Discrete design decisions

Real design problems involve discrete design decisions, such as layout of reactors and choice of control scheme, as well as selection of continuous design parameters, such as reactor sizes and controller tuning. The design tools pre-... 

One approach to making discrete design decisions is to pose a mixed-integer nonlinear program (MINLP) and apply a suitable optimization method to the selection of discrete design parameters. We have not adopted this approach for the following reasons. 

In a retrofit batch design, we optimize the batch plant profitability defined as the total production value minus the cost of any new equipment. The objective is to obtain a modified batch plant structure, an operating strategy, the equipment sizes, and the batch processing parameters. Discrete decisions correspond to the selection of new units to add to each stage of the plant and their type of operation. Continuous decisions are represented by the volume of each new unit and the batch processing variables which are allowed to vary within certain bounds. 

The activity of synthesis occurs throughout a design, from original process conception to construction and operation. Examples are 1) the synthesis of a control system for fixed plant configuration, 2) the synthesis of a start-up procedure for a new process, 3) the selection of materials of construction and 4) the layout of the plant to improve safety. All of these activities involve discrete decision making this aspect is a common denominator of all synthesis activities. 

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