Azeotropic distillation entrainer selection

The choice of a selective solvent is easier the more the components to be separated differ in their chemical structure. It would be difficult or impossible, for instance, to hnd a selective solvent for the separation of stereoisomers. Nevertheless, the restrictions on extractive distillation solvents are less severe than those on azeotropic distillation entrainers, because the solvent recovery problem is virtually nonexistent due to the wide gap between the boiling points of the solvent and the components to be separated. 

The hydrogen bonding grouping provides insight for screening suitable entrainers in the development of a feasible azeotropic distillation process. Selected entrainers are then tested experimentally for their quantitative effect on VLE. 

Adsorption regeneration (desoiption) (Continued) pressure-swing temp erature- sw ing cooling considerations heating considerations Alumina, activated Amines, tertiary Antoine equation Association reactions Atomic volumes Axial mixing Azeotropic distillation calculations selection of entrainer... 

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. 

If n-pentane is selected as the entrainer for an azeotropic distillation scheme, an ethanol product containing less water than that obtained in the extractive distillation method is easily obtained with entrainer-etha-nol ratios of 2.S-3.5, mole basis (10). For a ratio of 3.214, the water content of the ethanol is less than 3 ppm. Only 18 equilibrium trays are required in a column of less than 5 feet diameter. The heat loads in millions Btu/hour are about 10.7 for the reboiler and 11.3 for the condenser. A stripper is used to recover n-pentane and ethanol from the aqueous phase. The recovered n-pentane and ethanol can be recycled either to the feed or to the reflux stream of the azeotropic distillation column. 

The choice of an entrainer used to make a desired separation in an azeotropic distillation depends on the binary mixture being separated and the nonidealities of these components with the added entrainer. While several different entrainers might be used to provide a separation, the final selection may depend on the required purity of the product. If several entrainers can produce a product of desired purity, the final choice may depend on an economic evaluation of the several schemes. 

In azeotropic distillation, when two components are difficult to be separated due to the proximity of their boiling points or because they form an azeotrope, an entrainer is added that forms an azeotrope with at least one of the components to be separated. The entrainer must be selected so that the azeotropes that are formed have sufficiently spaced boiling points to facilitate their separation by distillation. Moreover, it must be verified that the azeotropes that are formed can themselves be separated by relatively straightforward and economical means. The number of suitable entrainers for a given situation is, therefore, quite limited, especially when other factors in the selection process are taken into account such as cost, stability, and safety. 

The above sequencing methods valid for zeotropic systems cannot be applied in the case of mixture with strong non-ideal character and displaying distillation boundaries, as those in the case of breaking azeotropes. Fortunately, the sequencing problem in this case has a different character. Most of the separations of multi-component non-ideal mixtures can be reduced by appropriate splits to the treatment of ternary mixtures, for which two or three columns are normally sufficient. The separation sequence follows direct or indirect sequence. The energetic consumption due to the recycle of entrainer dominates the economics. From this viewpoint preferred is that sequence in which the entrainer is recycled as bottoms. Hence, in azeotropic distillation the main problem is the solvent selection and not columns sequencing. 

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). 

While for azeotropic distillation the knowledge of the azeotropic points and of the miscibility gap is most important, for the selection of solvents (entrainers) for extractive distillation the knowledge of the influence of the entrainer on the separation factor is required. 

Table 11.5 Selected entrainers for the separation of ethanol (1)-water (2) and water (1)-acetic acid (2) by azeotropic distillation.

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. 

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. 

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