The Definition of Analysis Drying And The Factors Affecting The Time of Analysis Drying

The definition of analysis drying and the factors affecting the time of analysis drying

Analysis drying, also known as secondary drying, occurs after the first drying phase. After the primary drying, some adsorbed water (physically adsorbed on the capillary walls and polar groups of the dried material) and bound water (either bound to the crystal surface of crystalline products or included within the interior of amorphous products) remain. These waters are not frozen and cannot be removed during the primary drying. When present in sufficient quantities, they can provide conditions for the growth and reproduction of microorganisms and some chemical reactions. Studies have shown that even a small amount of moisture can have the same mobility and reactivity as in aqueous solutions. Therefore, to improve the storage stability of the product and extend its shelf life, it is necessary to remove these moistures. This is the purpose of analysis drying. The primary drying removes water in the form of ice crystals, so the temperature and pressure must be controlled below the eutectic point of the product to prevent the ice crystals from melting. However, adsorbed and bound waters have higher adsorption energy and require an increase in product temperature to be解析d. At this point, even if the product temperature exceeds the eutectic point, it will not melt, but it should be noted that the temperature should not destroy the product or cause denaturation due to overheating. Due to the temperature difference in heat transfer, the shelf temperature can generally be 2-5°C higher than the material temperature. Additionally, during this stage, the freeze-dryer must maintain a high vacuum to create a significant water vapor pressure difference inside and outside the product, providing enough driving force for the解析d water vapor to escape the product. Since adsorbed and bound water is subject to bond forces, its saturation vapor pressure is much lower than that of free water, resulting in a slower evaporation rate. After secondary drying, the residual moisture content in the product depends on the type and requirements of the product, generally between 1% and 5%.

Factors affecting the analysis drying time:

01 Type of product: Different products have different degrees of difficulty in drying, and different maximum allowable temperatures. Products with higher maximum allowable temperatures can maintain higher temperatures, and the analysis drying time can be correspondingly shortened.

02 Product water content: Products with low water content requirements have longer drying times. The residual moisture content of the product should be conducive to its long-term storage; too high or too low is not suitable. It should be determined based on experiments.

03 Performance of the freeze-dryer: Freeze-dryers that can achieve high vacuum in the later stage of analysis, have low condenser temperatures, and small shelf temperature differences can have relatively shorter analysis drying times.

04 Use of pressure control method: If the pressure control method is used, it can improve heat transfer, shorten the time for the product to reach the maximum allowable temperature, and also shorten the analysis drying time. Typically, the vacuum inside the freeze-dryer is controlled at 20-30 Pa at this time.