10t/h MVR Evaporation System – A Demonstration Project for Zero-Emission and High-Value Salt Resource Recovery of High-Salt, High-COD Mother Liquor in the Dye Industry

I. Project Background 

The centrifugal mother liquor composition of this dyeing and printing company is: NaCl 180-220 g/L, COD 30000 mg/L, color 8000 times, SO₄²⁻ 25 g/L, temperature 65℃. The original triple-effect evaporation + drum dryer consumed 0.38 t/t of steam and water, resulting in equipment coking, odor emission, and significant environmental pressure. In 2024, the company decided to build a new 10 t/h (evaporation water volume) MVR evaporation system, requiring steam consumption ≤0.08 t/t of water, salt whiteness ≥80, Cl⁻ ≥99%, and seamless integration with the existing DCS. Our company undertook the project using the EPC model, successfully starting up in February 2025. To date, it has been running continuously for over 7500 hours, reducing overall energy costs by 70% and saving 12 million yuan in annual operating costs.

II. Process Route

1. Raw Material Characteristics

Boiling point elevation Δb = 11℃ (23% NaCl, 70kPa·A)

High COD, high color, prone to foaming, Ca²⁺ + Mg²⁺ 1200mg/L, scaling factor RF = 0.82

Contains a small amount of aromatic amines, requiring closed-loop operation and tail gas treatment.

2. Process Summary

Mother liquor → Cooling and iron removal (≤30℃) → High-density precipitation (NaOH + Na₂CO₃, Ca/Mg ≤30mg/L) → Tubular microfiltration (SDI ≤3) → Two-stage preheating (condensate + secondary steam) → MVR forced circulation evaporator → Oslo crystallizer → Two-stage piston centrifuge → Fluidized bed drying → Automatic packaging

Secondary steam undergoes a 20℃ temperature rise via a speed-increasing box-type centrifugal compressor, with a compression ratio of 1.8, an evaporation temperature of 55℃, and a vacuum degree of -88kPa, avoiding the dye decomposition temperature zone.

3. Key Innovations

High-shear forced circulation + Oslo classification: Circulation rate 12000 m³/h, crystal residence ≥50 min, d50=0.85 mm, C.V. ≤5%.

Online defoaming: Side-jet + food-grade defoamer, foam height <200 mm, ensuring compressor surge-free operation.

Mother liquor reflux ratio adjustable from 0–30%, COD enriched to 80000 mg/L before pumping into the plant's RTO, no waste liquid discharge from the system.

Intelligent CIP: Alternating acid-base + oxidant cleaning triggered by a pipe wall temperature difference >2℃ or a vacuum drop of 2 kPa, extending the cycle from 3 days to 14 days.

III. Equipment Configuration

Evaporator: 2205 duplex steel, heat exchange area 1100 m², design margin 25%, ensuring five years of leak-free operation.

Compressor: One-piece milled impeller made of titanium, 1.3m diameter, linear velocity ≤280m/s, magnetic levitation bearing, design life 100,000h, vibration value <1.0mm/s.

Centrifuge: LW650×2800-N, separation factor 1600, screen gap 0.12mm, washing section uses 3℃ chilled mother liquor to reduce color entrainment

Drying: Closed-loop circulating fluidized bed, nitrogen protection, oxygen content <3%, product temperature ≤55℃ to prevent NaCl yellowing.

IV. Automation and Safety

DCS+APC: Multi-variable predictive control of evaporation rate, compression ratio, mother liquor density, and color online analyzer, fluctuation ±2%.

Dual Redundant SIS: 18 interlocks including compressor surge, oxygen content >8%, temperature >65℃, and color exceeding limits, response time <300ms.

V. Operating Data (Performance Assessment in May 2025)

• Evaporation Rate: 10000 kg/h (Design 10000)

• Live Steam: 0.08 t/t water (Design 0.10)

• Power Consumption: 27 kWh/t water (Compressor + Circulating Pump + Vacuum Pump + Defoaming Pump)

• Cooling Water: 1.5 t/t water (32℃→38℃)

• Output: NaCl 5.5 t/d, Whiteness ≥ 82, Purity 99.1%, Water Insoluble Matter ≤ 0.03%, Color ≤ 20 times, meeting the requirements for caustic soda salt for ion-exchange membrane systems

• System Recovery Rate: NaCl 98.5%, Water Reuse 100% efficiency, zero wastewater discharge throughout the plant

VI. Economic and Environmental Benefits

Annual steam savings of 33,000 tons, equivalent to 3,600 tons of standard coal, reducing carbon emissions by 9,500 tons. Annual operating cost savings of 12 million yuan (steam + outsourced disposal). The external sales price of refined industrial salt increased from 150 yuan/ton to 480 yuan/ton, resulting in an additional 6 million yuan in annual revenue.

VII. Customer Feedback

"The 10t/h MVR system has been running for seven months with zero compressor failures. The vacuum level remains at -88kPa even in summer, and the salt whiteness is consistently above 80, completely solving the problems of odor and energy consumption in dye mother liquor."