Key Takeaways
- H₂S is a weak diprotic acid (pKa₁ = 7.0, pKa₂ = 12.9) that requires pH control between 9 and 11 for efficient caustic scrubbing. Below pH 9, dissolved H₂S re-evaporates; above pH 12, NaOH consumption doubles without proportional benefit for most applications. A pH controller set to 10.5 ± 0.5 is essential.
- NaOH consumption is the dominant operating cost at $28,300/year for a 6,000 m³/h scrubber treating 500 ppm H₂S. Design consumption is 1.4–1.6 kg NaOH per kg H₂S (stoichiometric 1.18 plus 10–30% excess). If your supplier quotes a consumption rate below 1.3:1, either they are omitting the CO₂ co-absorption or they have underestimated the excess required to maintain pH driving force.
- HTU for H₂S in 15% NaOH with 2-inch Pall rings is 0.6–0.9 m — higher than HCl (0.4–0.6 m) or NH₃ (0.5–0.8 m) because the liquid-phase reaction is slower. Using a generic acid gas HTU value of 0.5 m will produce a column that is 30–50% short on packed depth.
- For biogas applications with 30–40% CO₂, a dual-loop caustic scrubber design can reduce total NaOH consumption by 20–35% compared to a single-loop design. If your biogas H₂S scrubber vendor recommends a single-loop design, ask for their CO₂ consumption calculation — a single-loop design that ignores CO₂ will cost you $5,000–10,000 per year in wasted caustic.
- A complete 6,000 m³/h H₂S caustic scrubber (PP, pH control included) costs $22,000–38,000 installed, with annual operating cost of $32,000–37,000 (NaOH + electricity + disposal). The scrubber typically pays for itself within 1–2 years compared to activated carbon replacement at the same flow and concentration.
A biogas plant in Ohio commissioned a caustic scrubber for their 500 kW anaerobic digester in 2022. Inlet H₂S: 2,000 ppm. Target: under 50 ppm for the gas engine manufacturer’s warranty. The scrubber vendor delivered a φ1.0m packed column with 1.2m of 2-inch Pall rings, recirculating 10% NaOH at 8 m³/h. On startup, outlet H₂S measured 180 ppm. The column was undersized — the NaOH concentration was too low for the H₂S loading rate, and the packed depth was 40% short of what the HTU calculation required. The fix: recirculate 18% NaOH and add a second packed bed. The vendor covered the $12,000 retrofit, but the plant lost six weeks of gas engine runtime at $2,800 per week in lost electricity revenue.
An H₂S scrubber design calculation differs from a standard acid gas scrubber because H₂S is a weak acid (pKa₁ = 7.0, pKa₂ = 12.9) that requires precise pH control to drive the absorption equilibrium. The caustic consumption rate is higher per mole than for HCl or HF, and the HTU is higher because the liquid-phase reaction is slower. This guide covers the full calculation sequence with H₂S-specific parameters, a complete worked example for a biogas application, and the common design mistakes that cause scrubbers to fail their outlet guarantees.
For the general packed bed sizing method, see our packed bed scrubber design calculation guide. For a comparison of H₂S removal methods beyond wet scrubbing, see our ammonia removal guide (the methodology applies to any contaminant).
H₂S Scrubber Chemistry and the pH-Driven Equilibrium
Hydrogen sulfide is a weak diprotic acid. When dissolved in water or a caustic solution, it dissociates in two steps:
H₂S (gas) ⇌ H₂S (aqueous) — physical dissolution
H₂S (aq) ⇌ H⁺ + HS⁻ — pKa₁ = 7.0 (first dissociation)
HS⁻ ⇌ H⁺ + S²⁻ — pKa₂ = 12.9 (second dissociation)
The EPA wet scrubber design reference identifies H₂S as a moderately soluble acid gas that requires chemical reaction in the liquid phase to achieve high removal efficiency. The overall reaction with sodium hydroxide proceeds in two stages depending on the pH:
H₂S + NaOH → NaHS + H₂O — at pH 9–11 (bisulfide stage)
H₂S + 2NaOH → Na₂S + 2H₂O — at pH >12 (sulfide stage)
The first reaction consumes 1 mole of NaOH per mole of H₂S and produces sodium bisulfide (NaHS), a soluble salt. The second consumes 2 moles of NaOH and produces sodium sulfide (Na₂S). In practice, most industrial H₂S scrubbers operate in the pH 9–11 range, targeting the bisulfide stage — it consumes half the NaOH of the full sulfide reaction and produces a byproduct (NaHS) that can sometimes be sold to mining operations for froth flotation at $200–400 per ton of NaHS solution (15% concentration).
At pH below 9, the equilibrium shifts back toward H₂S (aqueous), and dissolved H₂S can re-evaporate from the scrubbing solution — exactly the same problem as ammonia re-evaporation at high pH. At pH above 12, the reaction consumes twice the NaOH, but the driving force for absorption is maximized because the liquid-phase H₂S concentration is essentially zero.
OSHA’s Permissible Exposure Limit for H₂S is 10 ppm as an 8-hour TWA, and the Immediately Dangerous to Life or Health (IDLH) concentration is 100 ppm — 5 times more toxic than ammonia. This makes the design reliability of H₂S scrubbers even more critical than for other acid gases.
NaOH Consumption: The Key Operating Cost Driver
The NaOH consumption rate determines the largest single operating cost of an H₂S caustic scrubber. Unlike HCl or HF scrubbing — where the acid-base reaction is instantaneous and stoichiometry is straightforward — H₂S scrubbing has a more complex consumption profile because of the two-stage dissociation.
For a scrubber operating at pH 9–11 (bisulfide stage), the stoichiometric consumption is:
1 mole NaOH per 1 mole H₂S removed
NaOH molecular weight: 40 g/mol. H₂S molecular weight: 34 g/mol.
For each kg of H₂S removed: NaOH required = 40/34 = 1.18 kg NaOH per kg H₂S.
In practice, add 10–30% excess caustic to maintain the pH driving force and compensate for CO₂ co-absorption. Design consumption = 1.4–1.6 kg NaOH per kg H₂S.
Worked Example: NaOH Consumption for a Biogas H₂S Scrubber
Gas flow: 6,000 m³/h. Inlet H₂S: 500 ppm (approximately 700 mg/m³ at 35°C). Target outlet: 50 ppm (95% removal).
H₂S removed per hour = 6,000 × 700 × 0.95 / 1,000,000 = 4.0 kg/h
NaOH required (stoichiometric) = 4.0 × 1.18 = 4.7 kg/h
Design NaOH consumption (with 25% excess) = 4.7 × 1.25 = 5.9 kg/h
At 15% NaOH solution (150 g/L NaOH — a common delivered concentration):
Solution flow = 5,900 g/h / 150 g/L = 39 L/h of 15% NaOH
Annual NaOH cost (15% solution, $0.40–0.80 per kg of 100% NaOH): $5.9 kg/h × 8,000 h/yr × $0.60/kg = $28,300/year. This is the dominant operating cost — roughly 60–75% of total annual operating spend for a caustic scrubber.
For comparison, if the scrubber operates at pH >12 (sulfide stage), NaOH consumption doubles to 2.36 kg per kg H₂S. The higher operating cost is justified only when (a) the outlet specification is below 5 ppm, (b) complete conversion to Na₂S is required for downstream treatment, or (c) the CO₂ concentration in the gas is low enough that the second-stage consumption is not wasted on bicarbonate formation.
Sizing an H₂S Scrubber: Diameter, Height, and Wetting Check
The sizing sequence for an H₂S scrubber follows the standard packed bed method (Souders-Brown → HTU-NTU → MWR), but the HTU values and liquid properties differ from other acid gases because H₂S has lower solubility in water and requires chemical reaction for effective removal.
Column Diameter
Using the EPA Souders-Brown methodology with K = 0.06 m/s for 2-inch Pall rings, 15% NaOH solution (ρ_l ≈ 1,050 kg/m³), and gas density at 35°C (ρ_g ≈ 1.15 kg/m³):
u_sg (flooding) = 0.06 × √((1,050 − 1.15) / 1.15) = 0.06 × √911.2 = 1.81 m/s
Design velocity at 75% flooding: u_design = 1.81 × 0.75 = 1.36 m/s
For 6,000 m³/h at 35°C:
A = 6,000 / (1.36 × 3,600) = 1.23 m²
D = √(4 × 1.23 / π) = 1.25 m
Round to φ1.2 m (PP fabrication in 100 mm increments). Actual area = 1.13 m². Actual velocity = 6,000 / (1.13 × 3,600) = 1.47 m/s — 81% of flooding. Acceptable for preliminary design.
Packed Bed Height
For 95% removal (500 ppm inlet → 25 ppm outlet):
NTU = ln(500 / 25) = ln(20) = 3.0
HTU values for H₂S absorption (2-inch PP Pall rings):
- 15% NaOH (chemical reaction, pH 10–12): HTU ≈ 0.6–0.9 m (slower reaction than HCl, faster than physical absorption)
- Water only (physical absorption): HTU ≈ 1.5–2.5 m (H₂S has low water solubility — Henry’s law constant approximately 10× higher than NH₃)
- Iron chelate solution (chemical oxidation): HTU ≈ 0.8–1.2 m
For 15% NaOH at L/G = 1.5 L/m³: HTU ≈ 0.7 m.
Packed depth = 3.0 × 0.7 = 2.1 m
Add 0.3 m top + 0.3 m bottom: total packed section = 2.7 m. Total column height including sump (0.8 × D = 1.0 m) and mist eliminator: approximately 5.0 m. H/D ratio = 5.0 / 1.2 = 4.2 — acceptable.
Minimum Wetting Rate Check
For 2-inch PP Pall rings (a_p ≈ 100 m²/m³): MWR = 0.10 × 100 = 10 m³/(m²·h)
At φ1.2m (A = 1.13 m²): minimum liquid flow = 10 × 1.13 = 11.3 m³/h
NaOH solution flow from the consumption calculation is only 39 L/h (0.039 m³/h) — far below the MWR. The recirculation rate must be much higher. Design recirculation: 12 m³/h (L/G = 2.0 L/m³). The caustic is replenished by a dosing pump controlled by a pH meter — the bulk liquid recirculates, and only the consumed NaOH is replaced.
Liquid flux = 12 / 1.13 = 10.6 m³/(m²·h) — meets MWR with 6% margin.
pH Control and CO₂ Interference
pH control in H₂S caustic scrubbers is more complex than in acid gas scrubbers because the target pH range (9–11) is also where atmospheric CO₂ dissolves and consumes caustic. A scrubber treating a gas stream containing both H₂S and CO₂ — which includes biogas, refinery gas, and most industrial exhaust — will consume additional NaOH reacting with CO₂ to form sodium carbonate (Na₂CO₃) or sodium bicarbonate (NaHCO₃):
CO₂ + 2NaOH → Na₂CO₃ + H₂O (pH >10)
CO₂ + NaOH → NaHCO₃ (pH 8–10)
For biogas applications where CO₂ can be 30–40% of the gas stream by volume, the CO₂-related caustic consumption can equal or exceed the H₂S-related consumption. A dual-loop design — where the first loop uses partially spent caustic (low NaOH, high NaHS) and the second loop uses fresh high-concentration NaOH — can reduce total caustic consumption by 20–35% compared to a single-loop design, as documented in the Trimeric Corporation’s refinery caustic scrubber analysis.
Practical pH control strategy: Set the caustic dosing controller to pH 10.5 ± 0.5. This keeps the solution in the bisulfide-dominant range (HS⁻) where 1 mole of NaOH removes 1 mole of H₂S, minimizes CO₂ co-absorption (which accelerates at pH >11), and prevents H₂S re-evaporation (which occurs at pH <9). Use a dual-input pH/ORP controller with a dosing pump sized for 1.5× the peak NaOH consumption rate.
H₂S Removal Technologies: Comparison and Selection
Caustic scrubbing is the most common method for H₂S removal, but it is not always the best choice. The table below compares the main technologies used for hydrogen sulfide removal from gas streams.
| Technology | Removal Efficiency | Operating Cost | Best For | Key Limitation |
|---|---|---|---|---|
| Caustic (NaOH) wet scrubbing | 95–99.9% | $15,000–40,000/yr (NaOH) | Medium-high flow, 200–5,000 ppm inlet | NaOH cost, CO₂ co-absorption, spent caustic disposal |
| Iron chelate (Lo-Cat) scrubbing | 99–99.9% | $10,000–25,000/yr (catalyst replacement) | Natural gas, biogas, low O₂ streams | Higher capital cost, catalyst poisoning from heavy metals |
| Activated carbon adsorption | 90–99% | $5,000–15,000/yr (carbon replacement) | Low flow, intermittent use, <100 ppm inlet | Carbon saturation, disposal of spent carbon |
| Biofiltration | 80–95% | $2,000–8,000/yr (media + water) | Low concentration, consistent flow, large footprint OK | Large footprint, sensitive to temperature and load changes |
| Iron sponge (Fe₂O₃ media) | 90–99% | $8,000–20,000/yr (media replacement) | Small biogas, low flow, low capital budget | Media replacement labor, disposal of spent media |
| Chemical oxidant (H₂O₂ or NaClO) | 95–99.5% | $12,000–30,000/yr (chemical) | Low pH gas streams, no caustic handling | Chemical handling safety, oxidation byproducts |
For biogas applications (500–5,000 ppm H₂S, 30,000–40,000 ppm CO₂), caustic scrubbing works when the gas volume is above 500 m³/h and the H₂S concentration is above 300 ppm. Below 300 ppm, activated carbon or biofiltration is more cost-effective because the caustic consumption is low enough that the capital cost of the scrubbing system dominates the economics. Above 5,000 ppm H₂S, consider iron chelate (Lo-Cat) systems — they have higher capital cost but lower operating cost than caustic scrubbing at high sulfur loadings.
H₂S Scrubber Cost: Equipment and Annual Operating
The installed cost of an H₂S caustic scrubber depends on the column size, material (PP vs FRP vs SS), and whether the pH control and chemical dosing system are included. The table below provides budget-level ranges for NaOH recirculating packed bed scrubbers.
| Gas Flow (m³/h) | Diameter (m) | PP Cost (Ex-works) | FRP Cost (Ex-works) | Installed (×1.5–2.5) |
|---|---|---|---|---|
| 2,000 | φ0.8 | $5,000–8,000 | $8,000–14,000 | $10,000–18,000 |
| 4,000 | φ1.0 | $8,000–13,000 | $13,000–22,000 | $16,000–28,000 |
| 6,000 | φ1.2 | $11,000–18,000 | $18,000–30,000 | $22,000–38,000 |
| 10,000 | φ1.6 | $15,000–25,000 | $25,000–42,000 | $32,000–55,000 |
| 20,000 | φ2.2 | $25,000–42,000 | $42,000–70,000 | $55,000–95,000 |
These prices include the tower shell, 2-inch PP Pall rings (or equivalent), mist eliminator, liquid distributor, integrated sump, and pH control system. They exclude the recirculation pump, NaOH storage tank, fan, ductwork, and commissioning. Annual operating cost for a 6,000 m³/h caustic scrubber at 500 ppm H₂S and 8,000 operating hours per year: NaOH at $28,300/year (dominant), electricity at $2,500–4,500/year, and spent caustic disposal at $1,500–4,000/year. Total: approximately $32,300–36,800/year.
Frequently Asked Questions
What is the reaction between H₂S and NaOH in a scrubber?
The primary reaction is H₂S + NaOH → NaHS + H₂O (sodium bisulfide). This occurs at pH 9–11 and consumes 1 mole of NaOH per mole of H₂S. At pH above 12, the reaction proceeds further: H₂S + 2NaOH → Na₂S + 2H₂O, consuming twice the caustic.
What is the typical HTU value for H₂S in a caustic scrubber?
For 2-inch PP Pall rings with 15% NaOH at L/G = 1.5–2.0 L/m³: HTU ≈ 0.6–0.9 m. H₂S has a higher HTU than HCl (0.4–0.6 m) or NH₃ (0.5–0.8 m) because the liquid-phase reaction is slower. Confirming HTU with vendor data is essential — a wrong HTU produces an under-designed column by 30–50%.
Can I use water instead of caustic for H₂S removal?
Water alone achieves only 50–70% removal of H₂S because of its low water solubility. The EPA acid gas design reference catalogs H₂S as a moderately soluble gas requiring chemical enhancement for high-efficiency removal. Caustic (NaOH) or an oxidizing agent (H₂O₂, NaClO) is required to achieve >95% removal.
What is the OSHA limit for H₂S?
The OSHA Permissible Exposure Limit (PEL) for hydrogen sulfide is 10 ppm as an 8-hour TWA. The IDLH (Immediately Dangerous to Life or Health) concentration is 100 ppm. H₂S is approximately 5 times more toxic than ammonia (OSHA PEL of 50 ppm).
How does CO₂ affect caustic scrubber design?
CO₂ reacts with NaOH to form Na₂CO₃ or NaHCO₃, consuming caustic without removing H₂S. For gas streams with high CO₂ content (biogas at 30–40% CO₂), the CO₂-related caustic consumption can equal or exceed the H₂S consumption. A dual-loop design reduces this waste by using partially spent caustic in the first stage and fresh caustic only in the second.
What is the byproduct of H₂S caustic scrubbing?
The primary byproduct is sodium bisulfide (NaHS) solution, which can be sold to mining operations for froth flotation at $200–400 per ton of 15% NaHS solution. Alternatively, the spent caustic can be oxidized to sodium sulfate (Na₂SO₄) for safe disposal, or sent to wastewater treatment.
Conclusion
An H₂S scrubber design calculation must account for the weak-acid chemistry of hydrogen sulfide, the pH-dependent dissociation equilibrium, and the CO₂ co-absorption that competes for caustic. The sequence follows the standard packed bed method: Souders-Brown for φ1.2m diameter → HTU-NTU for 2.1m packed depth → NaOH consumption of 5.9 kg/h → recirculation of 12 m³/h to meet MWR → pH control at 10.5 ± 0.5. The NaOH cost dominates the operating budget at approximately $28,300/year for a 6,000 m³/h biogas scrubber at 500 ppm inlet. For applications with high CO₂ content or very high H₂S loading, consider dual-loop caustic designs or alternative technologies (iron chelate, biofiltration) that may offer lower total cost of ownership.