A 5t/h sodium chloride MVR evaporation crystallization system + mother liquor dehydrator system of a fine chemical company

Project Background

A listed fine chemical company produces approximately 40,000 tons of high-salt wastewater annually as a byproduct of its 120,000-ton-per-year ion-exchange membrane caustic soda production. The average flow rate is 5 tons per hour. The main components are: NaCl 18-20%, SO₄²⁻ 2,000 mg/L, Ca²⁺/Mg²⁺ 300 mg/L, CODcr 5,000-8,000 mg/L, pH 9-10, and trace amounts of benzene solvents. Direct discharge of this raw water will cause the characteristic pollutants to exceed standards. Traditional triple-effect evaporation consumes 0.42 tons of steam per ton of water, resulting in high operating costs and incomplete drying of the mother liquor. In 2024, the company decided to construct a new "5t/h sodium chloride MVR evaporation and crystallization system + mother liquor dehydrator system," requiring zero steam replenishment, a mother liquor moisture content ≤30%, condensate TDS ≤50 mg/L, and complete reuse for primary brine in caustic soda production. The investment payback period is ≤2 years.

Process Route

1. Water Quality and Thermodynamic Reassessment

Laboratory tests show that the boiling point elevation of this wastewater is Δbpe = 8.5℃ (20% NaCl, 90℃), which decreases to 6.2℃ under vacuum of -0.08MPa. MVR forced circulation + Oslo crystallization is suitable. The scaling index of Ca²⁺/Mg²⁺ is >2.5 when concentrated to 28%, requiring a double-layer system of softening and scale inhibitor.

2. Overall Process Overview

High-salinity wastewater → Equalization tank (50m³, air-stirred) → High-efficiency dissolved air flotation (DAF, oil removal rate ≥90%) → Resin softening (Ca²⁺/Mg²⁺ ≤20mg/L) → MVR evaporator → Oslo crystallizer → Two-stage piston pusher centrifuge → Fluidized bed drying (110℃, residence time 3s) → Ton bag packaging of refined salt.

The centrifuged mother liquor (containing 25-28% salt, COD 40,000 mg/L) enters the mother liquor dewatering system: high-pressure diaphragm filter press (800 m², feed pressure 1.2 MPa, pressing pressure 2.0 MPa) → low-temperature belt drying (90℃, residence time 40 min) → filter cake with a moisture content ≤28% of mixed salts, which is then sent to the cement kiln for co-processing; the filtrate is returned to the front end of the MVR, achieving a "zero mother liquor" cycle.

3. MVR Main Parameters: Rated evaporation capacity 5.0 t/h, feed NaCl 20%, output solid content 38-40%. Evaporation chamber: φ2200×5500mm, effective volume 45m³, constructed of 2205 duplex steel + titanium alloy composite plate, design pressure -0.09MPa;

Heater: Plate type, single plate area 1.5m², 200 plates total, total heat exchange area 300m², design temperature 95℃, heat transfer coefficient ≥4500W·m⁻²·K⁻¹;

Compressor: Single-stage centrifugal, titanium alloy impeller, flow rate 7.5t/h, compression ratio 1.9, installed power 450kW, speed regulation range 20-105%, isentropic efficiency ≥84%;

Circulation pump: Axial flow type, flow rate 5000m³/h, head 6m, frequency converter control, ensuring crystal slurry flow velocity 3.2m/s, scaling cycle ≥30 days.

Mother Liquor Dewatering System

1. High-Pressure Diaphragm Filter Press: 800m² filtration area, PP-EL diaphragm plates, feed pressure 1.2MPa, pressing pressure 2.0MPa, circulation cycle 60min, single batch production of 1.8t of mixed salt filter cake (28% water content).

2. Low-Temperature Belt Drying: Stainless steel mesh belt, hot air temperature 90℃, evaporation intensity 12kg water·m⁻²·h⁻¹, installed power 45kW, dried filter cake moisture content ≤15%, calorific value ≥2,000kcal/kg.

3. Filtrate: COD 50,000mg/L, after homogenization, returned to the front of the MVR, no high-salt liquid discharge from the system.

Equipment Selection and Materials

1. Corrosion Protection: Parts in contact with the feed liquid are made of TA2 titanium alloy/2205 duplex steel to avoid Cl⁻ pitting corrosion; secondary steam pipes are lined with PTFE to prevent COD condensate from corroding carbon steel. 

2. Anti-clogging: The falling film plate surface is electropolished to Ra≤0.4μm, and a removable titanium filter (60 mesh) is installed at the liquid distribution port, with periodic online backflushing.

3. Vacuum: Three-stage Roots + liquid ring pump combination, ultimate vacuum -0.098MPa, design vacuum can be established within 5 minutes, pumping capacity 150kg/h.

4. Online cleaning: The CIP system is equipped with 5% HNO₃, 3% NaOH and scale inhibitor (PAA-Na, 25ppm), automatically starting every 20 days of operation or when the system pressure drop >50kPa, cleaning time ≤3h.

Energy Consumption and Economy: The unit operates for 8000h per year, evaporating 40,000t of water, saving 16,800t of steam (0.42→0t/t water), which, at 220 yuan/t, translates to annual savings of 3.696 million yuan; electricity savings of 2.8GWh; and water savings of 32,000 tons. Annual new sales revenue (industrial salt + miscellaneous salt + recycled water) reached RMB 12.6 million, with a profit of RMB 5.2 million.

Environmental Protection and Safety

1. Exhaust Gas: After secondary steam condensation, non-condensable gases (benzene series ≤2mg/m³) enter a two-stage resin adsorption + plasma pyrolysis process, with VOCs emission concentration <20mg/m³.

2. Mother Liquor: Miscellaneous salt filter cake amount ≤5% of feed rate, all sent to the cement kiln for co-processing; the system has no high-salt liquid discharge.

Automation

Adopting the Kang Jinghui automatic control system with 2,800 I/O points; key indicators such as evaporation intensity, crystal particle size, mother liquor density, and filter cake moisture content are optimized in real time through control algorithms, with compressor current fluctuation <±1.0%. Production, energy consumption, and CIP plans can be remotely viewed, achieving an "unmanned operation + minimally staffed inspection" mode, reducing the number of personnel per shift from 12 to 3.

Operational Results

1. Production Capacity: The actual evaporation capacity of the unit is 5.2 t/h, exceeding the design capacity by 4%; by-product industrial salt is 1.0 t/h, with an annual production capacity of 8,000 tons. NaCl purity is ≥99.1%, and whiteness is ≥85.

2. Mother Liquor: The high-pressure diaphragm filter press produces 1.8 t/batch (dry basis), with an annual output of 1,200 tons of mixed salts, all of which is sent to the cement kiln.

3. Effluent: MVR condensate has a COD ≤50 mg/L and a conductivity ≤40 μS/cm, all of which is recycled for primary brine, with an annual recycling volume of 40,000 tons.

4. Reliability: The operating rate is 97%, the planned cleaning cycle is 30 days, and the actual operating time is as long as 45 days; the equipment failure rate is 0.3 times/month, far lower than the industry average of 1.2 times/month.