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Ind. Eng. Chem. Res. 1997, 36, 1954
Rebuttal to the Comments of Thomas J. McAvoy on “Simple Regulatory Control of the Eastman Process” William L. Luyben* Department of Chemical Engineering, Iacocca Hall 111, Lehigh University, Bethlehem, Pennsylvania 18015
Sir: I disagree with Professor McAvoy that the control objectives outlined in the paper by Downs and Vogel (1995) are “unambiguous”. There are several control objectives given in the paper, and, as is often the case, these objectives are conflicting. Downs and Vogel say flow variability of several streams (fresh feed stream 4 and product stream 11) is “of concern” and is to be “minimized”. On p 248 of the Downs-Vogel paper, the second control objective is to minimize variability of product flow rate. The third control objective is to minimize movement of fresh feed valves. Since these objectives are conflicting, some priorities must be established. My interpretation is that the objective mentioned first has the highest priority. The results of using the control
scheme proposed by McAvoy et al. (1995) show persistent oscillations in the flow rate of the product stream. My control structure holds it absolutely constant. I have no desire to enter into a prolonged debate about which control structure is “better”. I think the lesson to be learned from the several papers published on the Eastman problem is that the “best” control structure depends strongly on the control objectives. Literature Cited Downs, J.; Vogel, E. A Plant-wide Industrial Process Control Problem. Comput. Chem. Eng. 1995, 17, 245-255. McAvoy, T. J.; Ye, N.; Gang, C. Nonlinear Inferential Parallel Cascade Control. Ind. Eng. Chem. Res. 1996, 35, 130-137.
* To whom correspondence should be addressed. Telephone: (610) 758-4256. Fax: (610) 758-5057.
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