Continuous distillation at fixed throughput: As the external reflux ratio is increased, which operating cost component decreases?

Introduction / Context:
For a binary distillation column producing fixed top and bottom specifications at constant throughput, changing the reflux ratio trades capital against operating costs. Understanding these trade-offs is essential in economic optimization.

Given Data / Assumptions:

• Top and bottom purities and feed rate are fixed.
• Reflux ratio is varied from minimum upward.
• Column design scales with required stages and internal flows.

Concept / Approach:
Increasing reflux ratio from minimum reduces the number of theoretical stages (steeper operating line) and thus can initially reduce tower height and tray count, lowering fixed charges. However, higher reflux increases internal liquid and vapour traffic, raising both condenser and reboiler duties and therefore increasing utility costs.

Step-by-Step Solution:

Verification / Alternative check:
McCabe–Thiele constructions illustrate the reduction in stages as R increases; energy balances show rising condenser and reboiler loads with larger reflux flows.

Why Other Options Are Wrong:

• Cooling-water (b) and steam (d) costs increase, not decrease, with higher reflux.
• Internal vapour and liquid loads (c) increase, not decrease.

Common Pitfalls:
Assuming there is monotonic capital reduction; beyond a point, column diameter may increase due to hydraulic limits, partially offsetting earlier capital savings.

Final Answer:
Fixed charges initially (due to fewer theoretical stages required).