First, the definition and advantages of vacuum freeze drying:
The product is completely frozen, and the ice crystals are sublimated under a certain vacuum condition, thereby achieving the purpose of low-temperature dehydration. This process is called freeze-drying ( freeze-drying).
The lyophilized solid material exhibits a porous structure due to the sublimation of tiny ice crystals, and maintains the volume when it was originally frozen. It is easily dissolved and restored after adding water, and the temperature of the product is maintained at a lower temperature during the sublimation process (generally lower than -25 0 C), and thus it is particularly suitable for the drying of substances which are not heat resistant, such as enzymes, hormones, nucleic acids, blood and immunological products. The result of drying can discharge more than 95~99% of water, which is beneficial to the long-term preservation of the product. The drying process of the product is carried out under vacuum conditions, so it is not easy to oxidize. Lyophilization has been proved to be a very effective means for the chemical, physical and biological instability of some biochemical drugs. With the rapid development of biochemical drugs and biological agents, freeze-drying technology will increasingly show its importance and superiority.
Second, the structure and function of the medical freeze dryer:
The medical freeze dryer consists of a refrigeration system, a vacuum system, a heating system, and an electrical instrument control system. The main components are drying ovens, condensers, refrigeration units, vacuum pumps, heating/cooling devices, etc.
The lyophilization of the product is carried out in a dry box, the shelf in the drying box is made of stainless steel plate, and the media duct is distributed therein, and the product can be cooled or heated, and the vacuum sensor is installed behind the drying box, which will vacuum Turned into a telecommunication signal, the door is inlaid with a sealing rubber ring around it. It can be coated with vacuum grease before use to ensure the sealing of the box.
The condenser is good for winding cylindrical surface. There are many sets of cold air coils around the stainless steel cylinder. The working temperature is lower than the temperature of the products in the drying oven. The temperature is lower than -55 0 C, and the water is sublimated from the product. The vapor can be sufficiently condensed on the inner surface of the stainless steel cylinder surface in contact with the cold coil to ensure the smooth progress of the lyophilization process. The smooth cylindrical structure has the advantage of being easy to clean, after the lyophilization is completed. The frost layer can be removed by electric heating.
The rotary vane vacuum pump is used to evacuate the system. An electromagnetic vacuum valve with automatic deflation is installed at the 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, the air is deflated into the vacuum pump, which 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 freezers are used in parallel, and 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 tube, and then enters the capillary after throttling. The evaporator, due to the suction of the freezer, causes the pressure in the evaporator to drop, and the liquid refrigerant absorbs the heat of the environment and rapidly boils and evaporates. The low-pressure refrigerant gas is drawn back by freezing, and then compressed into a high-pressure gas to complete a refrigeration cycle. The cold-discharge pipe in the heating/cooling device is cooled by the cold-discharge pipe in the condenser and the cold-air coil in the condenser. In the middle of evaporation, they supply refrigerant through two different solenoid valves.
The heating system consists of an electric heating tube, a medium (silicone oil), a media pump, a media box, and the like, and a circulating pipeline is formed. After the silicone oil is heated by the electric heating tube, the medium pump is transported to the media conduit in the dry box shelf to warm the product. Sublimation heat is provided. When frozen, the silicone oil is cooled by the cooling tube, and the media pump is sent to the media conduit in the center of the drying oven shelf to cool and freeze the product.
The electrical instrument control system is designed for mechatronics and is controlled by a special industrial computer. All shelf temperature, media temperature and product temperature can be displayed on the centralized display. For specific adjustment control, please refer to the system manual.
Third, the basic principles and practice of freeze-drying
The quality requirements for freeze-dried products are: constant biological activity, uniform appearance and color, full shape, firm structure, fast dissolution rate and low residual moisture. To obtain high quality products, a comprehensive understanding of the theory and process of lyophilization should be obtained. The lyophilization process includes three stages of pre-freezing, sublimation and re-drying. Reasonable surface effectively shortening the freeze-drying cycle has obvious economic value in industrial production.
(1) Freezing of products
When the solution is frozen (10 to 50 ° C per minute), the crystal grains remain visible under the microscope; on the contrary, at slow freezing (1 ° C / min), the formed crystals are visible to the naked eye. The coarse crystal leaves a large gap in sublimation, which can improve the efficiency of lyophilization. The gap left by the fine crystal after sublimation is small, and the sublimation of the lower layer is hindered. The finished frozen particles are fine, uniform in appearance, large in specific surface area and porous. The structure is good, the dissolution rate is fast, and the wettability of the finished product is relatively stronger.
The medicine is pre-frozen in the freeze-drying machine in two ways: one is to cool the product and the drying box at the same time; the other is to cool the shelf to be dried to about -40 ° C, and then put the product, the former is equivalent to slow Freezing, the latter is between freezing and slow freezing, and is often used to balance freeze-drying efficiency with 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, the large-area sublimation may exceed the normal load of the condenser. This phenomenon is particularly noticeable in summer.
The freezing of the product is at rest. Experience has shown that the phenomenon of supercooling is easy to occur 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 the 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
When the saturated vapor pressure of ice at a certain temperature is greater than the partial pressure of water vapor in the environment, sublimation can be started; the suction and capture of water vapor by the condenser which is lower than the temperature of the product is the necessary condition for maintaining the rise.
The distance a gas molecule travels between two consecutive collisions is called the mean free path, which is inversely proportional to the pressure. Under normal pressure, the value is small, and the sublimated water molecules easily collide with the gas and return to the surface of the steam source, so the sublimation speed is very long. As the pressure decreased 13.3Pa or less, the mean free path is increased 105-fold, sublimation significantly faster, fly out of the water molecules rarely change their aspects, so as to form a vapor stream directed.
The vacuum pump acts as a permanent gas in the lyophilizer to maintain the low pressure required for sublimation. 1 g of water vapor is 1.25 L at normal pressure and 10,000 liters at 13.3 Pa, and it is impossible for a conventional vacuum pump to remove such a large volume per unit time. The condenser actually forms a vacuum pump that specifically traps water vapor.
The temperature at which the article and coagulation are typically between -25 ° C and -50 ° C. The saturated vapor pressure of ice at this temperature is 63.3Pa and 1.1Pa, respectively, so a considerable pressure difference is generated between the sublimation surface and the condensation surface, if the partial pressure of the non-condensable gas in the system is negligible. Except, it will promote the water vapor that is sublimated from the product, and will reach the surface of the condenser at a certain flow rate to form frost.
The sublimation heat of ice is about 2822 J/g. If the sublimation process does not supply heat, then the product only reduces the internal energy to compensate the sublimation heat until the temperature is balanced with the condenser temperature, and the sublimation stops. In order to maintain the temperature difference between sublimation and condensation, it is necessary to provide sufficient heat to the product.
(2) Sublimation process
During the * phase of warming (a large number of sublimation stages), the temperature of the article is below a range of its eutectic point. Therefore, the shelf temperature should be controlled. If the product is partially dry, but the temperature exceeds its eutectic point, the product will melt. At this time, the melted liquid is saturated with ice and not saturated with the solute. The dried solute will dissolve quickly, and after concentrating it into a thin, stiff, the appearance is extremely poor, and the dissolution rate is very poor. If the melting of the product occurs in the late stage of a large amount of sublimation, the dried pores are less due to the smaller amount of melted liquid. The solid is absorbed, resulting in a defect in the lumps after lyophilization. When dissolved in water, the dissolution rate is still slow.
In the process of mass sublimation, although the temperature of the shelf and the product is very different, since the plate temperature, the condenser temperature and the vacuum temperature are substantially unchanged, the sublimation heat absorption is relatively stable, and the product temperature is relatively constant. As the product is dried from top to bottom, the resistance to sublimation of the ice layer gradually increases. The temperature of the product will also increase slightly. Until the naked eye is not visible to the naked eye. At this time, more than 90% of the water has been removed. The process of mass sublimation has basically ended. In order to ensure the completion of the sublimation of the whole box of products, the temperature of the plate still needs to be maintained for one stage before the second stage of temperature rise.
The remaining moisture is called residual moisture, which is different from the free state water. The residual moisture includes chemically combined water and physical water, such as crystal water of crystallization, protein passing hydrogen Bonded water and adsorbed water on a solid surface or capillary. Since the residual moisture is restrained by the gravitational force, the saturated vapor pressure is reduced to a different extent, and the drying speed is significantly reduced. Although increasing the temperature of the product promotes vaporization of residual moisture, if the temperature exceeds a certain limit temperature, the biological activity may also drop sharply. The high drying temperature of the zui to ensure the safety of the product is determined by experiments. Usually we will keep the plate temperature +30 °C in the second stage and keep it constant. At the beginning of this stage, the temperature of the product rises rapidly because the temperature of the sheet rises and the residual moisture is small and it is not easy to vaporize. However, as the temperature of the product gradually approaches the plate temperature, heat conduction becomes slower and requires patience for a considerable period of time. Practical experience has shown that the residual moisture drying time is almost equal to the time of sublimation and sometimes even exceeds.
(4) Freeze-drying curve: The freeze-drying curve can be obtained by recording the change of shelf temperature and product temperature with time.
The typical freeze-drying curve divides the shelf temperature into two stages. The shelf temperature remains low during a large number of sublimation. According to the actual situation, it can generally be controlled between -10 and +10. In the second stage, the shelf temperature is appropriately adjusted according to the nature of the product. This method is applicable to products with a lower melting point. If the performance of the product is not clear, the machine performance is poor or the work is not stable enough, this method is also relatively safe.
If the eutectic point of the product is higher, the vacuum degree of the system can be kept good, and the condenser has sufficient cooling capacity, the temperature of the shelf can be raised to the allowable high temperature of the zui until the end of lyophilization. However, it is also necessary to ensure that the temperature of the product does not exceed the eutectic point when it is sublimated.
If the product is unstable to heat, the temperature of the second stage should not be too high. In order to improve the sublimation speed of the * stage, the shelf temperature can be raised once to the high temperature allowed by the product; when the mass sublimation phase is basically finished, the plate temperature is lowered to the allowable zui high temperature, and the latter two modes. Although there is some improvement in the speed of sublimation, the ability to resist interference is correspondingly reduced, and a sudden drop in vacuum and cooling capacity or power outage may cause the product to melt. A reasonable and flexible way to master the * is still the more common way.
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