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Multi-effect evaporator is often used in industrial wastewater treatment. The principle of multi-effect evaporation is to operate several evaporators in series, so that steam can be used many times. What is the process and characteristics of multi-effect evaporation?

I. Process of multi-effect evaporation

The process of multi-effect evaporation is distinguished by the flow direction of secondary steam and solution. Generally, the three-effect evaporation operation process is taken as an example. The first-effect evaporator uses raw steam as heating steam, while the other two-effect evaporators, the second-effect evaporator and the third-effect evaporator, use the secondary steam of the former effect as heating steam, thus greatly reducing the consumption of raw steam.

The secondary steam temperature of each effect is always lower than that of other heating steam, so the operating pressure of each effect and the boiling temperature of solution decrease in turn along the direction of steam flow during multi-effect evaporation.

II. Process Classification of Multi-effect Evaporation

The process of multi-effect evaporation can be divided into three types: parallel flow, counter flow and cross flow.

1. Multi-effect evaporation cocurrent flow: Solution and secondary steam pass through each effect in the same direction. Because the former effect pressure is higher than the latter effect, the material and liquid can flow by differential pressure. However, the heat transfer coefficient is low because of the high concentration and low temperature of the final effective solution and the high viscosity of the solution.

2. Countercurrent process of multi-effect evaporation: the direction of solution is opposite to that of secondary steam. It is necessary to pump the solution to the former effect with higher pressure. The influence of the concentration and temperature of each effective solution on the viscosity is almost offset, and the heat transfer conditions of each effect are basically the same.

3. Multi-effect evaporation advection process: the raw material liquid is added parallel to each effect, and the finished liquid is discharged from each effect separately. The flow direction of steam is still from the first effect to the last effect.

4. Cross-flow process of multi-effect evaporation: In a multi-effect evaporation process, the feeding mode can have both parallel flow and counter-flow, which becomes cross-flow method.

III. CHARACTERISTICS OF MULTI-EFFECT EVAPORATION PROCESS

The three kinds of multi-effect evaporation processes have their own advantages and disadvantages. The advantages and disadvantages of multi-effect evaporation process are briefly introduced below.

1. Advantages of the parallel flow process: Solution flows from the evaporator with higher pressure and temperature to the evaporator with lower pressure and temperature. Therefore, the pressure difference between efficiencies can be used to transport the solution between efficiencies without pumps. This process is relatively simple to operate and the process conditions are stable.

The disadvantage of the parallel flow process is that as the solution flows from the former one to the later one, its concentration increases while the temperature decreases, resulting in an increase in the viscosity of the solution and a decrease in the heat transfer coefficient of the evaporator. Therefore, the parallel flow process is not suitable for liquid materials whose viscosity varies greatly with the increase of concentration.

2. Advantages of countercurrent process: As the concentration of solution increases along the flow direction, its temperature also increases. Therefore, the effect of higher concentration on the increase of viscosity is roughly equal to that of lower viscosity due to higher temperature. Therefore, the viscosity of each effective solution is close and the heat transfer coefficients of each effective solution are roughly the same.

The disadvantage of countercurrent process: the flow of solution between effects is from low pressure to high pressure and from low temperature to high temperature. It must be pumped, so the energy consumption is high. In addition, the feed efficiency is lower than the boiling point, and there is no self-evaporation. Compared with the cocurrent method, the amount of secondary steam generated is smaller.

3. Advection process is more suitable for high viscosity and easy crystallization occasions. It can also be used for simultaneous evaporation of two or more different liquids.

4. Cross-flow process operation is more complex, each effect of liquid feed in and out separately, this process is suitable for liquid feed with crystalline precipitation.