Deviations from Ideal Flows

It is convenient to classify deviations from ideal flow conditions into two categories. 

Deviations from ideal flow can be classified in two types. In one type of deviation elements of fluid may move through the reactor at different velocities, causing channeling and dead spots. For such behavior to occur, the elements of fluid must not completely mix locally, but remain at least partially segregated as they move through the reactor. The other deviation refers to the extent of the local or micromixing. For exam.ple, there may be some mixing or diffusion in the direction of flow in a tubular reactor. 

Aside from these large-scale, macroscopic deviations from ideal flow patterns, nonideal F t) responses can arise from diffusion within the reactor, from velocity profiles in tubular reactors that deviate from the plug-flow pattern, or from combinations of the two effects. It is the sum combination of all such processes that constitute what we have called mixing effects on chemical reactor performance In what follows, we will first attempt to develop a model adequate for the types of F t) and C t) behavior illustrated in Figures 4.3(b) and 4.4(b), then attempt to extend these ideas to modeling some of the more pathological behavior illustrated in Figure 5.1. 

Here D, the axial dispersion (or diffusion) coefficient, is the parameter used to describe the deviations from ideal flow. If u is taken to be constant in the radial direction, the rightmost terms in equation (5-20) constitute the plug-flow mixing model and D (f Cld ) is a Fickian form of a diflusional correction term. 

Chapter 11 Deviations from Ideal Flow Conditions... 

Chapter 11 Deviations from Ideal Flow Conditions equation (11.1.59) may be rewritten as... 

---