Entrainment Jet Flooding

Entrainment flooding can be classified into spray entrainment flooding and froth entrainment flooding. Spray entrainment flooding is far more common. Froth entrainment flooding is only encountered when 

The entrainment flooding prediction methods described below are based primarily on the spray entrainment flooding mechanism. Considerations unique to froth entrainment flooding prediction are presented later in the section. 

From Eq. (6.9) the Souders and Brown flooding constant, CgB can be defined 

The Souders and Brown constant, Csb C-factor [E)q. (6.4)] at the flood point. Most modem entrainment tloodii correlations retain the Souders and Brown equation (6.10) as the correlation basis, but depart from the notion that Cgg is a constant. Instead, thqy express CsB as a function of several variables (below), which differ from one correlation to another. Depending on the correlation, Csb and are based either on the net area Ajv or on the bubbling area A.  

The increase in CgB with liquid loads at very low liquid loads co- 

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Entrainment (Jet) Flooding Froth or spray height rises with gas velocity. As the froth or spray approaches the tray above, some of the liquid is aspirated into the tray above as entrainment. Upon a further increase in gas flow rate, massive entrainment of the froth or spray begins, causing liquid accumulation and flood on the tray above. 

Figure 6.6 is a typical tray stability diagram. The area of satisfactory operation (shaded) is bound by the tray stability limits. These limits are discussed in the following sections. The upper capacity limit is the onset of flooding. At moderate and high liquid flow rates, the entrainment (jet) flooding limit is normally reached when vapor flow is raised, while the downcomer flooding limit is normally reached when liquid flow is raised. When flows are raised while the column operates at constant LIV (i.e., constant reflux ratio), either limit can be reached. At very low liquid rates, as vapor rate is raised, the limit of excessive entrainment is often reached. 

Flooding- mechanism Entrainment (jet) flood only Tray typee Sieve or valve trays only Pressure 1,5-500 psia (Note 1)... 

Figure 8-123 illustrates a typical sieve tray capacity chart. Entrainment by jet flooding or limitation by downcomer flooding are two of the main capacity limiting factors. The liquid backup in the downcomer must balance the pressure drop across the tray, with the process balance [209]. 

System limit flooding is similar to jet flooding, due to low surface tension and low density difference between liquid and vapor. Terminal velocity of some entrainment droplets is less than the upward vapor velocity, and hence they are carried up into the tray above, thus reducing tray efficiency and capacity. 

In the spray regime, flooding (usually called jet flooding) is caused by excessive entrainment of liquid from an active area to the tray above. It increases the tray pressure-drop, and the entrained liquid recirculates to the tray below. The larger liquid load in the downcomer and the increased tray-pressure-drop together cause the downcomer to overfill so the tray floods. 

One of the most frequent causes of flooding is the use of carbon steel trays. Especially when the valve caps are also carbon steel, the valves have a tendency to stick in a partially closed position. This raises the pressure drop of the vapor flowing through the valves, which, in turn, pushes up the liquid level in the downcomer draining the tray. The liquid can then back up onto the tray deck, and promote jet flood, due to entrainment. 

Have about 15 percent less capacity because, when vapor escapes from the slots on the bubble cap, it is moving in a horizontal direction. The vapor flow must turn 90°. This change of direction promotes entrainment and, hence, jet flooding. 

The liquid on the tray deck was at its bubble, or boiling, point. A sudden decrease in the tower pressure caused the liquid to boil violently. The resulting surge in vapor flow promoted jet entrainment, or flooding. 

The first two factors help make fractionation better, the last factor makes fractionation worse. How can an operator select the optimum tower pressure, to maximize the benefits of enhanced relative volatility, and reduced tray deck dumping, without unduly promoting jet flooding due to entrainment ... 

Equation (3.91) is the jet flood equation. The chief difference between this equation and the entrainment flood equations, (3.88) through (3.90), is the area references. Equation (3.91) is based on the total sieve tray hole area for gas passage, and Eq. (3.88) through (3.90) are based on the tray active area. Again, the tray active area is simply the tower cross-sectional area less the total downcomer area. 

Jet flooding occurs due to liquid entrainment induced by vapor jets passing through the liquid flowing on the tray. The entrained droplet may carry into the tray area above and reduce tray efficiency and capacity. 

The liquid can then back up onto the tray deck and promote jet flood due to entrainment. 

Weir loading in U.S. gallons per minute (GPM) per inch of weir length should be kept between 3 and 12. Above 13 or 14, a loss of tray efficiency due to entrainment (i.e., jet flood) can be expected. Below 3 GPM per inch of weir, the liquid flow across the tray will not be particularly uniform and a loss in tray efficiency will result. 

The flooding-over of the big can is rather similar to vapor or jet flood from a distillation tower tray. If the area of the tray is too small or if the vertical separation (tray spacing) between the tray decks is inadequate for a particular vapor velocity, then the distillation tower will flood due to excessive entrainment of liquid from the tray below to the tray above. 

The action on this type of tray seems to produce fewer jets of liquid froth than a bubble cap tray. The entrainment from the surface of the bubbling liquid-froth mixture is less (about K) than a bubble cap tray for the same superficial tower velocity and tray spacing. Generally the trays will flood before capacity reaches a limitation set by entrainment. 

Pinczewski and Fell [Trans. Inst. Chem Eng., 55, 46 (1977)] show that the velocity at which vapor jets onto the tray sets the droplet size, rather than the superficial tray velocity. The power/mass correlation predicts an average drop size close to that measured by Pinczewski and Fell. Combination of this prediction with the estimated fraction of the droplets entrained gave a relationship for entrainment, Eq. (14-202). The dependence of entrainment with the eighth power of velocity even approximates the observed velocity dependence, as flooding is approached. 

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