The Distillation of Essential oils Part 1 in Personal Care Magazine May 2012

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 in
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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.
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2. ESSENTIAL OILS
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
Molecules Molecules Molecules
water into a vapour. In distillation, heat is
of water of water of water
therefore converted energy in the form of vapour A and gas vapour vapour B
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 Liquid Liquid Liquid Liquid
must dissipate as it cannot disappear. water A water A oil B oil B
Heat can only travel from a hot body to a
cooler body, according to the second law Water Together Linalyl acetate
of thermodynamics. Thus heat from a 99.6˚C 99.6˚C 226˚C
carrier vapour can only dissipate into the
plant material (and sides of the still) during Figure 2: The composition of mixed vapours from immiscible liquids.
distillation. Fourier’s law of heat conduction
specifies that heat conducted from one molecules returning to the liquid. This is appearance. Wet steam will carry more
surface to another will occur at a rate an equilibrium state where the vapour is of these particles than dry steam. These
proportional to the contact area and the saturated. A decrease in temperature liquid cloud particles will vary between
magnitude 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 carry
transference of heat energy requires a and an increase in the temperature will microscopic liquid particles, as they are
temperature 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 invisible.
at a specific temperature according to pressure. Increases in temperature The aim of passing steam through a
a certain pressure. Below boiling point, thus increase the saturation level of the charge bin of plant material is to capture
the liquid will store the energy as heat. vapour space. and carry the volatile compounds with the
According to Fourier’s law the addition of The above behaviour is consistent steam through the charge to the
energy through plant material surfaces with Charles’ Law which states that the condenser. Thus distillation must create a
will cause the temperature of the liquid volume of a given gas is proportional to mixed vapour which behaves according to
inside the material to rise. With the its absolute temperature under constant Dalton’s law. Dalton’s law states that the
absence of additional energy, heat will pressure. If the temperature of a saturated total pressure of a mixture of two or more
dissipate through contact surfaces to the vapour is higher than the boiling point gases will be equal to the sum of all the
surrounding atmosphere, causing the of the parent mixture, it is called a individual pressures each component
temperature of the mass to decrease. superheated vapour. When superheated would exert, if it was alone as a single
The heat stored in the liquid is called vapours come into contact with their gas. This allows the boiling temperature
latent 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 points
become more active according to the is once again achieved. In essential oil will vary according to the surrounding
temperature until a point where they distillation, when steam enters a still with pressure. This is significant as oil vapour
separate 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 aromatic
surrounding space is closed, the new heat will dissipate to the surrounding materials to vapourise at lower
vapour 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 a
vapour pressure created will depend the steam becomes saturated again at mixture exposed to the vapour space in the
upon the physical characteristics of the a lower temperature. still will vapourise in similar proportions to
liquid/gas at prevailing temperatures. Steam is a two-phase mixture of air the liquid mixture. However, due to some
At constant temperatures, the number of gases and moisture molecules. Saturated aromatic molecules being more volatile
molecules escaping as vapour from the steam carries microscopic particles of than others and becoming more active
liquid 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 liquids
Figure 3: Method of oil release through latent heat transfer through plant material.
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3. ESSENTIAL OILS
application of heat, they will tend to escape
Ms. Teoh Ai Ling, School of Materials Engineering, University Malaysia Perli
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 Figure 4: The subcutaneous leaf and oil glands of Eucalyptu citriodora (magnified x200).
release themselves from plant material also
distort 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 the
distillation from plant material requires volatiles to the surface. water or vapour droplets on the surface
liberation of the oil from the glands and Many plant materials are able to act as of the plant material. This process most
tissue. Latent heat must be transferred a membrane through swelling that allows probably commences with existing moisture
from the steam to the plant material in the volatiles to escape the oil glands and within the plant material and is continued
still. This heat is transferred by tiny water moisture to enter. This is the process with new moisture penetrating the
droplets or vapour carried by the steam of osmosis promoted through the high membrane until all volatile materials
which 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 of
between the volatiles and steam, and the solubility of the oil with water. constituent vaporisation in the still is not
preventing them forming a mixed vapour. This allows the formation of an oil-in-water so much dependent upon volatility, but
In the case of many flowers, leaves and emulsion, which can permeate through solubility in water.
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4. ESSENTIAL OILS
A disadvantage of hydro-diffusion is
the effect of hydrolysis on some volatile
constituents within the plant material. Mixed vapour out
With prolonged heat, chemical reactions
between water and a number of Re- vapourisation
constituents of essential oils react and
convert to new compounds. For example, Re-condensation
esters which are formed from their parent
acids under hydrolysis can convert back
Re-vapourisation
to their parent acids and alcohols. This
problem is most acute in water distillation.
Steam distillation can lessen this reaction. Re-condensation
The process of hydro-diffusion is very
effective in assisting the exhaustion of Re-vapourisation
volatile constituents from plant material
during distillation. This is particularly so Re-condensation
of plant material where the oil glands
are superficial to the plant material and Re-vapourisation
exposed to the surface. These herbs would
include the mints and lavenders. Steam
Re-condensation
flow rates with these oils need not be fast,
as time is needed to condense water
droplets on the plant material for the Re-vapourisation
hydro-diffusion effect to set in. Wet and
superheated steam would be the most Re-condensation
effective in the distillation of these types
of plants. Steam in
Other plants store their oil well inside
their tissue and are considered
subcutaneous, as the oil is not exposed Figure 5: The vapourisation and re-condensation/re-vapourisation process during distillation.
to the surface. This would include the bark
of cinnamon and cassia, woods like at a low layer in the plant material and Each successive re-vapourisation will carry
sandalwood, 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 has
caraway and cardamom, roots and recondenses. This condensate will rest been exhausted. This process occurs at
rhizomes like ginger, angelica, orris, on the surface of the layer. With highly varying rates according to the absorption
calamus and vetiver and tough leaves like absorptive plant material, hydro-diffusion capacity of the plant material and height
eucalyptus and tea tree. With these plants, will occur through osmosis, where some of the still. Thus as the height of the still
the distillation process has to be assisted condensed vapour will be absorbed into increases, distillation time will also
through chopping, grating or crushing the the plant material, until it becomes increase. Figure 6 shows the time-steam
material, so as many of the plants’ oil saturated. Once the plant material is yield rate relationship for a distillation.6 P C
glands are exposed directly to steam during saturated, successive waves of mixed
distillation. 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: Ellis
into 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 Chem
height 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 Sci
Figure 6: The time-steam-yield rate relationship for a distillation. 1998; 26 (2): 175-87.
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