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Figure 10. Gas holdup reported by various sources for the riser of airlift reactors under conditions of little or no gas recirculation. The data correspond to different Ad/Ar ratios.

ment from the separator to the downcomer occurs. If it is assumed that the riser cross-sectional area Ar remains unchanged and the downcomer cross-sectional area Ad is increased, then it can be expected that the liquid velocity in the riser will increase as a result of the smaller resistance to flow in the loop, which in turn leads to a decrease in the riser gas holdup. An increase in Ad/Ar will result in a decrease in the liquid velocity in the downcomer, which leads to a decrease in the gas recirculation, since fewer bubbles are entrapped in the downcomer. The final outcome of increasing Ad/Ar is thus a decrease is the riser gas holdup. A similar argument can be applied in the discussion of the effect of reactor height on the riser gas holdup, that is, an increase in the height of the downcomer will result in a higher liquid velocity, which will in turn lead to a decrease, as in the former case, in the holdup in the riser. In contrast, an increase in Ad/Ar will lead to an increase in the extent of bubble entrapment in the downcomer, which will serve to inject some additional gas into the riser. On the other hand, an increase of gas holdup in the downcomer diminishes the driving force for recirculation, as shown in equation 1, and this will moderate the increase of liquid velocity generated by the larger height. This feedback control of the liquid velocity is one of the characteristics particular to ALRs.

Table 2 shows most of the expressions published for the correlation of experimental data obtained in external-loop ALRs. Some of these expressions are presented in Figure 11. The differences between the predictions obtained with the different correlations are probably due to the design of the gas separator. The equation given by Popovic and Robinson (63) seems to give an average of the proposed correlations.

Table 2. Gas Hold-up in External-Loop ALR

Formula

0.6q0.062q0'069iG;107

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