Optimization process flowsheets

To determine the optimal process flowsheet(s) with respect to the imposed performance criteria, we have to provide answers to the following key questions ... 

Note also that Ql involves discrete decision making while Q2 and Q3 make selections among continuous alternatives. As a result, the determination of the optimal process flowsheets) can be viewed conceptually as a problem in the mixed discrete-continuous optimization domain. 

In (i), rales of thumb based on engineering experience are applied so as to generate good initial process flowsheets which are subsequently improved via a set of evolutionary rales applied in a systematic fashion. The principle advantage of (i) is that near optimal process flowsheets can be developed quickly at the expense, however, of not being able to evaluate the quality of the solution. An additional drawback of (i) is the potential conflicting rales which imply arbitrary weighting schemes for the considered alternatives. 

Step 1 Representation of Alternatives A superstructure is postulated in which all process alternative structures of interest are embedded and hence are candidates for feasible or optimal process flowsheet(s). The superstructure features a number of different process units and their interconnections. 

The development of the appropriate superstructure is of major importance since the optimal process flowsheet(s) sought will be as good as the postulated representation of alternative structures. 

Stephanopoulos, G. and Westerberg, A.W., Studies in Process Synthesis—II. Evolutionary Synthesis of Optimal Process Flowsheets, Chemical Engineering Science, Vol. 31, pp 195-204, 1976. 

Although the algorithm described in next paragraphs is described for optimizing process flowsheets, it can be used for any black box model with or without noise. It guarantees a local minimum within a pre-specified tolerance. 

There have been several other proposed procedures for syaemetic generation of optimal process flowsheets without ua initial process structure. Of note is one developed by Mahalec and Motard36 and incorporated into a computer program they called BALTAZAR. Like AIDES, it can be used for die synthesis of an entire chemical plant but takes a more sophisticated approach to processing objectives and constraints. It is based on a systematic resolution of conflicts between a set of processing rales and die processing objectives. The procedure is based on a sequential depth-first approach which employs several structural... 

The economic optimal process flowsheet was obtained by minimization of the total annual cost (TAG) with five design variables IPA distillate composition (XD2) in the recovery column, total number of stages for the heterogeneous azeotropic column and the recovery column (Ni and N2), and the two feed stages (Api and Apa). The product specification of IPA is set to be ultrapure (99.9999 mol%) to be used in the semi-conductor industry. The product specification of water is set to be 99.9 mol%. In each simulation run, the IPA product specification is achieved by varying the reboiler duty of the heterogeneous azeotropic column, and the water product specification is achieved by varying the reboiler duty of the recovery column. The entrainer makeup flowrate will be very small to balance the entrainer loss from the two bottom streams. 

Figure 7.5 Optimized process flowsheets for the five esterification systems.

Kocis, G. R., and Grossmann, I. E., A Modeling/Decomposition Strategy for MINLP Optimization of Process Flowsheets, paper no. 76a, AIChE Meeting, Washingtonj D.C., 1988. 

Kocis GR and Grossmann IE (1988) A Modelling and Decomposition Strategy for the MINLP Optimization of Process Flowsheets, Comput Chem Eng, 13 797. 

Chen, H. S. and M. A. Stadtherr. A Simultaneous-Modular Approach to Process Flowsheeting and Optimization I. Theory and Implementation. AIChE J 30 1843-1856 (1984). 

Kisala, T. P. R. A. Trevino-Lozano J. F. Boston H. I. Britt et al. Sequential Modular and Simultaneous Modular Strategies for Process Flowsheet Optimization. Comput Chem Eng 11 567-579 (1987). 

Cuthrell, J. E., and Biegler, L. T., Simultaneous solution and optimization of process flowsheets with differential equation models, Chem. Eng. Res. Des. 64, 341 (1986). 

Kaijaluoto, S., Process optimization by flowsheet simulation, Technical Research Center of Finland, Publication 20(1984). 

Kocis, G. R., and Gro.ssmann, E. E., A modeling and decomposition strategy for the MINLP optimization of process flowsheets, Comp, and Chem. Engr. 13, 797 (1988). 

The answers to questions Ql and Q2 provide information on the topology/structure of the process flowsheet since they correspond to the selection of process units and their interconnections, while the answer to question Q3 provides information on the optimal values of the design parameters and operating conditions. 

Dynamic simulation, process control, real-time optimization Process synthesis, flowsheet convergence, simultaneous modular vs. equation-oriented... 

The conceptual development of a typical chemical process remains somewhat of an art. Usually, experience with similar process plants leads to an initial process flowsheet. Parameter optimization on that flowsheet can be used to determine the best economic design. Other flowsheets can be generated by adding/removing unit operations and process streams and changing the structure of the process flowsheet. A simplified depiction of this synthesis process is shown in Fig. 5. 

Parker, A. L., "Chemical Process Optimization by Flowsheet Simulation and Quadratic Approximation Programming", Ph.D. Thesis in Chemical Engineering, U. of Wisconsin, 1979. 

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