Mixing in the Gas Phase

For all practical purposes, the gas in the riser of an ALR exhibits plug flow behavior. Only for extremely high JG or hindered liquid circulation will the axial dispersion of the bubbles have some effect on the gas RTD. In the down-comer, the gas flow is almost plug-flow when the bubble recirculation is fully developed. But at the stage at which a stationary phase of suspended bubbles appears at the top of the downcomer, appreciable dispersion will occur. This zone has a large degree of mixing due to coalescence and consequent rise of larger bubbles amid smaller ones, with repeated events of breakup and coalescence. However, this type of operation has no relevance to practical applications. It is an operation mode to be avoided at all costs. Indeed, no data on mixing under these conditions have been reported. The main question related to the mixing of the gas phase is, in fact, related to gas recirculation. When a particular gas flow has developed in the downcomer, part of the gas is being recirculated (see "Liquid Velocity"). A pulse of gas tracer at the inlet would produce, as a response, a series of pulses, separated one from the other by the gas circulation time. In practice, not many of these pulses would be detected, due to dilution and disengagement of the tracer in each pass through the separator.

The only reported study on gas phase mixing in an ALR is that of Frehlich et al. (131). The distribution of the gas residence times in two reactors, one of 0.08 m3 and the other of 4 m3, was measured using pseudostochastic tracer signals and a mass spectrometer. The values of Bo were calculated from the first and second peaks, indicating the main gas stream and the recirculation. Figure 25 shows the Bo obtained for a laboratory-scale ALR. The values obtained in a pilot plant were of the same order.

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