Growth probability

When determining the product selectivities, all compounds of equal carbon numbers (paraffines, olefins, isomers, and oxygen compounds) were summarized to one product fraction. The chain growth probability was determined by the Anderson-Schulz-Flory (ASF) distribution ... 

Typical results of growth probability as a function of carbon number for FT synthesis on iron and cobalt7 26—both for different times on stream during catalyst... 

The ideal product distribution is defined by only one number, the value of growth-probability pg... 

The description of the product distribution for an FT reaction can be simplified and described by the use of a single parameter (a value) determined from the Anderson-Schulz-Flory (ASF) plots. The a value (also called the chain growth probability factor) is then used to describe the total product spectrum in terms of carbon number weight fractions during the FT synthesis. In the case... 

As indicated in Figure 10.2, there is a distinct change in the slope of the line at carbon numbers 8 to 12, and this has also been observed by other researchers.2-3 This change in the slope cannot be explained by the ASF model, which is based on the premise that the chain growth probability factor (a) is independent from the carbon number. Some further developments of the ASF model by Wojciechowski et al.3 made use of a number of abstract kinetic parameters for the calculation of a product spectrum. Although it still predicts a straight line for the a plot, they suggested that the break in the line is due to different mechanisms of chain termination and could be explained by the superposition of two ideal distributions. This bimodal distribution explained by two different mechanisms... 

Another explanation for the changing slope has been proposed by Schulz and Claeys,7 who suggest that the product olefins undergo secondary reactions and, because of changing product olefin solubility, result in chain length dependence on the chain growth probability (a). 

The carbon number distribution of Fischer-Tropsch products on both cobalt and iron catalysts can be clearly represented by superposition of two Anderson-Schulz-Flory (ASF) distributions characterized by two chain growth probabilities and the mass or molar fraction of products assigned to one of these distributions.7 10 In particular, this bimodal-type distribution is pronounced for iron catalysts promoted with alkali (e.g., K2C03). Comparing product distributions obtained on alkali-promoted and -unpromoted iron catalysts has shown that the distribution characterized by the lower growth probability a, is not affected by the promoter, while the growth probability a2 and the mass fraction f2 are considerably increased by addition of alkali.9 This is... 

Without doubt, the superimposed ASF distributions are the result of two incompatible mechanisms. In particular, the experiment with co-fed 13CH2N2 suggests that mechanism 1 assigned to distribution 1 and characterized by the growth probability a, is based on the insertion of CH2. It is assumed that mechanism 2 assigned to distribution 2 and characterized by the higher growth probability a2 is based on the insertion of CO. 

Chain propagation is started from a methylene group and terminated by desorption of 1-alkenes or alkanes. Propeller-type mobility of the olefin ligand renders possible CH3 branching of the growing chain, as demonstrated by the scheme. The growth probability is determined by the ratio of rates of formation of the alkyl intermediate and of the desorption of 1-alkenes, and to a minor extent of alkanes. 

Alkalization of iron catalysts causes two different effects. The selectivities of 1-alkenes are raised and both the growth probability a2 and the fraction f2 are markedly increased, as already shown in Figure 11.2. Detailed studies on the promoter effect of alkali have revealed the effect on 1-alkene selectivity to saturates at 1 mass% of K2C03, while the effect on f2 already begins at 0.2 mass% of K2C03.1213 This difference points to specific active sites in Fischer-Tropsch syn-... 

The mass fraction f2 increases strongly in the order Li-, Na-, K-, and Cs-oxide/ carbonate. However, the increase of the growth probability a2 is the same for all alkali promoters. The growth probability a and, consequently, mechanism 1 are not affected. Therefore, it is necessary to modify only mechanism 2 of the novel hypothesis for alkali ions to take part in the catalytic cycle.13... 

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. 

Detailed studies of co-feeding experiments with alcohol and aldehyde, first undertaken by Emmett and coworkers, have led to the conclusion that the distribution with the higher growth probability is with high probability due to a mechanism based on CO insertion. 

H2 content of the feed gas. As a consequence, the corresponding ASF distributions (Figure 16.4b) are strongly affected by the change in the H2/CO inlet molar ratio, so that an increase of the chain growth probability (corresponding to the exponential of the slope of the ASF diagram) is well evident as the H2/CO feed ratio decreases. 

This is a classical mechanistically developed equation, and the interpretation of the water effect is that water replaces surface carbon through a cleaning effect, and also acts as a source of hydrogen. The net effect is reported to be an increase in the population of CH.v-specics which in turn leads to higher CO consumption and also a higher chain growth probability. 

For maximum yield of liquid hydrocarbons and minimum yield of gases, FT synthesis is optimised to produce predominantly heavy products (heavy paraffins),7 i.e., producing hydrocarbon chains as long as possible at maximum hydrocarbon chain growth probability. 

A measure for the hydrocarbon growth is the chain growth probability . For optimum liquid hydrocarbon yields, it is typically in the order of 0.85 to 0.90. 

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... 

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