Martin Christ Beta 2-8 LSCplus - Manual (Page 19)

Beta 1-8 LSC&
Beta 2-8 LSC&
2 Layout and mode of operation
Version 04/2013, Rev. 1.1 of 16/12/2013 • sb
19
Translation of the original operating manual
Fig. 10: Shell freezing
Fig. 11: Cooling bath with spin freezing device Fig. 12: Spin freezing
NOTE
If the product that is to be dried contains solvents or high salt
concentrations, it may start to thaw during the drying process, which is
indicated by clearly visible foaming. In order to prevent this, the product
must be frozen as deeply as possible, e.g. with the aid of liquid nitrogen,
prior to putting it into the unit.
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2.2.2.3 Main drying
When the product is frozen to a sufficiently deep extent, the main drying
phase commences. The vacuum pump is switched on. The pressure inside
the drying chamber will be lowered to the value that corresponds to the
freezing temperature in accordance with the vapour pressure curve above
ice. At the same time, thermal energy will be added to the product. In the
case of products in round-bottom flasks, wide-neck bottles, etc., this is
realised through the environment that is considerably warmer (direct
contact heat), in the case of unheated shelves by way of thermal radiation
from the environment, and in the case of temperature-controlled shelves
directly via the shelves. As a result, the sublimation process starts.
At the beginning of the drying process, the maximum drying rate will be
reached. The more the sublimation area recedes into the product, the
further the produced water vapour must pass through the layers that have
already been dried.
Under certain conditions, it is possible that the vacuum inside the ice
condenser chamber increases during the main drying phase (e.g. from 0.63
mbar to 0.47 mbar) although the valve towards the vacuum pump is closed.
From a physical point of view, this is due to the pumping effect of the ice
condenser ("cryo-pumping effect").
The required drying time depends strongly on the drying vacuum. At 1.0
mbar, one gram of ice takes up a volume of 1 m
3 of vapour, at 0.1 mbar a
volume of 10 m 3 of vapour, and at 0.001 mbar a volume of 100 m 3 . The
closer the vacuum is to the solidification point, the smaller is the resulting
vapour volume. The drying rate increases and the drying time decreases.