Steam distillation for extracting essential oils from plant materials is a very simple operation. In fact, it is so simple that, for a long time, no one thought the way that the steam liberates the oil was worth investigating. So, until recently, this was very poorly understood and distillers were unaware of much inefficiency and waste built into stills and their operating procedures.
Because saturated equilibrium between a liquid surface and the overlying vapor space occurs almost instantly, it had always been assumed that oil-bearing herb merely had to be enveloped in an “atmosphere” of steam for any exposed oil to vaporize automatically in seeking to saturate the steam in the vapor space. Some of the observed evidence seemed to support this belief and it was generally accepted.
Investigation of the actual method by which the steam gathers up the oil, reveals that the factors involved are very different from what was previously assumed. Latent heats, and not the relative vapor pressures of steam and oil, are the dominant consideration. Understanding this, we now can improve our operating practices, and design our equipment with a scientific precision that previously was impossible.