Stage | Purpose | Typical hardware & controls |
1. Air conditioning | Cool, humidify, or remove coarse PM to protect the equipment | spray quenchor, venturi pre-unit |
2. Gas–liquid contact | Create large interfacial area for mass transfer | packed bed, spray tower, venturi, tray or rotary atomiser; ΔP ≈ 0.5–2.5 kPa |
3. Reaction zone | Absorbed compounds react and are neutralized, resulting in heat being released | recirculation tank with mixing and heat dissipation |
4. Droplet separation | Prevent liquid carry-over | mesh / chevron demister, cyclones |
5. Liquid conditioning | Maintain reagent strength and purge byproducts | pH or ORP-controlled make-up dosing, bleed/blow-down, solids separation |
Parameter | Typical set-point / range | Why it matters |
Liquid-to-gas ratio (L/G) | 1–20 L m⁻³ (spray/packed); ≥ 40 L m⁻³ (venturi) | governs mass-transfer driving force |
pH | 9–13 with NaOH or Ca(OH)₂ | ensures excess alkalinity for SO₂/HCl absorption |
ORP (oxidising service) | +150 … +350 mV with NaOCl/NaClO₂ | drives H₂S, mercaptan, or NOₓ oxidation |
Temperature | 15–55 °C; keep below reagent boiling/solubility limits | affects solubility and probe life |
ΔP across column | monitor for fouling or flooding | rising ΔP signals scaling or packing blockage |
Contaminant | Reagent Type | Main Reaction | pH/ORP Control |
SO₂, HCl, HF | NaOH, Ca(OH)₂ | Acid + base → salt + H₂O | pH 9–13 |
NH₃ | H₂SO₄, HCl | NH₃ + H⁺ → NH₄⁺ | pH 3–6 |
H₂S, mercaptans | NaOCl, ClO₂ | S-containing compound + oxidizer → SO₄²⁻/S⁰ | ORP +150…+350 mV |
Apparatus | When Preferred | Δp, kPa | HETP, m |
Packed column (structured or rings) | Low dust content, high efficiency; moderate pressure drop, standard fans | 0.5–2.5 | 0.4–1.0 |
Spray tower | Very high flow rates, strict limit on Δp; minimal resistance, i.e., almost no hindrance to flow, suitable for very large volumes | 0.2–0.8 | 1.5–3 |
Venturi | Simultaneous dust removal, particles ≤ 15 µm; much higher resistance, but allows capturing both dust and droplets at once | 5–25 | — |
Tray tower | Medium capacity, moderate foaming and/or presence of solids; requires a wide operating range without packing blockage; moderate resistance, standard fans | 1–3 | 0.6–1.2 |