The freeze-drying curve and freeze-drying sequence mainly determine the following data:
Prefreezing rate
The speed of the pre-freezing rate has a direct impact on the size of the grain in the product freeze, the survival rate of the live bacteria and the rate of sublimation. In general, the slow-frozen grains are large, the appearance of the product is rough, and it is not easy to damage the living bacteria, but the sublimation rate is fast, and the quick freezing is the opposite. Usually the freeze dryer does not adjust the freezing rate. If you need to freeze faster, first pre-cool the drying chamber (box) to a lower temperature and then freeze the product into the box. If the drying oven is cooled together with the product, the freezing rate is slower.
Pre-freezing temperature
There are slight differences depending on the pre-freezing method. In general, the shelf temperature should be less than 5 to 10 ° C of the melting point of the product.
Prefreezing time
The time required for pre-freezing is determined by different specific conditions. The general principle is that all parts of the product should be completely frozen. Generally speaking, the product is loaded in a large amount, the bottom of the packaged container is not flat, the heat transfer between the tray and the shelf is poor, the refrigeration capacity of the freeze dryer is small, the degree of subcooling of the product is small, and the temperature difference between the shelves is extended. Frozen time. On the contrary, the pre-freezing time can be shortened. Usually, the shelf lyophilizer, the shelf of the drying oven is reduced from room temperature 30oC to -40oC for about 2 to 4 hours. After the product sample temperature drops to the predetermined minimum temperature, it needs to be kept at this temperature for 1-2 hours. In order to sublimate.
Cooling time and temperature of water vapor condenser
The water vapor condenser should be cooled 30 to 50 minutes before the end of the product's pre-freezing (depending on its cooling capacity). When the temperature drops to about -40oC, the vacuum pump is started to pump vacuum. When the surface pressure of the product drops below 10-20Pa, the heating circulation pump is started to heat the product.
Vacuuming time
At the end of the pre-freezing, the vacuum is started. It is required to achieve a vacuum of 10 Pa at a time of about 0.5 h. The end of the pre-freezing is to stop the operation of the freeze-drying freezer, usually at the same time as vacuuming or vacuuming to the specified requirements. The operation.
Pre-freezing time
The end of the pre-freezing is to stop the cooling of the lyophilized box layer, usually stopping the cooling of the slab while vacuuming or vacuuming to the specified requirements.
Start heating time
It is generally believed that the time to start heating begins with a vacuum (actually the vacuum is applied and sublimation is started). The heating is started after the vacuum reaches 10Pa. Some lyophilizers use the vacuum relay to automatically turn on the heating, that is, when the vacuum reaches 10Pa, the heating starts automatically; some lyophilizers start heating half an hour after vacuuming. The degree of vacuum has reached 10 Pa or even higher.
Vacuum alarm working time
Since vacuum is extremely important for sublimation, the new freeze dryers are equipped with vacuum alarms. The operating time of the vacuum alarm is from the start of heating until the correction of the leak is used, or from the beginning until the end of lyophilization. Once the vacuum alarm occurs during the sublimation process and a vacuum alarm occurs, an alarm signal is issued on the one hand to automatically cut off the heating of the freeze-drying box. At the same time, the freezer of the freeze-drying box is also started to cool the product to protect the product from melting.
Vacuum controlled working time
The purpose of vacuum control is to improve the heat transfer in the freeze-drying box. It is usually used during the second stage of drying. After the product temperature reaches the maximum allowable temperature, it can be stopped and the vacuum state is continued. The length of use is determined by the product variety. The amount of loading and the degree of vacuum are determined. It can also be used during the first stage of drying.
Maximum allowable temperature for product heating
The maximum allowable temperature for lamellar heating is determined by the product. When sublimation, the heating temperature of the laminating layer can exceed the maximum permissible temperature of the product. At this time, the product stays in the low temperature stage, and the temperature of the laminating layer can promote sublimation; The layer temperature needs to be reduced to match the maximum allowable temperature of the product. Due to the temperature difference of heat transfer, the temperature of the ply can be slightly higher than the maximum allowable temperature of the product.
Total time of lyophilization
The total time of lyophilization is the pre-freezing time, plus the time of the ascending and the time of the second stage of work. The total time is determined and the freeze-drying end time is also determined. The total freeze-drying time is determined according to the product variety, bottle type, packing method, loading capacity, machine performance, etc. Generally, the freezing work takes a long time, about 18~24h, and some products take several days.
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