Feed rate-process yield relation

The first-order rate coefficient, k, of this pseudo-elementary process is assumed to vary with temperature according to an Arrhenius law. Model parameters are the stoichiometric coefficients v/ and the Arrhenius parameters of the rate coefficient, k. The estimation of the decomposition rate coefficient, k, requires a knowledge of the feed conversion, which is not directly measurable due to the complexity of analyzing both reactants and reaction products. Thus, a supplementary empirical relationship is needed to relate the feed conversion (conversion of A) to some experimentally accessible variable (Ross and Shu have chosen the yield of C3 and lighter hydrocarbons). It is observed that the rate coefficient, k, is not constant and decreases with increasing conversion. Furthermore, the zero-conversion rate coefficient depends on feed specifications (such as average carbon number, hydrogen content, isoparaffin/normal-paraffin ratio). Stoichiometric coefficients are also correlated with conversion. Of course, it is necessary to write supplementary empirical relationships to account for these effects. 

It was pointed out earlier that Equations 11 and 12 do not apply at feed conversion outside the range of commercial operating conditions. In this regime, feed conversion must be calculated from the rate constant integrated via Equation 10. It remains to complete the overall model by developing a correlation to relate pyrolysis yields to process operating parameters and feed conversion. 

Polymerization was the first refining process to produce larger molecules appropriate for gasoline. However, a chemically related process called alkylation is of more importance today [15]. This uses concentrated sulfuric acid or hydrofluoric acid as a liquid phase catalyst to form dimers or trimers from C3 and C4 olefins reacted with paraffinic hydrocarbons [24]. The yield of the alkylation process based on the olefin feed is about twice of that of polymerization. This factor and that the product has at least as good octane rating as polymer gasoline has made alkylation a dominant contributor to gasoline components. 

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