[Chemistry Class Notes] on Cracking Meaning Pdf for Exam

Cracking meaning in Chemistry pertains to such a process that involves the breaking down of high molecular weight compounds into smaller fragments. Such compounds are usually long-chained hydrocarbons.

This process finds extensive usage in the petroleum industry for the production of diesel and gasoline from kerogens and long-chain hydrocarbons. The large-scale commercial production yields oils, and a host of other materials like medium-ranged oils, gasoline (light oils), heavy oils, coke (carbon residues). Based on utility as well as molecular weight, the produced oil may be extracted directly or put another step of cracking for smaller fragments.  

Process of Cracking 

1. Initiation- Free radicals are important to undergo the process of Initiation. One molecule is broken into two free radicals. Out of the two, the smaller one undergoes the process of initiation. 

2. Hydrogen Abstraction-  Hydrogen is removed from the second molecule thereby making it a free radical. 

3. Radical Decomposition- Free radicals break into other free radicals to form alkene products. 

4. Radical Addition- Then the free radicals react with the alkenes. 

5. Termination- Two radicals react with each other to form a non-free radical product in the process of termination. 

Types of Cracking

The different types of cracking are thermal cracking and catalytic cracking. Here is a brief elaboration on the two –

1. Thermal Cracking 

The thermal method of cracking uses pressure and heat to break down large hydrocarbon molecules into lighter molecules. The process has been used for a long time in large scale industries. It is carried out at temperatures between 500 degrees celsius to 1000 degree celsius and at high pressures. This process can be further categorised into –

a. Steam Cracking 

This is an older process which uses the heat energy generated from steam. It is also referred to as Pyrolysis. Due to the presence of greater latent heat of energy in the steam, it is considered to be a useful technique for the production of alkenes.

 

The raw materials used in the process include propane-butane mixture, naphtha (obtained from directly distilling crude, and gas oil (in rare instances).  

b. Modern Thermal Cracking 

Both high pressure and high temperature are used for this method. Homolytic fission takes place among carbon bonds of hydrocarbon compounds. This reaction gains industrial importance with the production of burner fuels. 

By lowering the temperature, the delayed coking process takes place. It leads to the production of needle coke which is used for carbon electrodes. 

2. Catalytic Cracking

As understood from the term, a catalytic cracking process is undertaken in presence of a catalyst. The large molecules are broken in such a manner that it leads to the maximisation of gasoline yield. 

There are three sub-processes involved in this method –

1. Reaction

2. Regeneration 

3. Fractionation 

In the first step which is reaction, the feedstock or the hydrocarbon is kept in contact with the catalyst at a high temperature (480 degree celsius- 500 degree celsius). As the cracking reaction takes place, coke is formed which coats the catalyst further preventing it from promoting the reaction. The catalyst is regenerated in the second process called the regeneration by burning the coke off from its surface where it got deposited.  The cracked hydrocarbons are then separated in the final process of fractionation with the help of an equipment. 

Catalytic Cracking Can be Further Categorised Into –

a. Hydrocracking 

It involves a two-stage process that includes a combination of hydrogenation and catalytic cracking. In the presence of hydrogen, heavier feedstocks are being cracked to arrive at the desirable products. 

This process needs high temperature, high pressure, hydrogen, and a catalyst. The process is also largely dependent on relative rates of the two reactions and the nature of the feedstock. 

b. Fluid Catalytic Cracking 

Fluid Catalytic Cracking is the most popular catalytic process today. The process is a little distinct. The feedstock is subjected to moderate pressure and high temperature. It is subsequently brought into contact with powdered and hot catalysts. The long-chain molecules are broken down by such a catalyst into considerably shorter molecules. 

Difference Between Thermal Cracking and Catalytic Cracking

Importance in Cracking

There are two major functions of cracking:

• The supply of fractions is matched with the demand. Crude oil’s fractional distillation aids in the production of larger hydrocarbons as opposed to smaller hydrocarbons. Once cracking leads to conversion of smaller hydrocarbons from larger ones, fuels such as petrol are formed. The supply of fuel has improved. 

• The production of alkenes is also undertaken with the help of cracking. Given that alken
  es are more reactive as opposed to alkanes, the former acts as feedstock for the petrochemical industry. 

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