Number Of Control Loops Research Articles

A comparison between Claus and THIOPAQ sulfur recovery techniques in natural gas plants

A sulfur recovery process is one of the most important processes in the oil and gas industry to get rid of hydrogen sulfide (H2S) which is produced from the acid gas removal process of sour natural gas to convert it into sweet natural gas. Actual data from a gas field is used to obtain a realistic comparison between two sulfur recovery techniques, through which researchers and/or manufacturers can obtain information that will help them choose the most appropriate and cheapest method. A total feed acid gas flow rate of 5.1844 MMSCFD with an H2S concentration of 24.62% by mole percent was produced from amine acid gas removal units. Claus sulfur recovery technique is a traditional chemical process that uses thermal and catalytic reactors. Therefore, an acid gas enrichment unit is applied to increase the H2S concentration to approximately 50% mole to provide reliable and flexible operation in the thermal and catalytic reactors. Moreover, a tail gas treatment unit is applied to increase the overall conversion efficiency to 99.90% with the Claus technique instead of 95.08% without it to achieve high sulfur recovery and reliable operation through the conversion of carbonyl sulfide (COS) and mercaptans. Studies on the safety and simplicity of the Claus technique revealed many important hazards and a large number of transmitters (379) and control loops (128) in one Claus train. THIOPAQ sulfur recovery as a new technology is a biological desulfurization process that uses a natural mixture of sulfide-oxidizing bacteria. It is also a unique H2S removal process with an efficiency of 99.999%. In addition, studies on the safety and simplicity of the THIOPAQ technique have shown that the hazards, the number of transmitters (74), and the number of control loops (29) of a one THIOPAQ train are lower. The THIOPAQ technique showed higher efficiency, was safer, simpler, and had lower CAPEX and OPEX. This study was conducted using Aspen HYSYS V11 and actual data.

An Innovative Converterless Solar PV Control Strategy for a Grid Connected Hybrid PV/Wind/Fuel-Cell System Coupled With Battery Energy Storage

The proposed work addresses the modeling, control, energy management and operation of hybrid grid connected system with wind-PV-Battery Energy Storage System (BESS) integrated with Fuel Cell (FC) and Electrolyzer. A hybrid PV-Wind-FC with electrolyzer consisting of BESS with the least number of control loops and converters has been proposed. The proposed hybrid system presents a cost efficient solution for integrating PV into a hybrid system by eliminating the PV converter. This includes the design of controllers for grid-connected hybrid systems with a renewable distributed generator (Wind and PV) as a primary source, BESS as a secondary source and FC with Electrolyzer as a tertiary source. In addition, the lead compensator along with integrator is used for obtaining enough phase margin and removing steady state error completely. It increases the stability of the controller and adds phase shift φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>s</i></sub> at a cross gain frequency (ω <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>cut</i></sub> ). The Grid Side Controller (GSC) is capable of providing frequency support to the utility grid, when it is linked to the grid. In the proposed configuration, PV power is maximized and injected into grid through GSC. Rotor Side Converter (RSC) and GSC ensure the support for sharing the burden of the grid station. Moreover, the proposed controller of BESS with coordination of FC eliminates the effect of intermittency of power generated from wind and PV. Excess power production by renewable distribution generation is used by Electrolyzer to generate hydrogen. This hydrogen is further used by FC when there is not enough power generation due to unfavorable weather conditions. The energy management has been presented to fulfill the load profile, avoid BESS overcharging and to minimize the intermittency and fluctuation of Wind and PV sources. This method guarantees steady power flow and service continuity. The Simulink model of the proposed system results validate the efficiency of the proposed hybrid system as compared to the conventional hybrid system reported in the literature. The modeling of the proposed system and analysis has been demonstrated using the MATLAB Simulink model. Lastly, the energy management of the system has also been examined and compared with the conventional power system.

Dynamic controllability comparison of different side-stream extractive distillation processes with intermediate boiling entrainer

It is of great significance to simultaneously consider the economy and controllability for promoting the sustainability of the process industry. In the paper, the controllability of different energy-efficient side-stream extractive distillation (SSED) processes using intermediate boiling entrainer including two double column side-stream extractive distillation (DC-SSED) processes and one single column side-stream extractive distillation (SC-SSED) process are explored for separating the methanol/toluene azeotrope with triethylamine as entrainer. The side stream withdrawal locations of the DC-SSED processes are located in the rectifying section (DC-SSED (a)) and extraction section (DC-SSED (b)), respectively. The recycled intermediate boiling entrainer concentration is 45 mol% for the SC-SSED process. The two robust control strategies constructed for the DC-SSED (b) process are also suitable for the DC-SSED (a) process in resisting the same feed disturbances, which do not adopt the construction concept of the feasible control strategy for the SSED separation process with heavy entrainer. The difference between the two novel control strategies exists in how to control the methanol product purities via reflux ratios. Simultaneously, the two robust control strategies are constructed to resist the same feed disturbances for the SC-SSED process and compared to the two DC-SSED processes in terms of dynamic performances. The dynamic performances demonstrates that the DC-SSED (a) process with worst energy and economic performances is superior to the SC-SSED process and the DC-SSED (b) process from the perspective of the total integral squared error. However, the number of control loops used in the control strategies constructed for the SC-SSED process with optimal energy and economic performances is much smaller than that used in the control strategies constructed for the DC-SSED process, especially the number of composition control loops. Overall, the dynamic performances and economic performances of different processes may not be consistent, and the paper provides some advice for the construction of the robust control strategies for the SSED separation processes with intermediate boiling entrainer.

Three-Phase Single-Carrier PWM Inverter for Isolated Grid-Tied PV Applications

Owing to their dependency of weather conditions, distributed generation systems are integrated with utility grid through power converters. This paper proposes an isolated three-phase based flyback-inverter (TBFBI) that can be used as a central-type inverter for grid-tied PV applications. To alleviate DC-current ripples at the input side, the proposed inverter utilizes only a single LC-filter with small passive elements size, which reduces the system footprint and cost. Mathematical modeling of the TBFBI in addition to its control technique is presented in detail. Compared with other differential based converters, the proposed control technique is designed considering the least number of control loops and required sensors by using terminal voltage estimation strategy. Continuous-modulation-scheme (CMS) combined with static-linearization strategy (SLS) is utilized to diminish the low-order odd harmonics. In addition, harmonic compensation technique is utilized to eliminate negative-sequence harmonic component (NSHC) from the grid-injected currents. Also, identical three single-phase high-frequency transformers (HFTs) are designed based-on ferrite and nanocrystalline cores to compare the inverter operating efficiency. The TBFBI is experimentally validated via laboratory prototype-based 200 V, 1.6 kW, and switching frequency of 50 kHz. Experimental results of 1.6 kW power flow show that THD of the grid current is 3.95%, peak-to-peak current ripple at the input DC-side is 2.1% of the average DC input current that matches the IEEE-1547 standards for grid-tied PV applications.

A sparse loading-based contribution method for multivariate control performance diagnosis

An automatic and accurate control performance diagnosis algorithm is necessary for general chemical processes with a number of control loops when a performance change occurs. The popular contribution plots for control performance diagnosis have severe smearing effects of diagnosis and the problems are still not solved. A sparse loading based contribution plot is proposed in this paper to enhance the diagnosis performance. A complete iterative optimization procedure related to how to obtain the sparse loadings is developed. A numerical example and a period of practical industrial data are used to demonstrate the efficiency of the proposed method in comparison with the conventional contribution plots.

Clinical safety assessment of the Halcyon system.

The Halcyon consists of precommissioned linear accelerator and treatment planning algorithms that were designed to simplify the acceptance, commissioning, and clinical workflow for image-guided intensity-modulated radiotherapy. The purpose of this work was to perform a comprehensive safety assessment for the clinical use of the Halcyon. Systems-Theoretic Process Analysis was used as the safety assessment tool. As part of the analysis, a number of control loops and control actions are created to describe system function. Safety is assessed by determining unsafe control actions and a corresponding list of causal scenarios that leads to accidents. The scope of the analysis was from the acceptance of the Halcyon to routine patient treatments. All aspects of treating patients were considered including the roles of physicians, physicists, dosimetrists, and therapists. The analysis was completed by four physicists with input from other members of the radiation therapy team. The causal scenarios were summarized using the causality categories from the consensus recommendations for incident learning database structures in radiation oncology (Med Phys, Vol. 39, No. 12, Dec 2012). Twenty-three (23) control loops containing 52 control actions were created for the clinical use of the Halcyon. One hundred forty-four (144) unsafe control actions were identified with 385 associated causal scenarios. Twenty-seven percent (27%) of the causal scenarios were related to equipment technical issues, while 73% of the causal scenarios were predominantly related to procedural issues, human behavior, and organizational management. For routine clinical use of closed or largely automated radiation therapy equipment, the majority of safety concerns is related to nontechnical issues. The Halcyon and other similar systems may present opportunities to streamline, reduce, or eliminate some traditional equipment commissioning and routine quality assurance activities in exchange for an increased focus on issues related to organizational management, procedures, and human behavior.

ANALYSIS OF INVERTER FED MICROGRIDS FOR DIFFERENT MODES OF OPERATION IN MATLAB/SIMULINK

This paper presents analysis of control techniques of a converter fed microgrid in grid connected mode and islanded mode of operation. Here two control strategies are discussed , one for grid connected mode and the other for islanded mode of operation. In grid connected mode of operation the DG inverter system works in constant current control mode, and provide preset power to the grid, while in islanded mode of operation the DG inverter system goes to voltage control mode. Voltage control mode is simple and has a low number of control loops.

Control System Performance Monitoring

Product quality, operation safety, material and energy consumption, and thus the financial performance of industrial plants are directly or indirectly linked to the performance of control systems. Since process control systems are very complex, usually comprising different hierarchy levels, it is hopeless to maintain them on regular basis by plant personal. This is also the main reason why a large portion of industrial control loops has significant performance problems, as found out by audits carried out regularly since the 1990s. All these factors have contributed to the growing of the control performance monitoring (CPM) technology and applications in the last decade. CPM provides a framework for automatically and systematically assessing the performance of control loops, detecting and diagnosing root-causes of poor performance, as well as suggesting measures to improve control performance or avoid performance degradation. For this purpose, CPM techniques should be non-invasive, i.e., based on only routine operating data with limited or no additional process knowledge and without the need for any experimentation with the plant. This thesis deals with the complete CPM technology, from controller assessment (minimum-variance-control-based and advanced methods), over detection and diagnosis of control loop problems (process non-linearities, oscillations, actuator faults), to the improvement of control performance (maintenance, re-design of loop components, automatic controller re-tuning). It provides a contribution towards the development and application of completely self-contained and automatic methodologies in the field. Moreover, within this work, many CPM tools have been developed that goes far beyond available CPM packages. Industrial data from a large number of control loops in different industrial fields (building, chemicals, mining, mineral and metal processing) have been used to demonstrate the presented strategies and methods. Systematic procedures for automatic and continuous control performance monitoring, maintenance and optimisation are recommended, combining different control performance metrics and assessment, diagnosis and improvement methods. The main objective is to sustain top control performance during the whole life cycle of the control system, despite different and changing operational conditions.

Middleware for Control Kernel Implementation in Embedded Control Systems

Control tasks require a number of activities, not all of them with the same relevance and priority. The critical subtasks constitute what is denoted as the control kernel. The design of ECS should be structured, with a kernel unit providing the most basic features. Most of these activities, such as data acquisition or control action delivering, are common to a number of control loops implemented in the same CPU. In this paper, the architecture of a middleware (MW) for essential control activities to ensure economic, safe and reliable operation is discussed. It is specially designed for control purposes, interacting with the peripherals (sensors, actuators and communication channels), with the OS, and exchanging information with the bunch of control algorithm implemented in the application area. As part of the facilities of the middleware, some issues for improving in a transparent manner these characteristics are considered. Finally, an example of controller implementation by using this middleware is presented.

Tree mapping technology as a means of visualising control performance problems

Tree maps were intended to be visualisation aids for representing complex organisation charts, biological species, and so on. The objective of an effective controller performance visualisation chart is to present the information on the performance of the entire plant at a macro level and yet have the ability to display local (micro) details at the unit level or even the loop level. From a visual point of view it is important to provide an indication of the number of control loops in each unit rather than simply displaying them as nodes of the same size. Identifying specific control issues on a plant-wide basis and their root-causes is analogous to looking for a needle in a haystack. A relatively new technology called tree mapping may be a useful solution to this problem; it eases the visualisation challenge associated with monitoring hundreds or thousands of control assets.

CONTROL LOOP PERFORMANCE MEASURES IN THE EVALUATION OF PROCESS ECONOMICS

The number of control loops used in industry is growing continuously and there are problems in keeping them working at a satisfactory level. One reason for the poor performance is the lack of maintenance of control loops. Economical benefits can result from the better maintenance of control loops. In this paper the economic aspects of control loop performance measures are discussed. A strategy is proposed to evaluate the economics of control performance using control loop performance measures. In addition, a case study on an industrial process is also described. It is anticipated that the framework introduced in this paper may prove useful in presenting control quality in a common language understood by operators, engineers and management.

The Economical Aspects of Control Loop Performance Measures in the Remote Maintenance Center Concept

The number of control loops used in industry is growing continuously and there are problems in keeping them well tuned. In addition to maintenance, there is also always a need to improve the performance. Since the number of personnel is being reduced, companies are using, to an ever-increasing extent, a range of remote expert services for control loop performance evaluation. In this paper a set of performance indices appropriate to process monitoring and assessment is presented, and the economical aspects of these indices are evaluated. The economical evaluation has been mainly carried out from the remote center point of view. Finally, two case studies from industry are described.

Toward an understanding of DCS control operator workload

The relatively new technology of distributed control (DCS) has created a long list of new problems that must be solved and questions that must be answered. One perplexing question encountered by control system engineers is how to gauge DCS control operator workload and, more importantly, how to discern if the operator's workload is excessive. The introductions of local control networks and advanced control require more sophisticated metrics of workload than simply counting control loops and using the 200 control loops per operator rule of thumb. Studies have also shown that there is little correlation between the number of control loops and many parameters used to gauge operator workload (such as control moves per hour). This paper will investigate current issues of DCS control operator workload in three main topic areas. First, the paper will briefly define factors that contribute to control operator workload and introduce a model of control operator workload. Control operator workload is a multifaceted construct affected by a host of interacting variables including the operator/control system interface (display and alarm system design), process dynamics, operator training/experience, and crew interaction. Next, the paper will briefly describe strategies that can be used to assess operator workload. Techniques used to assess workload are traditionally divided into three categories: (1) Subjective measures (NASA-TLX, developed by NASA, and SWAT, developed by the U.S. Air Force); (2) Physiological measures (heart rate and eye blink rate); and (3) Performance based measures (such as the number of control moves or alarms per hour, or where performance is measured on a secondary task that the operator completes). Also, the paper will discuss which are the easiest and most appropriate techniques to use with control operators. Finally, the paper will present a workload study of a petrochemical plant DCS control operator job. The study represents a typical control operator loading and how the workload ratings change in response to increasing job demands. The paper will define performance based parameters which are indicative of excessive workload. Also, the paper will touch on the effects of advanced control on operator workload and will discuss stategies that the control system designer can use to reduce workload.