Falling Film Multiple-Effect Evaporator

I. Basic Concepts and Operating Mechanism

The falling film multiple-effect evaporator is a highly efficient and energy-saving concentration device operating under negative pressure. Its core principle lies in the series connection of multiple evaporation units to achieve stepwise utilization of thermal energy. The system requires only a small amount of fresh steam as the initial heat source; the secondary steam generated in the previous stage is introduced into the next stage as a heating medium, thus achieving an energy-saving mode of "one-time steam supply, multiple uses." The raw liquid is evenly distributed from the top of the equipment through a distribution device, forming a liquid film with a thickness of 0.1–0.5 mm along the inner wall of the heating tubes under gravity.  The liquid flows from top to bottom while being heated by steam outside the tubes, rapidly completing the evaporation process. The secondary steam generated in the last effect is recovered and processed by a condenser or MVR compressor. This process can control the steam consumption per ton of water evaporated to between 0.28–0.35 kg/kg, with energy consumption only about one-third of that of a single-effect evaporator, demonstrating outstanding energy-saving effects.

II. Key Components

1. Preheating Unit: Adopts a horizontal shell-and-tube structure, utilizing the low-temperature secondary steam from the last effect of the system to preheat the feed, reducing the heating load of the first effect and improving overall thermal efficiency.

2. Liquid Distribution Device: As a critical part of the system operation, it usually adopts a trough-type flow guide combined with nozzles or a spiral flow guide cone design to ensure that all heat exchange tubes are fully wetted, with a wetting rate of not less than 120%, effectively preventing coking caused by dry walls.

3. Heating Component: The heat exchange tubes can be made of 316L stainless steel, 2205 duplex stainless steel, or titanium alloy, with a diameter range of 25–50 mm and a length of 3–8 m, maintaining a length-to-diameter ratio of 80–120 to ensure the stability of liquid film flow; the shell side is equipped with steam-liquid separation baffles to prevent condensate from impacting the tube bundle.

4. Gas-Liquid Separation Chamber: Adopts a tangential gas inlet cyclone structure or a high-efficiency wire mesh demister to ensure that the concentration of entrained droplets in the secondary steam is ≤100 mg/Nm³, effectively reducing the COD value of the condensate. 5. Heat Energy Enhancement Device: Equipped with a thermocompressor, it can pressurize part of the secondary steam and return it to the first effect, achieving an additional energy saving of 12–18%; if integrated with MVR technology, the unit power consumption can be controlled within ≤18 kWh/t water.

6. Online Cleaning System:  Features a three-stage program of alkaline washing, acid washing, and passivation, combined with sponge ball circulation or rotating spray heads, allowing cleaning at low temperatures of 40°C, reducing downtime and ensuring continuous production.

III. Outstanding Technical Features

1. High Heat Transfer Efficiency: The liquid film is in close contact with the heating surface, and the heat transfer coefficient can reach 3000–5000 W/(m²·K), an improvement of more than 30% compared to traditional rising film equipment.

2. Short Material Residence Time: The single-pass time is only 5–30 seconds, suitable for the concentration of heat-sensitive substances such as vitamins, enzymes, and fruit and vegetable juices, maximizing the retention of active ingredients.

3. Strong Foam Control Capability:  Adopting a co-current flow design and a bottom suction structure, it can promptly break down foam and reduce entrainment, eliminating the need for additional defoaming agents.

4. Excellent Energy Saving Performance: With a triple-effect configuration and a thermocompressor, the steam consumption ratio (Q/S) is ≤0.33; when combined with MVR, the overall energy consumption can be as low as ≤27 kg standard coal/t water.

5. Compact Spatial Layout: Using a vertical stacking or "parallel" frame structure, it saves up to 40% of installation area compared to traditional single-effect equipment, adapting to space-constrained application scenarios.

IV. Main Application Areas

1. Industrial High-Salt Wastewater Treatment: Suitable for wastewater with a salt content of 12–25% from chemical, pesticide, dye, and lithium battery material mother liquor industries, it can be concentrated to near saturation, followed by crystallization, achieving "zero discharge" of wastewater and resource recovery of salts (NaCl, Na₂SO₄, KCl purity ≥97%).

2. Food and Pharmaceutical Fields: Used for low-temperature concentration of materials such as milk, maltose, and traditional Chinese medicine extracts, with an active ingredient retention rate of over 90%. 3. Seawater Desalination Project: As the core equipment of the Low-Temperature Multi-Effect Distillation (LTMED) system, it has a water production ratio of ≥10 and produces fresh water with a TDS of ≤5 mg/L, suitable for water supply in water-scarce areas.

4. Organic Solvent Recovery: Suitable for the recovery of low-boiling point solvents such as ethanol, isopropanol, and DMAC. The operating vacuum is controlled at 50–150 mbar, and the recovery efficiency is >98%, achieving resource recycling and environmental protection.

V. Key Points of Selection and Process Design

1. Determination of the Number of Effects: When the steam price is 0.25 RMB/kg, a three-effect system offers optimal economic efficiency; if the steam cost is higher than 0.35 RMB/kg, a four-effect or MVR integrated solution should be considered first.

2. Process Optimization: For materials prone to scaling, it is recommended to use a "forward flow + forced circulation" final effect design, maintaining a flow rate of ≥2 m/s, controlling the crystal particle size to <0.2 mm, and extending the cleaning cycle to more than 45 days.

3. Material Selection Basis: Select materials reasonably based on the corrosiveness of the medium – 304 stainless steel can be used for chloride ion concentrations <500 mg/L; 316L for 500–5000 mg/L; and 2205, 2507 duplex stainless steel or titanium is recommended for >5000 mg/L or when containing strong corrosive elements such as fluorine and bromine.

4. Temperature Difference and Vacuum Control: The first effect temperature is controlled at 75–90℃, the last effect is maintained at 40–45℃, and the temperature difference between each effect does not exceed 15℃ to prevent local high temperatures from causing coking or equipment damage.

VI. Operating Procedures and Operating Specifications

1. Startup Preparation:

Ensure stable cooling water, instrument air source, and live steam pressure (0.5–0.7 MPa);

Open the top vacuum breaking valve and slowly feed the material until the liquid level in each effect reaches the midline of the sight glass to avoid dry walls;

Start the vacuum system, and introduce live steam only after the vacuum degree of the last effect reaches ≥0.085 MPa. 2. Operation Monitoring:

Maintain a dynamic balance between feed rate, evaporation rate, and discharge rate to ensure stable liquid levels in each effect;

Control the fluctuation of live steam pressure to within ±0.02 MPa to prevent unstable operation of the heat pump;

Check the conductivity of the condensate every 2 hours. If it exceeds 150 μS/cm, immediately investigate for leaks.

3. Shutdown Procedure:

First, cut off the steam supply - release the vacuum - stop feeding - circulate and rinse with 85℃ hot water for 30 minutes - drain the remaining liquid - perform CIP alkaline and acid cleaning procedures to ensure the system is clean.

VII. Maintenance and Troubleshooting

1. Cleaning Plan:

Mild scaling: Circulate and clean with 50℃, 2% NaOH solution for 2 hours;

Sulfate-based hard scale: First, perform complexing cleaning with 5% EDTA + 1% citric acid at 60℃, then perform passivation treatment with 1% HNO₃;

Cleaning completion standard: pH difference between inlet and outlet < 0.2, conductivity difference < 50 μS/cm.

2. Common Problems and Solutions:

Decreased evaporation capacity: Check if the liquid distributor is blocked; if the scale thickness of the heat exchange tubes is > 2mm, mechanical cleaning is required;

Vacuum degree not meeting standards: Confirm that the cooling water temperature is ≤ 32℃ and the vacuum pump sealing water temperature is < 25℃;

Abnormal condensate COD: This may be due to damage to the demister or excessively high liquid level in the separation chamber; shutdown for inspection or replacement is required.

3. Regular Preventive Measures:

Inspect the liquid distribution nozzles during annual overhaul; replace them if the wear is > 1mm;

Perform 10% sampling inspection of the heat exchange tube wall thickness; if the remaining thickness is < 2.0mm, overall replacement is recommended;

Calibrate the instrument control system every six months to ensure accurate response of vacuum, liquid level, conductivity, and other interlocking signals.

VIII. Economic and Environmental Benefits Assessment

Taking a project processing 60 tons of high-salt wastewater per day as an example, using a triple-effect falling film + forced circulation system instead of the original single-effect equipment, the annual steam savings are 1.1 × 60 × 330 = 21,780 tons, equivalent to 2,480 tons of standard coal, reducing carbon dioxide emissions by approximately 6,500 tons. Simultaneously, approximately 1,600 tons of industrial salt can be recovered annually, generating additional economic benefits of approximately 720,000 RMB, with a payback period of less than 1.5 years. This demonstrates excellent economic and environmental value.

IX. Summary and Outlook

The falling film multi-effect evaporator, with its technical advantages of "high-efficiency heat transfer, low-temperature operation, and multi-stage energy saving," has become the mainstream choice in the fields of high-salinity wastewater resource utilization, heat-sensitive material concentration, and solvent recovery. By firmly grasping the three core elements of "liquid distribution design, material suitability, and cleaning strategy" during equipment selection, system integration, and operation management, enterprises can achieve long-term, stable, and low-cost operation of the device. This technology not only enhances the sustainability of industrial production but also provides practical technical support for achieving the national "dual carbon" strategic goals, making it one of the important pieces of equipment for promoting green industrial transformation.