L

1 2 3 4 5 6 7 8 Superficial gas velocity (cm/s)

Figure 13. Effect of liquid rheology on the gas holdup in the riser and in the downcomer of an internal-loop ALR with a non-Newtonian liquid. The apparent viscosity corresponding to the solutions used, calculated for y = 50 s_1, was 5-56 mPa s (65).

seen. This decrease is probably due to the onset of coalescence, which produces larger bubbles that ascend faster in the liquid and easily disengage in the gas separator. The viscosities corresponding to these solutions ranged from 4 mPa s to 14 mPa s. In Figure 13, the addition of carboxy-methyl cellulose (CMC) to water is shown (65). The change in CMC concentration had only a slight effect on the gas holdup for additions in the range 0.01 to 0.05% CMC. Only for solutions with concentrations higher than 0.5% CMC was an appreciable decrease in holdup seen.

Effect of Liquid Level. The influence of the liquid level Ct on the gas holdup is exerted as a consequence of changes in the extent of disengagement of the bubbles in the gas separator. This influence is therefore dependent on the geometric design of this section. Whether a bubble will disengage or will be entrapped into the downcomer depends on the interrelationship of several parameters—the free rising velocity of the bubble Ubro (a function of size and viscosity), the liquid velocity in the downcomer ULd (a function of the difference in gas holdup between the riser and the downcomer and frictional losses), and the residence time of the bubble in the gas separator (a function of geometric design and liquid height). For a given bubble size, if ULd is smaller than the Ubro corresponding to the smallest bubble, then there is no carryover. For smaller bubbles, the balance between the time required to cover the path from the end of the riser to the zone near the entrance of the downcomer and the time needed for disengagement will give the fraction of bubbles recirculated. It should nevertheless be kept in mind that this is a feedback process. A higher bubble disengagement rate leads to a lower gas holdup in the downcomer, which in turn increases the liquid velocity, enabling larger bubbles to be trapped, until the system eventually reaches a steady state.

Due to the above-explained influence of the geometric design, the influence of the liquid height is completely different in internal- and external-loop ALRs. In internal-loop reactors, an increase in Ct increases the zone of the separator in which the gas holdup is higher, and as a result, the gas holdup increases. The extent of this increase depends, as said earlier, on the free-rise velocity of the bubbles. Figure 14 (1) shows the gas holdup in the riser and downcomer of a 30-liter ALR for two values of the top clearance, Ct = 0.178 m and Ct = 0.308 m, for two different liquids, water and a 0.5% CMC solution. It can be seen that although for water Ct has a small effect, this is not so for the more viscous solution. In the latter, the lower rising velocity of the bubbles causes a greater number of bubbles to be entrained and carried down by the liquid. Thus, the residence time in the disengagement section becomes very important in determining the fraction of bubbles that recirculate. A lower Ct will give a shorter residence time in the separator, a larger bubble recirculation, and, hence, a larger gas holdup.

In external-loop ALRs, the opposite effect is obtained, as may be seen in Figure 15 (65). The data in Figure 15,

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