Effect of interaction multiplicity on control system design for a MTBE reactive distillation column

The control strategy of a reactive distillation for synthesis of MTBE is investigated. Although steady state multiplicities occur in the column, a linear control is still possible since a controlled and manipulated variable-pairing scheme that exhibits a sufficiently large range of near linear relations can be found, if we operate at constant reflux ratio. Reboiler duty is used to control the temperature of a stage just below the reaction section and near the top of the stripping section. Stoichiometric balance is controlled by a feed ratio plus internal composition control scheme, using a control valve installed on the C4 feed-line as the manipulated variable. Such a scheme is capable of maintaining the desirable steady state that achieves high product purity and reactant conversion. However, a similar scheme that uses a control valve installed on the methanol feed-line as the manipulated variable shows severe oscillation. It is caused by multiplicity in the interaction between the temperature and stoichiometric control loops.     

Two-point control of a reactive distillation column for composition and conversion

Reactive distillation is a hybrid process with dual process objectives: reactant conversion and product composition. Control schemes for reactive distillation frequently neglect the effect of the principal operating parameters on the reactant conversion, and this has a detrimental effect on the overall process profitability. An ETBE reactive distillation column has been used as a case study to show how a two-point control configuration, which recognises the importance of both composition and conversion, can be developed and implemented for a reactive distillation process. The combined composition and conversion control configuration was tested using SpeedUp dynamic simulations and proved to be effective in maintaining a high isobutylene conversion despite process disturbances. The two-point control scheme also had superior disturbance rejection capability, especially for feed rate changes, and composition set-point sensitivity compared with a one-point control scheme.     

动态流程模拟及其在精馏塔的操作分析中的应用

采用HYSYS流程模拟软件,对丙烯丙烷精馏塔进行动态流程模拟,分析了丙烯丙烷精馏塔在操作条件变化和进料流量及进料成分干扰变化时,整个塔的动态响应变化过程,研究了常规PID控制方案以及通常的先进控制方案所存在的问题,为改进的集实时优化与先进控制于一体的控制方案设计提供了依据。     

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.     

Regularized pole-placement adaptive control of a liquid-liquid extraction column

Some critical computations in pole-placement design and in that of many model reference adaptive systems are described. These numerical problems are associated with the resolution of the diophantine equation. They occur when the assumption of no common poles and zeros is violated. Regularization techniques which cope with ill-conditioning are presented. The resulting algorithm combines a standard indirect pole-placement adaptive control algorithm and a dimension-free regularization procedure of the design equations, thus avoiding the pole-zero cancellation problem and yet retaining the other properties of the algorithm. The application of this control scheme in a pulsed liquid-liquid extraction column is described. The control objective is to optimize the column behaviour. Extraction columns are subject to changes in feed compositions, feed flow-rates, physical properties of the solvent (the extractor) and the solute (liquid mixture) and various disturbances. The column exhibits highly non-linear and time-varying dynamics. The conductivity measured at a specific place in the column and the pulse frequency have been selected as control variables. Experimental results demonstrate the usefulness and the robustness of this regularized pole-placement adaptive control algorithm.     

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.     

Experimental investigation of conventional control strategies for a heterogeneous azeotropic distillation column

In this work, a laboratory scale sieve plate distillation column was constructed to investigate the conventional control strategies of an isopropyl alcohol (IPA), cyclohexane (CyH) and water (H₂O) heterogeneous azeotropic distillation column. Steady state process analysis showed that the optimal operation point should be located at a critical reflux, a transition point at which the distillation path switches from a route that passes through IPA+H₂O azeotrope to one that passes through IPA+CyH azeotrope. At this critical reflux, a high purity IPA product can be obtained with minimum energy consumption and maximum product recovery. However, the steady state is extremely sensitive to feed disturbances. A good control strategy must be able to maintain a steady column temperature profile that shows a plateau near 70°C to ensure passage around IPA+CyH azeotrope. In this study, an inverse double loop control strategy is recommended. Through experimental testing, the proposed control strategy was demonstrated to keep the product IPA purity at the desired high-purity level under all feed disturbance changes while other conventional control strategies fail.     

Online tuning of a steady state crude distillation unit model for real time applications

The steady state simulators, used for on-line performance prediction and for on-line optimization in crude distillation units are often sensitive to small variations in the feed composition, which is specified in terms of a True Boiling Point (TBP) vs volume percent distilled curve. The exact feed TBP is often not available during the plant operation. Also stratification of raw crude oil into layers in the large tank farm sections cause severe operating problems in terms of the stability of the column. If feed TBP can be predicted online, necessary feedforward action can considerably reduce the operating problems. A model has been developed for backcalculation of feed TBP using measured plant parameters. A heat balance is performed around an envelope encompassing the rectifying section of the fractionator and is followed by the calculation of Equilibrium Flash Vaporization (EFV) temperatures at six different locations of the column which are correlated with corresponding feed TBP temperatures. The second part of model tuning consists of calculating model parameters in the form of point efficiencies so as to minimize the discrepancy between the simulator predicted and measured column parameters which arises out of modelling approximations such as assumption of phase equilibria at each stage and use of imperfect thermodynamics correlations. The simulator results, after tuning, were found to match the plant measurements within two percent in all the cases investigated. The simulator output was used to predict various product properties using a Property Prediction package and these were also found to match well with those of laboratory measurements. Both the backcalculation of feed TBP and the efficiency tuning need to be implemented on-line for inferential control and supervisory optimization.     

Dynamic version of steady state optimizing control of a distillation column by trial method

The best optimizing control procedure using an automatic optimizing controller, based on a simple pattern-search type, steepest ascent method is presented for a distillation column. The control system is composed of a conventional feedback loop by which the composition of overhead product is maintained constant, and an optimizing control unit which holds the partial derivatives of the objective function with respect to control variables to zero. The objective function used in this study is the profit rate obtained from the distillation process, and the two control variables; feed flow rate and energy supplied to the plant, are used.     

基于线性集结的预测控制器

针对一类受约束的线性离散系统,研究了基于集结预测控制器的可行性问题．通过一个列满秩的集结矩阵将维数较小的控制变量序列映射成在线优化变量序列,在不缩短控制时域的情况下,降低了在线优化变量的个数．给出了优化变量可集结的充分条件,当集结矩阵满足这个条件时,在线求解优化问题总会得到可行解．最后,给出了一个仿真实例,仿真结果很好地验证了本文的结论．     

Variable selection using Gaussian process regression-based metrics for high-dimensional model approximation with limited data

In recent years, the importance of computationally efficient surrogate models has been emphasized as the use of high-fidelity simulation models increases. However, high-dimensional models require a lot of samples for surrogate modeling. To reduce the computational burden in the surrogate modeling, we propose an integrated algorithm that incorporates accurate variable selection and surrogate modeling. One of the main strengths of the proposed method is that it requires less number of samples compared with conventional surrogate modeling methods by excluding dispensable variables while maintaining model accuracy. In the proposed method, the importance of selected variables is evaluated using the quality of the model approximated with the selected variables only. Nonparametric probabilistic regression is adopted as the modeling method to deal with inaccuracy caused by using selected variables during modeling. In particular, Gaussian process regression (GPR) is utilized for the modeling because it is suitable for exploiting its model performance indices in the variable selection criterion. Outstanding variables that result in distinctly superior model performance are finally selected as essential variables. The proposed algorithm utilizes a conservative selection criterion and appropriate sequential sampling to prevent incorrect variable selection and sample overuse. Performance of the proposed algorithm is verified with two test problems with challenging properties such as high dimension, nonlinearity, and the existence of interaction terms. A numerical study shows that the proposed algorithm is more effective as the fraction of dispensable variables is high.

     

丙烯精馏塔关键组分软测量研究

丙烯精馏塔关键组分塔顶的丙烷浓度和塔釜的丙烯浓度的准确测量是乙烯生产企业提高丙烯收率的关键。鉴于丙烷浓度和丙烯浓度分析仪经常出现故障,提出以RBF神经网络加协同随机粒子群优化(PSO)算法的软测量建模法,即利用RBF神经网络的局部逼近能力来获得模型的结构,利用协同随机PSO算法的全局搜索能力来优化模型的参数,提高模型的逼近能力和泛化能力。该方法克服了BP网络对初始值和网络结构敏感,容易陷入局部最优的缺陷,以及RBF网络全局逼近能力差的缺点。仿真结果表明,此方法所得软测量模型精度高,泛化能力强。     

Combined internal model and inferential control of a distillation column via closed-loop identification

A procedure for designing distillation control systems with spe cified nominal properties is presented. The desired behaviour of the control system for both setpoint changes and disturbances in the feed flow rate and the feed composition can be specified. Both types of specifications can be handled because the disturbances can be inferred from the behaviour of the inventory control system. The control system is realized as a combined internal model and inferential control (CIMIC) system. A disturbance rejecting and decoupling (DRD) control structure is obtained as a special case. The performance of the control system is demonstrated experimentally on a pilot-scale distillation column. For comparison, experiments with pure internal model control (IMC) are also illustrated. A preliminary model of the distillation column was determined from step tests carried out in open-loop operation, but the final model used in the control system designs was obtained via a control-relevant closed-loop identification.     

Multivariable Control of a Pulsed Liquid-Liquid Extraction Column by Neural Networks

Modelling and control of chemical process systems are usual applications of artificial neural networks that have been explored so far with success. This paper deals with the potential application of neural networks to the multivariable control of a solvent extraction pilot plant. The pilot plant to be controlled is a pulsed liquid-liquid extraction column, which presents a non-linear behaviour and time-varying dynamics. Previous works have shown that the column could be maintained in its optimal behaviour by means of the control of conductivity by action on the pulse frequency. A given product specification can be obtained by the control of the product concentration in the outlet stream by acting on the solvent feed-flow rate. Owing to interactions between one variable and the other, a two input– two output control scheme has been developed and implemented. Promising experimental results have been obtained by using neural networks as an alternative tool for online control of chemical plant with dynamic changes.     

自适应模糊PID前馈补偿在机载挂飞摆扫转台控制中的应用

针对机载挂飞转台的摆扫速度控制问题,提出了一种利用模糊自适应PID技术进行前馈补偿的复合控制策略。首先根据实际应用提出摆扫转台的期望摆扫速度曲线,并对直流力矩电机驱动的摆扫转台进行了建模;然后根据扰动前馈补偿的控制原理,提出了模糊自适应PID前馈补偿方法,为摆扫转台的速度环设计了模糊PID控制器,并在此基础上设计了与之相适应的的自适应前馈补偿函数;最后进行了仿真结果验证。通过Matlab仿真结果表明,相对于模糊PID控制,所设计的模糊自适应PID前馈补偿控制器能有效的跟踪期望的转台摆扫速度,大幅地提高了在有稳定干扰和摆扫速度越变情况下的跟踪精度。     

Design and control of an ethyl acetate process: coupled reactor/column configuration

In this paper, design and control of a realistic coupled reactor/column process to produce ethyl acetate is studied. The process design is more complicated because the ethyl acetate product is neither the lightest nor the heaviest component in the system. A search procedure is proposed to obtain the optimum process design and operating condition of this process. The optimum process design is the one that minimize the Total Annual Cost (TAC) of this process while satisfying the stringent product impurity specifications. The optimum overall process design includes a continuous-stirred tank reactor (CSTR) coupled with a rectifier, a decanter, another stripper, and a recycle stream. After the process design is established, the next step is to use dynamic simulation to test the appropriate control strategy for this process. Sensitivity analysis is performed to obtain the suitable temperature control points for the columns. The proposed control strategy is very simple containing only one temperature control loop in each column. This recommended simpler control strategy uses the ratio of acetic acid feed rate to ethanol feed rate to control the 5th stage temperature of the rectifier and uses the stripper reboiler duty to control the 5th stage temperature of the stripper. The proposed control strategy does not need any on-line composition measurements and can properly hold product purity in spite of feed flow rate and feed composition disturbances. For small deviations of the product impurity compositions during disturbances, a slow cascade outer composition loop structure can be implemented using off-line composition measurements from the quality lab.     

Control of industrial gas phase propylene polymerization in fluidized bed reactors

The control of a gas phase propylene polymerization model in a fluidized bed reactor was studied, where the rigorous two phase dynamic model takes into account the polymerization reactions occurring in the bubble and emulsion phases. Due to the nonlinearity of the process, the employment of an advanced control scheme for efficient regulation of the process variables is justified. In this case, the Adaptive Predictive Model-Based Control (APMBC) strategy (an integration of the Recursive Least Squares algorithm, RLS and the Generalized Predictive Control algorithm, GPC) was employed to control the polypropylene production rate and emulsion phase temperature by manipulating the catalyst feed rate and reactor cooling water flow, respectively. Closed loop simulations revealed the superiority of the APMBC in setpoint tracking as compared to the conventional PI controllers tuned using the Internal Model Control (IMC) method and the standard Ziegler–Nichols (Z–N) method. Moreover, the APMBC was able to efficiently arrest the effects of superficial gas velocity, hydrogen concentration and monomer concentration on the process variables, thus exhibiting excellent regulatory control properties.     

Design and control of reactive-distillation process for the production of diethyl carbonate via two consecutive trans-esterification reactions

A reactive-distillation process to produce diethyl carbonate (DEC) and methanol (MeOH) from dimethyl carbonate (DMC) and ethanol (EtOH) via two consecutive trans-esterification reactions is proposed in this study. In the design study, an unusual feed location design of the reactive-distillation column is needed to suppress the formation of the intermediate (ethyl-methyl carbonate, EMC) while achieving near complete conversion of the limiting reactant. The heavier DMC feed should be located at the bottom of the reaction section, while the lighter EtOH feed should be located at the top part of the reaction section. The comparison of the TAC of this proposed design to that of a traditional process will be given which shows great benefit to utilize the reactive-distillation technology. Finally, a simple control strategy is proposed with only one temperature loop in each column. The proposed control strategy works effectively to hold the product compositions despite feed flow rate and feed composition changes.     

含氮天然气液化脱氮精馏塔液位控制模型和仿真

提出了用精馏塔对含氮天然气脱除N2生产液化天然气（LNG）,精馏塔塔釜液位的一种控制方法。该方法以脱氮分离效率和稳定的产品（LNG）出料温度为控制目标,建立了塔釜液位控制模型并仿真。整个闭环控制模型包括被控对象传递函数,调节阀传递函数,PID控制传递函数和检测传递函数。其中调节阀传递函数的建立,采用先计算阀位-Cv值（流量系数）曲线;然后通过系统辨识法[1]求得数学模型;最后利用泰勒级数把非线性化转化为线性化并求得传递函数。通过Matlab软件[2]的仿真,在阶跃干扰下,得到了合适的液位变化曲线和合适的阀位变化[3]。     

Online switching of entrainers for acetic acid dehydration by heterogeneous azeotropic distillation

Traditionally, a foreign chemical such as isobutyl acetate (IBA) is used as an entrainer in a heterogeneous azeotropic distillation (HAD) column to separate acetic acid (HAc) and water. However, the loss of this entrainer may contaminate the process of manufacturing terephthalic acid because of the recycle of product streams to an oxidation reactor and other parts of the process. In the study, use of p-xylene (PX), a component indigenous to the terephthalic acid process and already present in the feed to the HAc dehydration column, as the entrainer for the separation of HAc and water is investigated. Optimization results show that using the HAD process with PX as the entrainer not only overcomes the shortcomings of IBA as the entrainer but also requires less energy consumption. Moreover, we demonstrate that it is possible to dynamically transform the HAD column with IBA as the entrainer into the HAD column with PX as the entrainer, without the need of shutdown and startup. The key to successful implementation of such a dynamic transition is to maintain a proper PX inventory inside the HAD column. While simple sidedraw flow control can be used, it will create PX imbalance if there is a flow measurement bias. A temperature control scheme was found to accelerate such a transition and maintain HAc purities at their designed values.