Multiple-Effect of Evaporation is a mature and efficient wastewater treatment and concentration technology widely used in the industrial sector. By linking the operation of multiple evaporators, it maximizes heat recovery and utilization. This article will delve into the principles, features, and equipment classifications of multi-effect evaporation to help you better understand this technology.
Multiple-effect evaporation consists of a system composed of multiple single-effect evaporators. The system introduces secondary steam generated by the preceding effect evaporator into the next effect evaporator, achieving simultaneous heating and condensation into distilled water. This design allows the multiple-effect evaporation unit to utilize heat energy in a stepped manner, significantly improving the thermal efficiency of the evaporator. Multiple-effect evaporation relies mainly on phase change heat transfer, featuring high heat transfer coefficients and large evaporation intensity.
The steam consumption of multiple-effect evaporation is related to the number of stages, and its features include low power consumption and high concentration ratios. Additionally, multiple-effect evaporation has relatively high operational flexibility, making it adaptable to different industrial needs and material characteristics.
Table 1 Relationship Between Steam Consumption and Number of Effects in Multi-Effect Evaporation kg/kg
Number of Effects | I Effect | II Effect | III Effect | IV Effect | V Effect |
Steam Consumption | 1.1 | 0.57 | 0.40 | 0.30 | 0.27 |
The process flow of multiple-effect evaporation can be mainly divided into concurrent, countercurrent, mixed flow, and parallel flow methods.
In concurrent flow processes, the solution and vapor flow in the same direction, moving sequentially from the first effect to the last effect. The raw material is pumped into the first effect and flows into the next effect by relying on the pressure difference between the effects, with the finished liquid being pumped out from the last effect. The liquid flows between the effects by the pressure difference, avoiding the need to set up too many pumps.Concurrent flow processes are generally suitable for handling heat-sensitive materials at high concentrations.
In countercurrent flow processes, the raw material is pumped from the last effect to each preceding effect, with the finished liquid being discharged from the first effect, and the liquid flows counter to the direction of the vapor. Countercurrent flow processes are generally suitable for handling solutions with viscosity that varies greatly with temperature and concentration and are not easy to handle for heat-sensitive materials.
Mixed flow processes combine concurrent and countercurrent flow processes, leveraging the advantages of both while avoiding their disadvantages. However, they are more complex to operate and require higher levels of automation.
Parallel flow processes have lower thermal efficiency compared to concurrent flow but are suitable for the evaporation, concentration, and crystallization of saturated solutions prone to crystallization. Its characteristic is that raw materials are fed into each effect in the evaporator, and each effect discharges the finished liquid. Each effect will have crystals precipitating out, allowing prompt crystal separation.
Multiple-effect evaporation equipment comes in a variety of types to meet different material and concentration needs. Based on the orientation of the evaporation tubes, equipment can be categorized into vertical tube evaporators (VTE) and horizontal tube evaporators (HTE). Additionally, based on the flow type, they can be divided into forced circulation evaporators, rising film evaporators, and falling film evaporators.
Submerged tube evaporators are also a common type of multiple-effect evaporation equipment, known for their simple operation but more severe scaling issues, suitable for applications with no more than six effects. Vertical tube evaporators are noted for their high heat transfer efficiency, suitable for handling low-concentration solutions, and are commonly used in seawater desalination. Horizontal tube thin film evaporators, on the other hand, are compact in structure, effectively reducing heat loss, and are particularly suitable for low-grade heat energy utilization.
When selecting multiple-effect evaporation equipment, enterprises need to consider material characteristics, processing volume, and operating costs to ensure they choose the most suitable equipment and number of effects.
Multiple-effect evaporation technology, due to its efficiency, energy saving, and flexibility, has become an important treatment method in the field of industrial wastewater treatment and resource recovery. By understanding multiple-effect evaporation principles, features, and equipment types, enterprises can make more comprehensive considerations when selecting suitable evaporation equipment. Whether in improving processing efficiency or reducing operating costs, multiple-effect evaporation demonstrates its unique advantages. With the continuous advancement of technology in the future, multiple-effect evaporation will play an important role in more industries.