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Optimal Control of a Continuous Stirred-Tank Reactor

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Aris and Amundson [1–3] originally analyzed the design of a continuous stirred-tank reactor (CSTR) with a first-order, irreversible exothermic reaction. The reactor is controlled by varying the flow of a cooling fluid through a coil inside the reactor. The state equations for the CSTR are given by

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,

,

where and are the deviations from steady-state concentration and temperature, respectively, and is the control action.

This Demonstration finds the optimal control, which minimizes the performance index given by , where the normalized control action is and is a weighting parameter. Assume a final time of the control [4, 5]. You can set the initial conditions (i.e. both and ) and choose to apply upper and lower bounds on .

We plot the component of the state vector ( and shown in red and blue, respectively), the normalized control , and the costates. We also compute the value of the performance index .

For the case where is unbounded, if you select the initial conditions and , then , in close agreement with the result found in [4] and [5].

The costates and are plotted in magenta and cyan. The costates verify and (yellow dot).

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Contributed by: Housam Binous and Ahmed Bellagi (January 2016)
(King Fahd University of Petroleum & Minerals, KSA; University of Monastir, Tunisia)
Open content licensed under CC BY-NC-SA

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References

[1] R. Aris and N. R. Amundson, "An Analysis of Chemical Reactor Stability and Control—I: The Possibility of Local Control, with Perfect or Imperfect Control Mechanisms," Chemical Engineering Science, 7(3), 1958 pp. 121–131. doi:10.1016/0009-2509(58)80019-6.

[2] R. Aris and N. R. Amundson, "An Analysis of Chemical Reactor Stability and Control—II: The Evolution of Proportional Control", Chemical Engineering Science, 7(3), 1958 pp. 132–147. doi:10.1016/0009-2509(58)80020-2.

[3] R. Aris and N. R. Amundson, "An Analysis of Chemical Reactor Stability and Control—III: The Principles of Programming Reactor Calculations. Some Extensions", Chemical Engineering Science, 7(3), 1958 pp. 148–155. doi:10.1016/0009-2509(58)80021-4.

[4] R. Luus and L. Lapidus, "The Control of Nonlinear Systems. Part II: Convergence by Combined First and Second Variations," AIChE Journal, 13(1), 1967 pp. 108–113. doi:10.1002/aic.690130120.

[5] D. E. Kirk, Optimal Control Theory: An Introduction, Mineola, NY: Dover Publications, 2004.

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