Tray column performance: which of the following statements about plate (tray) efficiency and spacing is incorrect?

Introduction / Context:
Tray (plate) efficiency summarizes how closely real mass transfer approaches equilibrium in distillation/absorption columns. It depends on hydrodynamics: vapor and liquid contact time, dispersion, entrainment, weeping, and geometry (weirs, hole size, spacing). This item asks you to identify if any of the listed heuristics are incorrect, reflecting standard design and operating behavior of trayed columns.

Given Data / Assumptions:

• Sieve/valve/bubble-cap trays with conventional downcomers.
• Non-foaming or mildly foaming systems within normal operating ranges.
• “Efficiency” refers to overall or Murphree-like tendencies, not a single universal constant.

Concept / Approach:
Tray efficiency tends to rise with vapor rate up to an optimum before entrainment/flooding degrade performance. Adequate liquid depth over active area improves vapor dispersion and contact, especially when initial depth is shallow. Plate spacing must avoid excessive vapor velocities that cause carryover and poor contacting; too tight spacing often reduces disengagement and hurts efficiency.

Step-by-Step Solution:

Verification / Alternative check:
Design heuristics from tray vendors/handbooks show efficiency improvements with moderate vapor rates and proper liquid head. Typical plate spacing choices (e.g., 18–24 inches) ensure adequate disengagement and efficiency.

Why Other Options Are Wrong (as “wrong” choices):

• (a) is consistent with practice up to the entrainment limit.
• (b) reflects common experience: shallow heads underperform; increasing to a reasonable head often improves contacting.
• (c) is a known hydrodynamic limitation of overly tight spacing.

Common Pitfalls:
Assuming efficiency rises indefinitely with vapor rate (it does not), or that deeper liquid is always better (excessive head increases pressure drop and may cause other limits).

Final Answer:
None of these.