Difference Between Destructive Distillation and Fractional Distillation

Main Difference – Destructive Distillation vs Fractional Distillation

All naturally-occurring compounds are not pure since they contain several components mixed with each other. These unnecessary components in a sample are called impurities. There are several different techniques to separate or purify the substances in mixtures. Distillation is one such method. It is the action of purifying a liquid by a process of heating and cooling. This technique uses the differences between boiling points of the components in the liquid mixture. There are several different types of distillation such as destructive distillation and fractional distillation. The main difference between destructive distillation and fractional distillation is that destructive distillation involves the decomposition of a solid by heating it in a closed container and collecting the volatile constituents whereas fractional distillation includes the separation of important components according to the difference between their boiling points.

Key Areas Covered

1. What is Destructive Distillation
      – Definition, Explanation of a Simple Apparatus
2. What is Fractional Distillation
     – Definition, Steps of Procedure
3. What is the Difference Between Destructive Distillation and Fractional Distillation
     – Comparison of Key Differences

Key Terms: Boiling Point, Coal, Decomposition, Destructive Distillation, Distillation, Fractional Distillation, Volatile

Difference Between Destructive Distillation and Fractional Distillation - Comparison Summary

What is Destructive Distillation

Destructive distillation is the chemical process involving decomposition of a solid by heating it in a closed container and collecting the volatile constituents given off. This method is generally used for organic materials in the absence of air or in a limited amount of air. The destructive distillation cracks large molecules into small portions.

The most common application of this method is the destructive distillation of coal. When coal is heated in the absence of air, it does not burn, instead, many byproducts are formed. Some of the byproducts include coke, coal tar, amino acid liquor and coal gas. Following image shows a simple diagram of a destructive distillation apparatus from which coal gas is produced.

Difference Between Destructive Distillation and Fractional Distillation

Figure 1: A Simple Laboratory Apparatus for Destructive Distillation

Here, finely powdered coal is taken into a clean test tube and heated. With the rise of temperature, coal breaks down to form coke, coal tar, ammonia and coal gas. Coal tar is collected at the bottom of the second test tube. Coal gas escape through the side tube. The second tube contains water that can absorb ammonia produced here. This absorption forms ammonia liquor. The residue left in the first tube is coke.

What is Fractional Distillation

Fractional distillation is the process used to separate hydrocarbon components in crude oil. This method includes the separation of 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

  1. Crude oil is heated to a very high temperature at a high pressure.
  2. The crude oil starts to vaporize.
  3. This vapor then enters fractional distillation column from the bottom of the column.
  4. 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.
  5. There is a temperature gradient across the column. The bottom is filled with hot vapor, but the top of the column is cold.
  6. The vapor that passes through the column is cooled.
  7. 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.
  8. 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.
  9. The plates collect the condensed liquids. These liquids can be further cooled in condensers and are transferred to storage tanks for further processing.
Main Difference - Destructive Distillation vs Fractional Distillation

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 Destructive Distillation and Fractional Distillation

Definition

Destructive Distillation: Destructive distillation is the chemical process involving decomposition of a solid by heating it in a closed container and collecting the resultant volatile constituents.

Fractional Distillation: Fractional distillation is the process used to separate hydrocarbon components in crude oil.

Technique

Destructive Distillation: Destructive distillation involves the separation of components via decomposition of a solid.

Fractional Distillation: Fractional distillation involves the separation of components according to the differences in their boiling points.

Apparatus

Destructive Distillation: A simple destructive distillation apparatus includes two test tubes connected to each other via side tubes, one containing coal and other containing water.

Fractional Distillation: Fractional distillation consists of a fractional distillation column, a condenser, and two containers, one containing the sample mixture and other container is to collect the distillate.

Major Applications

Destructive Distillation: Destructive distillation is mainly used for coal distillation.

Fractional Distillation: Fractional distillation is mainly used to separate constituents in crude oil.

Conclusion

Distillation is a technique used to separate constituents in a mixture. Distillation can be found in several types. Destructive distillation and fractional distillation are two of them. The main difference between destructive distillation and fractional distillation is that destructive distillation involves the decomposition of a solid by heating it in a closed container and collecting the volatile constituents given off whereas fractional distillation includes the separation of the important components according to the difference between their boiling points.

Reference:

1. “Destructive Distillation of Coal.”  Tutorvista.Com, .
2. “Destructive distillation.” Wikipedia, Wikimedia Foundation, 14 Jan. 2018, .
3. “GCSE Bitesize: Fractional distillation.” BBC, .

Image Courtesy:

1. “Fractional distillation apparatus” By GOKLuLe – Own work via

About the Author: Madhusha

Madhusha is a BSc (Hons) graduate in the field of Biological Sciences and is currently pursuing for her Masters in Industrial and Environmental Chemistry. Her interest areas for writing and research include Biochemistry and Environmental Chemistry.

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