Packed absorption tower design data: Operating velocity is typically 40–50% of flooding. What pressure drop is normally used for design per metre of packing (mm H2O per m)?

Introduction:
Preliminary design of packed absorption columns requires an estimate of allowable pressure drop, which balances capacity, efficiency, and energy costs. Designers often reference a typical per-metre pressure drop under normal operating conditions (well below flooding) to select packing and tower dimensions.

Given Data / Assumptions:

• Packed column operating in the loading/preloading region at about 40–50% of flooding velocity.
• Clean service without excessive fouling.
• Standard random or structured packing with good liquid distribution.

Concept / Approach:
As gas superficial velocity increases toward flooding, pressure drop rises sharply. Normal designs target moderate pressure drops to avoid energy penalties and liquid maldistribution. Typical rule-of-thumb values commonly cited for packed absorption services are on the order of a few tens of millimetres of water column per metre of bed height.

Step-by-Step Solution:
Identify the regime: 40–50% of flooding → conservative operation.Typical design value range used in practice: ~20–40 mm H2O per metre of packing.Select the option that matches: 20–40.

Verification / Alternative check:
Vendor charts and handbooks show pressure drop per metre for common packings at preloading conditions in the tens of mm H2O/m, increasing rapidly near flooding.

Why Other Options Are Wrong:

• 1–5 or 5–10: Unrealistically low for typical gas rates.
• 100–150 and 1000–1500: Too high for normal operation; such drops indicate near-flooding or impractical design.

Common Pitfalls:
Designing too close to flooding to minimize column size; this sacrifices controllability and increases pressure drop.

Final Answer:
20–40