Reactions in parallel—two reactants

If a second reactant B is involved in a system of parallel reactions, then the same principles apply to B as to A. The rate equations are examined to see whether the order of the desired reaction with respect to B is higher or lower than that of the undesired reaction, and to decide whether high or low concentrations of B favour a high yield of desired product. 

There are three possible types of combination between the concentration levels of A and B that may be required for a high yield  

Alternatively a series of several continuous stirred tanks could be used in place of the tubular reactors in (a) and (d) 

These ways of matching reactor characteristics to the concentration levels required are illustrated in Fig. 1.23. 

Inspection of the rate equations shows that, with respect to A, the order of the desired reaction is unity, and the order of the undesired reaction is effectively zero because A does not participate in it. The desired reaction is therefore favoured by high values of CA. With respect to B, the order of the desired reaction is unity, and the order of the unwanted reaction is two. The desired reaction is therefore favoured by low values of CB. 

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