Tray hydraulics and efficiency: when the liquid path length on a tray exceeds about 5 ft, how does the overall column efficiency typically respond?

Introduction / Context:
On crossflow trays, liquid travels laterally from the inlet downcomer to the outlet downcomer while contacting rising vapor. The path length affects residence time, lateral gradients, and backmixing. Designers must balance area utilization with mass-transfer effectiveness to achieve high stage efficiency without inducing maldistribution or stagnant zones.

Given Data / Assumptions:

• Conventional single-pass crossflow trays.
• Liquid path length increased beyond ~5 ft.
• No drastic changes in foaming tendency or weeping/flooding regime.

Concept / Approach:
As path length increases, the liquid tends to exhibit a larger concentration and temperature gradient from inlet to outlet, and non-uniform froth quality can develop across the tray width. Excessive path length promotes backmixing, dead zones near the walls, and maldistribution, which reduce the approach to equilibrium and hence the effective tray (Murphree) efficiency. Therefore, very long paths are generally disfavored unless compensated by multiple passes or improved distribution.

Step-by-Step Solution:

Verification / Alternative check:
Multi-pass tray designs shorten individual path lengths while maintaining capacity; field experience shows efficiency recovery compared with single long paths.

Why Other Options Are Wrong:

• “Increases” and “Remains same” ignore well-documented maldistribution effects.
• “May increase or decrease … plate spacing” misattributes the dominant factor; spacing matters, but path-length-driven maldistribution is primary.

Common Pitfalls:
Assuming residence time alone controls efficiency; overlooking tray leveling and weir uniformity; pushing capacity at the expense of contacting quality.

Final Answer:
Decreases