Difference Between Fractional Distillation and Cracking

Main Difference – Fractional Distillation vs Cracking

A petroleum refinery is a process of chemical engineering in which natural crude oil is processed to obtain useful products. A petroleum refinery unit can be defined as a large factory complex. They are composed of several different processing units. Fractional distillation and cracking are two such processes. Fractional distillation is used to separate different compounds in crude oil while cracking is used to obtain smaller hydrocarbons from large hydrocarbons. The main difference between fractional distillation and cracking is that fractional distillation involves the separation of compounds through distillation according to their boiling points whereas cracking involves the breakdown of large molecules into smaller molecules.

Key Areas Covered

1. What is Fractional Distillation
      – Definition, Mechanism
2. What is Cracking
      – Definition, Mechanism
3. What is the Difference Between Fractional Distillation and Cracking
      – Comparison of Key Differences

Key Terms: Cracking, Crude Oil, Fractional Distillation, Hydrocarbons, Petroleum

What is Fractional Distillation

Fractional distillation is the process used to separate 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

1. Crude oil is heated to a very high temperature at a high pressure.
2. Then the crude oil starts to vaporize.
3. This vapour 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 vapour to pass through the column.
5. There is a temperature gradient across the column. The bottom is filled with hot vapour, but the top of the column is cold.
6. Therefore, the vapour that passes through the column is cooled.
7. At the point, the boiling point of vapour is equal to the temperature of the column, and the vapour is condensed to form the liquid.
8. The vapour is composed of a mixture of components that has 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.

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.

Figure 1: Fractional Distillation Column for Crude Oil

The above image shows a schematic diagram that shows a fractional distillation column. Here, the crude oil is first passed through a furnace. A furnace is a system that can be used to heat a compound to a very high temperature. From the top of the distillation column, gaseous compounds can be obtained.

What is Cracking

Cracking is the process used to break down large hydrocarbon molecules into small hydrocarbons. The cracking reaction is done for the fractions obtained from the fractional distillation of crude oil. The rate of cracking depends on the temperature and the catalysts present in the reaction mixture. We can obtain desired end products by changing these two parameters. The cracking reaction can be done in several ways.

• Thermal Cracking
  • Thermal cracking
  • steam cracking
• Catalytic Cracking
  • Fluid catalytic cracking
  • Hydrocracking

Figure 2: Catalytic Cracking of Petroleum Hydrocarbons

The cracking methods often involve the breakdown of long chain alkanes into small chain alkanes and alkenes. The thermal cracking methods are performed by either direct heating exposure to steam. Catalytic cracking methods involve the use of a catalyst to speed up the cracking reaction. In fluid catalytic cracking, a hot fluid catalyst is used whereas in hydrocracking, hydrogen gas is used at comparatively lower temperatures. Cracking reaction is very important in improving the octane rating of the fuel, which is used to measure the quality of a fuel and its contribution for knocking effect of engines.

Difference Between Fractional Distillation and Cracking

Definition

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

Cracking: Cracking is the process used to break down large hydrocarbon molecules into small hydrocarbons.

Technique

Fractional Distillation: Fractional distillation uses the difference between boiling points of components in a mixture.

Cracking: Cracking involves the production of small hydrocarbons to improve the octane rating.

Catalysts

Fractional Distillation: Fractional distillation does not use catalysts.

Cracking: Cracking can be done in the presence of catalysts.

Boiling Points

Fractional Distillation: Fractional distillation depends on the boiling points of hydrocarbons in crude oil.

Cracking: Cracking does not depend on the boiling points of hydrocarbons.

Conclusion

Crude oil is a major source of hydrocarbons that is used to produce fuels. Since crude oil is a mixture of different components, it should be processed to obtain a better product that has no impurities. Furthermore, it should be treated in order to obtain highly effective end products. Fractional distillation and cracking are two such methods used in the processing of crude oil. The difference between fractional distillation and cracking is that fractional distillation involves the separation of compounds through distillation according to their boiling points whereas cracking involves the breakdown of large molecules into smaller molecules.

References:

1. Freudenrich, Ph.D. Craig. “How Oil Refining Works.” HowStuffWorks Science, HowStuffWorks, 4 Jan. 2001, Available here. Accessed 30 Aug. 2017.
2. Lichtarowicz, Marek. “Cracking and related refinery.” The Essential Chemical Industry online, Available here. Accessed 30 Aug. 2017.
3. “Distillation, Cracking and Catalyzing; Refining Petroleum.” Watershed Sentinel, Available here. Accessed 30 Aug. 2017.

Image Courtesy:

1. “Crude Oil Distillation” By Users Psarianos, Theresa knott on en.wikipedia – According to Theresa knott’s user page (and her Image gallery), this image was originally created by her. – Mbeychok 23:26, 20 September 2007 (UTC) (CC BY-SA 3.0) via Commons Wikimedia
2. “FCC Chemistry” By Mbeychok – (Public Domain) via  Commons Wikimedia