L

50 75 100 125 150 175 200 Time (s)

50 75 100 125 150 175 200 Time (s)

Figure 28. Response of three probes, one located at the end of the riser, one at the inlet of the downcomer, and one at the bottom of a split-cylinder ALR. From Siegel et al. (143).

50 75 100 125 150 175 200 Time (s)

Figure 28. Response of three probes, one located at the end of the riser, one at the inlet of the downcomer, and one at the bottom of a split-cylinder ALR. From Siegel et al. (143).

the end of the riser, one at the inlet of the downcomer, and one at the bottom of a split-cylinder ALR. The difference between the figures stems from a different gas separator section, which changes the fluid dynamics in the system. In one case the system behaved as perfectly mixed, because of the large volume of the gas separator and the faster liquid circulation. In the other (closed system in the figure), the responses of the three electrodes were clearly different, calling for a different analysis. The validity of the criterion, originally proposed by Andre et al. (144), that compares the circulation time in the reactor (tc) with the characteristic time for mass transfer, which is the inverse of the mass transfer coefficient, was thus confirmed (143):

If this condition is not fulfilled, the bioreactor cannot be considered as a perfectly mixed volume, and more sophisticated ways of analysis are required. In this case, the mathematical model, and, consequently, the analysis ofthe experimental data become much more complicated. Nev-

ertheless, this approach has been used (48,122,145,146), and the mass transfer experiments render values of kLa for each of the regions of the ALR. Figure 29 shows the results reported by Hwang and Lu (146) in an internal-loop ALR. The graph shows that the mass transfer takes place at the highest rate in the riser. Values for the down-comer are 50% lower, and those for the gas separator are intermediate between the riser and the downcomer. The overall mass transfer rate is the result of the balance between the volumes and rates in the three sections.

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