The distillation of essential oils - Part 1

This first article will first briefly outline the scientific and engineering principles behind distillation, the second article will outline the various stages and types of distillation, and the final article will conclude with a brief discussion about applying these principles. The practice of distillation goes back to ancient times, perhaps as early as 484 BC, when Herodotus recorded the production of turpentine oil in his writings.1 Strong evidence also exists that the Arabs understood the distillation process, where the words chemistry, alcohol and alembic have their origins. It is most likely the Arabs inherited their knowledge of distillation techniques from the Syrian Empire.2 However, almost all distillation until midway through the nineteenth century was water distillation. Water distillation is where plant material is totally immersed in water, which is brought to a boil by a direct fire. Once the water is at boiling point steam begins to pass through a cooling coil (usually made of copper) to condense the distillate. Oil would then be collected from the top of the collection vessel upon separation with the water. Oils distilled within the geographically-centred and artisan-based perfumery industry at the time included rose, lavender, lavandin, rosemary, and herbs like thyme. Midway through the nineteenth century, the Germans and French in Grasse began experimenting to improve the distillation process. Equipment and techniques for watersteam and vacuum distillation were developed, greatly improving upon yields that were achieved through simple hydro or water distillation. Pre-distillation techniques, like comminution, were enhanced and fertilisers were applied to aromatic crops with dramatic results.3 However, it was only in the beginning of the twentieth century that steam from an external source to the charge bin was introduced, bringing in the method of steam distillation. The equipment and skills used for the distillation of aromatic materials from plant material is still very basic in many parts of the world. Many production centres, like the Australian eucalyptus and tea tree industries in the first half of the twentieth century, utilised available items like ship water tanks as charge bins to hold foliage during distillations.4 Even today in remote parts of the world, many stills adopt primitive designs and utilise very basic techniques in production, sourcing steam from direct fires.

The fundamental principles of distillation

Any understanding of the principals of distillation requires an understanding of the principles of the laws of thermodynamics and physical chemistry. Essentially, distillation enables the separation of volatile constituents contained in some form of plant material, through a parent carrier vapour (water) capturing other volatile materials from the plant material in the charge. To achieve this, the parent vapour must somehow capture these aromatic materials from the plant’s surface, through some form of contact and carry them up through the charge to the condenser for rectification. A number of fundamentals govern the behaviour of the dynamics of distillation. These can be summarised as follows: Heat is a form of energy which converts water into a vapour. In distillation, heat is therefore converted energy in the form of steam. This energy drives the distillation process and according to the first law of thermodynamics, this energy cannot be created or destroyed in a system of constant mass. Therefore, energy as heat must dissipate as it cannot disappear. Heat can only travel from a hot body to a cooler body, according to the second law of thermodynamics. Thus heat from a carrier vapour can only dissipate into the plant material (and sides of the still) during distillation. Fourier’s law of heat conduction specifies that heat conducted from one surface to another will occur at a rate proportional to the contact area and the magnitude of the temperature differential between the two surfaces. Thus the transference of heat energy requires a temperature gradient. Liquids will change into a gaseous state at a specific temperature according to a certain pressure. Below boiling point, the liquid will store the energy as heat. According to Fourier’s law the addition of energy through plant material surfaces will cause the temperature of the liquid inside the material to rise. With the absence of additional energy, heat will dissipate through contact surfaces to the surrounding atmosphere, causing the temperature of the mass to decrease. The heat stored in the liquid is called latent heat. When a liquid is heated, its molecules become more active according to the temperature until a point where they separate from the parent liquid into the vapour space above the liquid. If the surrounding space is closed, the new vapour molecules will exert pressure. This is called vapour pressure. The actual vapour pressure created will depend upon the physical characteristics of the liquid/gas at prevailing temperatures. At constant temperatures, the number of molecules escaping as vapour from the liquid will equal the number of vapour molecules returning to the liquid. This is an equilibrium state where the vapour is saturated. A decrease in temperature will cause more vapour molecules to condense and reduce the vapour pressure and an increase in the temperature will cause more molecules to vapourise than condense, thus increasing the vapour pressure. Increases in temperature thus increase the saturation level of the vapour space. The above behaviour is consistent with Charles’ Law which states that the volume of a given gas is proportional to its absolute temperature under constant pressure. If the temperature of a saturated vapour is higher than the boiling point of the parent mixture, it is called a superheated vapour. When superheated vapours come into contact with their parent liquid, the liquid will tend to vapourise until the saturation equilibrium is once again achieved. In essential oil distillation, when steam enters a still with greater space, its pressure becomes lower, which allows it to expand. The surplus heat will dissipate to the surrounding surfaces (both plant material and still walls) and vapourise surplus liquids until the steam becomes saturated again at a lower temperature. Steam is a two-phase mixture of air gases and moisture molecules. Saturated steam carries microscopic particles of liquid which give the gas a ‘cloudy’ appearance. Wet steam will carry more of these particles than dry steam. These liquid cloud particles will vary between 1% and 3% of the total steam mass. Superheated steam does not carry microscopic liquid particles, as they are completely vapourised and thus appears completely clear and invisible. The aim of passing steam through a charge bin of plant material is to capture and carry the volatile compounds with the steam through the charge to the condenser. Thus distillation must create a mixed vapour which behaves according to Dalton’s law. Dalton’s law states that the total pressure of a mixture of two or more gases will be equal to the sum of all the individual pressures each component would exert, if it was alone as a single gas. This allows the boiling temperature to drop according to the vapour pressures of the two mixtures, where boiling points will vary according to the surrounding pressure. This is significant as oil vapour pressure will always be less than water, thus enabling high boiling aromatic materials to vapourise at lower temperatures. Generally all aromatic molecules of a mixture exposed to the vapour space in the still will vapourise in similar proportions to the liquid mixture. However, due to some aromatic molecules being more volatile than others and becoming more active in the liquid mixture because of the application of heat, they will tend to escape into the vapour space more quickly than the less active ones. Thus in the early parts of a distillation there is a tendency for lower boiling compounds to vapourise more quickly than the higher boiling compounds. The vapour mixture will therefore have a higher proportion of lower boiling than the parent liquid.5 The extent of this fractionation phenomena depends upon the relative volatility of the respective compounds, which in the case of many terpenes, for example, is very low. Increasing distillation temperatures also changes the relative volatility of different aromatic molecules, thus preventing distillation occurring in a fractional manner. Other factors relating to the way volatiles release themselves from plant material also distort the principal of relative volatility. To extract volatile compounds during distillation from plant material requires liberation of the oil from the glands and tissue. Latent heat must be transferred from the steam to the plant material in the still. This heat is transferred by tiny water droplets or vapour carried by the steam which settle on the plant material, (Fig. 3). However, plant material acts as a barrier between the volatiles and steam, preventing them forming a mixed vapour. In the case of many flowers, leaves and non-fibrous plant materials, the process of hydro-diffusion assists in bringing aromatic volatiles to the surface. Many plant materials are able to act as a membrane through swelling that allows volatiles to escape the oil glands and moisture to enter. This is the process of osmosis promoted through the high temperatures of the distillation process, the permeability of the plant material and the solubility of the oil with water. This allows the formation of an oil-in-water emulsion, which can permeate through the membrane to the surface for vapourisation, once in contact with the water or vapour droplets on the surface of the plant material. This process most probably commences with existing moisture within the plant material and is continued with new moisture penetrating the membrane until all volatile materials have been exhausted from the oil glands. Thus to some degree, the speed of constituent vaporisation in the still is not so much dependent upon volatility, but solubility in water. A disadvantage of hydro-diffusion is the effect of hydrolysis on some volatile constituents within the plant material. With prolonged heat, chemical reactions between water and a number of constituents of essential oils react and convert to new compounds. For example, esters which are formed from their parent acids under hydrolysis can convert back to their parent acids and alcohols. This problem is most acute in water distillation. Steam distillation can lessen this reaction. The process of hydro-diffusion is very effective in assisting the exhaustion of volatile constituents from plant material during distillation. This is particularly so of plant material where the oil glands are superficial to the plant material and exposed to the surface. These herbs would include the mints and lavenders. Steam flow rates with these oils need not be fast, as time is needed to condense water droplets on the plant material for the hydro-diffusion effect to set in. Wet and superheated steam would be the most effective in the distillation of these types of plants. Other plants store their oil well inside their tissue and are considered subcutaneous, as the oil is not exposed to the surface. This would include the bark of cinnamon and cassia, woods like sandalwood, cedarwood and huon pine, dried flower buds like clove, seeds like caraway and cardamom, roots and rhizomes like ginger, angelica, orris, calamus and vetiver and tough leaves like eucalyptus and tea tree. With these plants, the distillation process has to be assisted through chopping, grating or crushing the material, so as many of the plants’ oil glands are exposed directly to steam during distillation. This is called comminution. Within a still charged with plant material, the vapourisation of volatiles into mixed vapours with the carrier steam vapour, occurs in layers. Thus charge height plays some importance in the distillation process. Oil is vapourised at a low layer in the plant material and carried vertically to a higher layer, where a proportion of the mixed vapour recondenses. This condensate will rest on the surface of the layer. With highly absorptive plant material, hydro-diffusion will occur through osmosis, where some condensed vapour will be absorbed into the plant material, until it becomes saturated. Once the plant material is saturated, successive waves of mixed vapours will pick up more oil and revapourise and move up to the next layer (Fig. 5). As oil is removed through the vapour to each successive layer, the plant tissue slowly exhausts its oil content into the re-condensing and re-vapourisation. Each successive re-vapourisation will carry less oil from the bottom layers, the oil-towater ratio will decrease, until all oil has been exhausted. This process occurs at varying rates according to the absorption capacity of the plant material and height of the still. Thus as the height of the still increases, distillation time will also increase. Figure 6 shows the time-steam yield rate relationship for a distillation.6