Types of Multi-Effect Evaporation Crystallization Processes

Multi-effect evaporation crystallization processes can be classified into several types based on different dimensions such as structure, process flow, and operation method. The following is a systematic classification:

I. Classification by Number of Effects

Based on the number of steam utilization times:

Double-effect evaporation crystallization: Steam is utilized twice, energy saving rate approximately 49%.

Triple-effect evaporation crystallization: Steam is utilized three times, energy saving rate approximately 68%, most widely used, balancing investment and energy consumption.

Quadruple/Five-Effect evaporation crystallization: Higher steam utilization rate, but significantly increased investment cost, suitable for large-scale salt production, etc.

II. Classification by Feed Flow (Core Classification)

1. Co-current (Parallel Flow) Feeding Flow

The solution and steam flow in the same direction (first effect → last effect), with gradually increasing concentration and decreasing temperature.

Advantages: The solution can flow automatically, the later effects have a self-evaporation effect, and the operation is simple.

Disadvantages: Concentration... Increased viscosity leads to a decrease in heat transfer coefficient.

Suitable for: Materials whose viscosity does not change much with concentration (e.g., NaCl solution).

2. Countercurrent feeding process: Solution and steam flow in opposite directions (last effect → first effect), concentration and temperature increase simultaneously.

Advantages: Stable viscosity, uniform heat transfer coefficient with similar values across effects.

Disadvantages: Requires pump delivery, small amount of secondary steam.

Suitable for: Materials whose viscosity changes greatly with concentration and temperature.

3. Parallel flow feeding process: Each effect has separate feed and discharge, steam passes through each effect sequentially.

Advantages: No need for inter-effect flow of solution, less prone to pipe blockage.

Suitable for: Materials that easily crystallize during evaporation.

4. Cross-flow (mixed flow) process: Secondary steam passes through each effect sequentially, with some liquid flowing between effects.

Features: High flexibility, suitable for complex liquids with crystal precipitation.

III. Classification by liquid distribution state

1. Thin-film evaporator crystallizer: The solution forms a liquid film on the heating surface, resulting in high evaporation efficiency: Falling film type: The solution forms a film from top to bottom, suitable for heat-sensitive, low-viscosity materials (such as juice and traditional Chinese medicine extracts). Rising film type: The solution forms a film from bottom to top, driven by steam, suitable for low-viscosity materials with large evaporation rates. Scraped type: A scraper forces film formation, suitable for high-viscosity materials prone to scaling.

2. Non-film evaporator crystallizer: The solution circulates in the pipeline: Forced circulation type (FC): A circulation pump drives the liquid at high speed (23 m/s) through the heating tubes, providing strong anti-scaling capabilities and is the standard configuration for mixed salt separation. Central circulation pipe type: Utilizes density difference for natural circulation; simple structure but lower efficiency.

IV. Classification by operating method:

1. Continuous evaporator crystallizer: Features: Continuous feeding and discharging, high production efficiency. Applications: Large-scale chemical production (such as salt production, zero wastewater discharge).

2. Intermittent Evaporation Crystallizer

Features: Batch operation, flexible control

Applications: Small batch, high-value products (e.g., pharmaceuticals)

V. Classification by Crystallizer Structure (Industrial Mainstream)

1. Forced Circulation (FC) Crystallizer

Structure: Separate crystallizer + circulation pump + heater

Principle: Supersaturation is generated in the heating tube and released in the crystallizer

Advantages: Strong anti-scaling ability, can handle large crystal particles

Applications: Coal chemical industry, environmental wastewater, sodium sulfate/sodium chloride separation

2. OSLO Crystallizer

Features: Clear liquid circulation, crystals do not participate in circulation, grow in a suspended bed

Advantages: Large product particles, high purity

Applications: Scenarios requiring large particle products

3. DTB Crystallizer

Features: With guide tube and baffle, internal circulation of crystal slurry

Advantages: Good mixing effect, narrow particle size distribution

Applications: General-purpose crystallizer

VI. Classification by Method of Obtaining Supersaturation

1. Evaporation Crystallizer

Achieves supersaturation by removing solvent, suitable for salts whose solubility changes little with temperature (e.g., NaCl).

Equipment Types: Boiler type (short tube vertical type), multi-effect evaporator group

2. Cooling Crystallizer

Achieves supersaturation by cooling, suitable for salts whose solubility changes greatly with temperature (e.g., Na₂SO₄).

Methods: Natural cooling or forced cooling

3. Vacuum Crystallizer

Cools down through adiabatic flash evaporation, combining evaporation and cooling effects.

Features: Low operating temperature, suitable for heat-sensitive materials

VII. Combined Process Types

1. Multi-effect + Salt Separation Crystallization

Flow: Multi-effect evaporation concentration → Nanofiltration salt separation → Separate crystallization

Results: Produces high-purity NaCl and Na₂SO₄, resource utilization rate >95%

2. TVR/Heat Pump Coupled Multi-Effect

Principle: The secondary steam from the final effect is compressed and reused using a steam jet pump or heat pump.

Effect: Water production ratio increases from 35 to 68, saving over 30% in energy.

In practical applications, triple-effect forced circulation evaporation crystallization is the mainstream choice in coal chemical industry, environmental protection, and other fields; co-current falling film evaporation is suitable for food and pharmaceutical industries; MVR technology continues to increase its penetration rate in areas with electricity price advantages.