Plant-wide control strategy applied to the Tennessee Eastman process at two operating points

This work presents a new plant-wide control strategy able to be applied on large scale chemical plants. It is based on an extension of the non square relative gain array (NRG) theoretical concepts, introduced by Chang and Yu (1990), and the generalized relative disturbance gain (GRDG) presented in Chang and Yu (1992). The extension of the NRG is useful for searching the best group of controlled variables (CVs) independently of the problem dimensionality. Meanwhile, the extension of the GRDG allows configure the loops pairing by considering the trade-off between servo and regulator behavior. It can be done thanks to define a proper function, named net load effect, accounting both set point and disturbances effects. Even though these concepts are not new, the main contribution of this paper is the selection of the adequate objective function. It is mathematically expressed in a new way, in terms of Frobenius norm of specific matrices related with the models of the plant and very useful for evaluating the process interaction. Then, it drives the search supported by genetic algorithms (GA), which evaluates all the possible combinations of input–output variables. It allows to solve successfully and with less computational effort the combinatorial optimization problem, even though the high dimension usually involved in large scale chemical plants. The use of the relative gain array (RGA) can also be considered for pairing purpose, but in some cases it could drive to a less effective structure. The use of relative normalized gain array (RNGA) for pairing the selected CVs with the most suitable MVs is able to lead to best control structures only if a dynamic model of the plant is available. Therefore, it must be emphasized that this approach is developed for working in cases where only steady-state plant information is available. However, if a dynamic model is disposable too the algorithm is extended to use it. In addition, a mathematical demonstration is presented so as to understand why is possible to find a well conditioned control structure. The methodology is tested in the Tennessee Eastman (TE) process at the base case proposed by Downs and Vogel (1992), and at an optimized working point presented by Ricker (1995). Both working points show two quite different scenarios. Thus, a set of dynamic simulations for both cases and the hardware requirements compared to the previous suggested are given to proof the capacity of this approach.     

DYNAMIC INTERACTION COMPENSATORS FOR MULTILOOP CONTROL STRUCTURE

This work presents a design procedure of a dynamic compensator for multivariable IMC control structure. This compensator can yield both faster transient response and zero steady state error to interaction effects. Several examples are given to demonstrate the availability of this methodology. They are chosen with different degrees of interactions, based on RGA (relative gain array) and RDG (relative disturbance gain). In addition, an 1 MC structure optimally decoupled is used as an upper bound for the performance obtainable to compare with the compensated interactive system. Finally, it is shown that this; procedure is easily extended to classical feedback controllers PI and P1D.     

Distillation Control Structure Selection for Energy‐Efficient Operations

Few studies have been reported concerning the energy efficiency of various distillation column control structures. The choice of an energy‐efficient control configuration by incorporating thermodynamics second law in the selection criteria is described. In addition to a relative gain array for assessing control loop interactions, a relative exergy array is used in evaluating the energy efficiency of various control structures. The preferred control structure should have both good operability and good energy efficiency as distillation columns are the major energy consumer in the chemical industry. The performance of the control structures in the dynamic mode is analyzed in detail. The proposed method is demonstrated on two binary distillation columns: methanol‐water separation and benzene‐toluene separation. Dynamic simulation results prove the efficiency of the proposed distillation control structure selection method.     

Evaluation of a two-temperature control structure for a two-reactant/two-product type of reactive distillation column

Several different control structures have been proposed for reactive distillation columns. The appropriate control structure depends on the flowsheet and on the type of reactions occurring in the column. If two reactants are involved and if it is desirable to operate the process without any excess of reactant, it is necessary to manage the fresh feed streams so that the stoichiometry is exactly balanced. A composition analyzer that measures an internal composition in the column is often required. However, if two products are produced, it is possible to avoid the use of an analyzer by using two temperatures in the column to adjust the two feed streams. This type of structure was proposed by Roat et al. [Roat, S., Downs. J., Vogel, E., Doss, J., 1986. Integration of rigorous dynamic modeling and control system synthesis for distillation columns. In: Chemical Process Control—CPC III. Elsevier, Amsterdam.] for the ideal reaction A+B↔C+D in one of the earliest papers dealing with reactive distillation control.The purpose of this paper is to explore the effectiveness of this two-temperature control structure for various column designs (number of reactive stages) to quantify the impact of design on controllability. We also discuss the issues of the selection of the trays whose temperatures are to be controlled and the tuning of the two interacting temperature controllers. Disturbances in production rate and fresh feed compositions are made to examine the rangeability of this control structure. Both an ideal reaction system and the methyl acetate system are studied. One of the main conclusions is that the locations of the temperature control trays should be made such that the two temperature controllers both have direct action (an increase in temperature increases feed), which requires negative openloop process gains for both loops.     

反应精馏隔壁塔生产乙酸正丁酯的优化与控制

对反应精馏隔壁塔生产乙酸正丁酯过程进行了模拟、优化与控制的系统研究。利用Aspen Plus软件模拟乙酸甲酯与正丁醇的酯交换反应过程,以年总费用（TAC）为目标函数进行过程优化,通过稳态敏感性分析及相对增益矩阵（RGA）判据得到不同的操纵变量与控制变量匹配关系,以此为基础,在Aspen Dynamics平台建立了若干控制结构并进行分析对比。结果表明,利用两股反应物呈比例进料可较为有效地抵抗进料扰动,最后提出的无再沸器热负荷与混合物进料量比值（Q_r/F）控制的改进控制结构CS3,在降低反应精馏隔壁塔控制过程超调量方面有较大的优越性。     

计算机辅助蒸馏过程模拟及控制(Ⅰ)——多组分复杂蒸馏控制的相对增益矩阵计算研究

本文应用Bristol的相对增益概念,对具有非线性多变量和相关多回路的复杂蒸馏过程,建立起5×5调节和被调参数对的相对增益计算方法,以研究它在设计中的控制策略问题.该方法改进了2×2相对增益子集分析方法的不足,达到了比通常应用的方法更准确和全面地反映出控制回路间存在的相关作用的目的,从而找到在全局意义下的控制策略.     

RGA ANALYSIS FOR DISTILLATION CONTROL

This paper presents the relative gain analysis of two industrial distillation columns. Both columns involve multicomponent separation and the purpose of the relative gain analysis has been to find proper pairing of manipulated and controlled variables. The matrix oriented method of Wang and Henke has been employed in the study, The study has investigated the effects of single-precision versus double-precision programming, step-size, and tightness of convergence, upon the relative gain array. The results with single-precision programming and double-precision programming have yielded significantly different results. The results confirm that a considerable reduction in the step-size of the manipulated variables may be necessary in some cases to achieve meaningful results. They also indicate that tight convergence may be required for some columns at a considerable expense in terms of CPU time while for others, loose convergence and moderate step-sizes may be adequate, The results of this study will be useful to those who are interested in the design of distillation control systems.     

Input–output pairing accounting for both structure and strength in coupling

Input–output pairing is an important problem in control system design and is often performed using the relative gain array (RGA) based approaches. While RGA‐based approaches have been very successful in many applications, they have some well‐known limitations. For example, they may give results which are not consistent with the physical topology since only the strength of interaction between inputs and outputs is taken into account in the RGA. In this work, we propose a new measure for input–output pairing that explores both strength and structural information in input–output coupling. Specifically, we take advantage of the tool of relative degree to measure the physical closeness of input–output pairs and to explore the strength of interaction progressively with respect to the relative degree. We call the proposed measure relative sensitivity array (RSA) between inputs and outputs. Detailed analysis is performed to reveal the relationship between the gain matrix used in the RGA and the sensitivity matrix in the RSA from a mathematical point of view. Since the RSA is an analog of the RGA, many existing pairing guidelines developed for the RGA can be used in the proposed RSA‐based pairing. The proposed RSA‐based approach is applied to two examples. The results show that pairs formed by the proposed approach are consistent with the physical topologies of the processes. Also, the results show that the proposed approach can handle larger systems that cannot be effectively handled by RGA‐based approaches. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1226–1235, 2017     

Control structure selection of increased-pressure extractive distillation process for DMC-MeOH azeotropic mixture

Producing dimethyl carbonate (DMC) as a green chemical with the desired purity is important in the industry. Although studies on the steady-state design of energy-efficient extractive distillation processes are important for the purification of DMC-methanol (DMC-MeOH) azeotropic mixtures, the dynamic controllability of these processes is also critical in the case of feed condition changes, and it should be investigated carefully. Results of the limited studies in the literature show that changing the operating pressures in extractive distillation processes might have different effects on the dynamic controllability of different systems. Thus, in this study, alternative control strategies are developed for a recently proposed increased-pressure extractive distillation process to separate DMC-MeOH mixture. All control structures are designed using inferential temperature controllers, which have a general acceptance in industrial applications. Effects of different ratio controllers are investigated by evaluating the dynamic responses of control structures for disturbances in feed flowrate and composition. Two metrics including integral absolute error and steady-state deviation of purities are used in the evaluation of alternatives. Results of dynamic simulations show that a control structure including reflux ratio controller is not a suitable strategy for this process. It is demonstrated that a control structure including reflux to feed ratio controller for both distillation columns is necessary for the robust and efficient control of a pressure-increased extractive distillation process. These efficient dynamic results support the economic advantage of increased-pressure extractive distillation process separating DMC-MeOH azeotropic mixtures.     

基于相对增益分析的换热网络旁路设计

提出一种考虑了可控性的换热网络旁路设计法。基于稳态模型增益的定量计算,推导换热网络稳态数学模型的求解过程,从可控性分析的角度逐一求解换热网络的非方相对增益矩阵,从中确定最优的旁路位置,使被控变量具有较高的可控性。突破相对增益矩阵仅用于控制配对的常规范畴,提出一种通过逐次求解换热网络非方相对增益矩阵优化选取最优旁路设置的方法,并给出了设置旁路的若干准则,以简化求算过程。分析稳态工作点变化后的情况,表明工况变化不影响上述得出的旁路设置。该法适用于大型复杂换热网络,满足生产控制要求,并能保证整个换热网络具有较高的可控性。以某常减压蒸馏装置脱盐前换热网络为例,验证方法的可行性和有效性。     

非线性系统两变量P控制下的分支特性

采用分支理论推导非线性系统两变量P控制下的分支特性,所导得的几个定理阐明了简单分支点和Hopf分支点的位置和性质.1个简化精馏塔模型用于演示这一过程,展示出精馏塔系统在闭环时可能出现多态.     

GAIN SCHEDULING FOR COMPOSITION CONTROL OF DISTILLATION COLUMNS

Composition control loops for high purity distillation columns are highly non-linear, particularly their gain characteristics. A simple gain scheduling approach suggested by Shinskey is evaluated. This approach involves a linear approximation to the process gain when product purity increases and the assumption of constant gain when purity drops. Total loop gain is kept constant when purity increases. Digital simulations show that gain scheduling yields significant improvements in control performance.     

DECOUPLER DESIGN AND CONTROL SYSTEM TUNING BY INA FOR DISTILLATION COMPOSITION CONTROL†

Six distillation column models have been used in studies of the usefulness of the Inverse Nyquist Array (INA) method for design of dual composition control for distillation. Five of the column models are experimental, one describing an industrial column, the others four pilot plants. One column model is obtained by modeling from first principles.

The control strategies investigated and compared are multiloop SISO, 2-way decoupling and l-way decoupling. The control variables are the standard ones, i.e. reflux flow and boilup.

In most of the cases INA has been found to be a useful tool for design of the loops and for comparison of the different control approaches. INA has also been found to be useful for simultaneous tuning of the decouplers and the feedback controllers.

For 2-way decoupling the criterion to minimize interaction at the critical frequency for each primary feedback loop has been found useful in most, but not all, cases. The scheme can be designed by pure gains in the decouplers and there is no reason to introduce dynamics into the decouplers—the systems studied are already sufficiently rich in dynamics.

INA has been found to be a suitable vehicle for the choice between the two possible l-way decoupling schemes.

With the design approach taken, l-way decoupling has been found to provide considerably better control quality than 2-way decoupling in two of the six systems, the differences in control quality being small in the other cases.

The effect of model mismatching on the results, caused e.g. by process nonlinearities, is discussed and simple rules to decrease the parameter sensitivity for 2-way decoupling schemes are given. When model mismatching is considered, the advantage of l-way decoupling over 2-way decoupling seems to increase.

One deficiency of the INA design as used in this paper is that it does not single out the design which is to be preferred on the grounds of robustness.     

Analysis and evaluation of the relative gains for nonlinear systems

By beginning with the most general implicit relationships between the controlled variables and the manipulated variables of a control system, formulas for computing the relative gain are developed. This approach leads to a more comprehensive analysis and consequently a better understanding of the conditions responsible for infinite relative gains. Also, the use of the Jacobian method of determinants to minimize the number of computer solutions required to evaluate the elements of the relative gain matrix is demonstrated by use of a numerical example.     

Eco-efficiency and control loop configuration for recycle systems

To integrate measurements of eco-efficiency with control loop configuration has become an important topic since all industrial processes/plants are requested to increase their eco-efficiency. The exergy eco-efficiency factor, a new measure of eco-efficiency for control loop configuration, has been developed recently [1]. The exergy eco-efficiency factor is based on the thermodynamic concept of exergy, which can be used to analyze a process in terms of its efficiency. The combination of the relative gain array (RGA), NI, CN, dynamic RGA, and the exergy eco-efficiency factor will help guide the process designer to find the optimal control design with low operating cost/eco-efficiency. In this paper, we validate the proposed exergy eco-efficiency factor for processes with recycles which are very common industrially.     

A Short Note About Energy‐Efficiency Performance of Thermally Coupled Distillation Sequences

In this work, we present a comparative study of the energy‐efficiency performance between conventional distillation sequences and thermally coupled distillation arrangements (TCDS) for the separation of ternary mixtures of hydrocarbons under the action of feedback control loops. The influence of the relative ease of separation of the feed mixture and its composition was analyzed. The feedback analysis was conducted through servo tests with individual changes in the set points for each of the three product streams. Standard PI controllers were used for each loop. The results show an apparent trend regarding the sequence with a better dynamic performance. Generally, TCDS options performed better for the control of the extreme components of the ternary mixture (A and C), while the conventional sequences offered a better dynamic behaviour for the control of the intermediate component (B). The only case in which there was a dominant structure for all control loops was when the feed contained low amounts of the intermediate component and the mixture had similar relative volatilities. The Petlyuk column provided the optimal choice in such case, which contradicts the general expectations regarding its control behaviour. In addition, the energy demands during the dynamic responses were significantly lower than those observed for the other distillation sequences. TCDS options, therefore, are not only more energy efficient than the conventional sequences, but there are cases in which they also offer better feedback control properties.     

萃取精馏分离苯/环己烷共沸体系的控制策略

将常规萃取精馏、差压热耦合萃取精馏以及隔壁塔萃取精馏技术应用于以糠醛为萃取剂的苯和环己烷共沸物分离过程。在稳态模型的基础上,利用Aspen Dynamics软件进行控制研究,对三工艺流程提出了若干控制策略。结果表明,对于常规萃取精馏过程,再沸器热负荷与进料量比值控制结构在降低控制过程超调量方面表现出明显优势;对于差压热耦合萃取精馏过程,带有压力-补偿控温策略的方案控制效果更佳;而对于隔壁塔,则选择了无隔板下方气液分离比控制的结构来作为较优的控制策略。     

Prediction of the steady-state relative gains by use of the total reflux model

The total reflux model is used to predict the relative gains for the steady states of a conventional distillation column both before and after an upset, gains for the control schemes wherein purity specifications on both the distillate and bottoms are evaluated by use of the reflux model and by use of the numerical solutions for an exact model of the column. A comparison of these results shows that the gains obtained by use of the predictive model are of sufficient accuracy to choose the most desirable control system.     

Integrated design and control of reactive distillation processes using the driving force approach

Superior controllability of reactive distillation (RD) systems, designed at the maximum driving force (design-control solution) is demonstrated in this article. Binary or multielement single or double feed RD systems are considered. Reactive phase equilibrium data, needed for driving force analysis and design of the RD system, is generated through an in-house property prediction tool. Rigorous steady-state simulation is carried out in ASPEN plus in order to verify that the predefined design targets and dynamics are met. A multiobjective performance function is employed to evaluate the performance of the RD system in terms of energy consumption, sustainability metrics (total CO₂ footprint), and control performance. Controllability of the designed system is evaluated using indices like the relative gain array (RGA) and Niederlinski index (N_I ), to evaluate the degree of loop interaction, as well as through dynamic simulations using proportional-integral (PI) controllers and model predictive controllers (MPC). The design-control of the RD systems corresponding to other alternative designs that do not take advantage of the maximum driving force is also investigated. The analysis shows that the RD designs at the maximum driving force exhibit enhanced controllability and lower carbon footprint than the alternative RD designs.