Sieve plate vs. bubble-cap plate: which statement about weir design and downcomer residence time is incorrect for practical distillation column design?

Introduction / Context:
Tray weir design—height and length—affects liquid head, residence time, and froth behavior. While sieve and bubble-cap trays share many hydraulic principles, certain design choices differ because cap trays rely on positive vapor sealing through slots and often operate with higher liquid depths for stable performance.

Given Data / Assumptions:

• Industrial columns operating near atmospheric to moderate pressures.
• Comparable tray spacings and capacities.
• Standard design practices for weir height: sieve plates typically lower than bubble-cap plates.

Concept / Approach:
Bubble-cap trays employ risers and caps that force vapor through submerged slots. To maintain a good liquid seal and reduce vapor backflow through clearances, cap trays frequently use a higher weir height than sieve trays. Weir length and downcomer sizing target similar residence times for liquid disengagement, so broad similarity can be expected there, but equal weir height across types is not a safe general rule.

Step-by-Step Solution:
Compare weir height norms: bubble-cap often 50–150 mm; sieve commonly lower.Assess downcomer and residence: both tray types require adequate seal and disengagement, leading to similar residence criteria.Weir length is a geometric parameter set by column diameter and hydraulics and can be similar between types.Thus, the claim of “same weir height” for both is incorrect.

Verification / Alternative check:
Pilot plant data and vendor guidelines confirm that higher weir heights are common on bubble-cap plates to promote sealing across a wider turndown and to stabilize froth depth.

Why Other Options Are Wrong:
(a) Reasonable—residence time aims are similar. (b) Plausible—length is often governed by column geometry. (d) True—bubble-cap weirs are typically higher. (e) True—both height and length are selected to meet sealing and disengagement requirements.

Common Pitfalls:
Assuming a single “best” weir height independent of tray type; ignoring tray turndown requirements; confusing length with height.

Final Answer:
Weir height on a bubble-cap plate is the same as that on a sieve plate