We have developed a systematic procedure for a limited class of chemical processes that includes control problems at the early stage of a flow sheet development. The procedure decomposes the control problem into a set of sub-problems. For the economic evaluations it is assumed that the raw-material and the recycle costs dominate the process economics, and therefore the variables which affect the input-output and the recycle material and energy flows are considered as the optimization variables (process-flow optimizations). The results obtained from this procedure will be helpful in the following areas: (1) Identifying potentially inoperable flow sheets due to the presence of trace component impurities in the feed streams or produced in the reactor, (2) Estimating the economically justified modifications (both in the flow sheet structure and the sizes of process units) to the optimum base-case design, (3) Generating alternative sets of process-flow, control structures (a set of controlled and manipulated variables, along with their pairing, that can drive the input-output and the recycle flows to their desired steady-states), (4) Estimating the magnitude of the overshoot of the manipulated variables during the transients, and changing the structure of the flow sheet, the equipment sizes, or the structure of the steady-state control structure to accommodate disturbances in an optimum way. Based on the process economics and the relative gain analysis, the optimum control structure is synthesized that would minimize the total operating cost in the face of disturbances. The optimum values of the sizes of the constrained unit designs (which restrict the process-flow optimization in the face of disturbances), and the optimum values of the process variables (i.e., the design variables and the process flows) are determined by solving a two-stage optimization problem. A method for developing approximate, dynamic models for the process flows for continuous chemical plants with recycle streams is described. (Abstract shortened with permission of author.)

Algorithmic and heuristic-based approaches are proposed for synthesizing integrated chemical systems. The former is used in the synthesis of reactor network and reactor-recycle-separator systems; the latter in the synthesis of integrated crystallization systems. In the algorithmic method, a network, or superstructure, which embeds all possible equipment to be used in the process and the potential interconnections among the equipment is generated. The procedure for generating the reactor network and the reactor-recycle-separator flow sheet structure is described. A nonlinear programming (NLP) problem is then formulated for the network. The optimal flow sheet and accompanying operational conditions are obtained by solving the NLP problem. For integrated crystallization process synthesis, a heuristic-based systematic procedure is developed. In a step-by-step manner, the procedure guides the user to generate alternative flow sheets for a given crystallization task. First, the required unit operations are determined by comparing the product specifications (production rate, product purity, and others) with the crystallizer effluent characteristics (occlusions, inclusions, crystal size, and others). Second, the destinations of the reaction solvent, mother liquor, wash liquid, recrystallization solvent, and drowning-out solvent are assigned. Then, the solvent recovery system is considered to recover the solvents and unconverted reactants, and to remove impurities from the system. Downstream system problems such as excessive filtration time and/or filter size are often caused by unfavorable crystal size. Various crystallizer designs to improve the crystal size distribution are discussed; short-cut equipment models are used to evaluate the alternatives for potential improvement. Issues related to minimization of inclusion impurities and heat integration are also examined. Guidelines are provided to help the user to add more details to the flow sheet at each level.

Process boundaries and difficult regions for separation units limit the feasible products and recovery of those products. When process boundaries are encountered, a separating agent and or combinations of different types of equipment are used. In this manner, a number of steps are used collectively to meet the separations objective. One type of equipment configuration, the distillation-membrane hybrid, has been studied for binary and multicomponent systems. In this hybrid, the distillation column performs the bulk of the separation, because of the favorable economics of distillation. The membrane is used to bypass the process boundaries and difficult regions. Methods for applying and screening these hybrids were developed. Several configurations were examined conceptually. Case studies were performed on the following systems: water-acetic acid, ethanol-water, propylene-propane, benzene-heptane-octane, methanol-ethanol-water. Separations synthesis using all possible separation units (crystallization, membranes, distillation, decantation, extraction, etc.) was then examined. A design methodology for generating flowsheets of process alternatives to separate multicomponent systems was developed based on representing process boundaries with linear hyperplanes. This approximation allowed the generation of process alternatives using relatively simple calculations.

Plantwide control refers to the control of entire plants, consisting of many interconnected unit operations. Synthesizing a plantwide control system requires evaluating numerous alternatives involving controlled variables, control structures, controller designs and tunings, etc. A hierarchical procedure for systematically synthesizing a plantwide control has been proposed by Zheng et al. [82]. In this procedure, the plantwide control problem is decomposed into six steps along which decisions are made based on economics. While many tools (e.g., a short-cut method for controlled variable selection, quantification of dynamic operability, etc.) have been developed by these authors to ease the implementation of the procedure on industrial processes, more tools need to be developed. For example, we need to develop systematic procedures for ensuring feasibility of the control structures, for selecting primary and secondary controlled variables and for designing controllers for systems with recycles. Furthermore, model uncertainty, which is important practically, needs to be taken into account to make these tools useful. The goal of this thesis is to accomplish these tasks. To this end, we address how model uncertainty affects the steady-state as well as the dynamic control structure.

Large chemical plants are usually designed by combining several individually well controlled process units into a whole. This practice produces satisfactory, controlled response if the dynamic interactions are "weak". However, high material and energy costs and increased market competition have driven chemical plants towards a higher degree of energy integration and material recycle. These "modern" plants become difficult to operate and control because strong interconnections generally degrade the performance of the individual units, and in some cases the total response for the plant may be unstable. Our study focuses on modifying the plant design and improving control configurations to regain stability and to obtain the original output trajectories of the units as closely as possible. Via a sequential procedure, we represent the plant by a hierarchical set of self-similar modules, in which the top level represents the entire plant. The analysis of the synthesis procedures depends on a proposed measure of interaction, $\phi$, whose minimization produces the required guidelines for stable design and control.