Azeotropic Distillation Sequences Research Articles

Process Assessment of Heterogeneous Azeotropic Dividing-Wall Column for Ethanol Dehydration with Cyclohexane as an Entrainer: Design and Control

The heterogeneous azeotropic dividing-wall column (ADWC) features significant reduction of capital investment and energy consumption. Conventional azeotropic distillation sequences can be thermally coupled into two types of configurations of ADWCs, namely, the original ADWC with simple azeotropic–recovery section and the other with recovery section also serving as preconcentrator. The ADWC with a combined recovery–preconcentrator section has been investigated in both steady-state design and dynamic controllability. However, the original ADWC has only been proven to be more energy efficient than the conventional azeotropic–recovery column sequence, not compared with the conventional azeotropic–stripper column sequence. Furthermore, the dynamic performance of this ADWC are still unclear. Note that all the benefits of the ADWC are reasonable only under good dynamic controllability. Thus, in this work, we develop a comparative study of optimal design and dynamic performance of the ADWC with two conventional s...

Optimal design of mixed acid esterification and isopropanol dehydration systems via incorporation of dividing-wall columns

This study explores the design of a reactive distillation system and that of a heterogeneous azeotropic system by incorporating dividing-wall column (DWC). The first system involves the esterification of mixed acid (acetic acid and propionic acid) with methanol. Simulation studies are carried out for conventional reactive distillation sequence as well as for reactive dividing-wall distillation system. Both systems are optimized by an iterative optimization procedure. Optimal design results show that the reactive dividing-wall system saves steam consumption by 45.2% and reduces total annual costs (TAC) by 34.5%. The second case investigated is a heterogeneous azeotropic distillation system involving dehydration of crude isopropyl alcohol with cyclohexane as entrainer. Two optimal separation systems are generated, including one with a single-dividing wall column and one with a double-dividing wall column. In comparison with an energy-efficient azeotropic distillation sequence containing two stripping columns by Chang et al. (2012) [1], simulation results show that the former two systems can cut steam usage further by 6.0%. The two systems save about 5.4–6.1% in terms of TAC. DWCs prove to be superior to the convention distillation systems with respect to both cost and energy efficiency.

Synthesis of Ternary Homogeneous Azeotropic Distillation Sequences: 2. Flowsheet Identification

This paper presents a systematic methodology for flowsheet generation for separating binary azeotropic mixtures using homogeneous azeotropic distillation. A new classification system for ternary mixtures using ‘standard distillation line maps’ defined in Sutijan et al. (2012) is employed. The new characterisation system is able to link candidate entrainers to flowsheet structures which can facilitate the separation. The sequences considered include pressure-swing distillation, two and three-column flowsheets with or without boundary crossing and the use of single and double-feed columns. For a given ternary mixture, suitable flowsheet structures that can facilitate the separation can be automatically identified. The method is illustrated using examples.

Economic Optimum Design of the Heterogeneous Azeotropic Dehydration of Ethanol

Several processes are available for the important operation of dehydrating ethanol/water mixtures to concentrations above the azeotrope (90 mol %). Heterogeneous azeotropic distillation has been studied using several entrainers: benzene, cyclohexane, isooctane, ethylene glycol, etc. A pioneering paper by Ryan and Doherty [Ryan, P. J.; Doherty, M. F. Design optimization of ternary heterogeneous azeotropic distillation sequences. AIChE J.1989, 35, 1592–1601.] explored several alternative process configurations and concluded that the three-column flowsheet with a preconcentrator (beer still), an azeotropic column, and a recovery column was the economic optimum. They used approximate ternary diagram methods. It appears that they arbitrarily assumed a beer still distillate composition with an ethanol concentration of about 88 mol %, which is quite close to the azeotropic composition of 90 mol %. Energy consumption in the beer still increases as its distillate composition gets closer to the azeotrope. On the ot...

Synthesis of Ternary Homogeneous Azeotropic Distillation Sequences: Entrainer Selection

This paper presents a methodology for automatic selection of entrainers for separating binary azeotropic mixtures using homogeneous azeotropic distillation. A new classification system for ternary mixtures based on the termini of distillation boundaries and the type (stability) of products and azeotropes is proposed. The new characterisation system is able to link candidate entrainers to flowsheet structures which can facilitate the separation. Existing entrainer selection criteria are extended to accommodate other promising entrainers, including light, intermediate and heavy-boiling entrainers.

Environmental Performance Assessment for Heterogeneous Azeotropic Distillation Partitioned Distillation Column

A heterogeneous azeotropic distillation partitioned distillation column (HADPDC) for alcohol dehydration was investigated compared to a conventional heterogeneous azeotropic distillation sequence (CHADS) from environmental performance perspective by using the software Aspen Plus. HADPDC can eliminate the condenser of the second column and decrease the degree of back-mixing. The optimal design of HADPDC guaranteed the minimum energy consumption, which is related to the minimum CO2 emissions.

Implementation of Ethanol Dehydration Using Dividing-Wall Heterogeneous Azeotropic Distillation Column

In this article, the design and optimization procedures of a dividing-wall column for heterogeneous azeotropic distillation (DWC-A) using cyclohexane as an entrainer for ethanol dehydration are investigated. The proposed procedures can detect the optimal values of the design variables and thereby guarantee the minimum energy requirements, which is related to the minimum CO2 emissions and the lowest total annual cost (TAC). Since ethanol and water form an azeotrope under atmosphere pressure, a conventional heterogeneous azeotropic distillation sequence (CHADS), including an azeotropic column and a recovery column, is usually used to perform the ethanol dehydration process. However, due to high energy requirements and equipment investments of CHADS, the TAC is at a relatively high level. DWC-A can be used to eliminate the condenser of the second column and decrease the degree of back-mixing. Both CHADS and DWC-A are simulated with Aspen Plus®, and the results show that DWC-A has an energy saving of 42.17% and the TAC reduction of 35.18% along with higher thermodynamic efficiency and reduction in greenhouse gas emissions.

Study on Heat Integrated Azeotropic Distillation

In this paper, the design and optimization procedures of a heterogeneous heat integrated azeotropic distillation sequence with side stripper (HIADS-SS) for the purification of isopropanol are investigated. The proposed procedures can detect the optimal values of the design variables and thereby guarantee the minimum energy consumption. The procedures are applied to the study of the separation of azeotropic mixtures using the two distillation sequences. In the HIADS-SS, the top end of the side stripper has both liquid and vapor exchange with the main column, which eliminates a condenser compared with the conventional heterogeneous azeotropic distillation sequence (CHADS). The results show that reductions in energy consumption can be obtained using the HIADS-SS.

Reducing Energy Consumption and CO2 Emissions in Thermally Coupled Azeotropic Distillation

AbstractThe design and optimization procedures of a heterogeneous thermally coupled azeotropic distillation sequence with a side stripper (TCADS‐SS) for the purification of isopropanol has been investigated. The proposed procedures can detect the optimal values of the design variables and thereby guarantee the minimum energy consumption, which is related to the minimum CO2 emissions and the lowest total annual cost (TAC). The procedures are applied to the study of the separation of azeotropic mixtures using the two distillation sequences. In the TCADS‐SS, the top end of the side stripper has both liquid and vapor exchange with the main column, which eliminates a condenser in contrast with the conventional heterogeneous azeotropic distillation sequence (CHADS). The results show that not only reductions in energy consumption and CO2 emissions but also higher thermodynamic efficiency can be obtained for the TCADS‐SS.

Startup of Homogeneous Azeotropic Distillation Sequences with Multiple Steady States

The operability of an azeotropic sequence, which consists of an azeotropic distillation column with multiple steady states and an entrainer recovery column with the corresponding recycle stream, is analyzed by dynamic simulation during its startup. The goal is to study the effect of the recycle stream on the start-up performance. The well-known benzene−heptane−acetone (BHA) system is studied. Given open-loop (manual) start-up strategies, the way in which the system reaches different steady states by applying different start-up configurations and/or policies can be observed through the temperature profile evolutions. These are generated using different initial charges in one or both columns, and by applying different schedules for the evolution of the operative variables. Indeed, a partition of the start-up policy space yielding different solutions is achieved. Therefore, it is possible to identify a set of critical values for the supervision of the startup. Based on these results, very simple closed-loop ...

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.

Multiple Steady States in Heterogeneous Azeotropic Distillation Sequences

In this paper multiplicity in heterogeneous azeotropic distillation sequences is studied. Two sequences, suitable for ethanol dehydration, are treated as sample problems and compared. As a basis the ∞/∞ analysis method (Petlyuk, F. B.; Avet'yan, V. S. Theor. Found. Chem. Eng. 1971, 5, 499−507; Bekiaris, N.; Meski, G. A.; Morari, M. Ind. Eng. Chem. Res. 1993, 32, 2023−2038), which assumes infinite reflux rate and infinite number of trays, is extended to and applied on heterogeneous azeotropic distillation sequences in order to determine steady-state bifurcation diagrams from thermodynamic considerations. The bifurcation diagrams are very different for the two sequences despite their similar structures. In particular, it is predicted that output multiplicity of the single azeotropic column, as recently experimentally verified by Müller and Marquardt (Ind. Eng. Chem. Res. 1997a, 36, 5410−5418), can induce output multiplicity of one sequence. It is further predicted that output multiplicity can be avoided by the choice of a different sequence structure. Furthermore, ∞/∞ analysis predicts state multiplicity in both heterogeneous azeotropic distillation sequences. This phenomenon in the ∞/∞ case is also analyzed and it is shown that state multiplicity in heterogeneous distillation sequences can be induced either by the corresponding single column behavior as reported by Gani and Jørgensen (Comput. Chem. Eng. 1994, 18, 55) or by closing the sequence. The predicted bifurcation diagrams and multiplicities are substantiated through rigorous simulation of column sequences operating at finite reflux and with a finite number of stages. Finally, the implications of the thermodynamics including the liquid−liquid plait point position are demonstrated to be important for obtaining reliable predictions.

Static Multiplicities in Heterogeneous Azeotropic Distillation Sequences

In this paper the results of a bifurcation analysis on heterogeneous azeotropic distillation sequences are given. Two sequences suitable for ethanol dehydration are compared: The ‘direct’ and the ‘indirect’ sequence. It is shown, that the two sequences, despite their similarities, exhibit very different static behavior. The method of Petlyuk and Avet’yan (1971), Bekiariset al. (1993), which assumes infinite reflux and infinite number of stages, is extended to and applied on heterogeneous azeotropic distillation sequences. The predictions are substantiated through simulations. The static sequence performances are compared for minimal impurities in both products, where the direct sequence exhibits output multiplicity, while the indirect sequence exhibits state multiplicity. The latter multiplicity may be avoided by accepting a slightly increased impurity in the ethanol product.

Influence of multicomponent mass transfer on homogeneous azeotropic distillation

Residue-curve and distillation-line maps are useful tools for the design of azeotropic distillation sequences. The calculation of residue curves uses the assumption that the vapour and liquid are always in equilibrium. The computation of distillation lines assumes that the vapour and liquid leaving a tray are in equilibrium. In this work, the equations defining residue curves and distillation lines are extended to take into account mass transfer effects that occur in real distillation columns. A general relationship between the composition of the vapour and liquid leaving a tray at total reflux is developed and used to calculate the maps. The curvature of the residue curves and distillation lines changes, significantly affecting the shape of the maps. The consideration of mass transfer in azeotropic distillation columns produces quantitative changes in the behaviour of the liquid composition path for distillation columns at total reflux, and in some cases, a qualitative change in the products formed can also be observed. No change has been found in the bounds on the product composition at minimum reflux when mass transfer effects are taken into account. The pinch-point curves or distillation limits remain the same. Entrainers should be selected considering not only how much they modify the vapour–liquid equilibrium, but also how much they affect the multicomponent mass transfer that takes place in real distillation columns.

Homogeneous Azeotropic Distillation. 1. Design Procedure for Single-Feed Columns at Nontotal Reflux

Residue-curve and distillation-line maps are useful tools in the design of azeotropic distillation sequences. In this work we make use of residue-curve and distillation-line maps to develop a procedure for the design of single-feed columns at finite reflux. The procedure is noniterative with respect to the reflux and boil-up ratios, instead using an ‘operation leaf' formed by the total and minimum reflux conditions. The operation leaf bounds the possible liquid composition profiles in a column section and is calculated from the product composition. The overlap of operation leaves is a necessary and sufficient condition for column feasibility. Special cases of operation leaves where the liquid composition profile strays outside the leaf are also presented. ‘Open leaves', where the product distillation lines or residue curves and the pinch-point curves end at different nodes are introduced. These open leaves can be used to facilitate the crossing of total reflux boundaries at finite reflux using side-column flow sheets. The use of side condensers and reboilers is shown to reduce the capital penalty in the crossing of total reflux boundaries. The concept of operation leaves can be applied to the retrofit or reuse of existing distillation columns by defining stage composition lines that bound the composition space within the leaf achievable with a given number of stages.

The curious behavior of homogeneous azeotropic distillation—implications for entrainer selection

AbstractWe examine the simplest homogeneous azeotropic distillation sequence of industrial relevance, where an entrainer is added to a binary azeotrope to recover both azeotropic constituents as pure products. Despite its apparent simplicity, such distillation columns can exhibit an unusual behavior not observed in zeotropic distillation: For some mixtures, separation as a function of reflux goes through a maximum. At infinite reflux, no separation is achieved. In some cases, achieving the same specifications with a larger number of trays requires a larger reflux. Sometimes the only feasible separation yields the intermediate component as a pure distillate, while the bottom product contains the light and heavy components. Sometimes the only feasible separation yields the intermediate component as a pure bottom product while the distillate contains the light and heavy components. While these unusual features can be regarded as curiosities, they are essential for proper entrainer selection and design. For a minimum boiling azeotrope, the existing and conflicting entrainer selection rules state that one should use a component that introduces no distillation boundary between the azeotropic constituents (Doherty and Caldarola, 1985), and either a low or high boiling component that introduces no additional azeotrope or a component which introduces new minimum boiling azeotropes (Stichlmair et al., 1989). By taking advantage of the curious aforementioned features, as well as our experience involving more than 400 mixtures, we have been able to analyze the assumptions behind these criteria, show when those assumptions break down, and therefore understand the limitations of the criteria.

Thermal integration of homogeneous azeotropic distillation sequences

AbstractExisting methods for synthesizing thermally‐integrated distillation sequences fail frequently when applied to nonideal and azeotropic systems, because increasing the column pressures often introduces new azeotropes and distillation boundaries into the mixture, which make some separation tasks infeasible. A new thermal integration procedure is presented that combines a bifurcation and residue‐curve map analysis with the methods of Andrecovich and Westerberg. The procedure is demonstrated with two commercially important separations: ethanol‐water‐ethylene glycol and methanol‐acetone‐water. Energy savings of 65% and 40%, respectively, over the optimized nonintegrated sequences are possible.

Design/optimization of ternary heterogeneous azeotropic distillation sequences

AbstractVarious strategies for sequencing heterogeneous azeotropic distillations have been explored. The design and optimization of these systems by simulation‐based methods is generally acknowledged to be a very difficult problem (Kingsley and Lucia, 1988). In this article, a simple and accurate design and optimization procedure has been developed. The method does not involve any recycle convergence loops and has been applied successfully to several test problems. Unlike the simulation‐based methods, our method is robust to changes in the system optimization variables (e.g., reflux ratio in the azeotropic column).In order to demonstrate the method, we present design and optimization results for the dehydration of ethanol using a benzene entrainer. Our optimal designs consume significantly less energy than state‐of‐the‐art designs reported in the literature.

Optimal design and synthesis of homogeneous azeotropic distillation sequences

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTOptimal design and synthesis of homogeneous azeotropic distillation sequencesJennifer R. Knight and Michael F. DohertyCite this: Ind. Eng. Chem. Res. 1989, 28, 5, 564–572Publication Date (Print):May 1, 1989Publication History Published online1 May 2002Published inissue 1 May 1989https://doi.org/10.1021/ie00089a011RIGHTS & PERMISSIONSArticle Views605Altmetric-Citations60LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (928 KB) Get e-Alerts Get e-Alerts