Detailed explanation of the basic principle of the freeze dryer - Database & Sql Blog Articles

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Freeze dryer

Detailed explanation of the basic principles

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Freeze-drying is a technique of drying by the principle of sublimation. It is a process in which the dried substance is rapidly frozen at a low temperature, and then the frozen water molecules are directly sublimated into a water vapor escape process under a suitable vacuum environment. The product obtained by drying is called a lyophilizer, and the process is called lyophilization. The substance is always in a low temperature (freezing state) before drying, and the ice crystals are uniformly distributed in the substance, and the sublimation process does not cause concentration due to dehydration, thereby avoiding side effects such as foaming and oxidation caused by water vapor. The dry material is porous in the shape of a dry sponge, the volume is basically unchanged, and it is easily soluble in water and restored to its original state. Physical and biological denaturation of dry matter is prevented to the greatest extent. The 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. It works by freezing the dried item below the triple point temperature and then sublimating the solid water (ice) in the item to water vapor under vacuum conditions, removing it from the item and drying the item. After the material is pretreated, it is sent to the quick freezing warehouse for freezing, and then sent to the drying chamber for sublimation dehydration, and then packaged in the post-treatment workshop. The vacuum system establishes a low pressure condition for the sublimation drying chamber, the heating system provides sublimation latent heat to the material, and the refrigeration system provides the required cooling capacity to the cold trap and the drying chamber. The equipment adopts high-efficiency radiant heating, the material is heated evenly; the high-efficiency water trap is used, and the defrosting can be realized quickly; the high-efficiency vacuum unit can be used to realize the separation of oil and water; the parallel centralized refrigeration system is adopted, and the multi-channel cooling is required on demand. Stable, energy-saving; artificial intelligence control, high control accuracy, easy to operate. Xinsheng Instrument Network's quality requirements for freeze-dried products are: constant biological activity, uniform appearance, 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 consists of three stages of pre-freezing, sublimation and re-freezing. Reasonable and effective shortening of the freeze-drying cycle has obvious economic value in industrial production.

Freezing of a product

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 Frozen, the latter is between quick freezing and slow freezing, so it is often used to balance lyophilization efficiency 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, 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.

Conditions and speed of the second 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 drops below 13.3 Pa, the mean free path increases by 105 times, causing the sublimation speed to increase significantly, and the water molecules that fly away rarely change their aspects, thus forming a directed steam flow. 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.

Three-liter process

In the first stage of heating (a large number of sublimation stages), the temperature of the product is lower than 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 finally concentrate into a thin, stiff, poor appearance, 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 porous solid is due to the small amount of molten liquid. After absorption, the block is damaged after lyophilization, and the dissolution rate can be found to be slow when dissolved with water. 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 bound by some 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 maximum drying temperature to ensure product safety is determined by experimentation. 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.

Four freeze-dried curve

The freeze-drying curve is obtained by recording the shelf temperature and the change in product temperature over 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 high, the vacuum degree of the system can be maintained well, and the condenser has sufficient cooling capacity, and the temperature of the shelf can be raised to the maximum allowable temperature 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 first stage, the shelf temperature can be raised once to the maximum temperature allowed by the product; when the mass sublimation phase is substantially finished, the plate temperature is lowered to the maximum allowable temperature, although the latter two methods A lot of sublimation speed is improved, but its anti-interference ability is correspondingly reduced, and a sudden decrease in vacuum and cooling capacity or power failure may melt the product. A reasonable and flexible way to master the first method is still the more common method.

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