Key Takeaways
- Acid fume scrubbers use the same packed bed design method as other gas scrubbers — Souders-Brown for column diameter, HTU-NTU for packed height — but the material selection is the differentiator. PP for HCl/HF below 60°C, FRP with vinylester resin for H₂SO₄ and NOx. A wrong material choice causes column failure within 6–12 months.
- HTU values for acid gases vary by a factor of 2: HCl at 0.3–0.5 m (instantaneous reaction), HF at 0.4–0.6 m (fast), H₂SO₄ at 0.5–0.8 m (mist absorption is slower). Using a default HTU of 0.5 m across all acid gases produces accurate results for HCl but underpredicts the packed depth for H₂SO₄ by 40–60%.
- NOx (oxides of nitrogen) cannot be removed by NaOH alone — a single NaOH scrubber for mixed acid exhaust containing HNO₃ will remove less than 30% of the NOx and produce a visible brown NO₂ plume. A reducing agent (sodium thiosulfate or urea) must be added to the scrubbing solution.
- NaOH consumption for acid fume scrubbing is typically very low — $200–600/year for an 8,000 m³/h system at 250 mg/m³ HCl inlet. This is 50–100× lower than H₂S caustic scrubbing because the inlet concentrations in ventilation applications are 10–50× lower than typical H₂S biogas concentrations.
- The complete installed cost for an 8,000 m³/h acid fume scrubber ranges from $18,000–28,000 (PP/HCl) to $35,000–55,000 (FRP/H₂SO₄). The material selection accounts for up to 60% of the total equipment cost difference.
A PCB etching plant in Taiwan installed an acid fume scrubber in 2020 for their HCl + HNO₃ (aqua regia) exhaust from gold recovery operations. The scrubber was sized for HCl only — 8,000 m³/h at 200 mg/m³, φ1.4m, 1.5m packed depth, NaOH scrubbing. But the process also released NOx fumes from the nitric acid. The NOx passed straight through the NaOH bed, oxidizing to NO₂ in the stack and forming a visible brown plume. The plant was cited by the local EPA within three months. The fix required a second stage with a reducing agent (Na₂S₂O₃), adding $22,000 to the project cost and three months of operating at reduced capacity while the system was retrofitted.
Acid fume scrubber design is straightforward when the acid gas is single-component (HCl, HF, or H₂SO₄ mist alone), but most industrial exhaust streams contain multiple acids, particulates, or temperature variations that require specific design adjustments. This guide covers the three most common acid fume scenarios — HCl pickling, HF etching, and mixed acid exhaust — with material selection, sizing calculations, and the configuration decisions that separate designs from field failures.
For the general packed bed sizing method, see our packed bed scrubber design calculation guide. For acid gas removal methods, see the EPA acid gas scrubber design reference.
Acid Fume Chemistry: HCl, HF, H₂SO₄, and NOx
Each acid fume type has distinct chemistry that affects the scrubbing solution selection, HTU, and material compatibility.
| Acid Fume | Common Source | Scrubbing Reaction | HTU (2″ PP Pall rings) | Best Material |
|---|---|---|---|---|
| HCl | Pickling, PVC production | HCl + NaOH → NaCl + H₂O | 0.3–0.5 m | PP |
| HF | Glass etching, semiconductor | HF + NaOH → NaF + H₂O | 0.4–0.6 m | PP or PVDF |
| H₂SO₄ mist | Battery, fertilizer, steel | H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O | 0.5–0.8 m | FRP (vinyl ester) |
| HNO₃ / NOx | Etching, metal finishing | NOx + NaOH + Na₂S₂O₃ → NaNO₂ + H₂O | 0.6–1.0 m | FRP (vinyl ester) |
| H₃PO₄ mist | Fertilizer, food processing | H₃PO₄ + 3NaOH → Na₃PO₄ + 3H₂O | 0.4–0.7 m | PP or FRP |
Key material alerts: HF attacks glass and silica-based FRP — use PP or PVDF with carbon-impregnated packing for HF service. H₂SO₄ mist above 80°C requires PVDF packing and FRP shell with a dual corrosion barrier. HNO₃/NOx requires a reducing agent (sodium thiosulfate or urea) in addition to NaOH because NOx is not an acid gas — it is oxidized in the scrubber and requires reduction to nitrite. A single NaOH scrubber for NOx will remove less than 30%.
HCl Scrubber Design Example
Gas flow: 8,000 m³/h. Inlet HCl: 250 mg/m³. Target outlet: 5 mg/m³ (98% removal).
NTU = ln(250/5) = 3.9. HTU ≈ 0.4 m (instantaneous reaction). Packed depth = 3.9 × 0.4 = 1.6 m.
Column diameter (Souders-Brown, K=0.06): φ1.4m. NaOH consumption: 250 mg/m³ × 8,000 m³/h × 0.98 × (40/36.5) / 1,000,000 = 0.21 kg/h. Negligible operating cost.
HF Scrubber Design Example
Gas flow: 5,000 m³/h. Inlet HF: 50 mg/m³. Target outlet: 1 mg/m³ (98% removal, below OSHA PEL of 3 ppm).
NTU = ln(50/1) = 3.9. HTU ≈ 0.5 m (fast reaction but slower than HCl). Packed depth = 2.0 m. Column: φ1.0m PP, with PVDF packing (PP degrades in HF above 60°C). NaOH consumption: 50 × 5,000 × 0.98 × (40/20) / 1,000,000 = 0.49 kg/h.
Material Selection and Cost Guide for Acid Fume Scrubbers
Material selection is the most consequential design decision for acid fume scrubbers — a wrong material choice can cause column failure within 6 months. The table below provides material compatibility for common acid fumes at typical operating temperatures.
| Acid Fume | Max Temp | Shell | Packing | Mist Eliminator | Ex-works Cost (8,000 m³/h) |
|---|---|---|---|---|---|
| HCl | 60°C | PP | PP Pall rings | PP mesh | $12,000–18,000 |
| HF | 60°C | PP | PVDF Pall rings | PP mesh | $15,000–22,000 |
| H₂SO₄ mist | 80°C | FRP (vinylester) | CPVC Pall rings | FRP chevron | $22,000–35,000 |
| HNO₃ / NOx | 70°C | FRP (vinylester) | PP Pall rings | PP mesh | $25,000–40,000 |
| Mixed acids | 70°C | FRP (vinylester) | PVDF Pall rings | PP/FRP | $28,000–45,000 |
PP is the default material for HCl and HF because it is low-cost and chemically resistant at temperatures below 60°C. For H₂SO₄ mist (which forms a high-temperature aerosol) and NOx (which produces nitric acid condensate), FRP with vinylester resin is required. For mixed acid exhaust containing both HF and H₂SO₄ — common in metal finishing — FRP is the safe choice because PP may fail at the hotter spots near the process source.
Annual operating cost for an 8,000 m³/h acid fume scrubber: NaOH at $200–600/year (very low for most acid gases because the stoichiometric consumption is driven by the inlet concentration, which is typically 50–500 mg/m³), electricity at $3,500–6,000/year, and maintenance at $1,000–2,500/year. Total: approximately $5,000–9,000/year — significantly lower than H₂S or NH₃ scrubbers because acid gas concentrations in ventilation exhaust are typically low.
Frequently Asked Questions
What is the difference between an acid fume scrubber and a packed bed scrubber?
There is no fundamental difference — acid fume scrubbers are packed bed scrubbers designed specifically for acidic gas streams. The distinction is in the material selection (PP or FRP, never carbon steel) and the scrubbing solution (alkaline, typically NaOH).
What is the best scrubbing solution for acid fumes?
NaOH (5–15% by weight) is the standard for most acid gases. For NOx, a reducing agent (Na₂S₂O₃ or urea) must be added. For HF, NaOH works but the NaF byproduct has limited solubility — maintain NaOH excess above the stoichiometric ratio to prevent NaF precipitation in the packing.
What material is used for acid fume scrubbers?
PP for HCl and HF below 60°C, FRP with vinylester resin for H₂SO₄ and NOx, PVDF for temperatures above 80°C. Carbon steel and aluminum must never be used in acid fume scrubber service.
How much does an acid fume scrubber cost?
An 8,000 m³/h PP packed bed scrubber for HCl costs $12,000–18,000 ex-works, or $18,000–28,000 installed. An FRP scrubber for H₂SO₄ mist costs $22,000–35,000 ex-works, or $35,000–55,000 installed.
What is the OSHA limit for acid gas exposure?
OSHA PELs: HCl 5 ppm ceiling, HF 3 ppm (8-hr TWA), H₂SO₄ 1 mg/m³ (8-hr TWA), HNO₃ 2 ppm (8-hr TWA), H₃PO₄ 1 mg/m³ (8-hr TWA).
Conclusion
Acid fume scrubber design is the most common and most straightforward application of packed bed scrubber technology. The key is matching the material selection to the specific acid chemistry — PP for HCl and HF, FRP for H₂SO₄ and NOx. The sizing method follows the standard packed bed sequence (Souders-Brown, HTU-NTU, MWR) with acid-specific HTU values of 0.3–0.8 m depending on the gas. Operating costs are low ($5,000–9,000/year for an 8,000 m³/h system) because the inlet concentrations in ventilation applications are typically 50–500 mg/m³. The most common design failure is using a single-material design for a mixed-acid exhaust — always verify the full gas composition before selecting materials.