A fine chemical plant's 1t/h sodium chloride double-effect evaporator

Project Background 

A fine chemical company mainly produces water-based coating additives, generating 8000 tons of sodium chloride wastewater annually as a byproduct. Typical components include: NaCl 12-15%, CODcr 8000-12000 mg/L, pH 6.5-7.5, and trace amounts of ethylene glycol, sodium benzoate, and suspended solids. The raw water was treated using an old unit consisting of flocculation, triple-effect evaporation, and centrifugation. Steam consumption was 0.48 tons of water per ton of water, and due to the treatment capacity being far below the design load, long-term low-load operation led to a decrease in heat transfer coefficient and blockage of the crystal slurry pipes. In 2024, the company decided to construct a new 1 t/h sodium chloride double-effect evaporator, requiring a footprint of ≤36㎡, steam consumption ≤0.35 tons of water per ton of water, condensate TDS ≤100 mg/L, and complete reuse in the polymerization process. The investment payback period was ≤2.5 years. The project successfully started up in June 2024 and is currently operating stably.

Process Route

1. Water Quality and Thermodynamic Reassessment

Laboratory tests show that the boiling point elevation of this wastewater is Δbpe = 5.8℃ (15% NaCl, 90℃), which decreases to 4.1℃ under vacuum of -0.075MPa. A combination of double-effect co-current flow + forced circulation + Oslo crystallization is suitable. A large amount of organic scum precipitates when COD is concentrated to 25%, requiring online scum removal.

2. Overall Process Overview

Raw material tank area (10m³, PE) → Basket filter (200μm) → Plate preheater (recovering sensible heat of condensate to 65℃) → First-effect evaporator → Second-effect evaporator → Oslo crystallizer → Piston pusher centrifuge (HR315-N) → Fluidized bed dryer (90℃, residence time 2s) → Ton bag packaging of refined salt.

Secondary steam is pumped via hydraulic jet and liquid ring vacuum pump to -0.075 MPa. The condensate, after passing through a polishing mixed bed, has a conductivity ≤40 μS/cm and is reused for cleaning the polymerization reactor. Mother liquor is periodically discharged to a spray drying pilot unit, achieving "zero" high-salt liquor discharge.

3. Dual-Effect Main Parameters: Rated evaporation capacity 1.0 t/h, feed NaCl 15%, output solid content 38-40%. **Effect 1:** Temperature 92℃, Vacuum -0.02MPa, Evaporation Rate 0.55t/h;

Effect 2: Temperature 72℃, Vacuum -0.075MPa, Evaporation Rate 0.45t/h;

Overall heat transfer coefficient: Effect 1 ≥ 3200W·m⁻²·K⁻¹, Effect 2 ≥ 2100W·m⁻²·K⁻¹; Steam consumption 0.32t/h (0.32t/t water), Cooling water circulation 12m³/h (Δt=6℃).

Equipment Selection and Materials:

1. Evaporator:The heating chamber uses φ25×2mm, 3m long 2205 duplex steel heat exchange tubes. The tube side of the first effect and the second effect are both 2205 to avoid chloride ion pitting corrosion. The shell side is Q345R, with a design pressure of 0.4MPa.Evaporation Chamber: φ1200×3000mm, top-mounted titanium alloy wire mesh demister (99% retention rate), bottom 60° cone angle + side-mounted forced circulation pump (flow rate 400m³/h, head 5m, frequency conversion), ensuring crystal slurry flow velocity of 2.5m/s and a scaling cycle ≥20 days.

2. Crystallizer: Oslo type, φ800×2500mm, built-in guide tube, residence time 1.8h, average particle size 0.45mm, coefficient of variation CV≤15%, wet salt content after centrifugation ≤3%.

3. Vacuum and Condensation: Two-stage hydraulic jet + liquid ring pump, ultimate vacuum -0.085MPa, installed power 7.5kW; surface condenser area 15m², using 304 thin-walled spiral tube, secondary steam condensate TDS≤50mg/L.

4. Online Cleaning: The CIP system is equipped with 5% HNO₃, 3% NaOH, and scale inhibitor (PAA-Na, 15ppm). It automatically starts every 15 days of operation or when the system pressure drop >30kPa, with cleaning time ≤2 hours.

Energy Consumption and Economy: The unit operates for 8000 hours annually, evaporating 8000t of water, saving 1280t of steam (0.48→0.32t/t water), which, at 220 yuan/t, translates to annual savings of 282,000 yuan; electricity savings of 15MWh; water savings of 8,000 tons; and an annual increase in sales revenue (industrial salt + recycled water) of 960,000 yuan.

Environmental Protection and Safety: 

1. Exhaust Gas: After secondary steam condensation, non-condensable gases (VOCs ≤20mg/m³) enter the activated carbon adsorption tank; benzene compounds were not detected at the emission outlet.

2. Mother liquor: The discharge rate is ≤3% of the feed rate. After small-scale spray drying (5kg/h), the residue is treated as solid waste and incinerated externally. The system has no high-salt liquor discharge.

3. Noise: Compressor soundproof enclosure + plant sound absorption; boundary noise during the daytime is ≤55dB(A).

Automation and Digitalization: Utilizes a Siemens S7-1200 PLC + touchscreen, with 180 I/O points. Key parameters such as evaporation intensity, mother liquor density, and centrifugal torque are optimized in real-time using PID + timed inversion algorithms, with steam flow fluctuation <±2%. One-button start/stop via touchscreen, supports remote software viewing, achieving an "unmanned operation + inspection" mode, reducing staffing from 2 people per shift to 1 person.

Operating Results:

1. Production Capacity: The actual evaporation capacity of the unit is 1.05t/h, exceeding the design capacity by 5%; by-product industrial salt is 160kg/d, with an annual production capacity of 53t, NaCl purity ≥98.8%, and whiteness ≥85.

2. Effluent: Condensate COD ≤ 60 mg/L, conductivity ≤ 40 μS/cm, all recycled for the polymerization process, annual recycling volume 8000t.

3. Reliability: 98% uptime, planned cleaning cycle 20 days, actual operating time up to 30 days; equipment failure rate 0.1 times/month, far lower than the industry average of 0.5 times/month.