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Superficial gas velocity (m/s)

Figure 37. Time constants calculated in an external-loop ALR as a function of the superficial gas velocity (187).

ment have been proposed. One of the earliest modifications was the two-staged ALR, proposed by Orazem and Erick-son (148). Their design was inspired by the improvement observed by them in the performance obtained with multistage sieve trays over single-stage bubble columns. They claimed that a substantially higher mass transfer coefficient was obtained, as was a better performance in terms of oxygen transferred per unit of energy invested.

The combination of a concentric-tube ALR with a marine propeller (1000 rpm) was studied by Pollard et al. (188) (Fig. 38). This modification, in which the stirrer was located near the bottom of the draft-tube, served to increase the circulation rate, which may be low for viscous liquids. This configuration showed enhanced oxygen transfer and more uniform distribution of the dissolved oxygen concentration throughout the reactor in comparison with that of the unmodified ALR. The liquid circulation was also intensified. However, the improvement was achieved at the cost of the introduction of a focus of energy input. As has been mentioned before, the absence of such a high shear region is one of the advantages of the ALR. This advantage is, in part at least, lost. It is interesting to note that a bakers' yeast culture (188) grown in the same type of modified reactor did not reveal any improvement. This may be related to the strong shear rate in the vicinity of the propeller.

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