Abstract
This paper deals with the freeze-drying process and, in particular, with the optimization of the operating conditions of the primary drying stage. When designing a freeze-drying cycle, process control aims at obtaining the values of the operating conditions (temperature of the heating fluid and pressure in the drying chamber) resulting in a product temperature lower than the limit value of the product, and in the shortest drying time. This is particularly challenging, mainly due to the intrinsic nonlinearity of the system. In this framework, deep process knowledge is required for deriving a suitable process dynamic model that can be used to calculate the design space for the primary drying stage. The design space can then be used to properly design (and optimize) the process, preserving product quality. The case of a product whose dried layer resistance, one of the key model parameters, is affected by the operating conditions is addressed in this paper, and a simple and effective method to calculate the design space in this case is presented and discussed.
Highlights
IntroductionThe freeze-drying process is commonly used to remove water (or, more rarely, an organic solvent) from a product, aiming at increasing its shelf life
The freeze-drying process is commonly used to remove water from a product, aiming at increasing its shelf life
When the ice sublimation is completed, it is necessary to remove the water bound to the dried product: product temperature is increased to cause water desorption, obtaining the target value of residual humidity in the final product [1,2,3,4,5]
Summary
The freeze-drying process is commonly used to remove water (or, more rarely, an organic solvent) from a product, aiming at increasing its shelf life. Afterwards, the temperature of the shelves is increased and the pressure in the drying chamber is lowered in such a way that ice sublimation occurs (primary drying). When the ice sublimation is completed, it is necessary to remove the water bound to the dried product (secondary drying): product temperature is increased to cause water desorption, obtaining the target value of residual humidity in the final product [1,2,3,4,5]. The process is indicated for water removal in case of products that are damaged by the higher temperatures of other drying processes, and it is widespread in the pharmaceutical industry where the high value of the manufactured products justifies the cost (and the duration) of a freeze-drying process
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