1. Freezing of Products
When the solution is quickly frozen (the temperature drops by 10~50℃per minute), the crystallites keep the size visible under the microscope; on the contrary, when frozen slowly (1℃/ min), the crystals formed are visible to the naked eye. Coarse crystals leave a large gap in sublimation, which can improve the efficiency of freeze-drying. Fine crystals leave a small gap after sublimation, which hinders the sublimation of the lower layer. The finished particles produced by rapid freezing are fine, with the merits of uniform in appearance, large in specific surface area, good in porous structure, fast in dissolution, and relatively stronger in moisture absorption of the finished products. Drugs are pre-frozen in freeze-dryer in two ways: one is that the products are cooled in the drying oven at the same time; The other is to wait for the shelf of drying box to be cooled to about -40℃ and then put the product in. The former is equivalent to slow freezing, while the latter is between quick freezing and slow freezing, so it is often used to give consideration to the freeze-drying efficiency and product quality. The disadvantage of this method is that when the product is put into the box, the water vapor in the air will condense on the shelf quickly. However, in the early stage of sublimation, if the temperature of the shelf rises faster, it is possible to exceed the normal load of the condenser due to the sublimation of large areas. This phenomenon is particularly pronounced in summer. Freezing of the product is in a static state. Experience has shown that the phenomenon of supercooling is easy to occur, which will lead to the product temperature reaching the eutectic point, but the solute is still not crystallized. In order to overcome the supercooling phenomenon, the freezing temperature of the product should be lower than a range below the eutectic point, and it should be kept for a period of time until the product is frozen completely.
2. Conditions and Speed of Sublimation
Sublimation can start when the saturation vapour pressure of the ice at a certain temperature is higher than the partial pressure of the water vapour in the surroundings; The suction and capture of water vapor by the condenser which is lower than the product temperature is the necessary condition for maintaining the sublimation. The distance traveled by a gas between two consecutive collisions is called the average free path, which is inversely proportional to the pressure. Under normal pressure, it is tiny and sublimated water molecules collide with the gas easily and then return to the surface of the steam source, so the speed of sublimation is very slow. As the pressure decreases below 13.3Pa, the average free path increases by 105 times, which makes the sublimation speed significantly faster. The water molecules flying out rarely change their own aspects, thus forming a directional steam flow. The vacuum pump plays the role of removing gas in the freeze dryer to maintain the low pressure necessary for sublimation. 1 gram of water vapor is 1.25L at normal pressure, but it expands to 10,000 L at 13.3Pa. It is impossible for ordinary vacuum pumps to pump such a large volume in a unit time. In fact, the condenser forms a special vacuum pump for capturing water vapor. The temperature of production and condensation is usually -25℃ and -50℃. At this temperature, the saturated vapor pressure of ice is 63.3Pa and 1.1Pa respectively, creating a significant pressure differential between the sublimation surface and the condensation surface. If the partial pressure of non-condensable gas in the system can be neglected at this time, it will promote the water vapor sublimated from the product to reach the surface of the condenser directionally at a certain flow rate and forms frost. The sublimation heat of ice is about 2822J/ g. If the sublimation process does not supply heat, the product can only reduce its internal energy to compensate for the sublimation heat until its temperature equals the temperature of the condenser and then sublimation will stop. Sufficient heat is supplied to the product to maintain the temperature difference between sublimation and condensation.
3. Sublimation Process
In the first stage of heating (mass sublimation stage), the product temperature should be lower than its eutectic point by a range. Therefore, the shelf temperature should be controlled. If the product has been dried partially but the temperature exceeds its eutectic point, the product will melt at this time. At this time, the melted liquid is saturated with ice, but not with solute, so the dried solute will dissolve into it quickly, and finally concentrate into a thin frozen block, which has a bad appearance and a poor dissolution rate. If the product melts at the later stage of sublimation, it will be absorbed by the dry porous solid due to the small amount of melted liquid, which causes the lump to be damaged after lyophilization, and the dissolution rate can also be found to be slow when adding water for dissolution. In the process of sublimation, although the temperature of shelf and product is quite different, the temperature of plate, condenser and vacuum are basically unchanged. So the sublimation heat absorption is relatively stable and the product temperature is relatively constant. As the product dries from top to bottom, the resistance of ice sublimation gradually increases. The product temperature will also rise slightly accordingly until the ice crystals are invisible to the naked eye. At this time, more than 90% of the water has been removed. So far, the process of sublimation has basically ended. In order to ensure the sublimation of the whole box of products, the shelf temperature still needs to be kept for one stage before the second stage of heating. The remaining percentage of water is called residual water, which is different from free water in physical and chemical properties. Residual water includes chemically combined water and physically combined water, such as the crystallization of combined crystal water, the water bonded by protein through hydrogen bonds, and the water adsorbed on solid surface or capillary. Saturated vapor pressure of residual water decreases in different degrees because of the resistance by a certain gravity, so the drying speed decreases obviously. Although increasing the temperature of the product can promote the gasification of residual water, if it exceeds a certain temperature, the biological activity may also drop sharply. The highest drying temperature to ensure the safety of products should be determined by experiments. Usually, in the second stage, the plate temperature is about 30℃ and kept constant. At the beginning of this stage, the temperature of the product rises rapidly as the plate temperature increases and the residual water evaporates less. However, as the product temperature gradually approaches the plate temperature, the heat conduction becomes slower, and it takes a long time to wait patiently. Practical experience shows that the drying time of residual water is almost equal to the sublimation time, and sometimes even exceeds it.
4. Lyophilization Curve
The lyophilization curve is obtained by recording the change of temperature of the shelf and the temperature of the product according to the change of time. The typical lyophilizationg curve divides the shelf temperature into two stages. The shelf temperature is kept low during mass sublimation, which can generally be controlled between -10℃ and 10℃ according to the actual situation. In the second stage, the shelf temperature will be raised according to the product properties appropriately. This method is suitable for products with low melting point. If the performance of the product is unknown and the performance of the machine is poor or its work is not stable enough, it is safer to use this method. If the eutectic point of the product is high, the vacuum degree of the system can be maintained well, and the refrigeration capacity of the condenser is sufficient then the certain heating rate can be adopted to raise the shelf temperature to the maximum allowable temperature until the lyophilization is finished. However, it is also necessary to ensure that the temperature of the product during mass sublimation should not exceed the eutectic point. If the product is unstable to heat, the second stage plate temperature should not be too high. In order to improve the sublimation speed in the first stage, the shelf temperature can be increased to above the maximum allowable temperature of the product at one time; When the mass sublimation stage is basically finished, the shelf temperature will be lowered to the maximum allowable temperature. Although the latter two methods have improved the speed of mass sublimation, the anti-interference ability will be reduced correspondingly, and the sudden decrease of vacuum degree and refrigeration capacity or power failure may melt the products. A reasonable and flexible approach to the first method is an increasingly common method used today.