There are 3 stages in a complete freeze drying process whether you use a commercial freeze dryer, industrial freeze dryer or a home lyophilizer.
The freezing step is the most important and critical in the whole freeze drying process. Because incomplete frozen product can be boiled and spoiled under vacuum environment. Keep in mind that freezing step guarantees the final products' form.
Products freezing can be done inside or outside of freeze dryers. The key is to cool the material below its eutectic point (lowest temperature that solid and liquid phases of the material can coexist). This ensures that sublimation rather than melting in the following steps.
Primary Drying Stage:
The primary drying step is to remove physical water from material. During this stage, pressure is lowered to a few mbars and proper heat is supplied to the material, so that there is enough energy for water to sublimation. In this stage, >95% water will be removed. So, Primary drying aim to remove physical water.
Primary drying takes longer time (several hours to several days) because you need add heat gradually. Too much heat add to material in a short time will cause melt. As well as, the vacuum degree, which can speed the sublimation need set considered.
Secondary Drying Stage:
The secondary drying step aims to remove bound water molecules. In this phase, more energy and high vacuum will be applied. Higher temperature (can be above 0C) and lower pressure (<30Pa) to encourage desorption. Secondary drying remove bound water effectively.
During the vacuum freeze drying, sublimated vapor will temporarily store in an external vessel which called ice bank, vapor collector or ice condenser. Once freeze drying process is complete, after vacuum broken, the condensed vapor (ice format) will be discharged after thaw.