Heuristics for Process Synthesis

This chapter returns to the steps of preliminary process synthesis in Section 3.4, in which a strategy is recommended that involves assembling the process operations in a specific order, as follows  

Chemical reactions (to eliminate differences in molecular type) 

Task integration (to combine operations into unit processes) 

In Section 3.4, as the operations are inserted into alternative flowsheets to manufacture vinyl chloride, rules of thumb or heuristics are utilized. For example, when positioning the direct chlorination operation, it is assumed that because the reaction is nearly complete at 90°C, ethylene and chlorine can be fed in stoichiometric proportions. Furthermore, when positioning the pyrolysis operation, the temperature and pressure are set at 500°C and 26 atm to give a 60% conversion. These assumptions and specifications are based on many factors, not the least of which is experience in the manufacture of vinyl chloride and similar chemicals. In this case, a patent by the B.F. Goodrich Co. [British Patent 938,824 (October 9, 1963)] indicates the high conversion of ethylene and chlorine over a ferric chloride catalyst at 90°C and recommends the temperature and pressure levels of the pyrolysis reaction. The decision not to use ethylene in excess, to be sure of consuming all of the toxic chlorine, is based on the favorable conversions reported experimentally by chemists. In the distillation operations, the choice of the key components, the quality of the feed streams and the distillate products, and the pressure levels of the towers are also based on rules of thumb. In fact, heuristics like these and many others can be organized into an expert system, which can be utilized to synthesize sections of this and similar chemical processes. 

and a complete material and energy balance is carried out, often using the process simulators discussed in Chapter 4. 

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R. Raman and I. E. Grossmann. Integration of logic and heuristic knowledge in MINLP optimization for process synthesis. Comp. Chem. Eng., 16 155,1992. 

Starting with the given thermophysical and kinetic data, a heuristic-numeric process synthesis has been carried out. This procedure led to the specific column configuration displayed in Fig. 10.18 for the production of DEC. 

Be able to implement the steps in Section 3.4 for process synthesis more effectively, using the many heuristics presented herein, and summarized in Table 5.2. The examples and exercises should enable him or her to gain experience in their application. 

The economics of the various methods that are employed to sequence multicomponent columns have been studied. For example, the separation of three-, four-, and five-component mixtures has been considered (44) where the heuristics (rules of thumb) developed by earlier investigators were examined and an economic analysis of various methods of sequencing the columns was made. The study of sequencing of multicomponent columns is part of a broader field, process synthesis, which attempts to formalize and develop strategies for the optimum overall process (45) (see Separation systems synthesis). 

The initial aim of the procedure is to generate a reasonable base case design that can be used for preliminary economic evaluation of the process. This can subsequently be optimized and/or compared with any process alternatives that are identified. The complete process is always considered at each decision level, but additional fine detail is added to the structure of the flowsheet at any stage. Established heuristics and equipment selection procedures are used together with new process synthesis insights to guide each flowsheet decision. 

In most chemical processes reactors are sequenced by systems that separate the desired products out of their outlet reactor streams and recycle the unconverted reactants back to the reactor system. Despite the fact that process synthesis has been developed into a very active research area, very few systematic procedures have been proposed for the synthesis of reactor/separator/recycle systems. The proposed evolutionary approaches are always based upon a large number of heuristic rules to eliminate the wide variety of choices. Many of these heuristics are actually extensions of results obtained by separately studying the synthesis problem of reactor networks or separator systems, and therefore the potential trade-offs resulting from the coupling of the reactors with the separators have not been investigated. 

S. H. Cheng and Y. A. Liu. Studies in chemical process design and synthesis 8. A simple heuristic method for systematic synthesis of initial sequences for sloppy multicomponent separations. I ECRes., 27 2304, 1988. 

Many of the aforementioned heuristic decentralized control synthesis approaches rely on engineering judgement rather than rigorous analysis. On the other hand, the implementation of advanced, model-based, control strategies for process systems is hindered by the often overwhelming size and complexity of their dynamic models. The results cited above indicate that the design of fully centralized controllers on the basis of entire process models is impractical, such... 

Most of the applications discussed here were based on algorithmic systematic generation approaches to process synthesis. There also exist a number of heuristic rule-based or expert-systems-based approaches including PIP for total flowsheet synthesis, SPLIT for separation schemes (Wahnschafft et ai, 1991),... 

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