Rapid freeze-drying cycle optimization using computer programs developed based on heat and mass transfer models and facilitated by tunable diode laser absorption spectroscopy (TDLAS)

Abstract

Computer programs in FORTRAN were developed to rapidly determine the optimal shelf temperature, Tf, and chamber pressure, Pc, to achieve the shortest primary drying time. The constraint for the optimization is to ensure that the product temperature profile, Tb, is below the target temperature, Ttarget. Five percent mannitol was chosen as the model formulation. After obtaining the optimal sets of Tf and P c, each cycle was assigned with a cycle rank number in terms of the length of drying time. Further optimization was achieved by dividing the drying time into a series of ramping steps for Tf, in a cascading manner (termed the cascading Tf cycle), to further shorten the cycle time. For the purpose of demonstrating the validity of the optimized Tf and Pc, four cycles with different predicted lengths of drying time, along with the cascading Tf cycle, were chosen for experimental cycle runs. Tunable diode laser absorption spectroscopy (TDLAS) was used to continuously measure the sublimation rate. As predicted, maximum product temperatures were controlled slightly below the target temperature of -25°C, and the cascading Tf-ramping cycle is the most efficient cycle design. In addition, the experimental cycle rank order closely matches with that determined by modeling. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association.