Fractional distillation is a type of distillation. In general, distillation is a process in which liquid is heated to its boiling point. As result, vapors of liquid are produced. They are cooled, condensed, and collected into a separated flask. For example, the mixture of water and salt can be separated or desalinated by boiling and condensing the water.
A mixture of volatile liquids is not easy to separate. If they have a slight difference in their boiling points, they can be separated by observing the temperature of the vapors. These vapors are produced by heating the mixture.
Fractional distillation is used to separate the mixture of volatile liquids. This is based on the difference in boiling points of the substances in the mixture. During heating of the mixture, it is passed through a fractionating column. A fractionating column is packed with glass beads or glass tubes that provide a larger surface area for a liquid to boil.
Fractional distillation allows the mixture to be distilled again and again in the column until the separation of components of the mixture takes place.
Apparatus
There are the following parts of the apparatus used in fractional distillation,
- Distillation flask
- Fractionating column
- Thermometer
- Condenser
- Distillation adapter
- Product receiver flask
Fractionating column
Fractionating column is used to separate the mixture of volatile liquids on the basis of boiling points. Its efficiency can be determined by the time of separation. There are the following types of fractioning columns used in chemical or pharmaceutical laboratories.
Vigreux column
This type of fractionating column is widely used in laboratories. It is made of glass. There is a series of small chambers inside the glass tube. The ends are joined from the bottom and make a slope known as indentation. The mixture passes through the column by moving from these indentations or depressions. That’s why the efficiency of the vigreux column has a moderate level.
Dufton column
This column is used for general purposes. It has better efficiency than the vigreux column. From inside, there is a spiral tube of glass surrounding a straight tube of glass. This gives benefits such as liquid having low volume retained inside the column.
Hempel column
Hempel column has better efficiency than dufton and vigreux column. It is a tube that is packed with glass or porcelain rings from inside. Porcelain rings give high-efficiency separation of the liquids from the mixture.
Pear bulb column
It contains pear-shaped bulbs inside the glass tube. These bulbs increase the surface area for cooling the mixture. The efficiency of the column can be increased by increasing the number of glass bulbs.
Working
Take a sample of the mixture in a distillation flask that is usually rounded from the bottom. Fit the fractionating column vertically with the flask. A thermometer can be placed on the top of the column for observing the temperature. Attach the condenser to the receiver flask for collecting the product liquid.
After setting the apparatus, a hot plate can be placed under the distillation flask. It starts to heat the sample mixture. Adjust the heat from the hot plate in such a way that one drop of the liquid passes in 2-3 seconds from the column. This makes fractional distillation more efficient. In this way, a better-distilled yield of product can be achieved.
Allows the vapors of liquid to rise at the thermometer bulb and adjust the temperature. In this way, the thermometer will remain bathed in liquid vapors or droplets as the mixture distills. Note the temperature on the thermometer, when the first component of the mixture starts to distill. Collect the distillate product from the condenser into the receiver flask.
Distillation is maintained by giving a small amount of heat to the distillation flask. The temperature should be continuously monitored. At a point, the first component of the mixture is finished distilling, then the temperature will rise suddenly to several degrees celsius. At this moment, remove the first receiver flask (collecting the first component distillate) from the condenser and replace it with the other receiver flask to collect the distillate of the second component. Stopper the first flask to prevent it from evaporation.
Record the temperature, when the second component begins to distill. Keep distilling the second component until 5ml to 6ml of liquid remains in the distillation flask. At the end of distillation, remove the hot plate but not the receiver flask. The collecting or receiver flask can be removed when distillation stops.
By using a graduated cylinder, measure the volume of each component of the mixture. Calculate the composition of the original sample to determine the percentages of low boiling and high boiling components of the sample. The percentage obtained is not completely accurate. This is due to the loss of distilled liquids and not being properly isolated.
Industrial distillation
Fractional distillation is used in oil refineries to separate the crude oil from the petroleum mixture and convert it into pure hydrocarbons products. This is the first step to refining the crude oil. This process occurs in a tower called a fractional distillation tower.
Besides crude oil, there are also other components of the mixture separated. They are known as fractions, such as kerosene oil, diesel, gasoline, bitumen, etc.
The process of industrial fractional distillation is the same as in the laboratories. The only difference is in the apparatus. Instead of fractionating columns, towers are used for the separation of products on large scale.
First, the crude oil is heated in a furnace and sent to the bottom of the distillation tower. There is about 400 oC temperature at the tower’s bottom. This converts the crude oil into vapors form. These vapors move upward direction in the tower and the temperature becomes decreases. Due to a decrease in the temperature, different hydrocarbons condense at different temperature levels.
After the condensing of hydrocarbons, they are collected into different containers. In this way, they are separated on the basis of their boiling points. The distillates obtained from crude oil are differentiated into three fractions below,
Light distillates
- They are light in weight and obtained from the top side of the distillation tower.
- Light distillates are more useful hydrocarbons and highly volatile.
- These distillates are ignitable and flowable.
- Their boiling points are very low, ranging from 70-200 oC.
- Light distillates are gasoline, kerosene oil, jet fuel, and paraffin.
Intermediate distillates
- They are found in the middle of the distillation tower.
- Their boiling points are high (200-350 oC).
- Intermediate distillates are gas oil and diesel fuel.
Heavy distillates
- These products are obtained from the lower side of the tower.
- The temperature of heavy distillates are ranging from 350-365 oC.
- They have low volatility and higher molecular weight.
- Heavy distillates are fuel oil.
Uses
- It is used for the purification of water.
- During polymerization, monomers start to form dimers that make the polymer chain weaker, fractional distillation is used to remove them from the main chain of the polymer.
- In the pharmaceutical industry, when a high-purity product is required, it is purified by fractional distillation.
- Unstable products must be made through fractional distillation. During organic synthesis, low boiling products are formed that can be separated through this process while reactants remain behind.
- It is used in chemical industries for the purification of organic compounds a large scale. In industries, the apparatus of fractional distillation is different from the laboratory apparatus.
Concepts Berg
What is the difference between simple and fractional distillation?
Simple distillation is used to separate a mixture of liquids that have large differences in their boiling points. However, if the boiling point difference of liquids is very close to each other then this mixture is separated in fractional distillation.
What is the principle of fractional distillation?
The principle of fractional distillation is the separation of miscible liquids on the basis of boiling points.
What are the 4 steps of fractional distillation?
The steps are below,
- Heating
- Evaporation
- Condensing
- Collection
What type of mixture is separated by distillation?
Distillation is used to separate liquids from other substances. That substance can be liquid, solid, etc.
What is the aim or purpose of fractional distillation?
Fractional distillation is used for the purification of chemicals. A liquid can be purified by fractional distillation of the mixture.
What are the advantages of fractional distillation?
It is a better technique compared to simple distillation. Fractionating column used for distillation in fractional distillation. This gives a large surface area for the separation of liquid from the mixture. It also increases the efficiency of separation.
In fractional distillation, why is the hottest part at the bottom?
After the collection of the first component of the mixture, the temperature abruptly increases. Because the second component has a higher boiling point as compared to the first component. The second component remains at the end of the flask which has the higher temperature.
What is the purpose of condensers in fractional distillation?
A condenser is used for cooling the vapors and converting them into a pure form of liquid. This liquid can be collected into the receiver flask.
Reference Books
- Experimental Organic Chemistry By Philippa B. Cranwell, Laurence M. Harwood, Christopher J. Moody
- Laboratory Techniques in Organic Chemistry By V. K. Ahluwalia, Pooja Bhagat, Renu Aggarwal
