Vacuum column operation: Typical absolute pressure (vacuum) maintained in a vacuum distillation unit processing reduced crude is closest to which value (in mmHg)?

Introduction:
Vacuum distillation lowers boiling temperatures to prevent cracking of heavy fractions. The absolute pressure in the vacuum column is a key design/operation parameter, typically expressed in mmHg (torr). The question asks for a representative operating level for reduced crude service.

Given Data / Assumptions:

• Service: atmospheric residue to vacuum column.
• Target: sufficiently low pressure to cut VGO and vacuum residue without thermal degradation.

Concept / Approach:
Typical absolute pressures in the flash zone and upper sections are on the order of 10–40 mmHg, achieved with steam ejectors and vacuum systems. 12 mmHg sits within this normal operating window. Values near 700 mmHg approach atmospheric pressure (ineffective), while 120 mmHg is too high for deep vacuum duty; 1.2 mmHg is achievable in lab/cryogenic systems, not common for large crude vacuum towers.

Step-by-Step Solution:

Verification / Alternative check:
Design texts show flash-zone pressures commonly around 20 mmHg with top pressures even lower; 12 mmHg is a representative figure.

Why Other Options Are Wrong:

• 1.2: Overly deep, impractical for crude towers.
• 120: Not deep enough for reduced crude separation.
• 700: Essentially atmospheric; defeats purpose.
• 380: Partial vacuum but far above typical crude vacuum conditions.

Common Pitfalls:
Confusing gauge vacuum with absolute pressure; this question uses absolute mmHg.

Final Answer:
12