Thin Film Evaporation

Image courtesy of LCI Corporation

Evaporators are used to separate the components of a process liquid based on volatility. The basic process involves applying heat to a liquid which causes it to evaporate, thereby leaving the less volatile substance behind. The more volatile – and now gaseous – substance is then condensed and collected at a separate location.

Traditionally, batch or continuous types of evaporators such as natural/forced circulation, falling and rising film, and climbing and falling plate evaporators have been used successfully with a range of process liquids. However, these evaporators have proven to be inefficient with high-boiling, viscous, fouling, and heat sensitive liquids.

The design principle of thin film evaporators allows them to successfully separate difficult-to-handle products. This method uses indirect heat transfer and mechanical agitation to evaporate a thin layer (0.1 mm to 1.0 mm) of flowing substance under controlled conditions.

HOW DO THIN FILM EVAPORATORS WORK?

The key to the efficiency of thin film evaporators is their ability to evaporate the target solution at relatively low temperatures with minimum residence time in the evaporator. This is due to operating the evaporator under vacuum conditions and the high speed rotor creating a thin liquid film.

The main component of the thin film evaporator consists of a rotor/wiper encased in a cylindrical heating jacket. The process can be broken up in several stages:

1. The product first enters above the heated zone where it is evenly distributed against the inner wall of the heating jacket by the rotating wipers.
2. A rotor then forms a thin turbulent layer of liquid, which creates ideal heat flux and mass transfer conditions.
3. The end result is rapid evaporation of the more volatile substance, which is then expelled from the heating cylinder to be condensed or subjected to further processing.
4. The less volatile materials are collected and sent to the discharge nozzle.