What primarily causes a liquid gradient (height difference) across a distillation tray during operation?

Introduction / Context:
Liquid does not flow perfectly level across a trayed distillation column stage. A liquid “gradient” from inlet (downcomer outlet) to outlet (weir) forms due to hydraulic resistance and vapor–liquid interaction. Understanding the origin of this gradient is important for tray sizing (weir height, weir length) and ensuring adequate residence time without excessive weir crest or entrainment.

Given Data / Assumptions:

• Standard sieve, valve, or bubble-cap trays with downcomers and outlet weirs.
• Normal operating vapor and liquid rates below flooding.

Concept / Approach:
As liquid crossflows over the active area, it encounters frictional resistance (from tray deck, valves/caps, risers) and a counter-current momentum exchange from vapor bubbling up through holes/slots. Both effects raise the static head required to drive liquid toward the outlet. The result is a higher height near the inlet and a lower height toward the outlet (superimposed on the outlet weir crest), i.e., a measurable gradient.

Step-by-Step Solution:

Verification / Alternative check:
Tray design correlations use gradient allowance in the overall tray pressure balance; vendors often include “gradient” or “static head loss” terms when sizing weirs and downcomers.

Why Other Options Are Wrong:

• Low gas velocity: Typically reduces interaction; not a primary cause of a larger gradient.
• Large plate spacing: A geometric parameter; it affects disengagement, not the fundamental gradient driver.
• Large reflux ratio: Alters liquid rate, but the gradient cause is resistance and vapor interaction, not reflux ratio alone.

Common Pitfalls:
Ignoring increased gradient at higher vapor loads (approaching loading/flooding) or when caps/valves are fouled; neglecting gradient allowances in downcomer sizing can lead to back-up and weeping issues.

Final Answer:
Resistance to liquid flow caused by caps and risers, combined with opposing vapor flow through the tray.