Industrial Cracking

This lesson covers: 

1. Why heavier fractions need to be cracked
2. The process of cracking hydrocarbons
3. Types of cracking; thermal and catalytic

Meeting demand through cracking

The process of refining crude oil by fractional distillation yields varying amounts of hydrocarbons. This produces larger quantities of long-chain hydrocarbons like bitumen, and smaller quantities of short-chain hydrocarbons, such as petrol, diesel, and naphtha.

Short-chain hydrocarbons are in higher demand than long-chain hydrocarbons. This is because they are more efficient as fuels and serve as crucial chemical feedstocks for producing in-demand consumer products, including plastics, fabrics, and packaging materials.

To balance this demand, the heavier, less useful fractions are converted into smaller, more valuable molecules through a process called cracking.

Cracking breaks long-chain alkanes down

Cracking is a key process that involves breaking the carbon-carbon bonds in larger alkane chains to form smaller alkenes and alkanes. It is a type of thermal decomposition reaction.

For instance:

Decane ➔ octane + ethene

C₁₀H_22 ➔ C₈H₁₈ + C₂H₄

Key points:

• Cracking transforms less useful long-chain alkanes into more valuable smaller hydrocarbons.
• The point at which the chain breaks is random, leading to a variety of product combinations.
• Cracking is also essential for producing alkenes, which are used in manufacturing plastics and polymers.

Thermal and catalytic cracking

Two principal methods of cracking exist:

1. Thermal cracking
2. Catalytic cracking

1. Thermal cracking

• Thermal cracking operates at very high temperatures (approximately 1,000°C) and pressures (about 70atm).
• This process generates a high yield of alkenes, which are vital for creating numerous valuable products.
• For example, ethene, a common product of thermal cracking, can be polymerised to manufacture polyethene, a widely-used plastic.

2.   Catalytic cracking

• Catalytic cracking employs a zeolite catalyst (a hydrated aluminosilicate mineral) and requires lower temperatures (around 450°C) and pressures than thermal cracking.
• This process primarily produces aromatic hydrocarbons and fuels for vehicles. Aromatic hydrocarbons contain highly stable benzene rings with delocalised electrons.

Catalysts enhance the efficiency of the cracking process in two ways:

• They allow for lower temperatures and pressures, reducing energy consumption and production costs.
• They increase the rate of reaction, enabling faster production of desired products.

The table below compares the conditions and products of thermal and catalytic cracking: