Tray Temperature Research Articles

COMBINING CONCEPTUAL AND REFERENTIAL REACTION METHODS FOR BATCH DISTILLATION CONTROL

The referential reaction empirically determines reduced-order models for designing and tuning feedback controllers dedicated to tracking non-stationary conditions in batch processes. Defining a feedback control system for a batch distillation implies first the selection of an appropriate tray temperature evolution. Then, the dynamics associated to the manipulated variable (distillate flow rate) is isolated from the main time-variable behavior desired for the operation, and used with available tuning rules for integrating systems. The effectiveness of this procedure is illustrated by implementing a feasible quasi-optimal recipe derived from pinch theory and valid for multicomponent mixtures.

Control System Design for a Single Feed ETBE Reactive Distillation Column

Reactive distillation (RD) is advantageous for the Ethyl Tert-Butyl Ether (ETBE) synthesis. The steady state model of an ETBE reactive distillation column created using the simulator HYSYS is analyzed to synthesize effective control structures. Since the column exhibits input multiplicity with the dual process objectives of ETBE RD (isobutene conversion and ETBE purity), inferential variables are selected. A control structure that organizes a sensitive tray temperature in the stripping section using the reboiler duty and maintains the temperature difference of reactive trays using the reflux flow, is found to be most suitable. A decentralized PI controller and constrained Model Predictive Controller (MPC) are implemented, and performances are compared for set point tracking and disturbance rejection. MPC control algorithms are implemented in MATLAB and interfaced with HYSYS. Constrained MPC (CMPC) is found to be effective for load disturbance rejection, which frequently occurs in the single feed configuration.

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.

Relay Feedback Tests for Highly Nonlinear Processes: Reactive Distillation

This paper is intended to demonstrate the effectiveness of the relay feedback test for the identification of highly nonlinear processes, namely, reactive distillation. Because of the multifunctional nature, reactive distillation exhibits strong nonlinearity and possible reversal in the steady-state gain of product composition and tray temperatures. A methyl acetate reactive distillation is used to illustrate performance of two different identification approaches: step test and relay feedback test. For the composition control, step tests reveal sign changes in steady-state gain as the step size varies from -10% to +10%. Moreover, step test is also sensitive to the direction of the step. On the other hand, consistent ultimate gain and the ultimate period can be identified for different relay heights with different initial step directions. The error in the high-frequency information is <5% for the reactive distillation column that has been studied. Good control performance can be obtained using simple PI controllers tuned with the ultimate gain and ultimate period. Comparison is also made for temperature control and, again, the results indicate that consistent high-frequency information can be obtained with the relay feedback test. Finally, relay feedback is applied to multivariable control of reactive distillation. The resultant PI controllers, designed based on the ultimate information, also gives satisfactory performance for set-point and load changes.

Offset‐Free Inferential Feedback Control of Distillation Compositions Based on PCR and PLS Models

AbstractAn offset‐free inferential feedback control strategy for distillation composition control using principal component regression (PCR) and partial least squares (PLS) models is presented in this paper. PCR and PLS model based software sensors are developed from process operational data so that the top and bottom product compositions can be estimated from multiple tray temperature measurements. The PCR and PLS software sensors are then used in the feedback control of the top and bottom product compositions. With this strategy the problem of substantial time delay in composition analyzer based control and of substantial bias in single tray temperature control can be overcome. A practically very important issue in software sensor based feedback control is that static control offsets often exist due to a static estimation bias, especially when the process operating condition changes. A technique for eliminating the static estimation bias and the resulting static control offsets through mean updating of process measurements is proposed in this paper. Applications to a simulated methanol‐water separation column demonstrate the effectiveness of this control strategy.

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.

Dynamics and Control of Reactive Distillation Configurations for Acetic Acid Esterification

Three different flowsheets have been proposed for acetic acid esterification of acetic acid with alcohols ranging from C1 to C5 according to the ranking of normal boiling points and immiscibility. This work explores the similarities and differences in the dynamics and control of these three types of flowsheets. The degree of process nonlinearity can be computed quantitatively based on the fraction of “sign reversal” for all tray temperatures or based on Allgower's nonlinearity measure. These measures provide useful information to the potential problems in closed-loop control. Next, a systematic design procedure is proposed to devise control structures for all three types of flowsheets for these five esterification systems. The simulation results reveal that reasonable control can be achieved for all five systems with different degrees of asymmetry in closed-loop responses as predicted by the nonlinearity measures. Simulation results clearly show that the simple decentralized control provides a workable solution for highly nonlinear reactive distillation columns under various flowsheet configurations.

INFERENTIAL CONTROL BASED ON A MODIFIED QPLS FOR AN INDUSTRIAL FCCU FRACTIONATOR

A modified Quadratic Partial Least Squares (MQPLS) algorithm based on nonlinear constrained programming is proposed. Sequential Unconstrained Minimization Technique (SUMT) is employed to calculate the outer input weights and the parameters of inner relationship. It was found that MQPLS can not only explain more of the underlying variability of the data, but also has improved modelling and predictive ability. An inferential control system is implemented on the Distribute Control System (DCS) of a fluid catalytic cracking unit (***FCCU) main fractionator. A soft sensor MQPLS–based was developed to estimate solidifying point of diesel oil. The controller was established via constrained Dynamic Matrix Control (DMC) algorithm. Real time application results demonstrated the performance of the inferential control system based on MQPLS was much better than the original tray temperature control system. This resulted in a 1.0% increase in production rate, and a significant increase in profit.

Control of different reactive distillation configurations

AbstractThree different flowsheets have been proposed for acetic acid esterification of acetic acid with alcohols ranging from C1 to C5 according to the ranking of normal boiling points and immiscibility. This work explores the similarities and differences in the dynamics and control of these three types of flowsheets. The degree of process nonlinearity is analyzed qualitatively based on the residue curve map and the boiling point ranking and it can be computed quantitatively based on the fraction of “sign reversal” for all tray temperatures or based on Allgower's nonlinearity measure. These measures provide useful information to the potential problems in closed‐loop control. Next, a systematic design procedure is proposed to devise control structures for all three types of flowsheets for these five esterification systems. The simulation results reveal that reasonable control can be achieved for all five systems with different degrees of asymmetry in closed‐loop responses as predicted by the nonlinearity measures. Dual‐temperature control and one‐temperature/one‐composition control are studied. Simulation results clearly show that the simple decentralized control provides a workable solution for highly nonlinear reactive distillation columns under various flowsheet configurations. © 2005 American Institute of Chemical Engineers AIChE J, 2006

Effect of Feed Composition on the Selection of Control Structures for High-Purity Binary Distillation

This paper discusses the selection of an effective control structure for a binary distillation column producing high-purity products. Results show that this selection depends on the feed composition. If the concentration of the light component is large, the reflux ratio is small. Control of a single appropriate tray temperature by manipulating the reboiler heat input and using a fixed feed-to-reflux ratio provides effective control for both feed-rate and feed-composition disturbances. The reflux-drum level is controlled by the distillate flow rate. For intermediate concentrations of the light component and for high-purity products, this study shows that a two-temperature control structure with reflux and reboiler heat input manipulated is required to handle feed-composition changes. Small concentrations of the light component yield small distillate flow rates and high reflux ratios. Conventional distillation control wisdom advises that the reflux-drum level should be controlled by the reflux flow rate when the reflux ratio is larger than ∼2. A control structure is frequently recommended in which the flow rate of the distillate is ratioed to the reflux. However, in many columns, a constant reflux-ratio strategy is not as effective as a constant reflux-to-feed strategy for maintaining product purity at both ends of the column in the face of feed-composition disturbances when a single tray temperature is controlled. An alternative control structure is proposed in this paper that achieves the preferred constant reflux-to-feed strategy by controlling the reflux-drum level with reboiler heat input and manipulating the small distillate flow rate to control a tray temperature.

Steady State Analysis of Reactive Distillation Using Homotopy Continuation

Homotopy continuation is applied to the industrially important MTBE and methyl acetate reactive distillation (RD) columns to analyse for steady state multiplicities. Continuation is applied in two distinct ways, with respect to tray catalyst mass and with respect to column inputs. Catalyst mass continuation provides a tool for convergence to a steady state solution of the highly non-linear governing equations. Also, the solution diagram reveals the occurrence of multiple steady states. For the MTBE column, up to five steady states in the kinetically controlled regime are detected. Of these, only three remain in the limit of reaction equilibrium on the trays. For the methyl acetate column, three steady states in both the kinetically controlled and reaction equilibrium regimes are obtained at fixed reflux rate and reboiler duty. If the column specification is changed to fixed reflux ratio and reboiler duty, only a single steady state solution is observed. The column operating policy thus substantially impacts the existence of multiple steady states. Column input continuation is used to obtain the complex relationships between input variables (reboiler duty, reflux rate, and so on) and output variables (tray temperatures, conversion, and so on). Steady state multiplicities are seen in both the MTBE and methyl acetate columns. A thorough understanding of these multiplicities is essential for devising robust control strategies.

Evaluation of criteria for selecting temperature control trays in distillation columns

The use of tray temperatures to infer compositions is widespread in distillation control. A number of criteria have been proposed for selecting which trays to hold at constant temperature. The most commonly used are (1) choosing a tray where there are large changes in temperature from tray to tray (“slope” of the temperature profile), (2) finding the tray where there is the largest change in temperature for a change in the manipulated variable (“sensitivity”), (3) using singular value decomposition (SVD) analysis, (4) selecting the tray where the temperature does not change as feed composition changes while producing the desired distillate and bottoms purities and (5) choosing the tray that produces the smallest changes in product purities when it is held constant in the face of feed composition disturbances. This paper provides a quantitative comparison of the effectiveness of these five alternative criteria. Several systems are considered, ranging from ideal binary to azeotropic multi-component. Results show that SVD analysis provides a simple and effective method for selecting temperature control tray location.

Comparison of Two Types of Two-Temperature Control Structures for Reactive Distillation Columns

The purpose of this paper is to compare the effectiveness of two different types of two-temperature control structures for reactive distillation columns. The first was a structure proposed by Roat et al. (Roat, S.; Downs, J.; Vogel, E.; Doss, J. Integration of rigorous dynamic modeling and control system synthesis for distillation columns. Chemical Process Control; CPC III; Elsevier: Amsterdam, The Netherlands, 1986) almost 2 decades ago in which two tray temperatures in the column are controlled by manipulating two fresh feed streams. The second was recently proposed by Yu and co-workers (Yu, C. C. Private communication, 2004) in which two tray temperatures are controlled by manipulating one of the fresh feed streams and vapor boilup. The second fresh feed stream is flow controlled. We discuss the issues of the selection of the fresh feed stream to be manipulated and the importance of the tuning procedure for the two interacting temperature controllers. Both an ideal reaction system and the methyl acetate system are studied. The effectiveness and robustness of these control structures are compared in the face of disturbances in the production rate and fresh feed compositions. One of the main conclusions is that the selection of the manipulated fresh feed stream in the second structure has an important role in the stability of the system. Sequential tuning of the interacting temperature controllers is sometimes necessary.

Influence of Feed Impurity on the Design and Operation of an Industrial Acetic Acid Dehydration Column

In this work, the design and operation of an industrial column for acetic acid dehydration via heterogeneous azeotropic distillation is investigated. Multiple column feed streams from various parts of the upstream process are fed into this column. The feed components besides acetic acid and water also contain small amounts of methyl acetate and another component. For proprietary reasons, this component will not be specified in this paper. This component during normal operation will not leave the column system through either a top decanter aqueous outlet stream or a column bottom stream. The accumulation of this component inside the column system is illustrated via rigorous dynamic simulation. A side stream is proposed to solve the accumulation problem of this component. The appropriate side-stream location and its flow rate are determined in this paper by total annual cost analysis. There is a great influence of this small feed impurity on the design and operation of the column. With the same purity specifications on both the top and bottom products, the column system with feed impurity requires much larger total annual cost and operating cost in comparison with the design of the no feed impurity case. A very simple overall control strategy is proposed for the column system with feed impurity and continuous side-stream draw-off. Because the optimal flow rate of the side stream is quite small, an alternative operating strategy in industry is to periodically purge out this impurity through the side stream. A practical automatic purging strategy is thus proposed in the paper. This automatic purging strategy does not rely on any online composition measurement but just needs information on some tray temperature measurements. Closed-loop dynamic simulation demonstrates that the proposed purging strategy can successfully keep the column under normal operation with both the top and bottom product purities at their specifications.

Nonlinear model-based control algorithm for a distillation column using software sensor

This paper presents the design of model-based globally linearizing control (GLC) structure for a distillation process within the differential geometric framework. The model of a nonideal binary distillation column, whose characteristics were highly nonlinear and strongly interactive, is used as a real process. The classical GLC law is comprised of a transformer (input-output linearizing state feedback), a nonlinear state observer, and an external PI controller. The tray temperature based short-cut observer (TTBSCO) has been used as a state estimator within the control structure, in which all tray temperatures were considered to be measured. Accordingly, the liquid phase composition of each tray was calculated online using the derived temperature-composition correlation. In the simulation experiment, the proposed GLC coupled with TTBSCO (GLC-TTBSCO) outperformed a conventional PI controller based on servo performances with and without measurement noise as well as on regulatory behaviors. In the subsequent part, the GLC law has been synthesized in conjunction with tray temperature based reduced-order observer (GLC-TTBROO) where the distillate and bottom compositions of the distillation process have been inferred from top and bottom product temperatures respectively, which were measured online. Finally, the comparative performance of the GLC-TTBSCO and the GLC-TTBROO has been addressed under parametric uncertainty and the GLC-TTBSCO algorithm provided slightly better performance than the GLC-TTBROO. The resulting control laws are rather general and can be easily adopted for other binary distillation columns.

Plant-Wide Control of a Complete Ethyl Acetate Reactive Distillation Process

By combining chemical reaction and multi-stage distillation in one column, a reactive distillation (RD) column can significantly save capital investment and make chemical conversion reach a much higher level. In a previous study, an optimal steady state design for the production of ethyl acetate (EtAc), consisting of two columns (one for RD and one for stripping column) and an overhead decanter, was reported. As a continuous study to that previous work, four alternative plant-wide control strategies for this designed process are studied in this paper. Feed compositions of both Acetic acid (HAc) and ethanol (EtOH) streams are treated as disturbances. These disturbances are used to test the closed-loop performance of each control scheme. It is found that control of the product qualities by modulating two tray temperatures in the RD column and one tray temperature in the stripper is most appropriate. By this control scheme, both of HAc and EtOH impurities in the product stream can be kept effectively within the acceptable product specifications.

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 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 is achieved with no on-line composition measurements. Tray temperature control loop will be designed to infer final product purity. From Aspen Dynamics simulation, the proposed control strategy performs very well in rejecting various disturbances while maintaining butyl acrylate product at high purity.

Two-Point Temperature Control Structure and Algorithm Design for Binary Distillation Columns

In this paper the problem of designing a two-point linear temperature controller for the regulation of the product compositions in binary distillation columns is addressed. The combination of feedforward, feedback, and internal model control concepts yields a control design methodology with (i) criteria to choose the (decentralized and one- or two-way) decoupling structure and the temperature measurements location, (ii) a linear control system that consists of a static interaction compensator and a pair of decoupled proportional−integral controllers with feed-temperature-based setpoint adjustment, (iii) an implementation that requires the static dependencies of the measured tray temperatures on the feed temperature, the slopes of the operating lines, and the temperature-gradient-to-holdup quotients in the measurement trays, and (iv) a conventional-like tuning scheme that resembles the ones employed in linear first-order controllers and filters. The linear controller recovers the behavior of a material-bal...

ANN based estimator for distillation—inferential control

Typical production objectives in distillation process require the delivery of products whose compositions meet certain specifications. The distillation control system, therefore, must hold product compositions as near the set points as possible in the faces of upset. Distillation column is generally subjected to disturbances in the feed and the control of product quality is often achieved by maintaining a suitable tray temperature near its set point. Secondary measurements are used to adjust the values of the manipulated variables, as the controlled variables are not easily measured or not economically viable to measure (inferential control). In the present paper, an artificial neural network (ANN) based estimator to estimate composition of the distillate is proposed. Nowadays with the advent of digital computers, the demand of the time is to amalgamate the control of various variables to achieve the best results in optimum time. It is therefore required to monitor all the desired variables and perform the control action (feed forward, feed back and inferential) as per algorithm adopted. The developed estimator is tested and the results are compared. The comparison shows that the predictions made by the neural network are in good agreement with results of simulation.

Design and control of acetic acid dehydration system via heterogeneous azeotropic distillation

Acetic acid dehydration is an important operation in the production of aromatic acid, such as terephthalic acid or in the manufacture of cellulose acetate. Although acetic acid and water does not form azeotrope, but using simple distillation to separate these two components is not practical. The reason is because the system has tangent pinch on the pure water end, thus it is more customary in industry to use an entrainer via a heterogeneous azeotropic distillation column system for the separation. In this study, a suitable entrainer is selected from three candidate acetates through rigorous steady-state simulation of this system. Optimum process design and operating condition are determined to keep high-purity bottom acetic acid composition and also keep a small acetic acid loss through top aqueous draw. Furthermore, the overall control strategy of this column system is proposed to hold both bottom and top product specifications in spite of feed rate and feed composition load disturbances. The proposed overall control strategy is very simple requiring only one tray temperature control loop inside the heterogeneous azeotropic column.