Distillation design — In plate (tray) columns using bubble-cap trays, the number of caps installed on a tray is primarily determined by allowable phase velocities to ensure capacity without weeping/entrainment. Which parameter set governs this most directly?

Introduction:
In tray distillation, bubble caps distribute vapor through the liquid on each tray to promote mass transfer. The number of caps per tray is a capacity and hydraulics decision: it must pass the required vapor flow without excessive pressure drop or entrainment and maintain adequate liquid holdup without weeping. This item tests understanding of which variables primarily control cap count and spacing.

Given Data / Assumptions:

• Bubble-cap tray operating within acceptable pressure drop and efficiency ranges.
• Flooding, entrainment, and weeping constraints apply.
• Tray diameter, downcomer design, and weir height are designed consistently with the same duty.

Concept / Approach:

Vapor capacity is set by allowable gas velocity through active area and cap slots; liquid handling is set by allowable liquid velocity across the tray and downcomer. The number of caps divides vapor among outlets to keep slot velocities below entrainment limits and to provide uniform vapor distribution. Thus, both allowable gas and liquid velocities govern the final number and layout, not feed composition directly (composition affects rates indirectly via V and L but not the primary criterion).

Step-by-Step Solution:

Verification / Alternative check:

Compare calculated tray pressure drop and approach-to-flood for several cap counts; the acceptable design meets both gas and liquid constraints simultaneously, confirming the dependence on both velocities.

Why Other Options Are Wrong:

A or B alone: considering only one phase can cause either weeping (too few caps) or entrainment (too many/small slots). D: Feed composition affects V and L indirectly but is not the primary governing parameter. E: Downcomer residence time is a check, not the primary determinant of cap count.

Common Pitfalls:

Oversizing cap count to reduce gas velocity without re-checking liquid backup; ignoring tray active area loss due to downcomers; neglecting turndown requirements.

Final Answer:

allowable gas and liquid velocities