Steam Distillation Process and Mass Relationship

Steam distillation is specially adopted in cases where the substances involved cannot withstand the temperature of distillation and decompose. Substances of a kind can be separated by reducing the partial pressure of the volatile component. 

This can be done by making use of an inert vapour that decreases the temperature of distillation. The inert vapour used should be practically immiscible with components to be distilled. 

Steam distillation is used

(I) for separating a high boiling component from the non-volatile impurities.

(ii) for separating a high boiling mixture into different fractions wherein the decomposition of material might occur if direct distillation were employed.

(iii) in cases where vaporisation temperature can not be reached by steam heat.

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Steam is widely used in distillation

It is immiscible with many organic compounds, it provides the required heat for vaporisation and it is readily available at a low cost. In steam, distillation steam is directly admitted into the liquid in the still (steam directly admitted into a pool of the liquid is called live or open steam). 

The mixed vapour containing the desired component is taken overhead and condensed and the desired component is separated from the water phase by gravity while the non-volatile material remains behind in the still. The necessary condition for employing steam distillation is that the solubility of steam in the liquid must be very low i. e. product must be practically immiscible with water. 

The process of steam distillation may make use of superheated steam which provides sufficient heat to vaporise the desired component without self-condensing or it may make use of saturated steam which provides sufficient heat to vaporise the desired component by partly condensing. 

In the latter case, the liquid  (water) phase is produced in the still. In either case, when the sum of the partial pressure of steam and desired component reaches the total pressure both substances pass over (as a vapour) in the molecular ratio of their partial pressures.

The mass relationship for steam distillation

mA/MA/mB/MB = pA/yB

= pA / P-pA

mA/mB = pA. MA/pB.MB

= pAMA/(P-pA) MB

Where MA and MB are molecular weights of the desired component and steam respectively. pB, pB and P are the partial pressure of the desired component, steam (called a carrier) and total pressure respectively. 

mA and MB are the masses of the desired component and steam in the vapour respectively. When the water phase is present and the effect of a non-volatile material on vaporization is neglected then for the system under consideration there will be two components and three phases.

According to the phase rule, F = C - P+2, there is only one variable that can be varied independently and set the temperature to adjust itself so that the sum of the partial pressures of two components equals one atmosphere. The temperature so reached is less than the boiling points of pure components. Thus if the pressure is atmospheric then it is possible to steam distil the material at a temperature lower than 100°C thus blocking/avoiding the destructive temperature.

>Simple Distillation | Differential Distillation 

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane