The Effect of Scale-up on Mass Transfer

The effect of reactor size on behavior was identified in the discussion of fluid dynamics and mass transfer in Chapter 2. Variation of velocity with position and the formation of boundary layers cause changes in mass transfer coefficients with electrode coordinates. We saw that for developing laminar flow, mass transfer coefficients and hence limiting currents decrease with increasing electrode length. Reactor operation should be at current densities 

Mass transfer as discussed in Chapter 2 provided means for predicting mass transfer rates, but were restricted to reactors devoid of entrance and exit effects (see Section 2.3.2). Real reactors, however, can be affected significantly by such effects. This section will discuss the problem by looking at the limited data about electrolytic cells in general and parallel plate geometry in particular. 

A reactor channel with a square cross section produces longitudinal variation of mass transfer and a spanwise distribution of the mass transfer coefScient (Fig. 5.15b). In the recirculating flow region mass transfer coefficients are highest at the center of the electrode and decrease toward 

FIGURE 5.15. Values of local wall mass transfer coefficients due to sudden expansion from a circular entry into a square channel, (a) Variation with downstream distance. Re=5000, expansion O 1 10, 1 6, A 1 4, 1 3, Al 2 (b) spanwise variation. Re=15670, 1 3 expansion parameters xjd. 

FIGURE 5.17. Distribution of mass transfer rates for small (catnoae area — 0.0225 m ) and large baffled cell (cathode area = 0.2025 m ). Re = 3900, A = smaller cell, O = larger cell the full symbols correspond to segments located at cell entry and where flow reversal occurs. 

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