High-salt wastewater treatment Difficult High difficulty! Do you understand the evaporation unit?

High-salt wastewater in the chemical industry is wastewater produced in the production processes of petrochemicals, fine chemicals, pharmaceutical chemicals, etc. It is currently the type of chemical wastewater with the largest production volume and the greatest processing difficulty.

Most of this type of wastewater is not a single component, but a mixed wastewater of multiple salts. It usually contains high levels of organic matter and volatile, easily coking substances. Due to its high salt content, it cannot be directly treated biologically. Instead, the wastewater is pre-treated, and the salt is separated by evaporation crystallization or cooling crystallization before entering the biological or other treatment units, finally achieving the standard for discharge or reuse as process water.

Because the wastewater contains high organic and easily coking components, pretreatment is required before entering the evaporator to separate the easily coking organic matter before entering the evaporation unit. The pretreatment requires a large amount of experimental research, and determining a suitable solution is the key to the long-term stable operation of the evaporation unit.

The evaporation unit is High-salt wastewater treatment the core process. There are various modes of evaporation. Currently, widely used evaporation systems are divided into three categories ：

1. Multiple Effect Evaporation (MEE)
2. Thermal Vapor Recompression Evaporation (TVR)

3. Mechanical Vapor Recompression Evaporation (MVR)

Multiple Effect Evaporation System

　 　Multiple effect evaporation (MEE) is based on the cascade utilization of steam to connect several evaporators to improve thermal energy utilization efficiency. The advantages of MEE are simple water pretreatment ； and flexible application. It can be used alone or in combination with other methods. ； The system operation is safe and reliable.

Thermal Vapor Recompression Evaporation System

　　 Thermal vapor recompression evaporation is based on the heat pump principle. Live steam passes through a Venturi steam jet heat pump, and a small amount of high-pressure steam is used as power to compress part of the secondary steam and mix it before entering the heating chamber as heating steam.

According to its efficiency characteristics, the energy saved by using a thermal vapor compressor is equivalent to the energy saved by adding an effect evaporator, so it is widely used.

　　In terms of equipment, TVR only adds a steam jet pump compared to the multiple-effect evaporation system. The multiple-effect + steam jet pump combination is more energy-efficient than pure multiple-effect evaporation. TVR technology operates according to the jet pump principle, has no moving parts, is simple and effective in design, and ensures high reliability of operation. Thermal Vapor Compression Evaporation System Still requires a continuous supply of a certain amount of fresh steam during operation.

Mechanical Vapor Recompression Evaporation System

　　 Mechanical vapor recompression is also known as mechanical heat compression. The evaporation principle is to use Roots type or centrifugal compressor to compress the secondary steam before entering the heating chamber as heating steam.

Live steam needs to be supplied during the start-up process of the device. After normal operation, no live steam supply is needed, thus improving thermal efficiency, reducing the demand for external heat sources, and reducing energy consumption. MVR adopts single-effect operation, with short material residence time, low energy consumption and operating cost, small footprint, and less supporting public utilities.

　　 After preheating by the preheater, the raw material is in state A (a), at which point the material is in a saturated liquid state; after preheating, the material enters the evaporator's evaporation chamber to absorb heat and boil and vaporize to produce secondary steam. The concentrated liquid flows out from the discharge pump, and the obtained secondary steam enters the inlet of the Roots compressor. The steam at this time is saturated steam under a specific evaporation pressure, and its gas state parameters are B (b); under the action of the compressor, the temperature of the secondary steam increases, and the pressure increases, and the enthalpy of the steam increases. In the ideal case (isentropic compression), the state is C (c). In the actual steam compression process, it is an irreversible process, and the system is not completely adiabatic. Its state parameters are a state D (d) with higher superheat. The superheated steam at this time is first saturated before entering the shell side of the evaporator to condense and release heat to obtain saturated condensate, which is in state E (e).

　　 The condensate is first stored in a temporary storage tank and then pumped out to preheat the raw material, realizing comprehensive energy utilization. When the system starts to operate, fresh steam needs to be introduced into the shell side of the evaporator. Once the system runs stably, the condensation latent heat of the compressed steam can be used to maintain the heat balance of the system, and only a small amount of fresh steam needs to be supplemented when the heat loss is large.

From the temperature-entropy diagram, it can be seen that the entire MVR heat pump system is under two pressures and three temperature states. The pressure change is the evaporation pressure before steam compression and the condensation pressure after steam compression. The temperature change is mainly the evaporation temperature, the superheated temperature after compression, and the compressed saturation temperature.

　 From the enthalpy-entropy diagram, it can be known that the essence of the operation of the MVR system is to use the compressor to do a small amount of work to increase the enthalpy of the steam, and then use the released latent heat of condensation of the compressed steam to reheat the raw material. Since the vaporization latent heat (or condensation latent heat) accounts for the vast majority of the enthalpy value of the steam, fully utilizing the latent heat of the steam is the key to the normal operation of the system.

　　The main technical feature of MVR is that all the generated secondary steam is compressed by the compressor to increase the pressure and temperature of the steam, obtaining high-quality steam, and then reused as heating steam. Therefore, the compression stage is the heart of the entire MVR evaporation system.

　　 Industrial wastewater generated during production processes generally has a high salt content, requiring the selection of a cost-effective evaporation method based on multiple factors such as region, energy, physical properties, and operating conditions.