toxic and hazardous substances, such as solvents, since they are operated as a closed system.
Vertical thin film dryer are contact dryers with minimum energy consumption and self-cleaning heating surfaces.
Thin film dryers are characterised by a mechanically agitated thin product layer. The film thickness is normally in the range of less than one up to a few millimetres. In vertical thin film dryers the wet feed is distributed over the heated wall by a distributing ring and evenly applied to it in a thin film by hinged pendulum blades. Running downward the product passes typically through an evaporation zone, then a slurry or crystallisation zone and finally a powder zone. Most of the liquid is removed in the evaporation and slurry zone while in the powder zone the surface moisture from the solid particles and some of the internal moisture is driven off. The volatile components driven out of the product by heat, pass upward through the dryer counter-current to the product flow and are then condensed in an external condenser. The powder obtained at the bottom is discharged continuously via a suitable gaslock system. The total residence time of the product is between 30 and 60 seconds.