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Sigurd Skogestad - One of the best experts on this subject based on the ideXlab platform.
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Control Structure design of a solid oxide fuel cell and a molten carbonate fuel cell integrated system top down analysis
Energy Conversion and Management, 2017Co-Authors: Prathak Jienkulsawad, Sigurd Skogestad, Amornchai ArpornwichanopAbstract:Abstract The integrated system of a solid oxide fuel cell and molten carbonate fuel cell theoretically has very good potential for power generation with carbon dioxide utilization. However, the Control strategy of such a system needs to be considered for efficient operation. In this paper, a Control Structure design for an integrated fuel cell system is performed based on economic optimization to select manipulated variables, Controlled variables and Control configurations. The objective (cost) function includes a carbon tax to get an optimal trade-off between power generation and carbon dioxide emission, and constraints include safe operation. This study focuses on the top-down economic analysis which is the first part of the design procedure. Three actively constrained regions as a function of the main disturbances, namely, the fuel and steam feed rates, are identified; each region represents different sets of active constraints. Under nominal operating conditions, the system operates in region I. However, operating the fuel cell system in region I and II can use the same Structure, but in region III, a different Control Structure is required.
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Control Structure design for stabilizing unstable gas lift oil wells
IFAC Proceedings Volumes, 2012Co-Authors: Esmaeil Jahanshahi, Sigurd Skogestad, Henrik Erring HansenAbstract:Active Control of the production choke valve is the recommended solution to prevent casing-heading instability in gas-lifted oil wells. Focus of this work is to find a simple yet robust Control Structure for stabilization of the system. In order to find suitable Control variables, a Controllability analysis of the system with different candidate Control variables and two alternative manipulated variables was performed. Moreover, to include robustness and performance requirements at the same time, the Controllability analysis was extended to a mixed sensitivity H1 optimization problem. A Control Structure using only the available top- side pressure measurements was found to be effective to stabilize this system.
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self optimizing and Control Structure design for a co2 capturing plant
Proceedings of the 2nd Annual Gas Processing Symposium#R##N#Qatar January 10-14 2010, 2010Co-Authors: Mehdi Panahi, Sigurd Skogestad, Mehdi Karimi, Magne Hillestad, Hallvard F SvendsenAbstract:In this thesis, the systematic plantwide procedure of Skogestad (2004) is applied to two processes; 1- Post-combustion CO2 capturing processes, 2- Natural gas to liquid hydrocarbons (GTL) plants, in order to design economically efficient Control Structures, which keep the processes nearoptimum when disturbances occur. Because of the large magnitude of energy consumption in both these processes, optimal operation is of great importance. The self-optimizing concept, which is the heart of the plantwide procedure is used to select the right Controlled variables in different operational regions, which when they are kept constant, indirectly give the operation close to optimum. The optimal is to reconfigure the self-optimizing Control loops when the process is entered into a new active constraint region, but we try to arrive at a simple/single Control Structure, which does not need switching, where a reasonable loss in operating economic objective function is accepted. The CO2 capturing process studied here is an amine absorption/stripping system. The chosen objective function for this process is first to minimize the energy requirement while fixed CO2 recovery of 90% is met. This leads to one unconstrained degree of freedom. Maximum gain rule is applied and a temperature close to the top of the stripper is found as the best Controlled variable. Further, we introduce penalty on CO2 amount released to the atmosphere, and this results in two unconstrained degrees of freedom. CO2 recovery and a temperature close to the top of the stripper are found as the best individual Controlled variables in low feedrate. In higher flue gas flowrates, stripper heat input saturates and the self-optimizing method is repeated to select the right Controlled variable for the remaining degree of freedom. We validate the propose Control Structures using dynamic simulations, where 5 different alternatives including decentralized Control loops and multivariable Controller are studied. We finally achieve a simple Control Structure, which handles a wide range of change in throughput and keeps the process close to optimum without the need for switching the Control loops or updating the Controlled variables setpoints by a costly real time optimizer. The GTL process modeled in this thesis includes an auto-thermal reformer (ATR) for synthesis gas production and a slurry bubble column reactor (SBCR) for the Fischer-Tropsch (FT) reactions. The FT products distribution is determined using a well-known Anderson- Schultz- Flory (ASF) model, where carbon component in CO (consumption rate is found based on the proposed rate by Iglesia et al.) is distributed to a range of hydrocarbons. ASF is a function of chain growth probability and the chain growth is a function of H2/CO ratio. We study different scenarios for chain growth and we arrive at a suitable model for optimal operation studies. The optimal operation is considered in two modes of operation. In mode I, natural gas feedrate is assumed given and in mode II, natural gas feedrate is also a degree of freedom. After optimization, in both modes, there are three unconstrained degrees of freedom. The best individual self-optimizing Controlled variables are found and since the worst-case loss value is rather notable, combination of measurements is done, which reduces the loss significantly. Mode II happens when oxygen flowrate capacity reaches the maximum and we show that operation in mode II in this case is in snowballing region where operation should be avoided. Operation at maximum oxygen flowrate capacity is where maximum practical profit can be achieved.
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Control Structure design for the ammonia synthesis process
Computers & Chemical Engineering, 2008Co-Authors: Antonio Carlos Brandao De Araujo, Sigurd SkogestadAbstract:This paper discusses the application of the plantwide Control design procedure of Skogestad [Skogestad, S. (2004a). Control Structure design for complete chemical plants. Computers and Chemical Engineering, 28, 219–234] to the ammonia synthesis process. Three modes of operation are considered: (I) given feed rate, (IIa) maximum throughput, and (IIb) “optimized” throughput. Two Control Structures, one for Mode I and
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Control Structure design for complete chemical plants
Computers & Chemical Engineering, 2004Co-Authors: Sigurd SkogestadAbstract:Control Structure design deals with the structural decisions of the Control system, including what to Control and how to pair the variables to form Control loops. Although these are very important issues, these decisions are in most cases made in an ad hoc fashion, based on experience and engineering insight, without considering the details of each problem. In the paper, a systematic procedure for Control Structure design for complete chemical plants (plantwide Control) is presented. It starts with carefully defining the operational and economic objectives, and the degrees of freedom available to fulfill them. Other issues, discussed in the paper, include inventory and production rate Control, decentralized versus multivariable Control, loss in performance by bottom-up design, and a definition of a the ‘‘complexity number’’ for the Control system. # 2003 Elsevier Ltd. All rights reserved.
William L. Luyben - One of the best experts on this subject based on the ideXlab platform.
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new Control Structure for feed effluent heat exchanger reactor systems
Industrial & Engineering Chemistry Research, 2012Co-Authors: William L. LuybenAbstract:Feed-effluent heat exchangers (FEHEs) are widely used in high-temperature exothermic adiabatic tubular reactor systems to conserve energy. The hot reactor effluent is recycled back to a feed preheater to provide all or a portion of the energy required to preheat the reactor feed to the optimum reactor inlet temperature. Several alternative flowsheets have been presented in the literature for this type of configuration. Some use bypassing of cold material around the FEHE, some use a furnace after the FEHE and before the reactor, and some use a cooler (steam generator) after the reactor. A number of Control Structures have been proposed to Control the reactor inlet temperature, which is critical for the steady-state performance (achieving the desired conversion and staying below some maximum temperature). It is also critical for dynamic performance since these systems can be openloop unstable as a result of the positive feedback of energy from the reactor back to the preheat system. The Control Structure mu...
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unusual Control Structure for high reflux ratio distillation columns
Industrial & Engineering Chemistry Research, 2009Co-Authors: William L. LuybenAbstract:Distillation Control wisdom suggests that reflux flow rate should be used to Control the reflux-drum level when the reflux ratio is large (>3). In the typical case where a single-end temperature Control Structure is used, reboiler heat input is used to Control a tray temperature at an appropriate location in the column. When the reflux ratio is large and reflux is used to Control the reflux-drum level, a frequently used scheme Controls the reflux ratio by measuring the reflux flow rate and rationing the distillate flow rate to the reflux flow rate. However, in many single-end distillation Control Structures, the inherent ability to handle feed composition disturbances is better for a fixed reflux-to-feed ratio than for a fixed reflux ratio. Using reflux to Control the reflux-drum level precludes the use of this Structure. This dilemma can be resolved by using an unusual Control Structure in which the reflux-drum level is Controlled by reboiler heat input and a tray temperature is Controlled by manipulatin...
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new Control Structure for divided wall columns
Industrial & Engineering Chemistry Research, 2009Co-Authors: Hao Ling, William L. LuybenAbstract:Industrial applications of the divided-wall column for the separation of ternary mixtures have increased in recent years with about 40 columns reported to be in service. The divided-wall column is a practical way to implement the topology of the Petlyuk column that features two columns (a prefractionator into which the feed is introduced and a main column from which a sidestream product is withdrawn) with interconnected vapor and liquid streams arising from a single reboiler and a single condenser. Many papers discuss the steady-state design issues and propose heuristic and rigorous design optimization methods. The dynamic Control of the divided-wall column has been explored in a relatively small number of papers. Control is more difficult than with a conventional two-column separation sequence because there is more interaction among Controlled and manipulated variables since the four sections of the column are coupled. The vapor split is fixed at the design stage and cannot be changed during operation, b...
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evaluation of a two temperature Control Structure for a two reactant two product type of reactive distillation column
Chemical Engineering Science, 2006Co-Authors: Devrim B Kaymak, William L. LuybenAbstract:Abstract Several different Control Structures have been proposed for reactive distillation columns. The appropriate Control Structure depends on the flowsheet and on the type of reactions occurring in the column. If two reactants are involved and if it is desirable to operate the process without any excess of reactant, it is necessary to manage the fresh feed streams so that the stoichiometry is exactly balanced. A composition analyzer that measures an internal composition in the column is often required. However, if two products are produced, it is possible to avoid the use of an analyzer by using two temperatures in the column to adjust the two feed streams. This type of Structure was proposed by Roat et al. [Roat, S., Downs. J., Vogel, E., Doss, J., 1986. Integration of rigorous dynamic modeling and Control system synthesis for distillation columns. In: Chemical Process Control—CPC III. Elsevier, Amsterdam.] for the ideal reaction A + B ↔ C + D in one of the earliest papers dealing with reactive distillation Control. The purpose of this paper is to explore the effectiveness of this two-temperature Control Structure for various column designs (number of reactive stages) to quantify the impact of design on Controllability. We also discuss the issues of the selection of the trays whose temperatures are to be Controlled and the tuning of the two interacting temperature Controllers. Disturbances in production rate and fresh feed compositions are made to examine the rangeability of this Control Structure. Both an ideal reaction system and the methyl acetate system are studied. One of the main conclusions is that the locations of the temperature Control trays should be made such that the two temperature Controllers both have direct action (an increase in temperature increases feed), which requires negative openloop process gains for both loops.
Subhashish Bhattacharya - One of the best experts on this subject based on the ideXlab platform.
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proportional integral resonant and dual loop current Control Structure comparison for grid connected converters in the rotating frame
Applied Power Electronics Conference, 2018Co-Authors: Srinivas Gulur, Vishnu Mahadeva Iyer, Subhashish BhattacharyaAbstract:Over the past few years, due the increasing penetration of renewable energy, there has been a steady rise in the harmonic content in grid voltages. In such circumstances, a proportional integral (PI) based current Control in the synchronous reference rotating frame (dq) for a grid connected voltage source converter may not be adequate to suppress the harmonic components and to precisely follow the fundamental frequency component with zero steady state error. Several current Control Structures have been proposed, with proportional integral — resonant Controller (PI-RES) based Structure being one of the most popular. In this paper, the PI-RES has been compared to the recently introduced dual loop current Control Structure. Both these current Control Structures have been compared in terms of their tracking, filtering and disturbance rejection capability. Robustness of both these Structures has also been analyzed under a grid impedance variation. Simulation and experimental results have been provided to validate the analysis presented.
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a dual loop current Control Structure with improved disturbance rejection for grid connected converters in the synchronous rotating reference frame
European Conference on Cognitive Ergonomics, 2017Co-Authors: Srinivas Gulur, Vishnu Mahadeva Iyer, Subhashish BhattacharyaAbstract:The increase in renewable energy penetration in the past few years has led to grave power quality issues in the utility grid. This has ushered in the need for a robust and stable Control system for reference tracking and disturbance rejection with respect to grid connected converters. Typically, a simple proportional integrator (PI) Controller is used in the synchronous reference frame (dq) for current Control of grid connected systems. However, the PI Controller in itself may be insufficient for disturbance rejection, when the utility grid voltages contain other harmonics in addition to the fundamental component. This paper introduces and analyzes a dual loop current Control Structure which utilizes two independent Controllers, one for reference tracking and the other for disturbance rejection in the dq frame. A small signal model of the dual loop current Control has been presented and its robustness under grid inductance variation, examined. Simulation and experimental results are presented to validate the dual loop strategy for improved disturbance rejection capability and filtering action during the presence of grid voltage harmonics and unbalances, without compromising on the reference tracking performance.
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a Control Structure for pwm Controlled static synchronous compensators under unbalanced conditions and grid faults
International Journal of Electrical Power & Energy Systems, 2015Co-Authors: Saman Babaei, Subhashish BhattacharyaAbstract:Abstract Grid connected Voltage Source Converters (VSCs) are the heart of many applications with power quality concerns due to their reactive power Controllability. Among the widely used grid-connected applications of the VSCs, the Static Synchronous Compensators (STATCOMs) are commonly used for compensating the voltage quality problems that come from voltage sag and swell. In spite of superior feature of fast voltage regulation and reactive power support functionality, VSC-based STATCOMs have the major drawback of being sensitive to the grid disturbances, especially the unbalanced condition and faults. Moreover, when the STATCOMs are used in the Distributed Generation (DG) applications or reactive power support of the sensitive industrial load, the unbalanced condition becomes even more intolerable. Protection system usually trips due to over current or highly distorted current caused by negative sequence current flow under unbalanced conditions and system faults. This paper propose an alternative Control Structure to keep the VSC-based STATCOM online during the unbalanced condition and system faults by limiting the negative sequence current. This eliminates the need to redesign/overdesign of the STACOM power components and over rating of the semiconductor switches to operate under fault current. Converter MVA rating reduction will decrease the cost significantly. Proposed Controller performance has been verified by simulation and Hardware-In-the-Loop test.
Amornchai Arpornwichanop - One of the best experts on this subject based on the ideXlab platform.
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Control Structure design of a solid oxide fuel cell and a molten carbonate fuel cell integrated system top down analysis
Energy Conversion and Management, 2017Co-Authors: Prathak Jienkulsawad, Sigurd Skogestad, Amornchai ArpornwichanopAbstract:Abstract The integrated system of a solid oxide fuel cell and molten carbonate fuel cell theoretically has very good potential for power generation with carbon dioxide utilization. However, the Control strategy of such a system needs to be considered for efficient operation. In this paper, a Control Structure design for an integrated fuel cell system is performed based on economic optimization to select manipulated variables, Controlled variables and Control configurations. The objective (cost) function includes a carbon tax to get an optimal trade-off between power generation and carbon dioxide emission, and constraints include safe operation. This study focuses on the top-down economic analysis which is the first part of the design procedure. Three actively constrained regions as a function of the main disturbances, namely, the fuel and steam feed rates, are identified; each region represents different sets of active constraints. Under nominal operating conditions, the system operates in region I. However, operating the fuel cell system in region I and II can use the same Structure, but in region III, a different Control Structure is required.
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Control Structure design and robust model predictive Control for Controlling a proton exchange membrane fuel cell
Journal of Cleaner Production, 2017Co-Authors: Narissara Chatrattanawet, Thanaphorn Hakhen, Soorathep Kheawhom, Amornchai ArpornwichanopAbstract:Abstract In this work, cell performance analysis, concept of Control Structure design, and Controller design for a proton exchange membrane fuel cell (PEMFC) were implemented. Steady-state analysis was performed to determine suitable operating conditions. The effects of the input parameters on cell voltage and cell temperature were analyzed to investigate the dynamic behavior of a PEMFC that is important for Control design. To obtain an efficient Control system, the Control Structure design of the PEMFC was also considered to find suitable Controlled and manipulated variables. Moreover, the selection of input-output pairings by considering the relative gain array (RGA) as a Controllability index was applied to the Control system design. Finally, model predictive Control and offline robust model predictive Control (MPC) based on a linear time-varying model were proposed for PEMFC Control. The results showed that the steady-state operating points were selected at the current density = 0.51 A/cm 2 , cell voltage = 0.59 V, power density = 0.30 W/cm 2 , and cell temperature = 332 K. The cell voltage and cell temperature depend on the inlet molar flow rates, temperature of hydrogen and air, and operating current density. According to the RGA, the inlet molar flow rates of air and hydrogen are manipulated variables that regulate the cell temperature and partial pressure of hydrogen, respectively. Furthermore, Controller design using MPC and robust MPC as a Controller can demonstrate good results. Robust MPC can guarantee the stability of the PEMFC.
Moonyong Lee - One of the best experts on this subject based on the ideXlab platform.
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Control Structure synthesis for operational optimization of mixed refrigerant processes for liquefied natural gas plant
Aiche Journal, 2014Co-Authors: Yuli Amalia Husnil, Moonyong LeeAbstract:The best Control Structures for the energy optimizing Control of propane precooled mixed refrigerant (C3MR) processes were examined. A first principles-based rigorous dynamic model was developed to analyze the steady-state and dynamic behaviors of the C3MR process. The steady-state optimality of the C3MR process was then examined in a whole operation space for exploring the feasibility of the energy optimizing Control for possible Control Structures. As a result, the temperature difference (TD) between the warm-end inlet and outlet MR streams was exploited as a promising Controlled variable to automatically keep the liquefaction process close to its optimum. The closed-loop responses were finally evaluated for every possible Control Structure candidate. Based on the steady-state optimality and the dynamic performance evaluation, several Control Structures with a TD loop were proposed to be most favorable for the energy optimizing Control of the C3MR process. The proposed optimality approach can be applied to any natural gas liquefaction process for determining a proper Controlled variable for optimizing operation. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2428–2441, 2014
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two point temperature Control Structure selection for dividing wall distillation columns
Industrial & Engineering Chemistry Research, 2012Co-Authors: Kwang Il Kim, Moonyong Lee, Sunwon ParkAbstract:Two-point temperature Control Structures of dividing-wall columns (DWCs) were investigated to overcome difficulties arising from their complicated natures. The Control performance of DWCs was assessed for nine different feed conditions characterized by the composition of an intermediate component and their relative volatility. Steady-state and dynamic simulations were performed to analyze the closed-loop responses for various two-point temperature Control Structures of DWCs. It is shown that proper Control strategies depend strongly on feed characteristics, particularly on the relative volatilities of the feed components. Control Structure evaluations follow from the rigorous simulations, and basic guidelines for selecting proper Control Structures for DWCs are proposed in relation to the feed conditions.
