Definition of Steam Distillation

Distillation is a commonly used method to separate liquids. Steam distillation is a process by which volatile organic compounds are separated from non-volatile inorganic salts. Most of the organic compounds break down at high temperatures. When steam is added to the distillation apparatus, the boiling point of the compounds comes down and this allows the organic compound to evaporate at lower temperature.

Principle

• Steam distillation is used for separating the liquid mixture which consists of compounds which are immiscible in water (i.e., that cannot undergo mixing or blending) but volatile in steam. The process is based on the principle that the vapor pressure above the mixture is equal to the sum of individual vapor pressure of the compounds. When vapor pressure is equal to atmospheric pressure, the liquid starts to boil. When steam is passed on to the mixture which contains the mixture of immiscible organic compounds, it gets heated by the steam and itself condenses into water. When vapor pressure above the mixture becomes equal to atmospheric pressure, the resulting mixture of organic compound and water starts to boil until one of the liquids completely distills out. The distillate which is a mixture of water and the organic compound separates into two layers because both the liquids are immiscible with respect to the other. These two layers are separated using a separating funnel.

Process

• Steam is generated in a boiler by heating water. It then flows to a separate container through a tube. It is necessary to separate both the containers since the heat used to produce the steam can decompose the compound which is to be distilled. After the steam is added to the distillation apparatus, the steam then passes through a condenser and the vapors are collected. The condenser uses water to cool the steam. The resultant liquid is again purified using simple distillation to separate the compound.

Apparatus

• A steam distillation apparatus has two parts. Steam is evolved from one part while the other part allows distillation of the compound or material. Both the parts receive separate heat treatments.

Production of Essential Oil

• The most common application of steam distillation is in the mass production of essential oil. The procedure involves allowing steam into a chamber which holds the plants from which the oil is to be extracted. The plants consist of small sacs which contain essential oil. When steam is passed, these sacs burst. The steam then carries the oil into a condenser which is kept chilled. Here, the steam condenses into water. The oil-water mixture is then separated using simple distillation process.

Other Applications

• Steam distillation is also used to extract eucalyptus oil from eucalyptus, citrus oils from lemon, and in the process of making perfumes.