For a bubble-cap tray column operating near atmospheric pressure, a typical design value for static submergence (liquid height over the tray deck before froth head) is approximately:

Introduction / Context:
Bubble-cap trays rely on a maintained liquid level (static submergence) to seal vapor passages and prevent weeping through the risers. Proper submergence ensures stable mass transfer, good contact between vapor and liquid, and robust operation across turndown ratios.

Given Data / Assumptions:

• Column operating near atmospheric pressure.
• Conventional bubble-cap design; not valve or sieve trays.
• Typical plate spacing (e.g., 18–24 inches) but focus is on submergence, not spacing.

Concept / Approach:
Static submergence is set by the weir height plus any additional liquid height maintained to prevent vapor blow-through and ensure caps remain submerged at all times. For bubble caps, practical design values are on the order of a couple of inches, striking a balance between seal integrity and hydraulic pressure drop across the tray.

Step-by-Step Solution:

Verification / Alternative check:
Tray design texts and vendor guidelines cite ~2–3 inches of static submergence for bubble-cap trays at low to moderate pressures.

Why Other Options Are Wrong:

• 0.5": Too small to reliably seal caps, prone to weeping/blow-through.
• 5": Overly conservative; increases liquid head and pressure drop without proportional benefit.
• 50% of plate spacing: Not a standard rule; would be excessive for common spacings.

Common Pitfalls:
Confusing static submergence with total froth height; the latter can be larger during operation due to vapor holdup.

Final Answer:
2.5"