A sodium chloride MVR evaporator for a fine chemical industry with a processing capacity of 100T/d for epoxy resin

Project Background

This fine chemical company has an annual epoxy resin production capacity of 180,000 tons, generating approximately 100 tons/day of high-salt wastewater (equivalent to 16-20% NaCl). The raw wastewater has a CODcr of 30,000-45,000 mg/L, pH 11-12, and contains aged resin, epichlorohydrin, toluene, glycerol, and trace amounts of bisphenol A. Direct discharge would cause exceedances of characteristic pollutants. Traditional triple-effect evaporation consumes 0.42 tons of steam per ton of water, resulting in high operating costs. In 2023, the company decided to adopt MVR (Mechanical Vapor Recompression) evaporation crystallization technology to construct a demonstration unit for a "100T/day sodium chloride MVR evaporator for the epoxy resin industry," achieving sodium chloride resource recovery, condensate reuse, and an operating cost of ≤30 yuan/ton of water. It has been operating stably and continuously to date.

Process Route

1. Water Quality Reassessment

Laboratory tests showed that when NaCl concentration ≥ 24%, the boiling point of the solution increased by 9.5℃; when COD > 35000 mg/L, the evaporation process easily generates a large amount of foam and leads to liquid slugging in the compressor. Therefore, a three-stage process of "pretreatment + MVR concentration and crystallization + mother liquor incineration" was designed to ensure long-term system stability.

2. Overall Process Overview

High-salinity wastewater → Equalization tank (30m³, air agitation) → High-efficiency dissolved air flotation (DAF, oil removal rate ≥ 90%) → Ozone catalytic oxidation (O₃/H₂O₂, COD removal rate 40-50%, reduces foaming) → Bag filtration (100μm) → MVR evaporator → Oslo-type crystallizer → Two-stage piston pusher centrifuge → Fluidized bed drying (120℃, residence time 3s) → Ton bag packaging of industrial salt.

Secondary steam is heated to 18°C by a titanium compressor and then reused. Condensate, after RO desalination, has a conductivity ≤80μS/cm and is entirely reused as makeup water for the circulating cooling water system. Mother liquor is periodically discharged to a spray dryer and rotary kiln for incineration. Waste heat from the flue gas is used in a waste heat boiler to generate 0.6MPa low-pressure steam for front-end ozone oxidation and equipment CIP.

3. MVR Main Parameters: Rated capacity 100T/d (4.2m³/h), feed NaCl 18%, output solids content 38-40%. 

Evaporation chamber: φ2800×6000mm, effective volume 35m³, constructed of 2205 duplex steel + titanium alloy composite plate, design pressure -0.09MPa;

Heater: Plate type, single plate area 1.0m², 120 plates in total, total heat exchange area 120m², design temperature 95℃, heat transfer coefficient ≥4200W·m⁻²·K⁻¹;

Compressor: Single-stage centrifugal type, titanium alloy impeller, flow rate 6.5t/h, compression ratio 1.8, installed power 280kW, speed adjustment range 20-105%, isentropic efficiency ≥82%;

Circulation pump: Axial flow type, flow rate 3000m³/h, head 4m, frequency conversion control, ensuring crystal slurry flow velocity 3.2m/s, scaling cycle ≥30 days.

Equipment Selection and Materials

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

2. Anti-clogging: The falling film plate surface is electrolytically polished to Ra≤0.4μm, and a removable titanium filter (40 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 120kg/h.

4. Cleaning: The CIP system is equipped with 5% HNO₃, 3% NaOH and scale inhibitor (PAA-Na, 20ppm), automatically starting every 15 days of operation or when the system pressure drop >50kPa, cleaning time ≤3h.

Energy Consumption and Economy: The actual evaporation rate of the unit is 3.6 t/h, equivalent to a steam consumption of 0.035 t/t water (based on standard coal equivalent for electricity), which is 88% more energy-efficient than the original triple-effect evaporator. It saves 28,000 tons of steam and 21,000 tons of water annually, with a comprehensive operating cost of 28 yuan/ton of water, lower than the industry average of 45 yuan/ton of water. Annual additional sales revenue (industrial salt + recycled water) is 12.6 million yuan.

Environmental Protection and Safety:

1. Tail Gas: After secondary steam condensation, non-condensable gases (toluene ≤ 2 mg/m³, epichlorohydrin ≤ 1 mg/m³) enter a two-stage resin adsorption + plasma pyrolysis process, resulting in VOCs emission concentration < 20 mg/m³.

2. Mother Liquor: The discharged mother liquor is ≤ 3% of the feed volume. After spray drying and rotary kiln (1100℃) incineration, the flue gas undergoes SNCR denitrification + rapid cooling + bag filter dust collection + alkaline washing before meeting emission standards. The waste residue is melted to produce glass, achieving "zero" landfill. 

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

4. Safety: Evaporator top equipped with rupture disc + safety valve for double pressure relief; 24-point ammonia/epoxychloropropane leak detection in the unit area, interlocked to start spray and fan.

Automatic Control: Employs Siemens dual-redundant control with 1800 I/O points; key indicators such as evaporation intensity, crystal particle size, and mother liquor density are optimized in real time through APC control algorithm, compressor current fluctuation <±1.5%. A mobile APP allows remote viewing of output, energy consumption, and CIP plans, achieving an "unmanned operation + minimally staffed inspection" mode, reducing staffing from 8 people per shift to 2.

Operating Results:

1. Capacity: Actual processing capacity of the unit is 105T/d, exceeding design by 5%; by-product industrial salt is 1.9t/h, with an annual capacity of 6300 tons. NaCl purity ≥98.5%, whiteness ≥80, meeting the requirements for refined industrial salt, and has been stably sold to chlor-alkali plants. 

2. Effluent: MVR condensate COD ≤ 80 mg/L, conductivity ≤ 80 μS/cm, all recycled for circulating cooling water, annual recycling volume 33,000 tons.

3. Reliability: 97% uptime, planned cleaning cycle 30 days, actual operating time up to 48 days; equipment failure rate 0.3 times/month, far lower than the industry average of 1.2 times/month.