Chemical Engineering Science
Volume 60, Issue 11,
June 2005
, Pages 2895-2909
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Abstract
A feasibility analysis is presented for the separation of close-boiling and azeotropic (minimum- and maximum-boiling) binary mixtures into pure components by the addition of an entrainer introducing a heterogeneous azeotrope. The analysis is done for both the conventional batch rectifier and the multivessel batch column. The analysis is theoretical and based on the assumptions of total reflux/reboil ratios and infinite number of stages. Two feasibility conditions are formulated that make it possible to investigate feasibility based on information coming solely from the distillation line map along with the binodal curve of the ternary mixture. Serafimov's classification is used for classifying the azeotropic phase diagrams. The feasibility analysis provides the necessary background and information for formulating rules for entrainer selection for the process. Two simple rules are then proposed, which make it possible to “screen” entrainers for heteroazeotropic batch distillation with minimum efforts.
Introduction
The separation of azeotropic and close-boiling mixtures is often faced in the organic chemical industry. Batch distillation is by far the most common unit operation in the pharmaceutical and fine/specialty chemical industries, where the production quantities are small and the objective and specifications of the separation task are often changing. Thus, investigating the possibilities of separating azeotropic and close-boiling mixtures in batch distillation columns is of great importance.
Distillation of binary azeotropic and close-boiling mixtures (AB) into pure components (light component A and heavy component B) requires the addition of a third component, the so-called entrainer (E), that enhances the separation. When the entrainer is heavy and is added continuously in the top section of the batch column, the process is called extractive batch distillation. When the entrainer forms a homoazeotrope with at least one of the original components and is added batchwise to the original mixture, the process is called homogeneous azeotropic or homoazeotropic batch distillation. When the entrainer forms a binary heteroazeotrope with at least one (and preferably with only one) of the original components or a ternary heteroazeotrope and is added batchwise to the original mixture, the process is called heterogeneous azeotropic or heteroazeotropic batch distillation. The topic of this work is heteroazeotropic batch distillation.
In another paper (Skouras et al., 2005),we presented a detailed analysis of the heteroazeotropic batch distillation process in the batch rectifier and the multivessel column together with simulation results. The two column configurations are shown in Figs. 1a and b, respectively. We mentioned that heteroazeotropic batch distillation, in the wide meaning of the term, can be performed under two operational modes which we called modes I and II. Mode I is a hybrid process, i.e., a combination of two different separation methods (homogeneous distillation and liquid–liquid split) realised in sequence. The column is refluxed with a mixture of both immiscible phases in the decanter and the liquid–liquid split is not introduced until at the end of the distillation step. Thus, mode I is governed by the rules of homoazeotropic distillation, while the post-operational split of the heteroazeotrope in the decanter gives us the additional possibility to “break” the azeotrope at the column top. On the other hand, mode II is a hybrid process, i.e., a combination of two different separation methods (distillation and liquid–liquid split) realised simultaneously. The liquid–liquid split is performed during the distillation step and we can reflux and withdraw or accumulate any combination of the two decanter phases. Thus, mode II is governed by special laws and is a more flexible process than mode I of heteroazeotropic distillation and homoazeotropic distillation, as was shown by the simulation results. We also analysed different separation strategies, “strategy A” and “strategy B”, for mode II of the process that were first mentioned by Koehler et al. (1995).
The studies in entrainer selection for heteroazeotropic batch distillation are limited, but valuable insight can be gained by the related literature for continuous columns. Pham and Doherty (1990a) studied the synthesis of continuous heteroazeotropic distillation and presented some general principles which could be used for distinguishing between feasible and infeasible entrainers for the process. An entrainer was considered to be feasible if the resulting residue curve map provided a feasible column sequence. Furzer (1994) screened entrainers for the process from a different point of view. The UNIFAC group contribution method was used for synthesising efficient entrainers for the heterogeneous dehydration of ethanol. Simple heuristic rules were developed that could be used in a knowledge database of an expert system and limit the extensive search of molecules that could be used as entrainers.
Rodriguez-Donis et al. (2001) were the first to provide entrainer selection rules specifically for batch columns. They pointed out that the rules for continuous columns can be only used as a basis for batch columns as they do not cover all the possible cases. This is because heteroazeotropic batch distillation is more flexible than its continuous counterpart. They studied all possible residue curve maps of heteroazeotropic mixtures under the assumptions of total reflux/total reboil ratios and infinite number of stages. The classification of Matsuyama and Nishimura (1977) with its 113 classes, which was later extended to 125 classes by Foucher et al. (1991), was adopted. The complete set of rules for the feasible entrainers was tabulated in tables.
The feasibility analysis by Rodriguez-Donis et al. (2001) relates to heteroazeotropic batch distillation with reflux of one or both immiscible phases in the decanter. Stripper configurations were also considered in their work. In contrast, the entrainer selection rules formulated in our work relate to reflux of the entrainer-rich phase only and no stripper configurations are considered. We will further comment on these issues later in our paper in order to better illustrate the differences between the two analyses.
Conclusively, our feasibility conditions and entrainer selection rules are a particular case of the more general feasibility analysis by Rodriguez-Donis et al. (2001). On the other hand, Rodriguez-Donis et al. presented many examples of feasible entrainers for the process, but did not formulate well-defined entrainer selection rules that would make it easy for someone to “screen” entrainers. Our objective is to formulate simple and clearly defined entrainer selection rules that can be used for preliminary “screening” of feasible entrainers with minimum efforts.
In a recent paper, Modla et al. (2003) presented results for heteroazeotropic and heteroextractive distillation in a batch rectifier. The separation of a close-boiling mixture by using a heavy entrainer (Serafimov's class 1.0–1b) was investigated. First, the feasibility of the process was addressed and then results from rigorous simulations verified the theoretical findings. The main findings of their feasibility analysis are in agreement with ours, as it will become obvious in the main parts of our paper.
By “feasibility” in this paper, we mean recovering the original component (B or A) miscible with the entrainer in pure form in the still, while the original component (A or B) immiscible with the entrainer and involved in the heteroazeotrope is recovered at the composition of the entrainer-lean phase ( or ) in the decanter. It is possible that a subsequent distillation task is required in order to recover a pure original component (A or B) from the entrainer-lean phase ( or ). This issue is discussed in the paper, but it is not covered by the feasibility conditions and entrainer selection rules developed. Moreover, it is usually not required to recover pure entrainer E, since it can be recycled to the next batch. However, cases where pure entrainer E can be recovered are also discussed.
Our objective is to derive simple conditions for feasibility that do not require a detailed analysis. The basis for this simplified analysis is the distillation line map along with the binodal curve of a ternary mixture. We initially use a working example in order to illustrate the principles of our feasibility analysis (Section 2). First, the differences in the feasibility regions for modes I and II of heteroazeotropic distillation are shown. After this we focus on mode II and illustrate the feasibility for separation strategies A and B in the rectifier column and the multivessel column. In Section 3, two general feasibility conditions are formulated that enable us to investigate feasibility based on minimum information coming from the distillation line map along with the binodal curve of the ternary mixture. In Section 4, we present the results from checking feasibility for various ternary diagrams. The original binary mixture (AB) can be (a) close-boiling, (b) minimum homoazeotropic and (c) maximum homoazeotropic and, in each case, the addition of various entrainers is investigated. These results provide the necessary background for the formulation of simple entrainer selection rules that can be used for preliminary “screening” of feasible entrainers for the process. These rules together with some guidelines for entrainer selection are given in Section 5.
Section snippets
Feasibility analysis for the working example
In this section, we present the principles of our feasibility analysis. The principles are general and apply to all mixtures studied in this paper. However, a working example is used for illustrative reasons. First, we discuss the feasibility for modes I and II of heteroazeotropic batch distillation mentioned in the introduction. Second, the feasibility for separation strategies A and B in the rectifier column and also in the multivessel column is addressed.
Suppose that an initial close-boiling
General feasibility conditions
In this section we attempt to address the question: “Given a distillation line map along with the binodal curve of a ternary mixture, how can we check if the separation is feasible”? We want to know, at a preliminary stage, if a separation is feasible without doing all the detailed feasibility analysis. Of course, if the separation is feasible, the detailed analysis should be done, in a later stage, in order to identify feasible regions, initial feed location, minimum amount of added entrainer,
Feasibility results for various cases
In this section, the validity of feasibility conditions 1 and 2 is checked for the distillation line maps of various mixtures. If the conditions are satisfied, the separation is feasible.
The following three general cases were studied:
Case a: The original mixture (AB) is close-boiling. Ten cases were analysed and the results are shown in Table 1.
Case b: The original mixture (AB) has a minimum-boiling homoazeotrope (AzAB). Nine cases were analysed and the results are shown in Table 2.
Case c: The
Entrainer selection rules
The objective of this section is to address the following issue: “Formulate some simple rules that enable us to screen entrainers for the process with minimum effort”. Based on the feasibility conditions 1 and 2 in Section 2 and the feasibility results in Table1, Table2, Table3, the following rules were formulated:
Entrainer selection rule 1: The entrainer (E) should form a heteroazeotrope (AzEA or AzEB) with one of the original components (A or B) and/or a ternary heteroazeotrope (AzEAB).
Conclusions
A feasibility analysis for heteroazeotropic batch distillation with reflux of the entrainer-rich phase only is provided for the rectifier and the multivessel column. The analysis is theoretical and based on the assumption of infinite reflux/reboil ratios and infinite number of stages. Under these assumptions, only information coming from the distillation line map and the binodal curve of the mixture is necessary for investigating feasibility. Two feasibility conditions were proposed for this
Notation
A light original component AzAB binary azeotrope of the two original components A and B AzEA binary azeotrope of the entrainer and the original component A AzEAB ternary azeotrope of the entrainer and the original components A and B AzEB binary azeotrope of the entrainer and the original component B B heavy original component D final product in the decanter when strategy B is implemented E entrainer F feed entrainer-lean phase final product of the entrainer-lean phase in the decanter in cases a5, a6, a9, b5,
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