Integration of Design and Control

A Decision-Based Approach to Plantwide Control Structure Synthesis

Authors: E. M. Vasbinder† and Karlene A. Hoo*
* Department of Chemical Engineering, Texas Tech University

ABSTRACT

The concept of plantwide control structure synthesis is not a new concept to the chemical process industry. The approaches used can be classified into two main categories, a process engineering approach and a mathematical-oriented approach. Within each category, there is no consensus as to one approach, which is not surprising given the size, complexity, and objectives of the plant. The work describes a modular decomposition of the plant flowsheet that is assessed based on a decision-based approach - the analytical hierarchical process, that consistently evaluates the merit of the decomposition among alternatives, and between neighboring modules to arrive at a prioritization of objectives for each module and for the integrated plant. An example system is provided to demonstrate the approach.

 

Publication Information: Ind. Eng. Chem. Res., 42, pp 4586-4598, 2003

Corresponding Author: Karlene A. Hoo

 

A Decision-Based Methodology for Plantwide Control Structure Synthesis

Authors: E. M. Vasbinder† and Karlene A. Hoo*
† ExxonMobil Chemical, Baytown, TX
* Department of Chemical Engineering, Texas Tech University

2004 AIChE National Conference (Austin, TX), Session 406, paper 406a

ABSTRACT

The synthesis of plantwide control structures is not a new area of study but presently there is no agreement on a unified approach to this problem. One reason for this is the large dimension of the problem that involves many manipulated and controlled variables. Recent work by [?, ?] has shown that the dimensionality of the plantwide control problem can be reduced by decomposing the process flowsheet into subsets of the flowsheet, or modules, using a decision-based approach known as the modified Analytic Hierarchical Process (mAHP) [?]. The natural interactions and dependencies inherent in the plantwide control problem dictate the decomposition process, while still retaining the integrity of the plantwide problem. The modules that result are more tractable to design a control structure without loss of performance of the overall process. Thus, the control structure is developed only as it pertains to the module and not the entire process, while satisfying the overall plantwide control objectives. In this work, the mAHP is used to demonstrate selecting between competing flowsheets and their respective plantwide control structures based on a quantitative analysis that incorporates more than purely economic considerations. This capability to assess the control objectives differently between alternatives provides flexibility that is not present in the traditional approaches, thus making the mAHP methodology more generalizable to various different processes. The results are demonstrated on two alternative process flowsheets that produce benzene by the hydro-deakylation (HDA) of toluene. The HDA process is a well studied example in the open literature [?] and therefore provides a good basis by which to demonstrate the usefulness of the methodology.

References:

  1. E. M. Vasbinder and K. A. Hoo. A decision-based approach to plantwide control structure synthesis. Ind. & Eng. Chem. Res., 42:4586–4598, 2003.
  2. E. M. Vasbinder, K.A. Hoo, and U. Mann. Synthesis of plantwide control structures using a decision-based methodology. In M. Georgadis and P. Seferlis, editors, The Integration of Design and Control, volume 1. Elsevier Science, Amsterdam, 2004.
  3. T. L. Saaty. Decision Making for Leaders. RWS Publications, Pittsburgh, PA, 1995.
  4. J. M. Douglas. Conceptual Design of Chemical Processes. McGraw-Hill, St. Louis, MO, 1988.

Corresponding Author: Karlene A. Hoo