Every environmental engineer eventually designs a wet scrubber. Whether it’s for acid gas removal (HCl, SO₂, HF), particulate control, or odor abatement, the fundamental design calculations are the same. This guide walks through the process from process conditions to vessel dimensions — with the actual equations you need.
Step 1: Define the Problem
Before touching a calculator, nail down these parameters:
| Parameter | Example Value | How to Obtain |
|---|---|---|
| Gas flow rate | 50,000 Nm³/h (wet) | Process data or stack measurement |
| Inlet gas temperature | 180°C | Process data |
| Target outlet temperature | 65°C (adiabatic saturation) | Determined by scrubber type |
| Contaminant | HCl | Process knowledge |
| Inlet concentration | 500 mg/Nm³ | Measurement or mass balance |
| Required outlet concentration | 10 mg/Nm³ | Emission limit (GB 31573-2015, EU IED, etc.) |
| Required removal efficiency | 98% | (In – Out) / In × 100 |
Step 2: Determine Scrubber Type
| Contaminant | Preferred Scrubber Type | Scrubbing Medium | Typical Efficiency |
|---|---|---|---|
| HCl, HF, SO₂ (acid gases) | Packed bed, counter-current | NaOH solution (pH 7-9) | 95-99% |
| SO₂ (high concentration) | Spray tower with limestone slurry | CaCO₃ slurry | 90-98% |
| PM₁₀ / PM₂.₅ | Venturi scrubber | Water | 90-99% (depends on ΔP) |
| Odor / VOCs | Packed bed with oxidizing solution | NaOCl or H₂O₂ | 70-95% |
| NH₃ | Packed bed with acid | H₂SO₄ solution (pH 2-4) | 95-99% |
For this example: HCl removal → counter-current packed bed with NaOH solution.
Step 3: Gas-Liquid Equilibrium and Minimum Liquid Rate
The scrubbing liquid must provide enough reactive capacity:
Calculate HCl Loading
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HCl mass flow = 50,000 Nm³/h × 500 mg/Nm³ × 10⁻⁶ kg/mg
= 25 kg/h HCl
Stoichiometric NaOH Requirement
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HCl + NaOH → NaCl + H₂O
36.5 g HCl reacts with 40 g NaOH
NaOH required = 25 kg/h × (40/36.5) = 27.4 kg/h NaOH (stoichiometric)
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Practical NaOH Flow
In practice, you need excess reagent — the reaction is fast but mass transfer limits the rate:
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Practical NaOH excess: 50-100% over stoichiometric
NaOH solution concentration: 5-10% wt (typical for HCl scrubbing)
At 100% excess, 5% solution:
NaOH mass flow = 27.4 × 2 = 54.8 kg/h
Solution flow = 54.8 / 0.05 = 1,096 kg/h ≈ 1.1 m³/h
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Check Liquid-to-Gas Ratio (L/G)
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L/G = Liquid mass flow / Gas mass flow
Gas mass flow ≈ 50,000 Nm³/h × 1.2 kg/Nm³ = 60,000 kg/h
L/G = 1,096 / 60,000 = 0.018 kg/kg = 18 L/1,000 Nm³
Typical L/G for packed bed acid gas scrubbers: 1.5-5 L/1,000 Nm³ per meter of packing. With 3 meters of packing: 4.5-15 L/1,000 Nm³. Our 18 L/1,000 Nm³ is slightly above the upper range, which is fine — higher L/G improves removal but increases pumping cost and pressure drop.
Step 4: Tower Diameter
The tower cross-section is determined by the flooding velocity:
Calculate Flooding Velocity (Eckert / Sherwood Correlation)
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Gas density at 65°C: ρg = 1.0 kg/m³ (approximate, saturated)
Liquid density: ρl = 1,050 kg/m³ (5% NaOH solution)
Flow parameter:
X = (L/G) × √(ρg/ρl)
= 0.018 × √(1.0/1,050)
= 0.018 × 0.031
= 0.00056
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For random packing (e.g., 50 mm Pall rings), the flooding line is approximately:
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Capacity parameter at flooding: Y_flood ≈ 0.18 (from Sherwood correlation chart)
(at X = 0.00056 for Pall rings)
Calculate Flooding Gas Velocity
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Y = (G'² × Fp × μl^0.2) / (ρg × ρl × g)
Where:
G' = gas mass velocity at flooding (kg/m²·s)
Fp = packing factor (50 mm Pall rings: ~25 m⁻¹)
μl = liquid viscosity (~1 cP for dilute NaOH = 0.001 Pa·s)
Rearranging for G':
G' = √(Y × ρg × ρl × g / (Fp × μl^0.2))
= √(0.18 × 1.0 × 1,050 × 9.81 / (25 × 0.001^0.2))
= √(1,854 / 6.3)
= √294
= 17.2 kg/m²·s
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Design at 60-75% of Flooding
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Design G' = 0.65 × 17.2 = 11.2 kg/m²·s
Calculate Tower Diameter
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Cross-sectional area:
A = Gas mass flow / Design G'
= (60,000 / 3600) / 11.2
= 16.7 / 11.2
= 1.49 m²
Diameter:
D = √(4A/π) = √(4 × 1.49/π) = 1.38 m
→ Select 1.4 m ID tower
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Rule of thumb check: Gas velocity should be 1.5-2.5 m/s for packed beds.
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Actual velocity = 50,000 / (3600 × 1.54) = 9.0 m/s ← too high!
(Recalculate at actual conditions: 50,000 Nm³/h at 65°C and saturated)
Actual volumetric flow = 50,000 × (338/273) × (101.3/101.3) × 1.05 (moisture)
≈ 65,000 m³/h
Velocity = 65,000 / (3600 × 1.54) = 11.7 m/s → still too high
This indicates the 1.4 m diameter is too small. Let's resize:
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For 2.0 m/s superficial velocity:
A = 65,000 / (3600 × 2.0) = 9.03 m²
D = √(4 × 9.03/π) = 3.4 m
→ Select 3.4 m ID tower. This is more realistic for 50,000 Nm³/h.
Step 5: Packing Height
Mass Transfer Approach
For absorption with chemical reaction (HCl + NaOH), the rate is controlled by gas-phase mass transfer:
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HTU = Gas-phase height of a transfer unit
NTU = Number of transfer units
Packing height Z = HTU × NTU
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Estimate HTU
For 50 mm Pall rings:
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HTUog ≈ 0.4-0.8 m for HCl absorption (depends on L/G and gas velocity)
At our design conditions: HTUog ≈ 0.6 m
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Calculate NTU
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NTUog = ln(Cin/Cout) = ln(500/10) = ln(50) = 3.91
Packing Height
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Z = HTUog × NTUog = 0.6 × 3.91 = 2.35 m
Add safety factor:
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Design packing height = 2.35 × 1.3 = 3.0 m
Check with Rules of Thumb
- Packed bed height ≥3.0 m for acid gas scrubbing ✓
- Bed height / diameter ratio: 3.0/3.4 = 0.88 (acceptable range: 0.5-3.0) ✓
- L/D ratio: Typically 1.0-3.0 for chemical absorbers ✓
Step 6: Pressure Drop
Use the Robbins correlation or packing manufacturer's curve:
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For 50 mm Pall rings at 65% flooding:
ΔP ≈ 20-40 mm H₂O per meter of packing
Total packing pressure drop:
ΔP_packing = 3.0 m × 30 mm H₂O/m = 90 mm H₂O ≈ 880 Pa
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Additional losses:
- Mist eliminator: 100-250 Pa
- Inlet/outlet duct transitions: 50-100 Pa
- Stack: Negligible for short stacks
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Total system ΔP ≈ 880 + 200 + 100 = 1,180 Pa
Fan Sizing
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Fan power = (Q × ΔP) / (η_fan × η_motor)
= (65,000/3600 × 1,180) / (0.75 × 0.92)
= 21,300 / 0.69
= 30.9 kW
→ Select 37 kW motor (standard size with margin)
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Step 7: Vessel Mechanical Design
Shell Thickness (Simplified)
For a 3.4 m diameter vessel at near-atmospheric pressure:
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Per ASME VIII-1 (or GB 150):
t = (P × D) / (2 × S × E - 0.2 × P) + CA
Where:
P = design pressure (atmospheric + 0.05 MPa = 0.15 MPa)
D = 3,400 mm
S = allowable stress (SS316L: 115 MPa at design temp)
E = joint efficiency (0.85 for spot radiography)
CA = corrosion allowance (3 mm for scrubbing service)
t = (0.15 × 3,400) / (2 × 115 × 0.85 - 0.2 × 0.15) + 3
= 510 / (195.5 - 0.03) + 3
= 2.6 + 3
= 5.6 mm
→ Select 8 mm SS316L (minimum practical thickness for this diameter)
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Internals
- Liquid distributor: Trough or pipe type, minimum 40 pour points/m²
- Packing support: Grid type for ceramic/plastic packing, gas injection type for metal
- Mist eliminator: Chevron (vane) type, SS316L, 100 mm thick
- Liquid redistribution: Every 5-6 m of packing height, or every 8-10 tower diameters, whichever is smaller. For 3 m of packing, no intermediate redistribution is needed.
Final Design Summary
| Parameter | Value |
|---|---|
| Scrubber type | Counter-current packed bed |
| Tower diameter (ID) | 3,400 mm |
| Packing type | 50 mm Pall rings (PP or SS316L) |
| Packing height | 3,000 mm |
| Scrubbing medium | 5% NaOH solution |
| Liquid circulation rate | 1-2 m³/h + recirculation (if used) |
| Superficial gas velocity | ~2.0 m/s (at actual conditions) |
| Total pressure drop | ~1,200 Pa |
| Fan motor | 37 kW |
| Material of construction | SS316L (shell), PP or SS316L (packing) |
| Removal efficiency | >98% (HCl) |
| Mist eliminator | Chevron type, SS316L |
Common Design Mistakes
- Undersizing the diameter: A scrubber that’s too small floods at turndown. Always check velocity at actual (hot, wet) conditions, not Nm³/h.
- Forgetting the quench section: If inlet gas is >200°C, you need a quench section (empty spray zone) before the packed bed. Otherwise you’ll thermally shock and crack the packing.
- Inadequate liquid distribution: For towers >1 m diameter, a single-point liquid inlet produces uneven wetting. Use a proper distributor — the cost increment is small, the performance impact is huge.
- Wrong material for the mist eliminator: PP mist eliminators soften above 80°C. If your gas is saturated at 65°C but there’s an upstream temperature excursion, you lose the mist eliminator. Use SS316L for the first 500 mm above the packing.
- No recirculation control: If you’re recirculating scrubbing liquid, you must monitor pH and blow down when salts build up (NaCl for HCl scrubbing, Na₂SO₄ for SO₂). Otherwise, salts crystallize on the packing.
This guide covers the fundamentals. Each scrubber application has specific nuances — always consult the packing manufacturer’s data and run a pilot test for new applications.
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