|technology > heating, cooling > freeze drying|
Active Freeze Drying - The new standard
Freeze drying (Lyophilisation) is a technique where a solvent (normally water) is being removed under very low pressure conditions. Unlike vacuum drying - with pressures down to 20 mbar - freeze drying is performed at pressures < 6 mbar.
|Parameters influencing the Freeze Drying process|
|Much higher capacities|
|No handling / no contamination|
|Active Freeze Drying applications|
The reason is physically: below 6 mbar water does not exist anymore as a liquid; it is either a solid or a gas. As the removal of water is accomplished, the water is sublimated, e.g. direct transfer from a solid state into a gaseous state.
The common technique of freeze drying in the Food- and Pharmaceutical industry is by using trays. The liquid to be freeze dried is poured on trays (normally up to 1 m2 and 1-2 cm height). The trays are driven with a trolley into the freeze drying chamber.
The other technique, where trays are applied as well, uses a grid in which dozens of vials are placed. Here each vial contains an exact amount of liquid, thus at a given concentration will end up to an exact amount (mg.) of powder. These vials are closed after operation with a stopper. Upon use an isotonic liquid is injected into the vial creating a ready to use active solution for injection in situ.
Due to the low pressure, the volumes which need to be removed are enormous. At atmospheric pressure 1 kg of water uses approximately 1,7 m3; at 0,1 mbar of pressure this equals to 11.000 m3 (See table below).
So whenever very low pressures are needed, large vacuum pumps are needed or an ice condenser is inserted to reduce the installed vacuum capacity. Also the diameter of the piping (due to large vapor volume flows) might become limiting. Another normal feature is the limiting value of the heat exchange.
|Values of 1 Kg of H2O|
|Pressure||Temperature [oC]||Volume [m3]||H2O / 1m3 [g]|
|1000 mbar||100||± 1.7||588.2|
|100 mbar||46||± 15||66.6|
|10 mbar||7||± 130||7.7|
|1 mbar||-20||± 1100||0.9|
|0.1 mbar||-42||± 11,000||0.09|
case to 6 mbar of course) and consequently will end up with much shorter drying times. For a 3 liter tray freeze drying system sublimation capacities can go up to 0,5 kg H2O/hour. In our Active Freeze Drying system this can be as high as 7 kg H2O /hour !
milling or classifying.
As a consequence of this unique feature, there is not only a significantly reduced handling, but perhaps more importantly there is hardly any contamination possible !
The active freeze drying method is not applicable for freeze drying materials in vials. A further limitation might be the presence of large contents of sugar(s). Here the reduction of the triple point and the eutectic point may cause problems. Please call Hosokawa Micron in case this aspect is of interest to you.
|Classic tray freeze dryer particles||Active freeze dryer particles|
milk derivatives, bacterial cultures, enzymes, yeast, and even soups.
For more information: Hosokawa Micron bv