Liquids, mixed, boiling points vapour pressures

The constant pressure diagram for this system is shown schematically in fig. 21.14. The boiling point of the mixture is independent of composition as shown by the horizontal dotted line at except when the second component disappears when, of course, the boiling point rises abruptly to that of the pure component T or T ), The line T E gives the composition of the vapour in equilibrium with pure liquid 1 as a function of temperature. The equilibrium temperature is lower than the boiling point of 1 as its partial pressure in the vapour phase is lower than total pressure. Similarly T E gives the composition of T mixed vapour in equilibrium with p liquid 2. At the eutectic point we have co-existence of the two liquid phases and vapour. The lines T E and T E are given by equations like (18.23) and (18.23 ). 

While we have studied the properties of binary mixtures at constant temperature so far we shall now examine the behaviour of these mixtures at constant pressure. The conditions are those found in distillation, which is normally an isobaric process tending to establish equilibrium between the liquid and vapour phases. A boiling point diagram shows the boiling points and the equilibrium compositions of binary mix-... 

The second stage of the styrene process involves the dehydrogenation of ethylbenzene. The reaction is carried out in the vapour phase at temperatures of 600—650°C over catalysts based on either ferric or zinc oxides with lesser amounts of other metallic oxides such as chromic, cupric and potassium oxides. The reaction is favoured by low pressure and in order to reduce the partial pressure of the ethylbenzene the feed is mixed with superheated steam before passage over the catalyst. Normally, a conversion of 35—40% per pass is achieved. The product is cooled and allowed to separate into two layers the aqueous layer is discarded. The organic layer consists of styrene (about 37%), ethylbenzene (about 61%) and benzene, toluene and tar (about 2%). The separation of styrene by distillation is difficult because of the susceptibility of the monomer to polymerization at quite moderate temperatures and because the boiling point of styrene (145°C) is rather close to that of ethylbenzene (136°C). It is necessary therefore to use specially designed columns and to add a polymerization inhibitor (commonly sulphur) before distillation and to distil under reduced pressure. In a typical process, a four-column distillation train is used. In the first column benzene and toluene are removed at atmospheric pressure in the second and third columns ethylbenzene is removed at about 35 mm Hg in the fourth column styrene is separated from sulphur and tar, also at about 35 mm Hg. Finally, an inhibitor is added to the styrene t-butyl catechol is preferred for this purpose rather than sulphur which leads to discoloration of the final polymer. Styrene is a colourless liquid with a characteristic odour. 

Comparison of Observed with Calculated Boiling Points. Closely Related Liquids.—We must now consider whether the actual boiling points of mixed liquids agree mth those calculated from the vapour pressures, and, in particular, whether the observed maximum deviations (Pg), agree with those deduced from the vapour pressures (Dj), or, when these are not available, from those calculated... 

Simplest Cases.—It has been seen that when two hquids are placed together in a closed vacuous space, the vapour pressure and the boiling point can only be accurately calculated from the vapour pressures of the components if (a) the liquids are non-miscible, or (6) they are miscible in aU proportions and show no change of temperature or volume when mixed together, this being generally the case when the substances are chemically closely related. A similar statement may be made with regard to the composition of the vapour evolved from the two liquids. 

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