I I I

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 Superficial gas velocity (m/s)

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 Superficial gas velocity (m/s)

Figure 18. Liquid velocity predicted by some of the proposed correlations from Table 3. ILR, internal-loop ALR; ELR, external-loop ALR.

superficial gas velocity, but its rate of increase is much lower at high superficial gas velocities. From Table 3, it can be seen that the riser-to-downcomer cross-sectional area ratio and the reactor height are the main parameters that affect the superficial liquid velocity at constant superficial gas velocity. The superficial liquid velocity increases with an increase in Ad/Ar.

The effect of the properties of the liquid, such as viscosity, surface tension, and ionic strength, on the superficial liquid velocity are much milder in ALR than in bubble columns (113). It is expected that increasing the liquid viscosity will decrease the liquid velocity because of frictional losses, but this, in turn, will increase the gas holdup in the riser and consequently increase the driving force for liquid recirculation. Hence, it seems that these two effects balance each other partially and result in a milder effect on the superficial liquid velocity. The effects ofthe surface tension and the ionic strength are also exerted via their influence on the gas holdup, as analyzed above.

As a rule, it can be recommended that the ZuberFindlay expression (equation 20) be used when the holdup is known and the liquid circulation velocity is high. For low liquid velocity, a correlation obtained in a piece of equipment as similar as possible to the one under design should be used.

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