Evaporation is a unit operation for concentrating solutions. Usually, solvents can evaporate, while most solutes have vapor pressures close to zero and cannot evaporate. Under boiling conditions, the solution removes some of the solvent by vaporization, and the process of concentration is called evaporation. In most cases, water vapor is used as the heating medium for the evaporator (usually called heating steam, primary steam, or fresh steam), which indirectly transfers heat to the solution through the metal wall. After being heated, the solvent boils and vaporizes, producing vapor (mostly water vapor), which is called secondary steam.
The material liquid enters the liquid distributor from the top. The liquid distributor distributes the material liquid evenly to each heating tube and makes it flow down the tube wall in a film shape. The liquid film evaporates and vaporizes under the heat transferred from the tube wall. When the heat transfer temperature difference is small, the vaporization occurs on the inner surface of the film, rather than at the interface between the heating tube and the liquid film (that is, the inner surface of the heating tube), making it less susceptible to scaling. The produced vapor usually flows down in parallel with the liquid film. Due to the large vaporization surface, the amount of liquid droplets carried in the steam is less, and the material liquid flows in a film shape on the tube wall, not filling the entire cross-section of the tube, so the amount of liquid passing through can be small.
The liquid distributor is a key component of the falling film evaporator, and the heat exchange intensity and production capacity of the falling film evaporator depend essentially on the uniformity of the liquid distribution along the heat exchange tubes. The so-called uniform distribution not only refers to the liquid being evenly distributed to each tube but also uniformly distributed along the entire circumference of each tube and maintained its uniformity throughout the length of the tube. When the liquid cannot wet the entire inner surface of the heating tube uniformly, the shortage or deficiency of the liquid surface may cause scaling due to evaporation. The scaling surface, in turn, hinders the flow of the liquid film, further deteriorating the heat transfer conditions in the adjacent area.
The process flow of the falling film evaporator has four forms: concurrent (co-current) flow, countercurrent flow, mixed flow (cross-flow), and parallel flow:
Co-current Flow
The solution and steam flow in the same direction from the first effect to the last effect. The feed material is pumped into the first effect and automatically flows into the next effect based on the pressure difference between the effects. The material liquid is then pumped out from the last effect (usually operated under negative pressure). As the pressure of the last effect is low, the boiling point of the solution is also low. When the solution flows from the previous effect into the last effect, it will flash part of the water, and the resulting secondary vapor is also plentiful. As the concentration of the last effect is higher and the operating temperature is lower than in the first effect, the heat transfer coefficient of the first effect is generally much higher than that of the last effect. The co-current process is generally suitable for processing heat-sensitive materials at high concentrations.
Countercurrent Flow
The material is pumped from the last effect to the first effect in turn, and the liquid flows in the opposite direction of steam. It is generally suitable for processing solutions whose viscosity changes significantly with temperature and concentration, but not for processing heat-sensitive materials.
Mixed Flow
A combination of co-current and countercurrent flows, it has the advantages of both and avoids their disadvantages, but the operation is complex and requires a high degree of automatic control.
Parallel Flow
The feeds and complete liquids, and crystals precipitate in each effect, which can separate the crystals in time. Generally used for the evaporation of saturated solutions.
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