Falling film evaporator

Falling film evaporation is to add the feed liquid from the upper tube box of the heating chamber of the falling film evaporator, evenly distribute it to each heat exchange tube through the liquid distribution and film-forming device, and form a uniform film to flow from top to bottom under the action of gravity, vacuum induction and air flow. During the flow process, it is heated and vaporized by the shell side heating medium. The generated steam and liquid phase enter the separation chamber of the evaporator together. After full separation, the steam enters the condenser for condensation (single effect operation) or the next effect evaporator as the heating medium, so as to achieve multi effect operation. The liquid phase is discharged from the separation chamber.

Evaporation is a unit operation of concentrated solution. Usually, the solvent can volatilize, while the vapor pressure of most solutes approaches zero and cannot volatilize. Evaporation is the process in which a solution is evaporated to remove part of the solvent and concentrate it under boiling conditions. In most cases, the evaporator water vapor is used as the heating medium (usually referred to as heating steam, primary steam or fresh steam) to indirectly transfer heat to the solution through the metal wall. After the solution is heated, the solvent boils and vaporizes, and the generated steam (also water vapor in most cases) is called secondary steam.

The vertical falling film evaporator and falling film reboiler are shown in Figure 1 below. The feed liquid enters the feed liquid distributor from the top. The material liquid distributor evenly distributes the material liquid into each heating tube and makes it flow down the inner wall of the tube in a film shape. The liquid film is vaporized by the heat transferred from the tube wall. When the heat transfer temperature difference is small, vaporization occurs on the inner surface of the strongly disturbed film, rather than at the interface between the heating tube and the liquid film (i.e. the inner surface of the heating tube), so it is not easy to scale. The steam produced usually flows down in parallel with the liquid film. Because the vaporization surface is large, the entrainment amount of liquid foam in the steam is small, and the material liquid flows in a film on the inner wall of the pipe, which does not fill the entire section of the pipe, so the amount of material liquid passing through can be small.

The material liquid distributor is the key component of the falling film evaporator. The heat exchange intensity and production capacity of the falling film evaporator essentially depend on the uniformity of the material liquid distribution along the heat exchange tube. The so-called uniform distribution not only means that the liquid should be evenly distributed in each pipe, but also evenly distributed along the whole periphery of each pipe, and its uniformity should be maintained over the length of the whole pipe. When the feed liquid can not uniformly wet the inner surfaces of all heating tubes, the surfaces with insufficient or little liquid may be scaled due to evaporation, and the scaling surface in turn blocks the flow of the liquid film, thus further worsening the heat transfer conditions in the adjacent areas.

technological process