Multicomponent Distillation Column Research Articles

A multirate parallel-modular algorithm for dynamic process simulation using distributed memory multicomputers

The use of a distributed memory message-passing multicomputer for dynamic simulation of chemical processes is investigated. The computational speed-up expected from the parallel implementation of the modular integration approach depends crucially on appropriate partitioning of the overall system into smaller sub-systems (modules), and the need for coordination and synchronization of module integrations to account for the interaction effects among the sub-systems. In this paper, a coordination algorithm based on a dynamic block Jacobi-like iteration is employed. Sufficient conditions for the finite-time convergence of the iterative solution scheme are established. These conditions are shown to be very useful in quantifying the convergence rate that can be used as a basis for adjusting the integration time horizon and selection of the best process system partitioning strategy. Timing results from simulation of the dynamics of a multicomponent distillation column on a distributed memory message-passing multicomputer demonstrate the potential of the proposed implementation. The results suggest that the execution time of modular dynamic process simulation can be reduced very significantly by distributing the module integrations onto several computing nodes communicating via high-performance communication networks.

A comparison of the equilibrium and nonequilibrium models for a multicomponent reactive distillation column

Apropriate models and computational algorithms were developed and a series of simulations for a reactive distillation tray column were performed in order to compare the equilibrium and nonequilibrium model. The homogeneous liquid phase reversible reaction between acetic acid and ethanol in production of ethyl acetate was employed as a test chemical reaction. Both the Newton–Raphson and homotopy-continuation methods were applied to solve the set of model equations. The homotopy-continuation method was found superior to the Newton–Raphson method in guaranteeing the desired solution for the nonequilibrium model. The comparison between the equilibrium and the nonequilibrium model reveals that the nonequilibrium model is to be preferred for the simulation of a tray column for reactive distillation because of the difficulty in the accurate prediction of tray efficiency for the equilibrium model. Close agreement in the prediction of the two models arises only when the tray efficiency can be correctly guessed for the equilibrium model. The effects of reaction rate and reflux ratio on the column performance were illustrated using the nonequilibrium model. The use of the heat generated by the exothermic reaction as an internal heat source in the column is discussed.

A composition estimator for multicomponent distillation columns — development and experimental test on ternary mixtures

A nonlinear extended Kalmanfilter has been developed to infer the outlet composition of a ternary distillation column from temperature measurements. The reliability of the developed estimator is discussed with respect to disturbances on either steam, reflux and feed flowrates. It is shown that the accuracy of the adopted vapor–liquid equilibrium model plays a crucial role. The performance of the estimator is tested by comparison with actual outlet compositions measured in a pilot plant, where the separation of a ternary mixture of ethanol, tert-butanol and water is carried out.

Inferential Control System of Distillation Compositions Using Dynamic Partial Least Squares Regression

In order to control product compositions in a multicomponent distillation column, the distillate and bottom compositions are estimated from on-line measured process variables. In this paper, inferential models for estimating product compositions are constructed using dynamic Partial Least Squares (PLS) regression, on the basis of simulated time series data. It is found that the use of past measurements is effective for improving the accuracy of the estimation. The influence of selection of measurements and sampling intervals on the performance is also investigated. From the detailed dynamic simulation results, it is found that the cascade control system based on the proposed dynamic PLS model works much better than the usual tray temperature control system.

Nonlinear Model-Based Control of a Batch Reactive Distillation Column

The inherent trade off between model accuracy and computational tractability for modelbased control applications is addressed in this article by the development of reduced order nonlinear models. Traveling wave phenomena is used to develop low order models for multicomponent reactive distillation columns. A motivational example of batch esterification column is used to demonstrate the synthesis procedure. Tight control of the column is obtained with the use of reduced model in a model predictive control algorithm, normalsize

Rigorous simulation of conventional and unconventional multicomponent batch distillation behavior for startup and production periods

A comparative study applied to multicomponent batch distillation using, for the reactive cases, conventional and unconventional column configurations is presented. Many other works in the literature have shown similar studies but much of them did not compare with experimental data nor use a rigorous software package either. Here the program called (ctive istillation namic imulator) is used for rigorous simulation purposes. This package permits the solution of a wide range of distillation problems accounting for simultaneous design, operation and control analysis, for single and multiple duties. Four example problems, two of them involving a reactive batch distillation column for the ethyl acetate production and its inverse configuration respectively. This last case was specially developed for this work in order to study such a system starting from the experimental data of the first case. The other two examples correspond to conventional multicomponent batch distillation columns, one of them using an optimal reflux ratio profile and operation conditions. The data employed for three examples are taken from experimental cases published in the literature. In addition the predicted dynamic behavior of a batch distillation column for separating a ternary mixture, obtained by, is compared with the results given by two well-known commercial software in order to evaluate its performance for implementing them in an inferential control structure inspired in the extended Luenberger observer design philosophy.

Multicomponent Distillation Design through Equilibrium Theory

A new tool for the design of multicomponent distillation columns is presented, which is based on analytical solutions of a suitable mathematical model. The assumptions on which it is built are (i) a large number of stages, which makes it possible to apply a continuous description of the column instead of the usual stage-by-stage equations; (ii) constant molar overflow; (iii) constant relative volatility; and (iv) attainment of vapor−liquid equilibrium everywhere along the column. The resulting model equations are solved in the frame of Equilibrium Theory, which was originally developed to describe chromatographic processes. Through this approach explicit results may be obtained and both the steady state and the dynamic behavior of high-purity columns can be analyzed. In particular, this work focuses on the former, thus obtaining the following results: (i) a complete picture of the different separation regions in the operating parameter plane spanned by the flow-rate ratios in the rectifying and stripping...

IDENTIFICATION OF THE TEMPERATURE-PRODUCT QUALITY RELATIONSHIP IN A MULTI-COMPONENT DISTILLATION COLUMN

In this paper, we show that in finding a mathematical expression to predict the relationship between temperatures measured inside a multi-component distillation column and the quality of the produced product at the top of the column, the application of a recently developed systematic procedure to identify Wiener nonlinear systems [20], supports the user in retrieving from the data accurate information about both the structure and initial parameter estimates of the model to be identified with iterative parameter optimization methods. This property enables the user to improve his prior knowledge instead of being dependent on it for getting parameter estimates as is the case in most existing parametric identification methods, A consequence of this dependency is that wrong prior information leads to models with poor prediction capability on one hand, and very little information on the other hand on how to modify the model structure in order to get improved results. The latter often results in very time consuming “trial-and-error” approaches that furthermore may yield poor results because of the possibility of getting stuck in local minima. The outlined approach has the potential to overcome these drawbacks. One common source of the use of wrong prior knowledge in the identification of multi-component distillation columns is the presence of a static nonlinearity of exponential type that can be removed by taking the logarithm of the measured product quality. It is shown that this “trick” to linearize the system decreases the accuracy of the predicted producted quality. The outlined approach is also compared to a simple NARX neural network black-box identification method that have the potential to approximate general nonlinear input-output behaviours. This comparison shows that the neural network approach easily requires twice as much observations compared to the Wiener identification approach applied in this paper when the variance of the predicted product quality needs to be the same. The real-life measurement used in this paper were collected at a refinery of the Dutch State Mines (DSM). Finally, in order to use the model obtained with one (training) data set under other operational conditions, that is to extrapolate the model a simple observer design is discussed and validated with real-life measurements.

Simultaneous optimization of design and operation of multicomponent batch distillation column—single and multiple separation duties

A method is presented for the simultaneous optimization of a batch distillation column design and its operation, for single and multiple separation duties, each involving different multicomponent mixtures and complex operations with intermediate cuts. For operation structures selected a priori, the formulation presented permits the use of general distillation design and cost models. The objective function and constraints include capital and operating cost. In particular, the number of internal plates is optimized along with the most significant operating variables (recoveries in various cuts and reflux ratio profiles and times). The multiple duty formulation presented accounts for the different importance of each duty and setup time between batches. Application of the method to single duty multicomponent separation from the literature shows that significant profit improvements can be achieved within acceptable computing times. For multiple separation duties (two binary mixtures), the method clearly shows the importance of including allocation time to each duty and setup time for each batch in the objective function.

Some Aspects in the Design of Multicomponent Reactive Distillation Columns with a Reacting Core: Mixtures Containing Inerts

Some aspects related to the design of reactive distillation columns with a reacting core are addressed in this paper. A set of transformed composition variables proposed in a previous paper is used to develop both the design equations and the reactive residue curve maps for quaternary reacting mixtures. Also, conventional residue curve maps are employed to achieve a better physical and conceptual insight into the relationships between the different constitutive parts of a column with a reacting core. Limitations in product specifications of entirely reactive distillation columns are explained by means of reactive residue curve maps, and a new feasibility criterion of a given separation for reactive columns with a reacting core is also developed on the basis of the two extreme operation conditions of a column. Finally, the behavior of column profiles for both high-conversion and low-conversion cases is analyzed and optimal product specifications are suggested to obtain high purity products. As we concluded in previous works, the inerts have a central role in both the design and synthesis of a reactive distillation column. All the concepts are applied to azeotrope-forming mixtures.

Evolution of Pinch-Point Zones in Multicomponent Distillation Columns. 1. Rectifying Section

The principles concerning the evolution of pinch-point zones with variation of the reflux ratio in the rectifying section of a multicomponent distillation column with an infinite number of stages are presented in this work. Analytical formulas are obtained for calculating the pinch-point zones as well as the distillate exactly. It has been found that the evolution of pinch-point zones in the stripping section is much similar to the evolution of pinch-point zones in the rectifying section. Based on these new findings, a new method for calculating the minimum reflux ratio of multicomponent continuous distillation, as well as a new shortcut method for the design calculation of a batch distillation process, can be developed.

Simulation of Multicomponent Batch Distillation with Chemical Reactions Processes

A well proven multicomponent, reactive, continuous distillation dynamic model has been enhanced to predict batch operations behavior, including simultaneous chemical reactions. This program has been implemented for personal (80486 based) microcomputers in a package called (ctive istillation namic imulator). Results obtained with the original model applied to multicomponent continuous distillation, startup, shutdown and multiple columns arrangements (distillation trains) were published in several recent papers, demonstrating its good performance. Due to the actual revival in batch operations interest the model has been adapted to handle batch reactive distillation operations. Simulated results for two examples considering both startup and production periods, are presented.

Composition Estimators for a Distillation Column: Theory and Experiments

This work is a contribution to the development of non-linear estimators based on the Extended Kalman Filter theory to monitor the dynamics of a continuous distillation column. The performances of the estimators are checked by comparison with the behavior of a pilot distillation unit where fractionations of ethanol-water binary mixtures are carried out. A simplified model of the column is presented, based on the orthogonal collocation approach, in order to extend the applicability of the estimator also to multicomponent distillation columns with high number of stages.

Modelling mass transfer in multicomponent distillation

This paper highlights inconsistencies in the modelling of mass transfer in multicomponent distillation columns. Models of flow and mass transfer are developed that seek to avoid these inconsistencies based on the usual assumptions of plug flow of vapor and of plug flow or axial dispersion of the liquid. These models are used together with a nonequilibrium model of trayed distillation columns. It is shown that the revised models are capable of sensible predictions of column performance. Methods of evaluating binary (Maxwell—Stefan) mass transfer coefficients are evaluated and it is found that mass transfer on sieve trays is best modelled with the Chan and Fair correlation. Mass transfer is multicomponent distillation should be modelled using the Maxwell-Stefan equations; simpler approaches may be significantly less demanding of computer time but columns designs based on such methods can be very different from those obtained using the more rigorous approach.

Some aspects in the design of multicomponent reactive distillation columns including nonreactive species

Some aspects related to the design of reactive distillation columns are addressed in this paper. A new set of transformed composition variables is proposed for mixtures including one or more components that are inert under the process conditions. This set corresponds to an extension of that suggested by Barbosa and Doherty (1988a, Chem. Engng Sci. 43, 1523–1537) and allows to compute by means of any traditional procedure, the concentration profiles along the column and therefore, the minimum reflux ratio. The compositions of product streams of a reactive distillation column are subject to constraints of thermodynamic nature that can be determined before any attempt to design the column. These thermodynamic constraints do not have a counterpart in conventional distillation and become an essential piece of information in order to select the design variables and specify their values. A parametric analysis of the simultaneous chemical reaction and liquid-vapor equilibrium is suggested as a very useful instrument to select thermodynamically feasible design specifications. In this work, we also present an initial discussion about possible columns sequences to obtain the reaction product free of inert species. The selection and calculation of the relevant variables that are common in the first steps of the design and synthesis of reactive distillation processes are discussed through examples. Finally, throughout the entire paper several interesting physical and operational conclusions regarding inert components in reactive distillation are given. The main conclusion is that the inerts play a key role in the design of a reactive column.

Screening tools for robust control structure selection

Screening tools for control structure selection in the presence of model/plant mismatch are developed in the context of the Structured Singular Value (μ) Theory. The developed screening tools are designed to aid engineers in the elimination of undesirable control structure candidates for which a robustly performing controller does not exist. The screening tools are examined on a multi-component distillation column control problem and compared with previously published methods such as the Condition Number Criterion.

Design and Control Selections of Cascade Loops in Distillation

A method to design cascade control schemes in multicomponent distillation columns when top and bottom compositions are controlled, is presented. It assists in the choices of the loop where the cascade control has to be located and of the pilot tray of the column. The method is based on two parameters which have a clear physical meaning and are computed from a steady state analysis. It is illustrated by its applications to two refinery columns: a gasoline stabilizer and a GPL splitter. A comparison of results allows to point out advantages of a correct selection of cascade control, according to the proposed method.

Multicomponent Distillation Column Design: A Semi‐rigorous Approach

AbstractA new semi‐rigorous approach for the design and simulation of multicomponent distillation columns is presented which combines a rigorous Naphtali‐Sandholm algorithm with Edmister's shortcut method. The column is assumed to comprise of single and multi‐tray units with the number of trays inside the multi‐tray unit treated as an iterative variable. The assumption of linear variation of flows inside a multi‐tray unit made by earlier workers (which often lead to convergence difficulties or erroneous results) has been replaced with a more rigorous calculation procedure. Examples are presented to demonstrate the usefulness of the new method for both design and simulation calculations. The semi‐rigorous model equations can be solved directly to obtain a reasonably accurate and practical design, whereas most other available methods require repeated calculations. Computer memory requirement is drastically reduced in this semi‐rigorous formulation as compared to rigorous methods.

多成分系蒸留システムの熱解析

The energy analysis of distillation systems, especially the identification of heating and cooling zones in a column, is useful in the application of various energy-saving technologies to the systems. A method of analyzing the heat demand and supply for a binary system has been established, but there are few practical applications for multicomponent systems. One of the reasons is that the installation of an intermediate heat exchanger in a multicomponent distillation system changes the composition profile. We propose an energy analysis method for multicomponent distillation systems. With this method we defined appropriate locations of an intermediate heat exchanger, that is, heating and cooling zones, and its maximum exchangeable energy load. Especially, the relationships between the patterns of heating and cooling zones and the locations of pinch zones are investigated. As a result, the upper and lower hypothetical pinch zones in a multicomponent distillation column terminate the heating zone at lower temperature, and the cooling zone at higher temperature, respectively. The conclusions are generalized by using graphical representations of the composition profiles

Experimental evaluation of a multivariable adaptive nonlinear predictive controller

Abstract A new multivariable adaptive nonlinear predictive controller is designed using a general nonlinear input-output model and variable transformations. The controller is similar in form to typical linear predictive controllers can be tuned analogously or by specifying a single parameters for each controlled variable. In addition, the design procedure is computationally efficient. The new controller is compared to a multi-loop proportional-integral (PI) controller with one-way static decoupling and to an adaptive linear predictive controller through tests on a simulated nonlinear distillation column. The new controller performed well in an experimental application to a multicomponent distillation column.