Which of the following fraction is used as raw material for the cracking process?

Analysis of the Question

The question asks for the specific fraction of crude oil that serves as the primary raw material for the cracking process. Cracking is a chemical process in petroleum refining where heavy, long-chain hydrocarbons are broken down into smaller, more useful, and more volatile molecules (like petrol/gasoline).

Concept Explanation

• Crude Oil Refining: Crude oil is separated into various fractions based on boiling points using fractional distillation.
• Cracking: This process is essential because the demand for lighter fractions (like petrol) is much higher than what is naturally produced during distillation.
• Raw Materials: Cracking typically uses "heavy" fractions—those with high boiling points and long carbon chains—that are less valuable or in lower demand. These include gas oils and diesel oils. By breaking these long chains, refineries convert them into high-demand products like petrol (gasoline) and alkenes.

Step-by-Step Solution

1. Evaluate the options: • Kerosene: Used primarily as fuel for jet engines and domestic heating/lighting. It is already a relatively light fraction. • Lubricating oil: Used for reducing friction in machinery. It is a very heavy fraction but is more valuable as a lubricant than as a feedstock for cracking. • Bitumen: The heaviest residue left at the bottom of the distillation tower. It is used for road surfacing and roofing; it is generally too viscous and complex to be the primary feedstock for standard cracking units. • Diesel oils (Gas oils): These are the middle-to-heavy distillates. They have the ideal chain length to be cracked into petrol and other lighter, high-value hydrocarbons.

2. Conclusion: In the context of industrial petroleum chemistry, diesel oils (often referred to as gas oils) are the standard feedstock fed into catalytic or thermal crackers to increase the yield of petrol.

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Exam Tip

When you see questions about "cracking," always look for the "heavier" fractions. Remember the hierarchy: Lighter fractions (like petrol) are the products of cracking, while heavier fractions (like diesel/gas oil) are the raw materials for cracking.