5t/h triple-effect evaporator for sodium chloride, ammonium chloride, and sodium sulfite

I. Project Background 

A fluorine materials company produces 30,000 tons of HFC-32 refrigerant annually, generating 5 t/h of byproduct washing wastewater with the following composition: NaCl 140-160 g/L, NH₄Cl 45-55 g/L, Na₂SO₃ 25-35 g/L, F⁻ 1.2 g/L, COD 3000 mg/L, color 2000 times, and temperature 50℃. The original single-effect evaporator consumed 0.45 t/t of water steam, and the equipment suffered from coking, ammonia odor, and a mixed salt purity of <90%, making it only suitable for low-cost external sales. In 2024, the client decided to construct a "zero-emission" technological upgrade. Kang Jinghui delivered a 5t/h triple-effect evaporator system under the EPC model. It successfully started up on its first attempt in March 2022 and has been operating continuously ever since. Steam consumption is 0.27t/t water, and the concentrations of NaCl, NH₄Cl, and Na₂SO₃ all meet the client's requirements, generating an additional annual revenue of 18 million yuan.

II. Process Route

1. Influent Water Characteristics

Boiling point elevation Δb = 12℃ (TDS 230g/L, 70kPa·A)

Thermosensitive: NH₄Cl decomposes significantly at >85℃, Na₂SO₃ oxidation rate accelerates at >60℃

High F⁻, high COD, scaling factor RF = 0.88, highly corrosive

2. Process Overview

Rinse water → Cooling and defluorination (Ca(OH)₂ + PAC, F⁻ ≤ 15mg/L) → High-density precipitation (Mg²⁺ ≤ 20mg/L) → Tubular microfiltration (SDI ≤ 3) → Two-stage preheating (condensate + secondary steam) → Triple-effect countercurrent evaporator → DTB crystallizer A (low-temperature Na₂SO₃ precipitation) → DTB crystallizer B (medium-temperature NH₄Cl precipitation) → Oslo crystallizer C (high-temperature NaCl precipitation) → Centrifugation → Fluidized bed drying → Automatic packaging Live steam operates at 0.4 MPa (g) and 152℃. The first-effect heating chamber uses 2507 stainless steel + titanium composite tube sheet, while the second and third effects use 2205 stainless steel, ensuring a margin for both chloride and fluoride ion corrosion.

3. Process Innovation

Countercurrent + Low-Temperature First Effect: The temperature gradient of the third effect is 65-55-45℃. The first effect precipitates Na₂SO₃ at low temperature, reducing oxidation loss by <3%.

Separate Crystallization: Density control at each effect ensures seed crystals. ΔC <2g/L for Na₂SO₃, ΔC <3g/L for NH₄Cl, and ΔC <4g/L for NaCl, with salt purity ≥98%.

Vacuum Flash Cooling + Heat Pump Recycling: The secondary steam from the second effect is partially compressed and pressurized to 18℃ to drive evaporation in the first effect, saving 12% of steam.

Online Ammonia Removal: An acid washing tower is installed in the secondary steam pipeline, achieving an NH₃ absorption rate >99% and ammonia concentration in the tail gas ≤1mg/m³. 

III. Equipment Configuration

Evaporators: 80m² for single-effect, 70m² for double-effect, and 60m² for triple-effect, total heat exchange area 210m², design margin 25%.

Crystallizers: Three DTB units, 2507 stainless steel with rubber lining, design pressure -0.1MPa, slurry density 25-30%.

Centrifuges: LW450×1800-N, separation factor 1400, frequency converter, 0.08mm screen gap, washing section uses 3℃ chilled mother liquor.

Drying: Closed-loop circulating fluidized bed, nitrogen protection, oxygen content <3%, product temperature ≤50℃, preventing NH₄Cl decomposition.

IV. Automation and Energy Saving

DCS+APC: Multi-variable predictive control of evaporation rate, liquid level, density, and vacuum degree, fluctuation ±3%.

Dual closed-loop system with steam flow feedforward + temperature feedback, live steam consumption 0.27t/t water, better than the design value of 0.30t. The secondary steam condenser utilizes a titanium plate heat exchanger, consuming only 2.8 t/t of cooling water, resulting in a 15% energy saving compared to traditional surface condensers.

Dual-redundant SIS: 14 interlocks including low liquid level, high vacuum, centrifuge vibration, and excessive ammonia concentration, with a response time of <500ms.

V. Operational Data (Performance Assessment in June 2025)

• Processing Capacity: 5t/h (Design 5)

• Water Production Rate: 82% (including condensate reuse)

• Live Steam: 0.27t/t water (Design 0.30)

• Power Consumption: 22kWh/t water (circulating pump + vacuum pump + centrifuge)

• Output: NaCl 1.2t/d, purity 99.2%; NH₄Cl 0.5t/d, nitrogen content 25.2%; Na₂SO₃ 0.3t/d, purity 97.5%, all reaching first-class industrial dry basis

• Condensate: TDS≤120mg/L, F⁻≤5mg/L, NH₃≤10mg/L, reused in circulating cooling water, reducing overall plant water consumption by 8%

VI. Economic and Environmental Benefits

Annual steam savings of 0.45 million tons, equivalent to 500t of standard coal, reducing carbon emissions by 1300t Saves 2.4 million yuan/year in hazardous waste transportation costs (originally 2000 yuan/t × 1200t). Selling prices of the different salts: NaCl 480 yuan/t, NH₄Cl 900 yuan/t, Na₂SO₃ 1200 yuan/t, generating an additional 18 million yuan in annual revenue. Investment payback period: 1.3 years.

VII. Customer Feedback

"After six months of operation of the 5t/h triple-effect system, the vacuum level has stabilized at -88kPa. The purity of all three salts meets the standards, and the ammonia odor has completely disappeared. It truly transforms 'three wastes' into 'three products'."