As an operator reduces the tower pressure, three effects occur simultaneously:
• Relative volatility increases.
• Tray deck leakage decreases.
• Entrainment, or spray height, increases.
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?
To answer this fundamental question, we should realize that reducing the tower pressure will also reduce both the tower-top temperature and the tower-bottom temperature. So the change in these temperatures, by themselves, is not particularly informative. But if we look at the difference between the bottom and top temperatures, this difference is an excellent indication of fractionation efficiency. The bigger this temperature difference, the better the split. For instance, if the tower-top and tower-bottom temperatures are the same for a 25-tray tower, what is the average tray efficiency? (Answer: 100 percent / 25 = 4 percent.)
Figure 5.5 illustrates this relationship. Point A is the incipient flood point. In this case, the incipient flood point is defined as the 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.
