Fischer-Tropsch synthesis chain growth probability

In Fischer-Tropsch synthesis the readsorption and incorporation of 1-alkenes, alcohols, and aldehydes and their subsequent chain growth play an important role on product distribution. Therefore, it is very useful to study these reactions in the presence of co-fed 13C- or 14 C-labeled compounds in an effort to obtain data helpful to elucidate the reaction mechanism. It has been shown that co-feeding of CF12N2, which dissociates toward CF12 and N2 on the catalyst surface, has led to the sound interpretation that the bimodal carbon number distribution is caused by superposition of two incompatible mechanisms. The distribution characterized by the lower growth probability is assigned to the CH2 insertion mechanism. 

Fischer-Tropsch synthesis, 28 80, 97, 103, 30 166-168, 34 18, 37 147, 39 221-296 activation energy and kinetics, 39 276 added olefin reactions, 39 251-253 bed residence time effects on chain growth probability and product functionality, 39 246-250... 

Fischer-Tropsch Technology FTS can be carried out in several different reactor types fixed bed, fluidized bed, or slurry phase and at different temperatures. The high-temperature Fischer-Tropsch (HTFT) synthesis runs at 320°C-350°C, at which temperatures typically all products are in the gas phase [22], HTFT is operated in fluidized-bed reactors, with iron catalysts. Selectivities correspond to chain-growth probabilities in the range of 0.70-0.75 and are ideal for gasoline production, but olefins and oxygenates are formed as well and are used as chemicals. 

Low-temperature Fischer-Tropsch (LTFT) synthesis runs at temperatures between 200°C and 250°C [23-25]. The chain-growth probability at these conditions is much higher than for the HTFT, and as a consequence, the product distribution extends well into the liquid waxes. LTFT reactors are thus three-phase systems solid catalysts, gaseous reactants, and gaseous and liquid products. Both cobalt and iron... 

Figure 10 Fischer-Tropsch synthesis with Ru/Na. Effect of catalyst potential (C/wr) on the probability of hydrocarbon chain growth. 

Liquid-phase Fischer-Tropsch synthesis has been investigated using a slurry-bed reactor. The catalytic activity of ultrafine particles (UFP) composed of Fe was shown to be greater than that of a precipitated Fe catalyst. The difference was interpreted as caused by the different nature of surface structure between these catalysts, whether porous or not. The obtained carbon number distributions over alkali-promoted Fe UFP catalysts were simulated by a superposition of two Flory type distributions. It is ascertained that the surface of alkali-promoted UFP catalysts possesses promoted and unpromoted sites exhibiting different chain growth probabilities. 

In research on the Fischer-Tropsch synthesis, FTS, at the Bureau, mechanisms of the growth of the carbon chain and the use of iron nitrides as catalysts were developments that were not anticipated by previous German work. Herington (2) in 1946 was the first to consider chain growth in FTS. He defined a probability 3 that the chain will desorb rather than grow at the surface, where... 

The Fischer-Tropsch s)mthesis is a process to convert synthesis gas (a mixture of carbon monoxide and hydrogen) to hydrocarbons that can be used as for instance transportation fuels. In the process all (straight chain) hydrocarbons fi om methane to heavy waxes are produced. In general this product distribution can be described by an Anderson-Schulz-Flory distribution based on a constant chain growth probability. As a consequence the selectivity towards diesel production is limited. When the diesel fraction is defined as CIO till C20, the maximum fraction of diesel that can be obtained is 39.4%, reached at a chain growth probability of 0.87. 

The readsorption of olefins is an important reaction in the Fischer-Tropsch synthesis that reverses the overall termination and increases the chain growth probability. Thus, readsorption results in heavier products. A larger time delay of the transient for the isotopic responses of olefin as compared with corresponding paraffin with increase in residence time (see Figure 51.10 for an example data on the ethane-ethene pair) is due to olefin readsorption. Transient experiments indicated that 1-olefins are the major candidates for readsorption on the catalyst surface, while the internal and iso-olefins readsorbed to a much less extent. 

Figure 6.11.2 Product distribution of Fischer-Tropsch synthesis as calculated by Eq. (6.11.15) for different values of the probability of chain growth a. ...

Hydrocarbon distributions in the Fischer-Tropsch (FT) synthesis on Ru, Co, and Fe catalysts often do not obey simple Flory kinetics. Flory plots are curved and the chain growth parameter a increases with increasing carbon number until it reaches an asymptotic value. a-Olefin/n-paraffin ratios on all three types of catalysts decrease asymptotically to zero as carbon number increases. These data are consistent with diffusion-enhanced readsorption of a-olefins within catalyst particles. Diffusion limitations within liquid-filled catalyst particles slow down the removal of a-olefins. This increases the residence time and the fugacity of a-olefins within catalyst pores, enhances their probability of readsorption and chain initiation, and leads to the formation of heavier and more paraffinic products. Structural catalyst properties, such as pellet size, porosity, and site density, and the kinetics of readsorption, chain termination and growth, determine the extent of a-olefin readsorption within catalyst particles and control FT selectivity. 

The first stage, Heavy Paraffin Synthesis (HPS), converts hydrogen and carbon monoxide into heavy paraffins by the Fischer-Tropsch process. The product distribution is in accordance with Schultz-Flory polymerization kinetics, which is characterized by, the probability of chain growth. 

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