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Figure 27. Steps in the determination of the mass transfer coefficient (kLa). From Merchuk et al. (141).

Figure 27. Steps in the determination of the mass transfer coefficient (kLa). From Merchuk et al. (141).

system, but a parameter of the model adopted. If total mixing is assumed in the model adopted, the mass transfer coefficient obtained will consequently be limited. The assumption of complete mixing is such a common practice that many reports do not even specify explicitly that this has been done, and in many texts the mass transfer coefficient kLa is defined by the equation:

It is often forgotten to state that this equation is valid only for perfectly mixed systems.

Strictly, the different sections of ALRs (riser, down-comer, and gas separator) have different flow characteristics, and the mass transfer coefficient may be expected to differ from one region to another. Some researchers (108) have assumed that the contribution of the downcomer to the overall mass transfer is negligible and have reported values of kLa that are, in fact, the entire mass transfer divided by the volume of the riser. The fact that the values of the mass transfer coefficient are calculated and reported in different ways introduces some ambiguity into the published data; special care should therefore be taken when comparing data or using published mass transfer coefficients for design purposes.

The assumption of perfect mixing in the liquid phase may be questionable in the case of tall reactors. One very simple method to verify this assumption is the simultaneous measurement of the response of the concentration to a step-change in the inlet. In a perfectly mixed system, the location of the probe should be irrelevant. Figure 28 (143) shows the response of three probes, one located at

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