Fluid Mechanics for Chemical Engineering

Chapter 9

Hydrodynamics and Residence Time Distribution – Stirring

We discuss in this chapter the link between the theory of turbulent diffusion/ dispersion presented in Chapter 8 and two classical tools of chemical engineering.

The first of these is the residence time distribution (RTD) method. This technique allows us to classify the dispersion properties of a constituent in a chemical reactor, with reference to ideal behaviors of simple reactors. The RTD theory does not explicitly associate a RTD with a flow configuration inside the reactor. We examine this particular issue when the flow is turbulent, by considering successively the cases of a tubular reactor with axial dispersion and of a continuous stirred tank reactor (CSTR). Matching up the dimensions of the reactor with the mean residence time and the velocity and length scales of turbulence allows the determination of the hydrodynamic conditions associated with either type of reactor, for which the RTD laws are recovered, using the turbulent dispersion concepts introduced in Chapter 8.

Section 9.2 of this chapter addresses the issue of stirring in a reactor. We introduce the primordial elements, from a mechanical point of view, for dimensioning a stirrer:

– The characterization of a stirrer by its power number and the link between this quantity and the rate of turbulent energy dissipation.

– The link between the mixing time and the rate of turbulent energy dissipation.

Both these concepts are recurrent in the dimensioning ...