Main Difference – Steam Distillation vs Fractional Distillation
Most naturally-occurring compounds are not pure since there are several components mixed with each other. We call these unnecessary components present in our sample as impurities. There are several different techniques to separate or purify the substances in mixtures. Distillation is one such method. Distillation is the action of purifying a liquid by a process of heating and cooling. There are four main types of distillation as simple distillation, steam distillation, fractional distillation and vacuum distillation. The main difference between steam distillation and fractional distillation is that steam distillation is used to separate heat-sensitive components whereas fractional distillation is used to separate hydrocarbon fractions in crude oil.
Key Areas Covered
1. What is Steam Distillation
– Definition, Principle, Technique
2. What is Fractional Distillation
– Definition, Steps of the Process
3. What is the Difference Between Steam Distillation and Fractional Distillation
– Comparison of Key Differences
Key Terms: Boiling Point, Distillation, Fractional Distillation, Impurities, Steam Distillation, Vapor, Vapor Pressure
What is Steam Distillation
Steam distillation is a process used to separate heat-sensitive components in a mixture. Hence it is used as a purification technique for compounds having impurities. In order to do this separation, the components in the mixture should be volatile.
The principle behind the steam distillation process is to separate the components by vaporizing them at temperatures below their actual boiling point. Unless otherwise, some compounds may decompose at their boiling points so the separation cannot be done accurately.
Figure 1: Steam Distillation Apparatus
In order to drop down the boiling point of components in the mixture, steam or water is added to the mixture. The mixture is filled to a round bottom flask and is then fixed to the distillation apparatus. Then the mixture is heated while agitation. This causes the components in the mixture to exert into their vapor phase, creating a vapor pressure inside the apparatus. This causes the vapor pressure inside the apparatus to increase. When the total vapor pressure coming from all components exceed the atmospheric pressure, the mixture starts to boil. Here the mixture is boiling not because the temperature equals the boiling point, but because of the increased vapor pressure. Therefore the boiling occurs at a lower temperature than the boiling point.
What is Fractional Distillation
Fractional distillation is the process used to separate hydrocarbon components in crude oil. This method includes the separation of the important components according to the difference between their boiling points. In other words, it uses distillation for the fractionation of crude oil.
Steps in Fractional Distillation Process
- Crude oil is heated to a very high temperature at a high pressure.
- Then the crude oil starts to vaporize.
- This vapor then enters fractional distillation column from the bottom of the column.
- This column is composed of plates that have tiny holes (the plates are in different levels or heights). These holes allow the vapor to pass through the column.
- There is a temperature gradient across the column. The bottom is filled with hot vapor, but the top of the column is cold.
- The vapor that passes through the column is cooled.
- At the point when the boiling point of vapor is equal to the temperature of the column, the vapor is condensed to form the liquid.
- The vapor is composed of a mixture of components that have different boiling points. Therefore, different components condense at different temperatures at different heights of the column.
- The plates collect the condensed liquids. These liquids can be further cooled in condensers and are transferred to storage tanks for further processing.
Figure 2: Fractional Distillation Apparatus
The liquid portions collected from different plates are called fractions of crude oil. From this fractional distillation method, one can separate a mixture of components that have even slight differences in their boiling points.
Difference Between Steam Distillation and Fractional Distillation
Definition
Steam Distillation: Steam distillation is a process used to separate heat-sensitive components in a mixture.
Fractional Distillation: Fractional distillation is the process used to separate hydrocarbon components in crude oil.
Technique
Steam Distillation: Steam distillation is done by distillation followed by condensation.
Fractional Distillation: Fractional distillation is done via repeated distillations and condensations.
Principle
Steam Distillation: Steam distillation is done by vaporizing chemical components at temperatures below their boiling point.
Fractional Distillation: Fractional distillation is done by vaporizing chemical components at their boiling point.
Major Use
Steam Distillation: Steam distillation is used to separate components that are heat-sensitive.
Fractional Distillation: Fractional distillation is used to separate different hydrocarbon fractions in the crude oil.
Conclusion
Distillation is the action of purifying a liquid by a process of heating and cooling. Steam distillation and fractional distillation are two types of distillation methods. The main difference between steam distillation and fractional distillation is that steam distillation is used to separate heat-sensitive components whereas fractional distillation is used to separate hydrocarbon fractions in crude oil.
References:
1. Helmenstine, Anne Marie. “Steam Distillation Definition and Principle in Chemistry.” ThoughtCo, Sep. 13, 2017, Available here.
2. “Steam distillation.” Wikipedia, Wikimedia Foundation, 22 Sept. 2017, Available here.
3. “Fractional Distillation.” [email protected], Available here.
Image Courtesy:
1. “Steam dist.” By Joanna Kośmider – Own work, Public Domain) via Commons Wikimedia
2. “Fractional distillation lab apparatus blank version” By William Crochot – Fractional_distillation_lab_apparatus.svg (CC BY-SA 3.0) via Commons Wikimedia
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