Entrainer Selection

When separating azeotropic mixtures, if possible, changes in the azeotropic composition with pressure should be exploited rather than using an extraneous mass-separating agent, since  

Occasionally, a component that already exists in the process can be used as the entrainer, thus avoiding the introduction of extraneous materials for azeotropic distillation. However, in many instances practical difficulties and excessive cost might force the use of extraneous material. 

There is always uncertainty and inaccuracy with vapor-liquid equilibrium data and correlations. Any errors in this data could mean an incorrect prediction of the location and shape of the boundary. 

All of the discussions so far regarding distillation lines, residue curves and distillation boundaries have assumed equilibrium behavior. Real columns do not work at equilibrium, and stage efficiency must be accounted for. Each component will have its own stage efficiency, which means that each composition will deviate from equilibrium behavior differently. This means that if nonequilibrium behavior is taken into account, the shape of the distillation lines, residue curves and distillation 

When introducing an entrainer, it will need to have a significant effect on the relative volatility between the azeotropic components to be separated, and it must be possible to separate the entrainer relatively easily. One way of making sure the entrainer can be easily separated is to choose a component that will introduce a two-liquid phase separation. Such entrainers typically introduce additional distillation boundaries, but the overall separation can be efficient if the two-liquid separation produces mixtures with compositions in the different distillation regions10. 

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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... 

Table 3.16 Criteria for entrainer selection for separations in one distillation field.

Table 3.17 Criteria for entrainer selection for systems with boundary crossing.

The following three conditions for the entrainer selection can be formulated [3] ... 

Sunol, A.K. Hugh, B. Chen, S. Entrainer Selection in Supercritical Extraction, in Supercritical Fluid Technology, Elsevier, New York 1985, 125-145. 

Guidelines for entrainer selection have been provided by Gerster9 and Berg,11 The Janer author has listed the following desirable properties of an antminer, for hydrocarbon separations ... 

N. Azuma, F. Toda, K. Endo, Chirality, 1997, 9, 220-224 H. Takahashi, R. Tamura, T. Ushio, T. Nakai, K. Hirotsu, F. Toda, Mol. Cryst. Liq. Cryst., in the press and by Collet s Entrainment (selective crystallization) A. Collet, M.-J. Brienne, J. Jacques, Chem. Rev. 1980, 80, 215-230 A. Collet in Comprehensive Supramolecular Chemistry, Vol. 10 (Ed. D. N. Reinhoudt), Elsevier, Oxford, 1996, Chapter 5. Both techniques require particular phase diagram conditions. 

The synthesis of separation sequences for non-ideal mixtures is handled nowadays by means of Residue Curve Maps. Major issues are feasibility and entrainer selection. However, there are still important unsolved problems (Chapter 9). 

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). 

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