Flowshop, chemical

Many chemical batch production facilities are dedicated to producing a set of products that require for their manufacturing a common set of unit operations. The unit operations are performed in the same sequence for each product. This type of production problem is often solved by configuring the available equipment so that each unit operation is carried out by a fixed set of equipment items that are disjoint from those used in any other operation. If one unit is assigned to each step, this is called a flowshop (Baker, 1974). 

The flowshop problem has been widely studied in the fields of both operations research (Lagweg et al., 1978 Baker, 1975) and chemical engineering (Rajagopalan and Karimi, 1989 Wiede and Reklaitis, 1987). Since the purpose of this chapter is to illustrate a novel technique to synthesize new control knowledge for branch-and-bound algorithms, we... 

The chemical flowshop, which we shall meet in Chapter 5, is similarly challenging to solve. Many different chemicals are synthesized within a single line of processing units in a defined sequence. The efficiency of the flowshop, which is determined by the length of time required to synthesize the complete group of chemicals, is strongly correlated with the order in which chemicals are made. Therefore, determining a near-optimum order is important in the operation of flowshops if the process is to be financially viable. For n chemicals, the number of possible sequences is n, a number... 

The success of the algorithm relies on the GA processing many generations solutions can only evolve if evolution is possible. Excessively large populations are especially problematic if evaluation of the fitness function is computationally expensive as it is, for example, in the chemical flowshop problem (section 5.10) in which evaluating the total time required for all chemicals to be processed by the flowshop in a defined order may take far more time than the execution of all other parts of the GA combined. 

A schematic of a small chemical flowshop. The circles and squares represent units, such as reactors, dryers, or centrifuges. 

The traveling gourmet problem is in itself not of much interest to physical and life scientists (or, more accurately, is probably of acute interest to many of us, but well beyond our means to investigate in any practical way), but problems that bear a formal resemblance to this problem do arise in science. The chemical flowshop is an example (Figure 5.29). ... 

Chemical flowshops are widely used in industry for the medium-scale synthesis of chemicals. In a flowshop, a number of different physical units, such as reactor vessels, ovens, dryers, and distillation columns are placed in a serial, or largely serial, arrangement. Rather than being devoted to the preparation of a single chemical, many different chemicals are synthesized one after another in a flowshop by feeding in the appropriate precursors and reagents as the intermediates pass through most or all of the units. This mode of operation is an effective use of the physical plant because no part... 

Choice of the optimum production order is a scheduling problem. Like the problem of the traveling gourmet, a GA solution for the flowshop consists of an ordered list. In the flowshop, this list specifies the order in which chemicals are to be made ... 

There has been a notable change in the way that the GA has been used as it has become more popular in science. Some of the earliest applications of the GA in chemistry (for example, the work of Hugh Cartwright and Robert Long, and Andrew Tuson and Hugh Cartwright on chemical flowshops) used the GA as the sole optimization tool many more recent applications combine the GA with a second technique, such as an artificial neural network. 

Cartwright, H.M., Long, R.A. Simultaneous Optimization of Chemical Flowshop Sequencing and Topology Using Genetic Algorihms, Ind. Eng. Chem. Res., 32,2706-2713 (1993). 

Abadi L, (1995). Flowshop scheduling problems with no-wait and blocking environments A mathematical programming approach, Ph.D. thesis. University of Toronto. Bjorkqvist J., Westerlund T., (1999). Automated reformulation of disjunctive constraints in MINLP optimization. Computers and Chemical Engineering Supplement, pp. Sll-S14... 

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