Separation of Zeotropic Mixtures by Distillation

For sequencing the separation of liquid mixtures, there are general heuristics, as shown in Table 3.11. More specific rules for the separations of zeotropic mixtures by distillation will be discussed later in this chapter. 

Selector analysis. In each selector, a logical diagram will guide the identification of the suitable separation method. A split becomes potential if it can be accomplished by at least one separation method. The selection of a separation method is supported by a ranked list of characteristic properties. For example, the relative volatility is a characteristic property for the separation of a mixture by simple distillation. However, it can be used only in for zeotropic mixtures, but not for azeotropic mixtures. 

The procedure known as the list-processing method allows the designer to identify feasible sequences for separating ideal or slightly nonideal zeotropic mixtures by means of simple distillation columns. Table 3.13 presents a list of more specific heuristics for sequencing, supplementary to those in Table 3.7. The first one shows... 

RCM is a powerful graphical tool for assessing the feasibility of separations by distillation. As a simple example, Figure 3.11 presents the generation of alternatives for separating a zeotropic mixture with A, B, C from lowest to highest boiler. The initial feed is the point F. Let us consider the segment d,b, as a representation of a simple distillation process (one feed and two products). This obeys the rules ... 

In a number of cases, if bubble temperatures of the components of the mixture under separation are very close to each other and the structure of their molecules is different, it is profitable to use extractive distillation, even atseparation of zeotropic mixtures, to decrease energy and capital expenditures (separation of mixtures of hydrocarbons of different homologous rows). The economy of expenditures on separation is being achieved at the expense of the fact that separation of one narrow-boiling mixture is replaced by separation of two wide-boiUng mixtures in two column at extractive distillation. 

Distillation is an important historical method for the separation of liquids. A mixture or solution of two liquids is heated until the vapor pressure of the lower boiling compound reaches the pressure of the surroundings. This may be ambient pressure or a lowered pressure caused by application of a vacuum. In either event, vaporization occurs, the vapors are condensed on a cold surface, and the condensed liquid is collected. If the boihng points of the two liquids are sufficiently different given the pressure and efficiency of the apparatus, separation may be achieved. A zeotrope is a mixture that can be separated by distillation. 

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. 

World Health Organization (WHO). A branch of the United Nations concerned with international health problems. Its interests are in the maintenance of nutrition, wholesomeness of foods, and consumer health, zeolite. Family of hydrated silicates which occur as porous mineral crystals used for their absorptive properties, as catalysts, separating agents, sequestrants in washing powders, etc. zeotrope. A solvent mixture whose vapor composition is different from its liquid composition regeneration by distillation is therefore difficult and its use for vapor degreasing may be limited. Ziegler. A type of polymerization catalyst, named after its inventor. 

An alternative method for removing water relies on the use of a solvent that forms an azeotrope with water. An azeotrope is defined as a constant boiling mixture that often boils at a temperature different from its components. To understand this definition, a zeotrope is defined as a mixture that can be separated by distillation. An azeotrope, therefore, is a mixtrue that cannot he separated by distillation. Ethanol, for example, forms an azeotrope with water that boils at 78.2°C, lower than the boiling point of ethanol (78.5°C) or water (100°C). This azeotrope is composed of 95.6% ethanol and 4.4% water. 

It is obvious that for the separation of three-component mixtures by means of extractive distillation, the mixtures of the type 4a (according to the classification given by Gurikov) that are widespread in practice are of the biggest interest. For these mixtures, according to Fig. 4.21, one can get pure component 1 as top product and zeotropic mixture 2,3 as bottom product. One can get the same result for the mixtures of type 4b. 

Figure 9.2 presents a RCM for an ideal zeotropic system benzene/toluene/ethyl-benzene in the form of an equilateral triangle. Pure components are set in vertices, while the sides describe binary mixtures. The space inside the triangle contains all possible ternary mixtures. Note that the boiling points of the pure components at the pressure of separation are marked in vertices. On the contrary, the boiling temperatures of the intermediate mixtures cannot be given explicitly, but their evolution during the distillation is clearly indicated by arrows. 

Figure 13.10 on the next page summarizes the general appearance of some relatively simple temperature-composition phase diagrams of binary systems. If the system does not form an azeotrope (zeotropic behavior), the equilibrated gas phase is richer in one component than the liquid phase at all liquid compositions, and the liquid mixture can be separated into its two components by fractional distillation. The gas in equilibrium with an azeotropic mixture, however, is not enriched in either component. Fractional distillation of a system with an azeotrope leads to separation into one pure component and the azeotropic mixture. 

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