On a trayed distillation column, bubble caps are typically arranged on an equilateral triangular pitch. Practical layout limits are commonly applied as:

Introduction / Context:
Bubble-cap trays employ multiple caps to distribute vapor through the liquid. The arrangement must avoid hydraulic interference between caps while providing enough open area for vapor flow and enough downcomer space for liquid. Designers therefore use pitch and clearance rules to place caps on an equilateral triangular grid within the active tray area.

Given Data / Assumptions:

• Conventional cap diameters and riser heights.
• Standard vapor and liquid loads (not extreme turndown).

Concept / Approach:
Two equivalent heuristics are popular: maintain a certain physical clearance (e.g., 25–75 mm) between neighboring caps to prevent hydraulic overlap, or define the cap pitch as a multiple of cap diameter (e.g., 1.3–2.0 times) to scale spacing with cap size. Both practices aim to ensure adequate froth development and avoid dead zones or flooding.

Step-by-Step Solution:

Verification / Alternative check:
Vendor layout manuals and historical tray designs use either form depending on house standards; both yield similar cap densities when cap sizes are in normal ranges.

Why Other Options Are Wrong:

• (a) alone or (b) alone omit the fact that both are legitimate approaches.
• (d) contradicts common industry practice.

Common Pitfalls:
Forgetting to de-rate cap count near downcomers and walls; crowding caps can cause maldistribution and entrainment.

Final Answer:
Either (a) or (b) are acceptable layout guidelines