Binary Mixtures of Immiscible Liquids: Steam Distillation

DISTILLATION

Distillation is a process in which a liquid mixture is separated into its component parts by vaporization. The vapor evolved from a boiling liquid mixture is normally richer in the more volatile components than the liquid with which it is in equilibrium. Distillation rests on this fact. Although multicomponent mix-tures are most common in distillation processes, an understanding of the operation can be based on the vapor pressure characteristics of two-component or binary mixtures. Binary systems in which the liquids are immiscible are discussed first. Discussion of the separation of miscible liquids by fractionation forms most of the remainder of the section.

Binary Mixtures of Immiscible Liquids: Steam Distillation

If the two components of a binary mixture are immiscible, the vapor pressure of the mixture is the sum of the vapor pressures of the two components, each exerted independently and not as a function of their relative concentrations in the liquid. This property is employed in steam distillation, a process particularly applicable to the separation of high–boiling point substances from nonvolatile impurities. The steam forms a cheap and inert carrier. The principles of the process, however, apply to other immiscible systems.

If a mixture of water and a high–boiling point liquid, such as nitrobenzene, is heated, the total vapor pressure increases and ultimately reaches the external pressure. The mixture boils, and the vapors evolved are condensed to give a liquid mixture, which separates under gravity. In practice, the vapors are pro-duced by blowing steam into the liquid in a manner that gives intimate contact between the phases. Since both components contribute to the total pressure, the boiling temperature must be lower than the boiling point of either component. In the case of nitrobenzene and water, the boiling point at atmospheric pressure is about 372 K. To distill nitrobenzene alone at this temperature, a pressure of 20 mmHg must be imposed. Steam distillation, therefore, permits the distillation of water-immiscible materials of high boiling point without the use of high temperatures, which might cause decomposition, or high vacua. The method, however, will only separate such materials from nonvolatile constituents. If volatile impurities are present, these will appear in the distillate.

The composition of the distillate is calculated in the following way. For two components, A and B, the total vapor pressure, P, is the sum of the vapor pressures of the components, P_A and P_B. Since the partial pressure of a com-ponent in a gaseous mixture is proportional to its molar concentration, the composition of the vapor is given by

where n_A and n_B are the number of moles of A and B in the vapor, respectively. If W_A and W_B are the weights of A and B in the vapor, then

where M_A and M_B are the respective molecular weights. The distillate obtained from the vapor is W_A + W_B. Therefore,

Percentage of A in the distillate = W_A  / (W_A + W_B) x 100 = P_AM_A  / (P_AM_A + P_BM_B)  x 100               (10:6)

The ratio of immiscible organic liquid to water in the distillate is increased if the former has a high molecular weight or a high vapor pressure.

Steam distillation under vacuum may be employed when the thermal stability of the material prohibits temperatures of about 373 K. A further variant is the introduction of unsaturated steam under conditions in which no con-densation to water takes place. Only two phases, the liquid being distilled and the mixed vapors, are then present. The external pressure no longer fixes the temperature, as in a three-phase system, and any convenient value can be chosen.

The chief uses of steam distillation are the purification and isolation of liquids of high boiling point, such as aniline, nitrobenzene, or s-dichlorobenzene, and the preparation of fatty acids and volatile oils. Many of the latter are pre-pared by introducing steam into a mixture of the comminuted drug and water. The method is also used to remove odoriferous elements, such as aldehydes and ketones, from edible oils. The dehydration of a material by adding a volatile, water-immiscible solvent, such as toluene, and distilling the mixture is a form of steam distillation. The solvent separates in the condensate and may be returned to the still.