Distillation is the process of separating components of a mixture by evaporating and then condensing the vapor into liquid, taking advantage of the fact that different elements or compounds have different boiling points. It has wide uses, from water purification and air distillation to extracting oils from organic matter and refining crude oil. Several distillation techniques have been developed over the years, including steam distillation.
Reasons for Using Steam Distillation
Traditional distillation techniques require direct heating of the mixture to evaporate its contents. While this works well for most inorganic solutions and a few organic ones, there are many organic compounds that decompose at high temperatures, including many natural essential oils and aromatic compounds. According to Steam Distillation (see Reference 1), to ensure that the required organic compounds aren't destroyed, they must be evaporated at lower temperatures.
Matter surface has high energy molecules that are in contact with the atmosphere, which exert a certain pressure against the atmosphere due to their internal energies, known as vapor pressure. If this pressure exceeds the atmospheric pressure, those molecules evaporate. Since heating increases the internal energy of those molecules, it also increases vapor pressure.
How It Works
Most complex organic compounds are immiscible with water, meaning they don't dissolve in water but form a mixture instead, which separates if allowed to settle as the water settles down and the organic compounds float on top. The steam distillation process works on the principle that when a mixture of two or more immiscible liquids is heated while ensuring that the surfaces of both liquids are in contact with the atmosphere, the vapor pressure exerted by the system is increased. This is because it now becomes the sum of the vapor pressures of all of the components of the mixture combined together. This allows for evaporation of elements with high boiling points at much lower temperatures merely by allowing them to form a mixture with water.
Steam is passed through the organic matter that contains the compounds to be separated. The steam condenses against that matter to form a mixture. That mixture gets heated further by more incoming steam, which continues to pass through the matter, evaporating the mixture. Due to the reduced vapor pressure, the required organic compounds also evaporate as a part of the mixture and are thus extracted from the organic matter.
The evaporated mixture of steam and the organic compounds is passed through jackets that have cold water coming in at one end. The evaporated mixture then passes out as hot water from the other end after cooling the mixture down. This condenses the mixture, which is then collected and allowed to settle. During the settling process, the extracted organic compounds come to the top, and they are then separated by filtering out the settled water from below.