Technical Sharing | High-Salt Wastewater Evaporation Crystallization Technology
High-salinity wastewater generally refers to wastewater with a total dissolved solids (TDS) mass fraction greater than 3.5%. In addition to containing a large amount of inorganic salts in the form of ions such as Cl-, SO42-, and Na+, this type of wastewater also contains ions such as Ca2+, Mg2+, NH4+, and HCO3- that are easily chemically changed into scale, as well as impurities such as chemical oxygen demand (COD) and suspended solids (SS). In 2015, the amount of high-salinity wastewater produced in China accounted for 5% of the total wastewater, with a discharge volume of approximately 975 million tons. If high-salinity wastewater is directly discharged into water bodies, it will cause eutrophication, rapid algae growth, resulting in water quality deterioration and mass mortality of fish and other organisms.
Evaporation (or distillation) is an ancient method, but due to continuous technological improvements and development, it remains a primary method for concentrating or desalination. The essence of the distillation process is the formation of water vapor; its principle is similar to how seawater evaporates to form clouds, and clouds cool down under certain conditions to form rain, which is not salty.
Based on the energy source, equipment, and process used, it can be mainly divided into multiple-effect evaporation, multi-stage flash evaporation, and mechanical vapor recompression (MVR) evaporation.
Multiple-Effect Evaporation Technology
Multiple-effect evaporation is one of the oldest desalination methods and was the dominant method for evaporation and concentration before the advent of multi-stage flash evaporation.
Multiple-effect evaporation consists of a system of single-effect evaporators. The secondary steam produced by the previous evaporator is introduced into the next evaporator as heating steam and condensed into distilled water in the next effect evaporator, and so on.
Raw water entry methods: countercurrent, cocurrent (entering each effect separately), parallel flow (entering from the first effect), and countercurrent preheating and parallel flow feed.
Advantages compared to multi-stage flash evaporation:
① The heat transfer process of multiple-effect evaporation is boiling and condensation heat transfer, which is phase-change heat transfer, so the heat transfer coefficient is very high. In general, the heat transfer area used in multiple-effect evaporation is less than that of multi-stage flash evaporation.
② Multiple-effect evaporation is usually a once-through evaporation, unlike multi-stage flash evaporation where a large amount of liquid circulates in the equipment, so the power consumption is less.
③ Multiple-effect evaporation has a high concentration ratio.
④ Multiple-effect evaporation has great flexibility.
3. Process Flow Classification
There are three main process flows for multiple-effect evaporation: cocurrent, countercurrent, and parallel flow.
Cocurrent: refers to the fact that both the feed liquid and the heating steam advance in the order from the first effect to the second effect.
The vacuum degree of multiple effects increases in turn, that is, the absolute pressure decreases in turn; therefore, the feed liquid does not need to be transported between the effects by a pump, but rather flows naturally to the later effects due to the pressure difference.
The temperature also decreases in turn, so when the feed liquid flows from the previous effect to the next effect, there is a superheating phenomenon, that is, flash evaporation occurs, producing some steam, i.e., fresh water.
For materials with high concentration and viscosity, the heat transfer coefficient of the later effects is relatively low; moreover, due to the high concentration, the boiling point is high, and it is difficult to maintain a large temperature difference between the effects, which is not conducive to heat transfer.
Parallel flow: Parallel flow means that each effect is fed separately in parallel, but except for the first effect, the other effects use secondary steam.
Suitable for: materials that easily crystallize, such as salt making, which quickly reach supersaturation and crystallize upon heating and evaporation.
In water treatment, the main purpose is to obtain fresh water, so countercurrent and parallel flow are not needed, and countercurrent and parallel flow do not have the high thermal efficiency of cocurrent flow.
Countercurrent: Countercurrent refers to the fact that the direction of feed liquid flow is opposite to the direction of heating steam flow. The raw material enters the system from the last effect with the highest vacuum degree and gradually flows to the previous effects, with the concentration increasing. Therefore, when the feed liquid is sent to the previous effect, not only is there no flash evaporation, but it also undergoes a preheating process before boiling.
It can be seen that the advantages and disadvantages are exactly the opposite of cocurrent flow. Countercurrent is more suitable for materials with high concentration and viscosity, because the last evaporation is in the first effect with the highest temperature. Therefore, although the concentration is high, the viscosity can still be reduced to some extent, and a relatively high heat transfer coefficient can be maintained. This is more commonly used in chemical production.
4. Process and Equipment Introduction
According to the analysis of the single-effect evaporator, the evaporation amount D/heating steam amount D0 = 0.91 or D0/D = 1.1, that is, 1 kg of steam can evaporate 0.91 kg of fresh water.
If the secondary steam is passed to the heating chamber of the second evaporator for heating, then the same 1 kg of secondary steam can evaporate 0.91 kg of fresh water.
By analogy, the more effects, the more fresh water can be evaporated using 1 kg of heating steam, which is advantageous in terms of heat utilization.
In fact, due to the boiling point elevation phenomenon of the solution and the flow resistance loss in the pipeline, there is a loss of temperature difference, and with the increase in the number of effects, even if the insulation is good, the heat dissipation area increases, and the heat loss also increases. Therefore, when the number of effects increases, the efficiency of heat utilization also decreases accordingly. Considering that the increase in the number of effects will increase the investment in equipment, there should be an optimal point for the actual number of effects used.
5. Classification of Multiple-Effect Evaporation Equipment
There are many types of multiple-effect evaporation equipment, and different materials and concentrations can use different evaporators.
According to the arrangement direction of the evaporation tubes: it can be divided into vertical tube evaporators (VTE) and horizontal tube evaporators (HTE).
According to the type of material flow: it can be divided into forced convection evaporators and film evaporators.
In film evaporators, according to the flow direction, they can be divided into rising film evaporators and falling film evaporators.
Falling film evaporators are divided into vertical tube falling film evaporators and horizontal tube falling film evaporators.
According to the direction of the combination of each effect, they can be divided into horizontally combined evaporators and tower evaporators.
There are various types of evaporators that make up a multiple-effect evaporation system. The following three are commonly used.
Immersion tube type (ST): This type of evaporator is a large category of evaporation equipment in which the heating tubes are immersed in the liquid.
Broadly speaking, there are many types of immersion tube evaporators. These include straight tubes, coiled tubes, U-shaped tubes, vertical tubes, and horizontal tubes. 。
The flow methods of the material in the evaporator include natural convection circulation and forced circulation. This type of evaporator appeared earlier and is easy to operate, but it has serious scaling, a high hydrostatic column of brine, and large temperature difference losses, so the number of effects should not be too many, generally less than 6 effects.
Vertical tube evaporation (VTE): This refers to an in-tube falling film evaporator.
Two main advantages are: firstly, because the vaporization inside the tube is in the form of a film, there is a phase change on both sides of the heat transfer wall, so the heat transfer coefficient is high. It also eliminates the temperature difference loss caused by the hydrostatic column of the material. The concentration rate of the system is relatively high. For low-concentration solutions such as seawater desalination, the currently designed number of effects is generally 11-13, and the water production ratio can reach 9-10.
Scaling problems, especially when the liquid distribution is uneven or the water volume is insufficient, dry areas may form on the inner wall of the tube, increasing the risk of scaling. Therefore, higher requirements are placed on anti-scaling and descaling. Generally speaking, the seeding method is not suitable for this type of evaporation system, mainly relying on chemical anti-scaling methods plus reasonable design of temperature and concentration.
Horizontal tube thin film type (HTE) In this type of evaporator, the circulating liquid forms a liquid film on the outside of the horizontal tube bundle through a spray device, and the heating steam (or secondary steam from the previous effect) condenses inside the tube.
It has the same advantages and disadvantages as the vertical falling film type, but the equipment height is much smaller than the vertical falling film type, the installation is compact, and all the tube bundles, spray pipes, and steam-water separators of each effect are installed in one cylinder, so the heat loss is small, and the energy consumption is low.
Due to the low temperature, scaling and corrosion are greatly reduced, ensuring a high heat transfer coefficient; in addition, the vapor phase resistance is small, and the static head loss is eliminated, so the heat transfer temperature difference can be very small, especially suitable for using low-grade heat energy.
Taking seawater desalination as an example
Multi-stage flash distillation technology
Flashing refers to the sudden vaporization phenomenon that occurs when water at a certain temperature is in an environment where the pressure is lower than the saturated vapor pressure corresponding to that temperature. The temperature of the flashed water decreases to balance its saturated vapor pressure with the ambient pressure.
MSF also utilizes this principle, allowing the material liquid heated to a certain temperature to be sequentially flashed and vaporized in a series of containers with gradually decreasing pressure, concentrating the raw material, and condensing the steam to obtain fresh water.
This method is developed based on multiple-effect distillation. Compared with the multiple-effect distillation method, multi-stage flash distillation reduces scale formation and is mostly used in the concentration of low-concentration materials.
Compared with other technologies, multi-stage flash distillation has the following advantages:
Because the heating and evaporation processes are separated in this method, the raw water does not actually boil (only surface boiling), thus greatly improving the scaling problem of general distillation;
The technology is mature and reliable, with high operational safety, especially suitable for large-scale concentration applications of low-concentration materials;
The equipment structure is simple, and the investment cost is relatively low.
A large amount of raw water circulation and fluid transportation leads to increased operating costs;
Compared with the multiple-effect distillation method, a larger heat transfer area is required;
1. Working principle of MVR
MVR (Mechanical Vapor Re-compression) refers to compressing the secondary steam (low temperature, low pressure, and unusable) generated in the evaporation (distillation, etc.) process with a compressor to increase its temperature and pressure, and reuse it as a heat source to heat the material to be evaporated, thereby achieving the purpose of recycling steam, so that the evaporation process does not require external steam; that is, using a small amount of electrical energy to obtain more thermal energy, thereby reducing the system's demand for external energy, which is a highly efficient energy-saving technology.
The role of MVR: improving the quality of steam without creating energy
2. Application in evaporation crystallization
In the evaporation and concentration process, the medium undergoes a "phase change": liquid phase → gas phase
The specific heat of water is 1 kcal/kg〃℃. 1 kg of water requires 1 kcal of heat for every 1 ℃ increase in temperature. To heat 1 kg of water from 0 ℃ to 100 ℃ boiling, only 100 kcal of heat is needed. To vaporize 1 kg of 100 ℃ water into steam of the same temperature requires 539 kcal of heat. The energy consumption is 539 times the heat required to raise the temperature of the same weight of water by 1 ℃.
Single-effect evaporation (taking 1 kg H2O as an example)
A large amount of heat from fresh steam → secondary steam → cooling water → atmosphere
The cooling tower consumes a large amount of circulating water and electrical energy (pump) operation, resulting in triple waste
3. Relationship between energy consumption and the number of effects (taking an evaporation amount of 1 t H2O as an example)
4. Development status of MVR technology
MVR is not a new technology.
The concept of the MVR heat pump was proposed as early as 1834 abroad, while the first product applying this technology was manufactured by a Swiss company in 1917. In 1925, Austria designed and installed a set of equipment, thus leading to the appearance of the MVR device in actual operation.
The oil crisis in the 1970s caused an energy crisis, and under the general trend of energy saving and consumption reduction, MVR heat pumps developed rapidly.
In the 1980s, Zhangjiaba Salt Chemical Factory introduced mechanical vapor recompression technology for salt production for the first time in China. In 2010, the 1.2 million tons/year refined salt MVR device introduced by Zhongyan Jintan has been successfully operating to this day. Currently, MVR technology has been recognized by numerous industries and enterprises.
