FEEDFORWARD CONTROL OF A BINARY HIGH PURITY DISTILLATION COLUMN

Feedforward and PID-SISO feedback control are compared on a pilot plant scale distillation column which is operated in the LV-configuration. For the rejection of disturbances in the feed concentration, a simple empirical feedforward control algorithm is presented. Final steady-state values of the manipulated variables are computed from quadratic equations in x_F, which are easily fitted to simulation or plant data. Experimental results for PID- and feedforward control are compared. The need for dynamic filtering of manipulated variables is demonstrated. While direct setting of the manipulated variables may disturbe the system unnecessarily, second order filtering achieves very fast disturbance rejection. The impact of errors in the disturbance measurement on the control performance is investigated. The feedforward algorithm is intentionally kept simple, to allow easy implementation on a standard decentralized control system     

CONTROL AND DYNAMIC ANALYSIS OF A PACKED DISTILLATION COLUMN BASED ON MODELING APPROACHES

In this work, dynamic analysis and control of a packed distillation column have been utilized theoretically and experimentally. In theoretical studies, two types of mathematical models stagewise (Frank model) and partial differential approaches (back-mixing model), were used. Packed distillation uses 1400 mm packing height, and packing type is rashing ring with 20-15 mm diameter. The reboiler was made from a 13 L glass container. Reflux ratio was adjusted by an on-line computer. The system temperature was measured with six thermocouples. For control studies, the reflux ratio and the reboiler heat dutywere chosen as manipulated variables. Perturbation in feed composition was utilized as the disturbance. Decoupling multivariable dynamic matrix control (DDMC) and Nondecoupling multivariable dynamic matrix control (NDMC) of overhead and bottom compositions were applied for control studies. Performance of the control system was tested by using an integral absolute error (IAE) criterion and it was also compared with decoupling multivariable PID control (DPID) and Nondecoupling multivariable PID control (NDPID).     

APPLICATION OF MULTIVARIABLE GENERALIZED PREDICTIVE CONTROL TO A PACKED DISTILLATION COLUMN

The present work deals with the application of Multivariate Generalized Predictive Control (MGPC) systems to a packed distillation column. The steady-state and dynamic behaviour of the system have been simulated using two film plug flow model. The model solutions have been obtained employing orthogonal collocation on finite element. The Jacobi polynomials within the finite element procedure was tested to determine the phase flow rates, the liquid and vapour composition profiles and the temperature profiles. All the theoretical results were compared with experimental data obtained from a pilot-plant packed distillation column distilling methanol-water mixture. Decoupling and MGPC control of overhead and/or bottom compositions were examined. Perturbation in feed composition and, reflux ratio and the reboiler heat duty were utilized as the disturbance and the manipulated variables respectively. Performance of these systems was tested by using an integral square of error (ISE and IAE) criterion.     

CLOSED-LOOP DATA RECONCILIATION FOR THE CONTROL OF A BINARY DISTILLATION COLUMN

Data reconciliation is a procedure that makes use of process models along with process measurements to give more precise and consistent estimates for process variables. Data reconciliation has been traditionally used to provide a more representative set of data to calculate steady-state inventories and process yields. For dynamic systems, the use of data reconciliation is relatively nascent. This article examines the potential use of data reconciliation in closed-loop control as a filter to attenuate the noise in measurements of the controlled variables so that the controllers can access more accurate sets of data. Data reconciliation filters were implemented in simulations of a PID control system for a binary distillation column. Results showed that data reconciliation could efficiently reduce the propagation of measurement noise in control loops, so that the overall performance of the controller is enhanced.     

CLOSED-LOOP DATA RECONCILIATION FOR THE CONTROL OF A BINARY DISTILLATION COLUMN

Data reconciliation is a procedure that makes use of process models along with process measurements to give more precise and consistent estimates for process variables. Data reconciliation has been traditionally used to provide a more representative set of data to calculate steady-state inventories and process yields. For dynamic systems, the use of data reconciliation is relatively nascent. This article examines the potential use of data reconciliation in closed-loop control as a filter to attenuate the noise in measurements of the controlled variables so that the controllers can access more accurate sets of data. Data reconciliation filters were implemented in simulations of a PID control system for a binary distillation column. Results showed that data reconciliation could efficiently reduce the propagation of measurement noise in control loops, so that the overall performance of the controller is enhanced.     

DIRECTIONS OF DISTURBANCES AND MODELING ERRORS ON THE CONTROL QUALITY IN DISTILLATION SYSTEMS

The effect of directions of disturbances and modeling errors on the control quality was studied with the aid of singular vectors. It is shown that the material balance type of control structure can handle larger size of set point changes than the conventional type of structure in almost all the directions of set point changes. On the other hand it is shown that the latter can handle larger size of feed disturbances than the former in large portion of the directions

The effect of modeling errors in the relative volatility and the stage efficiency on the robustness was discussed focusing on their directions. It is shown that the column with the high condition number of 57 remains stable even when both parameters were 10^° in error

Computer simulations were carried out to ascertain the results obtained.     

ON THE DESIGN OF ROBUST CONTROL SYSTEMS FOR DISTILLATION COLUMNS

The distillation column dynamics is very nonlinear, especially at high purity. Multivariable control system designs, which are essentially linear, may not be able to perform well at different operating conditions. This paper looks at three different multivariable design techniques—decoupling control, optimal state feedback and pole assignment as applied to distillation column control. Robustness of these techniques are analyzed by looking at the performance of these controllers at different operating conditions. Some interesting results are obtained.     

APPLICATION OF MULTIVARIABLE NONLINEAR ADAPTIVE GENERIC MODEL CONTROL TO A PACKED DISTILLATION COLUMN

Nonlinear adaptive generic model control and self-tuning PID control systems were applied to control the top and bottom product temperature of a packed distillation column separating methanol-water mixture. In the first control algorithm, an adaptive generic model control (AGMC) structure was proposed for dual temperature control of the system. In the second control algorithm, nonlinear self tuning PID (NLSTPID) control based on pole-placement technique was used to control the same system. For NLSTPID control purposes pseudo random binary sequence (PRBS) signal and recursive identification algorithm were used to estimate the relevant parameters of a polynomial NARMAX model. In this work, real-time application has been carried out. In both dynamic and control studies, perturbations in feed composition were utilized as the disturbance, and the reboiler heat duty and the reflux ratio were selected as the manipulated variables. The control performances have been obtained by using ISE and, in general, AGMC results were better than those of the STPID control algorithm.     

APPLICATION OF MULTIVARIABLE NONLINEAR ADAPTIVE GENERIC MODEL CONTROL TO A PACKED DISTILLATION COLUMN

Nonlinear adaptive generic model control and self-tuning PID control systems were applied to control the top and bottom product temperature of a packed distillation column separating methanol-water mixture. In the first control algorithm, an adaptive generic model control (AGMC) structure was proposed for dual temperature control of the system. In the second control algorithm, nonlinear self tuning PID (NLSTPID) control based on pole-placement technique was used to control the same system. For NLSTPID control purposes pseudo random binary sequence (PRBS) signal and recursive identification algorithm were used to estimate the relevant parameters of a polynomial NARMAX model. In this work, real-time application has been carried out. In both dynamic and control studies, perturbations in feed composition were utilized as the disturbance, and the reboiler heat duty and the reflux ratio were selected as the manipulated variables. The control performances have been obtained by using ISE and, in general, AGMC results were better than those of the STPID control algorithm.     

DYNAMIC SIMULATION OF A DISTILLATION COLUMN SEPARATING A MULTICOMPONENT MIXTURE

In this work, a dynamic model was developed to simulate the transient behaviour of a pilot scale, continuous, siagewise, 15 plate distillation column separating a mixture of ethanol, iso-propanol and isobutanol. The accuracy of the predictions was assessed by transient response data collected during experiments under step changes in reflux ratio, feed flow rate and feed composition.

Results demonstrated the feasibility of simulating multicomponent distillation under unsteady state conditions with fairly good success. Further improvements were suggested for a better accuracy     

酒精回收塔的计算机模拟与工业实践

在计算机上运用ＭｃＧｒａｗ－Ｈｉｌｌ公司的化工过程模拟软件，对酒精回收塔的精馏过程进行了模拟计算。模拟结果有利于提高酒精质量和产品回收率。采用新型高效规整填料，大大地提高了生产能力和分离效率。据此技术设计的精馏塔，具有极大的优越性，工业应用非常成功。     

FUZZY-BASED MODELING AND CONTROL OF AN INDUSTRIAL FLOTATION COLUMN

In the mineral production industry, separation of valuable from gangue minerals is generally carried out using the flotation process. Mineral recovery into the concentrate and the grade of the concentrate are the main metallurgical parameters of the flotation process. In the present investigation, a fuzzy model was developed to simulate the relationship between the process conditions (i.e., gas velocity, slurry solids %, frother dosage, and frother type) and the metallurgical performance of an industrial flotation column in a copper concentrator in Iran. Afterwards, an intelligent model–based control system was designed to control the process performance at the desired level using fuzzy logic rules. Simulation results show that the developed controller is capable of maintaining the process performance at its target level within a reasonable time.     

ON USING SUMS AND DIFFERENCES TO CONTROL DISTILLATION

The idea to use sums and differences of compositions (or temperatures) to improve dual-composition control of distillation has been suggested a number of times in the literature, at least since 1962. The main advantage of the scheme has been claimed to be that it results in noninteraction, i.e. the two composition control loops act as noninteracting SISO-Ioops. Most of the schemes suggested have used distillate flow D and vapor boilup V as manipulators for the composition loops, although also other manipulators have been suggested,

Some new theoretical results (Haggblom and Waller, 1987) show that noninteraction (triangular system matrix) at steady state is obtained for separations where the distillate flow D is equal to the bottoms flow B when the sum of product composition is controlled by D or by B. Dynamic simulations for one such case show that, compared to the usual approach of using D to control top composition and vapor boilup V to control bottoms composition, an improvement in control may be obtained when controlling the composition sum by D and the difference by V.

Also for another case with D + B an improvement is obtained although the controlled variables are not the product compositions but the compositions on plates 3 and 17.

When temperatures are the controlled variables or when D + B, noninteraction is not generally obtained. An experimental example illustrates that controlling the temperature sum and difference can degrade control quality. The hypothesis that the controlling of sums and differences would always result in noninteraction and improve control is thus falsified.     

基于软测量技术的先进控制在精馏塔上的应用

In this paper, the control of a butadiene distillation column is discussed and improved. At first, a neural network soft-sensor instrument of product quality was built based on abundant on-the-spot data collected by DCS and simulated data obtained by a theoretical model. Then, an inferential control scheme based on the soft-sensor was designed. By increasing logic and expert controllers in the inferential control arithmetic, the robustness of the system was enhanced. The practical application showed that the scheme could run smoothly over a long period and realized close-loop control of product quality.     

具有多塔功能的单塔半连续精馏过程研究

提出了具有多塔功能的单塔半连续精馏操作方法。综合了间歇精馏和连续精馏的优点，用一个塔处理多组元复杂料液获得多个产品，同时又避免了传统间歇精馏时料液在釜内受热时间过长的缺点。所用的新型卧式多层薄液蒸发釜，比立式降膜蒸发釜布膜率高，高度低。理论分析和实验结果均表明，该操作方法稳定可靠，对解决热敏性物料的精馏，具有较强的现实意义。     

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.     

多组份精馏塔数字仿真

描述多组份精馏塔要用众多的方程式,数字仿真是求得它的动态响应的有效方法,如果有J个组份,不是理论板,非恒分子流,在一块加料板上有气、液混合进料,并假设回流R及再沸器供热量Q_B恒定,在这样的条件下本文编排了仿真程序,仿真结果与文献[1]提供的数据一致,稍加更动后也可仿真其他情况。     

MULTIRATE MULTIVARIABLE SELF-TUNING CONTROL OF DISTILLATION COLUMNS USING KALMAN PREDICTOR

A novel method for designing Kalman Predictor (KP) based multivariable self-tuning controllers for Multi Input Multi Output (MIMO) systems has been proposed, and also applied to the control of two distillation columns. The objective is to maintain constant terminal compositions despite disturbances entering the system even when the controlled variables are not measured at the same sampling rate. The KP generates minimum variance estimates of the output variables. Simulation results show that KP based multirate multivariable self-tuning controller exhibits better performance than the earlier reported multirate controller for set point tracking, even in the presence of nonstationary disturbances. KP based self-tuning controller is, therefore, suitable for industrial applications.     

ADAPTIVE MULTIVARIABLE PID CONTROLOF A DISTILLATION COLUMN WITH UNKNOWN AND VARYING DEAD TIME

A new, reliable, and easy-to-use adaptive control strategy has been developed to overcome the long-existing difficulties in adaptive control practice caused by unknown and varying process dead time. A self-tuning PID control algorithm is adopted to control a distillation column possessing second-order-plus-dead-time dynamics. The self-tuning strategy is based on recursive least-squares estimation of process parameters. U-D factorization is applied to stabilize the parameter estimation calculations. A variable forgetting factor is used to alleviate wind-up in the estimator. A simulation study and an experimental evaluation demonstrate the capability of the adaptive algorithm.     

一种改进的多元精馏塔动态模型

综合了精馏塔的物料及能量平衡、塔板水力学方程、塔板效率及再沸器动态特性,给出一种改进的非理想多元物系精馏塔通用动态数学模型.增加了汽液平衡计算,从而弥补了文献[1]所提出的动态模型的缺陷,提高了其精度和通用性.模型仿真算法采用Gear积分方法,稳定性好,对解刚性比很高的精馏塔微分方程组十分有效.通过对某丙烯腈萃取精馏塔的仿真表明,该动态模型能够较好地反映对象的动态形为.