Plantwide Control System Design Research Articles

Optimal plant-wide control of the wet sulfuric acid process in an integrated gasification combined cycle power plant

This paper discusses plant-wide control system design of the wet sulfuric acid (WSA) process, which is part of an integrated gasification combined cycle power plant. The regulatory control structure, which ensures energy efficiency and operational stability, was designed by addressing the requirements and constraints of each key unit in the process. We used sensitivity analysis and simple stoichiometric calculation to obtain optimum set points for the controlled variables, which has a direct effect on the energy usage and production rate in the WSA process. In this work, we proposed several controller schemes that ensure efficiency and stability of the WSA process. The results of this work will contribute to improving the process efficiency, thereby facilitating energy efficient and stable operation of the WSA process.

Plant-wide control system design: Secondary controlled variable selection

This work is focused on the development of a rigorous, model-based approach for the selection of secondary controlled variables as part of a plant-wide control system design methodology. Secondary controlled variables should be easy to measure, easy to control, fast to respond to changes in the input variables, and lead to automatic, indirect control of the primary controlled variables. While much of the work on this subject has been based upon ad hoc approaches, here a systematic three-stage approach is proposed that addresses issues of controllability and economic performance of the control system. The first stage involves the generation of an initial set of candidate secondary controlled variables and the generation of selection constraints that are used to determine if manipulated variables can be used for control of candidate controlled variables. During the second stage, secondary controlled variables are selected to minimize integral absolute errors (IAEs) of the primary controlled variables subject to minimal loop interactions as determined by a relative gain array analysis. Finally, during the third stage, control performance of the secondary controlled variables is evaluated at off-design operations using a nonlinear process model. The proposed approach is then applied, as ongoing work in the application of plant-wide control, to an acid gas removal unit as part of an integrated gasification combined cycle power plant with CO2 capture.

Plant-wide control system design: Primary controlled variable selection

This work is focused on the development of a rigorous, model-based approach for the selection of primary controlled variables as part of a plant-wide control system design methodology. Controlled variables should be selected for their self-optimizing control performance and controllability while ensuring satisfactory performance in terms of dead-time and closed loop interactions. This work has considered both self-optimizing and control performance as well as has addressed issues related to loop-interactions and superstructure constraints. The new three-stage approach developed in this work results in a large-scale, constrained, mixed-integer multi-objective optimization problem. For solving this problem, a parallelized, bi-directional branch and bound algorithm with dynamic search strategies has been developed to solve the problem on large computer clusters. The proposed approach is then applied to an acid gas removal unit as part of an integrated gasification combined cycle power plant with CO2 capture.

Systematic top-down economic plantwide control of the cumene process

Plantwide control system design for economic operation over a wide through range (design throughput to maximum throughput) encompassing multiple active constraint regions, is studied for the cumene process. A unique feature of the process is that it recycles the heavy side-product to extinction. A novel top-down control system synthesis approach, where the control objectives for maximum throughput operation are first obtained using steady state optimization followed by control loop pairings with highest priority to economic objectives, is applied. The control structure thus obtained is unconventional with tight active constraint control requiring ‘long’ level loops that maintain the reflux drum and bottom sump levels of a column using the two process fresh feeds. This structure for maximum throughput operation is adapted for economic operation at lower throughputs. Rigorous dynamic simulations show that the structure provides acceptable process regulation for large disturbances despite the long level loops over the entire throughput range. More importantly, no back-off from the active hard equipment capacity constraints also ensure that the loss in throughput from the maximum achievable is negligible. This work is amongst the first reports illustrating the application of the top-down plantwide control system design approach for superior economic performance with robust process stabilization.

Plant-Wide Control System Design of an Alkylation Process Using Integrated Framework of Simulation, Heuristics, and Optimization

In recent years, increasing concern of process economics, strict product specifications, and stringent safety and environmental regulations has resulted in increased use of material recycle and energy integration in chemical process plants. These highly integrated plants need effective control systems for smooth and optimal operation. This has led researchers to propose plant-wide control (PWC) system design methodologies. The present contribution considers the application of one such recently proposed methodology, the integrated framework of simulation, heuristics, and optimization (IFSHO) [Vasudevan, S.; Rangaiah, G. P. Integrated Framework Incorporating Optimization for Plant-Wide Control of Industrial Processes. Ind. Eng. Chem. Res.2011, 50, 8122–8137]. This framework is applied to the autorefrigerated alkylation process for the production of high-octane C8 components, which is important in petroleum refining. The step-by-step application of IFSHO is discussed. The performance of the resulting control...

Economic plantwide control of the ethyl benzene process

Systematic plantwide control system design for economically optimal operation of the ethyl benzene process over a large throughput range is studied. As throughput is increased, constraints progressively become active with the highest number of active constraints at maximum throughput. An economic plantwide control system (CS1) is designed for operation at this most constrained operating point using a novel “top‐down” pairing approach with higher prioritization to the economic objectives over regulatory objectives. This structure is adapted for near optimal low throughput operation with constraints that go inactive taking up additional economic variable control. For comparison, a conventional plantwide control structure (CS2) with the throughput manipulator at a fresh feed and “bottom‐up” pairing for the control objectives is also synthesized. Four overrides are needed in CS2 to handle the hard equipment capacity constraints at maximum throughput. Rigorous dynamic simulations show that CS1 is dynamically and economically significantly superior to CS2. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1996–2014, 2013

Economic Plantwide Control of a C4 Isomerization Process

Plantwide control system design for economically optimal operation over a large throughput range of a C4 isomerization process is considered. The steady-state process degrees of freedom (dofs; eight in total) are optimized for a given throughput (mode I) and maximum throughput (mode II). At maximum throughput, all of the steady-state dofs are exhausted in driving as many constraints (inequality/equality) active. For this process, it is possible to synthesize a simple decentralized plantwide control system (CS1) for tight control of all of the active constraints at maximum throughput with negligible back-off due to transients with satisfactory inventory regulation and process stabilization. A unique feature of CS1 is the location of the throughput manipulator (TPM) inside the plant (in contrast to that at the process feed) and its relocation due to input saturation in a split range arrangement. A comparison with the conventional structure (CS2) with TPM at the process feed and conventional overrides for handling constraints demonstrates the economic and dynamic superiority of CS1.

Plantwide Control for Economic Optimum Operation of a Recycle Process with Side Reaction

Plantwide control system design for the economically optimum operation of a recycle process with side reaction, consisting of a continuous stirred tank reactor (CSTR) followed by two distillation columns, is studied. The steady-state operating profit is maximized over a large throughput range for an existing design. As the throughput is increased, constraints progressively become active until maximum throughput is reached where all the steady-state degrees of freedom are exhausted. A strategy of moving the throughput manipulator to the next constraint to become active as the throughput is increased is used to improve the control of the active constraint. This minimizes the variability and hence the back-off in the active constraint variables for economically optimal operation. The plantwide control system (CS1) so obtained is quantitatively compared with three other more conventional control systems (CS2–CS4) in terms of the required back-off to avoid hard constraint violation for disturbances. Results sh...

Plantwide Control System Design and Performance Evaluation for Ammonia Synthesis Process

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.

Plantwide control system design of the benchmark vinyl acetate monomer production plant

This paper addresses control system design of the vinyl acetate monomer production plant that has been introduced as a plantwide control benchmark problem by Luyben et al. Nonlinear analysis of a simplified model, which includes detailed reactor behavior and ideal separation with gas and liquid recycle, is performed to identify the optimal operating condition and to develop intuition for control system design. Control system is hierarchically constructed to realize the optimal operating condition, which is verified through simulation on a rigorous nonlinear model.

Plantwide Control System Design: Extension to Multiple-Forcing and Multiple-Steady-State Operation

A new approach to designing plantwide control systems that is based on a linear dynamic process model and output optimal control is extended in this paper. The design of a plantwide architecture is split into four stages, and the results from one stage are used as the input to the next. During the design process, transient responses are easily calculated, and they can be used to compare candidate architectures to one another to eliminate architectures with poor dynamic performance. The design methodology is facilitated through a user-friendly software package that makes use of the best currently available algorithms for solving output optimal control problems. In this paper, the design methodology is extended to cover cases where a set of step disturbances and/or step set-point changes is important, as well as to cases where a common control system is required for multiple-steady-state operation. The approach is illustrated on the Tennessee Eastman process.

Plantwide Control System Design: Methodology and Application to a Vinyl Acetate Process

A new approach to the design of plantwide control systems that is based on a linear dynamic process model and output optimal control is presented. The approach also makes use of engineering judgment in eliminating and evaluating candidate architectures. The design of a plantwide architecture is split into four stages, and results from one stage are used as the input to the next. During the design process, transient responses are easily calculated, and they are used to compare candidate architectures to one another so that those with poor performance can be eliminated. The methodology is applied successfully to a model of a vinyl acetate process that has 26 manipulated variables and 43 measurements. The methodology presented is facilitated through a user-friendly software package that makes use of the best currently available algorithms for solving output optimal control problems. A detailed discussion of the various algorithms used in the package is presented.

Synthesis of Plantwide Control Systems Using a Dynamic Model and Optimization

An optimization-based approach to synthesizing plantwide control architectures is discussed. The approach assumes that a dynamic model is available, and it extends earlier work. The plantwide controller is synthesized in three stages involving fast and slow safety variables to be controlled, followed by product variables. In each stage a mixed integer linear program is solved to generate candidate architectures. The objective function involves a tradeoff between manipulated variable moves and transient response area. The integers in the formulation determine the control architecture, and integer cuts are employed to generate second best, third best, etc., candidate solutions. Controlling component balances and adding unit operation controls completes the plantwide control system design. The Tennessee Eastman process is used to illustrate the synthesis procedure.

Plantwide regulatory control design procedure using a tiered framework

Two frameworks for process control design have been identified, and they are used to structure the control design decision sequence in a way which preserves the plantwide'' character of the problem and solution. A CSTR/column process example has been studied in depth, and a procedure for the design of the coupled system regulatory control structure (throughput, inventory, and product quality controls) has been developed. The procedure is based on a tiered framework for plantwide control system design and is justified and supported by an extensive set of dynamic simulations. Specific guidelines for designing inventory control structures are presented. The best-performing structures are shown to be those which are self-consistent'' and designed to minimize the propagation of disturbances through the system. The design procedure for throughput and inventory control design is believed to be fully applicable to other processes. Results from specific application examples will be presented in forthcoming publications (Price et al., 1993).