Distillation occurs in a number of definable stages. During the first stage, initial heating occurs through the charge where the plant material has been placed. Plant material will not be saturated with moisture and the temperature differential between the steam and the plant material will allow quick dissipation of latent heat from the steam to the plant material.
The initial steam, particularly if it is wet will tend to cool throughout the lower layers of the charge, where some condensation may occur. This has to be watched carefully, as too much condensation may flood the lower parts of the charge. Dry superheated steam may have the effect of drying out the plant material. Both flooding and drying out of the plant material are detrimental to efficient distillation. At this early stage steam is the only contributor to vapour pressure until saturation occurs and the layering process, discussed above begins. The second stage begins when the vapour reaches the condenser. At this point the oil-to-water ratio will be at its highest. During the second phase the distillation process will go through three sub-phases.
• In the early stage, due to some effect from relative volatilities of the various constituents, the most volatile constituents will tend to vaporise first and carry a higher than proportionate weight in the distillate as compared to the normal oil. It is also reported that oxygenated constituents also have a tendency to distil over before hydrocarbons.1 • During the middle stages, the oil will be distil over in the same proportions as the normal oil, and • During the third stage, the least volatile constituents of the oil will contribute a higher than proportionate composition in the distillate.
The distinct stages of the second stage distillation can be seen in Figure 1 showing the change in composition of sweet basil oil during distillation.2 The height of the still will influence this phenomenon. High stills tend to negate this effect because as the more volatile constituents from the bottom layer reach the top layer, the less volatile constituents from the bottom layer will have already started distilling over and reaching the upper layers of the charge. The final stage of distillation occurs when the water-to-oil ratio is very high as the charge becomes exhausted of oil. It will no longer be economical to continue with the distillation.
Steam and pressure
For efficient distillation, i.e., achieving the maximum amount of oil with the minimum quantity of steam, requires the maximum exposure between the steam flowing into the vat and the plant material surfaces area within the charge. The rate of steam flow per hour is subject to the amount of plant material in each charge. Daily throughput is thus limited by the amount of steam that can be generated on an hourly basis at a distillation plant. Steam must also be adjusted to suit the absorptive capacity of the plant material. This greatly varies between different herbs and the condition they are in before distillation. Steam produced by high pressure external boilers is usually very dry and can often be superheated. Steam produced by medium pressure boilers around 3 atmospheres pressure will contain moderate moisture content, but will not tend to superheat. Steam produced from evaporators at atmospheric pressure is relatively moist. In water-steam distillation array where steam is produced in the same vessel that the plant material is stored will produce very wet steam. Different steams can be utilised as a control mechanism to correct the conditions within the still housing the plant charge. Wet steam is a saturated vapour and is suitable for most distillation. In most cases, wet steam from a water bath produces ‘a richer oil’ with much shorter extraction time than other forms of steam.3 This is particularly the case with plant material that contains superficial oil glands. Increasing steam rates does not speed up distillation, especially if the process of hydro-diffusion is required. As plant material is already saturated with moisture and there is a constant diffusion rate. If herbs contain a lot of moisture, then there would be sufficient moisture in the plant material to commence hydro-diffusion and dry steam would be the most suitable to apply. However dry steam has less mass than wet steam and as such reduces the latent heat of steam, thus prolonging the distillation period. A superheated steam occurs when the temperature of the vapour is higher than that of the same saturated vapour at the same pressure. Superheated steam is independent of pressure and therefore advantageous in a number of situations, as it can be utilised at any temperature without increasing pressure, i.e. can increase the temperature of distillation without having to change the steam flow-rate due to changes in pressure. Superheated steam can be used for drying out a flooded charge vessel, where there is too much liquid around the plant material. However superheated steam is not advisable for general distillation as it would dry out the plant material, preventing the hydro-diffusion process occurring and is a poor conductor of latent heat – two of the important processes needed for efficient distillation. Superheated steam is most suitable for the distillation of glabrous herb material and can increase yields substantially. High boiling oils exert less vapour pressures and require relatively large temperature gradients to extract them during distillation. This leads to prolonged distillation periods. If the constituents are stable under long periods of heat, distillation can be performed under pressures above the atmosphere, which increases temperature. This cuts back on distillation time and saves energy. As increasing the still pressure, increases temperature, the temperature gradient will also increase between the vapour space in the still and the plant material. This assists in the vaporisation of high boiling volatile constituents. The advantage of pressure is that the temperature gradient increases, thus increasing the effects of latent heat exchange and reduces the likelihood of hydrolysis. However the use of high pressure distillation is limited by the extent that prolonged high temperatures will damage the composition of the essential oil. Inversely, as operating pressure is reduced, so does the temperature of the distillation. This method can be used for the extraction of heat sensitive constituents that would normally be damaged through exposure to excess heat. However distillation under reduced pressure has a number of limitations. Steam under reduced pressure is less dense, so requires more steam to carry out a distillation than under normal atmospheric pressure. The condenser system would need to be almost twice the size of a conventional still or a refrigeration system required in condensing the distillate. The recovery vessel and separator would have to be sealed within the closed system, which would lead to design and engineering difficulties. Recently a variation on the operation of pressure in steam distillation has been reported a number of times, utilising a technique called instantaneous controlled pressure drop (DIC). This is a technique where the foliage is first exposed to saturated steam and then the pressure drastically dropped to a vacuum level of around 5-50 KPa to provoke autovaporisation of the superheated volatile compounds through expanding and breaking up the cell walls with instantaneous cooling.4 Experiments have shown that results can be varied through changing time, pressures and the amount of moisture in the leaves.5
Wilting crops before distillation
As the moisture condition of the herb is a factor in the efficient distillation of the herb, wilting is often carried out to dry the crop before processing. The objective of wilting is to dry the herb enough to increase its absorptive surface. Many practitioners believe it is to dry out excess moisture so that distillation will be shorter and more efficient. However this is a fallacy. For example, with tough leaves like eucalyptus and tea tree, wilting does little to dry them out or increase their surface absorption, so wilting will have no benefit to the distillation process. Moisture in the plant material is actually of benefit to the distillation process.6 In fact there are risks with wilting in that the process may lead to losses in oil, not through evaporation, but through chemical reactions like oxidisation, resinisation and the formation of glycosides and enzymes in the materials. Excessive drying of moisture can remove necessary moisture breaking the contact between the oil component and surface of the plant material, thus hindering the promotion of hydro-diffusion during the distillation process. Which crops require wilting before distillation generally depends upon their natural surface absorption capacity. Nonabsorptive herbs like mint and basil need some wilting to promote absorption during distillation, as wilting to promote partial breakdown of the surface cellular structure of the leaf. In this case distillation would then commence with a wet steam fraction, followed by slightly drier steam once moisture has permeated into the leaf structure. Herbs with absorptive surfaces like lavender do not need to be wilted. In fact they do not have enough moisture to link the oil glands to the surface via a water interface, so wet steam fractions are needed during distillation. Grass crops like lemongrass and citronella contain enough moisture within their leaf structure to create a water-oil-surface interface for hydro-diffusion to occur during distillation, so wilting is not necessary. As moisture content is already sufficient in the leaf, dry steam would be suitable for the distillation process.
Water distillation involves distilling plant material totally immersed in water. Depending upon the specific gravity and charge mass in the still, the material will either float or sit totally immersed in the water. Heat is introduced by direct heating of the sides of the vat, a steam jacket, a closed system coil or in some cases a perforated steam coil. Water distillation was the only method used before the 20th century. Water distillation is useful for the distillation of flower materials which would normally congeal and form lumpy masses under steam distillation, where steam would not penetrate, like rose petals and orange blossoms. This method is also useful for fruit kernels that would form glutinous masses under steam distillation and powdered forms of plant material which need to be comminuted before distillation, like almond powder and huon pine saw dust. In water distillation there are a number of simultaneous processes that act to extract volatile constituents from plant material that are different from steam distillation. Essential oils contain a number of oxygenated constituents that are relatively soluble in water. This would include phenols, alcohols and some aldehydes. During the early stages of a water distillation, these compounds would dissolve in the surrounding water and become part of the boiling mixture and resulting mixed vapour. As water boils and converts to steam at the bottom of the vat and rises through the plant charge, it will come into contact with the plant material. Some oil is exposed on the surface of this material will be vaporised by the rising steam as it comes into contact with the plant surfaces. This steam carrying some volatile vapour will rise to the surface and carry over into the vapour space above the water until it reaches the still condenser. The boiling temperature of water at the bottom of the vat in water distillation is slightly less than the boiling point of water, due to the mixed liquid of solublised volatiles and water. Heat applied to the still will cause the creation of a small bubble of saturated mixed vapour from the liquid phase, where upon formation it rises to the top of the water in the charge. During the rise, the bubble’s pressure, temperature and proportion of oil to water decreases. The condensing volatiles, mostly being less dense than water, float to the top of the water and form a film on the surface of the water in the vessel. This lost oil tends to remain on top of the surface and cannot re-vaporise easily due to its higher boiling point and the generally cooler temperatures at the water surface. Most of the oil recovered in water distillation is the portion of the oil that does not condensate through this action. Observation shows that distillation undertaken with vigorous boiling produces better and quicker yields than mild boiling. Some distillers even install small propellers intruding into the side of the still to assist in agitation. This is most probably effective because the agitation in the charge tank prevents oil droplets clinging to the herb surfaces. It is necessary to generate enough steam in the water so that it will come in contact with as much of the oil on the plant surface material as possible during the distillation. The effects of hydro-diffusion are much slower in water than other types of distillation. Consequently, especially for wood materials extensive comminution must be undertaken so that particles in the charge are fine and as much oil as possible is exposed on the surface of the material. In water distillation, plant material is placed in a sealed vessel or retort that connects directly to a condenser. From the condenser the distillate runs into a separator. The rate of distillation is controlled by the intensity of the fire, the pressure of the vessel or retort and/or the rate of introduction of steam. As many woods contain high boiling compounds, pressure is vital to create high enough temperatures to vaporise the volatile constituents. These constituents may take many hours to boil out. Hydro-stills should generally be wide to maximise the evaporation area. Where particles are fine such as saw dust and powders some form of mesh or ‘P’ shape pipe arrangement should exist at the entrance to the condenser to prevent plant material from entry and possible clogging. Heavy charges and where heat coils are used in the still require a perforated grid to prevent plant material from directly coming into contact with the heating coils. With many materials, part of the oil dissolves in the water during distillation and forms a milky emulsion, as a number of aromatic constituents are soluble to some degree in water. This loss could range up to 25% of the essential oil.7 This means that the recovery of oil is incomplete and the recovered oil will be deficient in some constituents that would be the case with the oil recovered through steam distillation. Upon separation in water distillation, the water distillate is returned directly to the charge vessel to replace the decreasing water level due to evaporation. This is called cohobation. Sometimes the water distillate is redistilled in another vessel to extract the volatiles in emulsion. Salt is often added to the distillate to reduce the solubility of water. Whether this process is undertaken depends upon the probability of the constituents being damaged by further heat and the economics of re-distillation. Another method to recover the dissolved aromatic materials from the water distillate is to add a solvent. The mixture is then vigorously shaken to pick up dissolved constituents from the water into the solvent. These materials are then recovered through vacuum distillation of the solvent which results in a secondary essential oil.8 Some common water soluble aroma chemicals in essential oils are listed in Table 1. Another method that will contribute to minimising oil loss due to oil solubility in water during the separation phase is to control the outgoing distillate temperature from the condenser. Where oils are less dense than water, there will be an optimal temperature range where oil particles will freely float to the top of the distillate upon condensation. Some literature on distillation practices misses the point about the effect of condensation temperatures on oil yields.9 Based on private work,10 the higher the temperature of the outgoing distillate, the freer will be the oil particles to float to the top. For example, tea tree oil droplets will float to the top of the water distillate twice as fast at 60°C than at 40°C. The upper temperature limit will be restricted by the potential loss of low boiling volatiles during condensation. This has implications on the design of the specific condenser for specific crops and set range limits upon the temperature that distillation can take place, to achieve a specific outgoing distillate temperature range.
Water and steam distillation
Water and steam distillation involves the storing of the plant material above a water bath situated in the bottom of the charge vessel and heating the water either through direct fire, a steam jacket or a closed or open steam coil. Water and steam distillation produces saturated wet steam at the prevailing vessel pressure, which is usually atmospheric pressure. Within this configuration, unlike water distillation, only steam comes in contact with the plant material. Water and steam distillation is not very suitable for fine materials as steam will find a path of least resistance tending to create channels. This means that steam will not flow through the whole mass of plant material and an incomplete distillation will take place. If the plant material is loosely packed, the same effect will happen, as the material will offer no resistance to the steam. Water and steam distillation may take a long period of time to reach operating temperature as the plant material needs to be heated up with only saturated steam. This may cause early condensation and wetting of the plant material. Due to the limits on pressure that can be built up in the charge vessel, water and steam distillation will have only a limited effect on extracting high boiling materials from plant materials. However there is less opportunity for hydrolysis to occur than with water distillation. Water and steam distillation has another advantage over steam distillation as there are fewer decomposed products during the process due to less chance of plant material drying out. However water and steam distillation will take a lot longer. Water and steam distillation can produce very good results under reduced pressure. Water and steam distillation is much cheaper to set up than steam distillation facilities and lends itself to portable stills that can be transported from place to place.
Steam distillation employs an external steam generation system, external to the charge vessel. This configuration provides much more control (depending upon the boiler capacity) than water and steam distillation. This is because in steam distillation the wetness fraction, temperature and pressure can be manipulated according to needs and conditions. However, it is a misconception that greater steam volumes and increased pressures have positive effects on the process in all cases. As mentioned previously, dry and superheated steam has the effect of drying out plant material, which potentially halts distillation through the stopping of hydro-diffusion process. Faster steam flow rates do not necessary mean quicker recovery times. Fabricated steam boilers cost much more to run than water baths and may lead to high distillation costs, especially if they require petrochemical feed stocks. However if steam distillation facilities are designed and built with the correct steam ratings, they are much more economical to run than water and water and steam distillation systems. A comparison between water, water and steam and steam distillation is shown in Table 2.
The condenser system
A condenser in an essential oil distillation system is a heat exchange or dissipating device. The condenser must not only cool the condensate vapour into a liquid, but also cool the condensate to the temperature range where the oil will separate spontaneously from the water. The usual array for a condenser system is a tube or set of tubes running through a sealed water reservoir to cool the pipes. It is immediately attached to the top of the charge vessel to collect all vapour exiting the vessel. The design of the condenser must ensure that the vapour flow is turbulent inside the tubes to prevent high velocity vapour freely flowing through the condenser and maximise exposure to the cooler walls. A baffle is usually installed at the beginning of the condenser to disrupt a straight steam flow for this purpose. Failure to achieve those conditions would result in some vapour failing to condensate. Within the condenser system the flows of vapour and cooling water should be in opposite directions at the maximum possible speed. The condenser must be sensitive enough to react on the vapour flow very quickly. The required number of tubes and length of the condenser depends on the rate of distillate flow, the pressure, the temperature of the cooling water and the desired exit temperature range of the distillate. The condenser must remove the equivalent amount of heat that is needed to vaporise the distillate, plus the additional amount of heat to reach the optimal distillate temperature range of the condensate distillate exiting the condenser. The rate of which heat would be removed from the distillate can be represented by the following equation:
Q = UAt Where Q = the heat removed by unit of time U = a constant determined by operating conditions (condensing and cooling made up of a number of factors) usually a constant is used. A = the area available for heat removal t = the temperature difference between the vapour and the cooling medium.
U is made up of a number of factors including the flow rates of cooling water and vapours, the material that the condenser is constructed, and usually a constant is used due to the difficulty to calculate. The value of U increases as these factors increase. Thus according to the equation, the surface area can be as large or small as desired, as long as the other factors compensate. However the overall capacity of the distillation system will have great bearing on the condenser area. Condenser sizes will also vary in size according to the temperature of available cooling water on site, thus condensers in temperate and tropical areas will reflect this in size. Using a condenser system with the wrong capacity for the distillation system will have a number of operational consequences. A too efficient condenser system will deliver the distillate at a temperature below the optimum range, which could lead to cool air outside being sucked into the system. This outside cooler air in the condenser tubes could create expansion and contraction of vapours in the condenser leading to splattering and intermittent distillate outflow. This could also occur if the cooling water is too cold. If the condenser is too small for the distillation system, then the still must be operated with lower steam rates, which would lengthen distillation times and open up the possibility of hydrolysis to occur within the vessel.
The separator system
Before leaving the subject of distillation in this chapter, some words about the separator system must be mentioned. The function of the separator is to as quickly as possible separate the oil from the distillate water. As distillate water volume is much greater than oil, it is important that water can be removed continuously. Oil and water separates according to specific gravity forming two layers. Lighter than water oils will float to the top and heavier than water oils will sink to the bottom. This must be considered in separator design for water removal. If the specific gravity of oil and water is very close, the two components will not separate immediately. Distillate flowing into the separator must therefore not disturb the surface area and flow into the body of the water to prevent surface turbulence. The separator must also be large enough so that drained water does not carry away microscopic oil droplets. Temperature plays a crucial role in separation, where it should be moderately warm to increase the relative specific gravity differential of the oil and water. Raised temperature of the distillate water will allow the small oil particles to rise to the top of the separator quicker in a similar manner to the condenser exit temperature range where there will be an optimal separator water temperature range to promote oil-water separation.11 During distillation, the more volatile constituents tend to vaporise quicker and the less volatile constituents vaporise later in the distillation. This leads to an oil that will vary in constituents during the distillation period. By changing separation flasks at particular points during a single distillation, oils of different constituent profiles can be collected. This is important in ylang ylang and lavender distillation, where different oils profiles will have different uses and values to particular customers. Many distillers also use this principal to collect specific fractions during the distillation, which can later be blended together to create a whole oil that meets with certain specific specifications, such as a standard. The water distillate will always be saturated in oil and directly dumping it would lead to a loss in yield. For this reason some distillers (water and water and steam distillation) will channel the water distillate back into the still vessel for redistillation in what is called cohobation, mentioned previously. For this purpose the separator must be placed higher than the still vessel proper so distillate water in the separator can be fed back into the still through gravity.
Hydro-diffusion distillation is a variation on steam distillation where steam is introduced on the top of the vessel and condenses through the plant material in the still, where the distillate is collected and condensed under the plant material which rests on a grill or perforated tray. Through steam travelling down the still, there is more time for the volatiles and fatty acids floating on the plant material. In the case of wood and seeds that have many high boiling compounds, which are difficult to vaporise in an ordinary still, this system may be effective. This would be valuable when fatty acids contribute to the flavour of a material and it is desirable in the oil. Thus, hydro-diffusion distillation may return an oil more representative of the plant’s natural profile.12 It is reported that hydro-diffusion distillation gives quicker distillations with lower steam consumption than conventional steam distillation.13 However this process is governed by the physical laws that govern any other type of distillation and the fact that the steam travels downwards while cooling may affect the transfer of latent heat and thus increase, rather than decrease distillation time. The tendency to saturate the plant material near the bottom may also lead to hydrolysis and lead to lesser yields. However, hydro-diffusion distillation appears popular within the aromatherapy industry in Europe.
The final article in September will conclude with a brief discussion about applying these principles.
1 Johannes AK, Scheffer JC, Svendsen AB. Comparison of Isolation Procedures for Essential Oils, Z. Lebensm. Unters. Forsch, Vol. 168, 1979, pp. 106-111. 2 Private communication with Mr. Jon Bonnardeaux of Western Australia Department of Agriculture, early 1990s. 3 Denny EFK. Field Distillation for Herbaceous Oils, Lilydale, Tasmania, Denny McKenzie & Associates, P. 81, 1990. 4 Kristiawan M, Sobolik V, Al-Haddad M, Allaf K. Effect of pressure drop on the isolation of cananga oil using controlled pressure drop process, Chemical Engineering and Processing 2008; 47 (1): 66-75. 5 Rezzoug SA, Boutekedjiret C, Allaf K. Optimization of operating conditions of rosemary essential oil extraction by a fast controlled pressure drop using response surface methodology, Journal of Food Engineering 2005; 71 (1): 9-17. 6 Guenther E.The Essential Oils, Volume One: History – Origin in Plants, Introduction – Analysis, Malabar, Florida, Robert E. Krieger Publishing Company, P. 110, 1948. 7 Fleisher A. Water-soluble fractions of the essential oils, Perfumer & Flavorist 1991; 16 (3): 37-41. 8 Bouzid N, Toulgouate K, Villarem G, Gaset A. Analyse quantitative des fractions d’huile essentielle pouvant co-exister lors d’hydrodistillation de plants aromatiques, Rivista Ital Eppos 1997; 79: 15-25. 9 Boland DJ, Brophy JJ, House APN. Eucalyptus Leaf Oils: Use, Chemistry, Distillation and Marketing 1991; Melbourne, Inkata Press, P. 191. 10 Unpublished private work on tea tree distillation at Batu 9, Berseri, Perlis, Malaysia during 2004/5. 11 Hughes AD, (1952), Improvements in the Field Distillation of Peppermint Oil, Corvallis, Engineering Experimental Station Bulletin No. 31 1952; Oregon State College, 1-64. 12 Legast E, Peyron L. Hydrodiffusion Industrial Technology to Produce Essential Oils by Steam, in Proceedings of the 11th International Congress of Essential Oils, Fragrances and Flavors Vol. 2 1989; New Dehli, India, 69-73. 13 Hall R, Klemme D, Nienhaus J. The H&R Book: Guide to Fragrance Ingredients Vol. 4 1985 London, Johnson Publications, P. 13.
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