Fischer-Tropsch synthesis Schulz-Flory distribution

Keywords Cobalt catalyst. Kinetics, Modeling, Fischer-Tropsch synthesis. Hydrocarbon Product Distribution, Anderson-Schulz-Flory. 

FIGURE 5 Anderson-Schulz-Flory distribution of the linear hydrocarbons, linear oxygenates (n-alcohols, n-aldehydes, and linear carboxylic acids), and methyl alkyl ketones formed in the Fischer-Tropsch synthesis on an iron-containing Fischer-Tropsch catalyst operating at a temperature of 498 K (plotted using log(lO)). 

Any mechanistic proposal must comply with the following observations. (1) The Fischer-Tropsch hydrocarbon synthesis follows the formalism of polymerization kinetics with a Schulz-Flory distribution of the molecular weights. (2) a-Olefins and alcohols occur as the primary products. (3) The aliphatic final products are formed consecutively by hydrogenation of the olefins according to " C-labeling experiments [4 f, 30 b]. (4) Chain termination processes do not deactivate the catalyst centers because the chain-growth velocity stays constant for weeks. 

In 1951 Anderson[l] established the production distribution formulation of the Fischer-Tropsch synthesis (FTS), which is called Anderson-Schulz-Flory (ASF) formulation. Since then,for a long time it is almost always possible to describe FTS product distribution by ASF formulation,which has the following mathematical expression ... 

The hypothesis of multiple build-in where a chain can interact with a chain Cj leads one to reflect on the possibility of a chain termination by combination. If reactions were occurring in which termination could occur by simple desorption and also by combination, two peaks would be observed. The second maxima would have a center at approximately twice the value of the flrst, as doubling of the most prevalent adsorbed chain lengths is likely (25). Furthermore, secondary events such as those discussed above or chain transfer could cause the distributions of the two peaks to be different from one another. Thus the fact that secondary reactions during Fischer-Tropsch synthesis occur and that multiple build-in and termination by combination are viable propositions help rationalize distributions that do not follow the Schulz-Flory law and appear with more than a single maximum. 

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. 

Figure 15.14 (a) Product distribution of the Fischer—Tropsch synthesis predicted by Anderson—Schulz—Flory (ASF) polymerization model, (b) Product distribution obtained in the Fischer—Tropsch synthesis with different catalysts. 

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