Initialising distillation column models

Difficulties associated with the optimisation of distillation column models by non-linear programming are considered. The paper presents a systematic procedure to enable these difficulties to be overcome and proposes a particular formulation of the distillation column model. A certain limiting case of the column model is examined, that of infinite reflux or zero feed. This limiting case considerably simplifies the model and provides a system of non-linear equations that is readily solved. The solution of this problem gives useful information about the purity that can be achieved in the general case and the number of plates needed to attain a given level of purity. The limiting problem provides starting values for the solution of the general column and suggests a homotopy that can be followed if difficulties arise in obtaining convergence. To obtain a stable form of the limiting case requires the general column model to be formulated in a certain way, which to our knowledge has not previously been considered. The ideas have been successfully tested on various multi-column flowsheets involving distillation columns with heat integration.     

Design of a fully thermally coupled distillation column based on dynamic simulations

A dynamic simulation of a fully thermally coupled distillation column is conducted for the design of a possible operation scheme, and its performance is examined with an example process of butanol isomer ternary system. The outcome of the dynamic simulation indicates that the column can be operated by using a 3 × 3 control structure. The structure consists of three controlled variables of the compositions of overhead, bottom and side products and three manipulated variables of the flow rates of reflux and steam and liquid split ratio between a main column and a prefractionator. This paper was presented at The 5th International Symposium on Separation Technology-Korea and Japan held at Seoul between August 19 and 21, 1999.     

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).     

An experimental simulation of distillation column concentration profiles using a batch apparatus

There has been a growing interest in the use of residue curves for the preliminary design and sequencing of distillation columns. Residue curves are used not only to predict composition changes in separation processes, but also to determine the feasibility of proposed separations, and flowsheet development (Chem. Eng. Sci. 33 (1977) 281).An experimental technique has been developed for the measurement of these residue curves. (Distillation & absorption ’97, Inst. Chem. Eng. 1 (1997) 187). It can be shown that the time-dependent composition profiles obtained in a modified form of this apparatus are mathematically equivalent to the position-dependent profiles in a continuous distillation column. Hence, it is possible to experimentally simulate a distillation column profile in a small batch apparatus using only small quantities of material.The modified apparatus consists of a still immersed in a heated oil bath so that a liquid feed is continuously supplied to the still. Samples of liquid are then analysed over time using a gas chromatograph. The results from an experimental system have been compared to available information and simulations to determine the accuracy of the apparatus.This technique has several advantages over working with distillation columns, firstly in the sample size required, and secondly in the ease of operation. The method allows quick and low-cost measurements of the concentration variables that model a distillation column. The information obtained this way could prove useful for the selection of feasible systems and for finding minimum reflux requirements. It could also be very valuable for screening of complex systems where only small amounts of material are available and simulations may be very difficult.     

己二腈生产装置同系物精馏塔的模拟与优化

在1,3-丁二烯直接氰化法制己二腈工艺的基础上,用Aspen Plus软件对己二腈的分离与提纯过程进行模拟,同时考察塔板数、进料位置、回流比和塔压等因素对己二腈分离效果的影响,得到最佳工艺操作参数,在此条件下获得质量分数为99.90%的己二腈和99.50%的2-甲基戊二腈,计算结果与实验结果基本相符,可为该工艺的开发提供理论依据。     

Application of a thermally coupled distillation column with separated main columns to gas concentration process

A modified fully thermally coupled distillation column (FTCDC) for operability improvement is utilized in a gas concentration process. The column consists of a prefractionator and two separated main columns having high distillation efficiency and flexible control structure. The operability of the proposed column is evaluated by examining the open-loop dynamic responses of step input variations with the HYSYS simulation. The simulation result indicates that the modified system can give better control than the original FTCDC. The energy saving and reduction of construction cost are discussed, and the ease of vapor flow manipulation and the elimination of a compressor in the vapor transfer are also evaluated as possible improvements.     

MPC of distillation column with side reactors for methyl acetate

This paper focuses on the dynamic control of distillation column with side reactors(SRC)for methyl acetate pro-duction.To obtain the optimum integrated structure and steady state simulation,the systematic design approach based on the concept of independent reaction amount is applied to the process of SRC for methyl acetate produc-tion.In addition to the basic control loops,multi-variable model predictive control modular with methyl acetate concentration and temperature of sensitive plate is designed.Then,based on process simulation software Aspen Plus,dynamic simulation of SRC for methyl acetate production is used to verify the effectiveness of the control scheme.     

VKL70型刮膜式短程蒸馏小试装置的改进

对VKL70型刮膜式短程蒸馏装置在使用当中存在的问题进行了分析 ,采取了相应的改进措施 ,经实验验证结果明显好于改进之前     

Dynamic rate-based and equilibrium models for a packed reactive distillation column

Dynamic rate-based and equilibrium models were developed for a packed reactive distillation column for the production of tert-amyl methyl ether (TAME). The two types of models, consisting of differential algebraic equations, were implemented in gPROMS and dynamic simulations were carried out to study the dynamic behaviour of reactive distillation of the TAME system. The dynamic responses predicted by the two types of models are similar in general, with some differences in steady-state values. The influence of manipulated variables, such as distillate flow rate, reflux ratio, and reboiler duty, on the dynamic responses of the controlled variables (reactant conversion and product purity) was studied. The dynamic response of reactant conversion is very nonlinear and unconventional, but the response of product purity is well approximated by a linear first-order differential equation. The CPU time required to complete a dynamic simulation of the rate-based model is at least an order of magnitude higher than that for the equilibrium model. Therefore, the dynamic rate-based model is much more complicated than the equilibrium model, and simplification of the rate-based model is necessary for the implementation of model-based control. A new approach was proposed to simplify the dynamic rate-based model by assuming that the mass transfer coefficients are time invariant. This approach was demonstrated to be superior to the conventional simplification methods. It can reduce the number of equations by up to two-thirds and still accurately predicts the dynamic behaviour.     

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.     

Simulation and analysis of extractive distillation process in a valve tray column using the rate based model

Valve trays are becoming popular in the chemical process industries owing to their flexibility to handle a wide range of vapor throughputs. Using the rigorous rate based model, the importance of the non-equilibrium approach is demonstrated for a typical extractive distillation process in a Glitsch V-1 valve tray column. Simulation results based on an in-house developed code indicated that the rate based model predictions for a valve tray column operation showed significant differences relative to the equilibrium model. Even small errors in product purities translated into nonoptimal feed stage locations and inaccurate number of stages required. The counter-intuitive effect of high reflux ratio on separation is explained.     

Development on a coaxial heat integrated distillation column (HIDiC)

Various configurations of a column for performing the principle of HIDiC can be proposed, but a coaxial column which is installed a packing in an inside tube and outer side may be one of the simplest columns for HIDiC. In order to examine whether or not the configuration of the packed column mentioned above is appropriate to HIDiC, an experimental apparatus has been set up and heat and mass transfer rates have been measured by using benzene-toluene system. This paper was presented at The 5th International Symposium on Separation Technology-Korea and Japan held at Seoul between August 19 and 21, 1999.     

Experimental study on pressure drops in a dividing wall distillation column

Previous studies in the fields of process design and process control [1] have shown the potential benefits that can be achieved through the implementation of thermally coupled distillation sequences, in particular, the dividing wall distillation column. The dividing wall distillation column meets important goals of process intensification, including energy savings, reduction in carbon dioxide emissions and miniaturization. In this paper, an experimental study on the hydrodynamic behavior of a dividing wall distillation column is presented. Several different values for gas and liquid velocities were tested in order to measure pressure drops and identify operational regions; the air/water system was used as the basis for the experimental setup. Results regarding pressure drops (fitted to the model of Stichlmair et al.) provide operational limits for the operation of the packed dividing wall distillation column. According to the results, the experimental dividing wall column can be operated at turbulent regime that is associated to proper mass transfer.     

A simple,reliable and fast algorithm for the simulation of multicomponent distillation columns

This work has developed a simple, reliable and fast algorithm for the simulation of multicomponent distillation columns, where any equilibrium stage can accept a feed-stream and/or a vapor-side-stream and/or liquid-side-stream. The new scheme considers internal molar overflows and constant relative volatilities to avoid the need of heat balances and vapor–liquid equilibrium calculations. The solution scheme is founded on a Newton-based formulation in block algebra, which relies in a simple, reliable and fast algorithm. Although the proposed calculation scheme can be classified as an approximate method, it is very useful when accurate phase equilibrium and enthalpy data are lacking. Numerical experimentations show good agreement with the results obtained with well-known rigorous simulation approaches.     

Steady-state operation analysis of an ideal heat integrated distillation column

A systematic approach for the steady-state operation analysis of chemical processes is pro-posed.The method affords the possibility of taking operation resilience into consideration during thestage of process design.It may serve the designer as an efficient means for the initial screening ofalternative design schemes.An ideal heat integrated distillation column(HIDiC),without any reboileror condenser attached,is studied throughout this work.It has been found that among the various va-riables concerned with the ideal HIDiC,feed thermal condition appears to be the only factor exertingsignificant influences on the interaction between the top and the bottom control loops.Maximuminteraction is expected when the feed thermal condition approaches 0.5.Total number of stages andheat transfer rate are essential to the system ability of disturbance rejection.Therefore,more stagesand higher heat transfer rate ought to be preferred.But,too many stages and higher heat transfer ratemay increase the load of the compres     

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

An 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.     

Vapour- and liquid-side volumetric mass transfer coefficients measured in distillation column. Comparison with data calculated from absorption correlations

Volumetric mass transfer coefficients in liquid and vapour phases in distillation column were measured by the method consisting of a fitting of the concentration profile of liquid phase along the column obtained by the integration of a differential model to the experimental one. The mathematical model of distillation process includes mass and energy balances and the heat and mass transfer equations. The film model flux expressions with the convective transport contributions have been considered in the transfer equations. Vapour and liquid phases are supposed to be at their saturated temperatures along the column. Effect of changes of phase flows and physical properties of phases on the mass transfer coefficients along the column and non-ideal thermodynamic behaviour of the liquid phase have been taken into account. The concentration profiles of liquid phase are measured in the binary distillation of the ethanol-water and methanol-ethanol systems at total reflux on metal Pall Rings and Intalox saddles 25 mm in the column with diameter of 150 mm. The distillation mass transfer coefficients obtained by the fitting procedure are compared with those calculated from absorption data using Onda's, Billet's and Linek's correlations. The distillation heat transfer coefficients calculated from the model assuming saturated temperatures in both phases are compared with those calculated from the Chilton-Colburn and penetration model analogy between mass and heat transfer. The results have confirmed an agreement neither between distillation and from absorption correlations calculated mass transfer coefficients nor between analogy and from enthalpy balance calculated heat transfer coefficients. Also the concentration profiles obtained by the integration of the differential model of the distillation column using the coefficients from absorption correlation have differed from the experimental profiles considerably.     

Understanding the dynamic behaviour of middle-vessel continuous distillation columns

The fundamental dynamic behaviour of a middle-vessel continuous distillation column is studied in this paper. The interaction between the middle vessel (MV) design and control parameters is identified by means of a linear analysis. Transfer functions relating the dynamics of the product composition responses to changes in the disturbance inputs and manipulated inputs are derived. The role of the MV holdup and MV level controller gain in altering the product composition responses is identified. It is shown that the MV level controller gain can significantly affect the control performance when feed flow disturbances must be compensated for. However, it has a much lower impact in the control performance when feed composition disturbances need to be rejected; in this case, exploiting the “buffering” effect of the MV is more important for control. The analysis is carried out in detail for a LV control configuration, and is subsequently extended to an “on-demand” DB configuration. Finally, simple guidelines for the choice of the MV level controller gain and MV holdup are provided.     

Generalized modular framework for the synthesis of heat integrated distillation column sequences

The synthesis of simple and Heat Integrated distillation column sequences is a challenging problem of high economic importance due to its structural and physical complexities and to its economic impact on chemical industries. In this paper this synthesis problem is addressed through a novel systematic superstructure representation method extending the principles of the Generalized Modular Framework (Papalexandri and Pistikopoulos, 1996. A.I.Ch.E. Journal 42 (4), 1010-1032). The method proposes systematic structural and physical models addressing the problem's inherent complexities, generating considerably compact optimization problems while avoiding the use of simplifying assumptions. The method employed for ternary and quaternary non-azeotropic separations is demonstrated over illustrating case studies obtaining the most energy efficient distillation column sequences and Heat Integration schemes and generating substantial energy savings.