Heat and mass transfer scale-up issues during freeze-drying,I: Atypical radiation and the edge vial effect |
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Authors: | Shailaja Rambhatla Michael J Pikal |
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Institution: | (1) School of Pharmacy, University of Connecticut, 06269 Storrs, CT;(2) Biological Products, Bayer Corporation, 27520 Clayton, NC |
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Abstract: | The aim of this study is to determine whether radiation heat transfer is responsible for the position dependence of heat transfer
known as the edge vial effect. Freeze drying was performed on a laboratory-scale freeze dryer using pure water with vials
that were fully stoppered but had precision cut metal tubes inserted in them to ensure uniformity in resistance to vapor flow.
Sublimation rates were determined gravimetrically. Vials were sputter-coated with gold and placed at selected positions on
the shelf. Average sublimation rates were determined for vials located at the front, side, and center of an array of vials.
Sublimation rates were also determined with and without the use of aluminum foil as a radiation shield. The effect of the
guardrail material and its contribution to the edge vial effect by conduction heat transfer was studied by replacing the stainless
steel band with a low-thermal conductivity material (styrofoam). The emissivities (ε) of relevant surfaces were measured using
an infrared thermometer. Sublimation rate experiments were also conducted with vials suspended off the shelf to study the
role of convection heat transfer. It was found that sublimation rates were significantly higher for vials located in the front
compared to vials in the center. Additional radiation shields in the form of aluminum foil on the inside door resulted in
a decrease in sublimation rates for the front vials and to a lesser extent, the center vials. There was a significant decrease
in sublimation rate for goldcoated vials (ε≈0.4) placed at the front of an array when compared to that of clear vials (ε≈0.9).
In the case of experiments with vials suspended off the shelf, the heat transfer coefficient was found to be independent of
chamber pressure, indicating that pure convection plays no significant role in heat transfer. Higher sublimation rates were
observed when the steel band was used instead of Styrofoam while the highest sublimation rates were obtained in the absence
of the guardrail, indicating that the metal band can act as a thermal shield but also transmits some heat from the shelf via
conduction and radiation. Atypical radiation heat transfer is responsible for higher sublimation rates for vials located at
the front and side of an array. However, the guardrail contributes a little to heat transfer by conduction. |
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Keywords: | freeze-drying radiation heat transfer heat transfer coefficients vial array effects scale-up |
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