PID loop - long residence time
PID loop - long residence time
(OP)
Hi there,
I am trying to control a mass flowmeter to input Chemical A into a series of reactors, each with a residence time of 30 mins. I need to dope enough of Chemical A to remove environmentally harmful Chemical B from the reactors (need <1ppm). My analysis sampling point for Chemical B is after Reactor 2, so total residence time is 1 hr (after 2 reactors x 30 mins each).
I am using a PID controller to control the flow of Chemical A into Reactor 1 based on the ppm level of Chemical B out of Reactor 2. It takes approximately 10 mins to get a ppm reading of Chemical B via the online analyzer.
Once I get a ppm reading for Chemical B, I set the flowrate for Chemical A dosing. The problem is that if I change the flow of Chemical A, I do not see the effect of this change (lowered ppm reading of Chemical B) for at least 1 hr.
My knowledge of controllers is very limited; I realize that this problem may be very basic and so I appreciate any help/ideas I can get!
Thanks in advance!
I am trying to control a mass flowmeter to input Chemical A into a series of reactors, each with a residence time of 30 mins. I need to dope enough of Chemical A to remove environmentally harmful Chemical B from the reactors (need <1ppm). My analysis sampling point for Chemical B is after Reactor 2, so total residence time is 1 hr (after 2 reactors x 30 mins each).
I am using a PID controller to control the flow of Chemical A into Reactor 1 based on the ppm level of Chemical B out of Reactor 2. It takes approximately 10 mins to get a ppm reading of Chemical B via the online analyzer.
Once I get a ppm reading for Chemical B, I set the flowrate for Chemical A dosing. The problem is that if I change the flow of Chemical A, I do not see the effect of this change (lowered ppm reading of Chemical B) for at least 1 hr.
My knowledge of controllers is very limited; I realize that this problem may be very basic and so I appreciate any help/ideas I can get!
Thanks in advance!





RE: PID loop - long residence time
RE: PID loop - long residence time
RE: PID loop - long residence time
As JLseagull states, a Smith Predictor is a classic way to deal with delays
(see http://www
The model of the "Plant" has to be pretty good though for the Smith Predictor to work well as pointed out in the above article.
RE: PID loop - long residence time
If so add some feed forward to the loop so the flow of A is changed before the analyser sees the result.
The PID can then fine tune.
RE: PID loop - long residence time
In my world this is known as cascade control. I have not heard of Smith Predictor before, but I will be sure to school myself on that topic immediately. May be similar.
RE: PID loop - long residence time
Go Google feedforward cascade
RE: PID loop - long residence time
RE: PID loop - long residence time
RE: PID loop - long residence time
m777182
RE: PID loop - long residence time
RE: PID loop - long residence time
m777182
RE: PID loop - long residence time
http
Good luck,
PS. From practice I know deadtime like these (distance/velocity-lags) are very difficult to control.
A by-pass line, by-passing the big tanks, directly inserting into the reaction tank should be considered.
RE: PID loop - long residence time
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: PID loop - long residence time