Fischer-Tropsch reaction catalyst design

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). 

Summarizing, there are still many scientific challenges and major opportunities for the catalysis community in the field of cobalt-based Fischer-Tropsch synthesis to design improved or totally new catalyst systems. However, such improvements require a profound knowledge of the promoted catalyst material. In this respect, detailed physicochemical insights in the cobalt-support, cobalt-promoter and support-support interfacial chemistry are of paramount importance. Advanced synthesis methods and characterization tools giving structural and electronic information of both the cobalt and the support element under reaction conditions should be developed to achieve this goal. 

The hydroformylation of alkenes was accidentally discovered by Roelen [4] while he was studying the Fischer-Tropsch reaction (conversion of syn-gas to liquid fuels with a heterogeneous cobalt catalyst) in the late thirties. In what w s probably designed as a mechanistic experiment Roelen examined whether alkenes were intermediates in the Aufbau process for converting syn-gas (from coal, Germany 1938) to fuel. It took more than a decade before the reaction was taken further, but now it was the conversion of petrochemical hydrocarbons into oxygenates that was the driving force. It was discovered that the reaction was not catalyzed by the supported cobalt, but in fact by HCo(CO)4 which was formed in the liquid state. 

In a long and detailed paper on CO/H2 and CO2/H2 reactions on polycrystalline Rh, Sexton and Somorjai used a UHV-AES apparatus designed to allow sample scrutiny at low pressure yet to permit high-pressure (700 torr) reactions. They established good correlation of turn-over numbers between their results and results obtained on supported catalysts for the Fischer-Tropsch reaction. AES established that C was present on reactive surfaces yet this C (1 to 2 monolayers) did not influence rates of reaction or product distribution for high-pressure runs. It is however interesting to note that the most important influences on catalysis reported in this paper were found to be subsurface C and O, neither detectable by AES. The reactions studied at 250—300 °C showed that CO/H2 produced mainly Ci but also some C2, C3, and C4 hydrocarbons, whereas CO2/H2 produced CH4 exclusively. 

Gas-soUd catalytic reaction is the most common chemical process in the industry. However, owing to the limitation of this book, only three chapters were devoted to this theme. Chapter 11 describes the preparation of gold clusters and its application on the solid-gas biphasic catalytic reaction. The clarification of catalytic mechanism and reactive sites is very important for designing more efficient catalysts. So the identification of binding and reactive sites in metal cluster catalysis through imaging technique, kinetic study, and other methods are introduced in Chapter 9. To reflect the importance of theoretical calculation on catalysis, the molecular kinetics of the Fischer-Tropsch reaction by computational chemistry is introduced in Chapter 16. 

The principal advance ia technology for SASOL I relative to the German Fischer-Tropsch plants was the development of a fluidized-bed reactor/regenerator system designed by M. W. Kellogg for the synthesis reaction. The reactor consists of an entrained-flow reactor ia series with a fluidized-bed regenerator (Fig. 14). Each fluidized-bed reactor processes 80,000 m /h of feed at a temperature of 320 to 330°C and 2.2 MPa (22 atm), and produces approximately 300 m (2000 barrels) per day of Hquid hydrocarbon product with a catalyst circulation rate of over 6000 t/h (49). 

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... 

Table V shows the salient features of several Fischer-Tropsch processes. Two of these—the powdered catalyst-oil slurry and the granular catalyst-hot gas recycle—have not been developed to a satisfactory level of operability. They are included to indicate the progress that has been made in process development. Such progress has been quite marked in increase of space-time yield (kilograms of C3+ per cubic meter of reaction space per hour) and concomitant simplification of reactor design. The increase in specific yield (grams of C3+ per cubic meter of inert-free synthesis gas) has been less striking, as only one operable process—the granular catalyst-internally cooled (by oil circulation) process—has exceeded the best specific yield of the Ruhrchemie cobalt catalyst, end-gas recycle process. The importance of a high specific yield when coal is used as raw material for synthesis-gas production is shown by the estimate that 60 to 70% of the total cost of the product is the cost of purified synthesis gas.

The methanation reaction is a highly exothermic process (AH = —49.2 kcal/ mol). The high reaction heat does not cause problems in the purification of hydrogen for ammonia synthesis since only low amounts of residual CO is involved. In methanation of synthesis gas, however, specially designed reactors, cooling systems and highly diluted reactants must be applied. In adiabatic operation less than 3% of CO is allowed in the feed.214 Temperature control is also important to prevent carbon deposition and catalyst sintering. The mechanism of methanation is believed to follow the same pathway as that of Fischer-Tropsch synthesis. 

Available reaction-transport models describe the second regime (reactant transport), which only requires material balances for CO and H2. Recently, we reported preliminary results on a transport-reaction model of hydrocarbon synthesis selectivity that describes intraparticle (diffusion) and interparticle (convection) transport processes (4, 5). The model clearly demonstrates how diffusive and convective restrictions dramatically affect the rate of primary and secondary reactions during Fischer-Tropsch synthesis. Here, we use an extended version of this model to illustrate its use in the design of catalyst pellets for the synthesis of various desired products and for the tailoring of product functionality and molecular weight distribution. 

Solids play different roles in the different processes. In direct coal liquefaction, a part of the solid is dissolved in liquid (mainly in the preheater) and a part (i.e. mineral matter) may act as a catalyst for the hydrogenation reactions. In Fischer-Tropsch slurry processes, solids are catalysts. Finally, in chemical cleaning of coal, only a part of solid (i.e. sulfur) takes part in the reaction following the shrinking core diffusion/ reaction mechanism. The role of solids in the design and scaleup of the reactors for the three processes is therefore different. 

In the second stage, which constitutes the heart of SMDS, carbon monoxide and hydrogen are converted into paraffins via the Fischer-Tropsch (FT) reaction. The synthesis catalyst and the associated reactor design represent the essential technology of the SMDS process. Another and equally important feature in terms of the selective production of middle distillate fuels is, however, the two-stage, HPS-HPC concept as shown in Fig. 3. 

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