Continuous distillation: increasing the reflux ratio—what are the practical cost consequences for column fixed charges, reboiler duty, and condenser load?

Introduction / Context:
The reflux ratio is a key economic handle in continuous distillation. Raising reflux generally improves separation per stage and reduces the number of theoretical stages required, but it increases internal flows, which raises condenser and reboiler duties. The overall optimum balances capital and operating costs.

Given Data / Assumptions:

• Same product specifications (top and bottom purities) across cases.
• Column operates at the same pressure and feed condition.
• Heat-exchanger designs scale with heat duties and temperature approaches.

Concept / Approach:
As reflux ratio increases above the minimum, the required number of stages drops (capital saving potential). However, higher internal liquid and vapour traffic increases reboiler duty and condenser load (operating cost penalties). Consequently, fixed charges may fall (smaller column/shorter height), while utilities and cooling-water costs rise.

Step-by-Step Solution:
Increase reflux → fewer stages → potentially smaller diameter/height → lower fixed capital charges.Increase reflux → higher L/V ratios → higher condenser duty (more vapour condensed) → higher cooling cost.Increase reflux → higher boil-up to sustain internal flows → higher reboiler duty and fuel/steam cost.

Verification / Alternative check:
McCabe–Thiele or rigorous simulation confirms the trade-off: as reflux increases, approach the minimum-stage asymptote but with rising energy use; an economic optimum exists at intermediate reflux.

Why Other Options Are Wrong:
Claiming reduced variable costs in both exchangers conflicts with the physics of increased internal traffic at higher reflux.

Common Pitfalls:
Ignoring pressure drop effects on diameter; neglecting utility pricing or heat integration opportunities that can shift the optimum.

Final Answer:
All (a), (b) and (c)