Tray hydraulics: What is a typical maximum acceptable total liquid gradient (from inlet to outlet weir) across a distillation tray?

Introduction / Context:
On distillation trays, liquid flows crossflow from inlet downcomer to outlet weir. The liquid gradient is the level difference across the tray surface caused by friction and momentum. Excessive gradient can lead to weeping at the inlet, premature entrainment at the outlet, and maldistribution across active area.

Given Data / Assumptions:

• Standard valve/sieve tray operating under typical hydrocarbon service.
• Well-designed downcomer and active area with appropriate weir height.
• Seeking a commonly accepted design maximum.

Concept / Approach:
Design practices often target modest gradients to preserve tray efficiency and capacity. A practical upper limit near half an inch (about 12–13 mm) balances pressure drop and froth height while keeping backmixing under control. Much larger gradients signal hydraulic problems or excessive vapor/liquid rates.

Step-by-Step Reasoning:

Verification / Alternative check:
Tray design manuals and vendor guidelines cite gradients of about 10–13 mm as a maximum for good performance; higher values may be tolerated in special services but are not typical targets.

Why Other Options Are Wrong:

• 5.5 mm is conservative but not an upper bound.
• 30.5–75 mm indicate excessive hydraulic losses likely to impair efficiency.

Common Pitfalls:
Ignoring fouling/foam tendencies that aggravate gradients; underestimating the effect of vapor crossflow which can push liquid and increase outlet head locally.

Final Answer:
12.5 mm