Tray internals — bubble cap sizing:\nSelect the most common standard bubble-cap size combination used in industrial trays (cap diameter with riser diameter).

Introduction / Context:
Bubble-cap trays are legacy yet still-encountered distillation/absorption internals. Size standardization simplifies fabrication and performance prediction. Typical cap and riser diameters are chosen to balance pressure drop, capacity, and froth contact.

Given Data / Assumptions:

• Industrial-scale column trays with moderate vapor/liquid loads.
• Use of standardized caps and risers from historic catalogs.
• Focus on “most common” rather than niche or miniature lab sizes.

Concept / Approach:
A 6″ bubble cap with a 4″ riser is a widely referenced standard combination for production trays, offering adequate area and manageable pressure drop. Smaller sizes like 1″ with 0.5″ riser are laboratory or very light-duty; mismatched options (e.g., riser larger than cap) are impractical.

Step-by-Step Reasoning:

Verification / Alternative check:
Classic tray design references list common caps in the 4–8″ range with risers about 2–4″, with 6″/4″ frequently cited as standard.

Why Other Options Are Wrong:

• 1″/0.5″: too small for most industrial duties.
• 8″/1″: disproportionate; would restrict vapor severely.
• 4″ cap with 8″ riser: geometrically nonsensical.
• 3″/2″: used in small duties but not “most common.”

Common Pitfalls:
Assuming cap trays are universally optimal; modern designs often prefer sieve or valve trays or structured packing for efficiency and cost.

Final Answer:
6″ cap with 4″ riser