Evaporation Process Principles : Evaporation is the process used to concentrate a solution by removing the solvent (mainly water) in a purified form by the application of heat.
The evaporation process starts with a dilute solution and ends up with a more concentrated liquid. There are very few instances where the evaporated solvent is the required product.
Evaporator Process Applications
- Product concentration
- Dryer feed pre-concentration
- Solvent Volume reduction in a solution
- Water / solvent recovery
Evaporator Systems types
- Falling Film Evaporators
- Rising Film Evaporators
- Forced Circulation Evaporators
- Thermal and Mechanical Vapor Recompression (TVR & MVR)
Falling Film Evaporators : In falling film evaporators, liquid and vapors flow downwards in parallel flow. The liquid to be concentrated is preheated to boiling temperature. An even thin film enters the heating tubes via a distribution device in the head of the evaporator, flows downward at boiling temperature, and is partially evaporated. This gravity-induced downward movement is increasingly augmented by the co-current vapor flow.
Forced Circulation Evaporators: Forced circulation evaporators are used if boiling of the product on the heating surfaces is to be avoided due to the fouling characteristics of the product, or to avoid crystallization. The flow velocity in the tubes must be high, and high-capacity pumps are required.
The circulating liquid is heated when it flows through the heat exchanger and then partially evaporated when the pressure is reduced in the separator, cooling the liquid to the boiling temperature corresponding to this pressure.
The heat exchanger can be arranged either horizontally or vertically depending on the specific requirements in each case.
Rising Film Evaporators : These operate on a "thermo-siphon" principle. Feed enters the bottom of the heating tubes and as it heats, steam begins to form. The ascending force of this steam produced during the boiling causes liquid and vapors to flow upwards in parallel flow. At the same time the production of vapor increases and the product is pressed as a thin film on the walls of the tubes, and the liquid rises upwards.
Multiple Effect Evaporators with Thermal Vapor Recompression (TVR): In multiple-effect evaporators the heating medium in any subsequent effect is the vapor generated in the previous effect and thereby achieves a great steam economy.
The purpose of TVR in a multiple effect evaporator is to compress the product vapor from an effect, to a higher temperature level by means of a steam ejector (TVR) to use it as the heating medium in that effect itself thereby reducing the prime steam consumption.