Industrial distillation: for large columns of approximately 3–6 m diameter, what is the typical round bubble-cap size selected during tray design?

Introduction / Context:
Bubble-cap trays disperse vapor into liquid via caps mounted over risers. Cap size affects pressure drop, capacity, froth behavior, and tray hydraulics. Designers select cap diameters that balance mechanical practicality with mass-transfer performance on columns of a given size.

Given Data / Assumptions:

• Column diameter in the range of 3–6 m (typical major refinery and petrochemical towers).
• Standard cap geometries and spacing with reasonable vapor and liquid loads.

Concept / Approach:
Experience shows that cap diameters of about 75 mm (7.5 cm) are common on large columns, allowing practical cap counts, acceptable pressure drop, and good tray froth behavior. Very small caps (< 50 mm) would imply a very large cap count with potential maintenance complexity; very large caps (> 150 mm) reduce distribution quality and complicate spacing.

Step-by-Step Solution:

Verification / Alternative check:
Vendor guidelines and historical tray layouts often show cap sizes near 75 mm for large towers, with triangular pitch spacing to achieve target active area and turndown.

Why Other Options Are Wrong:

• 5 cm: leads to excessive cap count and maintenance burden.
• 15 cm: usually too large for good distribution on big trays unless loads are unusually high.
• 50 cm: impractical and hydraulically unsound.

Common Pitfalls:
Ignoring active area lost to downcomers and walls; cap size must be considered with pitch, clearance, and weir design.

Final Answer:
7.5 cm