The simultaneous solution of the design and control problems of chemical processes represents a complex task, given that several aspects should be fulfilled (e.g., although a design can be considered to be feasible, when attempting to control it might not be possible, or the control configuration could be quite complex). The interaction between design and process variables also must be considered while attempting to outline a control configuration. The second part of this communication shows how, through a model-based methodology, it is possible to solve the design and control problems in a systematic way, leading to a full understanding of a chemical process. The Tennessee-Eastman (TE) problem has been selected to highlight the application of the model-based methodology in a plantwide problem.

A three-stage systematic model-based analysis methodology for the integrated design-control for chemical process with a reaction-separation with recycle scheme is presented. At the core of the methodology, a model-based analysis of first-principles models of different complexities identifies the important interactions between the design and process variables, so that specified design targets may be achieved. Stage 1 in the model-based methodology simplifies the design-control problem, such that the main operations are identified. The corresponding model, which has been developed in terms of dimensionless variables, helps to identify the limiting values of a set of lumped (design process) variables and locates operational windows where process feasibility can be assessed. In stage 2, more-detailed models are developed by delumping the set of lumped variables and relaxing the assumptions of stage 1. The only solutions to be considered are those lying within the bounds (operation windows) defined in stage 1. The objective of stage 2 is to identify the location of an optimal operation based on a specified design target. Stage 3 is used as a verification stage, by means of more-rigorous models in steady-state and dynamic models. Through the solution of a hierarchical subproblems, the solution of a complex design-control problem is therefore obtained systematically, along with an understanding of the process behavior. The ethylbenzene production process is used to highlight the design features of the methodology.

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