Analyze Freeze-drying Pre-freezing Rate
Books that often introduce freeze-drying theory will mention that the greater the rate of cooling, the greater the degree of supercooling and supersaturation of the solution, the smaller the particle size of the critical crystallization, the faster the nucleation rate, and the easier the formation of smaller particles. Fine crystal. Therefore, after the ice crystals sublimate, the pore size formed in the material is small, the drying rate is low, but the rehydration after drying is good; on the contrary, the slow freezing tends to form large particles of ice crystals, and the water vapor escape passage formed by the ice crystal sublimation is larger in size. It is beneficial to increase the drying rate, but the rehydration after drying is poor.
Of course, there is nothing wrong with this, but don't forget that this theory is derived from the uniformity of heat. However, the freeze-drying conditions provided by the medical freeze dryers in our factory are not so ideal. It's not a matter of how fast the heat transfer oil cools down. Relatively speaking, I am more in favor of the reference of a forum comrades. He divided the quick freezing and slow freezing into the following categories: 1. The plate temperature dropped faster, and the plate temperature was much lower than the product temperature. The bottom of the product first freezes to produce crystals, but the upper liquid is still hot, so it will not be all in an instant. Crystallization, crystallization will grow slowly, and the effect of slow freezing is obtained. 2. The temperature of the plate is slower, and the temperature difference between the plate temperature and the product temperature is not large. The whole product is uniformly cooled and formed into a supercooling. When the energy accumulation is sufficient, all the crystals are instantaneously crystallized, and the effect of quick freezing is obtained. 3. The temperature of the plate drops very slowly and maintains (or slowly cools) at a suitable temperature below the eutectic point. The product forms a small degree of subcooling, and a small amount of crystallization occurs first in the liquid. These crystallization phenomena are further reduced in temperature. Continue to proceed slowly, so that you get a larger crystal, which is the real slow freezing. 4. The product is immersed in an ultra-low temperature environment (such as liquid nitrogen), and the whole crystal is instantly crystallized to form a very fine crystal (or in an amorphous state), which is the real quick freezing. I have observed most of the phenomena mentioned by this comrade-in-arms during the test. Therefore, I still prefer this method of division.
