Control Structure Selection Research Articles

On the selection of control structures using process operability analysis

This work aims to develop a generalizable framework for control structure selection using process operability analysis. Current approaches for selecting controlled variables in chemical processes are limited to assessing system attributes individually, focusing on controller performance or the economic impact based on a constant setpoint policy. However, the competitive industrial manufacturing market requires a holistic approach for control structure selection in large-scale plants that takes into account multiple factors. In particular, process operability can help to enable a generalizable approach that is able to select control structures that are operable considering economic and performance factors simultaneously. To achieve this goal, a framework that uses the Operability Index (OI) as a metric for ranking the achievability of the control objectives for the selected control structures is developed. To test the framework, a depropanizer distillation column is investigated as a case study associated with large-scale energy systems. This work thus introduces novel formulations and algorithms for the control structure selection problem, enhancing the design, operations, and synthesis of existing and future industrial systems.

Control structure selection of increased‐pressure extractive distillation process for DMC‐MeOH azeotropic mixture

AbstractProducing dimethyl carbonate (DMC) as a green chemical with the desired purity is important in the industry. Although studies on the steady‐state design of energy‐efficient extractive distillation processes are important for the purification of DMC‐methanol (DMC‐MeOH) azeotropic mixtures, the dynamic controllability of these processes is also critical in the case of feed condition changes, and it should be investigated carefully. Results of the limited studies in the literature show that changing the operating pressures in extractive distillation processes might have different effects on the dynamic controllability of different systems. Thus, in this study, alternative control strategies are developed for a recently proposed increased‐pressure extractive distillation process to separate DMC‐MeOH mixture. All control structures are designed using inferential temperature controllers, which have a general acceptance in industrial applications. Effects of different ratio controllers are investigated by evaluating the dynamic responses of control structures for disturbances in feed flowrate and composition. Two metrics including integral absolute error and steady‐state deviation of purities are used in the evaluation of alternatives. Results of dynamic simulations show that a control structure including reflux ratio controller is not a suitable strategy for this process. It is demonstrated that a control structure including reflux to feed ratio controller for both distillation columns is necessary for the robust and efficient control of a pressure‐increased extractive distillation process. These efficient dynamic results support the economic advantage of increased‐pressure extractive distillation process separating DMC‐MeOH azeotropic mixtures.

Optimal Control Structure Selection Based on Economics for Continuous Cross-Flow Grain Dryer

In this work, we present the control structure selection problem based on steady-state economics applied to a continuous cross-flow grain dryer model. The optimal selection of the controlled and manipulated variables and their set-points can be obtained for various external disturbance scenarios if an appropriate mathematical programming problem is formulated. Simulation results show that the economics performance of classical open loop and closed loop structure used for dryer control can be improved by the use of linear combination of controlled variables and also by the use of a hierarchical control structure. In addition, we study several alternatives (sub-optimal) control structures that show an acceptable economic performance.

Multi-Objective Optimal Design and Operation of Heat Exchanger Networks with Controllability Consideration

Controllability reflects the ease that a process can be controlled in practical operating environment. However, an unclear influence between the HEN synthesis and the control structure selection has been not investigated for the work of controllability of heat exchanger network (HEN). To address this challenge, this paper proposes a multi-objective optimization method by considering both the quantitative measures of economic and controllability, i.e., minimizing the total annual cost (TAC) and the relative gain array number (RGAn). This method is developed using a HEN synthesis procedure where a model-based superstructure is employed to involve the set of the network configuration alternatives and all the potential control structures. The effects of minimum approach temperature (ΔTmin) on the multi-objective optimization problem are investigated to distinguish the consistent and opposite variations of TAC and RGAn. The consistent change enables us to solve the single objective optimization problem for economical HEN design as well as for taking controllability into account. The opposite change prompts the Pareto front of the two objectives in order to develop a trade-off strategy. Results indicate that this method helps in the determination of the relationship-based nature between network configuration and control structure to yield a HEN design with economic and controllability considerations.

Accessibility, Observability, and Fault-Tolerant Control Structure Selection of Network Nonlinear Systems

An appropriate control structure is the prerequisite to guarantee the controllability and the observability of a system. This article addresses the issue of fault-tolerant control structure selection (CSS) problem for a class of network nonlinear systems (NNSs) with multidimensional nodes. First, based on differential algebra theory, necessary and sufficient accessibility/observability conditions are established, respectively, for NNSs, which fully reveal the relations between the accessibility/observability and the network structure as well as external inputs/outputs. Second, a fault-tolerant CSS-based recovering strategy is provided that adds minimal inputs/outputs/edges to recover the accessibility and observability of the faulty NNS. Finally, the new strategy is applied to examples of chemical reaction and multiple aircrafts to illustrate the effectiveness of the theoretical results.

Программное обеспечение систем управления «умным» жилым домом

The article deals with the issues of modeling and management of life support systems of a residential building. The resulting model of the liquid level in the tank allows you to establish the relationship between the level and the flow rate of the liquid. The results of the selection and justification of the fluid level control structure are presented. An algorithm that implements the operation of a virtual object is given. The structure of the automatic control system (ACS) of the liquid level is technically implemented in an open type. The results confirming the achievability of the proposed structural changes are obtained. The results of experimental studies are presented. The choice and justification of the method of controlling the heating system and the liquid level in the tank are considered. Programs for managing subsystems of a residential building in Assembly language, C++, and ladder diagrams are presented. A model for controlling the liquid level in a Multisim environment is proposed.

Analysis of Optimal Control Strategies for Efficient Operation of a Produced Water Reinjection Facility for Mature Fields

Abstract Produced water management policies in oil and gas industry have been moving towards zero emission, no ocean discharge and waste-to-value conversion. Produced water reinjection plays a key role in achieving these goals for mature fields as it derives value from produced water and is able to maintain environmental integrity. A central element in this, is optimal operation of pumping. Hydraulic efficiency and specific energy are generally the chosen indices used to evaluate operational energy efficiency in pumping systems. In this work, we investigate optimization strategies based on natural objective function candidates for a produced water reinjection system: (i) operation at best efficiency point; (ii) minimization of total shaft input power; (iii) maximization of overall pump hydraulic efficiency; (iv) minimization of total specific energy and (v) minimization of total delivered energy. In addition, we discuss the implication of the different solutions over control structure selection.

A quadratic approximation of the back-off methodology for the control structure selection problem

For almost three decades, the back-off methodology has been extensively developed and refined for addressing the control structure selection problem. Previous work has been based on a linear and a non-linear formulation. The linear formulation ensures quick determination of the optimal solution but with a potentially significant loss of accuracy in complex non-linear processes. The non-linear formulation develops higher accuracy and offers the opportunity of simultaneous consideration of design and control. This improved accuracy usually comes with an increase in computational cost and complexity. A new methodology based on linear models and a quadratic approximation of economics is proposed which demonstrates improved accuracy when compared with the linear approximation and reasonable increase in computational effort than the non-linear counterpart. Three case studies are studied in detail to examine the advantages of the new formulation.

Economic Control Structure Selection for Two-Layered Real-Time Optimization Systems

In industrial chemical plants, the selection of controlled variables, manipulated variables, and setpoint values directly impacts the economics of plant operation. The economic operation of a contr...

An internal model control design method for failure-tolerant control with multiple objectives

The selection of control structure is a critical step in the design of a control system since it can largely affect the achievable control performance. This paper presents a general internal model control (IMC) structure with multiple degrees-of-freedom for the design of control systems with multiple objectives (i.e., reference tracking and load and output disturbance rejection), so that controllers are designed independently of each other. The optimal performance of controllers is shown to remain intact when a controller is taken off-line, for example, due to actuator and/or sensor failures. This feature circumvents the need to redesign the remaining on-line controllers for optimal failure-tolerant control. The proposed control approach is used to derive IMC control structures for multi-loop cascade and coordinated control systems. The performance of the control approach is demonstrated on a simulated thin-film drying process in continuous pharmaceutical manufacturing for several multi-loop control structures and a variety of control-loop component failures.

Robust Multi-Variable Controller Design for Two-Input Two-Output Fourth-Order DC-DC Converter

The presented two-input two-output fourth-order integrated (TITO-FOI) DC–DC converter is capable of regulating DC-bus voltage and low voltage source current with only two switches and four energy storage elements. Each switch duty control signal in TITO-FOI converter system will affect the two output variables causing interactions. These interactions must be measured for proper controller structure selection. Interactions measurements may not be accurate for a converter system as some methods suggest centralized and some other suggest decentralized control schemes. Hence, there is an ambiguity in selection of suitable controller architectures. To overcome such an ambiguity a centralized interaction independent controller was designed for TITO-FOI DC–DC converter using loop-shaping design procedure. The effectiveness of TITO-FOI converter along with this centralized controller for the regulation of load division and bus voltage is demonstrated with simulation studies using a 36 V/12 V to 24 V, 100 W, prototype DC power distribution system.

Self-Optimizing Steady-State Back-Off Approach for Control Structure Selection

The selection of suitable control structures has an important influence on the economic performance of process systems in the presence of disturbances. Economics has been incorporated in the contro...

Loop Interaction Analysis and Control Structure Selection: Application to a Fluid Catalytic Cracking Unit

A simulation approach for the assessment of variables interaction and consequent control structure selection of a fluid catalytic cracking unit (FCCU) is presented in this paper. The simulator which was implemented in Matlab draws from an earlier mathematical model of the FCCU, was used as a virtual FCCU for studying the dynamic response of the riser temperature (T rx ), the regenerator temperature (T rg ) and the regenerator flue gas oxygen concentration (O d ) to step changes in air flow rate (F a ), regenerated catalyst flow rate (F rc ), gas oil feed rate (F gr ). The results show strong interaction in FCCU variables, with F a affecting T rg and O d ; F rc affecting T rx, T rg and O d ; F gr affecting T rx , T rg and O d . A linearised state-space model based on the first-principle model was deduced and transformed to a 3x3 input-output model. Three channel interaction measures: Relative Gain Array (RGA), Effective Relative Gain Array (ERGA) and the Normalized Relative Gain Array (RNGA) were applied to the selection of FCCU control structure. All the measures point to a diagonal scheme with the following pairings: (T rx /F gr ), (T rg /F a ) and (O d /F rc ) ,for the decentralized control of the riser temperature, the regenerator temperature and the flue gas oxygen concentration respectively. The suggested control structure offers a high promise of stability, with a Niederlinski index (NI) of 101.79. DOI : 10.7176/CTI/8-03

Decentralized stable and robust fault-tolerant PI plus fuzzy control of MIMO systems: a quadruple tank case study

Abstract This paper presents a typical design of fault-tolerant control using two decentralized PI plus fuzzy controllers to control the level of the lower two tanks in a nonlinear quadruple tank level process (QTLP). We also present some basic aspects of decentralized control design concerning stability and performance and illustrate them on a case study: a virtual model of a quadruple tank process. Control structure selection based on performance relative gain array (PRGA) is used, and its ability to evaluate the achievable performance is discussed. The controllers are designed based on a conventional PI controller plus fuzzy inference system technique. The relation between inputs/outputs was proved using relative gain array (RGA), and then, we divided the quadruple tank system into two subsystems and controlled each of them separately, both in minimum and non-minimum phases. Both the controllers were designed to control the nonlinear QTLP at any operating points. The proposed approach was compared with a decentralized fuzzy controller subject to actuator/sensor and system component faults. The simulation results show that the proposed decentralized fault-tolerant PI plus fuzzy controller has a more accurate tracking level and less computational time in both minimum and non-minimum phases.

Simultaneous Design and Control of a Ternary Reactive Distillation Column with Inert Material

AbstractProcess integration means significant challenges in operating a chemical process dynamically due to its complexity. One example is the ternary reactive distillation with inert material. Based on the heuristic approach, chemical process design and process control are performed independently one after another. This may lead to infeasible solutions either in process design and control or in both. The application of simultaneous design and control, formulated as mixed‐integer dynamic optimization (MIDO) problems, increased rapidly in the last decade. Simultaneous process design and control, including control structure selection and control parameters, is devised for the challenging benchmark problem of a ternary reactive distillation with inert material. The improved process design and control is obtained by solving the MIDO problem.

Solvent effects on design with operability considerations in post-combustion CO2 capture plants

This work presents an evaluation of the operation of alternative solvents for post-combustion CO2 capture under varying process conditions. The proposed methodology is part of a generalized design and operability framework and relies on the determination of the processes’ steady-state controllability as an inherent system feature, independent of the control structure selection. Assessment of such characteristics can shed light on the effect different capture solvents, process configurations and control structures have on the achieved dynamic responses and further associate them to the physico-chemical properties of the solvents. Three amine based solvents, namely monoethanolamine (MEA), diethanolamine (DEA) and 3-amino-1-propanol (MPA) are introduced to an advanced CO2 capture flowsheet previously designed for optimal steady-state operation. The dynamic characteristics of the investigated solvent-process systems under the influence of disturbances emulating real-time industrial conditions are then assessed and rank-ordered using an index that measures the departure of the process variables from the region of desired operation. As such, for a given disturbance, the drift from the acceptable steady-state operating point and the response of dynamic closed-loop simulations determine the suitability and applicability of each solvent in the employed CO2 capture process configuration. Results reveal the superiority of the dynamic behavior of MPA in terms of economic performance as well as expected process operability for variations in the inlet flue gas flowrate, greatly surpassing the performance of the benchmark solvent, MEA.

Selection of Control Structures and Phyisical Models for Robots in Open Environments

Selection of Control Structures and Phyisical Models for Robots in Open Environments

Applications of an Optimal Multi-Objective Technique for Integrated Control Structure Selection and Tuning

The controller tuning affects the control performance as much as the input/output (I/O) pairing. Thus, an optimal tuning should be sought simultaneously with the best I/O pairing, and not after the pairing has been defined. An optimal multi-objective technique for integrated control structure selection and tuning is proposed and applied to industrial benchmarks. The technique integrates the tasks of I/O pairing and controller tuning, and supports any multi-objective specification. It allows the direct employment of complex specifications of control problems, solving the I/O pairing problem simultaneously with the tuning problem. The technique is applied to two industrial benchmarks, a 3x7 heavy oil fractionator (HOF) and a 6x7 fluid catalytic cracking (FCC) units. The examples demonstrate the complete control project, including the PID I/O pairings, controller tuning, system identification and MPC implementation.

Demonstrating load-change transient performance of a commercial-scale natural gas combined cycle power plant with post-combustion CO2 capture

The present work aims to study the transient performance of a commercial-scale natural gas combined cycle (NGCC) power plant with post-combustion CO2 capture (PCC) system via linked dynamic process simulation models. The simulations represent real-like operation of the integrated plant during load change transient events with closed-loop controllers. The focus of the study was the dynamic interaction between the power plant and the PCC unit, and the performance evaluation of decentralized control structures. A 613MW three-pressure reheat NGCC with PCC using aqueous MEA was designed, including PCC process scale-up. Detailed dynamic process models of the power plant and the post-combustion unit were developed, and their validity was deemed sufficient for the purpose of application.Dynamic simulations of three gas turbine load-change ramp rates (2%/min, 5%/min and 10%/min) showed that the total stabilization times of the power plant’s main process variables are shorter (10–30min) than for the PCC unit (1–4h). A dynamic interaction between the NGCC and the PCC unit is found in the steam extraction to feed the reboiler duty of the PCC unit. The transient performance of five decentralized PCC plant control structures under load change was analyzed. When controlling the CO2 capture rate, the power plant performs in a more efficient manner at steady-state part load; however, the PCC unit experiences longer stabilization times of the main process variables during load changes, compared with control structures without CO2 capture rate being controlled. Control of L/G ratio of the absorber columns leads to similar part load steady-state performance and significantly faster stabilization times of the power plant and PCC unit’s main process variables. It is concluded that adding the PCC unit to the NGCC does not significantly affect the practical load-following capability of the integrated plant in a day-ahead power market, but selection of a suitable control structure is required for efficient operation of the process under steady-state and transient conditions.

Application of the dividing wall column to olefin separation in fluidization methanol to propylene (FMTP) process

Abstract Dividing wall column (DWC) is shown to be energy efficient compared to conventional column sequence for multi components separation, which is used for olefin separation in fluidization methanol to propylene process in the present work. Detailed design for pilot DWC was performed and five control structures, i.e. composition control (CC), temperature control (TC), composition-temperature control (CC-TC), temperature difference control (TDC), double temperature difference control (DTDC) were proposed to circumvent feed disturbance. Sensitivity analysis and singular value decomposition (SVD) were used as criterion to select the controlled temperature locations in TC, CC-TC, TDC and DTDC control loops. The steady simulation result demonstrates that 25.7% and 30.2% duty can be saved for condenser and reboiler by substituting conventional column sequence with DWC, respectively. As for control structure selection, TC and TDC perform better than other three control schemes with smaller maximum deviation and shorter settling time.