Packed absorption columns: the onset of flooding typically corresponds to a pressure drop of about how many millimetres of water per metre of packing height?

Introduction / Context:
Flooding in packed towers is a hydraulic limit where liquid holdup rises sharply and pressure drop increases rapidly, causing loss of mass-transfer efficiency and potential entrainment. Designers use pressure-drop criteria and capacity correlations to estimate safe operating envelopes and turndown.

Given Data / Assumptions:

• Random packing operation under typical gas–liquid services.
• Pressure drop referenced as mm H2O per metre of packed height.
• Interest is in an indicative range at the onset of flooding.

Concept / Approach:
Empirical practice suggests that flooding onset corresponds to modest pressure-drop levels, often within several tens of mm H2O per metre. A commonly referenced bracket is roughly 50–75 mm H2O/m at the approach to flooding for many ring or saddle packings, though exact values depend on fluid properties, packing size, and distributor quality.

Step-by-Step Solution:

Verification / Alternative check:
Packed-tower design literature and vendor curves align with similar onset ranges, adjusting for packing geometry and size.

Why Other Options Are Wrong:

• 25–50 can be too conservative for many packings.
• 200–250 or higher represent deep flooding, not onset.
• Very small values (5–10) are well below typical limits.

Common Pitfalls:
Ignoring distributor maldistribution; extrapolating one packing’s data to another; operating too close to flooding without adequate control.

Final Answer:
50–75