Freeze-drying, also known as sublimation drying, involves freezing a water-containing material below its freezing point, converting the water into ice. The ice is then removed by converting it into vapor under a high vacuum. For the freeze-drying of simple compounds, the eutectic point is a crucial material property that influences the success of the process. This quality attribute is influenced by the pre-freezing process and also by the primary sublimation process. However, from a broader perspective, the study of the freeze-drying process is more complex. Studies of the freeze-drying process for over a thousand injectable solutions have revealed that different types of material solutions, in addition to their dependence on the eutectic point, each have their own unique freeze-drying characteristics.

The ideal appearance of a freeze-dried injection should be a plump, white, fine, loose, and porous solid. However, in large-scale production, due to complex process parameters and the limited flexibility of equipment operation, the resulting injection's appearance is often unsatisfactory, often exhibiting blistering, delamination, surface unevenness, and discoloration, resulting in significant economic losses.

The vacuum system is a crucial component of the freeze dryer, typically consisting of a vacuum pump and Roots pump to accelerate vacuum. The vacuum system is connected to the condenser via sanitary stainless steel piping. A butterfly valve is installed near the condenser to isolate the condenser from the vacuum system. The vacuum system's function is to provide the necessary vacuum conditions for the freeze dryer, creating an environment for ice sublimation. The vacuum level of the vacuum system directly affects the sublimation rate of the product, and the vacuum system itself must have a low leakage rate. The accuracy of vacuum control directly impacts the operation of the pharmaceutical process.