Martin Christ Alpha 2-4 LDplus User Manual
Page 20
20 General Information on Freeze-drying
Operating Manual Freeze Dryer ALPHA 1-4 LDplus / ALPHA 2-4 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).