Comparing operating pressures: how does the typical bubble-cap diameter used in high-pressure distillation columns compare to that used in similar vacuum columns?

Introduction / Context:
Bubble-cap size affects slot area, vapor distribution, mechanical robustness, and hydraulic performance. Operating pressure changes vapor density and volumetric rates, which in turn influence appropriate cap dimensions for stable tray operation without excessive pressure drop or maldistribution.

Given Data / Assumptions:

• Comparing otherwise similar services and column diameters.
• High-pressure service has higher vapor density and lower volumetric flow for the same mass rate.
• Mechanical constraints require adequate thickness and rigidity of caps.

Concept / Approach:
At higher pressure, for the same mass flow, vapor volumetric flow is smaller, so less slot area is needed to maintain target vapor velocities. Designers therefore can use smaller bubble caps (or fewer/lower slot area) to achieve the desired vapor distribution and froth. Conversely, under vacuum, high volumetric flows require larger cap diameters and more open area to avoid excessive jet velocities and pressure drop.

Step-by-Step Solution:
Relate vapor volumetric rate to pressure: higher pressure → higher density → lower volume flow.Target slot velocity and pressure drop guide required open area.Thus, caps in high-pressure service can be smaller to provide the correct hydraulic performance.

Verification / Alternative check:
Vendor tray sizing charts show cap diameter/open area trending downward with increased operating pressure for the same capacity objective.

Why Other Options Are Wrong:
(b) Opposite to volume-flow logic; (c) ignores pressure effects; (d) and (e) over-simplify—weir height and metallurgy matter but do not solely determine cap size.

Common Pitfalls:
Forgetting that density changes with pressure alter vapor velocities; using vacuum-service cap sizes at pressure leads to over-capacity, maldistribution, or high ΔP.

Final Answer:
Smaller in high-pressure columns