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Low 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
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.
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.

  1. Vacuum dryer
  2. Vacuum filter
  3. Condenser
  4. Receptacle
  5. Vacuum pump
  6. Optional condenser
  7. Optional receptacle
  8. Heating cooling skid
  9. Control cabinet
  10. Operating panel

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
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 bv

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Hosokawa Micron
Specialist in POWDER PROCESSING TECHNOLOGY: Mixing - Drying -...