In tray hydraulics for distillation, what operating condition most commonly causes back-trapping (reverse liquid flow under the downcomer skirt or across the tray)? Provide the primary cause without assuming mechanical damage.

Introduction / Context:
Back-trapping on distillation trays is an undesirable hydraulic condition in which liquid flows backward under a downcomer skirt or across the deck, upsetting mass transfer and tray efficiency. Understanding the root causes—especially process operating variables such as vapor rate and liquid gradient—is essential for troubleshooting columns and preventing capacity loss or flooding.

Given Data / Assumptions:

• Conventional crossflow tray (sieve, valve, or bubble-cap) with a downcomer and outlet weir.
• Column is mechanically intact; no collapsed internals.
• Back-trapping refers to reverse liquid flow driven by excessive head differences.

Concept / Approach:
Liquid flows by gravity across the tray from inlet to outlet, passing over a weir into the downcomer. A large liquid head difference (high gradient) between adjacent regions creates a driving force that can push liquid under skirts or through clearances in the wrong direction, especially when local vapor distribution is uneven. The main controllable contributor is excessive liquid gradient caused by high froth heights, maldistribution, or too much residence time on the deck.

Step-by-Step Solution:
Recognize that liquid gradient = difference in froth/liquid head from one side of the tray to the other.Back-trapping occurs when this head exceeds sealing capacity at skirts/clearances, pushing liquid backward.Conditions that increase gradient include high weir loads, poor distribution, or incorrect bubbling area leading to localized high froth depth.Therefore the primary cause is excessive liquid gradient over the tray.

Verification / Alternative check:
Field observations show that leveling the tray, adjusting vapor/liquid rates, or reducing weir load (thus head) eliminates back-trapping without changing reflux ratio alone—supporting the head-driven mechanism.

Why Other Options Are Wrong:
(a) Low vapor velocity tends to cause weeping rather than back-trapping. (b) High vapor velocity increases entrainment and jet flooding, not primarily reverse liquid flow. (d) Low reflux ratio reduces liquid load and gradient, often helping, not causing back-trapping. (e) A very small weir height can affect sealing, but the dominant driver for back-trapping is head gradient across the tray.

Common Pitfalls:
Confusing back-trapping with weeping; blaming vapor rate alone without measuring liquid head; overlooking tray leveling and downcomer seal design.

Final Answer:
Excessive liquid gradient (large liquid head difference) over the tray