Distillation Boundaries Research Articles

Experimental Verification of the Maxwell-Stefan Formulation in Describing Composition Trajectories During Azeotropic Distillation

Experiments were carried out in a bubble cap distillation column operated at total reflux with the quaternary mixture: water (l)–ethanol (2)–methanol (3)–acetone (4). This system has a binary minimum-boiling azeotrope for the water–ethanol mixture and the distillation boundary is represented by a surface with its corners at pure acetone, pure methanol and the water–ethanol azeotrope. For certain starting compositions the measured distillation composition trajectories clearly demonstrate that crossing of the distillation boundary is possible. In order to rationalize the experimental results, the authors develop a rigorous nonequilibrium (NEQ) stage model, incorporating the Maxwell–Stefan diffusion equations to describe transfer in either fluid phase. The developed NEQ model anticipates the boundary crossing effects, and is in excellent agreement with a series of experiments. In sharp contrast, an equilibrium (EQ) stage model fails even at the qualitative level to model the experiments. The differences in the NEQ and EQ trajectories emanates from differences in the component Murphree efficiencies, which in turn can be traced to differences in the binary pair vapour phase diffusivities. It is concluded that for reliable design of azeotropic distillation columns interphase mass transfer effects must be taken into account in a rigorous manner.

Influence of interphase mass transfer on the composition trajectories and crossing of boundaries in ternary azeotropic distillation

Abstract This paper examines the influence of mass transfer on the composition trajectories in multicomponent azeotropic distillation. Simulations were carried out for three different ternary systems: methanol–isopropanol–water, water–ethanol–acetone, and water–methanol–methylacetate. Two different models were used to calculate the composition trajectories in a tray column: an equilibrium (EQ) stage model and a rigorous nonequilibrium (NEQ) stage model based on the Maxwell–Stefan diffusion equations. The simulations show that the EQ and NEQ model trajectories could follow different composition paths and could end up in completely different corners of the composition space. Furthermore, in all the three case studies the NEQ model trajectory was found to cross the distillation boundary even when the boundary is a straight line. The study has implications for the development of improved separation strategies.

Feasibility of heterogeneous batch distillation processes

AbstractSynthesis of heterogeneous batch distillation is discussed, which aims at splitting azeotropic mixtures by adding an entrainer partially miscible with one of the initial binary mixture components. Key operational parameters are identified such as the amount of entrainer added in the ternary feed, the reflux policy, and the vapor line position by examples. Synthesis and operation are less straightforward for heterogeneous batch distillation than those for the homogeneous case, but offer many advantages: more design alternatives, simplified distillation sequences, a lower consumption of entrainer, and a crossing of distillation boundaries by the still path. Feasibility is assessed using simplified modeling and confirmed using a commercial batch process simulator package. Synthesis expectations and simulated results are verified throughout bench‐plant experiments for the separation of the acetonitrile—water mixture using acrylonitrile as a light heterogeneous entrainer.

The need for using rigorous rate-based models for simulations of ternary azeotropic distillation

Experiments were carried out in a bubble cap distillation column operated at total reflux with the system: water–ethanol–methylacetate. This system has two binary azeotropes (water–ethanol and water–methylacetate), which gives a simple distillation boundary connecting the two azeotropes. All experiments were restricted to the homogenous region without liquid phase splitting. For certain starting compositions the measured distillation composition trajectories clearly demonstrate that crossing of the distillation boundary is possible. In order to rationalize our experimental results, we develop a rigorous nonequilibrium (NEQ) stage model, incorporating the Maxwell–Stefan diffusion equations to describe transfer in either fluid phase and a fundamental description of tray hydrodynamics. The developed NEQ model anticipates the boundary crossing effects, and is in excellent agreement with a series of experiments carried out in different composition regions. In sharp contrast, an equilibrium (EQ) stage model fails even at the qualitative level to model the experiments. The differences in the NEQ and EQ trajectories emanates from differences in the component Murphree efficiencies, which in turn can be traced to differences in the binary pair vapor phase diffusivities Ðy,ij. It is concluded that for reliable design of azeotropic distillation columns we must take interphase mass transfer effects into account in a rigorous manner.

Crossing of the Distillation Boundary in Homogeneous Azeotropic Distillation: Influence of Interphase Mass Transfer

Residue curve maps can be used to predict composition trajectories for packed and trayed distillation columns, provided that vapor and liquid phases are in thermodynamic equilibrium and the column is operating at total reflux. In the case of ternary azeotropic distillations, distillation boundaries divide the composition space into two, or more, regions. It has been mentioned in the literature that distillation boundaries that are straight lines cannot be crossed during actual column operation. The major objective of this paper is to show that interphase mass transfer in ternary azeotropic distillation leads to differences in the component Murphree efficiencies and that such differences can allow even straight-line distillation boundaries to be crossed. Experiments were carried out in a bubble-cap distillation column operated at total reflux. The investigated homogeneous azeotropic system water (1)−ethanol (2)−acetone (3) has a binary minimum-boiling azeotrope for the water−ethanol mixture, which leads to a simple distillation boundary between the azeotrope and pure acetone. Even though the distillation boundary is nearly straight, our experiments clearly demonstrate boundary-crossing phenomena. To rationalize our experimental results, we develop a rigorous nonequilibrium (NEQ) stage model, incorporating the Maxwell−Stefan diffusion equations to describe transfer in either fluid phase. The developed NEQ model anticipates the boundary-crossing effects and is in excellent agreement with a series of experiments carried out in different composition regions. In sharp contrast, an equilibrium (EQ) stage model fails even at the qualitative level to model the experiments. It is concluded that for reliable design of azeotropic distillation columns we must take interphase mass-transfer effects into account in a rigorous manner.

Topological analysis of vapor–liquid equilibrium diagrams for distillation process design

Residues curve and distillation line maps have become an important tool for conceptual distillation process design. These maps are based solely on thermodynamic information on the vapor–liquid equilibrium and are called vapor–liquid equilibrium (VLE) diagrams here. They give, among other, information on the composition regions, in which products of a distillation process can be found. The entire composition simplex can be divided into distinct distillation regions separated by distillation boundaries, which usually cannot be passed in distillative separations. Up to now, VLE-diagrams are mostly used for the well-studied case of ternary systems, for which analysis and visualization is straightforward. However, in many applications, systems containing more than three components have to be considered and, hence, multicomponent VLE-diagrams are needed. In the present work, two approaches are suggested for automated synthesis of VLE-diagrams, which can be applied to any system of interest without limitations to its complexity. The first method is hierarchical and based on a sequential consideration of all constituent subsystems of the system of interest and combining the results to synthezise the VLE-diagram. The second approach is evolutional, because the multicomponent system is considered as it is, but the structure of the diagram is changed from the simplest state of a zeotropic system to the end state with all azeotropes. Both approaches are in principle combinatorial, but the first one needs consistency criteria to discriminate the proposed diagrams, whereas the second one produces always consistent diagrams.

Heterogeneous Entrainer Selection for the Separation of Azeotropic and Close Boiling Temperature Mixtures by Heterogeneous Batch Distillation

The presence of azeotropes in multicomponent mixtures complicates the design of batch distillation separation processes widely used in pharmaceutical and speciality chemical industries. Most of those processes include the use of homogeneous entrainer to ease the separation. We describe novel methods to break azeotropes using an entrainer that is partially miscible with one of the initial binary mixture components. The core of the paper presents the rules for the selection of a heterogeneous entrainer leading to a feasible process for the separation of a minimum or maximum temperature azeotropic binary mixture or a close boiling binary mixture. Through several illustrating examples, we depict some of the advantages of heterogeneous batch distillation processes: more design alternatives for the separation of an azeotropic binary mixture than with homogeneous batch distillation, simplified distillation sequences as a consequence of fewer distillation tasks, crossing of batch distillation boundaries due to t...

Entrainer Selection Rules for the Separation of Azeotropic and Close-Boiling-Temperature Mixtures by Homogeneous Batch Distillation Process

Batch distillation of nonideal mixtures usually produces the azeotropes often associated with those mixtures. Among various techniques available to break azeotropes, azeotropic and extractive distillation processes have a place of choice. They rely upon the selection of a suitable entrainer. Entrainer screening is therefore a key step for the synthesis and design of these processes. From an analysis of all ternary residue curve maps under assumptions of a large number of stages and total reflux/reboil ratio, we devise in this paper a complete set of rules for the selection of a suitable entrainer enabling the separation of minimum- and maximum-boiling azeotropic binary mixtures and close-boiling-temperature binary mixtures. These rules complete previously published rules and expand by many times the set of entrainer alternatives previously considered. Feasible batch distillation processes can always be obtained considering two batch task sequences using rectifier and stripper configurations. The effect of distillation boundary curvature on the selection of entrainers is analyzed, and in this case, a sequence of up to a three batch distillation configurations must be used to separate the original mixture components. Several practical examples are shown to illustrate the application of the defined rules in each studied case.

Analysis of residue curve maps of reactive and extractive distillation units

The aim of this work is to focus in the simultaneous analysis of reactive and extractive distillation, including the solvent recovery system. Residue curve maps can provide valuable insights and design assistance for a variety of separation processes, in which reactive distillation has been incorporated. These graphical techniques reveal the sensitivity of design options by giving us a visual representation over the whole composition space and assist the engineer to detect separation constraints. Simulation results show the reaction effect in the diagram topology: existence and position of nodes, saddles and distillation boundaries (separatrices or surfaces). Advanced distillation synthesis and design techniques, which are now supported by available software tools, were used to develop case studies for extractive distillation, entrainer influence in reactive systems and reactive distillation analysis with consecutive and/or competitive reactions.

Synthesis, experiments and simulation of heterogeneous batch distillation processes

The presence of azeotropes in multicomponent mixtures complicates the design of batch distillation separation processes widely used in pharmaceutical and speciality chemical industries. Most of those processes include the use of a homogeneous entrainer to ease the separation. We describe novel methods to break azeotropes using an entrainer that is partially miscible with one of the initial binary mixture components. We depict some of the advantages of heterogeneous batch distillation processes: more design alternatives for the separation of an azeotropic binary mixture than with homogeneous batch distillation, batch distillation boundary crossing thanks to a controlled reflux of the entrainer-rich phase, simplified distillation sequences as a consequence of less distillation tasks. Three examples based on the separation of non-ideal azeotropic or close boiling point binary mixtures through heterogeneous batch distillation are simulated using a commercial batch distillation package. Experiments validate the simulated separation of a minimum boiling azeotropic mixture.

Crossing of boundaries in ternary azeotropic distillation: influence of interphase mass transfer

This paper examines the influence of mass transfer on the composition trajectories in multicomponent azeotropic distillation. Simulations were carried out for five different ternary systems: methanol — isopropanol — water, water — ethanol — acetone, acetone — chloroform — methanol, acetone — chloroform — ethanol, and water — cyclohexane — ethanol. Two different models were used to calculate the composition trajectories in a tray column: an equilibrium (EQ) stage model and a rigorous nonequilibrium (NEQ) stage model based on the Maxwell-Stefan diffusion equations. The simulations show that the EQ and NEQ model trajectories could follow different composition paths and could end up in completely different corners of the composition triangular space. Futhermore, in some of the case studies the NEQ model trajectory was found to cross the distillation boundary even with the feed on the convex side or when the boundary is a straight line. The study has implications for the development of improved separation possibilities.

Development of a dynamic nonequilibrium cell model for reactive distillation tray columns

In this paper we develop a nonequilibrium (NEQ) cell model to describe the dynamic operation of reactive distillation (RD) tray columns. The features of our model are: (1) use of the Maxwell–Stefan equations for describing mass transfer between fluid phases, (2) chemical reactions are assumed to take place only in the liquid phase, (3) coupling between mass transfer and chemical reactions within the diffusion layer is accounted for, and (4) the use of multiple well-mixed cells in the liquid and vapour flow directions accounts for staging in either fluid phase. When the chemical reactions are suppressed, our model describes the dynamic behaviour of conventional distillation columns. We demonstrate the utility of the dynamic NEQ cell model by means of three case studies: (1) metathesis of 2-propene in an RD column, (2) distillation of methanol– iso-propanol–water, and (3) synthesis of methyl tert-butyl ether (MTBE) in an RD column. For comparison purposes we have also carried out dynamic simulations using the equilibrium stage (EQ) model. These case studies help us to draw the following conclusions. The introduction of staging in the liquid and vapour phases not only influences the steady-state performance, by increasing reaction conversion and separation capability, but also has a significant influence on column dynamics. With multiple cells per stage the column dynamics becomes much more sensitive to perturbations. Furthermore, we show that even when the NEQ cell model and EQ stage model exhibit almost identical steady-state characteristics, the dynamic responses of an RD column could even be qualitatively different. When operating close to the distillation boundary for nonreactive, conventional, distillation, small disturbances in feed compositions can lead to completely different product compositions.

Synthesizing alternative sequences via a P-graph-based approach in azeotropic distillation systems

The present work proposes a P-graph based approach capable of systematically generating all combinatorially feasible alternative sequences for both continuous and batch azeotropic distillation systems when the feeds, products, and operating units and their concomitant intermediate materials are known. To facilitate synthesis, a highly effective method has been established to identify processing or operating units necessary for constructing azeotropic-distillation systems by means of first principles and logical sequencing of such units. The materials represented in the space of the residue curve map are partitioned into lumped materials bounded by thermodynamic boundaries and pinches, e.g. distillation boundaries and liquid–liquid phase equilibrium envelope. Subsequently, the operating units are identified on the basis of these lumped materials. The proposed approach automatically discards the operating units and intermediates whose inclusion will lead to infeasible sequences. The efficacy of the approach is illustrated with a realistic example.

Synthesis of distillation-based processes for non-ideal mixtures

Our understanding of how to design distillation-based processes to separate mixtures displaying azeotropic behavior has grown enormously in the past quarter century. We elegantly sketch distillation column behavior on a composition diagram where we display VLE and column behavior using residue and distillation curves. In the presence of azeotropes, these curves partition composition space into distillation regions that trap the performance of individual columns. We can view liquid–liquid behavior as a tearing of composition space, a tear that always spawns from a minimum binary azeotrope. A material balance across a column section leads to a difference point, which we can use to understand column tray-by-tray and limiting behavior for ordinary, extractive and even reactive distillation. We demonstrate how to synthesize alternative separation processes, concentrating on those that produce pure products. We use boundary curvature, solvent addition, extractive agents, liquid–liquid behavior and strategically placed reaction to step across distillation boundaries. We show that these processes always contain recycles to gain feasibility but also to have economically attractive processes.

Optimal design of complex azeotropic distillation columns

Geometric methods are valuable tools for process synthesis and design since they provide designers with intuition and quantitative results. The calculation and graphical representation of azeotropes, residue curves and distillation boundaries provide a wealth of knowledge about the entire composition space. This paper shows how these tools can be used for optimal design of complex azeotropic distillation columns. Two examples of the design of distillation systems of industrial importance are shown. In the first example, the second feed location of a two-feed distillation column with a top decanter is optimized. In the second example, the side stream draw-stage location of a distillation column with a top decanter and a side stream draw is optimized. In both cases, a feasible and near optimal structure has been designed based on information obtained by examining the whole composition space.

Influence of Mass Transfer in Distillation of Mixtures with a Distillation Boundary

This paper examines the influence of mass transfer on the composition trajectories during distillation of mixtures that exhibit distillation boundaries. Simulations for total reflux distillation of the systems methanol-iso-propanol-water and benzene-iso-propanol-n-propanol show that composition trajectories originating along the distillation boundary could end up in completely different corners of the composition triangle depending on whether or not diffusional interactions are taken into account. Using published experimental data it is concluded that for reliable design and simulation it is necessary to use a rigorous model for mass transfer based on the Maxwell-Stefan diffusion equations.

A general approach to generate distillation regions for azeotropic and heteroazeotropic multicomponent mixtures

The aim of this paper is to show how distillation boundaries can determined. The method developed in this paper can be applied to azeotropic, heteroazeotropic or reactive distillation. Our article is composed of three principal sections, each describing an essential step enabling us to built the distillation maps: determination of the azeotropes, as a function of the temperature and composition, and their nature; obtention and drawing of the residual curves, determination of distillation boundaries. To conclude this paper, some illustrative examples are given.

An aggregated MINLP optimization model for synthesizing azeotropic distillation systems

In this work an aggregated model for homogeneous distillation is proposed. The aggregated model can be considered only an approximation to the actual process, but the results shown that is accurate enough for being useful in the initial stages of the design and is computational convenient. We also introduce a systematic procedure for creating superstructures for homogeneous azeotropic systems. For this we need to known the characteristics of the system, that include the composition and nature of the azeotropes, distillation boundaries (if any) and distillation curves. With all this information we can determine qualitatively which separations are possible and then formulate a superstructure that includes all the possible separations.

Minimum Reflux for Batch Distillations of Ideal and Nonideal Mixtures at Constant Reflux

This paper presents a short-cut procedure for estimating the instantaneous separation performance in batch columns having an infinite number of stages that it is applicable to nonideal mixtures with any number of components. The model allows the estimation of the instantaneous top composition without resorting to stage-by-stage computation and therefore can be used both as a method for rapid simulation of the batch operation and as a procedure for calculating the minimum-energy demand. A key aspect of the method is the determination of the different regimes that can be found during the operation of batch columns. Each one of the different regimes is controlled by a pinch that determines the geometry of the internal profiles. For systems with constant relative volatilities, the presented approach is compared with the use of the Underwood equation to determine the instantaneous separation column performance. We show that both methods are equivalent. This analysis also gives an interpretation of the application of the Underwood equation in the context of batch rectification. Finally, we analyze the application of the model to highly nonideal mixtures with azeotropes and distillation boundaries and briefly discuss the modifications that have to be introduced to extend the method.

Bifurcation study of kinetic effects in reactive distillation

AbstractSimple distillation with multiple chemical reactions of arbitrary stoichiometry is described, with emphasis on effects of chemical kinetics. For low and high Damköhler number, the lightest and heaviest boiling components or azeotropes are known or can be found by known methods. In those limits, the products from a distillation can be anticipated and feasible separation structures can be identified. Feasible structures in these cases are not the same, however, and there are one or more transitions so that different structures can appear for different ranges of the Damköhler number. The transition points correspond to bifurcations, and using bifurcation analysis and continuation methods a systematic method that represents these changes was developed. The practical implication is that the feasibility and product purities for a reactive distillation may depend on production rate, catalyst level, and liquid holdup. The approach is illustrated for the production of methyl‐tert‐butyl ether (MTBE), where a critical Damköhler number for the disappearance of a distillation boundary is found. The influence of pressure on byproduct reactions in methyl acetate synthesis is also studied. A study of the synthesis of isopropyl acetate illustrates how the bifurcation method captures the essential kinetic effects in reactive distillation, including a critical Damköhler for the birth of a reactive azeotrope.