Thermal Separation

Pure Substances

The appearance of a pure substances as solid, liquid or gaseous depends on temperature and system pressure. For distillation processes we are particularly interested in the liquid and the vapour phase and the transitions between them (evaporation, condensation). As can be seen from the phase diagram, vaporization may be accomplished via increasing temperature and / or decreasing pressure. The contrary is true for condensation.

For a given temperature the boundary line between the liquid and the vapour phase designates the pressure at which the liquid boils, i.e., where the liquid's vapour pressure equals the ambient pressure. Apparently, a pressure decrease reduces the boiling point and vice versa.

The variation of the boiling point of stearic acid shows how pronounced the effect of pressure variation on the boiling temperature can be.

This is also important from the perspective that many substances may be sensitive to the application of heat, i.e., they may degrade, polymerize, or decompose as heat is applied. Pressure reduction may be an effective means to gently transfer such substances into the vapour phase. Pressure reduction may also be the method of choice to lower boiling temperatures into regions that are technically easier to handle.

Mixtures

The characterization of mixtures consisting of two or more components requires the consideration of the (molar or mass) fractions x_i of the respective components, where 0 ≤ x_i ≤ 1 and Σ x_i = 1.

Depending on the vapour pressures of the individual components the molar fraction of each component may be different in the liquid and in the vapour phase. In thermal equilibrium components with a higher volatility (= lower boiling point) tend to concentrate in the vapour phase whereas those with higher boiling points tend to concentrate in the liquid phase.

This opens the possibility to thermally separate liquid mixtures by distillation, i.e., evaporation and subsequent condensation, if the vapour pressures (~ boiling points) are sufficiently different.

The difference in volatility is usually captured with the so-called separation factor α_ij between components i and j. For a given temperature it is conveniently approximated by the ratio of the vapour pressures of the respective pure substances.

For a binary mixture (components A and B) the concentration of component A in the vapour phase as a function of its concentration in the liquid phase is shown in the adjacent diagram. The parameter α measures the relative volatility of component A compared to component B.