Plantwide control (PWC) methodologies have gained significant importance given the high and increasing degree of integration in chemical processes due to material recycle, energy integration, and stringent product quality control, all of which though economically favorable, pose tough challenges to smooth plant operation. As part of the continuing search for more effective PWC system design methods, an integrated framework of heuristics and simulation was proposed [Konda et al. Ind. Eng. Chem. Res. 2005, 44, 8300-8313]. The basic idea behind this development is to make effective use of rigorous process simulators to aid in decisionmaking during the development of the heuristic-based PWC structure. Konda and co-workers have successfully applied the procedure to the toluene hydrodealkylation process. Though the integrated framework is promising, there is still a need to test its applicability to other complex industrial processes. The present contribution considers the development of a PWC structure for the styrene monomer plant using the integrated framework. In addition, in order to gauge its effectiveness in comparison to the other plantwide control methods, two more methods are considered in this study. First, the heuristics procedure of Luyben and co-workers [Luyben et al. Plant-Wide Process Control; McGraw-Hill: New York, 1998], which is a popular heuristics-based methodology, is also applied to the same flowsheet, and is considered as the base case for performance assessment. Second, the self-optimizing control procedure [Skogestad, S. Comput. Chem. Eng. 2004, 28, 219-234] is also used in order to have a more comprehensive analysis of the effectiveness of the integrated framework. An analysis of the results indicates that while all the procedures give stable control structures, the integrated framework and self-optimizing control procedures give more robust control structures than the heuristics procedure. This is the first study to develop simulation models and complete PWC structures for the styrene plant, together with a detailed analysis of the relative performance of the resulting structures in order to evaluate the different PWC methodologies.

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Performance analysis of plantwide control (PWC) systems, which is one of the important areas of PWC of industrial processes, has not received much attention in the past. In this paper, many measures based on plant dynamics are described and discussed for evaluating the performance of different control structures in the presence of disturbances. These include overall process settling time, dynamic disturbance sensitivity, the total variation in the plant manipulated variables, net variation from the nominal profit, deviation from the production rate target, and the integral absolute error in product purity. These measures are developed and then applied to four alternative control structures of an important industrial process, namely, styrene monomer plant, in order to test their applicability. The results indicate that some of the presented measures are indeed effective in evaluating and comparing different PWC structures. The most important contribution of this paper is the presentation of some clear and easily computable criteria for the performance assessment of PWC systems.

The emphasis on process economics in modern chemical plants has led to the increased use of material recycle and energy integration that significantly alter process dynamics and add complexity. As a result, many plantwide control (PWC) methodologies have emerged. However, there are very few comparative studies to evaluate the effectiveness of alternate methodologies. In this study, a PWC structure for the ammonia synthesis process is developed using the integrated framework of simulation and heuristics [Konda et al. Ind. Eng. Chem. Res. 2005, 44, 8300−8313], which is then compared with that developed by Araújo and Skogestad [Araújo and Skogestad, Comput. Chem. Eng. 2008, 32, 2920−2932] using the self-optimizing control procedure. The different control structures are evaluated in a comprehensive manner including assessment of the dynamic performance and steady-state profit.

The reactor−separator−recycle (RSR) process, consisting of a reactor and a separator/distillation column with material recycle between them, simplifies and idealizes real chemical plants. It is an important test bed used in plantwide control (PWC) studies. Though it has been actively studied in the past 15 years, appropriate guidelines on control structure selection are lacking, and there has been no consensus on the best control system. Also, there is still a need to study RSR processes with real components in practical context (that is, with more complete flow sheets) using rigorous process simulators. Another important aspect that has not been considered is the significance and usefulness of the results from RSR studies for real complicated plants. The main aim of this paper is to study the applicability of the RSR results to designing a PWC structure for a complete plant with heat integration besides a recycle. Findings of gas-phase RSR studies are applied to the toluene hydrodealkylation, ammonia, and styrene processes. The performance of the alternative control structures is analyzed for each process. Based on a comprehensive analysis of the results, useful guidelines for PWC design for gas-phase processes are developed.