A chemical plant's 6t/h sodium sulfate solution MVR evaporation and crystallization system

I. Project Background 

This client produces 20,000 tons of H-acid annually. The centrifuged mother liquor contains 6–8% Na₂SO₄, COD 15,000 mg/L, and a color intensity of 3000 times. The original outsourced treatment cost was as high as 280 RMB/ton. The company decided to construct a new 6t/h sodium sulfate MVR evaporation crystallization system ---based on evaporation water volume, requiring "zero discharge," steam consumption <0.12t/t water, crystal purity ≥99%, and seamless integration with the existing DCS system. Our company undertook the project using the EPC model. It successfully started up in January 2025 and has been operating continuously for over 7200 hours, becoming a model for the treatment of high-salt, high-COD wastewater in the dye industry.

II. Process Route

1. Raw Material Characteristics

Boiling point elevation Δb = 10℃, 20% Na₂SO₄, 70kPa·A

Critical supersaturation ΔCmax = 5g/L, prone to explosive nucleation

High color, high COD, requires decolorization before evaporation

2. Process Summary

Raw material → Advanced oxidation --Ozone + Hydrogen peroxide, COD ↓ 60% → Tubular microfiltration --SDI ≤ 3 → Primary preheating --Condensate → Secondary preheating --Live steam → MVR forced circulation evaporator → Oslo crystallizer → Centrifugation → Airflow drying → Automatic packaging

The secondary steam experiences a temperature rise of 18℃ via a high-speed centrifugal compressor, with a compression ratio of 1.7 and an adiabatic efficiency of 82%. The evaporation chamber maintains -88kPa, and the evaporation temperature is 58℃, effectively avoiding the dye decomposition temperature zone.

3. Key Innovations

Integrated "falling film + forced circulation" composite structure: The falling film section handles 70% of the evaporation, while the forced circulation section is responsible for crystal growth, reducing the unit heat transfer area by 22%. Online washing of salt legs: Crystal residence time ≥50min, d50=0.8mm, C.V.≤5%, moisture content after centrifugation ≤3%.

Intelligent cleaning: CIP is triggered when the tube wall temperature difference >2℃ or the vacuum degree drops by 2kPa, alternating between acid/alkali/chelating agents, extending the cleaning cycle from 5 days to 15 days.

Mother liquor reflux ratio is adjustable from 0–30%, ensuring the system's tolerance to COD enrichment for a period ≥20 days.

III. Equipment Configuration

Evaporator: 2205 duplex steel, heat exchange area 1200m², design margin 20%, ensuring five years of leak-free operation.

Compressor: One-piece milled impeller of titanium material, diameter 1.1m, linear velocity ≤270m/s, bearings with magnetic levitation, design life 100,000h, vibration value <1.2mm/s.

Centrifuge: Horizontal spiral screen centrifuge with a screen gap of 0.12mm. The washing section is equipped with a 2℃ chilled mother liquor spray to reduce COD entrainment.

Drying: Closed-loop airflow drying with nitrogen protection. Oxygen content <3%, product temperature ≤55℃, to prevent decahydrate weathering.

IV. Automation and Safety

Kang Jinghui Automatic Control System: Multi-variable predictive control of evaporation rate, compression ratio, and mother liquor density, with fluctuations within ±2%.

Dual Redundant SIS: 16 interlocks including compressor surge, high liquid level, oxygen content >8%, and temperature >65℃, with a response time <300ms.

Explosion-proof Zoning: Compressor area ExdⅡBT4; all instruments, lighting, and maintenance sockets are explosion-proof certified.

V. Operational Data (Performance Assessment, April 2025)

• Evaporation Rate: 6000 kg/h Design: 6000

• Live Steam: 0.10 t/t water Design: 0.12

• Power Consumption: 26 kWh/t water Compressor + Circulating Pump + Vacuum Pump

• Cooling Water: 2 t/t water -32℃→38℃

• Anhydrous Na₂SO₄ Yield: 2.1 t/d, Whiteness ≥90%, Purity 99.3%, Cl⁻≤200 ppm, COD≤100 mg/kg

VI. Economic and Environmental Benefits

Annual Steam Savings: 18,000 tons equivalent to 1900 t of standard coal, Carbon Reduction: 5000 t

Annual Operating Cost Savings: 4.5 million RMB Steam + Outsourced Disposal Fees The external sales price of high-purity sodium sulfate has increased from 400 yuan/ton to 600 yuan/ton, generating an additional annual revenue of 1.5 million yuan. The investment payback period is 2.3 years.

VII. Customer Testimonials

"The MVR system has been running for seven months with zero compressor failures. The crystals are uniform in size and do not clump after drying. The project has truly achieved the dual goals of zero discharge of high-COD and high-salt wastewater and high-value utilization of sodium sulfate."