Chapter 1: Introduction
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Original instructions
Freeze Dry Rates
The efficiency of the Freeze Drying process is dependent upon the surface area
and the thickness of the sample, the collector temperature and vacuum obtained,
the eutectic point and solute concentration of the sample. It is important to
remember these factors when trying to obtain efficient utilization of your Freeze
Dry system. A listing of selected materials and their approximate drying times
are shown in Table 1 for your reference.
SAFE TEMPERATURE AND DRYING TIMES
FOR SELECTED MATERIALS
Material
10mm Thick
Safe
Temperature
°C
Collector
Temperature
°C
Hours
(Approx.)
Milk
-5
-40
10
Urea
-7
-40
10
Blood Plasma
-10 to -25
-40
16
Serum
-25
-40
18
Vaccinia
-30 to -40
-50
22
Influenza Vaccine -30
-50
24
Human Tissue
-30 to -40
-50
48
Vegetable Tissue
-50
-80
60
*Total sample quantities are contingent on various freeze dryer capacities.
Up to the point of overloading the system, the greater the surface area of the
sample, the faster the rate of freeze drying. By contrast, for a given surface area,
the thicker the sample the slower the rate of freeze drying. This is based on the
fact that the heat of sublimation is usually absorbed on one side of the frozen
sample and must travel through the frozen layer to vaporize water at the other
surface. In addition, as the sample is freeze dried, the water vapor must travel
through the layer of dried material. The thicker the sample, the greater the chance
that the dried layer may collapse which would cause an additional decrease in the
rate of freeze drying.
The surface area and thickness of the sample can usually be ignored when each
sample contains only a few milliliters. However, for larger volumes, the samples
should be shell frozen to maximize the surface area and minimize the thickness of
the sample. The volume of the freeze dry flask should be two to three times the
volume of the sample.
In order for lyophilization to occur, ice must be removed from the frozen sample
via sublimation. This is accomplished by the collector and the vacuum pump.
The collector, which should be at least 15 to 20°C colder than the eutectic
temperature (melting temperature) of the sample, traps vapor as ice. Since the
vapor pressure at the collector is lower than that of the sample, the flow of water
vapor is from the sample to the collector. Since this vapor diffusion process
occurs very slowly under normal atmospheric conditions, a good vacuum is
Table 1
