Packed towers with Raschig rings: liquid redistribution spacing should be every 6 m or approximately how many column diameters (whichever is lower)?

Introduction / Context:
Liquid maldistribution in packed towers degrades mass-transfer efficiency and can lead to channeling, flooding onset, and off-spec separation. Raschig rings, being relatively simple random packings, are more sensitive to distribution quality than modern structured packings. Designers therefore specify periodic liquid redistributors along the tower height.

Given Data / Assumptions:

• Random packing: Raschig rings.
• Nominal recommendation: redistribute every 6 m vertical height or a specified multiple of column diameter, whichever is smaller.
• Goal: maintain uniform wetting and minimize channeling.

Concept / Approach:
As liquid flows downward, maldistribution increases due to packing void variability and wall effects. Installing redistributors resets the liquid pattern. Common practice for rings is a spacing of roughly 5–7.5 column diameters. Taller spacing risks significant efficiency loss; closer spacing adds cost and pressure drop but may be justified for difficult services or very large diameters.

Step-by-Step Solution:

Verification / Alternative check:
Packed-tower design guidelines commonly present height or diameter-based rules; both generally point to comparable distances for Raschig rings under average services.

Why Other Options Are Wrong:

• 2.5–3.0 D is overly conservative for many services and increases capital unnecessarily.
• 10–12.5 D and above risk maldistribution and reduced efficiency before redistribution.

Common Pitfalls:
Ignoring fluid property effects (viscosity/foaming); using distributor designs with too few drip points; not leveling internals.

Final Answer:
5–7.5