back to top
| technology > heating, cooling > vacuum drying |
download articleLow temperature multi-purpose evaporation process
Article index
 | Key factors | |
| 1. Pressure | |
| 2. Temperature | |
| 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.
|
 |
Key factors
Vacuum drying cycle times are depending, beside product characteristics, on the following key factors:1. Pressure
The 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)
2. Temperature
The temperature of the dryer 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.3. Surface area
In a conical dryer, the surface area of the heated wall is less than proportional to the volume, so the drying time becomes shorter in a smaller dryer.4. Heat transfer coefficient
The heat transfer coefficient is a product depending parameter. Wet products can have 4 to 6 times bigger heat transfer coefficient than dry materials. For dry products the heat transfer coefficient is 25-30W/m2 /ºC. Since solvents other then water needs less input heat to evaporate, the drying capacity for these solvents can be much higher. For example the evaporation capacity of ethanol or methanol instead of water is twice as much. With acetone and ethyl acetate, the evaporation capacity will be even 4 times higher ( Table: physical properties various solvents )| Freezing point | boiling point |
Heat of vapourization |
||
| °C | °C | kJ/kg |
 |
|
| Water | 0 | 100 |
2250 |
|
| Ethanol | -114 | 80 | 853 | |
| Acetone | -94 | 57 | 563 | |
| Ethylacetate | -82 | 77 | 426 | |
| Methanol | -98 | 65 | 1128 |
The right rotor brings you the manufacturing process of tomorrow
One of the important design aspects of a vacuum dryer itself is the agitator. As every rotor has its own characteristics, it is essential to understand what is expected from the vacuum drying process. A large number of aspects have to be taken into account e.g. Process aspects, Cleaning hygienic, Attrition, Practical experience before being able to make the right choice. |
| Different type of agitators of Hosokawa vacuum dryers |
For more information: Hosokawa Micron B.V.
companies
