Tower Pressure Drop and Flooding

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 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 distillation towers. We have derived the following formula  

On the basis of hundreds of field measurements, we have observed 

18 to 0.25 Tray operation is close to its best efficiency point. 

35 to 0.40 Tray is suffering from entrainment—increase in reflux rate noticeably reduces tray efficiency. 

5 Tray is in fully developed flood—opening a vent on the overhead vapor line will blow out liquid with the vapor. 

00 The liquid level on the tray is zero, and quite likely the trays are lying on the bottom of the column 

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. 

Of course, any factor (dirt, polymers, gums, salts) that causes a reduction in the open area of the tray deck will also promote jet flooding. Indeed, most trays flood below their calculated flood point, because of these sorts of problems. Trays, like people, rarely perform quite up to expectations. 

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