Crude distillation practice: Why is crude oil subjected to vacuum distillation in the final stage of fractionation?

Introduction / Context:
Atmospheric crude distillation separates naphtha to gas oil cuts. Heavier fractions require higher temperatures to boil, which risks cracking and coking. Vacuum distillation enables recovery of heavy distillates without excessive thermal stress.

Given Data / Assumptions:

• Final stage uses reduced pressure (vacuum).
• Target products: vacuum gas oils and lube distillates.
• Thermal stability of heavy hydrocarbons is limited at atmospheric pressure boiling temperatures.

Concept / Approach:
Boiling point decreases with pressure. Operating under vacuum allows the column to vaporize high-boiling components at lower temperatures, reducing cracking/coking, preserving quality (viscosity index, color), and improving heat economy and run length.

Step-by-Step Solution:

Verification / Alternative check:
Standard crude/vacuum unit (CDU/VDU) design texts emphasize vacuum columns for lube distillates and VGO with minimized thermal cracking and coking propensity.

Why Other Options Are Wrong:

• Lighter products decompose more: In refineries, high-boiling material is more at risk during high-temperature exposure.
• Very high purity goal: Purity is not the primary reason; thermal protection is.
• None of these: Incorrect because there is a clear thermal sensitivity rationale.

Common Pitfalls:
Assuming vacuum distillation is for purity alone; confusing it with deep cut polishing or extractive operations.

Final Answer:
To avoid decomposition of heat-sensitive heavy products by boiling them at lower temperatures under vacuum.