ANALYSIS OF OPERATIONAL FLEXIBILITY IN CHEMICAL PROCESS DESIGN (UNCERTAINTY, OPTIMIZATION)

This thesis considers the problem of quantitatively analyzing chemical process operational flexibility. Variations in operating conditions are represented in terms of uncertain parameters which describe both internal and external sources of process variations such as exchanger fouling, catalyst deactivation, and changes in throughput or feed quality. The goal is to determine the range of parameter variations which a stated design can tolerate while satisfying performance specifications and constraints, given that process controls may be adjusted for different parameter realizations. An index of flexibility is proposed which provides a measure of the size of the region of feasible operation. Determination of this index provides actual bounds for the uncertain parameter values within which feasible operation can be guaranteed. Identification of the "worst-case" conditions which reach the performance limits of the design is also provided.; Mathematical formulations based on semi-infinite programming are developed to serve as a basis for computing the index, and their properties are studied. Sufficient conditions are established for the constraint functions under which the solution is guaranteed to lie at a vertex, i.e. with each parameter assuming an extreme value.; Two types of solution algorithms are proposed which rely on this behavior. The first is a direct search procedure, and features a heuristic variant that makes it possible to avoid the exhaustive enumeration of every vertex. The second algorithm employs an implicit enumeration scheme based upon a proposed lower bound which is rigorous for monotonic constraints. These algorithms are applied to solve several process example problems. The examples serve to demonstrate both the efficiency of the proposed algorithms and the use of the flexibility index in process design applications.