Multi-effect evaporators are primarily used to treat industrial wastewater with high concentration, high chromaticity, and high salt content. Simultaneously, they recover byproducts generated during wastewater treatment. They feature low steam consumption, low evaporation temperature, high concentration ratio, greater rationality, higher energy efficiency, and higher efficiency. This article will introduce the application of multi-effect evaporators in wastewater treatment.

Industrial wastewater is typically classified into three categories:

The first is by the chemical nature of the main pollutants in the industrial wastewater. Wastewater primarily containing inorganic pollutants is classified as inorganic wastewater, while wastewater primarily containing organic pollutants is classified as organic wastewater. For example, electroplating wastewater and wastewater from mineral processing are inorganic wastewater; wastewater from food or petroleum processing is organic wastewater.

The second is by the products and processing objects of industrial enterprises, such as metallurgical wastewater, papermaking wastewater, coking gas wastewater, metal pickling wastewater, chemical fertilizer wastewater, textile dyeing wastewater, dye wastewater, tanning wastewater, pesticide wastewater, power plant wastewater, etc.

The third is by the main components of pollutants in the wastewater, such as acidic wastewater, alkaline wastewater, cyanide-containing wastewater, chromium-containing wastewater, cadmium-containing wastewater, mercury-containing wastewater, phenol-containing wastewater, aldehyde-containing wastewater, oil-containing wastewater, sulfur-containing wastewater, organophosphorus-containing wastewater, and radioactive wastewater, etc.

The first two classification methods do not involve the main components of pollutants in the wastewater, nor do they indicate the harmfulness of the wastewater. The third classification method clearly indicates the components of the main pollutants in the wastewater, indicating a certain degree of harmfulness.

01 Technical Characteristics of Multi-effect Evaporation

Multi-effect evaporation is the earliest seawater desalination technology used. It has now developed into a relatively mature wastewater evaporation technology, solving the problem of serious scaling and is gradually being applied to the treatment of high-salt water.

Multi-effect evaporation has the following technical characteristics:

The heat transfer process of multi-effect evaporation is boiling and condensation heat exchange, which is a two-sided phase-change heat transfer, so the heat transfer coefficient is very high. For the same temperature range, the heat transfer area used in multi-effect evaporation is less than that of multi-stage flash evaporation.

Multi-effect evaporation consumes less power. Because the production of freshwater in multi-stage flash evaporation relies on the sensible heat absorbed by the saline water, and the latent heat is much greater than the sensible heat, therefore, to produce the same amount of freshwater, multi-stage flash evaporation requires much more circulation than multi-effect evaporation, so multi-stage flash evaporation requires more power consumption.

Multi-effect evaporation has great operational flexibility, with a load range from 110% to 40%, all of which can operate normally without reducing the water production ratio.

02 Process Flow of Saline Wastewater

Saline water first enters the condenser for preheating and degassing, and then is divided into two streams. One stream is returned to the sea as cooling water, and the other is used as feed for the distillation process.

After adding scale inhibitors, the feed saline water is introduced into the later stages of the evaporator. The liquid is evenly distributed onto the top header pipes of the evaporator through nozzles, then flows down the top header pipes in a thin film, and part of the water evaporates by absorbing the latent heat of the condensing steam inside the pipes.

The secondary steam condenses into product water in the next stage, and the remaining liquid is pumped to the next stage of the evaporator. The operating temperature of this stage is slightly higher than the previous stage, and the spraying, evaporation, and condensation process is repeated in the new stage.

The remaining liquid is pumped to the high-temperature stage and finally leaves the device as a concentrated liquid in the highest temperature stage.

Live steam is input into the evaporation tubes of the first stage and condenses in the tubes, and the saline water outside the tubes produces secondary steam that is basically equal to the condensation amount.

Since the operating pressure of the second stage is lower than that of the first stage, the secondary steam enters the heat transfer tubes of the next stage after passing through the vapor-liquid separator. The evaporation and condensation process is repeated in each stage, and each stage produces basically the same amount of distilled water. The steam from the last stage is condensed in the condenser by saline water.

The condensate from the first stage is returned to the steam generator, and the condensate from the other stages enters the product water tank, with the product water tanks of each stage connected. Due to the different pressures in each stage, the product water flashes, and the heat is returned to the evaporator.

In this way, the product water flows in a stepped manner and is cooled by stepwise flashing, and the recovered heat can improve the overall efficiency of the system. The cooled product water is transported to the product water storage tank by the product water pump. The product water produced in this way is pure water with an average salt content of less than 5 mg/L.

The concentrated brine flows from the first stage in a stepped manner into a series of concentrated brine flash tanks, and the overheated concentrated brine is flashed to recover its heat. After flash cooling, the concentrated brine is finally discharged back to the sea by the concentrated brine pump.

Non-condensable gases accumulate in the condenser and are extracted by a vacuum pump.

03 Technical Advantages of Low-Temperature Multi-effect Evaporation

From the above principles, the technical advantages of low-temperature multi-effect evaporation are reflected in the following aspects:

Due to the low operating temperature, corrosion and scaling of the equipment can be avoided or reduced.

Due to the low operating temperature, low-grade waste heat from power plants and chemical plants can be fully utilized. For low-temperature multi-effect evaporation technology, low-grade steam at 50℃-70℃ can be used as an ideal heat source, which can greatly reduce the impact of extracting backpressure steam on power plant power generation.

Pretreatment of feed saline water is simpler. Another major advantage of low-temperature operation is that it greatly simplifies the pretreatment process of saline water. Before entering the low-temperature multi-effect device, the saline water only needs to be filtered through a screen and a small amount of scale inhibitor added, unlike multi-stage flash evaporation, which requires acidification and degassing treatment.

The system has great operational flexibility. During peak periods, the desalination system can provide 110% of the design value of product water; during off-peak periods, the desalination system can stably provide 40% of the rated value of product water.

The system has low power consumption. The power consumption of the low-temperature multi-effect system for transporting liquids is very low, only about 0.9-1.2 kWh/m³. This can greatly reduce the water production cost of desalinated water, which is particularly important for areas with high electricity prices.

The system boasts high thermal efficiency. A temperature difference of over 30 degrees Celsius can achieve a heat transfer coefficient exceeding 12, resulting in a water production ratio of around 10.

The system operation is safe and reliable. In a low-temperature multiple-effect system, the process involves in-tube steam condensation and out-tube liquid film evaporation. Even if the heat transfer tube corrodes and leaks, because the vapor-side pressure is greater than the liquid film-side pressure, the brine will not flow into the product water; at most, only a small amount of steam leakage will occur, affecting the water production.

Refineries have a large amount of surplus low-temperature waste heat that can be utilized. The freshwater treated by low-temperature multiple-effect evaporation technology can be reused in multiple process stages, such as circulating water replenishment, achieving resource utilization of wastewater while efficiently utilizing low-temperature waste heat.

Therefore, introducing low-temperature multiple-effect evaporation technology into the refinery water treatment industry, utilizing its advantages of high water production ratio and good treated water quality, can organically combine low-temperature waste heat utilization and deep treatment of refinery wastewater, and solve the problems of difficult desalination and high energy consumption of high-salt wastewater in refinery wastewater.

As shown in the comparison table of low-temperature heat utilization technologies, compared with conventional heat pump technology and multi-stage flash evaporation technology, low-temperature multiple-effect evaporation has obvious advantages in heat utilization rate, technical process coupling wastewater treatment, etc., representing the development direction of related technical fields and is a key direction for carrying out waste heat utilization and wastewater treatment coupling technologies.

04 Multiple-effect evaporation process modes

Multiple-effect evaporation processes have the following process modes:

1. Co-current process flow

The flow directions of the solution and steam are the same, both flowing from the first effect to the last effect. The raw material liquid is pumped into the first effect, and relying on the pressure difference between the effects, it self-flows (if solids are produced during the concentration process or the solution viscosity is large, a transfer pump needs to be added) into the next effect for processing, and the finished liquid is extracted from the last effect by a pump.

The pressure in the latter effect is lower, and the boiling point of the solution is also relatively lower. Therefore, when the solution enters the latter effect from the former effect, it will evaporate spontaneously due to overheating, which is called flash evaporation. Therefore, the latter effect may produce more secondary steam than the former effect, but because the concentration of the latter effect is higher than that of the former effect, and the operating temperature is lower, the heat transfer coefficient of the latter effect is lower than that of the former effect, and the heat transfer coefficient of the first effect is often much higher than that of the last effect.

The co-current flow process is suitable for processing materials that are heat-sensitive at high concentrations.

Reasons for choosing the co-current process: The wastewater influent has low viscosity and does not contain a large amount of low-boiling substances, so there is no need to choose a counter-current mode for pre-condensation, and it does not affect the heat transfer coefficient.

Secondly, the salt concentration of the wastewater influent is not high, and the co-current feeding mode is only selected at extremely high concentrations.

2. Counter-current process flow

The raw material liquid is added from the last effect and pumped to the previous effect at once. The finished liquid is discharged from the first effect, and the material liquid and steam flow in opposite directions. As the solvent evaporates and the solution concentration gradually increases, the evaporation temperature of the solution also increases effect by effect, so the concentrations of the solutions in each effect are relatively close, making the heat transfer coefficients of each effect similar.

However, because the solution temperature is lower than the boiling point when the solution is transported from the latter effect to the former effect, additional heating is sometimes required; otherwise, the amount of secondary steam produced will gradually decrease. Generally speaking, the counter-current feeding process is suitable for processing materials whose viscosity changes significantly with temperature and concentration, but not for heat-sensitive materials.

3. Parallel flow process

Material liquid is added to each effect, and finished liquid is also drawn out. This process is used for the evaporation of saturated solutions (or solutions with high concentrations). Crystals are precipitated in each effect and can be separated in time. This method can also be used to simultaneously concentrate two or more aqueous solutions.

4. Cross-flow process

Also known as mixed flow process. It is a combination of parallel and counter-current flow processes. The characteristic of cross-flow is that it combines the advantages of parallel and counter-current flow while avoiding their disadvantages. However, the operation is complex, and complete automatic control instruments are required to achieve stable operation.