Packed columns with saddle packing: how frequently should liquid redistributors be installed (relative to column diameter) to maintain uniform wetting?

Introduction / Context:
In packed towers, uniform irrigation is critical for mass transfer efficiency. Maldistribution increases HETP and reduces capacity. Liquid redistributors are strategically placed to restore uniformity after maldistribution accumulates with height, especially with random packings like saddles.

Given Data / Assumptions:

• Random saddle packing (e.g., Berl/Intalox types).
• Typical process fluids with moderate surface tension and viscosity.
• Conventional irrigation density and nozzle layout.

Concept / Approach:
Liquid spreads non-ideally due to wall effects and channeling, so redistributors are added after a certain packed height. Industry guidelines commonly recommend spacing on the order of several column diameters, bounded above by a maximum absolute spacing (e.g., ≈6 m), to maintain acceptable distribution quality.

Step-by-Step Solution:
Identify accepted heuristic: use redistributors every 5–8 column diameters.Apply cap: do not exceed about 6 m between redistributors.Therefore, choose “Every 5–8 times the column diameter or 6 m, whichever is less.”

Verification / Alternative check:
Pilot and plant experience show marked performance improvements when redistribution intervals stay within this range, particularly for larger diameters and high turndown operation.

Why Other Options Are Wrong:
2–3 D is overly conservative and costly; 10–15 D or 20–25 D risks severe maldistribution; “not needed” is incorrect for most real towers.

Common Pitfalls:
Ignoring effects of liquid rate turndown; assuming initial distributor quality eliminates the need for periodic redistribution.

Final Answer:
Every 5–8 times the column diameter or 6 m, whichever is less