Nonlinear model based multivariable control of a debutanizer

The debutanizer column control is a major economic incentive to a refinery due to the importance of the product quality. The nonlinear model-based multivariable control on a debutanizer has shown to provide improved control performance of the top product and the bottom stream qualities using a steady-state process model with approximate dynamics. The control performance is much better than that achieved by the traditional PID-type control system. This paper outlines the non-linear control method applied to an industrial debutanizer and discusses the use of inferential models to predict the iso-pentane contents in the overhead stream and the Reid vapour pressure (RVP) in the bottom stream, in nonlinear control applications.     

Dynamics and control study on the low temperature methanation reactor with mass and heat recycle

Both nonlinear and linear dynamic characteristics of the low temperature methanation reactor with recycle loop are analyzed for the process safe operation and effective control. Aspen Dynamics tool is adopted to get transient behaviors of the reactor under step changes of the inlet temperature and CO mole fraction. A double-input-multi-output linearized system based on the nonlinear dynamic model is developed for further insight into the process stability, response rapidity and controllability. Similar responses are obtained in the nonlinear and linear models for commendable mutual authenticating. The presence of material recycle because of the unconverted reactants, together with the thermal recycle for energy saving, renders a more sensitive stability of the system, where thermal feedback has the major effect, but the existing of mass feedback may weaken this effect when considerable heat is recycled. Moreover, the transfer function of the linear model indicates that the integral term is necessary for the reactor variables control, and the proposed control strategy using a High Selector module in Aspen Dynamics can successfully maintain the hot spot temperature within the reactor by considering the migration of the peak temperature under the two disturbances.     

Multivariate statistical monitoring of two-dimensional dynamic batch processes utilizing non-Gaussian information

Dynamics are inherent characteristics of batch processes, and they may exist not only within a particular batch, but also from batch to batch. To model and monitor such two-dimensional (2D) batch dynamics, two-dimensional dynamic principal component analysis (2D-DPCA) has been developed. However, the original 2D-DPCA calculates the monitoring control limits based on the multivariate Gaussian distribution assumption which may be invalid because of the existence of 2D dynamics. Moreover, the multiphase features of many batch processes may lead to more significant non-Gaussianity. In this paper, Gaussian mixture model (GMM) is integrated with 2D-DPCA to address the non-Gaussian issue in 2D dynamic batch process monitoring. Joint probability density functions (pdf) are estimated to summarize the information contained in 2D-DPCA subspaces. Consequently, for online monitoring, control limits can be calculated based on the joint pdf. A two-phase fed-batch fermentation process for penicillin production is used to verify the effectiveness of the proposed method.     

Role of multiplicity in reactive distillation control system design

The impact of steady-state multiplicities on the control of a simulated industrial scale methyl acetate reactive distillation (RD) column is studied. At a fixed reflux rate, output multiplicity, with multiple output values for the same reboiler duty, causes the column to drift to an undesirable steady-state under open loop operation. The same is avoided for a fixed reflux ratio policy. Input multiplicity, where multiple input values give the same output, leads to “wrong” control action under feedback control severely compromising control system robustness. A new metric, rangeability, is defined to quantify the severity of input multiplicity in a steady-state input–output (IO) relation. Rangeability is used in conjunction with conventional sensitivity analysis for the design of robust control structures for the RD column. Results for the two synthesized control structures show that controlling the most sensitive reactive tray temperature results in poor robustness due to low rangeability causing “wrong” control action for large disturbances. Controlling a reactive tray temperature with acceptable sensitivity but larger rangeability gives better robustness. It is also shown that controlling the difference in the temperature of two suitably chosen reactive trays further improves robustness of both the structures as input multiplicity is avoided. The article brings out the importance of IO relations for control system design and understanding the complex dynamic behavior of RD systems.     

State estimation and inferential control for a reactive batch distillation column

An optimal reflux ratio profile is obtained for a reactive batch distillation system utilizing the capacity factor as the objective function in a nonlinear optimization problem. Then, an Artificial Neural Network (ANN) estimator system, which utilizes the use of several ANN estimators, is designed to predict the product composition values of the distillation column from temperature measurements inferentially. The network used is an Elman network with two hidden layers. The designed estimator system is used in the feedback inferential control algorithm, where the estimated compositions and the reflux ratio information are given as inputs to the controller to see the performance of the ANN. In the control law, a scheduling policy is used and the optimal reflux ratio profile is considered as pre-defined set-points. It is found that, it is possible to control the compositions in this dynamically complex system by using the designed ANN estimator system with error refinement whenever necessary.     

基于Cape-Open标准的间歇精馏过程模拟系统的开发

在合理假设的前提下建立间歇精馏过程的严格动态模型,此模型按精馏塔的结构分解为:再沸器、塔板、冷凝器、回流釜、分配器及产品罐模型等6个子模型,塔板子模型考虑了液相持液量。基于Cape-Open标准采用面向对象程序设计方法,实现各个子模型程序,将各个子模型的参数及解算方法封装在各自的类中,利用四阶龙格库塔积分方程,采用逐板计算法由塔釜至产品罐自下而上地求解该模型。最后引入一3组份混合物间歇精馏实例,模拟结果较好地反映了该物系间歇精馏的动态特征,表明该模型准确可靠,所开发的系统稳定。     

Qualitative Event-Based Expert Supervision for Transient Condition Monitoring

This paper addresses the following supervisory problem: a continuous plant (P) is to be supervised via symbolic (or quantised) actions. These symbolic actions suggest the set points for the lower level control loops. The system dynamic is analysed on the supervisory level (K) by a qualitative approach. The relationships between variables and the steady-state references are known. These problems are especially common in chemical process control. The supervisor handles start-up and shut-down procedures and takes appropriate action to solve the sequential or parallel tasks of a basic procedure. The object of this paper is to introduce an approach to solving the problem of how to derive a set of rules from a physical process.The solutions for supervising start-up and shut-down operations in close loop are suitable for large industrial systems, as are as the batch and semi-continuous processes used in order to maintain operations in a dynamic mode. This paper considers the qualitative event-based expert supervision approach to distillation column problems. The development of a general supervision in this work is based on an events generator and a corrective actions generator. The qualitative symbols are based on fuzzy sets. In particular, there are mechanisms for processing the changes in the system variables from qualitative symbols.     

醋酸脱水系统的动态模拟及其控制

通过Aspen Plus软件模拟PTA生产的醋酸脱水共沸精馏系统.稳态模拟结果与现场数据吻合较好.在稳态模型的基础上,设计两种控制方案,利用Aspen Dynamics进行动态模拟,测试这两种控制方案的性能,找出较好的控制方案.模拟的结果对生产操作和控制方案的设计具有重要的指导意义.     

Simulation and graphical animation of advanced manufacturing systems

Automated production systems, known as Flexible Manufacturing Systems (FMS), offer a promising solution for the low productivity of mid-volume, mid-variety batch production. These systems consist of integrated machine modules and material handling equipment under computer control.This paper describes a general purpose discrete-event Flexible Manufacturing Systems Simulator (FMSSIM). The package is programmed in FORTRAN and is capable of simulating different configurations, material handling systems, and topologies including bidirectional tracks. The simulator checks blockage of routes due to interference of carts and simulates random failures and repairs of the various components in the system. The simulator provides the user with a wide range of priority rules to select from, and enables the user to define his own rules if required. The simulator produces reports on various vital system performance statistics and also displays the movement of parts through the system on a refreshed CRT. This simulator is a modular, user-oriented package which allows the designer to evaluate a wide range of systems with varied design parameters and select an efficient flexible manufacturing system. Some general trends in the behaviour of the flexible manufacturing systems are identified and guidelines for their design and operation are suggested.     

Dynamics identification and control of nonlinear MIMO coupled plant using supervised neural gas and comparison with recurrent neural controller

The dynamics identification and subsequent control of a nonlinear system is not a trivial issue. The application of a neural gas network that is trained with a supervised batch version of the algorithm can produce identification models in a robust way. In this paper, the neural model identifies each local transfer function, demonstrating that the local linear approximation can be done. Moreover, other parameters are analyzed in order to obtain a correct modeling. Furthermore, the algorithm is applied to control a nonlinear multi-input multi-output system composed of tanks. In addition, this plant is a coupled system where the manipulated input variables are influencing all the output variables. The aim of the work is to demonstrate that the supervised neural gas algorithm is able to obtain linear models to be used in a state space design scenario to control nonlinear coupled systems and guarantee a robust control method. The results are compared with the common approach of using a recurrent neural controller trained with a dynamic backpropagation algorithm. Regarding the steady-state errors in disturbance rejection, reference tracking and sensitivity to simple process changes, the proposed approach shows an interesting application to control nonlinear plants.

     

无人驾驶车辆稳态漂移的无模型自适应控制

本文引入一种基于无模型自适应控制算法的无人驾驶车辆稳态漂移控制方案.首先,对车辆漂移动态过程进行动力学分析,其次,确定控制系统的控制输入量和控制目标,然后基于无人驾驶车辆稳态漂移控制系统的动态线性化数据模型,设计无模型自适应稳态漂移控制算法、伪雅可比矩阵估计整定算法和伪雅可比矩阵重置算法,从而实现无人驾驶车辆的稳态漂移...     

含执行机构未知动态的液压伺服系统输出反馈控制

针对含有未知动态(如:执行机构、负载等)液压伺服系统,提出一种基于未知系统动态估计器的输出反馈控制方法.该方法不依赖于函数逼近器和传统反步控制设计,且无需难以测量的系统内部状态.首先,为避免反步控制和系统全部状态,引入等价变换,将含液压执行机构的伺服系统高阶严格反馈模型转化为Brunovsky标准型,进而运用高阶滑模微分器观测转化后的系统未知状态.控制器设计中引入描述收敛速率、最大超调量和稳态误差的性能函数,保证预设控制系统稳态和瞬态控制性能.为补偿系统集总未知动态影响,设计一种仅含一个调节参数并保证指数收敛的未知系统动态估计器.该输出反馈控制器可以实现对系统输出的精确跟踪控制.最后,通过数值仿真结果表明了所提出算法的有效性.     

Comparison of PI and MPC for control of a gas recovery unit

This study compares PI and MPC controls via a computer simulation for a gas recovery unit (GRU), which consists of three distillation columns operated in series: a de-ethanizer, a depropanizer and a debutanizer. In addition, the de-ethanizer feed is preheated by the bottoms product from the de-ethanizer, which causes additional process coupling. Rigorous models are developed for the columns including column pressure dynamics and heat transfer dynamics. The process is a highly coupled system and has interactive constraints that determine the feasible operating regions. A decentralized PI control system with override controls for the constraints was designed and implemented on the GRU simulator and was compared with an industrial MPC controller. The MPC controller was observed to outperform the decentralized control system due to its multivariable constraint control capability. Since the simulator is available to other university researchers, it can serve as a challenge problem for multivariable control and identification. Three MPC controllers with different strategies for controlling the bottom level of the first column were implemented on the GRU process. The first MPC controller does not directly control the level, the second one moves the setpoint to the PI level controller, and the third one controls the level directly by manipulating the flow. The results show that including level into the MPC controller improves composition control for cases in which the manipulated variable for the level control has a significant impact on compositions.     

HDPE装置反应系统模型的应用

HDPE装置反应系统稳态及动态模型采用POLYMERS PLUS软件及ASPEN DYNAMICS,Aspen Custom Modeler软件进行开发。该模型可用于优化操作和增大装置生产能力,从而提高产品质量和产量。经验证模型准确,在此模型的基础上,研究了可提产的新工艺流程的可行性以及聚合反应中主要参数之间的相互影响关系,为改扩建方案的实施提供了依据。     

精馏塔多通道对角矩阵解耦控制仿真

在化工生产中,许多单元操作的控制涉及至少2个被控变量,因此产生至少2个以上的控制回路。在该类具有多个控制回路的多变量控制系统中,同一被控对象上的多个控制回路,在实现其各自的控制目标时,将发生相互作用和相互影响。精馏塔是化工等行业中广泛使用的高耗能分离设备,为满足工艺要求和节能,需将塔顶和塔底产品流控制在设计值,即双成分控制。通常,利用回流量和加热量作操纵变量分别控制塔顶和塔底产品的组成。不过,回流量的变化也会影响塔底产品的组成,加热量的改变也会影响塔顶产品的组成。当采用温度作为反映产品组成的间接指标时,塔顶和塔底温度控制回路具有强烈的耦合作用,会导致系统不稳定。一个简单有效的方法就是解出耦合,因此,本文先介绍了一种基于对角矩阵的温度解耦控制方案,并在MATLAB/Simulink平台进行了控制仿真。结果显示:与两个独立PID回路控制相比,对角矩阵解耦控制不仅能达到更优的控制效果,还具有极强的对象特性鲁棒性。     

Design and plantwide control of n-butyl acrylate production process

n-Butyl acrylate is commercially produced from acrylic acid and n-butanol using strong acidic homogeneous catalysts. To overcome problems related to corrosion, catalyst removal from product and catalyst disposal after neutralization, research based on solid catalysis received increased attention in the recent years. However, design and control studies of an entire plant are rare. In this paper, the design and control of two reaction-separation-recycle process alternatives are developed. Both use a fixed-bed reactor with Amberlyst 131 catalyst. The separation of n-butyl acrylate from the mixture with n-butanol and acrylic acid is difficult. One of the process alternatives achieves the separation by distillation at pressure above atmospheric, while the other conveniently employs extractive distillation with ethylene glycol. Both processes are controllable, the control system showing robustness when increase or decrease in production capacity is required, or operating conditions change. Aspen Plus and Aspen Plus Dynamics are used as efficient Computer-Aided Process Engineering tools.     

稀醋酸共沸精馏脱水工艺的模拟

醋酸是一种重要的化工原料,其水溶液广泛存在于各类工业过程中。由于醋酸与水会缔合使得两者的相对挥发度不大,生产中用于分离醋酸/水的工艺能耗较高,因此研究者和工业界都在寻求更好的分离方法。本文以醋酸正丙酯做挟带剂,应用Aspen Plus流程模拟软件,汽相逸度采用Hayden-O’Connell方程,液相活度系数采用NRTL方程计算,实现了稀醋酸共沸精馏脱水工艺的模拟。本文详细讨论了全塔理论板数、进料位置、回流比等因素对脱水塔性能的影响。结果表明,在理论板数50块,回流比3.2,醋酸进30块板,挟带剂与进料比0.15条件下,塔釜得到醋酸浓度高于95%。模拟结果对醋酸脱水工艺的设计和改造具有重要的指导意义。     

Plant-wide design and control of acetic acid dehydration system via heterogeneous azeotropic distillation and divided wall distillation

Energy-saving plant-wide design and plant-wide control of an acetic acid dehydration system with the feed containing methyl acetate and p-xylene are investigated in the study. A heterogeneous azeotropic distillation using isobutyl acetate as an entrainer is designed to obtain high-purity acetic acid at the column bottom and to keep a small acetic acid loss through the top aqueous draw. The accumulation of p-xylene in the column is avoided by adding a side product stream. The mixture in the aqueous phase of decanter, containing mostly water, methyl acetate, and isobutyl acetate is separated using a divided wall distillation column. The whole acetic acid dehydration system includes a heterogeneous azeotropic distillation column and a divided wall distillation column.The control strategies using temperature loops are proposed for this acetic acid dehydration system. For the heterogeneous azeotropic distillation column, the requirements for acetic acid compositions in both the aqueous phase of the decanter and the column bottom can be satisfied by designing entrainer inventory temperature control and cascade temperature control simultaneously. The stages of controlled temperatures are chosen by singular value decomposition and closed-loop analysis methods based on the criteria of minimum entrainer makeup. For the divided wall distillation column, steady-state analysis methods are used for the selection of proper controlled and manipulated variables and the determination of their pairings. Dynamic simulation results demonstrate that the proposed plant-wide control strategy can maintain product purities and reject external disturbances in feed flow and composition changes as well as internal disturbances such as changes in liquid and vapor splits.     

Attitude control of spacecraft simulator without angular velocity measurement

In this paper the attitude control of a spacecraft simulator using Reaction Wheels (RW) as the actuators is investigated. The main goal of the current study is to bring the RWs to the rest at the end of the maneuver without angular velocity measurement. A modified feedback linearization controller is applied by considering the Euler angles of the simulator as the output and the RWs angular momentums as the internal state variables. The stability of the proposed controller and the internal dynamics is analyzed using Lyapunov theory. Two modified sliding mode observers are designed to estimate the angular velocities of the spacecraft attitude control subsystem simulator. The proposed observers do not use the control input and the detailed knowledge of the model and thus it can be implemented easily. The global stability of the system is proved. The proposed controller and observers are finally evaluated numerically and experimentally on an attitude spacecraft simulator.