Distillation Columns

This lesson covers column hardware design. Learn trays (sieve, valve, bubble-cap) vs packing (random, structured), flooding and column diameter (Souders–Brown/Fair), tray spacing and efficiency, column height, pressure drop, weeping and entrainment, and internals like distributors and downcomers.

Stage calculations tell you how many theoretical stages and what vapour and liquid flows you need. Column hardware design decides the diameter (set by vapour velocity limits) and the height (stages ÷ efficiency × spacing), plus the internals that make vapour and liquid contact well.

Distillation columns can be over 100 m tall and 10 m in diameter.

An analogy

A tray column is like a multi-storey car park for vapour and liquid: each floor (tray) lets them meet briefly before liquid flows down the ramp (downcomer) and vapour rises through holes.

Trays vs packing

Trays (sieve, valve, bubble-cap) — robust, handle high liquid rates and fouling, easy to inspect. Random packing (rings, saddles) — small columns, corrosive service. Structured packing — low pressure drop and high efficiency, ideal for vacuum columns.

Diameter from flooding

Too much vapour lifts liquid up the column (flooding); too little lets liquid weep through holes. Diameter is set so vapour velocity is ≈ 75–85 % of flooding using Souders–Brown/Fair correlations (C_SB depends on tray spacing and flow parameter).

Height and efficiency

Real trays achieve 50–85 % of an equilibrium stage. Tray spacing 0.45–0.6 m (more for maintenance or foaming). Extra height for feed zones, reflux distribution, sump and vapour disengagement.

Hydraulics and troubleshooting

Operating window bounded by flooding, entrainment, weeping, downcomer backup and choking. Liquid distributors are critical in packed columns. Gamma scans, pressure-drop trends and temperature profiles diagnose problems like fouling, damage or foaming.

Formulas

• Flooding velocity: u_f = C_SB √((ρ_L − ρ_V)/ρ_V)
• Net area: A_n = Q_V / (f · u_f) (f ≈ 0.8)
• Column diameter: D = √(4 A_T/π), A_T = A_n/(1 − downcomer fraction)
• Height: H = (N_theo/E_o) × spacing + top/bottom allowances
• Packed height: Z = N_theo × HETP

Worked example: Column diameter

Given: Vapour 115.7 kmol/h, M = 80, ρ_V = 3 kg/m³, ρ_L = 800; C_SB = 0.08 m/s, 80 % flood, 12 % downcomer area

Find: Diameter.

• u_f = 0.08 × √(797/3) = 1.30 m/s → design 1.04 m/s
• Q_V = 115.7 × 80/3600/3 = 0.857 m³/s
• A_n = 0.822 m² → A_T = 0.934 m² → D = 1.09 m

Answer: D ≈ 1.1 m

Worked example: Column height

Given: 14 theoretical stages, E_o = 0.7; Tray spacing 0.6 m, +3 m ends

Find: Height.

• Actual trays = 14/0.7 = 20
• H = 20 × 0.6 + 3 = 15 m

Answer: ≈ 15 m

Common mistakes

• Wrong: Designing at 100 % flood. — Right: Use ≈ 75–85 % for safety margin and turndown.
• Wrong: Ignoring liquid distribution in packed columns. — Right: Poor distribution destroys packing efficiency.

Applications

• Refinery fractionators: Large valve-tray columns with high liquid loads.
• Vacuum and fine chemicals: Structured packing for low pressure drop.
• Revamps: Replacing trays with high-capacity internals to debottleneck.

Key takeaways

• u_f = C√((ρ_L − ρ_V)/ρ_V); design ≈ 80 %.
• D from net area + downcomer.
• H = N/E × spacing + allowances.
• Trays vs packing depends on service.

Going further

• Flow parameter: F_LV = (L/V)√(ρ_V/ρ_L) sets C_SB on Fair's chart.
• Gamma scanning: Radioactive scans reveal tray damage and flooding in operating columns.
• Dividing-wall internals: Internal wall creates two sections in one shell.