CSTR-PFR—A Problem in Comparison and Synthesis

Reactions are often complex. By that we mean that rather than having one reaction take place the reactants and products are involved in many different chemical transformations simultaneously. For example, at the temperature required to produce B from A, we may find 

In addition, whether more D or more B is produced depends on the values of the two rate constants for the two steps. 

Another case that is often encountered is one in which the product of the reaction of A to B may itself be reactive and at the same conditions will produce D. These reactions occur in series  

We will concern ourselves with this problem using the series network of reactions. We will explore the differences between the complete back-mixed CSTR and the axially distributed but radially well-mixed PFR. 

Consider this reaction network in more detail. Let us assume that D is in fact the product that we seek to produce, but that it must go through B. It is quite realistic to suppose that this is the only viable route to D, but that B is very undesired. For example, D may be a pharmaceutical or nutraceutical with special properties, whereas B is harmful when present in quantities above a given level. Impurity problems of this kind also show up in other chemical products, including specialties and materials. The presence of B above a certain threshold may deleteriously affect the performance of the product. Thus the impurity problem is one that is very real and that crops up across the industries in which chemical engineers participate. 

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