3t/h sodium chloride triple-effect evaporation crystallization system

Project Overview

A mid-scale chemical manufacturing facility in Liaoning Province, China, faced mounting pressure from stringent environmental regulations and rising wastewater disposal costs. The plant generated approximately 72 cubic meters per day of sodium chloride-rich wastewater from its chlor-alkali and dye intermediate production lines. The wastewater contained 8-12% NaCl, trace organics, and heavy metals, making direct discharge impossible and conventional treatment economically unviable.

In 2024, the facility commissioned a 3 tons per hour triple effect evaporative crystallization system designed specifically for sodium chloride recovery. This case study documents the system design, operational performance, and measurable outcomes that demonstrate why triple effect evaporative crystallization systems remain the preferred solution for chemical plant wastewater treatment in high-salinity applications.

Wastewater Characteristics and Treatment Challenges

The incoming wastewater presented three core challenges typical of chemical evaporative crystallization systems:

First, the high sodium chloride concentration (80,000-120,000 mg/L) required substantial energy input for water removal. Second, organic contaminants from dye intermediates created foaming and scaling risks during evaporation. Third, trace heavy metals including nickel and chromium demanded pretreatment to meet industrial salt purity standards.

The facility needed a solution that would achieve zero liquid discharge (ZLD), produce salable sodium chloride, and operate within constrained utility budgets. Single-effect evaporation would consume excessive steam; MVR technology, while efficient, carried higher capital investment and maintenance complexity for this mid-scale application. The triple effect evaporative crystallization system emerged as the optimal balance of capital efficiency, operational reliability, and energy performance.

System Design and Process Flow

The chloride sodium evaporative crystallization system employs a forward-feed triple effect configuration with dedicated crystallization capability.

Pretreatment Stage: Wastewater enters a reaction tank where caustic soda adjusts pH to 9.5-10.5, precipitating heavy metals as hydroxides. Organic sulfur precipitants target residual nickel and cadmium. Coagulation with PAC and PAM followed by lamella clarifier reduces suspended solids below 50 mg/L and heavy metals below 1 mg/L. Ozone oxidation degrades residual organics to prevent foaming in downstream evaporators.

Triple Effect Evaporation Stage: Clarified feed passes through a plate preheater recovering condensate sensible heat. The triple effect evaporative crystallization system operates with three falling-film/forced-circulation evaporators in series.

Effect One receives fresh steam at 0.3 MPa (133°C), concentrating feed from 10% to 18% NaCl. Vapor generated at 105°C becomes heating steam for Effect Two. Effect Two operates at 85°C, further concentrating brine to 24% NaCl. Its vapor at 80°C drives Effect Three at 65°C, achieving final concentration near saturation at 26-27% NaCl.

Each effect employs 2205 duplex stainless steel heat exchanger tubes for chloride resistance. Forced-circulation pumps maintain 2.5-3.5 m/s tube velocity, preventing crystal deposition and scaling. Inter-effect transfer occurs through pressure-driven flow, eliminating intermediate pumps.

Crystallization Stage: Concentrated brine from Effect Three enters an OSLO-type crystallizer operating at 50°C. Controlled cooling precipitates sodium chloride crystals with average particle size 0.4-0.6 mm. Centrifugal separation yields industrial salt with moisture below 3% and purity exceeding 98.5%, meeting GB/T 5462 Grade I standard.

Mother liquor partially returns to Effect Three maintaining system balance. Purged concentrated liquor undergoes drum drying for mixed salt disposal, representing less than 5% of total salt output.

Condensate Recovery: All effect condensates collect in a common receiver. Conductivity remains below 50 μS/cm, suitable for boiler makeup or process reuse. The facility achieves 95% water recovery, with only drum dryer exhaust and minor purges requiring treatment.

Energy Performance and Operating Economics

The triple effect evaporative crystallization system demonstrates compelling energy efficiency through multiple-pressure operation.

Steam consumption averages 0.38 tons per ton of water evaporated, compared to 1.1-1.2 tons for single-effect systems. Annual steam savings exceed 6,000 tons standard coal equivalent, reducing CO₂ emissions by 15,800 tons annually.

Electrical consumption totals 18 kWh per ton water, including circulation pumps, vacuum system, and centrifuge. The vacuum system employs two-stage steam ejectors with liquid-ring backup, maintaining final effect pressure at 8 kPa absolute.

Total operating cost, including steam, electricity, chemicals, and labor, amounts to 28 RMB per ton feed. With recovered sodium chloride sold at 280 RMB per ton and avoided wastewater discharge fees, the system achieves net positive operating cash flow. Capital payback period calculated at 2.3 years.

Operational Performance and Reliability

After 18 months continuous operation, the chemical evaporative crystallization system demonstrates consistent performance:

Evaporation capacity maintains 3.0-3.2 tons per hour against design 3.0 tons. Steam economy remains stable at 0.37-0.39 tons steam per ton water. Product salt purity averages 98.8%, with monthly heavy metal testing confirming compliance for industrial applications.

Cleaning cycles occur every 45 days using 5% citric acid circulation for 4 hours, restoring heat transfer coefficients to within 5% of baseline. Annual tube inspection reveals negligible corrosion on 2205 duplex steel, with wall thickness loss below 0.02 mm.

Downtime totals 168 hours annually for scheduled maintenance, yielding 98.1% availability. Automated PLC control with 4G remote monitoring enables off-site troubleshooting, reducing emergency callouts by 60%.

Environmental and Regulatory Compliance

The evaporative crystallization system delivers comprehensive environmental benefits. Wastewater discharge volume reduced from 72 m³/day to zero, eliminating regulatory non-compliance risk. Recovered 2,100 tons annually of industrial-grade sodium chloride replaces virgin salt purchases elsewhere in the facility's supply chain.

Condensate reuse reduces freshwater withdrawal by 23,000 m³ annually. Mixed salt residue, representing 3% of total solids, undergoes cement kiln co-processing rather than hazardous landfill, achieving full lifecycle waste valorization.

Third-party environmental audits confirm the facility now qualifies for local green manufacturing certification, unlocking tax incentives and preferential utility pricing.

Key Success Factors and Replication Guidance

Several design decisions proved critical for this chloride sodium evaporative crystallization system implementation:

Pretreatment intensity directly impacts evaporator performance. Insufficient organic removal causes persistent foaming, reducing vapor quality and compressor efficiency if MVR were employed. The triple effect configuration tolerates moderate foaming better than MVR due to larger vapor volumes and lower compression ratios.

Material selection for chloride service cannot be compromised. Initial consideration of 316L stainless steel was rejected due to pitting risk at chloride concentrations above 80,000 mg/L and temperatures exceeding 80°C. The 2205 duplex steel specification, while 25% higher capital cost, eliminates replacement risk and maintains heat transfer performance over the 15-year design life.

Crystallizer type selection depends on product specifications. OSLO crystallizers provide larger, more uniform crystals favorable for centrifugation and drying. For applications accepting smaller crystals, draft-tube-baffle (DTB) crystallizers offer lower capital cost and simpler operation.

Summary

This 3TPH triple effect evaporative crystallization system demonstrates that mid-scale chemical plants can achieve economic zero liquid discharge while generating salable byproducts. The technology balances capital efficiency against operating cost, delivering reliable performance without the maintenance complexity of mechanical vapor compression systems.

For facilities processing 50-150 m³/day of sodium chloride or similar inorganic salt wastewater, triple effect evaporative crystallization systems represent the proven, financeable pathway to regulatory compliance and resource recovery. The case facility continues to expand capacity, with a second identical line planned for 2026 to handle growing production volumes.

About Our Evaporative Crystallization Systems

CONQINPHI design and manufacture complete chemical evaporative crystallization systems including triple effect, MVR, and TVR configurations. Our sodium chloride evaporative crystallization systems serve chlor-alkali, dye, pharmaceutical, and food processing industries across 40 countries. Contact our engineering team for feasibility assessment and custom system design tailored to your wastewater characteristics and production goals.