Fisher-Tropsch

The mixture of carbon monoxide and hydrogen is enriched with hydrogen from the water gas catalytic (Bosch) process, ie, water gas shift reaction, and passed over a cobalt—thoria catalyst to form straight-chain, ie, linear, paraffins, olefins, and alcohols in what is known as the Fisher-Tropsch synthesis. 

Imperial Chemical Industries (ICI) operated a coal hydrogenation plant at a pressure of 20 MPa (2900 psi) and a temperature of 400—500°C to produce Hquid hydrocarbon fuel from 1935 to the outbreak of World War II. As many as 12 such plants operated in Germany during World War II to make the country less dependent on petroleum from natural sources but the process was discontinued when hostihties ceased (see Coal conversion PROCESSES,liquefaction). Currentiy the Fisher-Tropsch process is being used at the Sasol plants in South Africa to convert synthesis gas into largely ahphatic hydrocarbons at 10—20 MPa and about 400°C to supply 70% of the fuel needed for transportation. 

HTS catalyst consists mainly of magnetite crystals stabilized using chromium oxide. Phosphoms, arsenic, and sulfur are poisons to the catalyst. Low reformer steam to carbon ratios give rise to conditions favoring the formation of iron carbides which catalyze the synthesis of hydrocarbons by the Fisher-Tropsch reaction. Modified iron and iron-free HTS catalysts have been developed to avoid these problems (49,50) and allow operation at steam to carbon ratios as low as 2.7. Kinetic and equiUbrium data for the water gas shift reaction are available in reference 51. 

During World War II German scientists developed a method of hydrogenating soHd fuels to remove the sulfur by using a cobalt catalyst (see Coal CONVERSION processes). Subsequently, various American oil refining companies used the process in the hydrocracking of cmde fuels (see CATALYSIS SuLFUR REMOVAL AND RECOVERY). Cobalt catalysts are also used in the Fisher-Tropsch method of synthesizing Hquid fuels (21—23) (see Fuels, synthetic). 

Feed to recycle gas, ratio of fresh 127 Fisher-Tropsch... 

Industrially, transformation of syngas is normally carried out using Fisher-Tropsch (FT) method which utilizes either Fe or Co based catalysts to obtain useful fuels and chemicals. For wider applications, the usage of a support for these catalysts... 

Iglesia, E. 1997. Design, synthesis, and use of cobalt-based Fisher-Tropsch synthesis catalyst. Appl. Catal. 161 59-78. 

Zhang, Y., Li, Y., Yang, G., Sun, S., and Tsubaki, N. 2007. Effects of impregnation solvent on Co/Si02 catalyst for Fisher-Tropsch synthesis A highly active and stable catalyst with bimodal sized cobalt particle. Appl. Catal. A Gen. 321 79-85. 

Figure 4. Differential spectra of CO chemisorbed on alumina-supported Ni particles both before and after heating to 425 K. Very little surface hydrocarbon is seen to form on the Ni particles. This lack of surface hydrocarbon reflects the selectivity of such catalysts for methanation over Fisher-Tropsch synthesis.

The majority of the existing plants were designed and constructed to produce a synthetic gas, consisting primarily of hydrogen and carbon monoxide (CO), which is used for the production of hydrogen or Fischer-Tropsche (F-T) syncrude. Hydrogen is then used to produce a wide variety of chemicals and fertilizers. The Fisher-Tropsch syncrude is used to manufacture transportation fuels, lube oils, and specialty waxes. 

Fuel gas for gas turbines (IGCC) and as a replacement for natural gas or chemical substitute. Can be used as a feedstock to Fisher-Tropsch synthesis, methanation, methanol, and ammonia production. 

The world s largest concentration of Lurgi gasifiers is in South Africa, where Sasol operates three major complexes. The Sasol plants (Sasol I, II, and III) located in Seconda and Sasolburg gasify approximately 30 million ton/year of bituminous coal to synthesis gas, which is converted to fuels and chemicals via the Fisher-Tropsch process. It was recently announced (International Coal... 

Q dehydrocyclization, 29 311 ring enlargement, 29 311-316 Bifunctional Fisher-Tropsch/hydroformylation catalysts, 39 282 Bifunctional mechanism, 30 4 Bifurcation diagram, oscillatory CO/O, 37 233-234... 

H2 has many uses in hydrogenation, and CO has many uses in carbonylation. The mixture is used in methanol synthesis and in the Fisher Tropsch synthesis of hydrocarbons. 

We next return to another reaction of a CO + H2 mixture, which we called synthesis gas or syngas. It has this name because it is used to synthesize many chemicals such as methanol. Another synthesis reaction from CO and H2 is a polymerization process called the Fisher Tropsch synthesis of synthetic diesel fuel. 

Figure 11-9 Proposed mechanism for Fisher Tropsch polymerization of CO and H2 to form alkane polymer.

In Fisher Tropsch synthesis from CO and H2, different catalysts produce different types of product molecules. Referring to the figure of propagation in the FT process, sketch the termination steps that will lead to (a) alkanes, (b) a-olefms, and (c) alcohols. 

In contrast to the Fisher Tropsch synthesis of higher alkanes from CO and H2, Ni catalysts produce primarily methane, and Cu/ZnO catalysts produce mainly methanol. Sketch the mechanistic steps that favor these products rather than polymerization ... 

It has been reported that for diameters less than 7.62 cm, the gas holdup depends on the column diameter, whereas it is independent of it for diameters greater than 10.2 cm (Hughmark, 1967 Saxena, 1991). The same has been found in studies of the Fisher-Tropsch synthesis in slurry bubble columns, where it has been reported that the effect of the column diameter is negligible when foam is not present in the system (Fox and Degen, 1990). 

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