In distillation column design practice, what is the minimum recommended residence time for the liquid in the downcomer (downspout) to ensure adequate vapour disengagement and stable tray hydraulics?

Introduction / Context:
In tray distillation columns, liquid leaving a tray falls into the downcomer (also called the downspout) before re-entering the active area of the tray below. The liquid residence time inside the downcomer must be long enough to allow disengagement of entrained vapour and prevent hydraulic instabilities such as downcomer backup and weeping. A commonly used design thumb rule specifies a minimum residence time on the order of a few seconds to secure reliable operation without oversizing the downcomer.

Given Data / Assumptions:

• The question asks for a typical minimum recommended residence time, not a precise value for a particular system.
• Normal, non-extreme foaming tendency is assumed.
• Tray spacing and downcomer geometry are conventional (sieve/valve/bubble-cap trays).

Concept / Approach:
Design practices balance two needs: (1) enough time for vapour disengagement from the descending liquid, and (2) avoiding excessive downcomer volume that would penalize column diameter or tray active area. Empirical correlations and industry handbooks converge on several seconds as adequate for most non-foaming services, while highly foaming systems may require somewhat more.

Step-by-Step Solution:
Identify the lower practical bound used by many designers: several seconds per tray pass.Screen the options: 1 s is generally too short; 8 s is within widely accepted practice for minimum residence time.Large values like 80 s or 180 s are unnecessarily high and would be uneconomic.

Verification / Alternative check:
Heuristic ranges frequently cited: about 3–10 seconds for non-foaming systems; the lower end (≈3–5 s) is a hard minimum, and many conservative designs use ≈5–8 s. Thus, 8 seconds fits the commonly recommended minimum end of safe practice.

Why Other Options Are Wrong:
1 second: insufficient for vapour disengagement; risks backup. 80 or 180 seconds: unrealistic and uneconomic. 30 seconds: overly conservative for minimum guidance and not typically required.

Common Pitfalls:
Applying a single value irrespective of foaming tendency; ignoring that residence time interacts with downcomer velocity, froth height, and tray spacing; neglecting that very small columns may be constrained by geometry rather than residence time rules.

Final Answer:
8 seconds