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where the constant a depends on the friction losses in the loop, and b is usually a value between 0.6 and 0.7, as is illustrated in Figure 10 (65). The fact that neither the area ratio nor the top clearance affects the gas holdup demonstrates the role of the gas-liquid separator in determining the performance of the reactor in general. In the absence of gas recirculation, there is no effect on these variables. Moreover, this means that under conditions of no gas en-trainment from the separator to the downcomer, it is possible to predict the riser gas holdup as a function of the riser superficial gas velocity alone, which is of great importance for design purposes.

It is accepted that liquid velocity has a mild negative effect on gas holdup in the riser. This effect is usually studied by reducing the liquid flow; this is achieved by adding resistance to the liquid loop by means of a valve or other controlled obstruction (71,78,82) under conditions of low or nil gas recirculation. Such experiments, which are relatively simple in external-loop ALRs, indicate that the holdup decreases as the liquid velocity is increased from zero (bubble column) to 0.3 m/s (which is close to the bubble free-rise velocity). For higher velocities, the effect of UL is small. These findings add to our understanding of the fluid dynamics in the column. At liquid velocities that are smaller than the bubble free-rising velocity, the liquid transported in the wake of the bubbles, which must return downward to balance the mass flux, is the cause of the meandering and loops that typically appear in bubble column operation (83). As the overall liquid flux increases, the patterns straighten out, the bubbles begin to ascend in a straight pattern, and the holdup goes down. When the liquid velocity is higher than the free-rise velocity of the bubbles, piston flow of bubbles ensues in the tube, and the decrease in holdup for further increases in liquid velocity is due solely to the change in the ratio of gas-liquid volumetric flow rates.

When there is gas recirculation, the area ratio Ad/Ar becomes an important variable affecting gas holdup. The effect of Ad/Ar starts in the region in which gas entrain-

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