer-Tropsch process, using various carbonates as promoters and meteoritic iron as catalyst. The conditions used were D2/CO mole ratio = 1 1, temperature == 400°C, and time = 24-48 hr. Sodium, calcium, magnesium, potassium, and rubidium carbonates were tested as promoters but only potassium carbonate and rubidium carbonate produced fatty acids. These compounds are normal saturated fatty adds ranging from C5 to Cis, showing a unimodal Gaussian distribution without predominance of odd over even carbon-numbered aliphatic chains. The yields in general exceed the yields of aliphatic hydrocarbons obtained under the same conditions. The fatty acids may be derived from aldehydes and alcohols produced under the influence of the promoter and subsequently oxidized to the acids. 

The Fischer-Tropsch process utilises CO as a carbon source, with H2 as the reductant, for the production of hydrocarbons and oxygenates, especially in times of hmited crude oil supply (CO may be derived from methane or coal). Fischer-Tropsch systems do not, however, give carbocychc products, nor homologate CO under mild conditions (pressures typically used are >300 bar and temperatures >500 °C, in conjunction with either homogeneous or heterogeneous catalysts [96]). Carbocycles often form the backbone of many pharmaceutical drugs, therefore a catalytic process that could synthesise them from a non-crude oil source (e.g. CO) under mild or even... 

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