Specify the typical absolute pressure range (mm Hg abs) maintained in the vacuum distillation tower of a crude distillation plant to enable separation without thermal cracking.

Introduction / Context:
Vacuum towers operate at low absolute pressures to depress boiling temperatures for heavy hydrocarbons, preserving quality and limiting undesirable reactions. Selecting the right pressure window is key to balancing separation and equipment constraints.

Given Data / Assumptions:

• Pressure expressed as absolute mm Hg.
• Industrial-scale VDU with steam ejectors or dry vacuum systems.
• Goal: minimize thermal stress and coking.

Concept / Approach:
Operating in the tens of mm Hg absolute provides adequate temperature reduction for vacuum gas oils and residue cuts. Going too high compromises separation; going too low is impractical for capacity and cost. Hence, the canonical operating window is ~30–80 mm Hg abs in many designs.

Step-by-Step Solution:

Verification / Alternative check:
Refinery references report similar pressure bands at the flash zone and rectifying sections, enabling target cut points.

Why Other Options Are Wrong:

Common Pitfalls:
Mixing up absolute and gauge pressure reporting.

Final Answer:
30-80