Conceptual process designs hierarchical approach

Douglas, J. M., and Stephanopolous, G. (1995). Hierarchical approaches in conceptual process design framework and computer-aided implementation, In Foundations of Computer-Aided Process Design (L. T. Biegler and M. F. Doherty, eds.), pp. 183—197, AIChF Symposium Series, 91(304), New York. 

Figu re 2.1 Classical hierarchical approach of conceptual process design [2]. 

Consider the following industrial example of a process for the production of methyl acetate, first synthesized in the conventional conceptual process design manner, then modified using evolutionary approaches, and finally resynthesized using the lessons from the hierarchical process synthesis procedure experience. For simplicity here, only identity, amount, and composition differences are resolved (that is, temperature and pressure changers will be ignored). 

Douglas [1,2], among others, has proposed a hierarchical approach to conceptual process design. In this approach, the design process follows a series of decisions and steps. The order in which these decisions are made forms the hierarchy of the design process. These decisions are listed as follows. 

The flowsheet synthesis of continuous chemical-like process can be performed following a systematic strategy known as the hierarchical approach. The procedure, initially proposed by Jim Douglas and coworkers in the decade 1980-90 [1, 2], describes the conceptual design process as a logical sequence of analysis and synthesis steps grouped in levels. Each level involves a flowsheet development mechanism based on design decisions. The result is not a unique solution but a collection of alternative flowsheets from which an evaluation procedure eliminates the less attractive ones. 

The hierarchical approach is a simple but powerful methodology for the development of process flowsheets. It consists of a top-down analysis organized as clearly defined sequence of tasks aggregated in levels. Each level handles a fundamental conceptual problem input/output structure, reactor design, structure of separa-... 

Jim Douglas (1988) of the University of Massachusetts has devised a hierarchical approach to the conceptual design of process flowsheets. Although he primarily considers the steady-state aspects of process design, he has developed several useful concepts that have control structure implications. 

Douglas, J. M. Conceptual Design of Chemical Processes, New York McGraw-Hill11988,. Ponton. J. W., and Laing, D, M. A Hierarchical Approach to the Design of Process Control Systems, Cham.. Eng. Res. Des., 71, 181-188 (.1993). 

One of the most important factors that determines the column configuration is the formulation (or goals) of the separation task with respect to the total flow sheet. Although a mixture may consist of C components, it does not mean that all C products are necessary. The components contained in streams recycled into the process (e.g., unreacted reactants recycled to the reactor) usually do not have to be separated from each other. Also separation of streams that are later mixed (blended) should be avoided, if possible. The separation system needs to be optimized together with the entire plant, either simultaneously or in a hierarchical approach, as described by Douglas (The Conceptual Design of Chemical Processes, McGraw-ffill, New York, 1988). 

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