Hello readers, hope you are reading well! Today we are going to talk about steam distillation in depth. Needless to say it is foremost and traditional technique in the extraction of essential oils. Before landing on the steam distillation we need to understand what distillation is.
Distillation is a process that involves conversion of a liquid into vapor that later condense back to liquid form. Ancient nations observed that water when freely exposed to air may disappear. Rays and heat of sun hastened this conversion process of liquid. This process was reversed when there temperature drops and moisture takes forms of clouds, mist, fog or rain.
People even believed that vapor rising from the sea was source of clouds and rain. They even experienced that any cold body held in the steam from boiling liquid shows droplets of liquid on surface. This was first attempts at condensation and distillation.
This widespread phenomenon of condensation and distillation is something that we all have experienced. Distillation was termed after a Latin word ‘distillo’ from stills a drop which refers to dropping of liquid by human or artificial means. Thus the term was applied to any process where a liquid was separated in drops.
Distillation is basically used in separation of liquids from non-volatile solids like separation of alcoholic liquor from fermented materials. It should be noted that it is a physical separation process and not a chemical reaction. It is of high importance in industry, chemistry and food science.
Humans have been using the process of distillation since at least 3000 BC in the Indus valley of Pakistan. The process was known to be used by the Babylonians of Mesopotamia. You will be surprised to know, distillation initially was used in the making of perfume. Distillation of beverages came much later. Distillation is divided into many types like simple distillation, steam distillation, fractional distillation, and hydrodistillation. Our area of concern is steam distillation so will move quickly in knowing more about it.
So steam distillation is a type of distillation which deals in compounds which are heat sensitive such as natural aromatic compounds. So basically in this we pass dry steam through the plant material. These vapors then undergo condensation process and collected in receiver tank. You might question why specifically dealing with steam distillation?
The reason is temperature of steam is easier to control than the surface of heating element. This also allows high rate of heat transfer without providing heat at high temperature. Steam distillation is also preferred when the substance to be extracted has higher boiling point than that of water. Starting raw material cannot be heated at such high temperatures because of decomposition.
History of Steam distillation:
- Steam distillation was invented by the Persian Ibn Sina who is known as Avicenna in the West. Invention was done for the purpose of extracting essential oils which finds application in aromatherapy, perfumery and drinking industries.
- Direct fire that is “feu nu” was used to obtain the steam and first steam distillation stills were filled with water. This technique was in use until the end of 19th century. You can still see such stills in developing countries.
- Application of indirect steam (steam generator) was used in distillation of lavender by a German company from Leipzig.
- Steam distillation is one of the oldest extraction methods invented by the Arabs in the ninth century.
- Steam distillation involves separation step which can be performed using Florentine vessel or separator.
There are variations and techniques in this process which was done keeping in mind that essential oils are not water soluble. In some cases they are heavier than water and other case lighter.
Now we should head towards complete process of steam distillation.
Process of steam distillation:
The process of steam distillation is a separation process in which two liquids are generally immiscible (incapable of mixing) and chemically non-reactive with water. This is an alternative method of achieving distillation. The process is widely utilized in extraction of essential oils. Process implements low pressure steam which replaces volatile compounds from the intact raw material.
The apparatus of steam distillation consist of a chamber which contains multiple holes in bottom for passing steam with dried or fresh herbs. The chamber consists of a lid to avoid oil from diffusing into the air which generally occurs on applying steam. After the condensation process collects water and oil droplets. Further separation of oil and water mixture is done using filter or separation funnel.
Principle of steam distillation
Principle of steam distillation is an important segment and it is completely connected with chemistry. You should keep a note that simple distillation and steam distillation are quite identical but principle behind both separation methods differs.
Let’s take a look.
The whole principle of steam distillation process revolves around vapor pressure (pressure exerted by the substance against atmospheric pressure). When two immiscible liquids for example water and organic compound are heated and agitated, the surface of each liquid exerts their own vapor pressure.
The components act independently from each other and exert vapor pressure which proves as they do not mix together. This in result increases vapor pressure of the system. Eventually two liquids start to boil when vapor pressure of liquids matches the atmospheric pressure. Boiling point of the liquid remains lower than that of water i.e. 100°C.
Working and stages of steam distillation:
Now you know we go for steam distillation in case of immiscible liquids. Most of the complex organic compounds do not dissolve with water but rather forms a mixture. They generally separate as two layers where water settles down and organic compounds float on top. When two or more liquids are heated, vapor of the system increases. It now combines vapor pressure of all the components that are present in mixture.
This vapor pressure plays all game; it allows evaporation of element with high boiling point at much lower temperature. Stages of steam distillation involves phenomenon that takes place in whole process:
Evaporation: Evaporation is a common phenomenon which we all know and we all studied about. It is conversion of liquid to vapor state. It is the major pathway that takes place in nature by which water moves back to water cycle as water vapors. It is also important part of steam distillation process where evaporated mixture of steam and organic compounds passes through a chamber.
Condensation: Condensation is change of state from gaseous state to liquid state. It is often termed as opposite of vaporization. This process is necessary in steam distillation because the evaporated mix of steam and organic compounds get collected as liquid. In case of steam distillation of aromatic plant material that liquid contains essential oil.
Separation: After condensation process, the resulting fluid consists of a layer of water and the compound to be distilled. As the two liquids are immiscible in nature they get separated. We can further obtain compound using physical separation i.e. decanting or the use of separation funnel.
Steam distillation apparatus:
Now we know the principle, process and stages of steam distillation. To make the picture clearer, I am attaching an image of steam distillation. Have a look and recollect what we have covered so far.
Steam distillation apparatus commonly called as ‘still’ which consist of a vessel containing plant material and water. It consists of a condenser to cool and condense the vapor produced and a method of collection or receiver.
As it is evident in the image that the whole set up consists of a steam generator, vessel containing raw material, condenser and a receiver. On a lab scale, steam distillations are carried out using steam generated outside
the system and piped through macerated biomass or steam generated in-situ using a Clevenger-type apparatus.
Steam distillation of biomass generally yields two products: a high purity essential oil or volatile oil and an aqueous condensate which is known as hydrosol. The recovered essential oil consists of hydrophobic aromatic compounds that are produced in small quantities as secondary metabolites in plants. The hydrosol may also contain some secondary metabolites but these are hydrophilic in nature. You must have heard about rose hydrosol, remember? It is obtained during production of rose oil.
Steam distillation procedure
Okay so we know basic steam distillation apparatus set up. Let us have quick look on procedure to perform steam distillation:
- You need to be very sure about seals of glass connections and check that the joints are well lubricated or not.
- Start your mode to heat application. If you are using a hot plate then turn it on to a high setting.
- Water in boiling flask gradually starts to boil and it then reaches the still containing plant material. Steam will appear and start to condense in the still head after half an hour of heating.
- You have to keep track of the time when the first drop runs down the condenser and into the receiver.
- Keep a check on the water temperature that is draining out of the condenser outlet. If water is warm we need to increase the flow rate of cold water.
- Keep a close eye on the receiver while liquid collects. An oil layer will form on top of the hydrosol or water. Most of the essential oil collects in the first 10-20mins but more recovery occurs in the next 1-2 hours.
- Continue with the process until the essential oil layer has not grown. When the distillation is finished, the raw material or biomass will look like ‘spent’
- Turn off the connections at the end and let the entire set up cool for 30mins
- Disconnect the receiver from condenser with utmost care not to disorient the oil layer.
- You can label and weigh an empty vial for essential oil collection.
- With proper cover and lid, store oil for further analysis.
Application of Steam distillation:
Steam distillation is widely employed in the manufacturing of essential oils; you can take example of perfumes. It is also applied in the production of consumer food products and petroleum industries. It can also be helpful in separation of fatty acids from mixtures. Eucalyptus oil, Camphor oil and many other oils are obtained by this method on industrial scale.
Advantages of steam distillation:
- The process is highly recommended for temperature-sensitive compounds like aromatic plant materials.
- Steam distillation allows you to control the temperature and amount of steam. This prevents the degradation of essential oils.
- It is a relatively cheap process as compared to the CO2 process and so can be easily affordable.
Disadvantages of steam distillation:
- The process is not applicable to some of the citrus family plants. So usually the cold press method is used for such a process.
- The process has a high initial cost of investment in the equipment. One needs to be trained for operating the equipment.
I hope the information given in this article help you in best possible ways.
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