Re: Gain Consistency

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Posted by Graeme Taylor on September 28, 2000 at 15:32:25:

In Reply to: Re: Gain Consistency posted by Don Snowden on September 21, 2000 at 11:51:39:

Thank you for the reply, Simulate certainly shows the controller works more sensibly having forced the gain ratio to be constant, but having continued looking at the controller I have some further concerns.

I’ve been looking closely at the pressure and temperature curves for each tower (de-ethaniser, de-butaniser and naphtha splitter) and I am seeing similar problems, but this time with the material balance. For example on the debut the distillate product is a CV and again the raw gains suggest that it is possible to control say the C4s and the distillate product at limits with only the temperature and pressure. I doubt that this is true and again forcing the curves to have the same gain ratio solves the problem.

Having thought about this some more I am coming to the conclusion that this should not be a surprise, as pressure and temperature simply both set the cutpoint. Therefore shouldn’t the gain ratio of pressure and temperature for the qualities and material balance be constant for any lightends towers? In effect the controller will then simply implement pressure compensation of temperature and the ratio can be checked by the Antoine equation if desired. To match the gains I don’t have to change them very far, about the 5-10% that you mentioned and the ratios are very close to that suggested by the Antoine equation. I’d appreciate any thoughts on this.

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

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: 
: Thank you for the reply, Simulate certainly shows the controller works more sensibly having forced the gain ratio to be constant, but having continued looking at the controller I have some further concerns.  

: I’ve been looking closely at the pressure and temperature curves for each tower (de-ethaniser, de-butaniser and naphtha splitter) and I am seeing similar problems, but this time with the material balance.  For example on the debut the distillate product is a CV and again the raw gains suggest that it is possible to control say the C4s and the distillate product at limits with only the temperature and pressure.  I doubt that this is true and again forcing the curves to have the same gain ratio solves the problem.  

: Having thought about this some more I am coming to the conclusion that this should not be a surprise, as pressure and temperature simply both set the cutpoint. Therefore shouldn’t the gain ratio of pressure and temperature for the qualities and material balance be constant for any lightends towers?   In effect the controller will then simply implement pressure compensation of temperature and the ratio can be checked by the Antoine equation if desired.  To match the gains I don’t have to change them very far, about the 5-10% that you mentioned and the ratios are very close to that suggested by the Antoine equation.  I’d appreciate any thoughts on this.

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