Chapter 1: Introduction
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2
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.
Table 1
