Steam distillation
The most widely used method for extracting plant essential oil is steam distillation [4, 32]. steam distillation is the process of distilling plant material with the steam generated outside the still in a stand-alone boiler (figure 4). As the steam-water distillation system, the plant material is placed on top of a perforated gird above the steam inlet [23]. both water and steam in this method, are utilized, but the plant material is not in direct contact with water and steam generated outside the still in a boiler and flows through a pipe into the bottom of the steel. A mixture of water and oil are vapored and condensed. Finally, the oil is separated from the water by a separator. steam is always fully saturated, wet, and never superheated in this method [7, 33]. Also thermal degradation of components does not occur and amount of steam is adjustable. Of course, this method is more expensive than the hydro-distillation and water-steam distillation. Boutekedjiret is reported extraction of rosemary essential oil by steam distillation [34]. Also E. Cassel succeeded in extracting of rosemary, basil and lavender oils with 0.51%, 0.38% and 0.32% (w/w), respectively [35].