Chemistry for Every Man: Steam Distillation

Essential oils are water-immiscible volatile naturally-occurring substances that are used in the manufacture of perfumes and flavorings. In addition, essential oils have been used in so-called Aroma Therapy. There are a number of methods of processing plants to obtain essential oils. One frequently used in the citrus industry is expression, or cold-pressing. Another important technique is solvent extraction. A third, perhaps costlier technique, and the one discussed in this article, is distillation. Before discussing the varieties of distillation at the chemist's disposal, let's first consider what distillation actually is.

What Is Distillation?

Distillation is basically boiling a liquid so that it becomes a vapor, which can then travel to a location where it is then cooled back down, turning into a liquid once again. Moonshiners use a "still" generally made of copper to distill their moonshine. Housewives observe distillation everyday, when they cover a simmering pot with a lid, and then remove it, revealing droplets of water on its inside. This is because the steam strikes the cooler lid which then liquefies or condenses it back to liquid.

What is Steam Distillation?

Some organic compounds are perfectly alright at the boiling point of water, but if heated much higher than that, they may be unstable and break down. One way to get around this is to use steam distillation. Combining the mixture (complete with impurities) with water(1) gives it a lower boiling point. So the mixture boils at a temperature below the breakdown points of the chemicals being distilled. After distillation, the mixture is cooled and the purified substances can be isolated, undamaged.

So that's the end of the article, as all substances utilizing distillation can be handled by either simple distillation or simple steam distillation, right? Not right.

Using Partial-Pressure (Vacuum)

There are times when an organic compound may decompose even at the relatively low temperature of boiling water. All hope is not lost in this situation as there is a further modification that can be made. By applying vacuum to the steam distillation process, the pressure of the system is reduced, and the mixture boils at an even lower temperature. In that case, the combination of the impure compound and water will boil considerably below 100 degrees Celsius. Following cooling, the two can be separated in the usual fashion, and the pure compound recovered.

Steam distillation thus joins the repertoire of tools at the chemist's, the chemical engineer's, and the manufacturer's disposal. There are quite a few other tools, too, that deserve attention.(2)

(1) Sometimes, rather than mix water with the starting material it is deemed more efficacious to pump live steam into the reaction chamber. The same principles apply, whichever technique is applied.

(2) The author, a chemist, intends this article as the first of several discussing the various tools of the chemist.