In continuous distillation design, the optimum reflux ratio is selected primarily on the basis of which consideration?

Introduction / Context:
Choosing the reflux ratio is a central decision in distillation design. The minimum reflux reduces column duty but requires many stages; very high reflux reduces stages but greatly increases condenser and reboiler utilities. The optimum balances these opposing costs.

Given Data / Assumptions:

• Continuous distillation with specified product purities and feeds.
• Fixed separation duty; column hydraulics and tray/packing type chosen.
• Economic evaluation includes capital and operating costs.

Concept / Approach:
The total annualised cost comprises capital (column diameter/height, internals) and operating (steam, cooling water, electricity). Increasing reflux ratio generally decreases required stages (lower height, potentially smaller diameter) but increases condenser and reboiler duties. The optimum reflux ratio minimises the sum of these costs.

Step-by-Step Solution:

Verification / Alternative check:
Rules of thumb often choose R_opt ≈ 1.2–1.6 times R_min as a starting point, then refine using economics or simulation sensitivity studies.

Why Other Options Are Wrong:

• Maximum vapour velocity and flooding limits constrain hydraulics but do not alone define the economic optimum.
• “None of these” is incorrect since a clear economic criterion exists.
• Minimum stages alone ignores utility costs; the optimum is a trade-off, not an extreme.

Common Pitfalls:
Focusing solely on stage count reduction or solely on utility savings without total-cost optimisation can lead to uneconomic designs.

Final Answer:
Total cost trade-off (capital for column vs. utilities for condenser/reboiler)