Multi-effect evaporators are efficient and energy-saving wastewater treatment technologies specifically designed for handling high-salinity, high-concentration, and refractory industrial wastewater (e.g., from chemical, pharmaceutical, and food processing industries, as well as landfill leachate). Their core working principle lies in the recycling of steam latent heat, which significantly reduces energy consumption.

Below is a detailed breakdown of their working principles and key elements:

Core Concept: Steam Recycling

In single-effect evaporators, heating steam (primary steam) transfers heat to wastewater, causing it to boil and evaporate. The generated steam (called secondary steam) is directly condensed and discharged, with a large amount of latent heat it carries remaining underutilized.

The essence of multi-effect evaporators: Connect multiple evaporators (called "effects") in series. The secondary steam produced by the previous effect is reused as the heating steam for the next effect. In this way, the heat from one initial supply of primary steam can be repeatedly utilized across multiple effects to evaporate water.

Working Process (Taking Forward Flow as an Example)

1. First Effect

Preheated raw wastewater enters the first-effect evaporator.

Externally supplied primary steam enters the heating chamber of the first effect (one side of a shell-and-tube heat exchanger or plate heat exchanger), condenses outside the tube or plate walls, and releases latent heat.

Heat is transferred through the tube/plate walls to the wastewater flowing inside the tubes or between the plates, causing it to boil and evaporate.

The generated steam is referred to as secondary steam.

The concentrated wastewater (with increased solids content) is pumped to the second effect.

2. Second and Subsequent Effects

Secondary steam from the first effect is introduced into the heating chamber of the second effect, serving as its heating steam source.

The operating pressure of the second effect is lower than that of the first effect (typically maintained by a vacuum pump or condenser), so its boiling point is also lower. This allows the higher-temperature secondary steam from the first effect to condense and release heat smoothly in the second effect’s heating chamber.

The latent heat released during condensation is transferred to the concentrated wastewater (from the first effect) entering the second effect, causing it to boil and evaporate again at a lower temperature, generating new secondary steam.

This process repeats across the series of connected effects (which can be 3-effect, 4-effect, 5-effect, or more).

The concentrated liquid from each effect flows sequentially to the next, with its concentration increasing progressively (in forward flow mode).

Secondary steam from each effect is reused as heating steam for the next effect (except for the last effect).

3. Last Effect

Secondary steam produced by the last effect is no longer used as heating steam (as there is no subsequent effect).

This secondary steam enters a condenser, where it is condensed into liquid (condensate) by cooling water.

The condenser is usually connected to a vacuum pump to maintain the required vacuum (low pressure) at the end of the system.

The relatively pure condensate from the condenser is typically collected for reuse or discharge.

Products

Concentrate/Mother Liquor: High-concentration waste liquid discharged from the last effect. It may reach a supersaturated state and requires further treatment (e.g., crystallization, drying) or disposal as hazardous waste. In some cases, partial concentrate may also be discharged from intermediate effects.

Condensate: A mixture of condensed water from the heating steam of each effect (highly pure) and condensed secondary steam from the last effect (relatively pure, possibly containing trace volatile substances). It generally has good water quality and can be reused in boilers or production processes.

Non-Condensable Gases (NCGs): Incondensable gases (e.g., air, CO₂, or other volatile organic compounds) extracted by the vacuum pump along with secondary steam in the condenser. These gases must be treated before discharge.

Key Elements & Technical Considerations

1. Pressure Gradient (Temperature Gradient)

This is the foundation of multi-effect evaporation operation. The system pressure decreases progressively from the first effect (highest pressure and temperature) to the last effect (lowest pressure and temperature), typically achieved through the condenser and vacuum pump at the last effect. The pressure gradient ensures that the higher-temperature secondary steam from the previous effect can effectively condense and release heat in the next effect (at a lower temperature).

2. Boiling Point Elevation

As the concentration of wastewater increases across each effect, its boiling point also rises. This means that under the same pressure, concentrated solutions are more difficult to evaporate than pure water— a factor that must be considered in the design.

3. Temperature Difference Loss

Including boiling point elevation, static head loss (higher pressure and boiling point at the bottom due to liquid column height), and pipeline resistance loss. The effective heat transfer temperature difference decreases progressively across effects, limiting the number of effects that can be infinitely increased.

4. Feeding Modes

Forward Flow: Raw liquid and steam flow in the same direction (from the first to the last effect). High efficiency is achieved when the raw liquid has a high temperature and low viscosity.

Countercurrent Flow: Raw liquid enters from the last effect (low temperature and pressure) and flows to the first effect (high temperature and pressure); steam flows from the first to the last effect. Suitable for solutions with viscosity that increases sharply with concentration.

Parallel Flow: Raw liquid is added to each effect in parallel, and concentrated liquid is discharged separately from each effect. Suitable for scenarios where crystals precipitate during evaporation.

Mixed Flow: Combines two or more of the above modes.

5. Energy-Saving Principle

Theoretically, the amount of steam consumed to evaporate 1kg of water (in kg) ≈ 1 / number of effects. For example:

• Single-effect evaporation: ≈ 1.0 - 1.1 kg steam / kg water
• Double-effect evaporation: ≈ 0.5 - 0.6 kg steam / kg water
• Triple-effect evaporation: ≈ 0.33 - 0.4 kg steam / kg water
• Quadruple-effect evaporation: ≈ 0.25 - 0.3 kg steam / kg water

6. Preheating

Waste heat from the condensate and concentrate discharged from each effect is often used to preheat the raw wastewater entering the system, further improving thermal efficiency.

Application Advantages in Wastewater Treatment

• High Efficiency & Energy Savings: Significantly reduces steam consumption and lowers operating costs.
• High-Concentration Evaporation: Greatly reduces wastewater volume (volume reduction rate can exceed 95% in some cases).
• Water Recovery: The produced condensate has good quality and can be reused.
• Handles Complex Wastewater: Suitable for high-salt, high-COD, heavy metal-containing, and organic-rich wastewater—especially when salt recovery or crystallization is required.
• Reduces Subsequent Disposal Costs: Dramatically decreases the volume of concentrate requiring final disposal (e.g., incineration, landfilling).

Summary

By connecting multiple evaporation units (effects) in series and carefully establishing a pressure gradient (temperature gradient), multi-effect wastewater evaporators reuse secondary steam from the previous effect as the heating source for the next. This design maximizes the recycling of steam latent heat, significantly reducing the fresh steam required to evaporate unit volume of water.

As a key technology for treating high-concentration and refractory industrial wastewater, its core lies in the "cascaded utilization of thermal energy" and "establishing an effective temperature/pressure difference to drive heat transfer."

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