Column tray hydraulics: The main purpose of providing stepping on fractionation/absorption trays is to ________.

Introduction:
Tray design must accommodate specified vapor and liquid loads without flooding or weeping. “Stepping” (multi-pass or stepped trays) is one of several layout strategies used when large liquid rates must traverse each stage while maintaining adequate contacting area and controllable liquid gradients.

Given Data / Assumptions:

• Conventional cross-flow trays (sieve/valve/bubble-cap types).
• High liquid throughput relative to column diameter.
• Objective: maintain efficiency without excessive froth height or weir loading.

Concept / Approach:
When liquid rate is high, a single pass across the tray can yield too short a path, excessive velocity, and maldistribution. Stepped or multi-pass arrangements split the liquid into parallel paths, lowering path length and local velocities, improving froth uniformity, and keeping the tray within hydraulic limits. This is distinct from adjusting weir heights or downcomer areas alone.

Step-by-Step Solution:
Identify the limitation: high L requires larger active area and manageable velocities.Apply stepping/multi-pass trays to divide the flow and reduce hydraulic gradient.Result: improved capacity and operation at large liquid flows.

Verification / Alternative check:
Tray vendor catalogs show multi-pass trays specified specifically for high-L services to maintain efficiency and capacity.

Why Other Options Are Wrong:

• Residence time/pressure drop: may change as a consequence but are not the primary purpose.
• “Improve flow condition” is vague; the specific driver is large liquid load.
• Lowering weir height is a separate hydraulic adjustment.

Common Pitfalls:
Attempting to handle very high liquid loads by only increasing downcomer area; tray active area must also be managed with layout changes.

Final Answer:
facilitate handling of large liquid flow rate