Why are bubble-cap trays widely used in large distillation columns? Select the most appropriate reason.

Introduction / Context:
Tray selection in distillation balances hydraulics, efficiency, pressure drop, fouling resistance, and operability. Bubble-cap trays, while not always the lowest-cost option, have historically been favored in large columns for their robust operating window.

Given Data / Assumptions:

• Conventional tray column separating non-foaming to moderately foaming systems.
• Comparable vapor/liquid loads within turndown ranges.
• Focus on relative advantages of bubble caps.

Concept / Approach:
Bubble caps maintain a positive liquid seal via risers and caps, allowing good contact between vapor and liquid even at low vapor rates. This gives a broad turndown ratio and relatively stable efficiency when throughput varies. By contrast, sieve and valve trays can weep at low vapor rates unless carefully designed, reducing efficiency.

Step-by-Step Solution:
Option (a): Captures the flexibility and near-constant efficiency across a wide operating range — correct.Option (b): Bubble-cap trays usually have higher pressure drop than sieve or valve trays — so this claim is false.Option (c): No tray “eliminates” foaming/entrainment; these phenomena depend on system properties and hydraulics — false.Option (d): Not applicable since (a) is valid.

Verification / Alternative check:
Operational experience shows bubble caps remain effective during start-up, turndown, and off-design operation, justifying their continued use in certain services despite higher cost and pressure drop.

Why Other Options Are Wrong:
(b) Misrepresents pressure drop characteristics.(c) Overstates capability; tray design mitigates but cannot eliminate such issues.

Common Pitfalls:
Choosing bubble caps solely for new builds without weighing pressure drop penalties; ignoring that modern valve trays can also deliver good turndown.

Final Answer:
They offer flexibility with nearly constant efficiency over varying operating conditions.