Continuous distillation is one of the most important separation operation in the process industries.  It is also one of the most energy intensive processes.  Optimization is very important for this reason.  Some Helpful Hints are shown below.

• Packed towers should operate at 70% of flooding according to the correlation of Sherwood, Lobo, et. al.

• Reflux drums are usually in the horizontal configuration with a liquid holdup of 5 minutes when the drum is half full. In some cases 10 -15 minutes.

• For towers of about three feet, add four feet of straight side before the weld on the top head for vapor disengagement and a 6 inch mist elimination pad. Add six feet of straight side before the weld on the bottom head for liquid level and reboiler return.

• Reflux pumps should be oversized by 25% to handle transient higher flows during upsets, allow for times when a higher reflux ratio may be required, and allow for times when the column capacity is pushed to higher rates.

• The minimum number of trays is found with the Fenske- Underwood equation Nm, = log{[x/(1- x)]ovhd/[x/(1- x)]btms}/log α.

• Economically optimum reflux ratio is about 1.2 times the minimum reflux ratio Rmin.

• Pressure drop per tray is about  3 in. of water.

• Tower operating pressure is determined most often by the temperature of the available condensing medium, 100-120 °F if cooling tower water; or by the maximum allowable reboiler temperature, 150 psig steam, 366 °F.

• Tray spacings are usually 24 inches.  Then allows good access for tray installation and later on for routine inspections.

• Always inspect a column twice on a turnaround to insure there are no tools left in a column prior to startup. One of our engineers once found an old flashlight in a trayed column.  It had been in there for years.