Brief introduction of vacuum freeze drying technology principle
1. Definition and advantages of vacuum freeze-drying:
The product is completely frozen, and the ice crystals are sublimated under certain vacuum conditions, so as to achieve the purpose of low temperature dehydration. This process is called freeze drying (Freeze-drying), or freeze drying for short.
The lyophilized solid material exhibits a porous structure due to the sublimation of tiny ice crystals, and maintains the original volume when frozen. It is easily dissolved and restored after adding water. The temperature of the product is kept at a lower temperature during the sublimation process (generally lower than -250C), so it is especially suitable for the drying of heat-resistant substances such as enzymes, hormones, nucleic acids, blood and immune products. As a result of drying, more than 95 ~ 99% of water can be discharged, which is conducive to long-term storage of products. The product drying process is carried out under vacuum conditions, so it is not easy to oxidize. In view of the chemical, physical and biological instability of some biochemical drugs, freeze-drying has been proved to be a very effective method in practice. With the rapid development of biochemical drugs and biological agents, freeze-drying technology will increasingly show its importance and superiority.
2. Introduction to the structure and function of medical freeze dryer:
The medical freeze dryer is composed of refrigeration system, vacuum system, heating system, electrical instrument control system. The main components are drying oven, condenser, freezing unit, vacuum pump heating / cooling device, etc.
The freeze-drying of the products is carried out in a drying oven. The shelves in the drying oven are made of stainless steel plates, and there are media ducts distributed in them, which can cool or heat the products. A vacuum sensor is installed at the back of the drying oven, which will vacuum It turns into an electric signal, and a sealing rubber ring is inlaid around the door of the box. Before use, it can be coated with vacuum silicone grease to ensure the sealing of the box.
The condenser is best to be wound cylindrical type. There are multiple sets of air-cooled coils wrapped around the stainless steel cylinder. Its working temperature is lower than the temperature of the product in the drying oven, and the lowest temperature can reach -550C. The water vapor sublimated from the product can be fully It is condensed on the inner surface of the stainless steel cylinder in contact with the cold coil to ensure the smooth progress of the freeze-drying process. The biggest advantage of the smooth cylinder structure is that it is easy to clean. After the freeze-drying is completed, the frost can be removed by electric heating. Layer removed.
The rotary vane vacuum pump is used to vacuum the system. An electromagnetic vacuum valve with automatic deflation is installed at the air inlet of the mechanical pump. It is controlled by the same power source as the rotary vane pump. When the pump is stopped, the electromagnetic valve is automatically closed. At the same time, deflation into the vacuum pump not only protects the vacuum system, but also prevents the vacuum pump from returning oil to the system.
In the refrigeration system, two 2.2Kw semi-closed refrigerators are used in parallel. The high-pressure refrigerant liquid (fluorine-free refrigerant V55C) from the wind condenser passes through the drying filter and the solenoid valve to reach the capillary and enters after throttling In the evaporator, due to the suction effect of the refrigerator, the pressure in the evaporator drops, and the liquid refrigerant absorbs the heat of the environment and rapidly boils and evaporates. The low-pressure refrigerant gas is frozen and pumped back, and then compressed into a high-pressure gas to complete a refrigeration cycle. The cold exhaust pipe in the heating / cooling device, the cold exhaust pipe in the condenser, and the cold air coil in the condenser are compliant with the refrigeration system Medium evaporation, they are supplied with refrigerant through two different solenoid valves.
The heating system consists of an electric heating tube, media (silicone oil), media pump, media box, etc., forming a circulation pipeline. After the silicone oil is heated by the electric heating tube, it is transported by the media pump to the media conduit in the shelf of the dry box to heat the product. Provides sublimation heat. When frozen, the silicone oil is cooled by the cooling pipe, which is transported by the media pump to the media duct in the center of the shelf of the drying oven to cool and freeze the product.
The electrical instrument control system is a mechanical and electrical integration design, controlled by a dedicated industrial control motor. All shelf temperature, media temperature and product temperature can be displayed on a centralized display. For specific adjustment control, please refer to the system manual.
3. Basic principles and practice of freeze-drying
The quality requirements for freeze-dried products are: unchanged biological activity, uniform appearance and color, full shape, firm structure, fast dissolution rate, and low residual moisture. To obtain high-quality products, you should have a relatively comprehensive understanding of the theory and process of freeze-drying. The freeze-drying process includes three stages: pre-freeze, sublimation and re-freeze. Reasonably shortening the freeze-drying cycle has obvious economic value in industrial production.
(1) Freezing of products
When the solution is frozen quickly (10-50 ° C per minute), the grains remain the size visible under the microscope; on the contrary, when the solution is frozen slowly (1 ° C / min), the crystals formed are visible to the naked eye. Coarse crystals leave a larger gap in sublimation, which can improve the efficiency of freeze-drying. Fine crystals leave a smaller gap after sublimation, which hinders the sublimation of the lower layer, and the finished frozen particles are delicate, uniform in appearance, large in specific surface area, and porous The structure is good, the dissolution speed is fast, and the moisture absorption of the finished product is relatively stronger.
The drugs are pre-frozen in the freeze dryer in two ways: one is to cool the product and the drying box at the same time; the other is to wait for the shelf of the drying box to cool to about -40 ℃, and then put the product in, the former is equivalent to slow Freezing, the latter is between freezing and slow freezing, so it is often used to balance the efficiency of freeze-drying and product quality. The disadvantage of this method is that when the product enters the box, the water vapor in the air will quickly condense on the shelf, and in the early stage of sublimation, if the plate heats up faster, it may exceed the normal load of the condenser due to the large area of ​​sublimation. This phenomenon is particularly significant in summer.
The freezing of the product is at rest. Experience has shown that the phenomenon of supercooling easily occurs until the temperature of the product has reached the eutectic point. However, the solute still does not crystallize. In order to overcome the phenomenon of supercooling, the freezing temperature of the product should be lower than a range below the eutectic point, and it needs to be kept for a period of time until the product is completely frozen.
(2) Conditions and speed of sublimation
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