The principle, advantages and disadvantages of multi-effect evaporation (MED)

Multi-effect evaporator

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

In the multi-effect evaporation system, only fresh steam needs to be added at the first effect, and the secondary steam generated at the top of the previous first-effect evaporation tower is directly used as the heating medium of the subsequent first-effect evaporation tower reboiler, and 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 heat source.

▲Schematic diagram of multi-effect evaporation

Therefore, its biggest advantage is that the vaporization and condensation of secondary steam can be used multiple times, which can significantly reduce the consumption of fresh steam.

MED evaporator type

There are many types of MED evaporators, which are classified according to evaporation pressure, evaporator type, evaporation efficiency and material flow direction, with a total of four categories and fifteen types:

According to the steam pressure, it is divided into: atmospheric evaporation, pressure evaporation and vacuum evaporation;

According to the type of evaporator, it is divided into: tube evaporation, plate evaporation and tube-plate combined evaporation;

According to the number of effects, it is divided into: two, three, four, five and six evaporation;

According to the flow of materials, it is divided into: parallel flow, counterflow, mixed flow and advection flow.

So, how should MED evaporators be compared? 

3 principles:

1. The counter-flow and mixed flow effects are better than those of the parallel flow system, and the counter-flow multi-effect evaporation energy consumption is the smallest, and the parallel flow multi-effect evaporation energy consumption is the largest. The characteristics of the mixed-flow multi-effect evaporation system are better than those of the parallel flow multi-effect evaporation system.

2.The evaporation efficiency is not as much as possible, when the efficiency increases, the efficiency of heat utilization is also reduced, considering that the efficiency increases, the investment in equipment increases, so the actual use of the efficiency should have an optimal point. For example, for some high boiling point systems, only two-effect or three-effect evaporators can be used.

3. Considering the characteristics of the material, the heat balance and the degree of non-condensable gas interception and other factors in the selection of evaporation pressure, studies have shown that the pressure of each effect is not only related to the material and heat balance of the evaporator, but also related to the characteristics of the material and the size of the non-condensable flow saving degree of each effect.

Advantages and disadvantages of MED

The advantages of MED are mainly reflected in the following five aspects:

(1) The pretreatment is simple, the consumption of chemical agents is less, and the scale inhibitor can be added.

(2) The heating time is short, and the bilaterally variable heat transfer method of condensation in the tube and boiling outside the tube is mostly adopted, with a small heat transfer area and a high heat transfer coefficient.

(3) The operation flexibility is large, the system can provide 40%~110% of the design value of product water, while multi-stage flash evaporation and reverse osmosis do not have such great operation flexibility.

(4) The treatment effect is good, the salt analysis is thorough during the treatment process, and the salt content of the coolant can be removed by more than 90% after cooling, so that it is difficult for microorganisms to be inhibited by salt.

(5) The operation reliability is high, the whole process is fully automated, and the pressure in the pipe is greater than the pressure outside the pipe during the operation, even if the phenomenon of corroding the heat exchange tube occurs, the cooling water will not pollute the product water.

The disadvantages of MED are mainly reflected in the following two aspects:

(1) The pipe is easy to scale, and it needs to be cleaned once in about 10 days, and it needs to be descaled in time.

(2) The efficiency is increased, and the steam utilization rate is low. When the number of effects increases, the heat transfer temperature difference loss of each effect increases, such as the ratio of steam consumed per evaporation of 1t of water 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.

There are three common technical problems and countermeasures of multi-effect evaporation MED

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

1. Solve the problem of blistering in the device

Physical defoaming mainly includes high temperature and low temperature defoaming method, sonic defoaming method, liquid spray defoaming method and mechanical vibration method. Although physical defoaming is effective in the case of particularly large processing capacity, its device and its operating cost are high;

The chemical defoaming method mainly refers to the use of defoamers, but the use is affected by the high price, high production cost and complex production process of defoamers.

The mechanical defoaming method mainly uses rotation to change the pressure and shear force acting on the bubble to achieve defoaming, which is more popular because of its low cost and good defoaming effect.

2. Solve the scaling problem of the evaporator

Some researchers have pickled and neutralized the scale sample (sodium sulfate and calcium carbonate) on the outer wall of the evaporator, and pickled the scale sample (calcium carbonate) on the inner wall of the final heat exchanger, and the hanging piece analysis found that the average corrosion rate of each effect hanging piece was less than 1g/m2·h, and the total corrosion amount was less than 10g/m2.

3. Solutions to the problem of saline ion terminal effect steam corrosion equipment

Low chloride ion content condensate can be used for low-temperature, timed, quantitative replacement and replenishment, and high-efficiency corrosion inhibitors can be added to the circulating water.