Azeotropic entrainer selection

The majority of successful processes are those in which the entrainer and one of the components separate into two Hquid phases on cooling if direct recovery by distillation is not feasible. A further restriction in the selection of an azeotropic entrainer is that the boiling point of the entrainer is 10—40°C below that of the components. 

Wahnschafft OM and Westerberg AW (1993) The Product Composition Regions of Azeotropic Distillation Columns n. Separability in Two-Feed Columns and Entrainer Selection, Ind Eng Chem Res, 32 1108. 

I. Rodriguez-Donis, V. Gerbaud, X. Joulia, Entrainer selection rules for the separation of azeotropic and close boiling temperature mixtures by homogeneous batch distillation, Ind. Eng. Chem. Res 40 (2001) 2729-2741. 

In contrast, when dealing with azeotropic mixtures the feasibility of separation is not guaranteed. Entrainer selection and feasibility is the central problem. The... 

Table 3.16 gives the criteria for entrainer selection [7]. Medium-boiler entrainer is valid for both minimum and maximum azeotropes, but this condition is difficult... 

In the subsequent presentations, we will consider that A is more volatile as B. The entrainer must be selected such that both components to be separated belong to the same distillation field. The AB azeotrope must be a node, stable or unstable, or in other words does not belong to a distillation boundary. Criteria for entrainer selection have been proposed by Doherty Caldarola (1985), and are given in Table 9.1. The choice is organised as function of the azeotrope type and the relative volatility of the entrainer. Note that the mentioned requirements are minimum. Additional azeotropes may exist. 

The selection of an entrainer with boundary crossing is based on the observation diat in a RCM both constituents A and B are nodes, stable or unstable. In other words, both A and B can be separated either as overhead or bottom products. Table 9.3 gives a list of recommended heuristics for entrainer selection (Stichlmair and Fair, 1999). In all cases, the distillation boundary must be highly curved, although how much curved is not exactly known at the present time. The simplest choice is a low boiler for a minimum AB azeotrope, and a high boiler for a maximum AB azeotrope. 

The elementary problem analysed in a RCM is the separation of high-purity components from an A/B binary mixture by means of a Mass Separation Agent (entrainer). The key issue is the entrainer selection that will produce a favourable RCM for breaking flie azeotrope. In this respect a major decision is the application of only homogeneous azeotropic distillation, or considering also heterogeneous azeotropic distillation. 

The separation by homogeneous azeotropic distillation is severely constrained by distillation boundaries. The major concern is the place where the process takes place, namely in one or two distillation regions. The first situation is similar with zeotropic systems, but finding a suitable entrainer is problematic. In the second case, the distillation boundary has to be crossed. Since insufficient theoretical and experimental research is available, this is not guaranteed by only simulation. Heuristics have been formulated for the both situations for the proper entrainer selection. 

Azeotropic distillation. A further development involves the addition of an entrainer, either another solvent or water, to the mixture of liquids to be separated. The purpose of this material is to form a selected azeotrope with one of the components. This results in a difference in relative volatility between the azeotrope and the non-azeotropic component allowing separation to be achieved. Typically the azeotrope will be of higher volatility and becomes the distillate, although the azeotrope can be such that it is removed as bottoms. An effective entrainer therefore must be selective for the solvent to be recovered, stable under the conditions of use, chemically compatible with all components, relatively inexpensive, readily available and must be easily separable from the desired product. Water is an ideal entrainer when used to form azeotropes with solvents which separate on condensation. Guidelines for entrainer selection have been provided by Berg and Gerster [28,29]. Many examples of azeotropic distillation can be cited [23]. Examples include the separation of benzene from cyclohexane by the azeotrope of the latter with acetone followed by liquid-liquid extraction with water to yield the cyclic hydrocarbon. Similarly the use of methylene chloride as an entrainer for separation of an azeotropic mixture of methanol and acetone is achieved by addition of methylene chloride followed by the distillation of the selective azeotrope between the alcohol and chlorinated hydrocarbon. 

Laroche, L., N. Bekiaris, H. W. Anderson, and M. Morari, The Curious Behavior of Azeotropic Distillations—Inplications for Entrainer Selection, AIChE J., 38, (9), 1309 (Sept.1992). 

Approaches to the selection of an extractive distillation solvent are discussed by Beig," Ewell et al. and Tassios. In general, selection criteria follow those for azeotropic entrainers ... 

Wahnschafft, O. M. (1997). Advanced Distillation Synthesis Techniques for Nonideal Mixtures Are Making Headway in Industrial Applications. Presented at the Distillation and Absorption Conference, Maastricht, pp. 613-23. Wahnschafft, O. M., Kohler, X, Westerberg, A. W. (1994). Homogeneous Azeotropic Distillation Analysis of Separation Feasibility and Consequences for Entrainer Selection and Column Design. Comput. Chem. Eng., 18, S31-S35. Wahnschafft, O. M., Westerberg, A. W. (1993). Tie Product Composition Regions of Azeotropic Distillation Columns. 2. Separability in Two-Feed Columns and Entrainer Selection. Ind. Eng. Chem. Res, 32,1108-20. 

It is worth mentioning that, apart from the important factors above relating to the phase equilibrium behavior, other factors such as thermally stable, nontoxic, low price, and other physical properties should also be considered in the entrainer selection. A good paper by Gmehling and Mollmann used four examples to demonstrate the entrainer selection procedure for extractive and azeotropic distillation. 

Selection of an entrainer such that the desired produc ts all he within a single distillation region (the products may be pure components or azeotropic mixtures). 

So far, the separation of azeotropic systems has been restricted to the use of pressure shift and the use of entrainers. The third method is to use a membrane to alter the vapor-liquid equilibrium behavior. Pervaporation differs from other membrane processes in that the phase-state on one side of the membrane is different from the other side. The feed to the membrane is a liquid mixture at a high-enough pressure to maintain it in the liquid phase. The other side of the membrane is maintained at a pressure at or below the dew point of the permeate, maintaining it in the vapor phase. Dense membranes are used for pervaporation, and selectivity results from chemical affinity (see Chapter 10). Most pervaporation membranes in commercial use are hydrophyllic19. This means that they preferentially allow... 

Process synthesis and design of these non-conventional distillation processes proceed in two steps. The first step—process synthesis—is the selection of one or more candidate entrainers along with the computation of thermodynamic properties like residue curve maps that help assess many column features such as the adequate column configuration and the corresponding product cuts sequence. The second step—process design—involves the search for optimal values of batch distillation parameters such as the entrainer amount, reflux ratio, boiler duty and number of stages. The complexity of the second step depends on the solutions obtained at the previous level, because efficiency in azeotropic and extractive distillation is largely determined by the mixture thermodynamic properties that are closely linked to the nature of the entrainer. Hence, we have established a complete set of rules for the selection of feasible entrainers for the separation of non ideal mixtures... 

Selection of a suitable entrainer for the separation of w-hexane-ethyl acetate mixtures by heterogeneous azeotropic batch distillation... 

The extractive distillation profits from the capacity of an entrainer (solvent) to modify selectively the relative volatility of species. Normally, the entrainer is the highest boiler, while the component to be separated becomes heavier, being carried out in bottoms. For this reason, this operation may be regarded as an extractive absorption. Extractive distillation can be used for separating both zeo-tropic and azeotropic mixtures. The entrainer is fed near the top for a zeotropic mixture or a minimum-boiling azeotrope, or mixed with the feed for a maximumboiling azeotrope. The separation sequence normally has two columns, for extraction and solvent recovery [5]. 

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