Controlled nucleation during freezing step of freeze drying cycle using pressure differential ice fog distribution
A method of controlling and enhancing the nucleation of product in a freeze dryer, wherein the product is maintained at a predetermined temperature and pressure in a chamber of the freeze dryer, and a predetermined volume of ice fog is created in a condenser chamber separate from the product chamber and connected thereto by a vapor port. The ice fog has a predetermined pressure that is greater than that of the product chamber, and is rapidly conveyed through the vapor port into the product chamber for even distribution therein to create uniform and rapid nucleation of the product in different areas of the product chamber.
1. A method of controlling and enhancing the nucleation of product in a freeze dryer, comprising:
maintaining the product at a predetermined temperature and pressure in a chamber of the freeze dryer;
creating a predetermined volume of ice fog in a condenser chamber separate from the product chamber and connected thereto by a vapor port, the ice fog having a predetermined pressure that is greater than that of the product chamber; and
rapidly conveying the ice fog through the vapor port into the product chamber for even distribution therein to create uniform and rapid nucleation of the product in different areas of the product chamber.
2. The method of claim 1 wherein the vapor port has an isolation valve between the product chamber and the condenser chamber to open or close vapor flow therebetween.
3. The method of claim 1 wherein a vacuum pump is connected to the condenser chamber for selectively reducing the pressure within the product chamber and the condenser chamber when the isolation valve is opened.
4. The method of claim 1 wherein the pressure within the product chamber is about 50 Torr and the pressure within the condenser chamber is about atmospheric pressure when the ice fog is rapidly conveyed from the condenser chamber to the product chamber.
5. The method of claim 4 wherein the temperature of the product is about −5.0° C. and the temperature of the condenser chamber is about −53° C. to −85° C. when the ice fog is rapidly conveyed from the condenser chamber to the product chamber.
6. The method of claim 1 wherein a predetermined moisturized back fill gas is introduced into the condenser chamber to produce the ice fog.
7. The method of claim 6 wherein the condenser chamber has a release valve which is opened to enable the moisturized back fill gas to be introduced into the condenser chamber when the temperature of the condenser chamber is about −53 C to −85 C to produce the ice fog.
8. The method of claim 6 wherein the back fill gas is ambient atmospheric air and has a moisture content of about 50-80% by volume.