Crude distillation basics – topping unit The crude “topping” column (first-stage crude distillation) normally operates at what overall pressure regime?

Introduction / Context:
The first major refinery separation step is the crude distillation unit (CDU), often called the topping unit, which separates crude into light gases, naphtha, kerosene, gas oils, and atmospheric residue. Knowing its pressure regime is foundational for understanding downstream unit feeds and operating constraints.

Given Data / Assumptions:

• We refer to the atmospheric crude column (not the vacuum column).
• Minor pressure drops across trays/packing and overhead systems exist but do not change the classification.
• Units include preheat trains and a fired heater leading into the column.

Concept / Approach:
The topping column operates close to atmospheric pressure (slightly above to ensure positive flows and to accommodate pressure drops). Heavier fractions that would thermally degrade at their atmospheric boiling points are later separated under vacuum in a separate vacuum distillation column, not by raising pressure.

Step-by-Step Solution:

Verification / Alternative check:
Standard refinery flow diagrams label the first column as atmospheric and the second as vacuum; operating pressures hover near 1 atm.

Why Other Options Are Wrong:

• 10 atm / 3 atm: Far above normal CDU top pressure; would complicate equipment and not aid separation.
• Vacuum: This describes the VDU, not the CDU.

Common Pitfalls:
Thinking higher pressure improves fractionation; in distillation, pressure selection balances temperature limits and condenser duties.

Final Answer:
Atmospheric