Gas–liquid contacting design: commercial packed scrubbers are typically sized for what pressure-drop range per metre of packed height (expressed in mm of water column)?

Introduction / Context:
Packed scrubbers (absorption columns) remove gaseous pollutants using liquid contact over random or structured packings. A key hydraulic choice is allowable pressure drop per metre of packing, which trades off blower power against mass-transfer performance and flooding margin.

Given Data / Assumptions:

• Steady operation with clean service or modest fouling.
• Random packings or moderate-resistance structured packings.
• Design below the flooding point with a reasonable turndown.

Concept / Approach:
Pressure drop increases as gas rate rises toward flooding. Designers therefore select a moderate range that provides good liquid distribution and film renewal without penalizing operating power. Practical experience places typical design ΔP values in a few tens of millimetres of water per metre of packing for many air pollution and absorption duties.

Step-by-Step Solution:

Verification / Alternative check:
Pilot tests and vendor rating programs show curves with typical operation well below flooding, often giving per-metre drops in the 20–30 mm H2O region for common packings and loads.

Why Other Options Are Wrong:

• 4–8 mm H2O: too low for reliable contacting; risks maldistribution/inefficient transfer.
• 52–68 or 88–105 mm H2O: unnecessarily high for most services; increases blower power and may reduce turndown margin.

Common Pitfalls:
Ignoring liquid distribution quality; designing too close to flooding; not accounting for fouling which elevates pressure drop over time.

Final Answer:
17–34 mm H2O per m