Distillation occurs in a number of definable stages. During the first stage, initial heating occurs through the charge where the plant material has been placed. Plant material will not be saturated with moisture and the temperature differential between the steam and the plant material will allow quick dissipation of latent heat from the steam to the plant material.
The initial steam, particularly if it is wet will tend to cool throughout the lower layers of the charge, where some condensation may occur. This has to be watched carefully, as too much condensation may flood the lower parts of the charge. Dry superheated steam may have the effect of drying out the plant material. Both flooding and drying out of the plant material are detrimental to efficient distillation. At this early stage steam is the only contributor to vapour pressure until saturation occurs and the layering process, discussed above begins. The second stage begins when the vapour reaches the condenser. At this point the oil-to-water ratio will be at its highest. During the second phase the distillation process will go through three sub-phases.
• In the early stage, due to some effect from relative volatilities of the various constituents, the most volatile constituents will tend to vaporise first and carry a higher than proportionate weight in the distillate as compared to the normal oil. It is also reported that oxygenated constituents also have a tendency to distil over before hydrocarbons.1
• During the middle stages, the oil will be distil over in the same proportions as the normal oil, and
• During the third stage, the least volatile constituents of the oil will contribute a higher than proportionate composition in the distillate.
The distinct stages of the second stage distillation can be seen in Figure 1 showing the change in composition of sweet basil oil during distillation.2 The height of the still will influence this phenomenon. High stills tend to negate this effect because as the more volatile constituents from the bottom layer reach the top layer, the less volatile constituents from the bottom layer will have already started distilling over and reaching the upper layers of the charge. The final stage of distillation occurs when the water-to-oil ratio is very high as the charge becomes exhausted of oil. It will no longer be economical to continue with the distillation.
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