Vacuum distillation
The essential oil is obtained from plant materials under a vacuum varying in intensity depending on the material [37, 38]. The evaporation temperature of the material is lower under vacuum. As a result, materials can be separated by consuming less energy. Vacuum distillation is a suitable method for separating components which have a high boiling point and prevents the destruction of volatiles materials [17]. This method is rarely used to distill oil directly from plant materials.
The table 2 shows a general comparison between conventional distillation methods (hydro-distillation, water-steam distillation steam distillation). In this table, the construction, available temperature, available pressure, plant material suitability, use of Hydro-diffusion, hydrolysis condition and rate of distillation and yield are compared [17, 23].
Table 2-Comparison of Conventional distillation methods