Falling Film Multi-Effect Evaporators: A Game-Changer in Energy-Efficient Wastewater Treatment and Resource Recovery

The falling film multi-effect evaporator is rapidly emerging as a cornerstone technology for industries looking to maximize energy efficiency, reduce operational costs, and enhance resource recovery. Operating under negative pressure, this innovative system is designed to concentrate liquids, recover valuable salts, and treat high-salinity wastewater while significantly minimizing energy consumption. With its unique multi-effect design and forced circulation mechanism, this evaporator offers several advantages over traditional systems.

The principle of operation is based on the series connection of multiple evaporators. In this design, the secondary steam generated in one effect is used as the heating medium for the next, making efficient use of heat energy. The raw liquid is distributed evenly from the top of the equipment, forming a thin liquid film on the heating tubes. As the liquid flows downward, it rapidly evaporates due to the steam on the outside of the tubes. The secondary steam generated is then recovered and processed by a condenser or Mechanical Vapor Recompression (MVR) compressor, contributing to energy savings.

A key highlight of this evaporator is its outstanding energy-saving performance. With steam consumption per ton of water evaporated reduced to between 0.28–0.35 kg/kg, it consumes only about one-third of the energy required by single-effect evaporators. The inclusion of a preheating unit, liquid distribution device, and thermocompressor further enhances its efficiency, allowing for optimized heat exchange and additional energy savings.

Key Components and Features:

1. Preheating Unit: Utilizing low-temperature secondary steam from the last effect, this unit preheats the feed, reducing the heating load on the first effect and boosting overall thermal efficiency.

2. Liquid Distribution Device: Ensures uniform wetting of the heat exchange tubes, preventing dry spots that could lead to coking or inefficiencies.

3. Heat Exchange Tubes: Made of corrosion-resistant materials such as 316L stainless steel or titanium alloy, these tubes provide stable liquid film flow, maintaining high heat transfer efficiency.

4. Gas-Liquid Separation Chamber: Equipped with advanced cyclone or demister designs to ensure minimal entrained droplets, effectively reducing COD levels in the condensate.

5. Online Cleaning System: The three-stage cleaning process, including alkaline, acid washing, and passivation, ensures that the system stays clean with minimal downtime and reduced maintenance.

The multi-effect falling film evaporator is particularly suited for high-salinity wastewater treatment in industries like chemical production, pharmaceuticals, and food processing. It is highly effective in concentrating wastewater containing crystalline salts, such as NaCl, Na₂SO₄, and KCl, and can help achieve "zero discharge" by recovering and recycling salts. It also excels in handling heat-sensitive materials like vitamins and enzymes, where minimal temperature exposure preserves the active ingredients.

Key Applications Include:

• High-Salinity Wastewater Treatment: Ideal for treating wastewater with a salt content of 12-25%, including chemical, dye, and lithium industries, with excellent salt recovery rates.

• Food and Pharmaceutical Processing: Concentrates heat-sensitive substances such as milk, maltose, and Chinese herbal extracts, retaining over 90% of active ingredients.

• Seawater Desalination: Integral to Low-Temperature Multi-Effect Distillation (LTMED) systems for producing high-quality fresh water in water-scarce areas.

• Organic Solvent Recovery: Efficiently recovers low-boiling point solvents like ethanol and isopropanol with high recovery efficiency (>98%).

The economic benefits of adopting falling film multi-effect evaporators are compelling. For example, a plant processing 60 tons of high-salinity wastewater per day can save 21,780 tons of steam annually, reduce CO2 emissions by 6,500 tons, and recover up to 1,600 tons of industrial salts worth approximately 720,000 RMB. With a payback period of less than 1.5 years, these systems deliver significant long-term cost savings.

In conclusion, falling film multi-effect evaporators are an essential part of modern industrial processes, offering a combination of energy savings, resource recovery, and environmental benefits. They represent a key technology in the global shift towards sustainable, low-cost, and efficient industrial practices, making them a vital component in the push toward achieving national and global environmental goals.