|technology > heating, cooling > vacuum drying|
Low temperature multi-purpose evaporation process
There are many reasons to choose a batch vacuum dryer above other type of dryers. The toxicity of the materials, the containment measures required, the type of solvents to be evaporated, the risk of explosion under atmospheric conditions, etc. but the first of all we should consider is if the material is heat sensitive.
|3. Surface area|
|4. Heat transfer coefficient|
|The right rotor brings you the manufacturing process of tomorrow|
A vacuum dryer is characterized by its ability to easily remove liquids at low temperatures, typically around 30-40ºC, and to recover the liquids after removal.
Looking at the low drying temperature usually applied, the evaporation capacity of a single vacuum dryer is in the order of 100Kg/h for water evaporation. This will typically correspond to a solid production of about 400kg/h for a 6m3 dryer.
Vacuum drying is still so far a batch operation. This has the advantage to make it extremely flexible and multi-purpose. Making the essential assumption that we choose the right shape of vessel and rotor, a vacuum dryer can handle many materials going from thick slurries to solid shapes like wet powders, filter cakes, flakes, granules and fibrous or crystalline solids.
The liquid removal is a function of heat input, and ability of removal of the formed gases. When more heat input is given, more gases are formed. When more gases are removed, more heat input or more cooling down of the product is the result.
pressure in a vacuum dryer has a range from a few mbar to a few hundred mbar for evaporation of solvents like methanol, acetone. As a general rule, it is most economical to produce a vacuum by passing the vapors first through a surface condenser and then to a vacuum pump. In this way only a small flow goes to the vacuum pump. Most organic solvents have higher vapor pressures than water at the same temperature. So at a certain vacuum, the evaporation temperature of most organic solvents is lower. (see graph below)
wall depends on the maximum allowable temperature permitted by the solids. A water circulating heating system is usual most economical when the wall temperature is below 100 ºC.
|Freezing point|| boiling point
|| Heat of vapourization
|Different type of agitators of Hosokawa vacuum dryers|
For more information: Hosokawa Micron bv