Tray selection and performance in distillation: Which statement is WRONG regarding valve trays, sieve trays, and bubble-cap trays used in continuous distillation columns?

Introduction / Context:
Choosing a tray type balances capital cost, operating flexibility, capacity, efficiency, and maintenance. Bubble-cap, sieve, and valve trays each occupy niches depending on fouling tendency, load variability, and pressure drop limits.

Given Data / Assumptions:

• Conventional tray columns in refinery/petrochemical service.
• Comparison of qualitative features: cost, efficiency, turndown, and sealing at low vapour rates.
• We seek the statement that is incorrect (WRONG).

Concept / Approach:
Bubble-cap trays provide a positive seal and handle very low vapour loads but are complex and costly to build and maintain. Sieve trays are simple and inexpensive but have limited turndown. Valve trays offer adjustable openings, providing high turndown and broad operating windows, with maintenance generally lower than for bubble-caps.

Step-by-Step Analysis:
(a) True: bubble-caps ensure liquid retention and operate at low V.(b) True: efficiencies are similar; sieve/valve can reach higher peaks under ideal conditions.(c) False: bubble-caps are the most complex/maintenance-heavy; sieve and valve are simpler and typically cheaper to maintain.(d) True: valve trays indeed have the best turndown ratio among the three.

Verification / Alternative check:
Design handbooks consistently rank bubble-caps as highest in cost/complexity; valve trays as most flexible; sieve trays as simplest/lowest cost.

Why Other Options Are Wrong (i.e., actually correct statements):
(a), (b), and (d) accurately reflect standard tray characteristics and are therefore not the wrong statement.

Common Pitfalls:

• Assuming the simplest tray (sieve) has best turndown; it does not.
• Overlooking fouling or weeping limits that also govern tray choice.

Final Answer:
Maintenance cost for valve and sieve trays is comparatively higher than for bubble-cap trays due to their more complicated construction.