How to select MVR evaporation process for different materials—boiling point elevation and viscosity are key.

MVR evaporation process selection is a core aspect of evaporation system design, directly impacting equipment efficiency, energy consumption, and operational stability. Given the diverse properties of materials, boiling point elevation and viscosity are two key parameters determining the MVR process selection. The following section provides a detailed explanation, covering principle analysis, selection strategies, and practical case studies, to help engineers make informed decisions.

I. Impact of Boiling Point Elevation and Process Selection

1. Mechanism of Boiling Point Elevation: 

When materials contain high levels of salt or solute concentration, the boiling point of the solution under evaporation operating pressure is significantly higher than that of pure water, a phenomenon known as "boiling point elevation."

Boiling point elevation leads to a reduction in the effective temperature difference, affecting heat transfer efficiency and placing higher demands on the temperature rise capability of the MVR compressor.

2. Process Selection Strategies

For materials with a small boiling point elevation (<5℃): Conventional MVR film evaporators (rising/falling film) or plate evaporators can be used, offering low energy consumption and simple equipment.

For materials with a large boiling point elevation (5-15℃ and above): MVR forced circulation evaporation process is recommended. High flow rates reduce scaling and improve the heat transfer coefficient; if necessary, dual-effect or multi-effect MVR coupling can be used to alleviate temperature difference pressure.

For materials with extremely high boiling point elevation/easily crystallizing materials: MVR forced circulation + crystallizer should be used, with enhanced circulation pumps to prevent clogging, and a high-head compressor design to ensure evaporation efficiency.

3. Equipment and Operation Compatibility Considerations

Compressor selection should fully consider the temperature rise required due to increased boiling point to avoid insufficient pressure ratio.

Appropriately set operating pressure and evaporation temperature to optimize heat transfer temperature difference and reduce energy consumption.

II. Viscosity Influence and Process Selection

1. Viscosity Influence Analysis

High material viscosity results in poor flowability, easily causing material buildup and scaling on the heat transfer surface, reducing heat transfer efficiency and increasing pumping resistance.

As evaporation and concentration occur, viscosity continues to rise, posing challenges to evaporator type and circulation method.

2. Process Selection Strategy

Low viscosity materials (<500 cP):

Suitable for MVR film evaporation (rising/falling film). The material forms a uniform thin film on the heating surface, resulting in short heating time and high efficiency.

Medium viscosity materials (500-2000 cP):

Recommended: MVR plate evaporator or forced circulation evaporator. The plate structure is easy to clean, the flow channels are less prone to clogging, and the circulation pump ensures high-speed material flow, preventing localized concentration and scaling.

High-viscosity/fouling-prone materials (>2000 cP): 

Utilize MVR forced circulation evaporation, or even combine it with a scraped-film evaporator, to continuously scrape off material from the heating surface, preventing coking and clogging.

3. Equipment and Operation Compatibility Considerations: 

High-viscosity materials require a high-flow-rate, high-head circulation pump to ensure system flow and heat transfer.

Evaporator design should minimize dead zones and incorporate self-cleaning functionality for easy online cleaning and maintenance.

III. Case Studies of Material Type and Process Compatibility:

1. Low-boiling-point elevation, low-viscosity materials—such as pharmaceutical and food solutions: Recommended MVR falling film/plate evaporation process, energy-efficient and high-performance, ensuring product quality.

2. High-boiling-point elevation, medium-low viscosity materials—such as inorganic salt wastewater: Recommended MVR forced circulation evaporation, effectively addressing boiling point elevation and preventing crystallization clogging.

3. High-viscosity, high-boiling-point elevation materials—such as high-concentration organic wastewater and chemical mother liquor: Utilize a combined MVR forced circulation + scraped-film evaporation process, balancing the complex challenges of high viscosity and boiling point elevation.

IV. Comprehensive Selection Process and Recommendations

1. Material Characteristic Testing: Accurately measure the boiling point elevation and viscosity change curves at the initial and final concentration points to obtain key design parameters.

2. Process Scheme Comparison: Compare multiple schemes technically and economically, considering boiling point elevation, viscosity, heat sensitivity, and crystallization tendency.

3. Equipment and System Optimization: Optimize the configuration of core components such as compressors, circulating pumps, heaters, and separators based on key parameters.

4. Operation and Maintenance Design: Consider online cleaning, anti-clogging measures, and automatic control to improve system adaptability and stability.

V. Common Misconceptions and Warnings

• Ignoring the impact of boiling point elevation on compressor selection leads to insufficient temperature rise and low evaporation efficiency.

• Underestimating the impact of viscosity changes on flow and heat transfer results in undersized compressors, causing frequent scaling and downtime.

• Selecting compressors solely based on initial material characteristics without considering the dynamic changes in concentration, viscosity, and boiling point during evaporation.

VI. Conclusion

Selecting the appropriate MVR evaporation process for different materials requires a scientific analysis of the material characteristics, focusing on two core parameters: boiling point elevation and viscosity, and a rational matching of evaporator type and system configuration. Only by comprehensively considering the physicochemical properties of the material, the dynamic changes in the evaporation process, and economics can the efficient, energy-saving, and stable operation of the MVR system be ensured. For practical projects, it is recommended to combine laboratory-scale tests with professional simulations to provide reliable data support for process selection.