Definition
Homogeneous liquid mixtures of substances consisting of two or more components can be separated by distillation if the composition of the gaseous phase above the liquid phase is different from the composition of the liquid phase. Distillation means the partial evaporation of a liquid and a subsequent partial or total condensation of the vapors.

Phases
Whether a substance is solid, liquid or gaseous (i.e. vapor), depends on temperature and pressure. Distillation is using the phase transitions

to increase or to lower the concentration of one of the components of a mixture.

Boiling Temperature
The temperature at which a liquid is boiling, depends in the ambient pressure. One can experience this at the top of high mountains. Up there, water is not boiling at 100 °C, but at considerably lower temperature (e.g. at the top of Mount Everest water is boiling at 70 °C already). The reason for this is the fact that the ambient pressure is decreasing the higher we get. The dependency of the boiling temperature from the pressure is shown in boiling curves (see example at right).

Vacuum distillation benefits from this dependency in order to evaporate and condense mixtures of substances at low pressure - i.e. under vacuum - and thus at low temperatures.

The advantage of vacuum distillation is exactly this low temperature range as the thermal load of the material to be distilled is considerably reduced. Substances the chemical structure of which is affected at high temperatures - e.g. by decomposition or polymerisation - can be separated gently, i.e. at minimal thermal load, by means of vacuum distillation.

In the rough vacuum range the mean free path (i.e. the average distance which a vapor molecule has to move until it hits another molecule) is below 0.1 mm and the gas flow can be described by means of equations of "viscous flow".

In the fine vacuum range the mean free path is a few centimetres. Then neither the laws of "viscous flow" are valid, nor those of "molecular flow". It is a certain intermediate range which is called "Knudsen flow".

The vacuum range and its kind of gas flow is a very important aspect for the design of vacuum plants. While in the rough vacuum range relatively small pipe diameters can be used for the flow of vapors, much larger diameters are needed in the fine vacuum range to achieve the same mass transport, as the flow resistance in this pressure range is exponentially increasing with smaller diameters.

Thin Film Evaporators of UIC GmbH are mainly applied in the rough vacuum range, while Short Path Distillators have their particular advantages in the fine vacuum range. In both cases, evaporation is performed from a thin film, therefore we talk about film evaporation.