5t/h sodium chloride triple-effect falling film evaporator

I. Project Overview

In industries such as chemical, pharmaceutical, and environmental protection, the evaporation and concentration of sodium chloride-containing wastewater is a crucial step in achieving wastewater reduction and resource recovery. Triple-effect evaporation technology, with its mature reliability, moderate investment, and stable operation, has a broad application base in medium-scale treatment scenarios.

This project is a supporting facility for a fine chemical enterprise, designed to process 5 t/h of sodium chloride solution through evaporation and concentration. The raw material comes from high-salt mother liquor from the upstream synthesis reaction and washing processes, with a sodium chloride concentration of approximately 6%–10%. The system is required to operate continuously and stably, with an annual effective operating time of no less than 7200 hours. The concentrated liquid after evaporation should meet the feed requirements of the subsequent crystallization process, and the condensate should be recycled back into the production system.

II. Process Background and Technical Characteristics

2.1 Technical Positioning of Triple-Effect Evaporation

The core principle of multi-effect evaporation is to connect multiple evaporators in series, using the secondary steam generated in the previous effect as the heating heat source for the next effect, achieving cascade utilization of thermal energy. For evaporation units with a capacity of 5 t/h, triple-effect evaporation has the following technical characteristics in engineering practice:

Steam Economy: The theoretical value is approximately 1/3 that of single-effect evaporation. In actual operation, due to factors such as boiling point elevation, heat loss, and condensate flash recovery, the steam consumption per ton of water evaporated is typically in the range of 0.28–0.38 tons.

Investment and Operating Cost Balance: Compared to MVR systems, triple-effect evaporation does not require a steam compressor, resulting in lower initial investment and simpler maintenance. It is suitable for operating conditions with ample steam supply and high electricity costs.

Operating Flexibility: By adjusting the feed distribution, live steam volume, and vacuum level of each effect, stable operation can be achieved within a load range of 70%–110%.

Technological Maturity: Triple-effect evaporation technology has undergone decades of industrial validation, with a high degree of equipment standardization, rich operating experience, and controllable risks.

2.2 Evaporation Characteristics of Sodium Chloride Solution

The solubility of sodium chloride in water changes relatively slowly with temperature (approximately 35.7 g/100 g water at 0℃ and approximately 39.8 g/100 g water at 100℃). However, the following issues still need to be considered during the evaporation and concentration process:

* **Boiling Point Elevation:** The boiling point of sodium chloride solution increases significantly with increasing concentration, reaching 5℃～8℃ in the high-concentration range, directly affecting the heat transfer temperature difference and evaporation efficiency of each effect.

* **Scale Formation Tendency:** Trace amounts of calcium and magnesium ions and suspended solids easily form scale layers during high-temperature concentration, reducing the heat transfer coefficient.

* **Corrosivity:** High-temperature, high-concentration sodium chloride solution is highly corrosive to carbon steel; therefore, the equipment material must be stainless steel or a higher-grade corrosion-resistant material.

III. Process Design and Technical Solution

3.1 Process Flow

This project adopts a co-current triple-effect falling film evaporation process:

* **Co-current Feed:** The solution enters from the first effect and flows sequentially through the second and third effects using the pressure difference between each effect. No intermediate transfer pump is required, simplifying the process and reducing energy consumption.

Preheating System: The feed material is preheated in multiple stages using condensate from the third-effect reactor, condensate from the second-effect reactor, and a portion of the secondary steam from the first-effect reactor, gradually increasing the temperature to near the boiling point of the first-effect reactor to maximize the recovery of low-grade heat energy.

Condensate Recovery: Condensate from each effect is collected after recovering low-pressure steam in a flash tank, cooled, desalinated, and reused in the production process.

3.2 Process Parameters for Each Effect Parameter 

| First Effect | Second Effect | Third Effect Heating Steam Source | External Live Steam (0.3～0.5MPa saturated steam) | Secondary Steam from First Effect | Secondary Steam from Second Effect Operating Pressure | Atmospheric or Slight Positive Pressure | Slight Negative Pressure (approx. -0.02～-0.04MPa) | Negative Pressure (approx. -0.06～-0.08MPa) Operating Temperature | 100℃～110℃ | 78℃～88℃ | 58℃～68℃ Evaporation Rate Distribution | Approx. 40% (2.0t/h) | Approx. 35% (1.75t) /h) | Approximately 25% (1.25t/h)

3.3 Key Equipment Configuration

Falling Film Evaporator (Single Effect): Vertical shell and tube, heat exchange area approximately 80-100m², 316L stainless steel

Falling Film Evaporator (Second Effect): Vertical shell and tube, heat exchange area approximately 70-90m², 316L stainless steel

Falling Film Evaporator (Third Effect): Vertical shell and tube, heat exchange area approximately 60-80m², 316L stainless steel

Gas-Liquid Separator: Matched to each effect, high-efficiency demister structure to prevent secondary steam entrainment

Preheater: Shell and tube type, utilizing condensate... Waste heat from water is used for staged preheating of the feed.

Vacuum system: Water ring vacuum pump maintains triple-effect negative pressure.

Condensate recovery system: Flash tank + collection tank recovers low-pressure steam and condensate.

IV. Main Technical Parameters

1. Design evaporation capacity: 5t/h

2. Feed sodium chloride concentration: 6%～10%

3. Discharge concentration: 20%～25% (or adjusted according to subsequent crystallization requirements)

4. Evaporation process: Co-current triple-effect falling film evaporation

5. Live steam pressure: 0.3～0.5MPa (saturated steam)

6. Steam consumption per ton of water evaporated: 0.2 8～0.35t

7. Cooling water consumption: Approximately 80～120t/h (for vacuum pump and final-effect condenser)

8. Main body material: 316L stainless steel

9. Design annual operating time: ≥7200h

10. Load adjustment range: 70%～110%

11. Automation control: PLC control, online monitoring and alarm for temperature/pressure/liquid level

V. Design Considerations and Engineering Considerations

5.1 Engineering treatment for boiling point elevation

 The boiling point of sodium chloride solution gradually increases during the concentration process in each effect, leading to a decrease in the effective heat transfer temperature difference. The following compensation measures are adopted in the design:

* Use higher-pressure live steam (0.4～0.5MPa) in the first effect to increase the total heat transfer temperature difference;

* Optimize the evaporation area distribution of each effect, with the area of the first effect appropriately increased by 10%～15% compared to the second and third effects;

* Maintain a higher vacuum level in the third effect (-0.07～-0.08MPa) to lower the boiling point of the final effect and ensure the evaporation efficiency of the final effect.

5.2 Scale Prevention and Cleaning 

The liquid film distribution within the falling film evaporator is uniform, with the flow velocity controlled at 0.5–1.0 m/s to prevent localized dry walls and scale buildup. The discharge pipe and separator bottom are designed with a 1%–2% slope to prevent sodium chloride crystal deposition and blockage. The system has a pre-installed CIP online cleaning interface, supporting periodic water washing or dilute acid chemical cleaning to remove trace amounts of scale.

5.3 Material and Corrosion Control 

All flow-through components are made of 316L stainless steel, possessing excellent resistance to chloride ion corrosion. The heat exchange tube wall thickness is designed according to corrosion allowance (typically 1.5–2.0 mm allowance). The main equipment is designed for a service life of no less than 10 years.

5.4 Heat Loss Control 

The evaporator shell, pipes, and valves are covered with an insulation layer made of rock wool or aluminum silicate, with a thickness of 80–120 mm. The insulation layer is further protected with aluminum foil or color steel plate, with a surface temperature not exceeding 50℃. This reduces heat loss and improves the overall thermal efficiency of the system.

VI. Operational Results Since its commissioning, the system has operated stably and reliably. Key indicators are as follows:

|Indicator|Actual Operating Data|Remarks|

|Evaporation Capacity|5.0t/h|Achieved design load|

|Steam Consumption|0.30～0.35t/t Evaporated Water|Conforms to design value|

|Continuous Operating Cycle|Meets production plan requirements|Good condition under regular maintenance|

|Condensate Temperature|≤45℃|Meets reuse requirements|

|Condensate Conductivity|≤200μS/cm|Meets process water replenishment standards|

|Concentrate Concentration|20%～25%|Meets subsequent crystallization feed requirements|

|Equipment Corrosion Status|No significant corrosion|316L material operating well|

|Automation Rate|≥95%|Minimum manual intervention, convenient operation and maintenance|

Triple-effect evaporation technology, as a mature and reliable evaporation and concentration method, offers significant advantages at the 5t/h processing capacity level, including moderate investment, stable operation, and easy maintenance. This project, through rational process design, precise thermal calculations, and reliable equipment selection, achieved efficient concentration and low-energy operation of sodium chloride solution, providing a valuable engineering practice for similar applications. Our company, with its solid technical expertise and engineering implementation experience, provides customers with cost-effective evaporation solutions that meet their specific needs.