Plantwide Control Research Articles

Plantwide Control: A Review of Design Techniques, Benchmarks, and Challenges

This paper is a review of plantwide control, presenting both an introductory view for those new to the subject and also summarizing recent advances for those familiar with it. Previous reviews have been studied, summarized, and updated with new publications in the area. The major advances since the beginning of its formal studies are described, with highlighting of the most noticeable contributions. Plantwide control architectures, design techniques, and some important open research topics are addressed. Additionally, noteworthy literature and benchmarks are enumerated.

Optimization and control of a reactive distillation process for the synthesis of dimethyl carbonate

Abstract Dimethyl carbonate is an environmentally benign and biodegradable chemical. Based on integration of reactive distillation and pressure-swing distillation technologies, a novel process for synthesis of dimethyl carbonate through transesterification with propylene carbonate and methanol has been developed by Huang et al. In this work, the optimization of this process was performed by minimizing the total TAC. The results show that the optimal design flowsheet can save energy consumption by 18.6% with the propylene carbonate conversion of 99.9%. Then, an effective plant-wide control structure for the process was developed. Dynamic simulation results demonstrate that the temperature/flow rate cascade control plus with simple temperature control can keep not only product purity but also the conversion of the reactant at their desired values in the face of the disturbance in reactant feed flow rate and feed composition.

A Survey on Control Configuration Selection and New Challenges in Relation to Wireless Sensor and Actuator Networks

This survey on Control Configuration Selection (CCS) includes methods based on relative gains, gramian-based interaction measures, methods based on optimization schemes, plantwide control, and methods for the reconfiguration of control systems. The CCS problem is discussed, and a set of desirable properties of a CCS method are defined. Open questions and research tracks are discussed, with the focus on new challenges in relation to the emerging area of Wireless Sensors and Actuator Networks.

Flotation plant-wide control strategy evaluated on hybrid pilot plant

In this work, the implementation and use of a metallurgical simulator for a flotation plant, including rougher-cleaner-scavenger, RCS, is presented. The simulator can be fed by the user with input operating and metallurgical variables generated off line or in a hybrid manner. In the last case, the operating variables of all circuits are read from instrumentation installed in the pilot plants and combined with virtual variables, generated by the user, to run the metallurgical simulator in order to predict the characteristics of each virtual stream. The pilot plant is located at the Process Control Laboratory and includes a three pneumatic cells rougher circuit, a flotation column and a two pneumatic cell scavenger circuit. The regrinding and classification stages are simulated by using models based on literature.An expert control system is built in order to find out best operating points, in the sense of metallurgical results, mainly Cu global recovery and specified characteristics of the final concentrate. The expert system administrates the set points of the controllers of the operating variables of the plant (froth depths and air flow rates). The control strategy maximizes the rougher Cu recovery subject to the condition that the produced rougher concentrate can be adequately processed in the cleaner, regrinding, classification and scavenger circuits. To achieve this, the quality of the rougher concentrate (Cu grade over a given value and solid mass flow rate under a given value) is monitored jointly with the degree of saturation of manipulated variables in the rest of the circuits, the quality of the final concentrate and the recovery of the scavenger circuit. The result is to find out how to operate the whole plant to obtain both a final concentrate meeting specifications and the maximum Cu global recovery of the plant. Several examples are discussed. Copyright © 2017 IF AC.

Improved plantwide control structure for extractive divided-wall columns with vapor recompression

The conventional extractive distillation flowsheet for dehydration of low-concentration bioethanol uses three-columns: a pre-concentrator, an extractive column and a solvent-recovery column. A recent paper studied the control of a process using a single divided-wall column. The complex flowsheet uses both main and intermediate reboilers. Ethanol goes overhead, water is removed as a liquid sidestream and solvent is recovered from the base.The control challenges of such a complex multivariable interacting process are obvious. The published control structure is complex and dynamically very sensitive. Only slow ramp disturbances can be handled. The purpose of this paper is to develop a more simple control scheme that is robust and is capable of handling large step disturbances. The key feature of the modified process is to only compress what is needed in the side reboiler.

Optimization and control of a stand-alone hybrid solid oxide fuel cells/gas turbine system coupled with dry reforming of methane

This paper presents the hybrid solid oxide fuel cells (SOFC)/gas turbine (GT) system coupled with dry reforming of methane (DRM). The DRM is a syngas producer by consuming greenhouse gas. The stand-alone (off-the-grid) power system is developed by using a combination of a post-burner, recuperators and pressurized recycles in place of external energy supplies. To address the stand-alone operation and meet the complete combustion condition for the burner, the optimal operating conditions are initially determined by solving a constrained optimization algorithm for maximizing the hybrid power efficiency, and the dynamic control loops are implemented in a plantwide environment. In the proposed plantwide control strategy, the inventory control framework is added to regulate the plant component inventory, an air/fuel cross-limiting combustion control is added to ensure complete combustion and reduce heat loss, and the power and CO2 emission control configuration is added to achieve the quality control performance. Finally, the simulation shows that the IMC-based multi-loop control scheme can efficiently regulate the total system power and control CO2 emissions per kWh of electricity as well.

Design and control of a cryogenic multi-stage compression refrigeration process

Providing refrigeration at very low cryogenic temperatures requires the use of several stages of compression refrigeration with progressively lower boiling point refrigerants in the various stages. The cascade of cycles ultimately rejects heat to cooling water, but the working fluid at each stage removes heat from a lower-temperature source and rejects this heat plus the compressor work in that stage to a higher-temperature sink. The design of these systems appears to be only qualitatively discussed in the literature, and no discussion of their plantwide dynamic control has been found.This paper presents a quantitative design of a three-stage compression refrigeration process that uses methane, ethylene and propylene as the working refrigerant fluids in the three stages. Heat is removed in the condenser of a cryogenic distillation column separating carbon monoxide and methane. The bubble-point temperature of carbon monoxide at 13.9bar is −158°C. The distillation column condenser is cooled by evaporating boiling liquid methane at −163°C. The second stage has an evaporator with boiling ethylene at −106.7°C. The final stage has an evaporator with boiling propylene at −25.9°C. An effective plantwide control structure is developed and tested.

A NEW BENCHMARK FOR PLANTWIDE PROCESS CONTROL

Abstract The hydrodealkylation process of toluene (HDA) has been used as a case study in a large number of control studies. However, in terms of industrial application, this process has become obsolete and is nowadays superseded by new technologies capable of processing heavy aromatic compounds, which increase the added value of the raw materials, such as the process of transalkylation and disproportionation of toluene (TADP). TADP also presents more complex feed and product streams and challenging operational characteristics both in the reactor and separator sections than in HDA. This work is aimed at proposing the TADP process as a new benchmark for plantwide control studies in lieu of the HAD process. For this purpose, a nonlinear dynamic rigorous model for the TADP process was developed using Aspen Plus™ and Aspen Dynamics™ and industrial conditions. Plantwide control structures (oriented to control and to the process) were adapted and applied for the first time for this process. The results show that, even though both strategies are similar in terms of control performance, the optimization of economic factors must still be sought.

Plantwide Control for Maximum Throughput Operation of an Ester Purification Process

Plantwide control for maximum throughput operation of an ester purification process consisting of a liquid–liquid extractor (LLX), a distillation column, and material recycle is evaluated. The extractor uses water solvent and is necessary to cross a distillation boundary making pure ester product recovery feasible. Extraction capacity is the bottleneck constraint. A steady state analysis reveals the occurrence of steady state multiplicity induced infeasibility for too high fresh feed rates and for too low water feed rates. To avoid infeasibility due to the multiplicity behavior, we recommend using the extractor feed rate as the throughput manipulator (TPM) with the fresh feed rate adjusted as a makeup stream (CS1). The other control loop pairings are conventional. Rigorous closed loop dynamic results show that CS1 is significantly more robust than the conventional control structure (CS2) with the fresh feed rate as the TPM. In particular, CS1 achieves more than 25% higher maximum throughput compared to CS...

Continuous diisobutylene manufacturing: Conceptual process design and plantwide control

The complete process design cycle encompassing flowsheet synthesis, design and controllability evaluation is studied for continuous DIB manufacturing. The residue curve map tool is applied to synthesize two new flowsheets, FS1 and FS2, exploiting pressure swing distillation. A unique feature of these is the use of a decanter for recovery and recycle of water, which allows maintaining the reactor tert-butyl alcohol content at the desired level for suppressing the side reaction. Unlike the literature flowsheet (FS0), this innovation makes it possible to achieve both high conversion and high yield for an economically superior design. Between FS1 and FS2, FS1 is found to be superior with significantly lower capital and energy costs as well as lower fresh water consumption. A “smart” control strategy for holding the single-pass reactor conversion and the overall process yield towards economic process operation is also developed. Rigorous dynamic simulations demonstrate the controllability of FS1 and FS2.

Distributed plantwide control based on differential dissipativity

SummaryModern chemical plants are becoming very complex, often consisting of a number of nonlinear process units (subsystems) with strong interactions due to material recycle and energy integration. The operation setpoint may need to be adjusted from time to time based on the market demand. To address the aforementioned challenges, a plantwide distributed nonlinear control scheme based on differential dissipativity is proposed in this paper, which can ensure plantwide incremental exponential stability and achieve bounded incremental L2 gain performance. As a non‐unique property, the differential dissipativity of individual subsystem is shaped by a setpoint‐independent control structure – differential state feedback control. The dissipativity properties of subsystems and individual controllers are determined simultaneously as a large‐scale feasibility problem to ensure the plantwide stability and performance. It is converted into an LMI condition for plantwide supply rate planning and small‐scale sum‐of‐squares programming problems for individual subsystem dissipativity shaping, by using the alternating direction method of multipliers method. The proposed approach is illustrated using a chemical reactor network with a recycle stream. Copyright © 2016 John Wiley & Sons, Ltd.

Plant-wide control of coupled distillation columns with partial condensers

Conventional distillation control processes use vapor distillate flowrate to control column pressure and condenser heat removal to control the reflux drum level. These intuitive control systems work well for isolated columns or columns with total condensers. However, these controls are not effective when columns with partial condensers occur in series. The pressure and reflux drum level interact in such systems in ways that defeat conventional control systems, rendering them unable to maintain product purities in the presence of large feed flowrate and composition disturbances. This investigation documents a plant-wide control structure that can address this issue by controlling pressure through reflux heat removal rate and reflux drum level by reflux flow rate. This control system demonstrates its capability to handle large disturbances in throughput and feed composition through a series of Aspen simulations. This alternative system is no more complicated than the conventional system and should work on distillation columns of nearly all designs, not just the coupled partial condenser designs for which it is essential.Common natural gas processing provides a specific example of this alternative control system. Natural gas commonly includes high concentrations of CO2 that must be removed prior to pipeline or LNG distribution. The existence of a minimum-boiling temperature azeotrope between ethane, virtually always present in natural gas, and carbon dioxide complicates the separation of CO2 from the hydrocarbons. This separation commonly employs extractive distillation with high-molecular-weight hydrocarbons. Our recent paper Ebrahimzadeh et al. (2016) discusses in detail the steady-state economic design of a new extractive distillation strategy for the CO2–ethane azeotrope separation with three columns. This strategy shows a 5% reduction in capital and 15% reduction in operating costs when compared to optimized versions of the conventional process. The new strategy also produces CO2 in a liquid rather than a vapor phase, which simplifies transport, storage, and handling. Two columns of the proposed design use partial condensers and are the focus of this investigation.

Novel Design of Collaborative Automation Platform Using Real-Time Data Distribution Service Middleware for an Optimum Process Control Environment

Refining and petrochemical processing facilities utilize various process control applications to raise productivity and enhance plant operation. Client–server communication model is used for integrating these highly interacting applications across multiple network layers utilized in distributed control systems. This paper presents an optimum process control environment by merging sequential and regulatory control, advanced regulatory control, multivariable control, unit-based process control, and plant-wide advanced process control into a single collaborative automation platform to ensure optimum operation of processing equipment for achieving maximum yield of all manufacturing facilities. The main control module is replaced by a standard real-time server. The input/output racks are physically and logically decoupled from the controller by converting them into distributed autonomous process interface systems. Real-time data distribution service middleware is used for providing seamless cross-vendor interoperable communication among all process control applications and distributed autonomous process interface systems. Detailed performance analysis was conducted to evaluate the average communication latency and aggregate messaging capacity among process control applications and distributed autonomous process interface systems. The overall performance results confirm the viability of the new proposal as the basis for designing an optimal collaborative automation platform to handle all process control applications. It also does not impose any inherent limit on the aggregate data messaging capacity, making it suitable for scalable automation platforms.

Model Predictive Control (MPC)-Based Supervisory Control and Design of Off-Gas Recovery Plant with Periodic Disturbances from Parallel Batch Reactors

This paper proposes a plantwide control structure and a model predictive control (MPC) scheme for the effective separation of off-gas from a polysilicon plant with periodic disturbances in the feed. The target process studied in this work produces 10 000 MT of polysilicon per year with 28 chemical vapor deposition (CVD) reactors. CVD reaction kinetic models were constructed, and their parameters were identified using a genetic algorithm. Aspen Dynamics was used to model the off-gas recovery process and implement proportional–integral (PI) controllers for regulating the inventory levels. A centralized supervisory layer based on MPC, to minimize the fluctuation of the recovered concentration, was linked with regulatory controllers through a MATLAB/SIMULINK environment. Using a rigorous plantwide dynamic model, four cases were studied with different control schemes with and without buffer tanks. The proposed control strategy showed better performance, compared with fully decentralized classical proportional–integral–derivative (PID) schemes. It is thus expected that the developed control strategy will be able to achieve the design of batch-continuous processes with periodic disturbances. The benefits of using the multivariable MPC approach as a supervisory layer are shown and analyzed using Aspen Plus, Aspen Dynamics, and MATLAB/SIMULINK.

Williams-Otto Plant Control Based on Production Planning Associated to Coordinated Decentralized Optimization and Plantwide Control Techniques

Williams-Otto Plant Control Based on Production Planning Associated to Coordinated Decentralized Optimization and Plantwide Control Techniques

Economic plant-wide control design with backoff estimations using internal model control

Economic optimal operation typically involves operating as close as possible to the active constraints. However, in the presence of disturbances it is necessary to back-off from the constraints in order to avoid violating them. The backoff approach aims at selecting the control structure that minimizes the economic loss associated with the required constraint backoffs. This paper revisits the backoff approach and proposes a framework for estimating the constraint backoffs based on well-known elements of internal model control (IMC) theory, such as an automatic procedure for tuning the IMC low-pass filters, a stability condition, and an uncertainty representation based on diagonal input multiplicative uncertainty. Since the constraint backoffs are estimated using a linear dynamic model, the inclusion of input multiplicative uncertainty allows introducing conservatism in the estimation of the backoffs, which is required in order to avoid constraint violations. A forced-circulation evaporator benchmark problem is used to illustrate the approach.

Design and Control of a Reactive Distillation Process for Naphtha Hydrodesulfurization

Increasingly stringent regulations on the maximum permissible sulfur levels in transportation fuels has inspired considerable interest in reactive distillation (RD) based fuel hydrodesulfurization (HDS) in the refining industry. In this work, the steady-state economic design and plantwide control of an RD naphtha HDS process is studied. The process consists of an RD column followed by an absorber-stripper unit. Engineering heuristics are employed to develop a near-optimum economic process design. The important trade-offs in the design of the process are explained. A simple decentralized plantwide control system employing single-point temperature inferential control in the RD column with the fresh naphtha feed as the throughput manipulator is shown to provide effective process regulation for large changes in throughput, fresh naphtha thiophene composition, and feed hydrogen composition.

Plant-wide optimization and control of an industrial diesel hydro-processing plant

Diesel hydro-processing (DHP) is an important refinery process which removes the undesired sulfur from the oil feedstock followed by hydro-cracking and fractionation to obtain diesel with desired properties. The DHP plant operates with varying feed-stocks. Also, changing market conditions have significant effects on the diesel product specifications. In the presence of such a dynamic environment, the DHP plant has to run in the most profitable and safe way and satisfy the product requirements. In this study, we propose a hierarchical, cascaded model predictive control structure to be used for real-time optimization of an industrial DHP plant.

Control of parallel dry methane and steam methane reforming processes for Fischer–Tropsch syngas

The ratio of hydrogen to carbon monoxide in the synthesis gas required for feeding to the gas-to-liquid Fischer–Tropsch (FT) process to produce liquid transportation fuel is about two. The dry methane reforming (DMR) process feeds carbon dioxide and methane and produces a syngas with a H2/CO ratio of about unity. The steam methane reforming (SMR) process feeds water and methane and produces a syngas with a H2/CO ratio of about four. This paper studies the plantwide control of a process with DMR and SMR units operating in parallel to produce FT syngas. The total methane fresh feed is split between the two parallel processes in the appropriate fraction so as to produce the desired H2/CO ratio in the mixed syngas stream from the process.Both the SMR and DMR reactions are highly endothermic and the reactors are fired furnaces with combustion of fuel providing the endothermic heat of the reactions. Reactor exit temperatures are controlled by manipulating the flowrates of fuel to each furnace, with combustion air ratioed to the fuel. Dynamic reactor models assume tubular SMR and DMR reactors in which heat fluxes are determined by the heat generated in their associated fuel/air combustion reactors.The plantwide control structure effectively handles large 20% disturbances in throughput and large setpoint changes in the desired H2/CO ratio (1.7–2.3).

Slow feature analysis for monitoring and diagnosis of control performance

Recently, slow feature analysis (SFA), a novel dimensionality reduction technique, has been adopted for integrated monitoring of operating condition and process dynamics. By isolating temporal behaviors from steady-state information, the SFA-based monitoring scheme enables improved discrimination of nominal operating point changes from real faults. In this study, we demonstrate that the temporal dynamics is an additional indicator of control performance changes, and further exploit its unique efficacy in control performance monitoring. Because of its data-driven nature and ease from first-principle knowledge, the SFA-based monitoring scheme allows an overall assessment of the plant-wide control performance and is compatible with different control strategies. An attractive feature of the SFA-based approach compared to existing ones is that generic process monitoring indices are used, which renders contribution plots naturally applicable to real-time diagnosis of control performance. As a result, potential fault variables as root causes of control performance changes can be identified, including not only controlled variables (CV) but also manipulated variables (MV) and disturbance variables (DV). Simulated and experimental studies demonstrate the effectiveness of the proposed method.