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Posted by Brian Burgio on April 05, 2000 at 14:13:10:
In Reply to: Scope of design of MPC controllers posted by Gary Cera on March 29, 2000 at 07:55:32:
To expound on and reiterate what Don said in his reply…
The key principle is to make the controller big enough to capture the significant constraints to maximizing profitability of the unit in question and all significant interactions … but no larger than it has to be.
As an example…
Say you have a crude refining unit with an atmospheric tower, debutanizer and vacuum tower. The overheads of the atmospheric tower feeds the debutanizer and the bottoms feeds the vacuum tower. The scope of this controller my include all three towers, just the atmospheric tower and the debutanizer or just the atmospheric tower and the vacuum tower.
Lets start with the atmospheric tower and the dubutanizer. Say the debutanizer has a dP (loading) constraint and a constraint on the amount of butane in the bottoms and C5’s in the overhead. With certain crude slates the debutanizer can become overloaded. At first thought, you may think that the debutanizer should be included in the atmospheric tower controller scope since it can become overloaded at times and become a constraint. The question to ask is: “When the debutanizer becomes overloaded, what is the correct response of the unit operator based on plant operating objectives?” One response could be to cut back crude feed to the unit. Another response could be to give up on the product specification limits on the debutanizer. You can not give up on the dP constraint as then the tower would be in flood. If the answer is to reduce unit feed, then the debutanizer should be included in the scope of the atmospheric controller. If the answer is to give up on the debutanizer product specifications, one or both, and not cut unit feed, then the debutanizer should probably be left out of the atmospheric tower controller. If it is included, when the debutanizer becomes constrained the controller will use the only handle it has left to alleviate the constraint, which is unit feed.
Likewise, if the vacuum tower is going to take all the atmospheric tower bottoms no matter what, then it probably should not be included in the atmospheric tower controller. However, if for example the vacuum tower feed furnace is a constraint and the proper operating move is to reduce unit feed, then the vacuum tower should be included in the scope with the atmospheric tower.
A good exercise at the design phase is to draw the anticipated controller model matrix. Use this model to identify the expected interactions. Highlight the key constraints and consider how the unit should be operated when those constraints are active. This should help in laying out the initial scope of the controller. Remember, the scope of the controller can be changed as more information is gained about the process. The key is to make sure that all the necessary data is collected during the plant test phase of the project.