Gain Consistency

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Posted by Graeme Taylor on September 19, 2000 at 17:18:23:

I am currently implementing a DMC+ controller on a refinery light ends train and I have a question regarding the debutaniser. The models that I have identified for the C5s in the distillate and the C4s in the bottoms appear to have some inconsistencies.

The base control for the tower is:-
1. Tray temperature controller manipulating reboiler duty
2. Internal reflux controller manipulating the external reflux
3. Pressure controlled by the distillate product (the tower has a drumless flooded condenser).

The question that I have is with regard the models I have identified for the C4s and the C5s for moves in pressure and tray temperature. The gains for these variables would suggest that if I increased the tower pressure, whilst holding the C5s constant by adjusting the tray temperature that the C4s in the bottom product would decrease. I can’t think why this would happen, if anything I would expect the reverse due to a potential change in relative volatility (although I doubt I would see this impact as the moves we could make on the pressure during the test were fairly small).

I am concerned that if I put the controller online with these gains that the controller will at some stage try to control both the C4s and the C5s to their limits, even when the reflux was unavailable (say at an MV limit). As a result I am considering forcing the pressure and temperature gains to have a constant ratio for the C4s and the C5s. I hope this will mean that the controller will not try to control both qualities simultaneously with only pressure and temperature. Has any one else seen this potential problem and if so what have you done (if anything) to solve it?

Follow Ups:

Re: Gain Consistency Mike McCarty 12:05:52 9/28/00 (0)
Re: Gain Consistency Don Snowden 11:51:39 9/21/00 (2)

Re: Gain Consistency Graeme Taylor 15:32:25 9/28/00 (1)

Re: Gain Consistency Don Snowden 16:26:32 10/02/00 (0)

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: I am currently implementing a DMC+ controller on a refinery light ends train and I have a question regarding the debutaniser. The models that I have identified for the C5s in the distillate and the C4s in the bottoms appear to have some inconsistencies.
: The base control for the tower is:-
: 1. Tray temperature controller manipulating reboiler duty
: 2. Internal reflux controller manipulating the external reflux
: 3. Pressure controlled by the distillate product (the tower has a drumless flooded condenser).
: The question that I have is with regard the models I have identified for the C4s and the C5s for moves in pressure and tray temperature. The gains for these variables would suggest that if I increased the tower pressure, whilst holding the C5s constant by adjusting the tray temperature that the C4s in the bottom product would decrease. I can’t think why this would happen, if anything I would expect the reverse due to a potential change in relative volatility (although I doubt I would see this impact as the moves we could make on the pressure during the test were fairly small).
: I am concerned that if I put the controller online with these gains that the controller will at some stage try to control both the C4s and the C5s to their limits, even when the reflux was unavailable (say at an MV limit). As a result I am considering forcing the pressure and temperature gains to have a constant ratio for the C4s and the C5s. I hope this will mean that the controller will not try to control both qualities simultaneously with only pressure and temperature. Has any one else seen this potential problem and if so what have you done (if anything) to solve it?

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