Bubble-cap tower design: for towers with diameter greater than 3 ft, what is the typical ratio of total riser area to the tower cross-sectional area?

Introduction / Context:
On bubble-cap trays, risers convey vapor from below the tray into the liquid pool under the caps. The combined cross-sectional area of these risers determines the superficial vapor velocity through each riser and influences pressure drop, entrainment, and capacity. Designers select a modest fraction of the tower area for risers to maintain proper contacting without excessive restriction.

Given Data / Assumptions:

• Tower diameter > 3 ft.
• Conventional bubble-cap geometry; adequate downcomers and weirs.
• Normal hydrocarbon/aqueous service; no extreme foaming.

Concept / Approach:
If the riser area is too small, vapor velocity through slots and caps is too high, leading to entrainment and pressure drop. If too large, active bubbling area is lost and tray efficiency may suffer. Industry practice typically targets a riser-area fraction on the order of 10–20% of column cross-section to balance these effects across a wide range of services.

Step-by-Step Solution:

Verification / Alternative check:
Vendor rating calculations consistently yield riser fractions in this band for middle-of-the-road designs; extremes are tuned only for special services.

Why Other Options Are Wrong:

• 0.35–0.75 or 0.55–0.85: implausibly large, would severely reduce active bubbling area.
• 0.4–0.6: likewise unrealistically high for normal tray design.

Common Pitfalls:
Ignoring slot velocity limits; neglecting the accompanying need for sufficient downcomer area; assuming a single fraction suits highly foaming systems.

Final Answer:
0.1–0.2