MVR evaporators utilize mechanical recompression technology to compress and heat secondary steam for reuse, resulting in energy efficiency...
Forced circulation triple-effect evaporators are a top-tier technology for handling materials with high salt content, high hardness, easy scaling, and easy crystallization. When your materials have complex characteristics and conventional evaporators cannot operate stably, forced circulation is almost the only reliable option. Its value lies not in its highest energy efficiency, but in its ability to achieve long-term, stable, and continuous operation under extreme conditions, ensuring the smooth operation of the entire production or environmental protection process.
1. Core Design Concept and Working Principle
The design concept of the forced circulation evaporator is "boiling suppression and scale prevention, powerful circulation." Its fundamental difference from falling film and natural circulation evaporators lies in its use of a high-flow-rate, high-head axial-flow circulation pump to provide a powerful driving force for the material flow within the system.
2. Co-current Forced Circulation Triple-Effect Evaporator
Forced Circulation Establishment: Under the powerful drive of the circulation pump, the material flows through the heating chamber (shell and tube type) at a high velocity of 3-5 m/s.
Suppressing Boiling Inside the Tubes: The system is carefully designed with operating pressure and temperature so that the material is heated only to near its boiling point throughout its flow through the heating chamber, but never boils. This is the essence of its scale prevention design. Because boiling is a key step in the precipitation of crystal nuclei and their adhesion to the tube wall to form a scale layer, suppressing boiling prevents scale formation inside the heating tubes at its source.
Flash Evaporation: When the superheated liquid enters the evaporation separation chamber, the pressure drops sharply, causing a violent flash evaporation and rapid vaporization of water.
Vapor-Liquid Separation: In the separation chamber, secondary steam rises and is purified and separated by a demister. The concentrated liquid and most of the unevaporated liquid are drawn back into the circulation pump and enter the next cycle.
Multi-Effect Series Connection and Discharge: The crystal slurry reaching the predetermined concentration is partially drawn from the first-effect circulation pipeline and enters the second and third effects for further concentration and co-current flow, or directly enters the subsequent crystallizer and centrifugal separation system. The heat energy utilization between effects is the same as in conventional multi-effect evaporation, i.e., the secondary steam from the previous effect serves as the heat source for the subsequent effect.
3. Typical Application Areas
Forced circulation evaporators are one of the ultimate solutions for treating complex wastewater and products, widely used in:
Chemical Industry: Concentration and crystallization of easily crystallizing materials such as NaCl, Na₂SO₄, CaCl₂, and NaOH.
Environmentally Friendly "Zero-Emission" ZLD: Ultimate volume reduction treatment for extremely difficult-to-treat high-salt, high-hardness wastewater such as coal chemical wastewater, desulfurization wastewater, pharmaceutical wastewater, and RO concentrate; it is the core equipment of the ZLD system.
Metallurgical Industry: Evaporation of seed mother liquor in alumina plants.
Lithium Extraction from Salt Lakes: Concentration and crystallization process of lithium salt mother liquor.
4. Limitations and Selection Considerations
High Energy Consumption: The power consumption of the large-flow circulation pump is its main operating cost, resulting in relatively low energy efficiency.
High Investment Cost: High requirements for pump and heat exchanger materials and manufacturing processes, resulting in a large initial investment.
Not applicable to heat-sensitive materials: The material circulates many times in the system and has a long total residence time, which may not be applicable to materials that are extremely prone to thermal decomposition. Although it has the characteristics of high flow rate and low temperature, it still needs to be evaluated.
Technical Features and Advantages
Excellent Scale Prevention and Anti-fouling Capacity: This is its core advantage. The dual mechanism of high-flow-rate scouring and thermodynamic suppression of boiling within the tubes enables it to handle high-concentration, easily crystallizing, and easily scaling materials, such as CaSO₄, CaCO₃, and SiO₂.
Strong Adaptability and Stable Operation: Capable of handling high-viscosity, high-solids-content materials. The system operates under forced circulation, with intense convection, making it less prone to clogging. It has a wide operating window and is highly stable and reliable in operation.
Stable Heat Transfer Coefficient: Because the inner wall of the heating tube is constantly scoured by high-speed flowing liquid, the cleanliness coefficient is high, maintaining high heat transfer efficiency over a long period. Unlike other evaporators, heat transfer efficiency does not decrease sharply over time due to scaling.
Direct Production of Crystal Slurry: Ideal for use as an evaporative crystallizer, it can directly produce high-concentration solid-liquid mixed crystal slurry, facilitating subsequent solid-liquid separation processes.
Key System Components and Design Considerations
Axial Flow Circulating Pump: The "heart" of the system.
Characteristics: Low head, high flow rate, corrosion resistance, wear resistance, and temperature resistance.
Energy Consumption: This is the main energy consumer of the system, with considerable electricity consumption. This is a necessary price to pay for achieving scale prevention capabilities.
Heating Chamber: Typically a vertical shell-and-tube heat exchanger. The design must consider wear and corrosion under high flow rates. High-grade stainless steel, duplex steel, titanium, or even Hastelloy are commonly used for the tubing.
Evaporation Separation Chamber: The space needs to be large enough to reduce the flow rate and ensure that secondary steam can be fully separated from the liquid, avoiding mist entrainment.
Crystallization Slurry Treatment System: Includes crystallization slurry pumps, thickeners, centrifuges, etc., used to treat high-solids content materials discharged from the system.
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