Continuous rectification hydraulics: from theoretical sizing, how does the required column diameter of the stripping section compare to that of the enriching (rectifying) section?

Introduction / Context:
In binary distillation, the column above the feed tray is called the enriching (rectifying) section and the part below is the stripping section. Required diameter depends on vapor and liquid traffic (factors like reflux ratio, boilup, and feed condition) that determine flooding limits and allowable vapor velocity.

Given Data / Assumptions:

• Steady continuous operation with ideal mechanical design (same internals type in both sections).
• Similar tray/packing efficiency and allowable vapor velocity criteria in both sections.
• Typical feed conditions (not extreme cold/hot side streams).

Concept / Approach:
For many conventional designs, vapor rate is higher in the enriching section because overhead product plus reflux create larger upward traffic than boilup alone below the feed. Since allowable vapor velocity is set by entrainment/flooding limits, the higher vapor load usually dictates a larger diameter in the enriching section. Thus, the stripping section often requires a smaller diameter.

Step-by-Step Solution:

Verification / Alternative check:
Column rating tools commonly return a larger diameter above the feed tray for moderate reflux ratios; revamp case studies confirm this trend.

Why Other Options Are Wrong:

• More than / same as: contrary to the usual vapor traffic distribution for typical designs.
• “Either more or less depending only on relative volatility”: diameter depends on actual hydraulic loads, not solely on relative volatility.

Common Pitfalls:
Ignoring the impact of subcooled or superheated feed (which can shift loads); mixing different internals across sections without adjusting allowable velocities; neglecting pressure drop limits.

Final Answer:
Less than the enriching section