•The author believes that "low-molecular-weight hydrocarbons" refers to light hydrocarbons at near atmospheric pressure or under vacuum. The foam stability of light hydrocarbon distillation at medium and high pressure is best inferred from the Glitach criterion.
A downcomer must be sufficiently large to transport all of the liquid downflow without choking. If the friction losses in the downcomer and or downcomer entrance are excessive, liquid will back up onto the tray and eventually flood the column. This is termed downcomer choke. The prime design parameter is the downcomer top area, where friction losses are highest. Further down the downcomer, vapor disengages and the aerated liquid downflow is greatly reduced. With sloped downcomers, the downcomer bottom area is normally set at about 1.7 to 2 times less than the area at the top of the downcomer (1,8,9,10,48). This taper is small enough to keep the downcomer top area the prime downcomer choke variable.
Criteria for determining downcomer area are described below. The author recommends that the downcomer area is set large enough to satisfy all of these criteria. A more detailed discussion is available elsewhere (1).
Downcomer velocity. The most popular criteria for maximum velocity of clear liquid at the downcomer entrance are the Glitsch (7), Koch (8), and Nutter (9) correlations. Lockett (12) noted large differences between the predictions of these correlations, but made no recommendations on which correlation to use. Lockett's analysis shows that generally the Glitsch correlation tends to predict the highest downcomer velocities, while the Koch correlation tends to predict the lowest downcomer velocities, with the Nutter correlation giving intermediate values. The Glitsch correlation (7) is
Qc.m« = [(&>,roe«)i or 2 or (QDimJ3, whichever is lowest]
SF is the derating factor (Sec. 6.2.10); if no derating, SF = 1.0.
The Koch correlation (8) and Nutter correlation (9) are maximum residence time criteria (see below) which were converted into downcomer velocity criteria. The maximum downcomer velocity is calculated from
The values of tR to be used in Eq. (6.26) are given by Fig. 6.13.
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