Freeze-drying is a technique that uses the principle of sublimation to dry. It is the process of quickly freezing the dried material at a low temperature, and then in a suitable vacuum environment, the frozen water molecules are directly sublimated into the process of water vapor escape. Freezing The product obtained by drying is called lyophilizer, and the process is called lyophilization.
The substance is always at a low temperature (frozen state) before drying, and the ice crystals are evenly distributed in the substance. The sublimation process will not be concentrated due to dehydration, avoiding the side effects of foaming and oxidation caused by water vapor. The dry substance is porous and dry, the volume is basically unchanged, and it is easily dissolved in water and restored to its original state. To prevent the physical and chemical and biological denaturation of dry matter to the greatest extent.
The 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. Its working principle is to freeze the dried items to below the triple point temperature, and then to directly sublimate the solid moisture (ice) in the items into water vapor under vacuum conditions, and to exclude them from the items to dry them. After pre-processing, the materials are sent to the quick-freezing warehouse for freezing, and then sent to the drying warehouse for sublimation and dehydration, and then packed in the post-processing workshop. The vacuum system establishes low-pressure conditions for the sublimation drying bin, the heating system provides the latent heat of sublimation to the materials, and the refrigeration system provides the required cooling capacity to the cold trap and drying chamber. This equipment adopts high-efficiency radiant heating, materials are heated uniformly; adopts high-efficiency water-trapping trap, and can realize rapid defrosting; adopts high-efficiency vacuum unit, and can realize oil-water separation; adopts parallel centralized refrigeration system, multi-channel cooling on demand, working conditions Stable, energy-saving; artificial intelligence control, high control accuracy and easy operation.
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-drying. Reasonably and effectively shortening the freeze-drying cycle has obvious economic value in industrial production.
When the frozen solution of a product is quickly frozen (cooling down 10 to 50 ° C per minute), the crystal grains maintain the size visible under the microscope; on the contrary, when it is slowly frozen (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 quick 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.
Conditions and speed of two-sublimation ice can begin sublimation when the saturated vapor pressure of the ice at a certain temperature is greater than the partial pressure of water vapor in the environment; the suction and capture of water vapor by the condenser that is lower than the product temperature is maintenance Necessary conditions.
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, its value is very small, the sublimated water molecules easily collide with the gas and return to the surface of the steam source, so the sublimation speed is very diffuse. As the pressure decreases below 13.3Pa, the average free path increases by a factor of 105, which significantly accelerates the sublimation speed. The water molecules that fly out rarely change their own aspects, thus forming a directional steam flow.
The vacuum pump plays the role of removing permanent gas in the freeze dryer to maintain the low pressure necessary for sublimation. 1g of water vapor is 1.25L at normal pressure and expands to 10,000 liters at 13.3Pa. It is impossible for ordinary vacuum pumps to remove such a large volume per unit time. The condenser actually forms a vacuum pump that specifically captures water vapor.
The temperature of the product and coagulation is usually -25 ℃ and -50 ℃. 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 can be ignored at this time Regardless, it will promote the sublimation of the water vapor from the product to reach the surface of the condenser at a certain flow rate to form frost.
The sublimation heat of ice is about 2822J / g. If no heat is supplied during the sublimation process, then the product can only reduce the internal energy to compensate for the sublimation heat until the temperature is equal to the temperature of the condenser, and the sublimation will stop. In order to maintain the temperature difference between sublimation and condensation, sufficient heat must be provided to the product.
In the first sublimation process (the mass sublimation stage) of the three-sublimation process, the temperature of the product is within a range of its eutectic point. Therefore, the shelf temperature should be controlled. If the product has been partially dried, but the temperature exceeds its eutectic point, the product will melt at this time, and the melted liquid is saturated with ice and not saturated with solute, so The dried solute will quickly dissolve into it, and finally condensed into a thin stiff block. 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 number of sublimation, the amount of melted liquid is small, so the porous solid is dried Absorbed, resulting in defects after freeze-drying, the dissolution rate can still be found to be slow when dissolved with water.
In a large number of sublimation processes, although the shelf and the product temperature are very different, but because the plate temperature, the condenser temperature and the vacuum temperature are basically unchanged, the sublimation heat absorption is relatively stable, and the product temperature is relatively constant. As the product dries from top to bottom, the resistance to ice sublimation gradually increases. The product temperature will also increase slightly. Until the naked eye is no longer the existence of ice crystals. 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 complete sublimation of the whole box of products, the plate temperature still needs to be maintained for one stage before the second stage of temperature increase. The remaining percentage of water is called residual water, which is different from the free state water in physical and chemical properties. The residual water includes chemically combined water and physically combined water, such as combined crystal water crystals, protein through hydrogen Bonded water and adsorbed water on solid surfaces or capillaries. Because the residual moisture is bound by some kind of gravitation, its saturated vapor pressure is reduced to varying degrees, so the drying speed is significantly reduced. Although raising the temperature of the product promotes the vaporization of residual moisture, if it exceeds a certain limit temperature, the biological activity may also drop sharply. The maximum drying temperature to ensure the safety of products should be determined by experiment. Normally, in the second stage, we keep the plate temperature around + 30 ℃ and keep it constant. At the beginning of this stage, due to the increase in plate temperature, the residual moisture is low and it is not easy to vaporize, so the product temperature rises faster. However, as the product temperature and the plate temperature gradually get closer, the heat conduction becomes slower, and you need to wait patiently for a long period of time. Practical experience shows that the time for residual moisture drying is almost equal to the time for a large amount of sublimation, and sometimes even exceeds.
Four freeze-drying curves Record the changes of shelf temperature and product temperature with time to obtain freeze-drying curves. The typical freeze-drying curve divides the shelf heating into two stages. The shelf temperature is kept low during a large amount of sublimation. According to the actual situation, it can be generally controlled between -10 and +10. In the second stage, the shelf temperature is adjusted appropriately according to the nature of the product. This method is suitable for products with a lower melting point. If the performance of the product is unclear, the machine performance is poor or the work is not stable enough, this method is also more secure.
If the eutectic point of the product is high, the vacuum degree of the system can be maintained well, and the cooling capacity of the condenser is sufficient, you can also use a certain heating rate to increase the shelf temperature to the maximum allowable temperature until the end of freeze-drying. However, it is also necessary to ensure that the temperature of the product during mass sublimation must not exceed the eutectic point.
If the product is unstable to heat, the plate temperature in the second stage should not be too high. In order to increase the sublimation speed of the first stage, the shelf temperature can be raised to above the maximum allowable temperature of the product at one time; after the substantial sublimation stage is basically over, the plate temperature can be reduced to the maximum allowable temperature again. A large number of sublimation speeds have been improved, but its anti-interference ability has been reduced accordingly. Sudden reduction in vacuum and cooling capacity or power failure may cause the product to melt. Reasonably and flexibly mastering the first method is still the more commonly used method at present.
The substance is always at a low temperature (frozen state) before drying, and the ice crystals are evenly distributed in the substance. The sublimation process will not be concentrated due to dehydration, avoiding the side effects of foaming and oxidation caused by water vapor. The dry substance is porous and dry, the volume is basically unchanged, and it is easily dissolved in water and restored to its original state. To prevent the physical and chemical and biological denaturation of dry matter to the greatest extent.
The 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. Its working principle is to freeze the dried items to below the triple point temperature, and then to directly sublimate the solid moisture (ice) in the items into water vapor under vacuum conditions, and to exclude them from the items to dry them. After pre-processing, the materials are sent to the quick-freezing warehouse for freezing, and then sent to the drying warehouse for sublimation and dehydration, and then packed in the post-processing workshop. The vacuum system establishes low-pressure conditions for the sublimation drying bin, the heating system provides the latent heat of sublimation to the materials, and the refrigeration system provides the required cooling capacity to the cold trap and drying chamber. This equipment adopts high-efficiency radiant heating, materials are heated uniformly; adopts high-efficiency water-trapping trap, and can realize rapid defrosting; adopts high-efficiency vacuum unit, and can realize oil-water separation; adopts parallel centralized refrigeration system, multi-channel cooling on demand, working conditions Stable, energy-saving; artificial intelligence control, high control accuracy and easy operation.
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-drying. Reasonably and effectively shortening the freeze-drying cycle has obvious economic value in industrial production.
When the frozen solution of a product is quickly frozen (cooling down 10 to 50 ° C per minute), the crystal grains maintain the size visible under the microscope; on the contrary, when it is slowly frozen (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 quick 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.
Conditions and speed of two-sublimation ice can begin sublimation when the saturated vapor pressure of the ice at a certain temperature is greater than the partial pressure of water vapor in the environment; the suction and capture of water vapor by the condenser that is lower than the product temperature is maintenance Necessary conditions.
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, its value is very small, the sublimated water molecules easily collide with the gas and return to the surface of the steam source, so the sublimation speed is very diffuse. As the pressure decreases below 13.3Pa, the average free path increases by a factor of 105, which significantly accelerates the sublimation speed. The water molecules that fly out rarely change their own aspects, thus forming a directional steam flow.
The vacuum pump plays the role of removing permanent gas in the freeze dryer to maintain the low pressure necessary for sublimation. 1g of water vapor is 1.25L at normal pressure and expands to 10,000 liters at 13.3Pa. It is impossible for ordinary vacuum pumps to remove such a large volume per unit time. The condenser actually forms a vacuum pump that specifically captures water vapor.
The temperature of the product and coagulation is usually -25 ℃ and -50 ℃. 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 can be ignored at this time Regardless, it will promote the sublimation of the water vapor from the product to reach the surface of the condenser at a certain flow rate to form frost.
The sublimation heat of ice is about 2822J / g. If no heat is supplied during the sublimation process, then the product can only reduce the internal energy to compensate for the sublimation heat until the temperature is equal to the temperature of the condenser, and the sublimation will stop. In order to maintain the temperature difference between sublimation and condensation, sufficient heat must be provided to the product.
In the first sublimation process (the mass sublimation stage) of the three-sublimation process, the temperature of the product is within a range of its eutectic point. Therefore, the shelf temperature should be controlled. If the product has been partially dried, but the temperature exceeds its eutectic point, the product will melt at this time, and the melted liquid is saturated with ice and not saturated with solute, so The dried solute will quickly dissolve into it, and finally condensed into a thin stiff block. 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 number of sublimation, the amount of melted liquid is small, so the porous solid is dried Absorbed, resulting in defects after freeze-drying, the dissolution rate can still be found to be slow when dissolved with water.
In a large number of sublimation processes, although the shelf and the product temperature are very different, but because the plate temperature, the condenser temperature and the vacuum temperature are basically unchanged, the sublimation heat absorption is relatively stable, and the product temperature is relatively constant. As the product dries from top to bottom, the resistance to ice sublimation gradually increases. The product temperature will also increase slightly. Until the naked eye is no longer the existence of ice crystals. 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 complete sublimation of the whole box of products, the plate temperature still needs to be maintained for one stage before the second stage of temperature increase. The remaining percentage of water is called residual water, which is different from the free state water in physical and chemical properties. The residual water includes chemically combined water and physically combined water, such as combined crystal water crystals, protein through hydrogen Bonded water and adsorbed water on solid surfaces or capillaries. Because the residual moisture is bound by some kind of gravitation, its saturated vapor pressure is reduced to varying degrees, so the drying speed is significantly reduced. Although raising the temperature of the product promotes the vaporization of residual moisture, if it exceeds a certain limit temperature, the biological activity may also drop sharply. The maximum drying temperature to ensure the safety of products should be determined by experiment. Normally, in the second stage, we keep the plate temperature around + 30 ℃ and keep it constant. At the beginning of this stage, due to the increase in plate temperature, the residual moisture is low and it is not easy to vaporize, so the product temperature rises faster. However, as the product temperature and the plate temperature gradually get closer, the heat conduction becomes slower, and you need to wait patiently for a long period of time. Practical experience shows that the time for residual moisture drying is almost equal to the time for a large amount of sublimation, and sometimes even exceeds.
Four freeze-drying curves Record the changes of shelf temperature and product temperature with time to obtain freeze-drying curves. The typical freeze-drying curve divides the shelf heating into two stages. The shelf temperature is kept low during a large amount of sublimation. According to the actual situation, it can be generally controlled between -10 and +10. In the second stage, the shelf temperature is adjusted appropriately according to the nature of the product. This method is suitable for products with a lower melting point. If the performance of the product is unclear, the machine performance is poor or the work is not stable enough, this method is also more secure.
If the eutectic point of the product is high, the vacuum degree of the system can be maintained well, and the cooling capacity of the condenser is sufficient, you can also use a certain heating rate to increase the shelf temperature to the maximum allowable temperature until the end of freeze-drying. However, it is also necessary to ensure that the temperature of the product during mass sublimation must not exceed the eutectic point.
If the product is unstable to heat, the plate temperature in the second stage should not be too high. In order to increase the sublimation speed of the first stage, the shelf temperature can be raised to above the maximum allowable temperature of the product at one time; after the substantial sublimation stage is basically over, the plate temperature can be reduced to the maximum allowable temperature again. A large number of sublimation speeds have been improved, but its anti-interference ability has been reduced accordingly. Sudden reduction in vacuum and cooling capacity or power failure may cause the product to melt. Reasonably and flexibly mastering the first method is still the more commonly used method at present.
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