Method for controlling a freeze drying process

Details Method for controlling a freeze drying process Method for controlling a freeze drying process

2000-03-20

2000-12-26

Gravini, Stephen

Drying and gas or vapor contact with solids

Process

Freeze-drying

34292, 34 92, 34495, F26B 506

Patent

active

061639796

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The present invention relates to a method for controlling a freeze-drying process. It finds particular application in processes in which a frozen product is arranged on temperature adjustable surfaces in an air-evacuated chamber that is first subject to a main drying phase and subsequently to an after-drying phase. During the main drying phase, the temperature of ice enclosed in the product to be dried is continuously measured. The chamber pressure and/or the temperature of the storage surfaces are modified during a transition from the main drying phase to an after-drying phase.
Freeze-drying is a method for removal of water from a water-containing frozen product, for example from pharmaceutical products or food items. In general, the process is performed at an air pressure which is low vis-a-vis the water vapor pressure at the selected temperature of the ice. For example, an ice temperature of -20.degree. C. corresponds to a water vapor pressure (in equilibrium) of 1.03 mbar. In order for the water vapor to be able to flow from the surface of the ice into the drying chamber, the water vapor pressure in the drying chamber must clearly be lower than 1.03 mbar, e.g., 0.4 mbar. Thus, it is appropriate to select relative to said pressure value a low pressure, for example, 0.05 mbar. Freeze-drying is normally done in a chamber in which temperable storage surfaces are located with an attached evacuation device, for example, an ice condenser combined with a vacuum pump.
Basically, two drying phases are characteristic for the course of the drying process. As long as there is still crystallized (frozen) water within the product, said drying phase is called main or sublimation drying. If the shut-off device between the chamber and the evacuation device is cut off for only a brief period of time (a few seconds) during this drying phase, equilibrium water vapor pressure becomes established inside the chamber which corresponds to the prevailing temperature of the ice.
From the rise in pressure, a direct conclusion can be drawn with respect to the temperature of the ice. Said method for measuring the ice temperature is known under the concept of barometric temperature measuring and is described, for example, in DE-PS 10 38 988.
As long as solid ice is present in the product, i.e., during the main drying phase, the temperature of the product must not rise above certain values, ranging, in most cases, far below 0.degree. C. in order to avoid impairment of the quality and/or the properties of the product. With progressing drying, the ice nuclei present in the product continue to decrease. In the area of dry marginal zones, higher temperatures are already permissible.
When water is no longer present in the form of ice, the remaining water has been absorbed by the dry product or more or less firmly bonded thereto as well. Removal of this remaining water takes place during the after-drying or desorption drying phase. The quantity of water which can be desorbed during this phase depends upon the temperature of the product, the type of water bonding, and the quality of the still present water. The after-drying phase is initiated by another modification in the physical conditions governing the course of the drying process.
A method of the initially mentioned type is known from the reference DE-PS 10 38 988. For determining the transition from the main drying phase to the after-drying phase, measurements are taken by means that also serve to measure the temperature of the ice. To that end, the shut-off times, which last only a few seconds when measuring the temperature of the ice, are substantially lengthened, i.e., to two minutes or longer.
If, after shut-off times of this magnitude, there occurs an almost constant difference between the operating pressure and the saturation vapor pressure, it may be assumed that the solid ice has been completely removed from the product and that the main drying phase is, in fact, completed. The storage surface temperature and the pressure can be adjusted to the part

REFERENCES:
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