Downflow flooding

These two types of flooding are usuaUy considered separately when a plate column is being rated for capacity. For identification purposes they are caUed entrainment flooding (or priming ) and downflow flooding. When counterflow action is destroyed by either type, transfer efficiency is lost and reasonable design hmits have been exceeded. 

Downflow Flooding Columns can flood because of their inability to handle large quantities of liqmd. For crossbow plates this hmit on liquid rate Is evidenced by downcomer backup to the plate above. To avoid downflow flooding one must size the column downcomers such that excessive backup does not occur. 

Similarly, with the gas rate held fixed, the liquid rate may be increased until flooding occurs. In this type of flooding, called downflow flooding, the liquid in the downcomer of each plate backs up to the plate above and a sharp increase in pressure drop across the column is observed. 

In order to prevent downflow flooding, the trays must be adequately spaced and the downcomers must be appropriately sized. It is customary in many designs to take the tray spacing S to be equal to twice the liquid holdup H in the downcomer (S = 2H). However, this choice of S may be more conservative than necessary or it may be inadequate. If it is known that the flow over the outlet weir is relatively free of vapor, then the tray spacing may be taken approximately equal to H. On the other hand, if it is known that the downcomer is filled with froth having a much lower density than the liquid, a tray spacing equal to as... 

For distillations, it is often of more interest to ascertain the effect of entrainment on efficiency than to predic t the quantitative amount of liquid entrained. For this purpose, the correlation shown in Fig. 14-26 is useful. The parametric curves in the figure represent approach to the entrainment flood point as measured or as predicted by Fig. 14-25 or some other flood correlation. The abscissa values are those of the flow parameter discussed earher. The ordinate values y are fractions of gross hquid downflow, defined as follows ... 

Current designs for venturi scrubbers generally use the vertical downflow of gas through the venturi contactor and incorporate three features (I) a wet-approach or flooded-waU entry sec tion, to avoid dust buildup at a wet-dry pmction (2) an adjustable throat for the venturi (or orifice), to provide for adjustment of the pressure drop and (3) a flooded elbow located below the venturi and ahead of the entrainment separator, to reduce wear by abrasive particles. The venturi throat is sometimes fitted with a refractoiy fining to resist abrasion by dust particles. The entrainment separator is commonly, but not invariably, of the cyclone type. An example of the standard form of venturi scrubber is shown in Fig. 17-48. The wet-approach entiy section has made practical the recirculation of slurries. Various forms of adjustable throats, which may be under manual or automatic control. 

A heterogeneous tubular reaetor that ineorporates three phases where gas and liquid reaetants are eontaeted with the solid eatalyst partieles, is elassified as a triekle-bed reaetor. The liquid is usually allowed to flow down over the bed of eatalyst, while the gas flows either up or down through the void spaees between the wetted pellets. Co-eurrent downflow of the gas is generally preferred beeause it allows for better distribution of liquid over the eatalyst bed and higher liquid flow rates are possible without flooding. 

Particular conditions may occur for two-phase downflows in vertical or inclined channels (ducts) that are not completely described by the flow regime maps. Flooding occurs as the rising vapor completely blocks descending liquid. With lowering velocities of the vapor phase, this condition is preceded by the phenome-... 

Since a downflow in flooding is under the choked condition by the upward-moving void, the void may become stagnant along the wall. Thus the heat transfer... 

Current designs for Venturi scrubbers generally use the vertical downflow of gas through the Venturi throat and incorporate three features (1) a wet-approach or flooded-wall entry section to avoid a dust buildup at a wet-dry junction ... 

Low pressure-drop and no flooding (at least for cocurrent, downflow reactors). 

Figure 18.5. Entrainment from sieve trays in the units mols liquid entrained/mol of liquid downflow LM, is the weight rate of flow of liquid and VMv is the weight rate of flow of vapor. The flooding correlation is Figure 13.32(b). [Fair and Matthews, Pet. Refiner 37(4), 153 (195S)].

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... 

For decades, the Fair correlation [Pet/Chem. Eng., 33(10), 45 (September 1961)] has been used for entrainment prediction. In the spray regime the Kister and Haas correlation was shown to be more accurate [Koziol and Mackowiak, Chem. Eng. Process., 27, p. 145 (1990)]. In the froth regime, the Kister and Haas correlation does not apply, and Fair s correlation remains the standard of the industry. Fair s correlation (Fig. 14—34) predicts entrainment in terms of the flow parameter [Eq. (14-89)] and the ratio of gas velocity to entrainment flooding gas velocity. The ordinate values XF are fractions of gross liquid downflow, defined as follows ... 

Countercurrent Flow The gas flows up countercurrent with the downflow liquid. This mode of operation is not as widely used for catalytic reactions since operation is limited by flooding at high gas velocity at flooding conditions increasing the liquid flow does not result in increase of the liquid holdup. 

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. 

In general, when a fixed bed is selected, the issue whether to employ a concurrent upflow or downflow operation must be considered. Operating a randomly packed bed reactor in the countercurrent mode is usually not feasible because flooding occurs at gas velocities far below industrial relevance. In a concurrent upflow, complete catalyst wetting is obtained at the expense of much larger liquid holdup compared to a concurrent downflow. High liquid holdup... 

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