Homogeneous Azeotropic Mixtures Research Articles

A new metric for the evaluation and selection of energy-intensified extractive distillation sequences

Extractive distillation is a widely-used but energy-intensive method for separating homogeneous azeotropic mixtures into pure components. Numerous design alternatives have been proposed for reducing energy consumption in such processes, including alternatives with side-streams, column stacking and thermally coupled sequences. However, there is very little guidance in the literature about which alternative is likely to work the best in a given situation.To address this deficiency, in this work, seventeen industrially-relevant chemical systems comprising two species that form a minimum boiling azeotrope and a heavy entrainer are selected for study. Two ordinary and six energy-saving flowsheet alternatives for extractive distillation processes were also selected. Each of the eight flowsheet alternatives was optimized for each of the 17 chemical systems. A novel metric RCB (the ratio of the entrainer flow rate to the heavy key flow rate) is proposed to facilitate interpretation of the results. General trends in the relative attractiveness of the three energy-saving strategies (side streams, thermal coupling, column stacking heat integration) were found as RCB changes. An understanding of these trends can help engineers to understand the conditions for which various energy-savings strategies are most likely to be effective.

Evaluation of the possibility of separating commercial phenol from the phenolic fraction of coal tar

This article is about the possibility of concentrating commercial coal phenol with a concentration of 99% by the weight method of rectification from the phenolic fraction of coal tar. The sufficiency of phenol is ensured by the kumol method, however, the consumption of phenol increases. Modeling of the vapor-liquid equilibrium of double and triple mixtures of components using the NRTL model showed the presence of the following positive homogeneous azeotropic mixtures: phenol-indane, phenol-indene. Modeling of the vapor-liquid equilibrium of double and triple mixtures of components using the NRTL model showed the presence of the following positive homogeneous azeotropic mixtures: phenol-indane, phenol-indene. The compositions and temperatures of these azeotropes are determined. The authors propose the isolation of phenol from the fraction and its purification from indane and indene by a clear rectification method. The technological scheme consisting of four devices is based on the analysis of the component composition of the initial mixture and the existing azeotropes. Modeling of the technological scheme showed that this scheme provides the release of phenol by 99% of the mass, which meets the requirements for coal phenol. After optimizing the technological parameters of the distillation columns, the estimated extraction costs will amount to 5.64 Gcal per ton of commercial phenol.

Automatic Column Sequencing and Optimization for Homogeneous Azeotropic Mixtures

A method is developed for determining feasible sequences for separating mixtures with azeotropes and optimizing each column in every sequence (including two-column subsequences for azeotropic disti...

Comparative study of Heat Pump Assisted Distillation Column and Its Application for Pressure Swing Distillation Process

Two of the well-known heat pumps assisted distillation column scheme are direct vapor recompression and bottom flashing. The comparison between the two schemes was systematically studied for binary mixture of non-azeotropic benzene-toluene system. Simulation was carried out using Aspen Plus software. The best scheme was selected based on economic evaluation (utility cost and TAC reduction). The application of both schemes was also studied for pressure swing distillation process. Pressure swing distillation is one of the most common methods for separating pressure-sensitive azeotropic system. Direct vapor recompression scheme and bottom flashing scheme can be applied to enhance the energy savings. The minimum-boiling homogeneous azeotropic mixture of methyl acetate-methanol was chosen as representative in process simulation.

Computer-Aided Screening of Ionic Liquids As Entrainers for Separating Methyl Acetate and Methanol via Extractive Distillation

In the production of poly(vinyl alcohol), the raw materials methyl acetate (MeOAc) and methanol (MeOH) exist as a homogeneous azeotropic mixture. Twenty-five kinds of ionic liquids, composed of five types of cations and five types of anions, were studied using the COSMO-SAC method. The σ-profile data for each component and the selectivity at infinite dilution (S∞) were calculated and analyzed, respectively. 1-Hexyl-3-methylimidazolium chloride ([HMIM][Cl]) and 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]) were selected as suitable entrainers based on the COSMO-SAC method. The binary interaction parameters of the NRTL model of the MeOAc/ionic liquid and MeOH/ionic liquid systems were regressed. The conceptual design for the separation of MeOAc and MeOH using ionic liquids as entrainers was investigated. The comparison of two processes using two entrainers was carried out from an economic perspective. The total annual cost (TAC) of the process using [HMIM][Cl] as an entrainer can be reduced by 16.5% com...

Optimization of Pressure-Swing Distillation by Evolutionary Techniques: Separation of Ethanol-Water and Acetonitrile-Water Mixtures

Abstract Complete separation can be achieved in selective homogeneous azeotropic mixtures by exploiting the pressure sensitive nature of the system. In the present work the optimal number of trays, feed location and reflux ratio for sequential column systems encountered in continuous pressure swing distillation (PSD) have been determined by use of two evolutionary techniques. Two industrially relevant systems: ethanol-water and acetonitrile-water have been considered. The Napthali-Sandholm model is solved to obtain the concentration and temperature profiles. The objective is to minimize the total cost using Genetic Algorithm (GA) and Differential Evolution (DE) for the two azeotropic systems. The techniques offer attractive features like applicability to discontinuous and non-differentiable search spaces.

Globally Optimal Networks for Multipressure Distillation of Homogeneous Azeotropic Mixtures

In this article, a methodology for the globally optimal synthesis of a network of vapor–liquid equilibrium flash separators that can operate at multiple pressures and separate an azeotropic mixture is presented. The objective function minimized is the total flow entering the network flashes. The proposed synthesis methodology employs the infinite-dimensional state-space (IDEAS) conceptual framework, which is shown to be applicable to the problem under consideration. The resulting infinite linear programming (ILP) IDEAS formulation is shown to have several properties that allow its simplification. The approximate solution of this IDEAS ILP is pursued through the solution of a number of finite-dimensional linear programs (FLPs) of ever increasing size, whose optimum values form a sequence that converges to the ILP’s infimum. The proposed optimal design methodology is general in nature and can be used to separate any number of pressure-sensitive azeotropic mixtures, with or without use of an entrainer. The method is demonstrated on a first case study involving the dual-pressure separation of a methyl acetate/methanol binary mixture, which exhibits a minimum-boiling azeotrope, without using an entrainer, and a second case study involving the dual-pressure separation of a ternary mixture of water, methanol, and acetone that also exhibits a minimum-boiling azeotrope for the methanol/acetone binary mixture, again without using an entrainer. The IDEAS-generated globally optimal design is shown to be 31.54% better than an optimized, dual-pressure, traditional, two-column design for the binary mixture (case 1) and 15.15% better for the ternary mixture (case 2).

Pressure-Swing Distillation for Minimum- and Maximum-Boiling Homogeneous Azeotropes

Pressure-swing azeotropic distillation uses two columns operating at two different pressures to separate azeotropic mixtures by taking high-purity product streams from one end of the columns and recycling the streams from the other end with compositions near the two azeotropes. This configuration can be economically used when changes in pressure significantly shift the composition of the azeotrope. The larger the shift, the smaller the required recycle flow rates, so the smaller the energy requirements in the two reboilers. Pressure-swing distillation can be applied to both minimum-boiling and maximum-boiling homogeneous azeotropic mixtures. With minimum-boiling systems, the distillate streams are recycled. With maximum-boiling systems, the bottoms streams are recycled. Intuition would lead us to expect that recycling distillate streams would be more energy intensive than recycling bottoms streams. A distillate recycle must be boiled up in the column. A bottoms recycle stream is not boiled up. Therefore, ...

Efficient Optimization-Based Design of Distillation Processes for Homogeneous Azeotropic Mixtures

Rigorous optimization is a valuable tool that can support the engineer to tap the full economic potential of a distillation process. Unfortunately, the solution of these large-scale discrete-continuous optimization problems usually suffers from a lack of robustness, long computational times, and a low reliability toward good local optima. In this paper, the rigorous optimization of complex distillation processes for azeotropic multicomponent mixtures is achieved with outstanding robustness, reliability, and efficiency through progress on two levels. First, the integration within a process synthesis framework allows a reduction of the complexity of the optimization superstructure and provides an excellent initialization by shortcut evaluation with the rectification body method. Second, the reformulation as a purely continuous optimization problem enables a solution with reliable and efficient NLP solvers. Moreover, the continuous reformulation considers a particular tight column model formulation such that the introduction of special nonlinear constraints to force integer decisions could be largely avoided. A careful initialization phase and a stepwise solution procedure with gradually tightened bounds facilitate a robust and efficient solution. Different superstructures for the tray optimization of distillation columns are tested. The methods are illustrated by three demanding case studies. The first case study considers the conceptual design as well as the rigorous optimization of a curved boundary process for the complete separation of an azeotropic four-component mixture. The rigorous optimization of a pressure swing process for the separation of a highly nonideal five component mixture is presented in the second case study. Finally, the third case study covers the rigorous optimization of an extractive separation within a complex column system. All case studies could be robustly solved due to the favorable initialization phase. The continuously reformulated problems required significantly less computational time and identified local optima of better quality as compared to the mixed-integer nonlinear programming techniques (MINLP) solution.

Azeotropic mixtures as mobile phase for thin-layer chromatography of organic fluorescent dyes

Reproducible R f values in the thin-layer chromatography (TLC) of various fluorescent dyes (amino derivatives of naphthoylenebenzimidazole, naphthalimide, benzanthrone, phenalenone, including phosphorus-containing luminophores) were shown to be obtained by elution with binary homogeneous azeotropic mixtures of solvents of different polarity. Some trends in the chromatographic mobility were established governed by the structural features of the groups of compounds under study.

A new energy-integrated pervaporation distillation approach

Hybrid-pervaporation-distillation processes have been intensely studied in the last years and considered as a good alternative in the separation of homogeneous azeotropic mixtures. Compared with the techniques in use in the chemical industry, they can reduce the energy consumption and avoid the use of entrainers. However, due to the evaporation enthalpy, a temperature drop is presented along the membrane and external heat exchangers are required between two consecutive pervaporation steps. In the present paper, a process approach has been developed in order to avoid the external energy support and the temperature drop along the module. The novel energy integration concept utilizes the distillate vapor stream of a distillation column as heating medium in the membrane. The vapor condenses partially and energy is released directly into the module. For the study of the new energy integrated process, a novel membrane configuration is developed and a mathematical model is built. Simulation results show that the new process can yield to higher permeation fluxes and smaller temperature drop in the membrane, what also means a reduction in the required membrane area, if a constant purity in the product is considered. Due to the energy integration concept, there is no need of external heat supply in the pervaporation and the permeate flux rises, which can reduce the operation and investment costs.

Pressure Swing Batch Distillation for Homogeneous Azeotropic Separation

The separation of the homogeneous azeotropic mixture acetonitrile/water by pressure swing distillation (PSD) is considered. In this work, the PSD is operated as a discontinuous (batch) process and two basic batch modes, regular and inverted, are investigated. The processes are analysed and a rigorous dynamic model for both batch PSD processes is formulated. The model takes a cold and empty column as an initial condition. Because of the lack of experimental data, in particular for the inverted batch distillation new experiments were carried out. The regular and the inverted batch PSD consist of two steps, a low pressure and a high pressure run, and experimental results are shown for each step. The simulations fit the experiments with good accuracy. Using the validated model, a simulation study to discuss different batch processes is carried out and an alternative inverted batch process is proposed.

Separation of Homogeneous Azeotropic Mixtures by Pressure Swing Distillation - Analysis of the Operation Performance

The complex separation of a homogenous azeotropic mixture by pressure swing distillation is analyzed based on a rigorous dynamic model considering the system acetonitrile/water. The model is validated with our own experimental data. Investigations showed that the process can be operated in a very stable and robust manner even when large disturbances set the feed concentration to the other distillation region.

Recycle Selection for Homogeneous Azeotropic Distillation Sequences

To recover all the pure components from a homogeneous azeotropic mixture using distillation, one or more recycles are needed. These recycles may have compositions close to those of singular points (i.e., pure components and azeotropes) or may have compositions of mixtures of singular points. The selection of the recycle structure and of the recycle flow rates for a given distillation sequence is a design challenge, as recycles impact on both the feasibility and performance (e.g., the number of stages and the reflux requirement) of separations. This work analyzes recycles with singular point compositions in terms of their effects on the feasibility and performance of splits. On the basis of this analysis, a set of rules and a systematic procedure are developed for selecting recycles with singular point compositions and for identifying promising recycles comprising a mixture of singular points. These rules and procedures allow the recycle structure to be proposed efficiently for a distillation sequence separating a multicomponent homogeneous azeotropic mixture into pure component products.

Shortcut Design Method for Columns Separating Azeotropic Mixtures

A shortcut method is proposed for the design of columns separating homogeneous azeotropic mixtures. Azeotropes are treated as pseudocomponents, and a C-component system with A azeotropes is treated as an enlarged (C + A)-component system. This enlarged system is divided into compartments, where each compartment behaves like a nonazeotropic distillation region formed by the singular points that appear in it. The compartment boundary is linearly approximated. A procedure is proposed for transforming vapor−liquid equilibrium behavior in terms of pure components into that in terms of singular points, allowing relative volatilities to be characterized in terms of singular points. The classical Fenske−Underwood−Gilliland method can then be used to design columns separating azeotropic mixtures. This method is extremely computationally efficient and can be applied to homogeneous azeotropic mixtures with any number of components. The results of the shortcut design method are useful for initializing rigorous simulations using commercial software.

Shortcut methods for nonideal multicomponent distillation: 3. Extractive distillation columns

AbstractExtractive distillation is a common process for the separation of homogeneous azeotropic mixtures with a minimum boiling azeotrope. In this process entrainer feed flow rate and reflux ratio of the extractive column represent the crucial design degrees of freedom that govern feasibility and operating cost. In this work feasibility and operational stability of the extractive column are related to the nonlinear analysis of the extractive section. A fully automated conceptual shortcut design method for the simultaneous determination of minimum entrainer feed flow rate and minimum reflux ratio is presented. The method is applicable to arbitrary multicomponent mixtures and allows direct screening of design alternatives and comparison of different entrainers. © 2004 American Institute of Chemical Engineers AIChE J, 50: 1129–1149, 2004

Influence of unequal component efficiencies on trajectories during distillation of a homogeneous azeotropic mixture

The overall objective of this work is to examine the influence of interphase mass transfer on the composition trajectories in homogeneous azeotropic distillation. A total of 38 experiments were carried out in a bubble cap distillation column operated at total reflux with the system: water–ethanol– tert-butanol. The experiments were carried out in the two regions on either side of the distillation boundary connecting the ethanol–water and t-butanol–water azeotropes. In order to model the composition trajectories, a rigorous nonequilibrium (NEQ) stage model is developed. The NEQ model incorporates the Maxwell–Stefan diffusion equations to describe the intraphase transfers in the vapor and liquid phases. The only adjustable parameter in the NEQ model is the size of the vapor bubbles on trays. A choice of a bubble diameter of 4 mm in the developed NEQ model gave the best agreement with the experimental results for all of the 38 experimental runs. The Murphree efficiencies of the constituents in the ternary mixture were found to be significantly different from one another for all the runs. In order to ascertain the influence of unequal component efficiencies on the column composition trajectories, the experimental results were also simulated with an equilibrium (EQ) stage model assuming a uniform, constant efficiency for all components on all the trays. The value of this constant efficiency for any experimental run was obtained by averaging the individual component efficiencies for all the three components on all the trays, calculated by the rigorous NEQ model. The predictions of the EQ model leads to significantly worse predictions of the column composition trajectories for each of the runs, when compared to the NEQ model. It is found that the column composition trajectories are significantly altered due to differences in the component efficiencies. From a design view point, it is shown that for the water–ethanol– tert-butanol system, the attainment of a desired ethanol purity in the top product may require significantly larger number of stages than that anticipated by the EQ model incorporating constant component efficiencies.

A New Catalog of the Most Promising Separation Sequences for Homogeneous Azeotropic Mixtures I. Systems without Boundary Crossing

This paper presents a systematic procedure for generating a catalog of the most promising separation sequences for homogeneous azeotropic mixtures without boundary crossing. First, the azeotropic mixtures are classified into classes of azeotropic systems. This is followed by the generation of feasible distillation sequences for each class of azeotropic system. The sequences generated are screened using new heuristics developed here by means of geometric reasoning, as well as some well-established sequencing heuristics, to give the most promising distillation sequence for each class of azeotropic system. The procedure results in a new catalog of classes of azeotropic systems with their corresponding most promising separation sequences.

Design and economic optimization of azeotropic distillation processes using mixed-integer nonlinear programming

Distillation is the most important separation technique even for nonideal and azeotropic mixtures. A hybrid approach for the design and optimization of such distillation processes using thermodynamics and mathematical programming is presented. In a first step, a superstructure is derived from a knowledge of the feed composition and the phase equilibrium. The basic idea is to establish a superstructure that consists of a sequence of preferred separations, i.e.distillations with minimum energy input. Thus, an upper bound on the number of columns is determined. The column interconnections allow for direct heat integration and recycling of intermediate fractions. This ensures that even complex column structures are included. In a second step the rigorous stage-model of the superstructure is optimized by means of mixed-integer nonlinear programming (MINLP). The objective of the mixed-integer optimization is a function based on a detailed description of both the capital and the utility costs. Optimizing the whole set of variables with respect to total annualized costs leads to an economic optimal process design. The methodology is applicable to zeotropic as well as to homogeneous azeotropic mixtures. Superstructures for ternary and quaternary azeotropic mixtures and examples for the optimization of such a superstructure are given.

Superstructures for the mixed integer optimization of nonideal and azeotropic distillation processes

Distillation is the most important separation technique even for nonideal and azeotropic mixtures. A methodology for the synthesis of superstructures for the structural optimization of such distillation processes by means of mixed integer nonlinear programming (MINLP) is presented. The superstructure is derived knowing the phase equilibrium and the feed composition. The basic idea is the description of the distillation columns under minimum reflux conditions by the model of the preferred separation. Each column is supposed to perform a nonsharp split at minimum reflux ( preferred split). Thus, an upper bound on the number of columns can be determined. The column interconnections allow for direct heat integration and recycling of intermediate products. This ensures that even complex process alternatives are included in the superstructure. The methodology is applicable to zeotropic and homogeneous azeotropic mixtures with at least three components. Examples for the separation of a ternary mixture with a minimum azeotrope and a distillation boundary are given.