what is falling film evaporator?

Falling film evaporation is the material liquid from falling film evaporator heating room on the tube box to join, through the liquid distribution and film-forming device, uniform distribution to each heat exchanger tube, in gravity and vacuum induced and air flow, into a uniform film flow from top to bottom. During the flowing process, it is heated and vaporized by the shell process heating medium, and the steam generated enters the separation chamber of the evaporator together with the liquid phase, and the steam and liquid are fully separated, and the steam enters the condenser to condense (single effect operation) or enters the next effect evaporator as heating medium, thus realizing multi-effect operation, and the liquid phase is discharged from the separation chamber.

brief introduction

Evaporation is the unit operation of a concentrated solution, usually the solvent can be volatile, while the vapor pressure of most solutes tends to be near zero and cannot be volatile. The process of concentrating a solution under boiling conditions by vaporizing it and excluding some of the solvent is called evaporation. In most cases the evaporator uses water vapor as the heating medium (usually called heating steam, primary steam or fresh steam), and indirectly transfers heat to the solution through the metal wall. After the solution is heated, the solvent boils and vaporizes, and the resulting steam (in most cases, also water vapor) is called secondary steam.

Vertical falling film evaporator and falling film reboiler are shown in figures (a) and figure (b) below. The feed liquid enters the liquid distributor from the top. The liquid distributor distributes the liquid evenly to each heating tube and makes it flow down along the tube wall in the form of a film, and the liquid film is evaporated by the heat transferred from the tube wall. When the heat transfer temperature difference is not large, 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 form scale. The steam generated is usually flowing downward in parallel with the liquid film. Since the vaporization surface is large, the amount of liquid foam entrained in the steam is small, and the liquid material flows in a film-like pattern on the inner wall of the tube, not filling the entire cross section of the tube, so the amount of liquid material passing through can be small.

The heat exchange strength and production capacity of the falling film evaporator depend on the uniformity of the liquid distribution along the heat exchange tube. The so-called uniform distribution not only means that the liquid should be evenly distributed to each tube, but also uniformly distributed along the whole periphery of each tube and keep its uniformity over the whole length of the tube. When the liquid does not uniformly wet all the inner surface of the heating tube, the lack of liquid or less liquid surface may be scaled due to steaming dry, and the scaled surface in turn blocks the flow of the liquid film thus further deteriorating the heat transfer conditions in the adjacent area.

Process flow

The process flow has four forms: downstream (parallel flow), counter flow, mixed flow (wrong flow), and advection.

Downstream

Solution and steam flow in the same direction, both from the first effect of the sequence of flow to the final effect. The raw material is pumped into the first effect and automatically flows into the next effect depending on the pressure difference between the effects, and the finished solution is pumped out from the last effect (usually operated under negative pressure). As the pressure of the latter effect is low, the boiling point of the solution is also low, and the solution will flash some water when it enters the latter effect from the former effect, and the secondary steam produced is also more. Downstream process is generally suitable for handling heat-sensitive materials in the case of high concentration.

Counter flow

The raw material is sent from the end effect to the front effect by the pump in turn, and the finished liquid is discharged from the first effect, and the liquid and steam flow in the opposite direction. Generally suitable for processing the solution whose viscosity changes with temperature and concentration, not easy to deal with heat-sensitive materials.

Mixed flow

It is a combination of the flow with and against the flow and has the advantages of the flow with and against the flow to avoid its disadvantages, but the operation is complex and requires a high degree of self-control.

Flat flow

Each effect is charged with the finished liquid, each effect has crystallization precipitation, can be separated in time crystallization, generally used for the evaporation of saturated solutions.

Features

1) The material liquid of falling film evaporator is added from the top of the evaporator, and it descends along the wall of the tube in the form of film under the action of gravity and evaporates and thickens in the process, and the concentrated liquid is obtained at its bottom. The falling film evaporator can evaporate higher concentration and larger viscosity materials.

2) As the solution flows in a single process evaporator in the form of membrane, the heat transfer coefficient is high.

3) Short retention times, not easy to cause material deterioration, suitable for handling heat-sensitive materials.

4) The liquid retention is small, and the falling film evaporator can take fast operation according to the change of energy supply, vacuum, feed quantity, concentration, etc. Near constant,

5) Since the process fluid flows only under the action of gravity and is not driven by high temperature difference, low temperature difference evaporation can be used.

6) Falling film evaporators is suitable for evaporation and concentration of foaming materials, because the material liquid evaporates in the heating tube in the form of film, that is, the formation of vapor-liquid separation, at the same time, at the bottom of the effect body, most of the material liquid will be pumped away, only a small part of the material liquid and all the secondary steam into the separator to strengthen the separation, the whole process of material liquid does not form too big impact, to avoid the formation of foam.

Scope of application

The falling film evaporator is widely used for evaporation and concentration of water or organic solvent solution in pharmaceutical, food, chemical, light industry and other industries, and can be widely used for liquid waste treatment in the above industries. Especially suitable for heat-sensitive materials, the equipment is operated continuously under vacuum and low temperature conditions, with high evaporation capacity, energy saving, low operating costs, and can ensure that the material does not change in the evaporation process.

Wenzhou Ace Machinery Co., Ltd. is one of the manufacturers of falling film evaporator in China. If you have any need, you can contact us for customization and technical support.