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Ind. Eng. Chem. Res. 1999, 38, 1444-1455
Dominant Variables for Partial Control. 2. Application to the Tennessee Eastman Challenge Process Bjo1 rn D. Tyre´ us† DuPont Central Research & Development, Experimental Station, P.O. Box 80101, Wilmington, Delaware 19880-0101
A thermodynamically motivated method for the identification of dominant variables used in partial control is applied to the Tennessee Eastman challenge process. The method shows that the reactor temperature together with the vapor-phase composition of component A are dominant variables. It is also shown that the temperature in the separator is dominant for this unit and that the overhead vapor composition is a dominant variable for the stripper. When a complete partial control structure is formed by feedback control of all the dominant variables, it turns out that an insufficient number of manipulated variables remain to satisfy inventory and component balances. A reduced partial control structure is thus suggested. It is demonstrated that this design is superior to other control solutions to the Eastman process in that it can easily attain and hold the plant at its maximum production rate. Introduction In the first part of this series1 it was demonstrated how a thermodynamically motivated method can be used to identify the dominant variables in a process. Dominant variables enter into the feedback control loops that form the core of partial control structures. The reason the proposed method works is that in many processes the economic parameters are related to the flow and production rates in the process. The rates also determine the magnitude of the power release in each unit and hence the energy exchange between the plant’s different energy carriers. By identifying the variables that affect the power release, we also know what affects other rates and therefore the economic objectives. In this paper the method is applied to the Tennessee Eastman challenge process. The Tennessee Eastman challenge process was published by Downs and Vogel2 as a realistic example of an industrial process for which a control strategy was sought. Several authors have offered decentralized control solutions to this problem including McAvoy and Ye,3 Lyman and Georgakis,4 Ricker,5 and Luyben.6 These solutions were established using a variety of approaches resulting in different control structures. For a qualitative discussion of the differences between these structures see Luyben.6 No claim has been made that any of the proposed solutions should be a partial control strategy, nor is it obvious, in hindsight, that this is the case. A schematic of the Eastman process is shown in Figure 1. It consists of three main processing units; a reactor, a gas separation/recycle system, and a product purification unit (stripper). There are four fresh feed streams carrying the reactants, A, C, D, and E, to the process. A small amount of inert component, B, also enters the process with the feeds. The reactants are used to form two desired components, G and H along with a byproduct, F. The liquid product containing components G and H exits the process from the stripper base. A † E-mail:
[email protected]. Tel.: 302-6958287. Fax: 302-695-2645.
Figure 1. Schematic of the Tennessee Eastman challenge process.
purge stream is also taken from the recycle gas line to prevent buildup of components B and F. This process has three unusual features. First, the two-phase reactor has no liquid exit stream so that all products and excess reactants have to leave with the vapors. Second, the reaction rates are determined by the conditions in the vapor phase of the reactor. Third, one of the fresh feed streams (C feed) acts as a stripping agent for the purification unit. The process objectives are provided by Downs and Vogel2 and are summarized in Table 1. As was pointed out in part 1 of this series,1 the objectives for the Eastman process center around production rate, product composition, yield, and constraints on intensive variables. Most of these variables are rate-related and it thus appears logical to search for dominant variables with a method that examines the rate of energy transfer within the process. Thermodynamic Analysis Figure 2 shows some of the currents involved in a thermodynamic description of the process. Entropy is generated in the reactor from the chemical reactions releasing power. Part of this entropy leaves the process
10.1021/ie980620x CCC: $18.00 © 1999 American Chemical Society Published on Web 03/04/1999
Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999 1445 Table 1. List of Economic Objectives for the Eastman Process product
constraints
costs
rate specified to 14 076 pph with a purity of 50% G rate specified to 14 076 pph with a purity of 10% G rate specified to 11 111 pph with a purity of 90% G maximum achievable production rate at 50% G maximum achievable production rate at 10% G maximum achievable production rate at 90% G
none < reactor pressure < 2895 kPa 50% < reactor level < 100% none < reactor temperature < 150 °C 30% < separator level < 100% 30% < stripper level < 100% product flow rate variability
