Packed column practice: In industrial packed columns, small packings are usually dumped, but packings larger than which nominal size are typically stacked individually to control bed porosity and lower gas pressure drop?

Introduction / Context:
How packing is installed affects void fraction, liquid distribution, and pressure drop. Random dumping is convenient for small elements, while large pieces are stacked or oriented to ensure uniform channels and predictable performance.

Given Data / Assumptions:

• Random packings such as Raschig rings, Pall rings, or saddles.
• Concern is industrial practice for large sizes.
• Goal: minimize maldistribution and reduce pressure drop.

Concept / Approach:
As element size increases, random dumping can create large voids and poor liquid distribution. Stacking large packings creates ordered pathways and consistent porosity. A commonly cited threshold for random versus stacked installation is about 75 mm nominal size.

Step-by-Step Solution:
Identify operational issue: maldistribution and channeling with large elements.Engineering remedy: stack or orient individual large pieces.Rule-of-thumb cutoff ≈ 75 mm → choose 75 mm.

Verification / Alternative check:
Vendor literature notes stacked installation for very large rings/saddles (≥ 70–90 mm) to stabilise porosity and capacity curves.

Why Other Options Are Wrong:
25 or 50 mm remain in the typical random-dumped range; 150 mm is beyond normal sizes and would almost always be stacked but is not the threshold.

Common Pitfalls:

• Ignoring distributor quality; even stacked packings require good liquid distribution.
• Assuming pressure drop only depends on size; wetting and loading matter too.

Final Answer:
75 mm