With the increasing maturity and development of evaporation crystallization technology, its application in various industries has become very common. The evaporator is a major equipment in the evaporation crystallization system. Evaporators have different principles and operating methods according to different classification methods.

• Classification of evaporators •

Evaporators can be classified according to evaporation of methods It is divided into natural evaporation and boiling evaporation. A typical application of natural evaporation is solar salt production, where the solution evaporates below the boiling point. Industrial evaporation operations are basically boiling evaporation, where the solution is heated to the boiling point and evaporated in a boiling state.

By adding heat methods, evaporators can be divided into direct heat source evaporation and indirect heat source evaporation. The latter requires a heat transfer process and is more widely used in industry.

By Operating pressure classification, evaporators can be divided into atmospheric pressure, pressurized, and reduced pressure (vacuum) evaporators. Heat-sensitive materials should be evaporated under reduced pressure, while high-viscosity materials should be evaporated under pressure.

Evaporators according to evaporator number of effects can be divided into single-effect and multiple-effect evaporators. If the secondary steam produced by evaporation is directly condensed and not reused, it is called single-effect evaporation. If the secondary steam is used as the heating steam for the next effect, and multiple evaporators are connected in series, this evaporation process is called multiple-effect evaporation.

By The solution stays in the evaporator can be divided into circulating evaporators and single-pass evaporators. In circulating evaporators, the solution circulates in the evaporator. According to the different causes of circulation and different operations, they can be further divided into central circulation tube evaporators, externally heated evaporators, and forced circulation evaporators.

• Common evaporators •

Central circulation tube evaporator As the name suggests, its heating chamber consists of a vertical bundle of heating tubes, and there is a larger diameter tube in the center of the tube bundle, called the central circulation tube, whose cross-sectional area is generally 40~100% of the total cross-sectional area of the heating tube bundle. The advantages of this type of evaporator are its compact structure, easy manufacturing, good heat transfer, and reliable operation. The disadvantages are that the circulation speed is below 0.4~0.5m/s, and cleaning and maintenance are inconvenient.

 

Central circulation tube evaporator

 

Externally heated evaporator Its structural characteristics are that it uses long heating tubes, and the heating chamber is separated from the separation chamber, which not only facilitates cleaning and replacement but also reduces the total height of the evaporator. Because its heating tubes are longer, and because the solution in the circulation tube is not heated, the circulation speed of the solution is high, reaching 1.5m/s.

Forced circulation evaporator is a more commonly used evaporator now. In order to increase the circulation speed, a pump is used for forced circulation, which is more necessary for materials with high viscosity, easy crystallization, and easy scaling. The evaporation circulation speed can reach 1.8～5m/s, and the size can be adjusted, and the heat transfer coefficient is large. The heating chamber of this type of evaporator can be vertical, horizontal, or a plate heat exchanger.

Forced circulation evaporator

 

Single-pass evaporator Its characteristic is that the solution passes through the heating chamber once in the form of a liquid film without circulation. After the material passes through the heating chamber, it evaporates to the specified concentration. Its advantages are that the solution residence time is short, so it is especially suitable for evaporating heat-sensitive materials; the temperature difference loss is small, and the surface heat transfer coefficient is large. Its disadvantage is that it is not easy to form a film if the design or operation is improper, and the heat flow will decrease significantly. According to the different film forming methods, this type of evaporator can be divided into rising film evaporators and falling film evaporators.

Single-pass evaporator

 

Rising film evaporator The heating chamber consists of one or several vertical long tubes, and the raw material liquid is preheated and then enters from the bottom of the evaporator, and the heating steam condenses outside the tube. When the solution is heated and boils rapidly, the generated secondary steam rises rapidly in the tube, driving the liquid to flow upward in a film along the inner wall of the tube, and the rising liquid film continues to evaporate due to heating. Therefore, the solution is gradually concentrated during the process of rising from the bottom to the top of the evaporator, and the concentrated solution enters the separation chamber and is separated from the secondary steam, and then discharged from the bottom of the separator. Its main performance characteristics are suitable for evaporating large amounts (i.e., dilute solutions), heat-sensitive and easily foaming solutions, but not suitable for high-viscosity, crystallizing, or scaling solutions.

Falling film evaporator The difference from the rising film evaporator is that the raw material liquid is added from the top of the heating tube. The solution flows down in a film along the inner wall of the tube under its own gravity and is evaporated and concentrated, and the gas-liquid mixture enters the separation chamber from the bottom of the heating tube, and after gas-liquid separation, the finished liquid is discharged from the bottom of the separator. In order to make the solution form a uniform film on the wall, a liquid distributor must be set at the top of each heating tube. There are many types of distributors, and the three most commonly used ones are: using a cylindrical body with a spiral groove as a guide tube, the liquid rotates and flows down along the groove and is distributed on the entire inner wall of the tube; the lower part of the guide tube is a cone, and the bottom of the cone is concave inward to prevent the liquid flowing down along the cone slope from gathering in the center again; the liquid forms a film along the heating tube inner wall through the tooth gap and descends. The main advantages of the falling film evaporator are its high heat transfer coefficient, and compared with the rising film, it can evaporate solutions with higher concentrations, and it is also suitable for materials with higher viscosity. Its main disadvantages are its complex structure and high requirements for installation accuracy.