Tray hydrodynamics: In actual operation of a distillation column, why is vapor flow often not uniformly distributed among bubble caps on a tray?

Introduction / Context:
Even on well-designed bubble-cap or valve trays, actual vapor flow rarely splits evenly among all active devices. Understanding the causes of maldistribution helps troubleshoot capacity, efficiency, and weeping/entrainment issues.

Given Data / Assumptions:

• Crossflow tray with an inlet downcomer and an outlet weir.
• Liquid flows across the tray from inlet to outlet.
• Vapor rises through caps from the plenum beneath the deck.

Concept / Approach:
As liquid traverses the tray, a hydraulic gradient (liquid height gradient) develops, typically deeper near the outlet weir. This gradient changes the local hydrostatic head above caps, altering the pressure drop that vapor must overcome. Caps under shallower liquid (upstream) pass more vapor; those under deeper liquid (downstream) pass less, causing maldistribution.

Step-by-Step Solution:

Verification / Alternative check:
Tray rating tools model vapor–liquid hydraulics showing non-uniform froth height. Field gamma scans often reveal higher activity at the inlet side for this reason.

Why Other Options Are Wrong:

• Low skirt clearance, low static submergence, or small downcomer seal are individual settings but not the primary driver of cap-to-cap maldistribution; the liquid gradient is.

Common Pitfalls:
Assuming perfect distribution; ignoring revamps that alter weir loading; overlooking tray leveling which can exacerbate gradients.

Final Answer:
Due to liquid height gradient across the tray