Azeotropic mixture constant distillate composition

At z in the curve, however (the minimum of vapour pressure), the solution and vapour are in equilibrium and the liquid at this point will distil without any change in composition. The mixture at z is said to be azeotropic or a constant boiling mixture. The composition of the azeotropic mixture does vary with pressure. 

The breaking up of azeotropic mixtures. The behaviour of constant boiling point mixtures simulates that of a pure compound, because the composition of the liquid phase is identical with that of the vapour phase. The composition, however, depends upon the pressure at which the distillation is conducted and also rarely corresponds to stoichiometric proportions. The methods adopted in practice will of necessity depend upon the nature of the components of the binary azeotropic mixture, and include —... 

The phenomenon is illustrated for HF and HCl in Fig. 17.5. Conversely, when more concentrated aqueous solutions are boiled, the concentration of HX in the vapour is greater than that in the liquid phase which thereby becomes progressively diluted by distillation until the azeotropic mixture is again reached, whereupon distillation continues without change of composition and at constant temperature. The bps and azeotropic compositions at atmospheric pressure are listed below, together with the densities of the azeotropic acids at 25°C ... 

In most cases, systems deviate to a greater or less extent from Raoult s law, and vapour pressures may be greater or less than those calculated from it. In extreme cases, vapour pressure-composition curves pass through maxima or minima, so that attempts at fractional distillation lead finally to the separation of a constant-boiling (azeotropic) mixture and one (but not both) of the pure species if either of the latter is present in excess. 

Pure water and hydrogen chloride boil at 100° and -85°, while their constant boiling mixture (azeotropic mixture) containing 20.25% of hydrogen chloride boils at 108.5°, under a pressure of 1 atmosphere. If a solution containing less than 20.25% of HC1 is distilled, (i.e., between points A and M), water will pass over as the distillate and the residue left behind in the flask will consist of 20.25% solution of HC1 in water. Thus, pure HC1 can not be obtained. Similarly, if a solution containing more than 20.25% HC1 is distilled, then pure HC1 will pass over as distillate and the residue left behind in the flask contain a mixture of the same constant composition, viz., 20.25% HC1 in water. 

Not all liquids form ideal solutions and conform to Raoult s law. Ethanol and water are such liquids. Because of molecular interaction, a mixture of 95.5% (by weight) of ethanol and 4.5% of water boils below (78.15°C) the boiling point of pure ethanol (78.3°C). Thus, no matter how efficient the distilling apparatus, 100% ethanol cannot be obtained by distillation of a mixture of, say, 75% water and 25% ethanol. A mixture of liquids of a certain definite composition that distills at a constant temperature without change in composition is called an azeotrope 95% ethanol is such an azeotrope. The boiling point-composition curve for the ethanol-water mixture is seen in Fig. 4. To prepare 100% ethanol the water can be removed chemically (reaction with calcium oxide) or by removal of the water as an azeotrope (with still another liquid). An azeotropic mixture of 32.4% ethanol and 67.6% benzene (bp 80.1 °C) boils at 68.2°C. A ternary azeotrope (bp 64.9°C) contains 74.1% benzene, 18.5% ethanol, and 7.4% water. Absolute alcohol (100% ethanol) is made by addition of benzene to 95% alcohol and removal of the water in the volatile benzene-water-alcohol azeotrope. 

If the vapor pressures of the two pure constituents are close together, then any appreciable positive deviation from Raoult s law will lead to a maximum in the total vapor pressure curve similarly, a negative deviation will, in the same circumstances, be associated with a minimum in the curve. In any event, even if the vapor pressures of the pure constituents are appreciably different, marked positive or negative deviations can lead to a maximum or a minimum, respectively, in the total vapor pressure curve. Such maxima and minima are the cause of the formation of the familiar constant boiling mixtures or azeotropic mixtures. A liquid mixture having the composition represented by a maximum or a minimum will distil without change of composition, for the proportions of the two constituents are then the same in the liquid and vapor phases. That this must be the case will be shown in the next section. 

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. 

Azeotrope Mixture of two or more liquids in such a ratio that its composition cannot be changed by simple distillation also known as constant-boiling mixture. 

An azeotropic mixture is sometimes called a constant-boiling mixture, since it distills without any change in composition. It is impossible to distill from one side of an azeotrope to the other. For example, ethanol and water at 1.00 atm have an azeotrope at an ethanol mole fraction equal to 0.90. Any mixture of ethanol and water can be distilled to this composition, but no further. 

Occasionally, you ll run across liquid mixtures that cannot be separated by fractional distillation. That s because the composition of the vapor coming off the liquid is the same as the liquid itself. You have an azeotrope, a liquid mixture with a constant boiling point. 

Problem 13.56 The attempt to remove water from ethanol by fractional distillation gives 95% ethanol, an azeotrope that boils at a constant temperature of 78.15 °C. It has a lower boiling point than either water (100 °C) or ethanol (78.3 °C). A liquid mixture is an azeotrope if it gives a vapor of the same composition. How does boiling 95% ethanol with Mg remove the remaining H O ... 

Azeotropic Systems. An azeotropic system is one wherein two or more components have a constanl boiling point at a particular composition. Such mixtures cannot be separated by conventional distillation methods. If rhe constant boiling point is a minimum, the system is said lo exhibit negomv azeotropy if it is a maximum, positive azeotropy. Consider a mixture of water and alcohol in the presence of the vapor. This system of two phases and two components is divarianl. Now choose some fixed pressure and study the composition of the system at equilibrium us a function of temperature. The experimental results arc shown schematically in Fig. 5. 

---