K hw kom hJ2620

and Eq. (6.19) becomes of aerated liquid (6.17)

Calculation of each of the individual terms in Eq. (6.21) is described in Sees. 6.3.1 to 6.3.4.

6.2.8 Downcomer aeration

The downcomer aeration factor 4>dc is defined by Eq. (6.18). It describes the fractional volumetric liquid holdup in the downcomer.

Mechanism. Vapor enters the downcomer with the froth that flows over the outlet weir. Additional vapor is entrained into the liquid due to the impact of the falling liquid on the liquid surface in the down-corner, in the same manner as a waterfall induces air entrainment into the pool below it. Inside the downcomer, vapor disengages from the liquid due to its higher buoyancy. The driving force for vapor disengagement is the density difference between the liquid and the vapor.

Figure 6.12a shows the structure of the fluid mixture in a downcomer operated at low liquid rates with a nonfoaming mixture. At the upper (froth) zone of the downcomer, the vapor fraction is high and of the same order as in the tray froth. As the mixture travels downward, much of the vapor is disengaged. The froth zone transforms into an aerated liquid zone where vapor bubbles rise through a liquid pooL Upon further vapor disengagement, the aerated liquid zone transforms into a clear liquid zone.

Factors affecting downcomer aeration

■ Foaming tendency: Vapor disengagement is easy in nonfoaming low-pressure systems. Vapor disengagement from downcomer liquid in foaming systems is difficult as the liquid hangs on to the entrained vapor. At high pressure, vapor disengagement from liquid is difficult because of the smaller vapor-liquid density difference (i.e., lower vapor buoyancy). In distillation systems, the decrease in surface tension as pressure (and, therefore, equilibrium temperature) rises promotes foaminess, and further retards vapor disengagement.

■ Liquid flow rate: At low liquid flow rates, with nonfoaming systems, the line of demarcation between zones in Fig. 6.12a is fairly sharp (42). As liquid rate increases, so does the fractional gas holdup in the downcomer liquid. Lockett and Gharani (43) found

Downcomer ApronDowncomer Apron
Downcomer superficial velocity, (l/s

Hgura 6.12 Downcomer aeration, (a) Structure of rwo-phase mixture in the downcomer; (6) dependence of downcomer and downcomer underflow gas fraction on downcomer liquid velocity, air-water tests. (Data for part b based on M. J. Locketi and A. A. W. Gkarani, I. Ghent. E. Symp. Ser. 56, p. 2.3/43, The Institution of Chemical Engineers UK, 1979.)

that this continues up to a certain critical velocity, beyond which the fractional gas holdup (and therefore downcomer aeration) becomes constant and independent of the liquid load (Fig. 6.126). They also observed that when this point was reached, pressure drop on the tray started increasing rapidly suggesting downcomer-choke flooding {Sec. 6.2.9).

Downcomer vapor underflow. In the past, it was thought that down-comer velocity needed to be kept small to avoid vapor being carried under the downcomer apron to the tray below. Thomas {42) and Lockett and Gharani (43), however, showed that some vapor underflow exists even at velocities considered "safe." The latter authors showed that the gas fraction in the downcomer underflow depends on downcomer superficial liquid velocity in a manner similar to downcomer gas fraction (Fig. 6.126).

Downcomer vapor underflow ("vapor entrainment" or "gas recycle") is analogous to liquid entrainment. It reduces both tray capacity and efficiency (17,44,45). In low- and medium-pressure distillation systems, where gas density is significantly lower than liquid density, it takes only a small quantity of gas to generate volumes comparable to the liquid volumetric flow rate. The quantity of gas recycle is therefore small, and it has little effect on tray performance. At high pressures, the quantity of gas recycled is significant. An analysis of some FRI data (44) for iC4-nC4 distillation showed vapor entrainment increases from about 7 percent at 165 psia to about 50 to 60 percent at 400 psia on a molar basis.

Downcomer aeration factor prediction. The fractional liquid holdup varies from about 0.3 in the froth zone to close to unity in the clear liquid zone (Fig. 6.12a). The height of each zone is a complex function of system properties, operating conditions, and downcomer geometry. This makes it practically impossible to theoretically predict the average downcomer aeration factor <j>dc Correlations in the literature (e.g., 46) are based on limited data obtained in atmospheric pressure simulator work with small downcomers. It is therefore difficult to recommend them for commercial-size applications. Zuiderweg (17) presented a plot of downcomer aeration factors derived theoretically from commercial-scale high-pressure flood data. However, the plot is based on a handful of data and is therefore difficult to recommend for general aeration factor prediction.

Popular aeration factor prediction criteria are rules of thumb based on the foaming tendency of the system (Table 6.4). The three criteria listed are supplementary, with one criterion adding examples not listed by the others. The author recommends applying the criteria accordingly. For instance, an aeration factor of 0.4 is appropriate to either mineral oil absorbers or for systems whose vapor density exceeds 3 lb, ft3. The criterion of Fair et al. (18) is perhaps a little conservative compared to the others.

table 6.4 Criteria for Downcomer Aeration Factors

Foaming tendency

Bolles' criterion (47)

Glitsch's criterion (7)

Fair et al's criterion (18)




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