Flexibility, Reliability and Maintenance Considerations in Batch Plant Design under Uncertainty

I N T R O D U C T I O N While issues related to the operating pattern (scheduling) of batch plants have been extensively studied in the open literature, the problem of accounting for equipment (un)availability and systems effectiveness under changing conditions has received much less attention despite its enormous practical significance (Straub and Grossmann, 1992; Subrahmanyam et al., 1994; Sanmarti et al., 1995). Recently, Ierapetritou and Pistikopoulos (1995a,b) considered the problem of batch plant design and operations with uncertain demands; however, equipment deterioration and maintenance considerations were not taken into account. Thomaidis and Pistikopoulos on the other hand in their FRAMS development (1994, 1995a,b), have proposed generic integrated tools for the simultaneous assessment of operability objectives, such as flexibility, reliability and availability. The aim of this work is to integrate such operability objectives in the design and scheduling of multiproduct/multipurpose batch plants. The paper is organized as follows. First, we show how reliability aspects can be formally introduced in stochastic batch plant design and scheduling formulations; availability considerations are also included based on Markovian corrective maintenance principles. Finally, we briefly discuss how condition-based maintenance can be implemented based on an operability criticality analysis. A multiproduct batch plant design model is used throughout the text to illustrate the concepts (for ease in the presentation). B A T C H P L A N T D E S I G N A N D F L E X I B I L I T Y R E L I A B I L I T Y O P T I M I Z A T I O N Consider a simplified multiproduct batch plant design model involving N products, operating in single campaign without intermediate storage, in M stages (such as the one shown in Figure 1 involving two products and three stages). Equipment are assumed to be available at discrete sizes; typical reliability models are associated with each equipment to describe its deterioration as a function of time (for example, a Weibull function); forecasted product demands are described by given probability distributional forms. The consideration of feasibility and equipment deterioration in the batch plant design stage can be achieved based on the following ideas. First, for feasibility consideration, a numerical integration scheme, such as a Gaussian Quadrature formula, is employed to approximate the multiple integral of the expected 1To whom correspondence must be addressed; emaih e.pistikopoulos~ic.ac.uk ¢a~ 2o:1~ (I)-P S1209 S1210 European Symposium on Computer Aided Process Engineering--6. Part B STAGE I STAGE II STAGE lII