Designing Packed Bed Wet Scrubbers with Liquid Caustic Soda NaOH 32% for HCl, SO2, HF & Cl2 Acid Fume Abatement (QCVN 19:2024/BTNMT)

Table of contents

Counter-current Packed Bed Wet Scrubbers circulating Liquid Caustic Soda NaOH 32% are the primary Air Pollution Control (APC) barrier for steel pickling bays, electroplating lines, PCB etching rooms, and boiler flue gas systems. To maintain stack emissions strictly below QCVN 19:2024/BTNMT Column B thresholds (HCl ≤ 50 mg/Nm³, SO₂ ≤ 300 mg/Nm³, HF ≤ 5 mg/Nm³, Cl₂ ≤ 10 mg/Nm³), the scrubber must operate at a superficial gas velocity of 1.2–1.8 m/s, a liquid-to-gas ratio L/G = 1.5–3.0 L/m³, and an automated recirculation setpoint locked at pH 8.5–9.5—achieving over 98.5% acid capture while preventing carbonate scaling across the packing bed.

Actual 25kg jerrycans of Liquid Caustic Soda NaOH 32% at Loc Thien Chemical warehouse

1. Two-Film Mass Transfer Kinetics & Instantaneous Neutralization Stoichiometry

In metal pickling and surface finishing lines using Hydrochloric Acid HCl 32%, Sulfuric Acid H₂SO₄ 98%, or mixed HNO₃ + HF, acid vapors collected by lip-slot hoods enter the bottom of the packed tower and flow upward counter-currently against a downward spray of NaOH liquor distributed over high-surface-area random packing (110–220 m²/m³).

Under Whitman’s Two-Film Mass Transfer Theory, because free hydroxyl ions (OH⁻) in the liquid film react instantaneously and irreversibly with absorbed acid molecules, the equilibrium back-pressure of the acid gas at the gas–liquid interface drops to zero, making the absorption rate entirely gas-film controlled:

Hydrochloric Acid Vapor Neutralization: HCl (gas) + NaOH (aq) → NaCl + H₂O (ΔH = -55.8 kJ/mol)
Hydrofluoric Acid Vapor Neutralization: HF (gas) + NaOH (aq) → NaF + H₂O
Sulfur Dioxide Absorption: SO₂ (gas) + 2NaOH (aq) → Na₂SO₃ + H₂O
Free Chlorine Gas Absorption: Cl₂ (gas) + 2NaOH (aq) → NaOCl + NaCl + H₂O
Nitrogen Dioxide Absorption: 2NO₂ (gas) + 2NaOH (aq) → NaNO₂ + NaNO₃ + H₂O

Table 1: QCVN 19:2024/BTNMT Emission Limits & Specific Consumption of NaOH 32%

Target Acid Gas Pollutant	Molar Mass (`M`, g/mol)	Soluble Salt Product in Sump	`QCVN 19:2024/BTNMT` Column B Limit (`mg/Nm³`)	Theoretical `NaOH 100%` per `1 kg` Gas (`kg/kg`)	Practical `NaOH 32%` Dosing Rate (incl. `1.12` safety factor)
Hydrochloric Acid (`HCl`)	`36.46`	Sodium Chloride (`NaCl`)	≤ 50	`1.097 kg`	`3.84 kg NaOH 32%` / kg HCl
Hydrofluoric Acid (`HF`)	`20.01`	Sodium Fluoride (`NaF`)	≤ 5	`1.999 kg`	`7.00 kg NaOH 32%` / kg HF
Sulfuric Acid Mist (`H₂SO₄`)	`98.08`	Sodium Sulfate (`Na₂SO₄`)	≤ 35	`0.816 kg`	`2.86 kg NaOH 32%` / kg H₂SO₄
Sulfur Dioxide (`SO₂`)	`64.06`	Sodium Sulfite (`Na₂SO₃`)	≤ 300	`1.249 kg`	`4.37 kg NaOH 32%` / kg SO₂
Free Chlorine (`Cl₂`)	`70.91`	`NaOCl + NaCl` mixture	≤ 10	`1.128 kg`	`3.95 kg NaOH 32%` / kg Cl₂
Nitrogen Oxides (`NOₓ` as `NO₂`)	`46.01`	`NaNO₂ + NaNO₃` mixture	≤ 400	`0.869 kg`	`3.04 kg NaOH 32%` / kg NO₂

2. Hydraulic Sizing, Packing Depth & Mist Elimination Standards

Table 2: Engineering Design Criteria for Counter-Current Packed Bed Wet Scrubbers

Design Parameter	Unit	Standard Design Range (`HCl / HF / SO₂`)	Engineering Consequence of Improper Sizing
Superficial Gas Velocity (`vg`)	`m/s`	`1.2 – 1.8`	If `≥ 2.2 m/s`: liquid flooding and alkaline mist carryover out the stack. If `≤ 0.8 m/s`: oversized vessel diameter and poor gas distribution.
Liquid-to-Gas Ratio (`L/G`)	`L/m³` gas	`1.5 – 3.0`	If `≤ 1.0 L/m³`: dry channeling across packing surfaces, allowing unneutralized acid fumes to slip through.
Packed Bed Height (`Z = NTU × HTU`)	`m`	`1.5 – 2.5`	Provides `5–8` Number of Transfer Units (`NTU`), ensuring `98.5–99.5%` removal efficiency.
Random Packing Media	—	PP Pall Ring / Tellerette `D50 – D76 mm`	High void fraction (`90–93%`), specific surface area `110–190 m²/m³`, low pressure drop (`150–350 Pa/m`).
Demister Pad Thickness	`mm`	`150 – 200`	Knitted PP mesh or Chevron blades capturing `99%` of droplets `≥ 15 µm`.
Sump Liquid Retention Time	`minutes`	`5 – 10`	Allows complete reaction degassing and prevents pump cavitation.

Engineering Calculation Example: 15,000 m³/h HCl Pickling Exhaust Scrubber

Technical Illustration (Reference Engineering Model): A steel wire pickling bay exhausts Qg = 15,000 m³/h (4.167 m³/s) containing an average inlet HCl concentration Cin = 850 mg/Nm³, targeting stack outlet Cout ≤ 15 mg/Nm³ (η = 98.2%).

Tower Internal Diameter (D): Selecting design velocity vg = 1.5 m/s gives cross-sectional area A = 4.167 ÷ 1.5 = 2.778 m² → Internal diameter D = 1.88 m (select standard fabrication diameter D = 1,900 mm).

Recirculation Pump Flow Rate (QL): Selecting L/G = 2.0 L/m³ gives QL = 15,000 m³/h × 2.0 L/m³ = 30,000 L/h = 30 m³/h.

Hourly NaOH 32% Dosing Rate:

Hourly HCl mass load captured: M(HCl) = 15,000 × (850 - 15) × 10⁻⁶ = 12.525 kg HCl/h.

Commercial NaOH 32% (d = 1.35 kg/L) dosing rate: 12.525 × 3.84 = 48.10 kg/h (35.6 L/h).

3. Why Locking pH at 8.5–9.5 Prevents Carbonate Scaling (Na₂CO₃ / NaHCO₃)

A widespread operational mistake is manually dumping excessive caustic soda into the scrubber sump until pH reaches 12.5–13.5. Because over 99% of the gas drawn into the hood is ambient factory air containing ~400 ppm Carbon Dioxide (CO₂), running the scrubber above pH 10.8 forces the caustic liquor to scrub CO₂ into sparingly soluble Sodium Bicarbonate (NaHCO₃, solubility only 9.6 g/100 mL at 20 °C):

Step 1: CO₂ (gas) + 2NaOH (aq) → Na₂CO₃ (aq) + H₂O
Step 2: Na₂CO₃ (aq) + CO₂ (gas) + H₂O → 2NaHCO₃ (solid scale ↓)

Best-Practice Control Protocol from Loc Thien Engineers:

Lock the automated pH Controller setpoint at pH = 8.5 – 9.3: Strong acid gases (HCl, HF, H₂SO₄) are still absorbed at > 99% efficiency, while parasitic CO₂ absorption is suppressed—saving 18–25% of NaOH 32% consumption and keeping the packing media scale-free for 6–12 months.

Automated Conductivity Blowdown (EC = 65–80 mS/cm): Bleed 15–20% of the concentrated salt brine (~8–10% TDS) to the wastewater neutralization tank whenever EC hits 70 mS/cm.

Dual NaOH + Na₂SO₃ Dosing for Chlorine (Cl₂) Scrubbing: When scrubbing Cl₂ gas, maintain ORP ≤ +150 to +250 mV with minor sodium sulfite/thiosulfate addition so intermediate NaOCl cannot re-release toxic Cl₂ gas if pH transiently dips below 7.5.

ZDHC MRSL v3.1 compliant IBC storage at Loc Thien Chemical warehouse

4. ZDHC MRSL v3.1 Level 1 Certified NaOH 32%–50% Supply by Loc Thien Chemical

Loc Thien Investment Development Co., Ltd. (LOC THIEN INVESTMENT DEVELOPMENT COMPANY LIMITED — Tax ID: 0313650856) is a premier commercial supplier and distributor of industrial chemicals across Vietnam:

Official ZDHC MRSL Version 3.1 Conformance Level 1 Certification: NaOH (Sodium Hydroxide) supplied by Loc Thien holds CERTIFICATE (ZDHC MRSL Conformance Level 1) No. ZDHC1 127736 0001 Rev. 00 (Test Report No. 2486017672-Q24VPS-CHM2869) issued by TÜV SÜD Product Service GmbH / TÜV SÜD Vietnam.
6 Strategic Warehouses (3,000+ tons/month, 5–30-ton specialized tanker fleet): Ba Ria - Vung Tau (Phu My 1 IP), Dong Nai (Long Thanh IP), Binh Duong (Di An City), Da Nang (Hoa Khanh IP), Can Tho (Tra Noc IP), and Bac Ninh (Tien Son IP). Legal Headquarters: 452/6B Tinh Lo 10, Binh Tri Dong Ward, Binh Tan District, Ho Chi Minh City.
Quotation & Delivery SLA: Technical quotation within 15 minutes (unit price structured around 5 factors: 32% vs 50% concentration, order volume, IBC vs 5–30-ton tanker delivery, distance, and contract schedule). Delivery within 4 hours in inner HCMC, 24–48 hours across Southern & Mekong Delta industrial parks, and 2–3 days in Northern & Central Vietnam. Contact hotline 0979 891 929 for detailed delivery procedures.

Product Pages: Liquid Caustic Soda NaOH 32%–50% | Caustic Soda Flakes NaOH 99%
GHS Safety Data Sheets (MSDS): MSDS Sodium Hydroxide NaOH | MSDS Hydrochloric Acid HCl 32%
Technical Inquiry & 15-Minute Quotation: Hotline 0979 891 929 (tel:0979891929) — Email: [email protected].