On tray columns, a “calming section” before the downcomer is provided primarily to achieve which objective?

Introduction / Context:
Tray hydraulics aim to balance good mass transfer with stable liquid handling. A “calming section” (also called a quiet zone) is a region near the outlet weir where froth subsides, allowing vapor disengagement from the liquid prior to entry into the downcomer. This improves downcomer capacity and reduces choking and bypassing.

Given Data / Assumptions:

• Conventional tray with outlet weir and a downcomer.
• Significant froth on the active tray area upstream of the weir.
• Normal distillation service without special foaming inhibitors.

Concept / Approach:
In the active area, vapor bubbles through liquid creating froth. If this froth enters the downcomer, the entrained vapor occupies volume, reduces liquid residence time, and can cause downcomer backup or flooding. The calming section allows bubbles to disengage, converting froth to clearer liquid before overflow over the weir and entry into the downcomer.

Step-by-Step Solution:

Verification / Alternative check:
Tray design handbooks specify quiet-zone length as a fraction of tray chord; poor quiet-zone design correlates with downcomer choke and premature flooding.

Why Other Options Are Wrong:

• Reducing discharge fluctuation is not the primary function.
• Better vapor–liquid contact occurs in the active area, not in a calming zone.
• Cooling is unrelated; temperatures are set by column hydraulics/thermodynamics.

Common Pitfalls:
Too short a calming section; insufficient outlet weir length; allowing froth to spill directly into the downcomer.

Final Answer:
Permit release of entrained vapor from the liquid before it enters the downcomer