SP VirTis BenchTop 2K, Operator'S Manual

Page: 19 / 56

Chapter
3
Revision 004, 12/09
9
© SP Industries, Inc. 2009
Freeze-Drying Concepts
The first step of lyophilization (freeze-drying) is typically to freeze the product to a
minimum of -40 °C. After the product is frozen, moisture can be extracted through
sublimation. The sublimation process takes a solid (
i.e. , ice) and converts it to a gas
( i.e. , vapor), bypassing the liquid stage. The moisture is converted to a gas, while
maintaining the material’s crystalline structure.
The second step involves the migration of moisture from the product. BenchTop
series freeze dryers facilitate the migration of moisture by using a vacuum to extract
non-condensable vapors in the product chamber or container. The freeze dryer
creates a low-pressure zone to allow moisture to flow easily from the product. Since
vapor naturally travels toward cooler surfaces, the condenser chamber is cooled to at
least 15 °C below that of the product to attract product moisture.
Many products require a secondary drying process to remove moisture that did not
convert to vapor during the first process. During secondary drying, BenchTop units
remove the remaining bound moisture by slightly warming the product.
Note: To prevent meltback, the product should not be warmed above its eutectic point until all
free ice has sublimated.
Elements of Freeze-Drying
Freeze
The product must be completely frozen before freeze-drying can begin. Product
freezing can be accomplished directly in the condenser chamber, or through pre-
freezing methods such as dry-ice baths or a laboratory freezer. The product’s frozen
temperature should be at least -40 °C. To minimize freeze-drying time and protect the
product against meltback, the product should be frozen in the thinnest cross section
possible.
Vacuum
Adding vacuum assists in removing air and other non-condensable vapors from the
chamber to facilitate vapor migration. The vacuum system creates a nearly pressure-
free environment to allow the vapor from the frozen product to flow toward the cold
condenser surface easily.