Mixture with boiling point maximum

The appearance of azeotropic points has important consequences for the distillation of the mixtures concerned. First let us consider a system with a boiling point maximum (Fig. 14.23). A liquid mixture having the composition x boils at temperature Ti and its corresponding vapor is enriched by the more volatile component B (xf). If the vapor is removed continuously from equilibrium by simple distillation, meaning by condensation in a receiver, the composition of the... 

Azeotropes are of great importance to distillation and rectification. At the azeotrope gas and liquid have the same concentration y = x) and, in turn, no driving force for interfacial mass transfer exists. Azeotropic mixtures behave in some respects like pure substances. They cannot be fractionated by simple distillation. Azeotropes can exhibit a boiling point minimum (minimum azeotropes) or a boiling point maximum (maximum azeotropes). In multicomponent mixtures saddle point azeotropes with intermediate boiling temperature can also exist. 

Mixtures of Minimum Boilings Point.—In the great majority of cases where the formation of mixtures of minimum boiling point has been observed, one of the two liquids is a hydroxyl compound—an alcohol, an acid or water—and water also forms such mixtures with all the lower alcohols, except methyl alcohol. It is well known that the molecules of these liquids are more or less associated in the liquid state, and we may therefore conclude that mixtures of minimum boiling point (maximum vapour pressure) are most readily formed, and that has the... 

Carbon disulfide is completely miscible with many hydrocarbons, alcohols, and chlorinated hydrocarbons (9,13). Phosphoms (14) and sulfur are very soluble in carbon disulfide. Sulfur reaches a maximum solubiUty of 63% S at the 60°C atmospheric boiling point of the solution (15). SolubiUty data for carbon disulfide in Hquid sulfur at a CS2 partial pressure of 101 kPa (1 atm) and a phase diagram for the sulfur—carbon disulfide system have been published (16). Vapor—Hquid equiHbrium and freezing point data ate available for several binary mixtures containing carbon disulfide (9). 

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. 

For a solution or mixture of two or more distinct liquid components, an azeotrope is that composition (typically measured in mole fractions or percent weight and referred to as the azeotropic solution) with which there is either a maximum point (a negative azeotrope) or a minimum point (a positive azeotrope) in a boiling point versus composition diagram at constant pressure. 

Solvents used here for a general liquid-liquid extraction method were selected from Snyders solvent selectivity triangle. As extraction liquids have to be composed of mixtures of three solvents which may enter into maximum interaction with the analyte, three solvents had to be selected that represent a wide variety of selective interactions. In addition, the solvents should be sufficiently polar to ensure quantitative extraction. Besides selectivity and polarity requirements, the solvents should also meet a few other criteria, mainly for practical reasons they should not be miscible with water, have low boiling points (for relatively fast evaporation procedures) and have densities sufficiently different from the density of water, for pure solvents as well as for selected binary or ternary mixtures of solvents. 

In some cases, liquids with different boiling points cannot be separated in this way, owing to the formation of constant boiling mixtures. Such mixtures cannot, therefore, be separated by distillation. The excess of each constituent beyond the constant boiling proportion would, of course, pass over, until the composition reached that of the constant boiling mixture, which has either a maximum or minimum boiling point compared with any other mixture of the substances. 

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. 

This disposition is in conformity with that shown in Fig. 61 for a mixture of volatile liquids whose boiling point passes through a maximum value it has not, on the contrary, any analogy with that shown in Fig. 104. When, therefore, the temperature approaches 69 , we cannot suppose that the two layers into which the mixture of water and phenol is divided approach two distinct solutions having the same composition and as there is no doubt that they tend to have the same composition, we are constrained to admit that they have as limiting state, not two distinct solutions, but a single solution. 

We predicted their behavior earlier using infinite-dilution /f-values, with the results at 1 atm shown in Table VIII. Only the acetone and chloroform appear to display azeotropic behavior. With this information and that for pure species boiling points at the pressure of interest, we can sketch the ternary diagram for this mixture. We can also use a computer code to generate it, which was done for Fig. 25. We see that there is one maximum-boiling azeotrope between acetone and chloroform. 

EDP or EPW is a mixture of ethylene-diamine (ED or E), pyrocatechol (P), and water (W). This solution usually operates at 110-120 °C (about boiling point) [93], The typical compositions and etch rates can be seen in Table 5. Figure 23 shows the etch rate as a function of the water content [92]. No etching occurs in solutions without the presence of water and the maximum etch rate is observed with a water molar fraction of about 0.6. The etch rate increases with increasing pyrocatechol content to about 5 mol% above which etch rate becomes constant. There is a definite etch rate without addition of pyrocatechol,... 

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