Towards Integration of Controllability into Plant Design

In process design practice the plant piping and instrumentation diagram evolves iteratively using mainly experience and process reasoning to address questions related to plant controllability. It would be desirable to be able to address such questions more quantitatively at di erent abstraction levels during process design such that controllability evaluation can be integrated into the design process. Only few attempts have been reported towards integrating controllability investigations into early stages of plant design. This paper reviews literature on controllability from the perspective of controllability assessment with the aim of identifying tests, which may be used at di erent stages of plant design. First de nitions of terms used within process exibility design and controllability assessment for control structure design are given. Methods for controllability evaluation are reviewed for some modes of process operation. Basically two types of evaluation and design methods prevail. One type is based on linear model analysis, whereas another type is based on physical chemical insight and thus provides nonlinear information. Control structure development for controllability is illustrated on an energy integrated distillation plant by using a heuristic process knowledge based method to develop a basic control structure and subsequently using an optimisation based approach for selecting a product purity control structure. Controllability properties of a more energy e cient process design alternative is discussed to illustrate the potential trade o by choice of the most energy e cient design that however has the lowest controllability. Based on the review and the examples a procedure for integrating controllability assessment for control structure development into plant design is proposed.