Operating Manual Freeze Dryer
ALPHA 1-4 LD-2 / 2-4 LD-2
, Version 0203, Page 27
4
General Information on Freeze Drying
Freeze drying is the most gentle process for drying biological and chemical products. It
is based on the physical phenomenon of sublimation i.e. the direct conversion from
solid to gaseous state. The frozen product is placed in the vacuum chamber for drying.
The ice condenser can also be described as a vapour pump as the moisture which
evaporates under vacuum during drying freezes onto the ice condenser. Consequently
the vacuum pump is only intended to remove the air from the drying chamber (=gas
pump) but not the vapour. In order to start the sublimation process, heat must be
supplied to the product. This takes place during drying in round bottom flasks or wide-
neck closed filter bottles etc. due to the much warmer environment (direct heat
contact), on unheated shelves by means of heat radiation from the environment and
directly by means of the shelves when heatable. Once the
“
free water
”
has been
removed from the product, it is also possible to remove the crystalline bound water by
means of very low vacuum. This part of the drying process is referred to as final drying
(desorption).
Construction of the freeze dryer
The components of a freeze dryer are :
•
Drying chamber and heating accessories
a
heatable and unheatable shelves for drying in dishes
b
shelves with sealing device for drying in bottles
c
rubber valves for connecting round bottom flasks, wide-neck filter
bottles, etc.
d
manifold for connecting round bottom flask, wide-neck filter
bottles, etc.
•
Pumps for air and water vapour
a
vacuum pump to evacuate the drying chamber (= gas pump)
b
ice condenser with temperatures from -50
°
C to -110
°
C (depending on
type of unit) to condense the water vapour (= vapour pump ).
Sublimation
The principle of sublimation is briefly explained using the phase diagram of water
(freeze drying of mainly aqueous solutions, see vapour pressure curve).
If the atmospheric pressure is higher than 6.11 mbar, water passes through all three
phases (solid, liquid, gas) when the temperature is lowered or raised. At 6.1 mbar the
melting pressure curve, vapour pressure curve and sublimation pressure curve meet in
one point, the triple point. At this point all three phases occur in parallel
(simultaneously). Below this point, i.e. the vacuum is lower than 6.11 mbar, the ice is
converted directly from a solid to a gaseous phase on reaching the sublimation
pressure curve.
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