The reflux and feed stream introduction into a tower is a critical aspect of distillation column design. Unfortunately, it is also an often overlooked aspect, especially for trayed towers. This is because trays are normally forgiving pieces of equipment and can conceal the effects of poorly designed feed and reflux entries. As stated by Lieberman (1988), "... the top tray of a fractionator acts as an excellent liquid distributor...." However, when one tries to push a tower to its hydraulic limit, poor entry design can penalize the performance of the trays and result in a lower final capacity. A lot of attention is given to tray hydraulics, but the fundamentals of reflux and feed entry design are equally as important. Ultimately, failure to recognize this can lead to premature flooding of trays, mechanical failure and overall poor unit reliability.
Normally, new towers are not as susceptible to entry design problems as ones being revamped. This is because new towers usually have some degree of capacity over sizing. Standard design practices used for new columns having spare capacity, though, may not be suitable for revamped towers.
Our objective is to elaborate on the basic principles of reflux and feed entry design, good practices to follow, and poor practices to avoid. A case study to illustrate the potential pitfalls associated with incorrectly designed reflux and feed entry arrangements is shown.
The case study was a large diameter light hydrocarbon splitter revamped with high capacity trays. The objective of the revamp was to increase the fractionator throughput capacity by approximately 35 percent while maintaining existing product specifications. A post revamp audit of the unit revealed that the column capacity had increased by only 20 percent. Detailed analysis showed that the root cause of the lower than expected capacity was the poorly designed feed and reflux return configurations. The premature flooding problems and overall poor reliability of the unit caused by the feed and reflux entry design configurations could not be tolerated and led to a rework of the internals. After modifications addressing the reflux and feed systems were implemented, an additional 9 percent capacity increase resulted.
It is shown that good design practices are founded on a set of simple questions that the designer must investigate and answer. Common sense is also an important ingredient in achieving a sound design. These simple but important points that must be addressed are:
10 pages including 11 figures
Electronic version available in Adobe Acrobat PDF format file 030.PDF 818k.
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