Figure 15. Gas holdup in the riser of an external-loop ALR for several top clearances. Adapted from Hallaile (65).

ALR. The liquid velocity depends mainly on the difference in holdup between the riser and the downcomer, and it in turn influences the gas holdup in the riser. Despite the importance of recirculation, very little quantitative data are available on this phenomenon. Siegel et al. (35) evaluated the gas recirculation in a split-vessel ALR by an indirect method based on holdup measurements. From their results, shown in Figure 16, it may be seen that the recirculation rate remains fairly constant for changing gas flow rates in the riser for high values of the last variable. Thus, the recirculation rate is determined largely by the geometric configuration of the gas-liquid separator and the liquid level in the separator.

Three zones are evident in Figure 16; they represent operating conditions giving oscillating, borderline, and straight bubble flow in the downcomer. Oscillating flow patterns produce much larger fractions of gas retained in the downcomer, but they are much more sensitive to JG. At low superficial gas velocity, the recirculation increases very sharply with JG. The bubbles exhibit an oscillating swirling flow pattern, with some larger bubbles escaping toward the top. The borderline condition is defined as oscillatory bubble flow at a low gas flow rate that shifts to straight bubble flow with increasing input gas flow rate. The straight flow operation zone is distinguished by bubble flow in a straight, well-defined flow pattern for all the input gas flow rates studied. If a straight bubble flow pattern is desired, the reactor should be operated at high riser gas flow rates, at which the reactor will shift toward stable operation.

Lubbert et al. (97) attempted to evaluate the recirculation of gas during the cultivation of yeast (Saccharomy-ces cerevisiae) on waste from a starch factory in a 4-m3 pilot plant. They used microprocessor-aided pseudostochastic tracer input and cross-correlation techniques, which facilitated very reduced tracer feeds due to a high signal-to-noise ratio. The response to a pulse of helium was measured directly at the surface of the liquid in the separator by a quadruple mass spectrometer. The peak obtained showed pronounced shoulders (Fig. 17) which could be interpreted as superimposition of a second peak that represents the helium tracer one loop after. The fitting of such

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