Nonlinear Model Predictive Control Of A Distillation Column Using Hammerstein Model And Nonlinear Autoregressive Model With Exogenous Input.

Turus penyulingan adalah unit proses penting dalam industri penapisan petroleum dan kimia. Ia perlu dikawal hampir dengan keadaan-keadaan pengendalian yang optima demi insentif- nsentif ekonomi. Distillation column is an important processing unit in petroleum refining and chemical industries, and needs to be controlled close to the optimum operating conditions because of economic incentives

Improved multi model predictive control for distillation column

Model predictive control (MPC) strategy is known to provide effective control of chemical processes including distillation. As illustration, when the control scheme was applied to three linear distillation columns, i.e., Wood-Berry (2x2), Ogunnaike-Lemaire-Morari-Ray (3x3) and Alatiqi (4x4), the results obtained proved the superiority of linear MPC over the conventional PI controller. This is however, not the case when nonlinear process dynamics are involved, and better controllers are needed. As an attempt to address this issue, a new multi model predictive control (MMPC) framework known as Representative Model Predictive Control (RMPC) is proposed. The control scheme selects the most suitable local linear model to be implemented in control computations. Simulation studies were conducted on a nonlinear distillation column commonly known as Column A using MATLAB® and SIMULINK® software. The controllers were compared in terms of their ability in tracking set points and rejecting disturbances. Using three local models, RMPC was proven to be more efficient in servo control. It was however, not able to cope with disturbance rejection requirement. This limitation was overcome by introducing two controller output configurations: Maximizing MMPC and PI controller output (called hybrid controller, HC), and a MMPC and PI controller output switching (called MMPCPIS). When compared to the PI controller, HC provided better control performances for disturbance changes of 1% and 20% with an average improvement of 12% and 20% of the integral square error (ISE), respectively. It was however, not able to handle large disturbance of + 50% in feed composition. This limitation was overcome by MMPCPIS, which provided improvements by 17% and 20% of the ISE for all of types and magnitudes of disturbance change. The application of MMPCPIS on a single model MPC strategy produced almost similar performance for both types of disturbances, while its application on MMPC yielded better results. Based on the results obtained, it can be concluded that the proposed HC and MMPCPIS deserve further detailed investigations to serve as linear control approaches for solving complex nonlinear control problems commonly found in chemical industr

Dynamics, Control and Extremum Seeking of the Rectisol Process

Pendant la dernière décennie, les bioraffineries basées sur la gazéification ont fait l’objet de nombreuses études dans le cadre des efforts mondiaux visant à remplacer les combustibles fossiles qui produisent de l’énergie et des produits chimiques à valeur ajoutée. Une partie importante de ces bioraffineries est l’unité de purification des gaz de synthèse issus de l’oxydation partielle, qui enlève le CO2 et l’H2S. Un des procédés de purification considéré dans ces études est le Rectisol. Ce procédé est utilisé car il est plus environnemental et requière moins de coûts d’investissement et d’opération par rapport à d’autres procédés similaires. Afin de faire l’étude dynamique de ce procédé, une simulation en régime permanent à d’abord, été menée à l’aide du logiciel Aspen plus R. ----------ABSTRACT Gasification based biorefineries have been studied in the past decade as part of a global e↵ort to replace fossil fuels to produce energy and added value chemicals. An important part of these biorefineries is the acid gas removal units, that remove CO2 and H2S from the produced synthesis gas. One of the acid gas removal processes associated in these studies is Rectisol. Rectisol has been chosen since it’s environmental friendly and requires a lower amount of operational and capital costs compared to its opponents. To carry out a dynamic study of the process, as a first step, a steady-state simulation was carried out in Aspen Plus

Practical Solutions to the Non-Minimum Phase and Vibration Problems Under the Disturbance Rejection Paradigm

This dissertation tackles two kinds of control problems under the disturbance rejection paradigm (DRP): 1) the general problem of non-minimum phase (NMP) systems, such as systems with right half plane (RHP) zeros and those with time delay 2) the specific problem of vibration, a prevailing problem facing practicing engineers in the real world of industrial control. It is shown that the DRP brings to the table a refreshingly novel way of thinking in tackling the persistently challenging problems in control. In particular, the problem of NMP has confounded researchers for decades in trying to find a satisfactory solution that is both rigorous and practical. The active disturbance rejection control (ADRC), originated from DRP, provides a potential solution. Even more intriguingly, the DRP provides a new framework to tackle the ubiquitous problem of vibration, whether it is found in the resonant modes in industrial motion control with compliant load, which is almost always the case, or in the microphonics of superconducting radio frequency (SRF) cavities in high energy particle accelerators. That is, whether the vibration is caused by the environment or by the characteristics of process dynamics, DRP provides a single framework under which the problem is better understood and resolved. New solutions are tested and validated in both simulations and experiments, demonstrating the superiority of the new design over the previous ones. For systems with time delay, the stability characteristic of the proposed solution is analyze

Practical Solutions to the Non-Minimum Phase and Vibration Problems Under the Disturbance Rejection Paradigm

This dissertation tackles two kinds of control problems under the disturbance rejection paradigm (DRP): 1) the general problem of non-minimum phase (NMP) systems, such as systems with right half plane (RHP) zeros and those with time delay 2) the specific problem of vibration, a prevailing problem facing practicing engineers in the real world of industrial control. It is shown that the DRP brings to the table a refreshingly novel way of thinking in tackling the persistently challenging problems in control. In particular, the problem of NMP has confounded researchers for decades in trying to find a satisfactory solution that is both rigorous and practical. The active disturbance rejection control (ADRC), originated from DRP, provides a potential solution. Even more intriguingly, the DRP provides a new framework to tackle the ubiquitous problem of vibration, whether it is found in the resonant modes in industrial motion control with compliant load, which is almost always the case, or in the microphonics of superconducting radio frequency (SRF) cavities in high energy particle accelerators. That is, whether the vibration is caused by the environment or by the characteristics of process dynamics, DRP provides a single framework under which the problem is better understood and resolved. New solutions are tested and validated in both simulations and experiments, demonstrating the superiority of the new design over the previous ones. For systems with time delay, the stability characteristic of the proposed solution is analyze

Inferential active disturbance rejection control of distillation columns

PhD ThesisThe distillation column is an important processing unit in the chemical and oil refining industry. Distillation is the most widely employed separation method in the world’s oil plants, chemical and petrochemical industrial facilities. The main drawback of the technique is high energy consumption, which leads to high production costs. Therefore, distillation columns are required to be controlled close to the desired steady state conditions because of economic incentives. Most industrial distillation columns are currently controlled by conventional multi-loop controllers such as proportional-integral-derivative (PID) controllers, which have several shortcomings such as difficulty coping with sudden set-point jumps, complications due to the integral term (I), and performance degradation due to the effect of noise on the derivative term (D). The control of ill-conditioned and strongly non-linear plants such as high purity distillation needs advanced control schemes for high control performance. This thesis investigates the use of active disturbance rejection control (ADRC) for product composition control in distillation columns. To the author’s knowledge, there are few reported applications of ADRC in the chemical industry. Most ADRC applications are in electrical, robotics and others. Therefore, this research will be the first to apply the ADRC scheme in a common chemical processing unit, and can be considered as a first contribution of this research. Initially, both PI and ADRC schemes are developed and implemented on the Wood–Berry distillation column transfer function model, on a simulated binary distillation column based on a detailed mechanistic model, and on a simulated heat integrated distillation column (HIDiC) based on a detailed mechanistic model. Process reaction curve method and system identification tools are used to obtain the 2×2 multi-input multi-output (MIMO) transfer function of both binary and HIDiC for the purpose of PI tuning where the biggest log-modulus tuning (BLT) method is used. Then, the control performance of ADRC is compared to that of the traditional PI control in terms of set-point tracking and disturbance rejection. The simulation result clearly indicates that the ADRC gives better control performance than PI control in all three case studies. The long time delay associated with product composition analysers in distillation columns such as gas chromatography deteriorates the overall control performance of the ADRC scheme. v To overcome this issue an inferential ADRC scheme is proposed and can be considered as a second contribution of this research. The tray temperatures of distillation columns are used to estimate both the top and bottom product compositions that are difficult to measure on-line without a time delay. Due to the strong correlation that exists in the tray temperature data, principal component regression (PCR) and partial least square (PLS) are used to build the soft sensors, which are then integrated into the ADRC. In order to overcome control offsets caused by the discrepancy between soft sensor estimation and actual compositions measurement, an intermittent mean updating technique is used to correct both the PCR and PLS model predictions. Furthermore, no significant differences were observed from the simulation results in the prediction errors reported by both PCR and PLS. The proposed inferential ADRC scheme shows effective and promising results in dealing with non-linear systems with a large measurement delay, where the ADRC has the ability to accommodate both internal uncertainties and external disturbances by treating the impact from both factors as total disturbances that will then be estimated using the extended state observer (ESO) and cancelled out by the control law. The inferential ADRC control scheme provides tighter product composition control that will lead to reduced energy consumption and hence increase the distillation profitability. A binary distillation column for separating a methanol–water mixture and an HIDiC for separating a benzene–toluene mixture are used to verify the developed inferential ADRC control scheme.Petroleum Development of Oman (PDO) for their generous support and scholarshi

Controlling trace impurities in a dividing wall distillation column

Dividing wall distillation columns (DWCs) separate a feed mixture into three pure product streams using one column shell. Though attractive due to capital and operational savings, DWCs have yet to gain widespread industrial acceptance. One notable concern is controllability. The research within this document examines a four component feed mixture to evaluate the operational flexibility of a fixed-design DWC through experimental and simulation-based studies. A pilot DWC was successfully controlled at multiple operating points, and a dynamic model was developed to reflect the pilot dividing wall column. As a form of process intensification, DWCs have a higher risk for controller interaction making conventional PID control potentially inadequate. This work successfully used two PID temperature controllers to maintain the column at steady state, transition the column between steady states, and reject feed disturbances without controller interaction. These controller pairings were determined using conventional controller design techniques. Therefore, for this chemical system and column design, traditional approaches to distillation control are sufficient to handle the intensified nature of DWCs. Because more components are present in DWCs in larger amounts, there is concern that temperature control will no longer imply composition control. Temperature control proved successful in this study. Controlling two temperatures maintained column operation against feed disturbances. In addition, prefractionator temperature correlated well with reboiler duty for multiple feed qualities therefore serving as a promising control variable though more disturbances such as feed composition should be examined. The minimum energy controller was not tested experimentally. A steady state model with heat transfer matching the pilot data was scaled to the size of an industrial tower and used to generate a minimum energy response surface for different vapor and liquid split values. In summary, this research investigated the operational flexibility of a fixed-design DWC using a four component mixture, tested the ability of conventional distillation control design techniques to determine control structures for a DWC, and created a minimum energy operating surface that could be used to examine control structures. A technique to determine the overall heat transfer coefficients was developed, and the model closely matched experimental steady state data.Chemical Engineerin

Control and operation of a spinning disc reactor

PhD ThesisThe aim of the present research is to assess the control and operation of a Spinning Disc Reactor (SDR), carried out via four separate investigations. Firstly, the effect of equipment size reduction on control is studied by comparing the performance of a PID controller applied to simulated intensified and conventional processes. It was found that superior control performance in terms of Integral of Absolute Error (IAE) is achieved for the simulated intensified system. However, the results showed that intensified systems are more susceptible to disturbances and the controlled variable exhibits larger overshoots. Furthermore, the frequency response analysis of the two systems showed that the simulated intensified system has reduced stability margins. The second part of the research investigates the task of pH control in a SDR using a PID controller by means of simulation and experimental studies. The effectiveness of a disturbance observer (DO) and a pH characteriser to compensate for the severe pH system nonlinearity is also explored in detail. The experimental studies showed that a PID controller provides adequate setpoint tracking and disturbance rejection performances. However, sluggish transient responses prevailed and the effluent pH limit cycled around the setpoint. There were indications of unstable behaviour at lower flowrates, which implied more advanced control schemas may be required to adapt to various operating regions dictated by the complex thin film hydrodynamics. The addition of the DO scheme improved the control performance by reducing the limit cycles. In the third segment of the investigations, the potential of exploiting the disc rotational speed as a manipulated variable is assessed for the process of barium sulphate precipitation. A PI controller is successfully used to regulate the conductivity of the effluent stream by adjusting the disc rotational speed. The results are immensely encouraging and show that the disc speed may be used as an extra degree of freedom in control system design. Finally, the flow regimes and wave characteristics of thin liquid films produced in a SDR are investigated by means of a thermal imaging camera. The film hydrodynamics strongly affect the heat and mass transfer processes within the processing films, and thus the intensification aspects of SDRs. Therefore, effective control and operation of such units is significantly dependent on the knowledge of film hydrodynamics and the underlying impact of the operating parameters and the manipulated variables on a given process. The results provided an interesting insight and unveiled promising potentials for characterisation of thin liquid film flow and temperature profiles across the disc by means of thermographic techniques. The present study reveals both challenges and opportunities regarding the control aspects of SDRs. It is recommended that equipment design and process control need to be considered simultaneously during the early stages of the future developments. Furthermore, intensified sensors and advanced controllers may be required to achieve an optimum control capability. Currently, the control performance is inhibited by the lack of sufficient considerations during the SDR design and manufacturing stages, and also by the characteristics of the commercially available instrumentation.EPSRC Doctoral Training Awar

Modeling and Control of the Falling Film Evaporator Process

Wegen ihres energieeffizienten Betriebs und flexiblen Designs haben Fallfilmverdampfer (FFV) eine breite Anwendung in der Industrie. Neben Fragen zur Konstruktion sind dominante Totzeiten herausfordernd bzgl. Prozessmodellierung und -regelung. Insbesondere erfordert die Automatisierung von Produktionssystemen digitale Zwillinge, d.h. Anlagenmodelle, um Betreiber zu schulen oder den Designprozess zu beschleunigen. Das Herz eines FFV besteht aus Rohren, an deren Innenseiten verdampfender Flüssigkeitsfilm hinabläuft. Daher sind die Rohre primäre Quelle für Totzeiten, welche sich vornehmlich auf den Transport wichtiger Prozessgrößen wie Liquidkonzentration und Massenstrom beziehen. Allerdings ist die Modellierung des entsprechenden dynamischen Verhaltens schwierig. Aus Sicht der Regelung erzeugen Totzeiten Schwingungen der Ausgangskonzentration - im Speziellen während der Anfahrprozesse. Zusätzlich verkomplizieren starke Kopplungen zwischen Ausgangsmassenstrom und -konzentration die in modernen Produktionen erforderliche Mehrgrößenregelung. Die vorliegende Arbeit präsentiert Lösungen für alle genannten Herausforderungen. Durch Gliederung des FFV-Prozesses in Teilsysteme sind verschiedene Designs in einfacher Weise simulierbar. In diesem Kontext erfolgt die Validierung eines bestimmtes Anlagenmodell auf Basis von Realdaten, was zum digitalen Zwilling führt. Zur Entwicklung neuer Transportmodelle verdampfender Flüssigkeitsfilme werden Bilanzgleichungen ausgewertet, sodass Systeme hyperbolischer partieller Differentialgleichungen entstehen. Mittels des Charakteristikenverfahrens gelingt eine Transformation in Totzeitgleichungen; letztere sind für Simulation und Reglerentwurf vorteilhaft. Pilotanlagenexperimente zur Identifikation und Validierung eines ausgewählten Modells unterstreichen die Eignung des Ansatzes, das gemessene Ein-/Ausgangsverhalten abzubilden. Zur Beantwortung regelungstechnischer Fragen wird ein einfaches Prozessmodell entwickelt, das Zuordnungsproblem gelöst und ein Mehrgrößenregelungskonzept entworfen