Chain-length-independent growth

The FT growth mechanism implies that only methane (n = 1) and polymethylene (n — 00) can theoretically be obtained with 100% selectivity. Intermediate products of a specific carbon number range can only be made at substantially lower selectivities. A way to circumvent this limitation is to combine the chain-length-independent growth process with a selective cracking process, which has to meet the following two conditions ... 

The chain propagation occurs by a series of successive steps, all of which may be assumed to be governed by the same rate constant kp, independent of chain length. The growth of the chain may be... 

Here, n is the number of carbon atoms in products. Wn represents the weight fraction of products containing n carbon atoms, and a is the chain growth probability. The parameter a is assumed chain-length independent. 

The CO consumption rate is independent of the rate of termination. When the ratio of the rate of chain growth to the rate of termination changes, the average chain length of the hydrocarbon will be affected, but not the rate of CO consumption. Even without chain termination, hydrocarbon chain growth will still occur. 

Assuming that the reactivities of end groups or growth centres are equal and independent of the chain length, then... 

The kinetic scheme of the ideal Fischer Tropsch polymerization model is given in Fig. 8. Chain growth is possible only with one type of reaction step and this is assumed to be independent of chain length. Only one sort of... 

The fact that constant growth parameters will predict the isomer distribution data reasonably is remarkable. It is not necessary that the kinetic constants governing chain growth are independent of chain length and structure but that certain ratios of these parameters are constant. The fraction of tertiary carbons has been reported to decrease with carbon number beyond Cio (i7). The SCG scheme predicts a maximum and subsequent decrease, but the maxima occur at C12-C14 for products considered in this chapter. For the cobalt product, all schemes predict yields of dimethyl species that are often too large by factors of two to four. The simple schemes with constant growth parameters as described here are unable to predict the isomer distribution sensibly for products from fixed-bed iron (16) and from fixed-bed nickel... 

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