Analysis of primary drying of poly-γ-glutamic acid during vacuum freeze drying

Abstract

This paper analyzes primary vacuum freeze drying of poly-γ-glutamic acid (γ-PGA), a biomaterial, by considering the variation in tortuosity under different operating conditions. The results indicate that lower heating-plate temperatures and thinner sample thicknesses increase tortuosity, but the influence of heating-plate temperature on tortuosity is relatively small compared to that of sample thickness. A tortuosity model reflecting mass-transfer characteristics of the pore structure in a dried γ-PGA sample was derived, and the drying process was numerically analyzed via a moving-grid system and the finite-difference method. Numerical simulation results using the developed drying model matched well with the experimental results, with an error of less than 10 %. Decreasing heating-plate temperature or increasing sample thickness increases the time required to complete primary drying. In particular, increases in sample thickness result in greater changes in drying time than those in heating-plate temperature. The sample thickness range that resulted in the fastest drying rate was approximately 14–19 mm depending on heating-plate temperature. Lower heating-plate temperature resulted in smaller optimum sample thickness. The maximum drying rate in this study was observed at a heating-plate temperature of −5 °C and a sample thickness of 19 mm.