Explain why high-boiling fractions such as atmospheric residue are distilled under vacuum at comparatively low temperatures: at elevated temperatures, which undesired phenomenon tends to predominate?

Introduction / Context:
Very heavy crude fractions cannot be distilled at atmospheric pressure without exposing them to extreme temperatures. Refiners employ vacuum distillation to lower boiling points and protect product quality and equipment.

Given Data / Assumptions:

• Feed: atmospheric residue/high-boiling components.
• Objective: separate into vacuum gas oils and residue safely.
• Concern: avoid thermal degradation at high temperatures.

Concept / Approach:
At sufficiently high temperatures and residence times, large hydrocarbon molecules undergo thermal cracking, producing lighter gases and coke precursors. Vacuum operation reduces required temperatures, limiting the rate of these reactions and preserving desired yields and properties.

Step-by-Step Solution:

Verification / Alternative check:
Process design literature states vacuum distillation is chosen to avoid thermal cracking that would occur at atmospheric pressures for these boiling ranges.

Why Other Options Are Wrong:

Common Pitfalls:
Using “coking” as the primary reason; while coke is a severe outcome, the underlying driver is thermal cracking which vacuum aims to suppress.

Final Answer:
Thermal cracking