From emission reduction to emission limits and then to zero emissions, wastewater discharge standards are gradually increasing. To achieve "zero emissions", the key is to achieve the complete recovery of high-salt wastewater, which essentially means separating water and salts in wastewater.

Currently, concentration technology, crystallization technology, and the combined technology of the two are more commonly used to achieve zero emissions in the recovery of high-salt wastewater. Of course, sometimes, depending on the actual situation of high-salt wastewater, pretreatment technology needs to be added before the technology to provide better processing conditions for subsequent processes.

Concentration, as the core process for the resource utilization of high-salt wastewater, is divided into thermal concentration and membrane concentration according to different treatment objects and applicable scopes.

Among them, thermal concentration technology is suitable for treating wastewater with high TDS and COD up to hundreds of grams per liter. It concentrates ions in high-salt wastewater by heating, mainly including multi-stage flash MSF, multi-effect evaporation MED, and mechanical vapor recompression evaporation MVR.

Multi-stage flash (MSF)

Multi-stage flash technology started in the 1950s. High-salt wastewater heated to a certain temperature is successively flashed and vaporized in a series of containers with gradually decreasing pressure, and then the steam is condensed to obtain fresh water.

Multi-stage flash (MSF), as the earliest distillation technology applied, has mature technology and reliable operation, suitable for large-scale applications, but its thermodynamic efficiency is relatively low, energy consumption is high, and scaling and corrosion of equipment limit the MSF primary steam temperature, affecting the operating cost.

This technology will not be described in detail in this article. Let's focus on multi-effect evaporation MED and mechanical vapor recompression evaporation MVR.

Multi-effect evaporation (MED)

1. Basic principle:

The principle of multi-effect evaporation (hereinafter referred to as MED) is to connect multiple evaporators in series. The secondary steam from the previous evaporator is used as the heating steam for the next evaporator, and the heating chamber of the next evaporator is the condenser of the previous evaporator.

Three-effect evaporator process flow

In the multi-effect evaporation system, fresh steam is only added in the first effect. The secondary steam generated at the top of the previous evaporation tower is directly used as the heating medium for the reboiler of the subsequent evaporation tower. The evaporation tower after the first effect does not need to introduce fresh steam, and the steam at the top of the last effect tower can be used as a low-pressure level heat source.

Therefore, its greatest advantage is that it can significantly reduce the consumption of fresh steam by repeatedly using the vaporization and condensation of secondary steam.

2. Types of MED evaporators

There are many types of MED evaporators, which are classified into four categories and fifteen types according to evaporation pressure, evaporator type, number of effects, and material flow direction:

According to steam pressure: atmospheric pressure evaporation, pressure evaporation, and vacuum evaporation;

According to evaporator type: tube evaporator, plate evaporator, and tube-plate combined evaporator;

According to the number of effects: two-effect, three-effect, four-effect, five-effect, and six-effect evaporation;

According to material flow: parallel flow, countercurrent flow, mixed flow, and flat flow.

So, how should MED evaporators be selected? Three principles:

1. Countercurrent and mixed flow are better than parallel flow systems. Countercurrent multi-effect evaporation has the lowest energy consumption, and parallel flow multi-effect evaporation has the highest energy consumption; the characteristics of mixed flow multi-effect evaporation systems are relatively better than parallel flow multi-effect evaporation systems.

2. The number of evaporation effects is not always better. When the number of effects increases, the efficiency of heat utilization also decreases. 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. For example, for some high-boiling point systems, only two-effect or three-effect evaporators can be used.

3. Consider material properties, heat balance, and the degree of non-condensable gas retention to select the evaporation pressure. Studies have shown that the pressure of each effect is related not only to the material and heat balance of the evaporator, but also to the characteristics of the material and the degree of throttling of non-condensable gas above and below each effect.

3. Advantages and disadvantages of MED

1. The advantages of MED are mainly reflected in the following 5 aspects:

Simple pretreatment, less chemical reagent consumption, and scale inhibitors can be added.

Short heating time, mostly using double-sided heat transfer methods of in-tube condensation and out-tube boiling, small heat transfer area, and high heat transfer coefficient.

Large operating flexibility, the system can provide 40%~110% of the design value of product water, while multi-stage flash and reverse osmosis do not have such large operating flexibility.

Good treatment effect, the salt analysis is thorough during the treatment process, and after cooling, more than 90% of the salt in the coolant can be removed, making it difficult for microorganisms to be inhibited by salt.

High operational reliability, the whole process uses fully automated operation, and the pressure inside the tube is greater than the pressure outside the tube during operation. Even if corrosion of the heat exchange tube occurs, the cooling water will not pollute the product water.

2. The disadvantages of MED are mainly reflected in the following 3 aspects:

Scaling is easy to occur inside the tube, and it needs to be cleaned once every 10 days. Descaling treatment is required.

The increase in the number of effects reduces the steam utilization rate. When the number of effects increases, the heat transfer temperature difference loss of each effect increases. For example, the ratio of steam consumption per ton of water evaporated is 1.1 for one effect, 0.57 for two effects, 0.4 for three effects, 0.3 for four effects, and 0.27 for five effects, and the production capacity of the equipment decreases.

4. Three common technical problems and countermeasures of multi-effect evaporation MED

Generally speaking, multi-effect evaporation MED often encounters three major problems: foaming in the device, scaling of the evaporator, and corrosion of the equipment by salt ions in the last effect steam.

1. For the foaming problem in the device, the solution—

Physical defoaming methods mainly include high-temperature and low-temperature defoaming, ultrasonic defoaming, liquid spraying defoaming, and mechanical vibration, etc. Although physical defoaming is effective when dealing with large volumes, its equipment and operating costs are high;

Chemical defoaming mainly refers to the use of defoamers, but its use is affected by the high price of defoamers, high production costs, and complex production processes;

Mechanical defoaming mainly uses rotation to change the pressure and shear force acting on the bubbles to achieve defoaming. Because of its low cost and good defoaming effect, it is more popular now.

2. Regarding the scaling problem of the evaporator, the solution is -

Some researchers have conducted acid washing and neutral cleaning on the evaporator outer wall scale samples (sodium sulfate and calcium carbonate), and acid washing on the inner wall scale samples (calcium carbonate) of the last-effect heat exchanger. The hanging plate analysis shows that the average corrosion rate of each effect hanging plate is less than 1 g/m2·h, and the total corrosion amount is less than 10 g/m2.

It is worth mentioning that this method is superior to the "Quality Standard for Chemical Cleaning of Industrial Equipment" (HG/T2387-2007) and the "Standard for Preparation, Cleaning and Evaluation of Corrosion Samples".

3. Regarding the problem of equipment corrosion by steam containing salt ions in the last effect, the solution is -

Low-temperature, timed, and quantitative replacement and supplementation can be performed using condensate water with low chloride ion content, and a high-efficiency corrosion inhibitor can be added to the circulating water.

Mechanical Vapor Recompression (MVR)

I. Basic Principles

Mechanical vapor recompression technology (hereinafter referred to as MVR) is an energy-saving technology that utilizes the secondary steam and its energy generated by the evaporation system itself to raise low-grade steam to high-grade steam heat source through the mechanical work of the compressor. This continuously provides thermal energy to the evaporation system, thereby reducing the demand for external energy.

MVR Process Flow

In this system, the heat source in the preheating stage is provided by the steam generator until the material begins to evaporate and produce steam.

The secondary steam produced by heating the material is compressed by the compressor into high-temperature and high-pressure steam. The high-temperature and high-pressure steam generated here is used as a heating heat source. The material in the evaporation chamber is continuously evaporated by heating, and the high-temperature and high-pressure steam that has passed through the compressor is cooled into condensate water through continuous heat exchange, which is the treated water.

The compressor, as the heat source of the entire system, realizes the conversion of electrical energy into thermal energy, avoiding the system's dependence on and intake of external live steam.

II. MVR System Equipment Composition

From the MVR evaporation process flow, it is not difficult to see that the MVR evaporation system is composed of various equipment connected in series. The equipment must be cleverly matched in terms of thermodynamics and heat transfer to achieve the best effect for the entire system. The main equipment in the system includes the following four:

1. Compressor. There are two main types of MVR compressors: Roots compressors and centrifugal compressors.

Roots blowers are often used to compress small flow rates of steam. They are positive displacement compressors, providing small air volume, large temperature rise, and are suitable for materials with small evaporation capacity and large boiling point elevation.

Centrifugal compressors are pressure-difference blowers, providing small pressure difference, large flow rate, small temperature rise, uniform exhaust, and pulse-free airflow. They are suitable for materials with large evaporation capacity and small boiling point elevation.

In summary, the stability of centrifugal compressors is superior to that of Roots compressors, but centrifugal compressors sometimes experience surge, which can lead to compressor instability.

2. Evaporator. The types of evaporation treatment devices are generally divided into rising film evaporation and falling film evaporation.

The selection is mainly based on the characteristics and energy consumption of the processed materials. Currently, falling film evaporation is mainly used in China.

3. Heat exchanger. In the MVR heat pump evaporation process, the heat exchangers used are mostly wall-type heat exchangers.

In this type of heat exchanger, the hot and cold fluids do not directly contact but exchange heat through a wall. Commonly used wall-type heat exchangers in production include: shell and tube heat exchangers, corrugated heat exchangers, and spiral heat exchangers.

4. Gas-liquid separator. The gas-liquid separator is the place where the material and secondary steam are separated.

Its main function is to aggregate the solution in the mist into droplets and separate the droplets from the secondary steam. It is worth mentioning that the design of the separator should fully consider factors such as evaporation capacity, evaporation temperature, material viscosity, and separator liquid level.

III. Technical Advantages of MVR

1. Compared with traditional evaporation systems, the MVR system only needs to introduce live steam as a heat source at startup. When secondary steam is generated and the system is running stably, it will not require an external heat source. The system's energy consumption is only the energy consumption of the compressor and various pumps, so the energy-saving effect is quite significant.

2. The energy consumption of the MVR evaporator system is mainly the electricity consumption of the compressor, the operating costs are greatly reduced, the maintenance costs are low, and because the system does not require industrial steam, the safety risks are lower, and the operation is simple.

3. Under the same evaporation treatment capacity, the floor space required by the MVR evaporator is much smaller than that of traditional multi-effect evaporation equipment.

IV. Common Technical Problems in MVR Treatment of High-Salt Wastewater

Although MVR technology has achieved good results in the treatment of high-salt wastewater, some technical problems still affect the operation effect during operation.

1. System scaling problem

Scaling on the heat exchanger walls is one of the main reasons for the decrease in system evaporation efficiency. This is mainly because the heating heat source uses secondary steam, and scaling and coking will reduce the heat transfer effect, reducing the evaporation capacity per unit time, which reduces the amount of usable compressed secondary steam and has a more significant impact on production capacity.

Due to the special nature of MVR evaporators, it is common for equipment cleaning to be delayed, which is one of the reasons for unstable production capacity.

2. Temperature rise issues

Temperature rise issues in the MVR system are an important factor affecting its application in the treatment of saline wastewater.

When using MVR technology to treat high-concentration saline wastewater, due to its high concentration and significant boiling point elevation, the corresponding steam compressor needs to increase the temperature to overcome the impact of the boiling point elevation, which puts high demands on the compressor and significantly increases system energy consumption.

Studies have shown that when using MVR evaporation technology, a reasonable temperature rise range is 8℃～20℃. If the boiling point elevation exceeds 18℃, MVR technology will lose its advantages.

3. Material properties matching issues for MVR selection

Due to the different sources of industrial wastewater, MVR should be selected according to the properties of different materials.

Material property analysis mainly includes: the components contained in the material; whether crystallization occurs during the evaporation process; the viscosity, specific heat, density, and boiling point elevation of the material.

Parameters for single materials can be obtained by consulting relevant tables, but industrial high-salt wastewater is mostly a mixture, and its relevant data can only be estimated through simulation. Therefore, accurately analyzing and calculating the material properties is a key factor in ensuring the normal operation of the MVR device. Generally speaking,

For materials with a large increase in boiling point temperature, MVR single-effect evaporation is generally used;

High-concentration materials need to use forced circulation to prevent the material flow rate from being too slow and causing coking;

Heat-sensitive materials require that the residence time in the evaporator be as short as possible.

In summary, current evaporation technology is widely used, but it also has problems such as high energy consumption, high operating costs, and easy scaling and blockage. Therefore, high efficiency and energy saving must be considered, and multi-effect evaporation (MED) and mechanical vapor recompression evaporation (MVR) are recommended high-efficiency energy-saving technologies.

Among them, the initial investment of MVR evaporation equipment is relatively large, and the energy consumption is relatively low. However, with the continuous improvement of technology and production processes of domestic steam compressors, the price is also continuously decreasing; the number of effects of multi-effect evaporation equipment increases, so the investment in multi-effect evaporation equipment will also increase, but energy consumption can also be reduced to a certain extent.

Therefore, both MVR evaporation equipment and multi-effect evaporation equipment have certain relative advantages, and multi-faceted comparisons should be made based on applicability, investment, operation, consumption, labor, and land occupation.