Choke Flooding

Downcomer Choke Flooding This is also called downcomer entrance flood or downcomer velocity flood. A downcomer must be sufficiently large to transport all the liquid downflow. Excessive friction losses in the downcomer entrance, and/or excessive flow rate of gas venting from the downcomer in counterflow, will impede liquid downflow, initiating liquid accumulation (termed downcomer choke flooding) on the tray above. The prime design parameter is the downcomer top area. Further down the downcomer, gas disengages from the liquid and the volumes of aerated liquid downflow and vented gas... 

To avoid downcomer choke flooding, downcomers are sized to give a liquid residence time of not less than 3-7 sec, depending on the tendency of the liquid to form a stable foam. 

Figure 6.7 Common flooding mechanisms in tray columns, (a) Spray entrainment flood (ft) froth ontrainment flood (c) downcomer backup flood Id) downcomer choke flood. (Parle a and ft reproduced from Dr. D. C. Hausch, Discussion of Paper Presented In the Fifth Session, Proceedings of the International Symposium on Distiliation, the Institution of Chemical Engineers (London), I960, reprinted courtesy of the Institution of Chemical Engineers, UK- Parts c and d from H. Z. Kister. Distillation Operation. Copyright C 1990 6y McGraw-Hill, Inc. reprinted by permission.)...

Downcomer choke flooding (Fig. 6.7d). As liquid flow rate increases, so does the velocity of aerated liquid in the downcomer. When this velocity exceeds a certain limit, friction losses in the downcomer and downcomer entrance become excessive, and the frothy mixture cannot be transported to the tray below. This causes liquid accumuletion on the tray above. 

Capacity restriction mechanism(s). Column throughput is restricted by one of several different mechanisms. These include spray entrainment flooding, froth entrainment flooding, downcomer backup flooding, downcomer choke flooding, excessive entrainment and excessive pressure drop. Optimum tray and downcomer layouts vary with the mech-... 

Downcomer choke flooding (Fig. 14.1d). As liquid flow rate increases, so does the velocity of aerated liquid in the downcomer. When this veloc-... 

Prediction. Prediction methods for spray entrainment, froth entrainment, and downcomer backup are available in distillation texts and articles (e.g., 73, 193, 201o, 243, 257, 319, 371, 404, 409). Criteria for predicting downcomer choke flooding are discussed in Sec. 6.16. 

---