Freeze drying, also known as lyophilization, is a dehydration technique that removes water or other frozen solvents from a material through sublimation and desorption. In this process, the product is first completely frozen, typically in vials, flasks, or trays - then placed under a deep vacuum, well below the triple point of water. Heat energy is carefully added, causing the ice within the product to sublime directly into vapor without passing through the liquid phase. This phase change, known as sublimation, is essential to freeze drying and requires precise thermal control to ensure the quality of the final product.
As a method, freeze drying was known as early as the late 19th century but only gained industrial relevance after World War II, with the pharmaceutical industry leading its commercial adoption. It remains a critical process for preserving heat-sensitive materials such as proteins, antibiotics, blood plasma, and living cells. In the food industry, freeze drying began to gain traction in the late 1950s, valued for its ability to preserve flavor, color, appearance, and nutrients that are often degraded by conventional drying methods. Despite its advantages, freeze drying is an energy-intensive and expensive process, typically reserved for high-value products where maintaining structural and sensory qualities justifies the higher production cost.
The material balances are based on the evaporation percentages of the components designated as volatile (solvent). These percentages can either be specified by the user or calculated by the program from the difference between the initial and the final LOD%.
A cooling duty is calculated to reduce the material temperature to the specified freezing point, during which a user-defined percentage of the solvent is assumed to freeze. This calculation incorporates the enthalpy of fusion, which may also be specified by the user. Following freezing, a heating duty is determined to raise the material temperature from the freezing point to the sublimation temperature, at which the frozen solvent is assumed to sublimate. The enthalpy of sublimation, likewise user-defined, is used in this calculation. Finally, the remaining material is heated to the designated exit temperature. If the amount of solvent sublimed is insufficient to achieve the target final LOD percentage, the program assumes that the residual solvent is evaporated at the exit temperature, and the associated energy demand is included in the overall energy balance.
In Design Mode, the program will calculate the sublimation capacity as well as the required area based on either the wet cake depth, the specific evaporation rate or the specific sublimation rate. If the calculated sublimation capacity exceeds the maximum, the program assumes multiple units operating in parallel with a total sublimation capacity equal to the calculated one.
In Rating Mode, the user specifies the number of parallel units, their tray area and their sublimation capacity and the program calculates the wet cake depth, the specific evaporation rate and the specific sublimation rate. If the operating sublimation capacity is higher than the rated sublimation capacity or the calculated wet cake depth exceeds the maximum allowable value, a warning is generated.
The overall process time is the sum of the freezing time the heating/drying time and the setup time. The freezing and setup times are always specified by the user. In design mode the user has to specify the heating/drying time as well but in rating mode it can be calculated based either on the specific evaporation rate or the specific sublimation rate.
See Vacuum Pump Auxiliary Equipment Calculations.
1. Geankoplis, C.J. (1983). Transport Processes and Unit Operations, Allyn and Bacon, 2nd ed., pp. 554-557.
2. Perry R.H. and D.W. Green (1984). Perry’s Chemical Engineers’ Handbook, 6th ed. McGraw-Hill, section 20 pp. 22-23.
The interface of this operation has the following tabs:
● Oper. Cond’s, see Freeze Drying: Oper. Conds Tab
● Utility Data, see Freeze Drying: Utilities Tab
● Labor, etc, see Operations Dialog: Labor etc. Tab
● Description, see Operations Dialog: Description Tab
● Batch Sheet, see Operations Dialog: Batch Sheet Tab
● Scheduling, see Operations Dialog: Scheduling Tab