Hydro distillation is a traditional method for the removal of essential oils. This method is used to isolate essential oils from aromatic and medicinal plants. The conventional method for the extraction of essential oils is hydrodistillation (HD), in which the essential oils are evaporated by heating a mixture of water or other solvent and plant materials followed by the liquefaction of the vapors in a condenser. The instrumentation is made up of a condenser and a decanter to collect the condensate and separate essential oils from water, respectively (Fig. 1.1). The mechanism of plant material involves three physicochemical processes:
- Hydro diffusion,
- Decomposition by heat.
The principle of extraction is based on isotropic distillation. Water distillation is carried out at reduced pressure (under vacuum) to reduce the temperature to less than 100°. When the condensed material cools down, the water and essential oil are separated and the oil is decanted to be used as an essential oil. There are three types of hydro distillation namely water immersion, direct vapor injection, and water immersion and vapor injection. The distillation time depends on the plant material being processed.
- The method extracts finely powdered material or plant parts that, by contact with live steam, would otherwise form lumps through which the steam cannot penetrate.
- The equipment is inexpensive, easy to construct, and suitable for field operation.
- The complete extraction is not possible. Besides, certain esters are partly hydrolyzed and sensitive substances like aldehydes tend to polymerize.
- The high-boiling and water-soluble oil constituents are not completely vaporized so the process becomes uneconomical.
- It is a slower process than either water and steam distillation or direct steam distillation.
Applications: Traditional Method of Producing Attar, the distillates obtained by hydrodistillation of flowers (such as saffron, marigold, rose, jasmine, pandanus) in sandalwood oil or other base materials like paraffin.
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