The Distillation of Essential Oils Part 1


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The Distillation of Essential Oils Part 1

  1. 1. Murray Hunter – University Malaysia Perlis ESSENTIAL OILS The distillation of essential oils 1: PART The purpose of this series of articles is to briefly review the theory and practice of distillation of essential oils. Although many producers use some form of distillation to extract essential oils, very few thoroughly understand the theory behind the practice that lays the basis for the practices they utilise. 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 Figure 1: An old simple ‘bush’ distillery in Australia. the Syrian Empire.2 However, almost all distillation until midway through the Oil would then be collected from the top centres, like the Australian eucalyptus and nineteenth century was water distillation. of the collection vessel upon separation tea tree industries in the first half of the Water distillation is where plant material with the water. Oils distilled within the twentieth century, utilised available items is totally immersed in water, which is geographically-centred and artisan-based like ship water tanks as charge bins to hold brought to a boil by a direct fire. Once the perfumery industry at the time included foliage during distillations.4 Even today in water is at boiling point steam begins to rose, lavender, lavandin, rosemary, and remote parts of the world, many stills adopt pass through a cooling coil (usually made herbs like thyme. primitive designs and utilise very basic of copper) to condense the distillate. Midway through the nineteenth century, techniques in production, sourcing steam the Germans and French in Grasse began from direct fires. experimenting to improve the distillation process. Equipment and techniques for The fundamental principles watersteam and vacuum distillation were of distillation developed, greatly improving upon yields Any understanding of the principals of that were achieved through simple hydro distillation requires an understanding of the or water distillation. Pre-distillation principles of the laws of thermodynamics techniques, like comminution, were and physical chemistry. Essentially, enhanced and fertilisers were applied to distillation enables the separation of aromatic crops with dramatic results.3 volatile constituents contained in some However, it was only in the beginning of form of plant material, through a parent the twentieth century that steam from an carrier vapour (water) capturing other external source to the charge bin was volatile materials from the plant material introduced, bringing in the method of the charge. To achieve this, the parent steam distillation. vapour must somehow capture these The equipment and skills used for the aromatic materials from the plant’s distillation of aromatic materials from surface, through some form of contact and plant material is still very basic in many carry them up through the charge to the Lavender. parts of the world. Many production condenser for rectification. May 2012 P E R S O N A L C A R E 1
  2. 2. ESSENTIAL OILS A number of fundamentals govern thebehaviour of the dynamics of distillation.These can be summarised as follows: Heat is a form of energy which converts Molecules Molecules Moleculeswater into a vapour. In distillation, heat is of water of water of watertherefore converted energy in the form of vapour A and gas vapour vapour Bsteam. This energy drives the distillationprocess and according to the first law ofthermodynamics, this energy cannot becreated or destroyed in a system ofconstant mass. Therefore, energy as heat Liquid Liquid Liquid Liquidmust dissipate as it cannot disappear. water A water A oil B oil BHeat can only travel from a hot body to acooler body, according to the second law Water Together Linalyl acetateof thermodynamics. Thus heat from a 99.6˚C 99.6˚C 226˚Ccarrier vapour can only dissipate into theplant material (and sides of the still) during Figure 2: The composition of mixed vapours from immiscible liquids.distillation. Fourier’s law of heat conductionspecifies that heat conducted from one molecules returning to the liquid. This is appearance. Wet steam will carry moresurface to another will occur at a rate an equilibrium state where the vapour is of these particles than dry steam. Theseproportional to the contact area and the saturated. A decrease in temperature liquid cloud particles will vary betweenmagnitude of the temperature differential will cause more vapour molecules to 1% and 3% of the total steam mass.between the two surfaces. Thus the condense and reduce the vapour pressure Superheated steam does not carrytransference of heat energy requires a and an increase in the temperature will microscopic liquid particles, as they aretemperature gradient. cause more molecules to vapourise than completely vapourised and thus appears Liquids will change into a gaseous state condense, thus increasing the vapour completely clear and a specific temperature according to pressure. Increases in temperature The aim of passing steam through aa certain pressure. Below boiling point, thus increase the saturation level of the charge bin of plant material is to capturethe liquid will store the energy as heat. vapour space. and carry the volatile compounds with theAccording to Fourier’s law the addition of The above behaviour is consistent steam through the charge to theenergy through plant material surfaces with Charles’ Law which states that the condenser. Thus distillation must create awill cause the temperature of the liquid volume of a given gas is proportional to mixed vapour which behaves according toinside the material to rise. With the its absolute temperature under constant Dalton’s law. Dalton’s law states that theabsence of additional energy, heat will pressure. If the temperature of a saturated total pressure of a mixture of two or moredissipate through contact surfaces to the vapour is higher than the boiling point gases will be equal to the sum of all thesurrounding atmosphere, causing the of the parent mixture, it is called a individual pressures each componenttemperature of the mass to decrease. superheated vapour. When superheated would exert, if it was alone as a singleThe heat stored in the liquid is called vapours come into contact with their gas. This allows the boiling temperaturelatent heat. parent liquid, the liquid will tend to to drop according to the vapour pressures When a liquid is heated, its molecules vapourise until the saturation equilibrium of the two mixtures, where boiling pointsbecome more active according to the is once again achieved. In essential oil will vary according to the surroundingtemperature until a point where they distillation, when steam enters a still with pressure. This is significant as oil vapourseparate from the parent liquid into the greater space, its pressure becomes lower, pressure will always be less than water,vapour space above the liquid. If the which allows it to expand. The surplus thus enabling high boiling aromaticsurrounding space is closed, the new heat will dissipate to the surrounding materials to vapourise at lowervapour molecules will exert pressure. surfaces (both plant material and still temperatures.This is called vapour pressure. The actual walls) and vapourise surplus liquids until Generally all aromatic molecules of avapour pressure created will depend the steam becomes saturated again at mixture exposed to the vapour space in theupon the physical characteristics of the a lower temperature. still will vapourise in similar proportions toliquid/gas at prevailing temperatures. Steam is a two-phase mixture of air the liquid mixture. However, due to someAt constant temperatures, the number of gases and moisture molecules. Saturated aromatic molecules being more volatilemolecules escaping as vapour from the steam carries microscopic particles of than others and becoming more activeliquid will equal the number of vapour liquid which give the gas a ‘cloudy’ in the liquid mixture because of the Region of oil vapour elution Vapour phase 98˚C saturated mixed vapour Herb surface Water liquid Mixed liquids Oil liquids Water liquidsFigure 3: Method of oil release through latent heat transfer through plant material.2 P E R S O N A L C A R E May 2012
  3. 3. ESSENTIAL OILSapplication of heat, they will tend to escape Ms. Teoh Ai Ling, School of Materials Engineering, University Malaysia Perliinto the vapour space more quickly thanthe less active ones. Thus in the early partsof a distillation there is a tendency forlower boiling compounds to vapourise morequickly than the higher boiling compounds.The vapour mixture will therefore have ahigher proportion of lower boiling thanthe parent liquid.5 The extent of thisfractionation phenomena depends uponthe relative volatility of the respectivecompounds, which in the case of manyterpenes, for example, is very low.Increasing distillation temperatures alsochanges the relative volatility of differentaromatic molecules, thus preventingdistillation occurring in a fractional manner.Other factors relating to the way volatiles Figure 4: The subcutaneous leaf and oil glands of Eucalyptu citriodora (magnified x200).release themselves from plant material alsodistort the principal of relative volatility. non-fibrous plant materials, the process of the membrane to the surface for To extract volatile compounds during hydro-diffusion assists in bringing aromatic vapourisation, once in contact with thedistillation from plant material requires volatiles to the surface. water or vapour droplets on the surfaceliberation of the oil from the glands and Many plant materials are able to act as of the plant material. This process mosttissue. Latent heat must be transferred a membrane through swelling that allows probably commences with existing moisturefrom the steam to the plant material in the volatiles to escape the oil glands and within the plant material and is continuedstill. This heat is transferred by tiny water moisture to enter. This is the process with new moisture penetrating thedroplets or vapour carried by the steam of osmosis promoted through the high membrane until all volatile materialswhich settle on the plant material, (Fig. 3). temperatures of the distillation process, have been exhausted from the oil glands.However, plant material acts as a barrier the permeability of the plant material Thus to some degree, the speed ofbetween the volatiles and steam, and the solubility of the oil with water. constituent vaporisation in the still is notpreventing them forming a mixed vapour. This allows the formation of an oil-in-water so much dependent upon volatility, butIn the case of many flowers, leaves and emulsion, which can permeate through solubility in water. 1 ⁄2 PAGE AD May 2012 P E R S O N A L C A R E 3
  4. 4. ESSENTIAL OILS A disadvantage of hydro-diffusion isthe effect of hydrolysis on some volatileconstituents within the plant material. Mixed vapour outWith prolonged heat, chemical reactionsbetween water and a number of Re- vapourisationconstituents of essential oils react andconvert to new compounds. For example, Re-condensationesters which are formed from their parentacids under hydrolysis can convert back Re-vapourisationto their parent acids and alcohols. Thisproblem is most acute in water distillation.Steam distillation can lessen this reaction. Re-condensation The process of hydro-diffusion is veryeffective in assisting the exhaustion of Re-vapourisationvolatile constituents from plant materialduring distillation. This is particularly so Re-condensationof plant material where the oil glandsare superficial to the plant material and Re-vapourisationexposed to the surface. These herbs wouldinclude the mints and lavenders. Steam Re-condensationflow rates with these oils need not be fast,as time is needed to condense waterdroplets on the plant material for the Re-vapourisationhydro-diffusion effect to set in. Wet andsuperheated steam would be the most Re-condensationeffective in the distillation of these typesof plants. Steam in Other plants store their oil well insidetheir tissue and are consideredsubcutaneous, as the oil is not exposed Figure 5: The vapourisation and re-condensation/re-vapourisation process during the surface. This would include the barkof cinnamon and cassia, woods like at a low layer in the plant material and Each successive re-vapourisation will carrysandalwood, cedarwood and huon pine, carried vertically to a higher layer, where less oil from the bottom layers, the oil-to-dried flower buds like clove, seeds like a proportion of the mixed vapour water ratio will decrease, until all oil hascaraway and cardamom, roots and recondenses. This condensate will rest been exhausted. This process occurs atrhizomes like ginger, angelica, orris, on the surface of the layer. With highly varying rates according to the absorptioncalamus and vetiver and tough leaves like absorptive plant material, hydro-diffusion capacity of the plant material and heighteucalyptus and tea tree. With these plants, will occur through osmosis, where some of the still. Thus as the height of the stillthe distillation process has to be assisted condensed vapour will be absorbed into increases, distillation time will alsothrough chopping, grating or crushing the the plant material, until it becomes increase. Figure 6 shows the time-steammaterial, so as many of the plants’ oil saturated. Once the plant material is yield rate relationship for a distillation.6 P Cglands are exposed directly to steam during saturated, successive waves of mixeddistillation. This is called comminution. vapours will pick up more oil and References Within a still charged with plant revapourise and move up to the next 1 Wells FV, Billot M. Perfumery technology: art,material, the vapourisation of volatiles layer (Fig. 5). science, industry, 2nd edn. Chichester, UK: Ellisinto mixed vapours with the carrier steam As oil is removed through the vapour Horwood, 1981.vapour, occurs in layers. Thus charge to each successive layer, the plant tissue 2 Forbes RJ. The evolution of the still. Proc Chemheight plays some importance in the slowly exhausts its oil content into the Soc 1962; 7: 237-42.distillation process. Oil is vapourised re-condensing and re-vapourisation. 3 Poucher WA. Poucher’s perfumes, cosmetics and soaps, Vol. 2: The production, manufacture and Water passed (litres) application of perfumes, 9th edn. London: 20 40 60 80 100 120 Chapman & Hall, 1993: 37-8. 100 4 Penfold AR, Morrison FR. Guide to the extraction of eucalyptus oil in the field. Technology Museum 80 Bulletin No. 4. (4th edn). Sydney, New South Oil produced (%) Wales Government Printer, 1945. 60 5 Billet R. (Translated by Wulfinghoff M.) Distillation engineering. New York: Chemical 40 Publishing Company, 1979. 6 Based on trials with tea tree oil distillation, see 20 Ahmad AW, Mansor P Abdul Malek Y, Jaafar H. , Distillation of tea-tree (Melaleuca alternifolia) oil. 0 Establishment of basic parameters and standard 0.5 1.0 1.5 2.0 2.5 3.0 conditions for a test distiller and evaluation of Time (hours) two prototype distillers. J Trop Agr Food SciFigure 6: The time-steam-yield rate relationship for a distillation. 1998; 26 (2): 175-87.4 P E R S O N A L C A R E May 2012