As a DCS Operator, what parametric and/or qualitative changes suggest the following problems?
Flooding, dumping, jet flooding, entrainment, weeping and foaming.
Answers
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11/08/2020
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A:
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Bruce Carr, Suncor Inc, matukaze1@hotmail.com
There are a couple of "rule of thumb" indicators that a console operator can use to determine how a tower is operating. First off understanding the terms... Flooding... When one or more trays are full of liquid and impinging on the tray above normally happens during an upset, internal damage or too much liquid traffic v's vapour traffic. Can be indicated by a high tower differential pressure (not top flooding, I'll talk about that in a minute). Dumping happens when there is not enough vapour traffic V's the liquid traffic and the liquid drops through the holes in the tray. Entrainment would be indicated by bad separation can have several causes. Weeping is a mild form of dumping. Foaming occurs in only certain services...
Certain things a Console operator should know and use as a guide.... (I'm assuming the tower has a reboiler and is top temperature controlled)....
First... If you suspect a tower is flooded and a high differential pressure indicates this... cut your reflux...empty it from the top down.... In the past operators have been told to cut the reboiler heat.... The vapour from the reboiler is holding the weight of the liquid on the trays. If you cut the heat too quickly you run the risk of the trays collapsing....(It only takes 1/2# differential pressure across a tray to upset it) Take your time don't be in a rush.
Secondly...Make this your mantra.. "Reflux comes from the Reboiler".... If you increase the reflux flow to try to increase overhead product purity this will only work until you hit what is called the "Incipient Flood Point". Any time you increase the reflux the heat medium to the reboiler must increase also. If you increase the reflux and the Reboiler duty goes down you are in a "Top Flood" the top tray and overhead will be full of liquid.... The tower differential pressure will be low as only one tray is flooding. Reduce your reflux to below what caused it happen. If you don't the top circuit will just keep circulating the same product and will steadily get worse..
Maximize the differential temperature between the top and bottom of the tower for best separation.... Another indication you have passed the incipient flood point will be the Delta T will drop.
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