Fluid Mechanics for Chemical Engineering

Chapter 10

Micromixing and Macromixing

10.1. Introduction

In this chapter and in the following chapters of the second part of this book, we discuss the mixing of chemical constituents by turbulence and their chemical reaction. The focus is first on the case of a reactor filled with a fluid in which two chemical constituents are present (Figure 10.1), but we assume provisionally that they do not react together. Initially, constituents A and B are dissolved in two distinct volumes separated by an interface; they are in contact only through that interface. We denote by V_A and V_B the volumes containing A and B, respectively, and by C_A0 and C_B0 the associated initial concentrations.

At time t = 0, stirring is initiated in the vessel using a moving body. Ideally, stirring is assumed to be homogeneous. This means that the mixing is performed in the same way at the center of the reactor as at its corners. It also means that, after a while, constituents are dispersed in identical fashion throughout the reactor. The concentration of constituents A and B, measured in a sample, is the same wherever the sample is taken in the reactor. If the concentrations of constituents A and B are represented by the yellow and blue colors, we would expect to see, at the macroscopic level, a green color in the reactor. However, this observation does not mean that the two products are in fact mixed, as illustrated in Figure 10.1, using varying grey levels. It is necessary to examine the distribution in space ...