Composition of liquid and vapour

The points a, /3, on the horizontal lines, represent the compositions of liquid and vapour phases in equilibrium at a given temperature or pressure. 

It is a consequence of the Gibbs-Konowalow rule that the compositions of liquid and vapour (i.e., the residue and distillate, respectively) alter in the sense of falling and rising parts of the curve, respectively. One may imagine (Ostwald, loc. cit.) the composition of the liquid to be represented by a heavy mobile... 

If a binary liquid mixture is contained in a cylinder under a piston, and the latter slowly raised, evaporation proceeds, and the quantities and compositions of liquid and vapour gradually change. According to the conditions, the evaporation may proceed at constant temperature (p variable), or at constant pressure (T variable). The latter case usually occurs in practice (cf. 163-164). 

Figure 11.55. Compositions of liquid and vapour streams from plates...

Fid. 27. — Composition of Liquid and Vapour Phases of Solutions of Perchloric Acid of different Boiling Points. 

This tells us that the vapour is richer in the more volatile component (B) than the liquid with which it is in equilibrium. Using this relation we can now construct another curve, the lower solid line in Fig. 6.2, which tells us the composition of the vapour in equilibrium with liquids of various compositions. It is this difference in the compositions of liquid and vapour that enables us to separate components of a mixture by distillation. If we take a solution of composition xB(l) and allow it to boil, the initial composition of the vapour will be xB g), which is richer in component B. If the vapour is condensed and collected the process can be repeated and a liquid richer in component B, of composition x B(g), may be collected. When the original liquid is boiled, since relatively more of component B is lost to the vapour, the remaining liquid becomes richer in component A. 

An azeotrope is a mixture of two liquids that distils unchanged, the composition of liquid and vapour being the same. Unlike a pure substance, the composition of the azeotropic mixture depends on pressure. 

In order to relate compositions of liquid and vapour streams on successive plates it is necessary to carry out material balances around the top and bottom sections of the column. Consider a balance around the top of the column down to plane n (above the feed plate) as shown in Figure 7.35. 

Methods Employed.—The difficulties attending the experimental determination of the composition of liquid and vapour are, in most cases, very considerable and, unless great care be taken, erroneous and misleading results may be obtained. The chief methods which have been employed are the following —... 

Second Method.—Determinations of the relative composition of liquid and vapour by passing a known volume of air through a mixture at constant temperature have been carried out by Winkelmann (9), Linebarger (6), Gahl (10), and others. 

It seems desirable that very eareful determinations of the relative composition of liquid and vapour should be made with the same pair of substances by all the different methods. 

Data required.—If, for mixtures of any two substances, the curve representing the relations between boiling point and molecular composition has been constructed, and if the relation between the composition of the liquid mixture and of its vapour is known, the boiling point of the distillate may be read from the curve. We have seen that if the two substances are closely related we may safely assume that the boiling points of mixtures may be calculated from. the vapour pressures of the components, and that Brown s formula, taking the mean ratio of the vapour pressures for the value of the constant, c, gives the relation between the composition of liquid and vapour with fair accuracy. 

Benzene and Toluene.—In the case of benzene and toluene, the boiling points of mixtures have been found to agree very closely with those read from the theoretical curve, but the relation between the composition of liquid and vapour has not been experimentally determined. The mean ratio of the vapour pressures at temperatures between 80° and 110° is about 2 5, and we may probably assume that the relation is expressed with sufficient accuracy by the formula o b/ a = 2 5 Wb/ a ... 

Graphically from Composition of Liquid and Vapour.—If the relative composition of vapour and liquid has been determined for a series of mixtures, the composi-... 

As an example of the first method, Lord Rayleigh s determinations of the composition of liquid and vapour for... 

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