Distillation column design practice: for a column with diameter less than 3 ft, what is the typical minimum tray spacing specified (in inches) to ensure workable hydraulics, access, and mechanical clearance?

Introduction / Context:
Tray spacing is a key mechanical and hydraulic parameter in a distillation column. It sets the vertical clearance between adjacent trays and directly influences vapor head, downcomer residence time, liquid froth height, capacity, pressure drop, access for installation/cleaning, and allowance for manways or sampling devices. For small-diameter towers (less than 3 ft), industry practice converges to a practical minimum that balances performance and maintainability.

Given Data / Assumptions:

• Distillation column diameter < 3 ft.
• Conventional single-pass trays with downcomers (e.g., sieve, valve, or bubble-cap trays).
• Normal hydrocarbon or aqueous service at moderate pressure; no unusual fouling or foaming requirements.

Concept / Approach:
Minimum tray spacing must be high enough to accommodate the froth height on the tray plus a vapor space to prevent excessive entrainment and to allow predictable disengagement. It must also provide clearance for downcomer lips, startup surges, and fabrication tolerances. While larger columns may use standard spacings between 18 and 24 inches, small columns typically adopt the lower bound (about 18 inches) to maintain a compact tower while still meeting hydraulic and mechanical needs.

Step-by-Step Solution:

Verification / Alternative check:
Vendor catalog guidelines and tray rating tools commonly flag spacings below ~18 inches as risky for froth carry-over and maintenance, especially when considering downcomer hydraulics and tray hardware height.

Why Other Options Are Wrong:

• 6 inches: far too small for froth, downcomers, or hardware installation; severe entrainment risk.
• 24 or 34 inches: feasible but not a minimum; these increase height and cost without necessity for small towers.

Common Pitfalls:
Ignoring foaming tendencies (may demand larger spacing), neglecting internal hardware height, or attempting aggressive spacings that reduce operability and tray efficiency.

Final Answer:
18