DESIGN AND IMPLEMENTATION OF STATE OBSERVERS IN REACTIVE DISTILLATION

This study demonstrates that state observers can be developed and applied to infer the composition profiles of reactive distillation columns from noise-contaminated temperature measurements. The design and implementation of a Kalman filter (KF) and a Luenberger observer (LO) are carried out, and their performances are quantitatively assessed. The reliability, accuracy, and robustness of the two designs method are examined and compared quantitatively. The design and implementation of a Luenberger observer are simpler and easier to carry out than those of a Kalman filter. On the other hand, a Kalman filter is found to be more robust to a noisy measurements, erroneous initial estimates, and model uncertainties. A Luenberger observer could be used for composition estimation of reactive distillation when an ideal model of the system can reasonably approximate the real system; otherwise, a Kalman filter is recommended to be applied in more practical situations.     

Composition estimation of multicomponent reactive batch distillation with optimal sensor configuration

An inferential state estimation scheme based on extended Kalman filter (EKF) with optimal selection of sensor locations using principal component analysis (PCA) is presented for composition estimation in multicomponent reactive batch distillation. The properties of PCA are exploited to provide the most sensitive dynamic temperature measurement information of the process to the estimator for accurate estimation of compositions. The state estimator is supported by a simplified dynamic model of reactive batch distillation that includes component balance equations together with thermodynamic relations and reaction kinetics. The performance of the proposed scheme is evaluated by applying it for composition estimation on all trays, reboiler, reflux drum and products of a reactive batch distillation column, in which ethyl acetate is produced through an esterification reaction between acetic acid and ethanol. This quaternary system with azeotropism is highly nonlinear and typically suited for implementation of the proposed scheme. The results demonstrate that the proposed EKF estimation scheme with optimal temperature sensor configuration is effective for inferential estimation of compositions in multicomponent reactive batch distillation.     

基于自适应神经模糊推理系统的间歇反应精馏组分估算(英文)

Composition estimation plays very important role in plant operation and control. Extended Kalman filter (EKF) is one of the most common estimators, which has been used in composition estimation of reactive batch distillation, but its performance is heavily dependent on the thermodynamic modeling of vapor-liquid equilibrium, which is difficult to initialize and tune. In this paper an inferential state estimation scheme based on adaptive neuro-fuzzy inference system (ANFIS), which is a model base estimator, is employed for composition estimation by using temperature measurements in multicomponent reactive batch distillation. The state estimator is supported by data from a complete dynamic model that includes component and energy balance equations accompanied with thermodynamic relations and reaction kinetics. The mathematical model is verified by pilot plant data. The simulation results show that the ANFIS estimator provides reliable and accurate estimation for component concentrations in reactive batch distillation. The estimated states form a basis for improving the performance of reactive batch distillation either through decision making of an operator or through an automatic closed-loop control scheme.     

Nonlinear model based control of two-product reactive distillation column

Nonlinear feedback control scheme for reactive distillation column has been proposed. The proposed control scheme is derived in the framework of Nonlinear Internal Model Control. The product compositions and liquid and vapor flow rates in sections of the reactive distillation column are estimated from selected tray temperature measurements by an observer. The control scheme is applied to an example reactive distillation column in which two products are produced in a single column and the reversible reaction A+B=C+D occurs. The relative volatilities are favorable for reactive distillation so that the reactants are intermediate boilers between the light product C and the heavy product D. Ideal physical properties, kinetics, and vapor-liquid equilibrium are also assumed. It is shown that the proposed control scheme keeps tight product composition control.     

Studies in the control of tubular reactors—III stabilization by observer design

The stabilization of an unstable nonlinear distributed chemical reactor system is examined when concentration measurements are not possible. The linearized form of the finite dimensional approximate model developed in Parts I[1] and II[2] is used to show that the observability index is equal to two. Furthermore it is shown that the dynamical characteristics of the reactor are such that, by a proper design of the Luenberger observer, the concentration estimation at a given collocation point can be made independently of the estimation at other collocation points.For purposes of control a one-dimensional observer is designed to directly estimate the control variable. Simulations of the nonlinear model of the reactor show that the observer design is quite successful in the stabilization of an unstable steady state when only temperature measurements along the reactor are available.     

Selection of internals for reactive distillation column—case-based reasoning approach

The design of reactive distillation (RD) systems is considerably more complex than design of the individual, conventional reactors and distillation columns. One of the main issues in the design of RD column is a definition of type and specification of geometric features of the internal devices. In this work, there is described a design supporting tool for pre-selection of packing type for RD column. Case-based reasoning has been applied as a basic method for decision support. The computer system for pre-design of reactive column internals has been built as an implementation of developed methodology. The acid catalysed reaction of 2-methylpropylacetate synthesis was used to test the system. The CBR system can significantly reduce the development time for design of reactive distillation column and suggests a good start points for further design activity.     

Advances and selected recent developments in state and parameter estimation

This paper deals with two topics from state and parameter estimation. The first contribution of this work provides an overview of techniques used for determining which parameters of a model should be estimated. This is a question that commonly arises when fundamental models are used as these models often contain more parameters than can be reliably estimated from data. The decision of which parameters to estimate is independent of the observer/estimator design, however, it is directly affected by the structure of the model as well as the available data. The second contribution is an overview of recent developments regarding the design of nonlinear Luenberger observers, with special emphasis on exact error linearization techniques, but also discussing more general issues, including observer discretization, sampled data observers and the use of delayed measurements.     

反应精馏过程图解法设计策略研究

从反应精馏过程图解法设计的理论研究出发,探讨了反应空间、转换组成变量以及剩余曲线等应用范围和局限性;通过分析反应精馏操作线方程的变化,提出了图解法设计反应精馏过程的策略;（1）用奇异点判断法结合实验数据可判定应用反应精馏过程的可行性;（2）借助反应空间可判断该过程能否在单塔中得到完全分离;（3）图解法设计的多种设计结果及相应对策。采用实例验证了该策略的可行性。     

A novel process for the synthesis of unsaturated polyester

Traditionally polyester production is done in a batch reactor equipped with a separation column for a batch distillation. A promising alternative for the intensification of this process is reactive distillation. The aim of this paper is to study the conceptual design of reactive distillation and to find out whether reactive distillation is potentially interesting compared to batch reactor process. Therefore, a reactive distillation model is developed and sensitivity analysis is used to obtain the design and operational parameters for the reactive distillation process. These parameters are the required number of stages, required residence time, feed ratio, reflux ratio and temperature of feed stream. The model predicts the polymer attribute, isomerization and saturation composition of the polymer in the range of industrial polyester production data. The simulation study shows that the total production time of polyester in a continuous reactive distillation system is reduced from 12 h to 1.5-2 h compared to the industrial batch reactor process. The equilibrium conversion is also raised by 7% compared to the conventional process. The model demonstrated that reactive distillation has the potential to intensify the process by factor of 6-8 in comparison to the batch reactor.     

反应/分离集成系统塔板组成线设计法研究

反应/分离系统的集成是对现有工业过程进行强化的主要途径之一。文中在精馏塔板组成线的基础上,推导出了反应精馏塔板组成线基本方程及设计可行性判据,提出了给定进料和一个目标产品组成的塔板组成线反应精馏设计的新策略,并对非理想性较强的MTBE反应体系进行了实例设计计算。结果表明,提出的设计策略能同时获得多组可行设计方案,可有效地对单一和集成型反应精馏塔进行设计,结果与Aspen P lus的模拟结果吻合较好。     

酯化反应蒸馏过程的模型化及模拟研究

In this paper,a generalized model of the reactive distillation processes was developed via rate-based approach. The homotopy-continuation method was employed to solve the complicated nonlinear model equations efficiently. The simulation on the reactive distillation processes was carried out with the profiles of stage temperature,composition and flow rate for both vapor and liquid phases obtained. Based on careful analysis of the simulation results, the pitfalls in experimental design were detected. Finally, a software package for the simulation of reactive distillation processes was developed.     

A dead time compensation approach for multirate observer design with large measurement delays

In our previous work, we developed a multirate observer design method in linear systems with asynchronous sampling based on a Luenberger observer design coupled with inter-sample predictors. In this article, the problem of multirate multidelay observer design is addressed where both asynchronous sampling and possible measurement delays are accounted for. The proposed observer adopts an available multirate observer design in the time interval between two consecutive delayed measurements. A dead time compensation approach is developed to compensate for the effect of delay and update past estimates when a delayed measurement arrives. The stability and robustness properties of the multirate observer will be preserved under nonconstant, arbitrarily large measurement delays. A mathematical example and a gas-phase polyethylene reactor example demonstrate good performance of the proposed observer in the presence of nonuniform sampling and nonconstant measurement delays. © 2018 American Institute of Chemical Engineers AIChE J, 65: 562–570, 2019     

Prozesse der Reaktivdestillation

Reactive Distillation Processes Reactive distillation, i.e. the simultaneous implementation of reaction and distillation in a counter–current column, is a promising alternative to the conventional sequential processes. In a preceding paper the effective phase equilibrium of a reactive distillation was studied with a special focus on the determination of so–called reactive azeotropes. In this contribution, a knowledge of these thermodynamic fundamentals is applied to the conceptual design of reactive distillation processes. Furthermore, some process examples of technically important processes are presented.     

Multi‐rate observer design for process monitoring using asynchronous inter‐sample output predictions

In this article, the problem of observer design in linear multi‐output systems with asynchronous sampling is addressed. The proposed multi‐rate observer is based on a continuous‐time Luenberger observer design coupled with an inter‐sample predictor for each sampled measurement, which generates an estimate of the output in between consecutive measurements. The sampling times are not necessarily uniformly spaced, but there exists a maximum sampling period among all the sensors. Sufficient and explicit conditions are derived to guarantee exponential stability of the multi‐rate observer. The proposed framework of multi‐rate observer design is examined through a mathematical example and a gas‐phase polyethylene reactor. In the latter case, the amount of active catalyst sites is estimated, with a convergence rate that is comparable to the case of continuous measurements. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3384–3394, 2017     

反应精馏隔壁塔的模拟研究

以乙酸甲酯酯转换体系为例提出了一种反应精馏隔壁塔的设计和优化方法,应用该方法可将常规双塔反应精馏序列转化为反应精馏隔壁塔并保证各操作参数的最优值.首先通过在反应精馏塔与甲醇塔之间交换汽液相物流来实现反应精馏隔壁塔的简捷设计;然后利用Aspen Plus模拟软件,对常规反应精馏序列和反应精馏隔壁塔进行了模拟分析;最终以2...     

Design and control of butyl acrylate reactive distillation column system

In this study, the design and control of a reactive distillation column system for the production of butyl acrylate has been investigated. The proposed design is quite simple including only one reactive distillation column and an overhead decanter. The optimal design is selected based on the minimization of total annual cost (TAC) for the overall system. At this optimized flowsheet condition, output multiplicity was found with reboiler duty or feed ratio as the bifurcation parameter. The highest purity stable steady state was selected as the base case condition for the control study. The overall control of this system can be achieved with no on-line composition measurements. Simple single-point tray temperature control loop is designed to infer final product purity. From results of dynamic simulation, the proposed control strategy performs very well in rejecting various disturbances while maintaining butyl acrylate product at high purity. One of the important finding in this paper is that it is better to operate this reactive distillation column not at the exact feed stoichiometric balance point for better operability reason. The control performances of the proposed operating point and the operating condition right at the exact stoichiometric balance point will be compared.     

An experimental evaluation of Kalman filtering

The effectiveness of the stationary form of the discrete Kalman filter for state estimation in noisy process systems was demonstrated by simulated and experimental tests on a pilot plant evaporator. The filter was incorporated into a multivariable, computer control system and resulted in good control despite process and/or measurement noise levels of 10%. The results were significantly better than those obtained when the Kalman filter was omitted or replaced by conventional exponential filters. In this application the standard Kalman filter was reasonably insensitive to incorrect estimates of initial conditions or noise statistics and to errors in model parameters. The filter estimates were sensitive to unmeasured process disturbances. However this sensitivity could be reduced by treating the noise covariance matrices R and Q as design parameters rather than noise statistics and selecting values which result in increased weighting of the process measurements relative to the calculated model states.     

A NONLINEAR OBSERVER BASED ON HYBRID MODELLING OF CHEMICAL REACTORS

The successful design of an observer for inferring the outlet composition from a chemical reactor heavily relies on the goodness of the adopted kinetic rate model (Baratti et al., 1993). On the other hand, often, it is difficult to dispose of a simple, but, exhaustive kinetic model because of the complexity of the reaction scheme one has to deal with.

In this work, we explore the possibility to represent global (lumped) reaction rate laws by the use of neural network models. The aim is to develop a nonlinear observer (extended Kalman filter, EKF) of an heterogeneous gas-solid reactor that relies on a grey model where the “neural reaction rate” law is integrated within a first principles model. The procedure is outlined for the case of the catalytic oxidation of carbon monoxide over Pt-alumina catalyst. The results show that neural networks (NN) can be effectively used in representing lumped reaction rates since NN are able to capture the essential characteristics of the functional relationship relating the state variables.     

Observer-based control algorithms for a distillation column

This paper studies the synthesis of nonlinear observer-based globally linearizing control (GLC) algorithms for a multivariable distillation column. Two closed-loop observers/estimators, namely extended Kalman filter (EKF) and adaptive state observer (ASO), have been designed within the GLC framework to estimate the state variables along with the poorly known parameters. Exactly same basic model structure was used for developing the observers. The model structure is so simple that the estimator design was performed based on only two component balance equations around the condenser-reflux drum and the reboiler-column base systems of the distillation column. To construct these observers, the poorly known parameters, namely component vapor flow rate leaving top tray, component liquid flow rate leaving bottom tray and distribution coefficient in the reboiler, were considered as extra states with no dynamics. The comparative study has been carried out between the proposed GLC in conjunction with ASO (GLC-ASO) and that coupled with EKF (GLC-EKF). The GLC-ASO control scheme showed comparatively better performance in terms of set point tracking and disturbance as well as noise rejections. The control performance of GLC-ASO and a dual-loop proportional integral derivative (PID) controller was also compared under set point step changes and modeling uncertainty. The proposed GLC-ASO structure provided better closed-loop response than the PID controller.