Extraction Technologies for Medicinal and Aromatic ... - Capacity4Dev
Extraction Technologies for Medicinal and Aromatic ... - Capacity4Dev Extraction Technologies for Medicinal and Aromatic ... - Capacity4Dev
7 DISTILLATION TECHNOLOGY FOR ESSENTIAL OILS still must be higher than the temperature at which the oil boils in the presence of water on the surface of the plant material, otherwise there would not be a temperature gradient to take the latent heat from the condensing steam to vaporize the oil droplet. Thus, the energy from the steam in form of heat as latent heat of vaporization converts the oil into a vapor. But, as the boiling point of the oil is higher than that of water, the vaporization takes place with steam on the basis of their relative vapor pressures. It is imperative to note that a liquid always boils at the temperature at which its vapor pressure equals the atmospheric or surrounding pressure. For any two immiscible liquids, the total vapor pressure of the mixture is always equal to the sum of their partial pressures. The composition of the mixture in the vapor phase (in this case, oil and water) is determined by the concentration of the individual components multiplied by their respective partial pressures. For example, if a sample of an essential oil comprised of component A (boiling point, 190° C) and water (boiling point, 100° C) is boiled, after some time, once their vapors reach saturation, the temperature will immediately drop to 99.5° C, which is the temperature at which the sum of the two vapor pressures equals 760 mmHg. In other words, the oil forms an azeotropic mixture with water. Thus, any essential oil having high boiling point can be evaporated with steam in a ratio such that their combined vapor pressures equal the atmospheric pressure; the essential oil can be recovered from the plant by the wet distillation process. 7.3 Methods for Distillation The following four techniques for the distillation of essential oils from aromatic plants are employed: 1. Water distillation (or hydrodistillation) 2. Water and steam distillation 3. Direct steam distillation 4. Distillation with cohobation 7.3.1 Hydrodistillation Hydrodistillation is the simplest and oldest process available for obtaining essential oils from plants. Hydrodistillation differs from steam distillation mainly in that the plant material is almost entirely covered with water in the still which is placed on a furnace. An important factor to consider in water distillation is that the water present in the tank must always be enough to last throughout the distillation process, otherwise the plant material may overheat and char. In this method, water is made to boil and the essential oil is carried over to condenser with the steam which is formed. Water-distilled oil is slightly darker in color and has much stronger still notes than oils produced by other methods. The stills based on this principle are 116
EXTRACTION TECHNOLOGIES FOR MEDICINAL AND AROMATIC PLANTS simple in design and are extensively used by small-scale producers of essential oils. Care should be taken during distillation of powdered herbs, as they tend to settle on the bottom of the still and get thermally degraded. Also, for plant material that tends to form mucilage and increase the viscosity of the water, the chances of charring are greater. For plant material that has a tendency to agglomerate or to agglutinate into an impenetrable mass when steam is passed through (like rose petals), water distillation is the preferred method of oil isolation. The primitive, traditional Indian system of essential oil distillation, bhapka method, is also based on water distillation (Figure 1). In this process, the plant material is entirely covered with water in a distillation still, which is made of copper and is known as deg. This deg is placed in a brick furnace. Another copper vessel with a long neck is placed in a water tank or natural pond to serve as a condenser. A bamboo pipe is used as the vapor connection and mud is used to seal the various joints. The water is boiled, the oil vapors along with steam are condensed in the copper vessel, and oil is separated. The capacity of one deg is around 40 kg/batch. These types of units are still being used in Kannauj in Uttar Pradesh and in the Ganjam district of Orissa, India for the preparation of rooh and attars of gulab, kewda, khus, rajnigandha, and bela. These units can easily be transported from one place to another, but are not suitable for large-scale distillation of aromatic crops like grasses and mints. Figure 1: Traditional Indian deg bhapka method Although hydrodistillation (water distillation) is still being used, the process suffers from the following serious drawbacks: 117
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7 DISTILLATION TECHNOLOGY FOR ESSENTIAL OILS<br />
still must be higher than the temperature at which the oil boils in the presence<br />
of water on the surface of the plant material, otherwise there would<br />
not be a temperature gradient to take the latent heat from the condensing<br />
steam to vaporize the oil droplet. Thus, the energy from the steam in <strong>for</strong>m<br />
of heat as latent heat of vaporization converts the oil into a vapor. But, as<br />
the boiling point of the oil is higher than that of water, the vaporization takes<br />
place with steam on the basis of their relative vapor pressures.<br />
It is imperative to note that a liquid always boils at the temperature<br />
at which its vapor pressure equals the atmospheric or surrounding<br />
pressure. For any two immiscible liquids, the total vapor pressure of<br />
the mixture is always equal to the sum of their partial pressures. The<br />
composition of the mixture in the vapor phase (in this case, oil <strong>and</strong> water)<br />
is determined by the concentration of the individual components multiplied<br />
by their respective partial pressures. For example, if a sample of<br />
an essential oil comprised of component A (boiling point, 190° C) <strong>and</strong><br />
water (boiling point, 100° C) is boiled, after some time, once their vapors<br />
reach saturation, the temperature will immediately drop to 99.5° C, which<br />
is the temperature at which the sum of the two vapor pressures equals<br />
760 mmHg. In other words, the oil <strong>for</strong>ms an azeotropic mixture with water.<br />
Thus, any essential oil having high boiling point can be evaporated with<br />
steam in a ratio such that their combined vapor pressures equal the atmospheric<br />
pressure; the essential oil can be recovered from the plant by<br />
the wet distillation process.<br />
7.3 Methods <strong>for</strong> Distillation<br />
The following four techniques <strong>for</strong> the distillation of essential<br />
oils from aromatic plants are employed:<br />
1. Water distillation (or hydrodistillation)<br />
2. Water <strong>and</strong> steam distillation<br />
3. Direct steam distillation<br />
4. Distillation with cohobation<br />
7.3.1 Hydrodistillation<br />
Hydrodistillation is the simplest <strong>and</strong> oldest process available<br />
<strong>for</strong> obtaining essential oils from plants. Hydrodistillation differs from steam<br />
distillation mainly in that the plant material is almost entirely covered with<br />
water in the still which is placed on a furnace. An important factor to consider<br />
in water distillation is that the water present in the tank must always be<br />
enough to last throughout the distillation process, otherwise the plant material<br />
may overheat <strong>and</strong> char. In this method, water is made to boil <strong>and</strong> the<br />
essential oil is carried over to condenser with the steam which is <strong>for</strong>med.<br />
Water-distilled oil is slightly darker in color <strong>and</strong> has much stronger still notes<br />
than oils produced by other methods. The stills based on this principle are<br />
116