Freeze-dryer Alpha 1-4 LDplus
Freeze-dryer Alpha 2-4 LDplus
2 Layout and mode of operation
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Version 11/2006, Rev. 2.2 of 17/10/2016
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Translation of the original operating manual
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2.2
Mode of operation
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2.2.1
General information on freeze-drying
What is freeze-drying?
Freeze-drying or lyophilisation is a procedure for the gentle drying of high-
quality products. The product is dried by
sublimation
without passing
through the liquid phase.
What are typical applications for freeze-drying?
As far as their sheer quantity is concerned, foodstuffs are the major
application for freeze-drying. One widely known example is the production
of granulated instant coffee or the drying of fruit, e.g. for breakfast cereals.
Other areas of application are the restoration of water-damaged documents
or the drying of archaeological artefacts.
Another important area of application is the drying of biotechnological and
pharmaceutical products, e.g. tissues and tissue extracts, bacteria,
vaccines, and sera. Products that would not keep well when they are
dissolved in water can be preserved by freeze-drying. During this process,
the biological properties of these sensitive substances are preserved. The
compounds remain unchanged from a qualitative and quantitative point of
view. After the addition of water, the products will have the same
characteristics as the original products.
How does freeze-drying work?
Freeze-drying is a very gentle procedure for the extraction of water from a
product in the frozen state. The drying process takes place through
sublimation
, i.e. the direct transition of a product from the solid phase to the
gas phase. This happens under vacuum.
The following section describes the process of sublimation based on the
example of water, since most products that are processed by freeze-drying
are aqueous solutions. Their behaviour is based on identical fundamental
principles.
The vapour pressure curve above ice describes the phase transition as a
function of the pressure and temperature. The higher the temperature is,
the higher the vapour pressure.
• If the vapour pressure is higher than 6.11 mbar (A), water passes
through all three phases: solid, liquid, and gas (see the illustration).
• If the vapour pressure is below 6.11 mbar (B) and energy is added, the
ice will be directly converted into water vapour once the sublimation
curve is reached.
This transition is called “sublimation”. If thermal
energy is added to pure ice with a temperature of less than
–30°C at a
pressure of 0.37 mbar, it will be converted into water vapour once it
reaches
–30°C (see figure).
The vacuum prevents the melting of ice when energy is added. If thermal
energy is added to a frozen product under vacuum, thawing of the product
will be prevented and the water that is contained within the product will be
released in the form of water vapour.
