From water inlet to crystallization: A complete breakdown of the MVR evaporator process

MVR (Mechanical Vapor Recompression) evaporators are core equipment for "zero-discharge" treatment of industrial wastewater, and their process flow covers the entire process from raw water feeding to final crystallization. From an engineering practice perspective, the following systematic breakdown of the entire MVR evaporator process helps to understand its efficient, energy-saving, and resource-efficient treatment mechanism.

I. Influent and Pretreatment Stage

Raw Water Collection and Conditioning

Oil-containing wastewater, high-salinity wastewater, and other raw water first enter the conditioning tank to balance water quality and quantity, avoiding shock loads.

Multi-stage Pretreatment

• Physical Removal: Screening, grit removal, oil separation, etc., remove large suspended solids and oil droplets.

• Softening and Hardness Removal: Chemical reaction precipitation removes calcium and magnesium ions, reducing the risk of scaling.

• Filtration: Multi-media filtration, ultrafiltration/microfiltration, etc., further remove fine suspended solids and colloids.

• Advanced Oxidation or Adsorption: For recalcitrant organic matter, Fenton oxidation, ozone oxidation, or activated carbon adsorption can be selected.

• Resin Adsorption/Ion Exchange: Used to remove specific heavy metals or harmful ions.

• Purpose: To ensure the quality of influent water and reduce scaling, clogging, and corrosion problems in subsequent evaporators.

II. MVR Evaporation and Concentration Stage

Feeding and Preheating: Pretreated wastewater is pumped to the MVR evaporator. It undergoes multi-stage preheating in the preheater using system waste heat (such as condensate or secondary steam), reducing evaporation energy consumption.

Evaporator Main Process

• Heating and Evaporation: After preheating, the wastewater enters the evaporator heating chamber, where it exchanges heat with high-temperature steam through the heat exchange tube walls. The wastewater boils, the water evaporates, and secondary steam is generated.

• Vacuum-Liquid Separation: The evaporated vapor-liquid mixture enters the separator to separate the steam from the concentrate. The steam enters the compressor, while part of the concentrate is recycled and part enters the next cycle for further evaporation.

• Vacuum Recompression: The secondary steam is pressurized and heated by the compressor. After its enthalpy is increased, it is recycled as heating steam, significantly reducing external energy consumption.

• Forced Circulation/Falling Film Evaporation Synergy: For high-salt, high-concentration materials, a forced circulation pump is often used to maintain high-speed flow to prevent scaling and clogging. Falling film evaporation is used for heat-sensitive materials to ensure gentle evaporation.

Condensate Treatment

Steam condensation produces distilled water, which, after cooling, can be reused in production processes or discharged in compliance with standards. The condensate water quality is excellent, with COD and salinity levels typically far below discharge standards.

III. Deep Concentration and Crystallization Stage

Supersaturation and Crystal Nucleation

As evaporation and concentration occur, the salt content in the wastewater reaches supersaturation, and tiny crystals begin to precipitate spontaneously.

Crystal Growth and Control

Oplo and DTB crystallizers are used, and temperature, stirring speed, and residence time are controlled to ensure continuous crystal growth to a suitable particle size.

Some processes employ "clear-turbidity separation" or "staged crystallization" to achieve selective separation and high-quality recovery of different salts.

Solid-Liquid Separation

The crystallized slurry enters a centrifuge or filter to achieve efficient separation of crystals and mother liquor.

The mother liquor can be recycled back to the upstream stage for further concentration or returned to the evaporator for further treatment, while the crystals proceed to the next drying stage.

IV. Product Processing and Resource Recovery

Crystal Drying and Packaging

The separated crystals are dried in a dryer to remove surface moisture, yielding dried salt products, which are then packaged, stored, or sold.

Salt can be recovered as industrial-grade sodium chloride and sodium sulfate, turning waste into treasure.

Valuable Substance Recovery: Heavy metal-containing wastewater can recover metals such as nickel and chromium; organic-containing wastewater can recover organic solvents, improving economic efficiency.

Mother Liquor Reuse and End-of-Line Treatment: Mother liquor is recycled back to the evaporation system, minimizing residual liquid volume. Some difficult-to-treat residual liquids can be solidified, achieving true "zero discharge."

V. Automated Control and Online Maintenance: 

Full-process automation

Utilizing a PLC/DCS system, real-time monitoring and automatic adjustment of parameters such as temperature, pressure, liquid level, and flow rate are achieved, ensuring stable operation.

Online Cleaning (CIP) and Preventive Maintenance

Regular automatic cleaning of heat exchange surfaces, dynamic monitoring of scaling trends, and timely adjustment of operating parameters extend the continuous operating cycle of the equipment.

Data Management and Remote 

Operation and Maintenance: Supports data recording, analysis, and remote monitoring, enabling intelligent early warning and optimized operation.

VI. Typical Case Process Analysis

• Oilfield Produced Water Treatment

Pretreatment (oil separation, softening, filtration) → MVR evaporation and concentration → Salt crystallization → Centrifugal separation → Drying and salt recovery, distilled water reuse.

Zero Discharge of High-Salinity Electroplating WastewaterHeavy Metal Removal Pretreatment → MVR Evaporation → Staged Crystallization → Heavy Metal and Salt Recovery, Mother Liquor Reuse, 100% Wastewater Reuse.

• Pharmaceutical Organic Wastewater Treatment

Pretreatment (advanced oxidation, resin adsorption) → MVR low-temperature evaporation → Organic solvent recovery and salt crystallization, distilled water reuse.

VII. Key Points and Challenges of Process Optimization

Pretreatment is crucial, significantly reducing the risk of scaling and clogging.

Evaporation and crystallization processes need to be flexibly designed according to water quality, balancing energy saving and resource recovery.

Automation and intelligent operation and maintenance improve operational stability and economy.

To address challenges such as boiling point elevation and high corrosivity, forced circulation, corrosion-resistant materials, and multi-stage compression are employed.

In conclusion, the MVR evaporator process, encompassing influent pretreatment, evaporation and concentration, crystallization separation, and resource recovery, achieves efficient wastewater purification and resource utilization, representing a crucial technological pathway for industrial "zero emissions" and a circular economy. Scientific process design and intelligent operation and maintenance management will help enterprises save energy, reduce costs, and increase efficiency, promoting green, low-carbon, and sustainable development.