Detailed Explanation of MVR Evaporation Crystallization Process for Coking Wastewater

Coking wastewater has a complex composition, containing high concentrations of phenols, cyanides, ammonia nitrogen, organic matter, and salts, making it a typical example of difficult-to-treat industrial wastewater. With increasingly stringent environmental standards and "zero discharge" requirements, MVR (mechanical vapor recompression) evaporation crystallization technology has become an important solution for the deep treatment and resource recovery of coking wastewater.

I. Process Principle 

The core of the MVR evaporation crystallization process is the closed-loop recycling of steam energy:

1. Initial Evaporation: Pretreated coking wastewater enters the evaporator and boils at a low temperature (typically 60-90℃) under vacuum, generating secondary steam.

2. Steam Compression Upgrade: The secondary steam is compressed by a mechanical compressor (such as a centrifugal compressor or Roots compressor), increasing its pressure and temperature, transforming it into high-grade steam.

3. Heat Recycling: The high-temperature, high-pressure steam is returned to the evaporator as a heat source to heat the wastewater, and then condenses itself into high-purity condensate. The wastewater continues to evaporate and concentrate, achieving efficient energy recycling.

4. Crystallization Separation: The concentrated liquid enters the crystallizer, where temperature and supersaturation are controlled to promote the crystallization of salts (such as sodium chloride and sodium sulfate). After centrifugation and drying, industrial salt products are obtained.

II. Typical Process Flow

1. Pretreatment Stage: Adjusting pH, removing suspended solids, oils, and easily scale-forming ions (such as calcium and magnesium softening), and performing advanced oxidation when necessary to improve the evaporability and crystallization quality of the wastewater.

2. MVR Evaporation and Concentration: Wastewater enters a falling film/forced circulation evaporator, where a compressor enhances the secondary steam heat energy, achieving low-temperature evaporation with an evaporation rate exceeding 80%, significantly reducing wastewater volume.

3. Crystallization and Separation: The concentrated liquid enters a crystallizer, where stepwise crystallization or freeze crystallization is used based on salt characteristics to separate sodium chloride, sodium sulfate, and other crystals.

   Centrifuges and dryers dehydrate and dry the crystals, yielding marketable industrial salt.

4. Condensate and Mother Liquor Treatment: Evaporation condensate can be reused or further treated with membrane technology before reuse.

   Mother liquor can be further concentrated or treated as miscellaneous salts, achieving near-zero wastewater discharge.

5. Miscellaneous Salts and Resource Recovery: Miscellaneous salts can be further processed or sorted for recycling, reducing hazardous waste and creating economic benefits.

III. Technical Features and Advantages: 

1. Extreme Energy Saving and Low Operating Costs

   Energy consumption is only 1/3 to 1/2 of traditional multi-effect evaporation, with electricity costs of approximately 8-12 yuan per ton of water treated, achieving an energy saving rate of 60%-80%.

2. Low-Temperature Operation, High Adaptability

   Vacuum low-temperature operation reduces the decomposition of heat-sensitive substances and equipment corrosion and scaling, suitable for high-salt and high-organic wastewater.

3. High-Efficiency Resource Recovery

   Salt separation and crystallization technology can recover industrial salts such as sodium chloride and sodium sulfate, reducing the volume of miscellaneous salts and hazardous waste, achieving resource utilization.

4. High Automation and Stability

   Intelligent control system enables unattended operation and precise control, ensuring long-term stable operation.

5. Small Footprint and Flexible Installation

   High equipment integration, occupying only 1/3 to 1/2 of the area of traditional processes, suitable for companies with limited space.

IV. Application Scenarios and Cases

1. Zero Discharge of Wastewater in the Coking Industry: Achieves wastewater reduction, salt crystallization, and water resource reuse. A typical example is a coking group in Shanxi, which recovers tens of thousands of tons of industrial salt annually, saving hundreds of thousands of cubic meters of water.

2. High-Salt, High-COD Wastewater Treatment: Adaptable to complex wastewater with COD of 2000~10000ppm and salt content of 3.5%~25%.

3. Combined application with membrane technology and advanced oxidation: Achieve end-to-end treatment with a permeate reuse rate exceeding 95%.

V. Technical Challenges and Countermeasures

1. Complex salt content and scaling risks

   Optimize pretreatment, combining fractional crystallization or freeze crystallization to reduce scaling and improve crystal quality.

2. High COD and viscosity impacts

   Control the concentration ratio, rationally segment the treatment, and, if necessary, discharge part of the mother liquor or use mixed salt evaporation.

3. Compressor performance and domestic production

   Optimize high-efficiency compressors, break through high-speed centrifugal compressor technology, and improve equipment efficiency and lifespan.

4. Balancing economics and investment

   Large-scale production and domestic substitution reduce equipment costs and increase application adoption among SMEs.

VI. Future Development Trends

1. Technology integration: Combined with membrane separation, advanced oxidation, etc., to achieve a complete "membrane concentration + MVR evaporation + fractional crystallization" solution.

2. Intelligent upgrade: Introduce AI algorithms and IoT to achieve remote monitoring and intelligent control, improving energy efficiency and stability.

3. Green and low-carbon development: Reduce energy consumption and greenhouse gas emissions, contributing to the "dual-carbon" strategy and green factory construction.

4. Policy-Driven: Stricter environmental policies have led to MVR evaporation crystallization becoming the standard for wastewater treatment in energy-intensive industries.

VII. Conclusion 

The MVR evaporation crystallization process for coking wastewater, with its core advantages of high efficiency, energy saving, resource recovery, and near-zero emissions, has become a key technology for wastewater treatment and green transformation in the coking industry. With process optimization, intelligent upgrades, and policy support, its application will become more widespread, helping enterprises achieve a win-win situation of environmental compliance and economic benefits, and promoting the sustainable development of the industry.