Multi-effect evaporation technology

Multi-effect evaporation technology, as a highly efficient energy-saving technology, utilizes the secondary steam generated in each effect as the heat source for the next effect, achieving efficient energy recovery and utilization. This technology is not only widely used in the chemical, pharmaceutical, and food industries, but it is also highly regarded for its significant energy-saving effects. 
The basic principle of multi-effect evaporation technology lies in the series operation of multiple evaporators, forming a unique energy recovery system. In this system, the secondary steam generated by the previous evaporator is fully utilized as the heat source for the next evaporator. This design allows for efficient energy recovery and utilization during evaporation, significantly reducing energy consumption. 

Therefore, the greatest advantage of multi-effect evaporation technology is its ability to repeatedly utilize the vaporization and condensation processes of secondary steam, thus significantly reducing the consumption of fresh steam. 
Diversity of MED Evaporator Types

MED evaporators can be classified in various ways based on different criteria. First, based on steam pressure, they can be divided into three categories: atmospheric evaporation, pressure evaporation, and vacuum evaporation. Second, according to the type of evaporator, they can be classified as tubular evaporators, plate evaporators, and tube-plate combined evaporators. In addition, based on the number of effects, these evaporators can be classified as double-effect, triple-effect, quadruple-effect, quintuple-effect, and sextuple-effect evaporators. Finally, according to the flow direction of the material, they are further divided into types such as co-current, counter-current, mixed-flow, and cross-flow. 
When faced with a diverse selection of MED evaporators, how should we make comparisons? The following three principles may offer some guidance: 
1. Counter-current and mixed-flow systems are superior to co-current systems 
Counter-current multi-effect evaporation performs best in terms of energy consumption, while co-current multi-effect evaporation has the highest energy consumption. In comparison, the mixed-flow multi-effect evaporation system shows better performance. 
2. The number of evaporation effects is not always better 
As the number of evaporation effects increases, the efficiency of heat utilization gradually decreases. At the same time, an increase in the number of effects will also lead to increased equipment investment costs. Therefore, choosing an appropriate number of evaporation effects is crucial in practical applications. For example, for high-boiling point systems, usually only double-effect or triple-effect evaporators are needed to meet the requirements. 
3. The selection of evaporation pressure should comprehensively consider material properties, heat balance, and the degree of non-condensable gas retention 
Studies have shown that the pressure of each effect in the evaporator is not only affected by the material and heat balance but is also closely related to the unique properties of the material and the degree of throttling of non-condensable gases between the effects. 
Advantages and Disadvantages Analysis of MED Technology

MED technology, or multi-effect evaporation technology, has many advantages. First, its pretreatment process is relatively simple, requiring only the addition of a small amount of scale inhibitor to reduce the consumption of chemical agents. Second, this technology adopts a bilateral heat transfer method combining in-tube condensation and outside-tube boiling, resulting in a small heat transfer area and a high heat transfer coefficient, thus shortening the material's heating time. In addition, MED technology also has excellent operational flexibility, capable of providing product water at 40%~110% of the design value, far exceeding the operational flexibility of multi-stage flash and reverse osmosis. In terms of processing effect, MED technology also performs well, able to completely precipitate salts and remove more than 90% of salts through the cooling process, effectively inhibiting microbial growth. Finally, this technology has high operational reliability, with fully automated operation ensuring product quality and safety.

However, MED technology also has some disadvantages. Scaling is easy to occur in the tubes, requiring regular cleaning, which may cause some inconvenience in operation. At the same time, as the number of effects increases, the steam utilization rate gradually decreases, which to some extent affects the production capacity of the equipment. 
Common Problems and Countermeasures for Multi-effect Evaporation MED Technology

Multi-effect evaporation MED technology often faces three major technical problems during application. The first is the foaming phenomenon in the device, which may affect the evaporation efficiency. The second is the scaling problem of the evaporator, as salts easily accumulate in the tubes, requiring regular cleaning to maintain efficiency. The last is the corrosion of the equipment by the salt-containing ions in the final effect steam, which is also a problem that needs to be addressed. Corresponding countermeasures need to be taken to address these problems to ensure the stable operation and long-term benefits of MED technology. 
1. Solutions for Foaming Problems in the Device

Various methods can be used to address the foaming phenomenon in the device. Physical defoaming methods include high-temperature defoaming, low-temperature defoaming, ultrasonic defoaming, liquid spray defoaming, and mechanical vibration methods. These methods are effective in dealing with large amounts of foam, but the equipment and operating costs are relatively high. On the other hand, chemical defoaming mainly relies on the use of defoamers, but this method may be limited by the high price of defoamers, production costs, and the complexity of the production process. Currently, mechanical defoaming is receiving widespread attention. It achieves defoaming by changing the pressure and shear force acting on the bubbles through rotation, with the advantages of low cost and good defoaming effect. 
2. Solutions for Evaporator Scaling Problems

To address the scaling problem of evaporators, researchers have proposed an innovative solution. They first acid-washed the scale samples on the outer wall of the evaporator, followed by neutral cleaning, effectively removing scaling substances such as sodium sulfate and calcium carbonate. At the same time, acid washing was also used to treat the calcium carbonate scale samples on the inner wall of the final effect heat exchanger. Through plate analysis, it was found that the average corrosion rate of each effect plate was less than 1 g/m²·h, and the total corrosion amount was less than 10 g/m². This result is significantly better than the requirements stipulated in "Industrial Equipment Chemical Cleaning Quality Standard" (HG/T2387-2007) and "Preparation, Cleaning, and Evaluation Standard of Corrosion Samples". 
3. Addressing the Corrosion Problem of Equipment by Salt-Containing Ions in the Final Effect Steam

To address the corrosion challenge of equipment by salt-containing ions in the final effect steam, a series of measures can be taken. One of them is to use low-chloride ion content condensate water for low-temperature, timed, and quantitative replacement and replenishment to reduce the corrosion effect on the equipment. At the same time, adding highly effective corrosion inhibitors to the circulating water is also an effective means of enhancing the corrosion resistance of the equipment. 
The core principle of Mechanical Vapor Recompression (MVR) technology lies in utilizing the secondary steam generated by the evaporation system itself and its inherent energy. By using a compressor to mechanically work on low-grade steam, it is upgraded to high-grade steam as a heat source, continuously providing the necessary thermal energy for the evaporation system. This technology effectively reduces dependence on external energy sources, achieving the goal of energy saving and emission reduction. 

In a Mechanical Vapor Recompression (MVR) system, the steam generator provides the heat source during the preheating phase until the material begins to evaporate and produce steam. Subsequently, this secondary steam generated by the material heating is compressed by the compressor, converting it into high-temperature, high-pressure steam. This high-temperature, high-pressure steam is then used as a heating source, causing the material in the evaporation chamber to continuously evaporate. At the same time, the high-temperature, high-pressure steam passing through the compressor gradually cools through the heat exchange process, eventually becoming condensate, i.e., treated water. Throughout the process, the compressor plays a key role in converting electrical energy into thermal energy, enabling the entire system to eliminate its dependence on external live steam. 
Equipment Composition of the MVR Evaporation System

The MVR evaporation system achieves an efficient steam recompression process through the clever series connection of various equipment. To ensure optimal performance of the entire system, these devices must achieve a delicate match in terms of thermodynamics and heat transfer. The main components of the system include the following four key devices: 
1. Compressor

In the MVR evaporation system, the choice of compressor is crucial, as it directly affects the system's performance and stability. Common types of MVR compressors include Roots blowers and centrifugal compressors. Roots blowers are suitable for compressing small volumes of steam; their characteristics are small air volume but large temperature rise, especially suitable for materials with small evaporation capacity and large boiling point elevation. Centrifugal compressors provide small pressure difference but large air flow, with small temperature rise and uniform exhaust, no airflow pulsation, more suitable for materials with large evaporation capacity and small boiling point elevation. Although the stability of centrifugal compressors is usually superior to that of Roots blowers, care must be taken to prevent surging during operation to ensure the stable operation of the compressor. 
2. Evaporator

Evaporation treatment devices are generally divided into two types: rising film evaporation and falling film evaporation. The choice of method mainly depends on the characteristics of the material being processed and energy consumption considerations. Currently, in China, the falling film evaporation method is widely used. 
3. Heat Exchanger

In the MVR heat pump evaporation process, the plate heat exchanger is a commonly used device. This heat exchanger design allows for indirect heat exchange between hot and cold fluids through a partition wall without direct contact. Common types of plate heat exchangers include tube-in-tube, corrugated, and spiral types, which are widely used in production. 
4. Gas-Liquid Separator

The gas-liquid separator plays a crucial role in the MVR heat pump evaporation process. It is specifically designed for the separation of materials and secondary steam. Its core function is to agglomerate the solution in the mist into droplets and effectively separate these droplets from the secondary steam. When designing a gas-liquid separator, multiple key factors must be considered comprehensively, including evaporation capacity, evaporation temperature, material viscosity, and separator liquid level, to ensure its efficient and stable separation function. 
1. Compared with traditional evaporation systems, the MVR system has unique advantages. During startup, it only requires live steam as a heat source. Once secondary steam is generated, the system can operate stably without additional heat sources. Therefore, its energy consumption is mainly concentrated in the compressor and various pumps, making it extremely energy-efficient.

2. The energy consumption of the MVR evaporator system is mainly the compressor power consumption, which significantly reduces operating costs and maintenance costs. Since the system does not rely on industrial steam, the safety risk is lower and the operation is simple and easy to understand.

3. Under the same evaporation processing capacity, the floor space required for the MVR evaporator is much smaller than that of traditional multiple-effect evaporation equipment. This feature has a significant advantage in saving space resources. 
Application and Challenges of MVR Technology in the Treatment of High-Salt Wastewater 
MVR technology, with its excellent energy-saving effect and simple operation method, has been widely used in the field of high-salt wastewater treatment. However, in actual operation, some technical problems are still encountered, which pose certain challenges to the stable operation and treatment effect of the MVR system. 
1. System scaling problem 
In MVR technology, scaling on the heat exchanger wall is a problem that cannot be ignored. Since the heating heat source mainly uses secondary steam, scaling and coking will significantly reduce the heat transfer effect, thus affecting the evaporation capacity per unit time. This in turn reduces the amount of compressible secondary steam available, significantly impacting overall production capacity. In addition, due to the special structure of the MVR evaporator, the equipment is inconvenient to clean, which is also an important factor affecting the stability of production capacity. 
2. Temperature rise problem 
In the MVR system, the temperature rise problem has a significant impact on the application of high-salt wastewater treatment. Due to the significant increase in the boiling point of high-concentration saline wastewater, the steam compressor needs to provide a higher temperature to meet this challenge, which undoubtedly places more stringent requirements on the compressor and also leads to a significant increase in system energy consumption. Through research, we found that the suitable temperature rise range for MVR evaporation technology should be controlled between 8℃ and 20℃. Once the boiling point elevation exceeds 18℃, the energy-saving advantages of MVR technology will be weakened. 
3. Material properties matching problem for MVR selection

Due to the wide range of sources of industrial wastewater, when selecting an MVR system, it is necessary to carefully match it according to the physical properties of different materials. These property analyses include: the component composition of the material, whether crystallization may occur during evaporation, and key parameters such as viscosity, specific heat, density, and boiling point elevation of the material. For single materials, these parameters can be obtained by consulting relevant tables; however, for complex mixed liquids such as industrial high-salt wastewater, the relevant data often needs to be obtained through simulation and estimation. Therefore, accurate and detailed analysis and calculation of material properties are crucial to ensuring the stable operation of the MVR device.

In practical applications, for materials with a significant increase in boiling point temperature, it is usually recommended to use MVR single-effect evaporation technology; for high-concentration materials, forced circulation is needed to prevent coking due to slow flow rate; at the same time, for heat-sensitive materials, the residence time in the evaporator should be shortened as much as possible.

In summary, although evaporation technology is widely used in industry, its high energy consumption, high operating costs, and problems such as scaling and blockage cannot be ignored. Therefore, when considering efficient and energy-saving solutions, multiple-effect evaporation (MED) and mechanical vapor recompression evaporation (MVR) technology have become the preferred choices. Among them, although MVR evaporation equipment has a larger one-time investment, its energy consumption is lower, and with the continuous progress of domestically produced steam compressor technology, its price is also gradually decreasing; while the investment of multiple-effect evaporation equipment increases with the increase in the number of effects, its energy consumption can also be reduced within a certain range. Therefore, when choosing, it is necessary to comprehensively consider the applicability, investment cost, operating efficiency, energy consumption, labor cost, and floor space based on the actual situation. 
In the industrial field, the selection and application of MVR technology is a complex decision-making process that requires consideration of multiple factors. From the fine matching of material properties to the reasonable selection of evaporation technology, every link is crucial. By thoroughly understanding and comparing the advantages and disadvantages of various evaporation technologies, we can find the most suitable solution for different industrial wastewater treatment scenarios. At the same time, with the continuous advancement of technology and the gradual reduction of costs, the future application prospects of MVR evaporation equipment will be even broader.