General Principles of
Freeze Drying (The Lyophilization Process)
This illustrates a very high rate advantageous
to shorter drying times. But, as the product becomes lighter
(the kinetic energy of the vapor is proportional to the square
rate), the crossing of an already dry boundary carries product
in the flow (as much as 10 to 20%), and deposits itself on the
condenser where it is virtually irrecoverable. In addition,
these light product particles can pollute the chamber area,
where sensitive sensing equipment may be located.
When a non-condensable gas is introduced to raise
the pressure level, say to 0.2 torr, the volume occupied by
one gram of water (about 5m3), the speed through the neck of
the container is about 50 m/s. In short, the kinetic energy
of the vapor is divided by 100, and the amount of product entrained
in the vapor flow is reduced to negligible amounts.
With Pressure x Volume
Constant:
For one (1) gram of water at:
- Atmospheric pressure (760 torr) we derive:
760 x 22.4 (l) 1000
- At P=1 torr, V=1000 l or 1m3
- At P=10-1, V=1000, = 10,000 l, or 10m3
- At P=10-2 V=100m3
- At P=10-3 V=1000m3
Pressure Vapor Velocity, Vapor Velocity,
Kinetic Energy
- Product container neck vapor (1/2mV2)
- 2x10-2 torr 17.5 m/s 500 m/s 100X
- 2x10-1 torr 1.75 m/s 50 m/s 1000X
- At the end of the sublimation phase, there
is sufficient time to re-establish very low pressure to create
the pressure differential conditions needed for terminal drying.
The Role of the Condenser:
Given the thousands of cubic meters of water vapor
emitted from the product in a freeze dryer, no vacuum pump,
regardless of rate, could remove these volumes directly. Therefore,
a vapor trap, or condenser is essential to condense the evolving
water molecules.
As the role of the vacuum pump is limited to
eliminating the tiny traces of non-condensable gases, one could
conclude that the real vacuum effect is derived from the ice
condenser (via the pressure differential created by the surface
temperature of the product and the condenser).
If you examine the curve of Fig 4, you can note
that each temperature value for condensed ice corresponds to
a vapor pressure, the value of which is always lower than the
chamber pressure.
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