Incipient flood point

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. 

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. 

Experienced plant operators often refer to this situation as the column s "prime point," "jugging," or "optimum point." My term for these symptoms is incipient flood. For many refinery naphtha splitters, debutanizers, or depropanizers, the incipient flood point (i.e., the capacity at which the tower works best) should be reached at 80% to 90% of the calculated flooding capacity. Many towers, however, reportedly work best at a capacity of only 60% to 70% of flood. 

From this pressure drop, the gas rate at flood for any particular liquid rate can be calculated from a generalized pressure drop correlation. However, as these authors state, most of the data is based on an incipient flood point rather than a fully developed flooding condition. 

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. 

Note, 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 ... 

Figure 6.5 illustrates this relationship. Point A is the incipient flood point. In this case, the incipient flood point is defined as the... 

Incipient flood point The vapor-liquid loading at which a distillation tray develops its maximum efficiency. 

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