What Makes PFRs and CSTRs Ideal

In Chapter 4, we defined the characteristics of two ideal continuous reactors, the ideal plug-flow reactor (PFR) and the ideal continuous stirred-tank reactor (CSTR). We use the term ideal to refer to these reactors because the conditions of mixing and fluid flow in them are defined very precisely. To recap 

We might be tempted to explain the absence of gradients normal to flow in a PFR by invoking intense mixing normal to flow. However, it is difficult to visualize a mechanism that would create intense mixing normal to flow, but no mixing in the direction of flow. 

The two continuous, ideal reactors represent limiting cases of fluid mixing. The behavior of a real reactor will often approximate that of one or the other of these ideal reactors. However, this is not always the case. 

---