Trays best efficiency point

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 ... 

K = 0.18 to 0.25 tray operation close to its best efficiency point K = 0.35 to 0.40 tray suffering from entrainment—increase in reflux rate, noticeably reduces tray efficiency K = >0.5 tray is in fully developed flood—opening a vent on the overhead vapor line will blow out liquid, with the vapor K = 0.10 to 0.12 tray deck is suffering from low tray efficiency, due to tray deck leaking... 

For a tray to function reasonably close to its best efficiency point, the dry tray pressure drop must be roughly equal ( 50 percent) to the hydraulic tray pressure drop ... 

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. 

K = 0.18 to 0.25 Tray operation is close to its best efficiency point. 

However, the largest operating cost for many process units is the energy supplied to the reboilers. We should therefore avoid high reflux rates, and try to achieve the best efficiency point for distillation tower trays at a minimum vapor flow. This is best done by designing and installing the tray decks and outlet weirs as level as possible. Damaged tray decks should not be reused unless they can be restored to their proper state of levelness, which is difficult, if not impossible. 

However, tray efficiency is usually unknown. In this case, a sample lab test may be warranted. It is recommended to take a side sample one tray away from the feed tray. The feed point in simulation is one stage away from the theoretical stage, which matches the sample composition the best. Taking the sample from the feed tray would give compositions that are highly influenced by the feed and hence cannot truly represent the internal compositions inside the column. 

Tbe best-established theoretical method for predicting E is that of tbe AlCbE [Buhhle-Tray Design Manual, American Institute of Chemical Engineers, New York, 1958). It is based on tbe sequential prediction of point efficiency, Murpbree efficiency, and overall column efficiency ... 

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