Maximum boiling point azeotrope

This example clearly shows good distribution because of a negative deviation from Raonlt s lawin the extract layer. The activity coefficient of acetone is less than 1.0 in the chloroform layer. However, there is another problem because acetone and chloroform reach a maximum-boiling-point azeotrope composition and cannot be separated completely by distillation at atmospheric pressure. 

Maximum boiling point azeotrope occurs for 68.8 wt% HNO, at a boiling point of 122°C. ... 

In systems with negative deviation from ideal behavior, maximum-boiling-point azeotropes can occur. This is illustrated in Fig. 8 for the chloroform-acetone system, treated in Example 1. This system shows negative deviation from ideal behavior due to the possibility of hydrogen bonds between chloroform and acetone, which cannot occur with the pure components. 

For both minimum- and maximum-boiling-point azeotropes, fractional distillation can only separate a solution into a pure component and the azeotrope, not into the two pure components. It is not possible to distill past an azeotropic concentration. Absolute alcohol cannot be prepared by simply distilling aqueous solutions obtained by fermenting grains. 

Maximum boiling point azeotrope (negative azeotrope)... 

Figure 3.6. Maximum-boiling-point azeotrope, acetone-chloroform system, (a) Partial and total pressures at 60°C. (i>) Vapor-liquid equilibria at 101 kPa. (c) Phase diagram at 101 kPa pressure. [Adapted from O. A. Hougen, K. M. Watson, and R. A. Ragatz, Chemical Process Principles, Part //, 2nd ed., John Wiley and Sons, N. Y. ...

A maximum-boiling-point azeotrope results from a slight attraction between the component molecules. This attraction leads to lower combined vapor pressure than... 

Some maximum-boiling-point azeotropes are listed in Table 15.3. They are not nearly as common as minimum-boiling-point azeotropes. 

Examples of azeotropic mixtures of maximum boiling point are tabulated below these are not as numerous as those of minimum boiling point. 

An important system in distillation is an azeotropic mixture. An azeotrope is a liquid mixture which when vaporized, produces the same composition as the liquid. The VLE plots illustrated in Figure 11 show two different azeotropic systems one with a minimum boiling point and one with a maximum boiling point. In both plots, the equilibrium curves cross the diagonal lines. 

The mixture with the maximum boiling point is called maximum bailing azeotrope and behaves as if it is a pure chemical compound of two components, because it boils at a constant temperature and the composition of the liquid and vapour is the same. But the azeotrope is not a chemical compound, because its composition is not constant under conditions and rarely corresponds to stoichiometric proportions. 

Two types of azeotrope, maximum boiling point (Figure 3.9(a)) and minimum boiling point (Figure 3.9(b)), can be represented on this type of diagram. 

Consider the system shown in Fig. 14.9, which exhibits a maximum boiling point. If a mixture described by point a, having the azeotropic composition, is heated, the vapor will first form at temperature t that vapor has the same composition as the liquid ... 

Mixtures forming an azeotrope with maximum, boiling point distillate the comj)o-nent in excess, pure bottom product azeotropic mixture of the two components. 

Azeotropes with a minimum boiling point (for examples, see Fig. 43, column 3/III) are far more numerous than those with a maximum boiling point (Fig. 43, column o/III). According to the tables of Lecat [20], who li.sts 6287 azeotro]ies and 700 > iion-azeotropes, the ratio is about 9 to I. 

Fig. 231 shows the equilibrium curves of the azeotropic nii.xture acetone-chloroform to which have been added various amounts of the extracting agent, ineth>l-isobntylketone. (The quantities are mole fractions.) The binary S3 stem gives an azeotrope with maximum boiling point at 34.5 mol%. This disappears when 30 mol , of the additive is present further additions cau.se a still larger increase in the relative volatility [50]. 

Mixtures that display a maximum in the boiling point-composition curve can lead to initial separation of pure A on fractionation but as the composition of the liquid moves towards B and reaches the maximum, a constant boiling mixture L3 is reached that will distill over unchanged. An example of an azeotropic mixture of maximum boiling point is water (b.p. 100°C) and hydrogen chloride (b.p. -80°C), the azeotrope being 80% water and boiling at 108.6°C. 

Unlike ideally behaved systems, the acetone/benzene/chloroform system exhibits an azeotrope. An azeotrope is a point in the compositional space where a liquid mixture has a constant boiling point because the vapor has the same composition as the liquid. Azeotropes can occur between two or more species, and the boiling temperature determines the nature of the azeotrope. In this case (refer to Figure 2.5b), the azeotrope is a high (or maximum) boiling binary azeotrope between acetone and chloroform. 

This plot allows one to identify nodes from a different perspective. Rather than tracking the origin, direction, and termination of residue curves, one can now analyze the temperature contours to classify nodes. For the system at hand, the lowest temperature is observed at pure acetone, while the highest is at pure benzene. Also, the binary azeotrope is a maximum-boiling binary azeotrope between acetone and chloroform, and this is the point of highest temperature along that binary boundary (horizontal boundary of MET). However, this temperature is still lower... 

A liquid mixture of two or more substances that boils at a constant minimum or maximum boiling point lower or higher than that of its constituents is called azeotropic. The liquid and the vapour produced on boiling have the same composition and, in this, the mixture acts like a single substance. Chlorotrifluoromethane and trifluoromethane forms such a mixture. 

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