A 2t/h triple-effect evaporator for the pharmaceutical industry to process sodium chloride.

I. Project Overview 

In the fields of fine chemicals, pesticide intermediates, and high-salinity wastewater treatment, the evaporation, concentration, and crystallization recovery of sodium chloride solution are crucial steps in achieving zero wastewater discharge and resource recycling. As one of the most common inorganic salts, the quality of sodium chloride's crystalline product directly impacts its downstream application value, while the energy consumption and stability of the evaporation system are related to the long-term operating costs of enterprises.

This project is a supporting facility for a fine chemical enterprise, designed to process 5 t/h of sodium chloride solution through evaporation, concentration, and continuous crystallization. The raw material comes from the high-salt mother liquor from the upstream neutralization reaction and washing processes. The system is required to have a continuous and stable production capacity of no less than 8000 hours per year, with the crystalline product meeting industrial-grade sodium chloride standards, and the condensate meeting the requirements for reuse in the production process.

II. Process Background and Challenges Analysis

2.1 Material Characteristics

Concentration Fluctuation: The sodium chloride concentration in the incoming solution is affected by the upstream production conditions, typically fluctuating between 5% and 15%. The system needs to have a wide feed adaptability.

Contains Trace Impurities: The solution may contain small amounts of organic matter, calcium and magnesium ions, and suspended solids, which can easily form scale on the heat exchange surface during long-term operation.

Salt Formation Tendency: Sodium chloride solubility changes relatively steadily with temperature, but in high concentration areas, it is prone to crystallization and precipitation in pipe dead zones and heat exchange surfaces, causing blockage.

Moderate Scale: A processing capacity of 5 t/h is considered a medium-scale evaporation unit, requiring a balance between equipment investment and operating energy consumption.

2.2 Technical Challenges

1. Scaling and Blockage: Trace amounts of calcium and magnesium ions and organic matter easily form a mixed scale layer at high temperatures, affecting heat transfer efficiency and even blocking pipelines;

2. Crystallization Particle Size Control: Industrial-grade sodium chloride has certain requirements for crystal particle size, and excessive fine crystals must be avoided to prevent difficulties in centrifugal separation and severe mother liquor entrainment;

3. Thermal Economy and Investment Balance: Compared with MVR systems, triple-effect evaporation has the advantages of moderate investment and stable operation at a scale of 5t/h, but further steam consumption needs to be reduced through optimized design;

4. Continuous Operation Reliability: Chemical production has high requirements for continuous system operation, and it is necessary to reduce the frequency of downtime caused by scaling and cleaning.

III. Solutions and Process Routes

To address the above requirements, our company adopts an integrated process solution of "triple-effect falling film evaporation + forced circulation concentration + continuous crystallization," with the core process as follows:

3.1 Pretreatment and Preheating: After suspended solids are removed by a precision filter, the incoming liquid enters a multi-stage preheating system. The preheater sequentially utilizes the waste heat from the condensate of each effect and secondary steam to progressively raise the temperature of the feed, maximizing the recovery of low-grade heat energy and reducing the external steam demand of the first-effect evaporation section.

3.2 Triple-Effect Falling Film Evaporation The preheated solution enters the triple-effect falling film evaporation system, where heat energy is utilized in stages:

First Effect: Heated with external saturated steam, operating temperature 85℃～95℃, the solution forms a uniform liquid film and evaporates rapidly within the falling film tubes;

Second Effect: Utilizes the secondary steam from the first effect as a heat source, operating temperature 68℃～78℃, for further evaporation and concentration;

Third Effect: Utilizes the secondary steam from the second effect as a heat source, operating temperature 48℃～58℃, for deep concentration under negative pressure.

The triple-effect series design significantly reduces fresh steam consumption and significantly improves heat energy utilization.

3.3 Forced Circulation Concentration and Continuous Crystallization The sodium chloride solution concentrated by the triple-effect evaporation enters a forced circulation evaporator for further concentration to a supersaturated state, and then enters a continuous crystallizer (OSLO type). By precisely controlling the crystallization temperature, residence time, and stirring rate, sodium chloride is uniformly nucleated and grown under stable supersaturation, forming crystals of suitable particle size.

3.4 Crystal Slurry Separation and Drying After thickening in a thickener, the crystal slurry undergoes solid-liquid separation via centrifuge, with the moisture content of the wet salt controlled at 3%–5%. The separated mother liquor is either returned to the system for recycling or sent to the mother liquor treatment unit, depending on its quality. The wet salt can be further dried in a fluidized bed dryer before packaging, or sold directly as a wet salt product.

3.5 Condensate Recovery and Vacuum System Condensate generated at each effect is collected after waste heat recovery in a flash tank, cooled, and desalinated before being reused in the production process. The system uses a water ring vacuum pump set to maintain triple-effect negative pressure, ensuring smooth flow of low-temperature evaporation and secondary steam.

IV. Key Equipment and Technical Parameters

1. Design Evaporation Capacity: 5t/h

2. Feed Source: High-salt mother liquor from fine chemical industry

3. Feed Sodium Chloride Concentration: 5%～15% (depending on upstream conditions)

4. Evaporation Process: Triple-effect falling film evaporation + forced circulation concentration

5. Crystallization Method: Continuous crystallization (OSLO type) + thickener + centrifugal separation

6. First Effect Temperature: 85℃～95℃

7. Second Effect Temperature: 68℃～78℃

8. Third Effect Temperature: 48℃～58℃

9. Main Body Material: 316L stainless steel/2205 duplex steel

10. Steam consumption per ton of water evaporated: Approximately 0.28～0.35t (fresh steam)

11. Designed annual operating time: ≥8000h

12. Load adjustment range: 70%～110%

13. Automation control: PLC+DCS full-process automatic monitoring, online detection of temperature/pressure/liquid level/density

V. Project Highlights

5.1 Triple-effect cascade heat utilization 

The temperature decreases progressively from the first to the third effect, ensuring full cascade utilization of heat energy. The fresh steam consumption per ton of water evaporated is controlled at 0.28～0.35 tons, saving approximately 65%～70% energy compared to single-effect evaporation. A good balance between investment cost and operating energy consumption is achieved at a processing scale of 5t/h.

5.2 Integrated Evaporation and Crystallization

The system organically integrates triple-effect evaporation, forced circulation concentration, and continuous crystallization processes. The entire process, from low-concentration feed to crystallized product discharge, is continuous, reducing intermediate storage tanks and transfer links, thus lowering heat loss and operational complexity.

5.3 Anti-Scaling Design

Falling film evaporator: Thin liquid film and short residence time reduce the risk of coking of organic matter in the high-temperature zone;

Forced circulation section: Utilizes a reasonable circulation velocity (≥1.5m/s) to create turbulence within the heating tubes, inhibiting the deposition of sodium chloride and other salts on the tube walls;

Online cleaning: The system has a pre-installed CIP cleaning interface, supporting periodic chemical cleaning and hydraulic flushing, extending the continuous operation cycle.

5.4 Controllable Crystallization Quality

The OSLO continuous crystallizer, combined with stable supersaturation control and crystal slurry circulation ratio adjustment, produces sodium chloride crystals with uniform particle size and a white appearance, meeting industrial-grade standards. It also features high centrifugal separation efficiency and minimal mother liquor entrainment.

5.5 Reliable and Durable Materials

Key components such as the evaporator heating tubes, separator, and circulating pump flow parts are made of 316L stainless steel or 2205 duplex steel, possessing excellent resistance to chloride ion corrosion. The main equipment is designed for a service life of no less than 10 years.

5.6 Automation and Water Conservation

The PLC+DCS control system enables automatic feeding, concentration control, automatic discharge, fault alarms, and interlock protection, and supports remote monitoring. Condensate is treated and reused in production, significantly improving the system's water recycling rate and reducing wastewater discharge.

VI. Operational Results

1. Evaporation Capacity: Stably reaches the design load of 5t/h, with timely adjustment response to fluctuations in the influent concentration.

2. Steam Consumption: Steam consumption per ton of water evaporated is controlled between 0.28 and 0.35t, which is at a superior level in the triple-effect evaporation industry.

3. Continuous Operation Cycle: With proper maintenance and regular cleaning, the single-cycle continuous operation time meets the continuous production requirements of chemical enterprises.

4. Crystallized Product Quality: Sodium chloride crystals have a concentrated particle size distribution, and their whiteness and purity meet industrial-grade standards, with good centrifugal separation effect.

5. Condensate Reuse: The conductivity and temperature of the condensate meet reuse standards and can be used as process makeup water or circulating cooling water.

6. Automation Level: The system operates stably with minimal manual intervention, a low failure rate, and convenient operation and maintenance.

The successful commissioning of the 5t/h sodium chloride triple-effect evaporation crystallization system demonstrates the economic efficiency and reliability of triple-effect evaporation technology in the resource utilization of medium-scale, high-salinity wastewater. Faced with challenges such as fluctuating feed concentration, scaling tendency, and crystallization quality requirements, our company provides customers with energy-efficient, stable evaporation crystallization solutions through reasonable thermal calculations, equipment selection, and process integration, helping chemical enterprises achieve multiple goals such as wastewater reduction, resource recovery, and cost reduction and efficiency improvement.