Lhj

Figure 21. Bodenstein numbers as a function of the superficial gas velocity in an external-loop ALR. ALR-exp, experimental values for the whole reactor; ALR-calc, calculated values for the whole reactor. Adapted from Vorlaan et al. (80).

The extent of mixing in the gas separator of internal-loop ALRs can probably be compared to that in a bubble column of the same dimensions. In the case of external-loop ALRs, the extent of mixing depends on the geometric design. In special designs, like the channel-loop reactor (125), a different approach must be taken. However, in general, a rule of thumb may be recommended: consider that all the mixing takes place in the gas separator, and cal-

Figure 22. Simple model of an ALR representing the riser and the downcomer as plug-flow reactors and the gas separator as a perfectly mixed region. From Merchuk and Siegel (67).

Gas in

Figure 22. Simple model of an ALR representing the riser and the downcomer as plug-flow reactors and the gas separator as a perfectly mixed region. From Merchuk and Siegel (67).

Figure 23. Schematic representation of the effect of a tall slender gas separation region in a concentric-tube ALR.

culate the degree of mixing in it using published data for bubble columns. Correlations available for bubble columns can thus be used. Godboole and Shah (126) recommend the use of the correlation proposed by Deckwer et al. (127):

where DZ is the dispersion coefficient and D is the column diameter, or the relationship proposed by Joshi (128):

where JL is the superficial liquid velocity and Uc is the cell circulation velocity given by:

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