Distillation column sizing parameter:\nWhich factor is primarily used to set (determine) the diameter of a distillation column during design?

Introduction / Context:
Column design separates plate-to-plate (or height) requirements from hydraulic capacity. Once the number of theoretical stages is set by equilibrium, the diameter is chosen to prevent flooding and ensure adequate disengagement at the allowable vapor velocity.

Given Data / Assumptions:

• Specified vapor and liquid traffic at design conditions.
• Tray or packing type known to estimate capacity factors.
• Acceptable approach to flooding set by safety margins (e.g., 70–85%).

Concept / Approach:
Superficial vapor velocity determines entrainment, weeping, and downcomer backup. Correlations (Souders–Brown for packings and trays, or vendor capacity factors) translate allowable velocity into required cross-sectional area; diameter follows from A = V̇ / v_s.

Step-by-Step Solution:

Verification / Alternative check:
Hydraulic rating software and design manuals produce the same diameter when fed with vapor rate and chosen percent of flooding.

Why Other Options Are Wrong:

• Theoretical plates affect height, not diameter.
• Static submergence and weir length tune tray hydraulics but are not the primary diameter driver.
• Reboiler duty influences vapor rate, but diameter is ultimately set by allowable velocity, not duty alone.

Common Pitfalls:
Ignoring vapor surges and turndown; failing to subtract downcomer area from active area during sizing.

Final Answer:
Allowable vapour (vapor) velocity through the active area