What Is Distillation Used For?

At its most basic level, distillation is simply the heating of a substance or a combination of substances, whether in a vessel or not, to drive off one or more volatile components. In distillation, the substance distilled (called the "distillate") is usually collected by cooling in a condenser, which converts the vapors into a liquid or liquids.

Examples Requiring Distillation

Distillation is a tool whereby liquids are separated and/or purified. Bourbon grain "mashes" are distilled. Water for steam irons is distilled. Essential oils are distilled. Additional examples are listed below.

Ordinary Atmospheric Pressure Distillation

The most common distillations are performed from a closed vessel possessing a single exit port. The vessel contains one or more volatile components and may or may not include solids. The vessel is heated to just past the boiling point of the lowest boiling volatile component. The resultant vapors of that component are passed through a cooling "condenser" so the purified liquid can be collected.

If there are other liquid components in the distillation vessel (or "still"), the temperature is then allowed to rise so that the second-lowest boiling component can be collected. This process is repeated until all the desired components are collected. If necessary, this can be done while feeding an inert gas into the distillation system.

Fractional Distillation

If more than one component is to be collected and the boiling points of the components are closer together than about 70 degrees Celsius, a process termed "fractional distillation" is used, requiring special condensers.

Fractional distillation is used industrially to separate petroleum fractions. Liquefied air is separated into its components by fractional distillation.

Vacuum Distillation

Some liquids decompose before they can reach a temperature at which boiling occurs. Since boiling points of substances lower with decreasing pressure, by lowering the pressure, an otherwise unstable liquid may be distilled without decomposition. The application of this process is called "vacuum distillation." If vacuum alone is insufficient, a combination of earlier mentioned techniques can be used; however, some substances simply cannot be separated using any distillation technique.

Steam Distillation

Immiscible liquids (ones that won't mix together), if heated, will boil together at a lower temperature than either component boils separately. This principle is put to practical use in industry by passing steam into a vessel containing the substance to be distilled. Perfumes and essential oils are substances commonly steam-distilled.

References

• University of Minnesota Morris; "Production of Ethanol"
• Chemical Heritage Foundation; "Fractional Distillation"
• Chemguide; "Immiscible Liquids and Steam Distillation"

Resources

• University of Colorado at Boulder; Distillation