The selectivity of competitive-consecutive reactions

We consider the reaction scheme of eq. (3.55), that is frequently encountered in organic syntheses, e.g., in partial oxidations, chlorinations, reductions, etc., of compounds that can be converted further. In principle, conditions must be found where the desired reaction has a much higher rate than the undesired one. Also, in general an excess of reactant B will be applied to avoid the undesired conversion of the product particularly towards the end of the process. 

We will consider simple second order kinetics for both reactions  

TTie perfectly mixed reactor does not appear to be particularly suitable for this type of reactions, but it may be used when the ratio of the rate constants is very high, and when B is fed in sufficient excess. Also, there may be other reasons for selecting a CSTR, such as good heat transfer and thermal stability. 

A relatively simple solution of the problem can be given the case of constant density. The mass balances are then 

The mass balances (for A, B, and P) combined with the two kinetic equations, lead to a relation between the selectivity and the conversion. The results of a numerical solution are shown in figures 3.12 a and b. We can see from these figures that for a given value of the ratio of the reaction rate constants, a high selectivity can only be obtained for low degrees of conversion. Of course, die selectivity is improved by applying an excess of reactant B. 

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In this case, the desired reaction is an intramolecular reaction, and the undesired intermolecular reaction might be avoided under these high-dilution conditions. The high-dilution technique might also be effective for improving the selectivity of competitive consecutive reactions, although the overall reaction time should be greatly reduced in comparison with that required under normal conditions. 

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