Technical Briefs

Model for Heat and Mass Transfer in Freeze-Drying of Pellets

[+] Author and Article Information
Ioan Cristian Trelea1

UMR 782 Génie et Microbiologie des Procédés Alimentaires, AgroParisTech, INRA, 1 Avenue Lucien Brétignières, 78850 Thiverval-Grignon, Francecristian.trelea@agroparistech.fr

Stéphanie Passot, Michèle Marin, Fernanda Fonseca

UMR 782 Génie et Microbiologie des Procédés Alimentaires, AgroParisTech, INRA, 1 Avenue Lucien Brétignières, 78850 Thiverval-Grignon, France


Corresponding author.

J Biomech Eng 131(7), 074501 (Jun 04, 2009) (4 pages) doi:10.1115/1.3142975 History: Received October 07, 2008; Revised April 14, 2009; Published June 04, 2009

Lyophilizing frozen pellets, and especially spray freeze-drying, have been receiving growing interest. To design efficient and safe freeze-drying cycles, local temperature and moisture content in the product bed have to be known, but both are difficult to measure in the industry. Mathematical modeling of heat and mass transfer helps to determine local freeze-drying conditions and predict effects of operation policy, and equipment and recipe changes on drying time and product quality. Representative pellets situated at different positions in the product slab were considered. One-dimensional transfer in the slab and radial transfer in the pellets were assumed. Coupled heat and vapor transfer equations between the temperature-controlled shelf, the product bulk, the sublimation front inside the pellets, and the chamber were established and solved numerically. The model was validated based on bulk temperature measurement performed at two different locations in the product slab and on partial vapor pressure measurement in the freeze-drying chamber. Fair agreement between measured and calculated values was found. In contrast, a previously developed model for compact product layer was found inadequate in describing freeze-drying of pellets. The developed model represents a good starting basis for studying freeze-drying of pellets. It has to be further improved and validated for a variety of product types and freeze-drying conditions (shelf temperature, total chamber pressure, pellet size, slab thickness, etc.). It could be used to develop freeze-drying cycles based on product quality criteria such as local moisture content and glass transition temperature.

Copyright © 2009 by American Society of Mechanical Engineers
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Grahic Jump Location
Figure 1

Schematic representation of heat transfer during freeze-drying of pellets

Grahic Jump Location
Figure 2

Model validation based on product temperature. Measured product temperature (symbols), product temperature calculated by the presented model (solid), product temperature calculated by a compact layer model (dash-dotted), shelf temperature (bold), and chamber temperature (dotted).

Grahic Jump Location
Figure 3

Model validation based on partial vapor pressure in the chamber. Measured vapor pressure (symbols), vapor pressure calculated by the presented model (solid), vapor pressure calculated by a compact layer model (dash-dotted), and total (vapor+inert gas) pressure in the chamber (bold).




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