5 replies to this topic

[USR]

Dear members,
From what i know about flooding,it can happen in two ways either when the liquid feed(feed +reflux) into the column increases at constant vapour load or when vapour load to the column increases(higher reboiling) for constant liquid load.In either cases one observes an increase in the differential pressure across the column.Can one infer about flooding in distillation columns only through dp across column.I once attended a lecture by Norman Lieberman in which he had said,"high dp across column is not a very reliable inference for flooding".Can anyone clarify??

Edited by USR, 07 June 2012 - 01:41 AM.

[katmar]

Have you ever seen one of those joke weather stations consisting of a small rock hanging from a piece of rope? The instructions that go with the weather station say things like "If the rock is wet then it is raining", "If the rock is swinging it is windy", "If the rock is casting a shadow it is sunny" etc.

The pressure drop across a column can be a bit like that because the pressure drop over a tray is not very sensitive to the flowrates, but when the tray floods the pressure drop increases very rapidly. So you could say "If the pressure drop across the column is high then it is flooded". But when you are operating a column you really want to know before the column is actually flooded that a problem is building up.

Because the pressure changes are small, and because the pressure does change slightly with flow rate any way, it is hard to recognize quickly what the pressure drop is telling you. Unfortunately there isn't much else that can give an indication of incipient flooding. If you are watching the column carefully and you are aware of any changes in flow rates then you definitely can see the start of flooding, particularly if you know the columns characteristics well.

So I would have to both agree and disagree with Norman Lieberman. The pressure drop isn't the easiest indicator to read, but because it is about the only one we have, we have to learn to make the most of it. One thing that certainly does help is if you can plot the trends for the pressure drop together with the main flows (feed and reflux) and the steaming rate all together on your monitor then a diverging of the lines can warn you that a problem is building up.

[sheiko]

USR,
I have also attended NL's course and also read his book and, on the contrary, I believe he encourages to rely on pressure drop through the column as an indicator to monitor flooding. Read the first 4 chapters of "A Working Guide to Process Equipment".

Edited by sheiko, 08 June 2012 - 06:03 PM.

[Art Montemayor]

I concur with Sheiko.

I know Norm has been quoted out of context many times. Additionally, many people have totally misinterpreted what he writes and says in his seminars. To confirm this, I take the liberty to quote Norm directly on this subject - from one of his books:

A Working Guide to Process Equipment
Norm & Elizabeth Lieberman
Page 22

"Distillation tower turndown
The problem we have been discussing – loss of tray efficiency due to low vapor velocity – is commonly called turndown. It is the opposite of flooding, which is indicated by loss of tray efficiency, at high vapor velocity. To discriminate between flooding and weeping trays, we measure the tower pressure drop. If the pressure drop per tray, expressed in inches of liquid, is more than three times the weir height, then the poor fractionation is due to flooding. If the pressure drop per tray is less than the height of the weir, then poor fractionation is due to weeping or dumping."

[katmar]

Thanks to Art for posting that extract from the Lieberman book. It is a book that I have not used, but from this small extract I suspect it is likely to be a solid practical resource, clearly written by an engineer who has "been there, done that". It is also clear that his intent is very much what I compared with the joke weather station. Asking whether the pressure drop is >3x the weir height or less than the weir height is analogous to asking "Is the rock casting a clear shadow, or is it covered in snow". Once you have got to the SNAFU position the pressure drop will tell you whether you are flooded or weeping, but it has to be interpreted with care and a bit of experience to give a real-time indication of an impending problem.

[Raj Mehta]

What measures can we take before hand to determine the incipient condition of flooding? As Mr. katmar also highlighted in post#2, about plotting the graphs, is it possible ? When should one do that i.e. during the manufacturing stage or commissioning stage ? Varying the reflux and finding the point (maximum reflux point) beyond which flooding occurs (keeping feed & vapor load constant) then repeating the same for vapor load. I believe finding this 2 target points and keeping a track of these values will help a great way to avoid column from flooding. Correct me If I am wrong. Raj Mehta