Triple-effect tubular falling film evaporator

Working Principle

1. Three-Effect Series Process:

• First Effect: The feed liquid, preheated by the preheater, enters the top of the first-effect evaporator and is evenly distributed on the inner wall of the heating tubes by the liquid distributor, forming a liquid film. Heating steam (usually fresh steam) condenses and releases heat outside the tubes, causing the liquid film to evaporate and generate secondary steam. The concentrated liquid and secondary steam enter the gas-liquid separator. The separated concentrated liquid enters the second effect, where the secondary steam serves as the heat source.

• Second Effect: The concentrated liquid from the first effect enters the second-effect evaporator, utilizing the secondary steam generated in the first effect as the heating medium. Similarly, the liquid film evaporates, generating new secondary steam. The concentrated liquid enters the third effect, where the secondary steam serves as the heat source.

• Third Effect: The concentrated liquid from the second effect is further concentrated in the third effect. The heating steam for the third effect comes from the secondary steam from the second effect. Finally, the concentrated liquid is discharged from the third effect, and the secondary steam generated in the third effect enters the condenser for condensation. Non-condensable gases are discharged through the vacuum system.

2. Energy Cascade Utilization: 

The evaporation temperature and pressure decrease progressively in each effect (usually designed for vacuum operation), ensuring that steam flows naturally to the next effect as a heat source, forming a "temperature and pressure gradient."

The temperature difference between effects drives heat transfer, and the material concentration gradually increases during the progressive concentration process, ultimately achieving the desired concentration ratio.

3. Material and Steam Flow Direction:

Material flow direction: Usually co-current (in the same direction as the steam), i.e., flowing sequentially from the first effect to the third effect, with the concentration increasing progressively.

Steam flow direction: Counter-current (in the opposite direction to the material), i.e., fresh steam enters from the first effect, is utilized progressively up to the third effect, and finally condenses.

Core Structure and Components

1. Three-Effect Evaporation Unit:

Each effect contains an independent tubular falling film evaporator (heating tube bundle, liquid distributor, gas-liquid separator).

2. Preheater: Utilizes condensate or low-temperature steam from each effect to preheat the feed liquid, improving thermal efficiency.

3. Vacuum System: Maintains a low-pressure environment within each evaporator effect, lowering the boiling point of materials and adapting to heat-sensitive materials.

4. Control System: Integrates temperature, pressure, and liquid level sensors to automatically adjust parameters such as feed flow rate, steam pressure, and vacuum level for each effect.

5. Circulation Pump: Used for material transfer between effects, ensuring stable flow of the concentrate.

6. Condensation System: Steam from the last effect is condensed through a condenser (such as a shell-and-tube or plate condenser) to recover condensate or solvent.

Significant Advantages

1. Extreme Energy Saving: Through triple-effect evaporation, the steam generated in each effect is utilized by the next effect, significantly reducing fresh steam consumption. Compared to single-effect evaporators, energy savings can reach over 70%.

2. High Concentration Ratio: Multi-stage evaporation enables high-concentration material concentration, suitable for applications such as crystallized salt recovery and high-salt wastewater treatment.

3. Strong Applicability to Heat-Sensitive Materials: Falling film evaporation's short residence time (seconds) and low-temperature operation (boiling point lowered under vacuum) prevent material decomposition or deterioration. 

4. Stable Operation and Automation: The PLC system enables automatic parameter adjustment, such as temperature, liquid level, and pressure control for each effect, reducing manual intervention.

5. Environmental Benefits: Reduces steam and cooling water consumption, lowers carbon emissions, and meets green production requirements.

Application Areas

1. Chemical Industry: High-salinity wastewater concentration and zero discharge; organic solvent recovery (e.g., alcohols, acids).

2. Food and Beverage: High-concentration concentration of fruit juices, dairy products, and syrups, preserving flavor and nutrition.

3. Pharmaceutical and Bioengineering: Concentration of heat-sensitive intermediates; separation of antibiotic extracts.

4. Seawater Desalination and Resource Recovery: Seawater concentration for salt production, or recovery of valuable salts from wastewater.

5. New Energy Materials: Concentration of lithium battery electrolytes; separation of rare earth elements, etc.

Key Design and Operational Considerations

1. Temperature and Pressure Matching Between Effects: Accurate calculation of the evaporation temperature and pressure difference between each effect is required to ensure natural steam flow and avoid "evaporation to dryness" or insufficient heat transfer. 

2. Scaling Prevention and Cleaning: For materials prone to scaling, design wide-channel tube bundles or online cleaning systems for regular chemical or mechanical cleaning.

3. Material Flowability Control: Maintain a stable liquid film thickness within each effect using circulating pumps or liquid level control to prevent dry walls or liquid film rupture.

4. Vacuum System Maintenance: Ensure stable operation of the vacuum pump to prevent air leakage that could reduce evaporation efficiency.

Comparison with MVR Evaporators

1. Energy Source: Triple-effect evaporators rely on fresh steam for startup and consume only a small amount of steam during long-term operation; MVR evaporators rely entirely on electricity to drive the compressor and require no external steam.

2. Applicable Scenarios: Triple-effect evaporators are suitable for scenarios with low steam prices and ample supply; MVR evaporators are suitable for situations with high steam costs or limited steam availability.

3. Initial Investment and Energy Consumption: The initial investment for triple-effect evaporators is lower than that for MVR, but their long-term energy consumption is slightly higher.

Development Trends

1. Intelligent Upgrade: Integrate AI algorithms to optimize parameter control, achieving dynamic adjustment and fault warning.

2. Material Innovation: Utilizing corrosion-resistant alloys or ceramic coatings to handle highly corrosive materials.

3. Multi-Effect Coupling Technology: Combined with heat pumps, membrane separation, and other technologies, further improving energy utilization.

Summary: 

The triple-effect tubular falling film evaporator, through multi-effect series connection and falling film evaporation technology, achieves rapid energy saving, high concentration ratio, and stable operation. It is a core piece of equipment for processing large-scale, complex materials through evaporation and concentration. Its balance between energy saving and economy makes it irreplaceable in many industrial fields.