Tray design for bubble-cap columns: Which variable primarily determines the number of bubble caps per tray?

Introduction / Context:
Bubble-cap trays distribute vapor through caps and slots into liquid on each tray. Proper sizing ensures adequate contact area while avoiding excessive pressure drop and entrainment. The number of caps must match the vapor traffic that the tray must handle at design conditions.

Given Data / Assumptions:

• Conventional bubble-cap geometry and slot design.
• Design at steady-state capacity with some turndown margin.

Concept / Approach:
Vapor load sets the total required open area for vapor passage. That total area is provided by the combined slots of all caps. Thus, for a given cap type (slot area per cap) and allowable slot velocity, the number of caps N must satisfy N * A_slot_per_cap * v_allowable ≥ total vapor volumetric flow. Tray diameter and liquid load influence layout and hydraulics, but the count of caps scales directly with vapor flow requirement.

Step-by-Step Solution:

Verification / Alternative check:
Vendor rating methods for bubble-cap trays perform the same balance between vapor flow, slot area, and slot velocity, yielding the number of caps.

Why Other Options Are Wrong:

• Permissible slot velocity is a design parameter used in the calculation, not the driving plant load.
• Tray diameter constrains physical placement but does not set the required count by itself.
• Liquid load affects weir/load hydraulics but not the primary sizing of cap count.

Common Pitfalls:
Ignoring maldistribution and maintenance access; practical layouts may require rounding up the cap count and adjusting spacing.

Final Answer:
Vapor load