Martin Christ Beta 2-8 LDplus - Manual (Page 22)

22 General Information on Freeze-drying Operating Manual Freeze Dryer BETA 1-8 LDplus / BETA 2-8 LDplus
With increasing pressure (not vacuum!) the rate of sublimation
rises and the drying period is shortened.
The water vapour generated during the main drying phase is not
pumped off by the vacuum pump but collected by the ice
condenser.
The purpose of the vacuum pump is to lower the partial pressure
of the non-condensable gases so that the water vapour can be
transported from the product to the ice condenser.
During the main drying phase the moisture is removed by
sublimation, during final drying the bound moisture is removed by
desorption.
The residual moisture of the dried product depends mainly
on:
the temperature of the dried product during the final
drying process,
the final vacuum reached during the final drying
process.
The end of the main drying phase is reached, when the product
temperature is nearly the same as the shelf temperature
(temperature difference between shelf and product approx. 3 K to
5 K). If the adsorptively bound water is to be removed from the
product, the final drying phase can be started.
The following picture shows the drying process for a product
containing approx. 10 % solid matter. During the first quarter of
the main drying phase 50 % of the water content is condensed.
During the next quarter of the main drying phase 50 % of the
remaining water content is condensed. This continues until the
drying curve approaches the time axis asymptotically. This typical
drying curve is due to the fact that the area of sublimation recedes
into the product and the water vapour still to be extracted has to
pass through the already dried layers. During the drying process
the inner resistance increases. Thus the drying curve is primarily
determined by the latent heat of sublimation and the water vapour
transport speed. In order to increase the specific heat conduction
properties of the product to be dried and to keep the water vapour
volume as low as possible it is necessary that drying takes place
as close as possible to the solidification point (eutectic point).