Equipment in distillation towers must do more than separate the feed stream into distinct products. It must accommodate the entry of streams into and draw of streams from a column. Unfortunately, this aspect is also often overlooked when the engineer concentrates solely on matters such as tray efficiency and packing HETP (height equivalent theoretical plate).
Getting material into a tower involves three separate cases: feed entry, reflux return, and reboiler return. Drawing material from a tower includes liquid draws, vapor draws, partial draws, and total draws. Each of these has specific areas of concern. In addition, drawing and feeding on packing versus trayed towers adds even more permutations.
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 here is to present general issues on feeding and drawing streams and then follow this with specific practical aspects applicable to both trayed and packed towers. Three case studies, one for a packed tower and two for trayed towers will illustrate the relative importance of the points raised. Ultimately, the failure to take feed and draw design properly into account can lead to premature tower flooding, lower efficiency, and reduced capacity.
We will elaborate on the basic principles of feed and draw design, good practices to follow, and poor practices to avoid. The case studies will illustrate the potential pitfalls associated with incorrectly designed arrangements.
As a basis of good practice in design, we will show that following a set of basic, yet critical, criteria leads to successful feed and draw design:
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