Multiple-Effect Evaporator

The Multiple-Effect Evaporator (MEE) is an energy-saving concentration system that connects several single-effect evaporators in a series. It uses the secondary steam generated in the previous stage (pre-effect) to drive the next stage (sub-effect), allowing 1 kg of fresh steam to be reused multiple times, thus multiplying the evaporation capacity. This article provides a systematic overview from seven aspects: principle, process type, structural composition, performance indicators, application areas, advantages and disadvantages, and selection points.

I. Working Principle

1. Cascaded Thermal Energy: Live steam enters only the heating chamber of the first effect, where the solution is heated to generate secondary steam. Although the temperature and pressure of this secondary steam are slightly lower, it can still serve as a heat source for the next effect, and so on, stage by stage.

2. Vacuum Decrease: To ensure a temperature difference for heat transfer at each stage, the final effect is connected to a vacuum system, causing the operating pressure and boiling point of each effect to decrease sequentially.

3. Increasing Concentration: The solution concentration increases with each effect, ultimately reaching the target concentration in the nth effect; condensate and concentrate are continuously discharged separately. 4. Energy Saving Effect: Theoretically, n effects can amplify the evaporation capacity of 1 kg of live steam to n kg; in practice, due to heat loss and boiling point elevation, five effects consume approximately 0.3 tons of fresh steam to evaporate 1 t of water.

II. Process Flow (Feeding and Steam Flow)

1. Co-current Flow: The feed liquid and steam flow in the same direction. The pressure is high in the first effect, and the subsequent effects can flow in by gravity without a pump; however, as the concentration increases and the temperature decreases, the viscosity increases, and the heat transfer coefficient decreases.

2. Counter-current Flow: The feed liquid is pumped from the last effect to the first effect. The temperature is also highest at the highest concentration, which can reduce viscosity. This is suitable for materials whose viscosity increases rapidly with concentration; however, it requires inter-effect pumps, making control complex.

3. Parallel Flow: Each effect has separate feed and separate concentrate output. This is often used in situations where crystallization occurs during the evaporation process, preventing crystal blockage in the pipes.

4. Mixed Flow: A combination of the above methods, such as parallel flow in the first few effects and counter-flow in the later effects, balancing energy consumption and viscosity control.

III. System Structure and Key Components

1. Effect Unit: Each effect contains a heating chamber (tube/plate) and a separation chamber (flash tank), sometimes combined into one unit.

2. Condenser: Secondary steam from the last effect enters a surface or mixing condenser, where it is condensed using circulating water and a vacuum is maintained.

3. Vacuum Pump: Pumps non-condensable gases during startup/operation to ensure the vacuum level of the last effect.

4. Preheater: Utilizes the waste heat from the condensate of each effect to raise the feed temperature, reducing live steam consumption.

5. Forced Circulation Pump (Optional): For easily crystallizing or high-viscosity materials, maintains a tube-side flow rate of 2–4 m/s within the effect, preventing scaling and improving the heat transfer coefficient.

IV. Main Performance Indicators

1. Steam Economy: Evaporation rate per kg of water / kg of live steam: 1.8–2.0 for two effects, 2.5–2.8 for three effects, and approximately 4.5 for five effects. 2. Specific heat transfer area: Increases with the number of effects, resulting in a linear increase in investment; operating costs decrease while equipment costs increase, indicating an optimal number of effects. Industrial applications commonly use 2-3 effects, with a maximum of 6 effects.

3. Temperature difference loss: The effective temperature difference ΔT per effect is approximately 8-15℃; with a fixed total temperature difference, the more effects, the smaller the ΔT per effect, requiring a larger area.

4. Energy consumption comparison: Compared to single-effect evaporators, triple-effect evaporators can save over 60% of steam; compared to MVR (Mechanical Vapor Recompression), multi-effect evaporators have lower initial investment and lower requirements for steam quality, but higher long-term power consumption.

V. Typical application areas

1. Chemical industry: Concentration of caustic soda, soda ash, phosphates, and dye intermediates.

2. Food industry: Fruit juice, whey, sugar solutions, soy sauce, and maltodextrin.

3. Pharmaceutical industry: Concentration of traditional Chinese medicine extracts, antibiotic fermentation broths, and vitamin solutions. 4. Environmental Protection: Zero discharge of high-salinity wastewater, landfill leachate, and electroplating mother liquor; recovery of by-product salts such as NaCl and Na₂SO₄.

5. Seawater Desalination: Coupled with low-temperature multi-effect electrochemical treatment (LT-MEE) and cogeneration, resulting in high water production ratio and low scaling tendency.

6. Petrochemical and Metallurgical Applications: Pickling wastewater, catalyst mother liquor, and oil-water separation.

VI. Advantages and Limitations

Advantages

Significantly reduces live steam consumption; operating costs decrease with scale-up.

Low requirements for inlet steam pressure and electricity, suitable for plants with existing low-pressure steam.

Mature technology, high operational flexibility, capable of handling non-heat-sensitive, high-concentration, and crystalline materials.

Modular system; 2–6 effects can be freely combined, facilitating capacity expansion.

Limitations

Increasing the number of efficiency effects leads to an exponential increase in heat transfer area, steel consumption, and initial investment;

Materials with significant boiling point rise (BPR) (such as NaOH and sugar solutions) will rapidly erode the temperature difference, limiting the number of efficiency effects;

Circulating cooling water and vacuum pumps are required, and water and electricity consumption remain;

The temperature is still too high for heat-sensitive materials, requiring coupling with falling film, MVR, or TVR (thermal recompression).

VII. Selection and System Design Considerations

1. Material Characteristics: Boiling point rise, viscosity, crystallization habit, heat sensitivity, and corrosiveness determine the number of efficiency effects, process flow, and material (304/316L/2205/Ti).

2. Utilities: On-site steam pressure, cooling water temperature, and electricity price; if steam prices are high and electricity prices are low, MVR can be used; if steam is inexpensive, multi-efficiency systems are the most stable choice.

3. Operation Mode: Continuous operation has the lowest energy consumption; semi-continuous or cyclic batch operation can be used for intermittent or small-batch operations. 4. Scale Prevention and Cleaning: For sparingly soluble salts such as CaSO₄ and SiO₂, cleaning interfaces and online chemical dosing are provided; for crystallization efficiency, Oslo-type (OSLO) or DTB crystallizers are used.

5. Automation: DCS/PLC controls liquid level, pressure, density, and condensate conductivity to achieve automatic concentrate removal and automatic in-line cleaning (CIP).

6. Economic Trade-offs: Using "Annual Total Cost = Depreciation + Steam Cost + Water and Electricity Cost + Maintenance Cost" as the objective function, 2–4 effects typically reach a minimum value; for scales >10 t/h, three or more effects show significant advantages.

Conclusion: Multi-effect evaporators fully utilize heat energy through "multi-stage steam relay," making it one of the most mature and reliable large-scale concentration technologies currently available. With the surge in demand for zero-discharge of high-salinity wastewater, new lithium battery materials, and seawater desalination, multi-effect evaporators are being upgraded towards larger scale, high vacuum, low temperature difference, forced circulation, and MVR/TVR coupling, continuing to play a core role in the global industrial energy conservation and resource recycling field.