Downcomer vapor underflow

Downcomer vapor underflow, In the past, it was thought that downcomer 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 recycle1 ) 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. 

Equation (6.59) was derived from the orifice equation with an orifice coefficient of 0.6 (3), and assuming pure liquid is passing under the downcomer. Tests by Lockett and Gharani (43) showed that Eq. (6.59 gives conservative predictions, even under conditions of significant vapor entrainment in the downcomer underflow (Sec. 6.2.3). 

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