Industrial Utility Efficiency

How to Choose Vacuum Pumps for Chemical Distillation

To determine which type of vacuum pump will be most suitable for an application, it is important to understand the distillation process, the factors that make distillation work, and the features of the different pump types.


How Does Distillation Work?

Distillation, degassing, drying, filtration, membrane separation, adsorption, and crystallization are all among the separation processes that rely on the differences in the physical properties of substances in a mixture. Distillation relies on the differences in boiling point or in vapor pressure versus temperature characteristics. Heating, evaporation, and condensing are the tools used in distillation that separate the liquid constituents in a liquid mixture.

For separating liquid constituents with differences in boiling points of less than 30 °C, a fractionating column with plates or packing is normally recommended. This provides repeated condensing and re-evaporation of the reflux liquid as it progresses up the column for better separation of the constituents. 

Destilation Column

Depicted is an example of distillation column set-up for separation processing.

More volatile liquids have lower boiling points or higher vapor pressure versus temperature curves and more readily evaporate. The vapor phase mixture is richer in the more volatile compounds and can then be condensed, contained, and returned for further separation and purification, if necessary. The greater the difference in the volatility of a component from the mixture, the more easily it is separated.


Understanding Volatility

Volatility is the concentration of a substance in a solution (or its mole fraction) in the vapor phase compared to the concentration of the same substance in its liquid phase. Volatility is also a substance’s pure component pressure compared to its total pressure. The relative volatility of two substances is the ratio of their pure component vapor pressures.

Volatility of substance “i” is defined as:  Ki = yi/xi. Where,

Ki is the volatility of the i component.
yi is the mole fraction of the i component in the vapor phase.
xi is the mole fraction of the i component in the liquid phase.
(Mole fraction is the ratio of the number of moles of a substance to the total number of moles in a solution.)

The ratio is the same between a substance’s pure component pressure and its total pressure:

Since yiP = xiPvi, where P is the total pressure and Pvi is the pure component vapor pressure: then yi/xi = Pvi/P.

The relative volatility “α” of two substances is: α = K1/K2 = (y1/x1)/(y2/x2) = Pv1/Pv2

For a simplified binary mixture that behaves as an ideal liquid, a phase diagram at constant pressure can be drawn with the mole fraction of the more volatile component on the horizontal axis and the temperature on the vertical axis.

The lower curve is normally referred to as the “bubble point” where, for a given mole fraction of liquid mixture, the liquid begins to boil at a given temperature. The higher curve is normally referred to as the “dewpoint,” which indicates the different temperatures where the different mole fractions of the vapor would start to condense.

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