Incipient flood

When the spray height from the tray below hits the tray above, this is called the incipient flood point, or the initiation of jet flooding. 

though, that jet flood may be caused by excessive downcomer backup. It is simple to see in a glass column separating colored water from clear methanol how tray separation efficiency is reduced as soon as the spray height equals the tray spacing. And while this observation of the onset of incipient flood is straightforward in a transparent tower, how do we observe the incipient flooding point in a commercial distillation tower  

The reason I can write with confidence on this subject is that I worked with a 4-inch demonstration transparent column at the Chevron Refinery in Port Arthur, Texas, in 1989.1 used the little distillation tower to explain to plant technicians how distillation towers worked. 

The tower s feed was windshield wiper fluid with blue dye. The alcohol went overhead and the blue water was the bottoms product. 

The amount of butane in the overhead propane product drops. 

The reboiler duty increases, to restore the tower-bottom temperature to its set point. 

The weight flow of vapor and the vapor velocity through the tray increase. 

The spray height, or entrainment, between the trays increases. 

When the spray height from the lower trays impacts the upper trays, the heavier, butane-rich liquid contaminates the lighter liquid on the upper trays with heavier butane. 

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There is not a specific flood curve a pressure drop of 1.50 in HgO/ft is considered to represent an incipient flooding condition, although... 

Lieberman gives two rules of thumb for troubleshooting fractionators that could also be used as checks on a design. First, the pressure drops across a section of trays must not exceed 22% of the space between the tray decks, to avoid incipient flood. Mathematical , hold... 

Figure 1.10 illustrates this point, from plant test data obtained in a Texas refinery. Point A is called the incipient flood point, that point in the towers operation at which either an increase or a decrease in the reflux rate results in a loss of separation efficiency. You might call this the optimum reflux rate, that would be an alternate description of the incipient flood point, neglecting the energy cost of the reboiler steam. 

It is a characteristic of process equipment, that the best operation is reached, at neither a very high nor a very low loading. The intermediate equipment load that results in the most efficient operation is called the the best efficiency point. For distillation trays, the incipient flood point corresponds to the best efficiency point. We have correlated this best efficiency point, for valve and sieve trays, as compared to the measured pressure drops in many chemical plant and refinery distillation towers. We have derived the following formula ... 

Figure 3.5 illustrates this relationship. Point A is the incipient flood point. In this case, the incipient flood point is defined as that operating pressure that maximizes the temperature difference across the tower at a particular reflux rate. How, then, do we select the optimum tower pressure, to obtain the best efficiency point for the trays Answer—look at the temperature profile across the column. 

An increase in reflux rate, assuming that the reboiler is on automatic temperature control, increases both the tray weir loading and the vapor velocity through the tray deck. This increases both the total tray pressure drop and the height of liquid in the tray s downcomer. Increasing reflux rates, with the reboiler on automatic temperature control, then will always push the tray closer to, or even beyond, the point of incipient flood. 

If the liquid holdup is too low, fractionation efficiency will be bad. We say that the height equivalent to a theoretical plate (HETP) will be high. If the liquid holdup is too high, fractionation efficiency will also be poor. We again say that the HETP will be high. This idea is expressed in Fig. 7.6. When the holdup rises above the point that corresponds to the minimum HETP, we can say that the packing is beginning to flood. The minimum HETP point on Fig. 7.6 corresponds to the point of incipient flood, discussed in Chap. 1. 

Reducing the pumparound duty increases the tray loadings on trays 1 through 7. But in so doing, the trays operate closer to their incipient flood point. This is fine. The incipient flood point corresponds to the optimum tray performance. But if we cross over the incipient flood point, and trays 5, 6, and 7 actually start to flood, their fractionation efficiency will be adversely affected. Then, as we decrease the pumparound heat-removal duty, the mutual contamination of diesel and gas oil will increase. 

From an operating standpoint, we can see when this flooding starts. As we decrease the pumparound duty, the temperature difference between the diesel- and gas-oil product draws should increase. When these two temperatures start to come together, we may assume that we have exceeded the incipient flood point, and that trays 5, 6, and 7 are beginning to flood. 

Equation (14-142) permits finding the pressure drop curve in Fig. 14-55 or 14-56 at which incipient flooding occurs. 

But another important complication is related to a not uniform distribution of humidity inside the electrolyte and on the catalyzed electrodes, and to the eventuality of incipient flooding phenomena, due to an excess of water bulked inside the fuel cell module that could dangerously limit the reactant entrance to active sites of catalyzed electrodes. This phenomenon could be present also without external water addition, because dry air streams entering into the ceU module gradually moisturize creating the possibility of local flooding at the end of their path [28]. 

Entrainment flooding. At incipient flooding, die minimum column diameter is fixed. 

FIGURE 13.13 Capacity of plate-type absorbers and strippers. Curves denote the incipient flooding condition, and for design must be discounted. [Fair, J. R., 1961. Petro/Chem. Engineer 33 (9) 57.]... 

The maximum allowable capacity of a plate absorber or stripper is represented by an incipient flooding condition. The prudent operating capacity is based on about 85% of the predicted flooding condition. The first step is to calculate the dimensionless ow parameter for the point in the column being investigated ... 

As mentioned above, whan the vapor rare i very high, downswing liquid lands to be held up by the drag of the vapor. This reduces the net available cross section for vapor flow and causes the pressure drop to increase. If vapor raw continues to increase, liquid will be carried overhead and the column will reach a stale of incipient flooding. The same situation can arise if there is a large increase in liquid rate, when vapor rate is kept constant, and excessive rates of liquid also can Iced to a state of incipient flooding. 

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