Comparison of Evaporation Principles and Processes
Evaporation principle and core mechanism
Evaporation is a unit operation that uses thermal energy to vaporize the solvent (usually water) in a solution, thereby increasing the concentration of the solute in the solution. Its physical essence is a liquid-gas phase transition process. In industrial evaporation systems, an external heat source (steam or hot water) transfers heat to the feed liquid. When the feed liquid reaches its boiling point, water molecules gain enough kinetic energy to overcome the binding force of the liquid surface and escape from the liquid phase as vapor. As water continues to evaporate, the solute concentration gradually increases until crystals precipitate.
The core performance indicators of industrial evaporation systems include: ① Evaporation intensity (kg/m²·h), reflecting the evaporation efficiency per unit heat transfer area; ② Steam economy (kg water evaporation/kg steam consumption), reflecting energy consumption level; ③ Non-condensable gas emissions, which are related to scaling and corrosion control. Multi-effect evaporation achieves cascaded steam utilization by introducing secondary steam from the preceding effect into the subsequent effect as a heat source. Typically, a three-effect system can achieve a steam economy of 2.0~2.5. Mechanical thermal compression (MVR) technology uses a compressor to pressurize and reuse low-pressure secondary steam, achieving a COP of 10~20, significantly reducing energy consumption per ton of water. Furthermore, different flow modes such as forced circulation, falling film, and rising film have a significant impact on heat transfer coefficients, scaling tendency, and applicable concentration range. Process selection must comprehensively consider factors such as feed liquid properties, concentration ratio, scaling tendency, and operating costs.
Comparative Analysis of Mainstream Evaporation Processes
Currently, there are five main evaporation processes commonly used in industrial wastewater treatment, each with its own applicable scenarios and technical limitations:
| Process type | Working principle | advantage | shortcoming | Applicable Scenarios |
| Multi-effect evaporation (MED/MEE) |
The secondary steam from the first stage is used as the heat source for the subsequent stage, and multiple stages are connected in series to utilize the heat of the steam. | The technology is mature and the investment is relatively low; steam is economical. | It requires external steam supply, resulting in higher energy consumption than MVR; the system is complex and occupies a large area. | Preliminary concentration of high-salinity wastewater and pretreatment for seawater desalination |
| Mechanical thermocompression evaporation (MVR) |
The compressor pressurizes the secondary steam and returns it to the evaporator chamber as a heat source. | Extremely low energy consumption (30~80 kWh per ton of water); no external steam required; stable operation. | High investment in compressors; unsuitable for extremely high salinity or highly corrosive liquids. | RO concentrate and low-to-medium salinity wastewater evaporation and concentration |
| Thermal compression evaporation (TVR) |
The jet-type hot compressor ejects and pressurizes the secondary steam. | Simple structure, no moving parts, low maintenance cost | A stable high-pressure steam source is required; steam economy is lower than MVR. | Chemical and mining applications with inexpensive steam sources |
| Forced Circulation Evaporation (FC) |
The circulating pump forces the liquid feed through the heating tube at high speed, and evaporation is completed in the flash chamber. | Suitable for high-viscosity, easily crystallizing liquids; less prone to scaling and pipe clogging. | The circulating pump consumes a lot of electricity; the heat transfer temperature difference requirement is large. | High-salt crystallization, mine water evaporation crystallization end |
| Falling film evaporation (FF) |
The liquid forms a thin film on the inner wall of the pipe, resulting in high heat transfer efficiency. | High heat transfer coefficient, small temperature difference; short residence time, suitable for heat-sensitive materials. | It requires high uniformity of liquid distribution; it is not suitable for liquids that easily crystallize. | Heat-sensitive wastewater such as pharmaceutical wastewater and food wastewater |
