Falling Film Evaporators Essay

Falling film evaporators are widely used in many industrial applications for continuous processing of heat-sensitive viscous and fouling products. Such processes include concentration of beverages such as orange juice and milk to reduce transport costs.1 The falling film provides a short contact time between the heated surface and the liquid that's being processed, low-pressure drops during the heating process, high evaporation rate, and high heat transfer coefficient. Short contact time is very critical for the evaporation process because it not only prevents secondary reactions when processing heat sensitive products such as fruit juices, milk, and coffee, but also avoids undesired chemical and physical changes of the products fed in from overheating. In order for the vapor and liquid to be separated in an evaporator, heat must be applied to the liquid feed through a heat exchanger, which will raise the temperature of the liquid to its saturation temperature. In the falling film evaporator, the liquid to be concentrated will be preheated and pumped to the top of the heating tubes and distributed uniformly as a thin film when the liquid flowing down the inside of the tube walls. The downward movement of the product flow caused by gravity, and enhanced by the downward flow of vapor, which is more efficient because there is no driving force limitation. The liquid and vapor formed are separated in a vapor-liquid separator, in which the concentrated liquid is pumped out and water vapor goes to the condenser.2
Falling film evaporator can be used to separate volatile and nonvolatile materials when the feed has low viscosity. For example, the concentration of yeast extract, manufacturing of gelatin, and the extraction of fruit juices and coffee use falling film evaporators. The heat-sensitive product milk can also be concentrated using a falling film evaporator due to its distinct characteristics such as being able to operate at any low temperature difference, having a short residence time, high flow velocity and small deposit on the walls, which benefit the dairy industry in both economically and environmentally. Due to small amount of deposit on the walls, the cleaning period can be shortened with longer operating time. Pre concentrated milk before drying will reduce the operating energy and cost by reducing storage volume for liquid products, which also reduce the transportation and distribution cost for the company. The evaporation process is not only used for improving the quality of the final products, but also used to remove excess moisture, which improves product stability and increase the solid contents. In large companies, evaporation process can be used to cut down waste-handling cost because other methods that are compatible with environmental guidelines are more expensive. Evaporation processes can remove moisture efficiently, which helps the industry to reduce the amount of waste product that must be processed.3
The experimental design allows for the study of the separation efficiency of ethanol and water mixture, and thermal efficiency of water in a falling film evaporator. Separation and thermal efficiency can be predicted through different trials of experimentation.