A 72m³/d mixed salt triple-effect evaporator for a fine chemical manufacturer

Project Background

The owner produces 6,000 tons of high-end fluorescent whitening agent annually, generating 72 m³/d (3 m³/h) of high-salt process wastewater as a byproduct. The wastewater has a complex composition: NaCl 10-12%, Na₂SO₄ 6-8%, organic amine salts 1-2%, CODcr 15,000 mg/L, color 800 times, pH 9-10, and trace amounts of methanol and xylene. Direct discharge of the raw water into the industrial park's wastewater treatment plant causes significant color impact, while the existing single-effect evaporator + centrifugal unit consumes 0.52 t/t of steam per ton of water, resulting in high operating costs and severe color inclusions in the crystals. In 2024, the company plans to construct a new "72 m³/d mixed-salt triple-effect evaporator," requiring steam consumption ≤0.35 t/t of water, condensate COD ≤500 mg/L, color ≤20 times, NaCl and Na₂SO₄ separation rate ≥85%, and an investment payback period ≤2 years.

Process Route

1. Water Quality and Phase Diagram Reassessment

Laboratory tests, combined with the NaCl-Na₂SO₄-H₂O ternary phase diagram, confirmed that Na₂SO₄·10H₂O preferentially precipitates at -15℃, and NaCl reaches saturation at 70℃. Based on this, a two-stage process of "triple-effect co-current flow + cryogenic denitrification + sodium chloride crystallization" was designed to achieve high-value separation of mixed salts.

2. Overall Process Overview

High-salinity wastewater → Equalization tank (50m³, FRP) → High-efficiency dissolved air flotation (DAF, oil removal rate ≥90%) → pH adjustment (H₂SO₄) → Effect I → Effect II → Effect III → Centrifugal separation (NaCl) → Thickening → Drying → Packaging. The mother liquor from the third-effect reactor is cooled and crystallized under vacuum (-5℃, vacuum degree 0.09MPa) to obtain Na₂SO₄·10H₂O, which is then further processed by melting crystallization and evaporation crystallization to obtain anhydrous Na₂SO₄ product. The chilled clear liquid is returned to the third-effect reactor, and the system has no high-salt mother liquor discharged.

3. Main parameters of the third-effect reactor: Rated evaporation water capacity 3.0t/h, total feed salt 18%, output solid content 35-38%. Effect I: Temperature 95℃, Vacuum -0.02MPa, Evaporation rate 1.15t/h;

Effect II: Temperature 80℃, Vacuum -0.06MPa, Evaporation rate 1.00t/h;

Effect III: Temperature 62℃, Vacuum -0.085MPa, Evaporation rate 0.85t/h; Overall heat transfer coefficient: Effect I ≥ 3500W·m⁻²·K⁻¹, Effect III ≥ 2200W·m⁻²·K⁻¹; Steam consumption 1.05t/h (0.35t/t water), Cooling water circulation 60m³/h (Δt=8℃).

Equipment Selection and Materials

1. Heater: Effect I uses φ38×3mm, 4m long 2205 duplex steel heat exchange tubes, shell side Q345R, design pressure 0.4MPa; Effects II and III are both 2205 to avoid chloride ion pitting corrosion.

2. Evaporation Chamber: φ1600×5000mm, top equipped with a titanium alloy wire mesh demister (99% retention rate), bottom 60° cone angle + side-mounted forced circulation pump (flow rate 800m³/h, head 6m, frequency conversion), crystal slurry flow velocity 2.8m/s, scaling cycle ≥15 days.

3. Vacuum and Condensation: Two-stage hydraulic jet + liquid ring pump, ultimate vacuum -0.090MPa, installed power 15kW; surface condenser area 40m², secondary steam condensate COD ≤400mg/L, color ≤15 times, all recycled for workshop equipment cleaning.

4. Online Cleaning: CIP system equipped with 5% HNO₃, 3% NaOH and scale inhibitor (PAA-Na, 20ppm), automatically starts every 10 days of operation or when system pressure drop >30kPa, cleaning time ≤2.5h.

Energy Consumption and Economy: The unit operates for 330 days annually, evaporating 9900 tons of water, saving 1683 tons of steam (0.52 to 0.35 tons/ton of water), which translates to annual savings of 370,000 yuan at 220 yuan/ton; it also saves 18 MWh of electricity and 6,000 tons of water, generating an additional 2.1 million yuan in annual sales revenue (NaCl + Na₂SO₄ + recycled water).

Environmental Protection and Safety:

1. Tail Gas: After secondary steam condensation, non-condensable gases (VOCs ≤ 20 mg/m³) enter the two-stage resin adsorption process; toluene was not detected.

2. Mother Liquor: The mother liquor from the cryogenic denitrification process is recycled, resulting in no high-salt discharge from the system; sodium sulfate production is 0.35 tons/day, sold to glass manufacturers.

Automatic Control: The system employs a Siemens S7-1200 PLC with a touchscreen and 260 I/O points; key parameters such as evaporation intensity, mother liquor density, and centrifugal torque are optimized in real-time using a PID + timed inversion algorithm, with steam flow fluctuations < ±2%. The touchscreen allows for one-button start/stop and supports remote monitoring, enabling an "unmanned operation + inspection" mode, reducing staffing from 3 per shift to 1 person.

Operating Results

1. Production Capacity: The actual evaporation capacity of the unit is 3.2 t/h, exceeding the design capacity by 6%; by-product NaCl is 0.45 t/h, with an annual production capacity of 3300 tons, purity ≥98.8%, and whiteness ≥80; Na₂SO₄ is 0.12 t/h, with an annual production capacity of 900 tons, purity ≥99%.

2. Wastewater: Condensate COD ≤400 mg/L, color ≤15 times, all recycled in the workshop, with an annual recycling volume of 9900 tons.

3. Reliability: Operating rate is 96%, planned cleaning cycle is 15 days, actual operating time is up to 25 days; equipment failure rate is 0.2 times/month, far lower than the industry average of 0.8 times/month.