INSIGHT - COMMENTS
Vol. 6 No. 2 - The Importance of Units and Terms in Lyophilization
Comment on the INSIGHT “The Importance of Units and Terms in Lyophilization”
Thomas A. Jennings Ph.D. presented a scientific and a comprehensive analyze of the importance of units and terms in lyophilization, subject used as title for his last INSIGHT. In that INSIGHT, Thomas A. Jennings Ph.D. takes into discussion the paper “Freezing and Annealing Phenomena in Lyophilization: Effects upon Primary Drying Rate, Morphology, and Heterogenity”, published by Theodore W. Randolph and J.A. Searles in the “American Pharmaceutical Review”.
The freeze - drying (lyophilization) is a very complicated process based on the simultaneous heat and mass transfer at low pressure and low temperatures. Process improvements can be obtained if an accurate interpretation of phenomena involved is available.
I agree with T. A. Jennings’s notes, and I would like to go further on the same topic.
For the readers of a scientific paper it is useful to find a list of the symbols and their units to avoid any confusion. Sometimes, a physical model if is presented, helps readers to understand easily the presented phenomena.
Regarding Eq. (2), in terms of mass transfer, in my opinion, the sublimation rate does not imply the resistance terms. In a general form, the mass transfer could be written as:
B/p is a simplex taking into account the chamber pressure influence.
The Kv coefficient have to be determined in such a way so that it will describe the viscous, transition and molecular gas flow.
The term of conductance proposed by T. A. Jennings [1] shows directly the special character of the gas flow, at very low pressure, and its dependence of the Knudsen number.
Of course, in the special literature, we find different terms to describe the diffusion or the mass transfer of the vapor through a porous material named as: effective diffusivity [2], permeability [3].
For the value Pproduct , vapor pressure of water at the sublimation front, the authors indicate the equation (3), represented by Antoine equation. Getting of the Pproduct from this equation, and using it in getting of Equation (4), leads to a confusion - the equation (4) is not homogeneous dimensional, as Jennings demonstrated in his INSIGHT.
I would suggest the using of the Clapeyron’s equation, as following:
In this way, it is visible that the Pproduct has a pressure unit and one avoids any confusion.
Finally I do appreciate any discussion making clear different aspects in the special literature.
Ana Bacaoanu Ph.D.
Chemical Engineering Department
Technical “Gh. Asachi” University of Iasi, Romania
E-mail: abacaoan@ch.tuiasi.ro
References:
1. T. A. Jennings, Lyophilization - Introduction and Basic Principles, Interpharm Press, Buffalo Grove, IL 1999.
2. I. Athanasios, Liapis and Roberto Bruttini, Handbook of Industrial Drying, Second edition, Edited by Arun S. Mujumdar, New York, 1995.
3. Y. Sagara, Measurement of transport properties for the dried layer of coffee solution undergoing freeze drying, Drying Technology, 12 (5), 1081 -1103, 1994
March 2003
Response by T.A. Jennings, Ph.D.
Thank you Professor Bacaoanu for your very thoughtful comment and response. I do hope that others will also make their views know on this topic for only by such discussions will lyophilization advance from an art to a science.
