Difference Between Thermal Cracking and Catalytic Cracking

Main Difference – Thermal Cracking vs Catalytic Cracking

Petroleum refining is the processing of crude oil in order to obtain desired products. There are several petroleum refining processes that are helpful in converting crude oil into useful products. A refinery is a large industrial area that is composed of a number of processing units. The reactions that take place in a refinery includes distillation, cracking reactions, reforming reactions, polymerization, isomerization, etc. Thermal cracking and catalytic cracking are such reactions that are used to break down large molecules into smaller compounds. The main difference between thermal cracking and catalytic cracking is that thermal cracking uses heat energy for the breakdown of compounds whereas catalytic cracking involves a catalyst to obtain products.

Key Areas Covered

1. What is Thermal Cracking
      – Definition, Mechanism, and Examples
2. What is Catalytic Cracking
      – Definition, Mechanism, and Examples
3. What is the Difference Between Thermal Cracking and Catalytic Cracking
      – Comparison of Key Differences

Key Terms: Catalyst, Catalytic Cracking, Cracking, Crude Oil, Isomerization, Hydrocracking, Liquid Phase Catalytic Cracking, Petroleum, Refinery, Polymerization, Thermal Cracking, Vapor Phase Catalytic Cracking

What is Thermal Cracking

Thermal cracking is the process of breaking down large compounds into small compounds at high temperatures and high pressures. The end products of the thermal cracking are small hydrocarbon molecules. The temperature used for this process is about 500-700^oC. The pressure is about 70 atm.

Thermal cracking involves the breaking of carbon-carbon bonds and carbon-hydrogen bonds. The products of thermal cracking are always smaller than reactants. Most of the times, the end products are small alkanes and alkenes. But sometimes, small unsaturated molecules such as alkynes are also given.

Figure 1: An Oil Refinery

When chemical bonds form, energy is released. Likewise, in order to break a chemical bond, energy is required. Thus, the reactions including bond breaking require energy from outside, and thermal cracking is highly endothermic. The change in enthalpy is a large positive value. Due to the formation of small molecules from large molecules, the entropy is also increased.

Modern refineries use thermal cracking processes for three major applications. They are visbreaking, thermal gasoline production, and delayed coking. Visbreaking is a process used to reduce the viscosity of the fuel. Thermal gasoline production involves both reductions of the viscosity the recovering of a maximum amount of gasoline. The goal of delayed coking is to maximize the formation of cracking products.

What is Catalytic Cracking

Catalytic cracking is the breakdown of large compounds into small hydrocarbons using an acid catalyst. This cracking process can be done at a less temperature and pressure condition. Therefore, the operating of the processing unit is much easier than that of thermal cracking.

Figure 2: A Fluid Catalytic Cracker

Modern crackers use Zeolite as the catalyst. Zeolite is a complex aluminosilicate. When zeolite is used for this cracking process, we can use moderate temperatures such as 450^oC and moderate pressures.

Catalytic cracking can be done in two major ways. They are liquid phase cracking and vapor phase cracking. In liquid phase catalytic cracking, the reaction mixture is maintained at a temperature of about 500^oC and 20 atm pressure. Silica or related compounds are often used as the catalyst. This process results in octane numbers ranging from 65 to 70. In vapor phase catalytic cracking, about 600^oC temperature and 10 atm pressure is used. The catalyst used is alumina. This cracking is done in the presence of hydrogen gas. It is also called hydrocracking. Here, the carbon-carbon bonds are broken down.

Difference Between Thermal Cracking and Catalytic Cracking

Definition

Thermal Cracking: Thermal cracking is the process of breaking down large compounds into small compounds at high temperatures and high pressures.

Catalytic Cracking: Catalytic cracking is the breakdown of large compounds into small hydrocarbons using an acid catalyst.

Method

Thermal Cracking: Thermal cracking involves cracking by applying high temperatures and pressures.

Catalytic Cracking: Catalytic cracking involves cracking by adding catalysts along with moderate temperature and pressures.

Temperature

Thermal Cracking: The temperature used in thermal cracking ranges between 500-700^oC.

Catalytic Cracking: The temperature used in catalytic cracking ranges between 475-530^oC.

Pressure

Thermal Cracking: The pressure used in thermal cracking is about 70 atm.

Catalytic Cracking: The pressure used in catalytic cracking is about 20 atm.

Applications

Thermal Cracking: Thermal cracking is used for visbreaking, thermal gasoline production, and delayed coking.

Catalytic Cracking: Catalytic cracking is used to obtain fuel with octane number 65-70.

Conclusion

Thermal cracking and catalytic cracking are two major processes used in petroleum refineries in order to obtain useful products out of crude oil distillates. Both techniques have advantages as well as drawbacks. The main difference between thermal cracking and catalytic cracking is that thermal cracking uses heat energy for the breakdown of compounds whereas catalytic cracking involves a catalyst to obtain products.

References:

1. “Thermal Cracking.” Chemical Engineering Processing, Available here. Accessed 18 Sept. 2017.
2. “Cracking.”  [email protected], Available here. Accessed 18 Sept. 2017.
3. “Cracking alkanes – thermal and catalytic.” Chemguide, Available here. Accessed 18 Sept. 2017.

Image Courtesy:

1. “Imperial Oil Refinery” By The Kurgan (CC BY-SA 3.0) via Commons Wikimedia 
2. “Fluid Catalytic Cracker” By Valero Energy Corporation/TX – (Public Domain) via Commons Wikimedia