Freeze-drying in a fluidized-bed atmospheric dryer and in a vacuum dryer: Evaluation of external transfer coefficients

Abstract

The factors which influence freeze-drying are classified as either internal or external. Internal factors derive from the properties of the material undergoing drying, while external factors relate to the physical conditions under which drying occurs. This paper looks at the external factors and compares them for two different freeze-drying methods: a vacuum dryer and an atmospheric fluidized-bed dryer containing an adsorbent. First, the ratio between the external heat and mass transfer coefficients, h/K, was evaluated: this ratio for the fluidized-bed dryer was constant at 398·8 kcal torr kg −1 °C −1 but varied for the vacuum dryer within the range 9·5 to 16·2 kcal torr kg −1 °C −1. The individual heat and mass transfer coefficients were calculated. The mass transfer coefficient, K, for the vacuum dryer was 1·0 kg h −1 m −2 torr −1; for the atmospheric fluidized-bed dryer, K varied slightly with temperature, being 0·7, 0·8 and 1·0 kg h −1 m −2 torr −1, at fluidized-bed temperatures of 0, −10 and −20°C, respectively. At these temperatures, the heat transfer coefficients for the fluidized-bed dryer were 287, 321 and 402 kcal h −1 m −2 °C −1, respectively. For the vacuum dryer h was 9·5–16·2 kcal h −1 m −2 °C −1. Thus, K values were comparable in the two drying systems but h in the fluidized-bed dryer was some 20–40 times greater than in the vacuum dryer. The study helped to elucidate the mechanisms involved during ice sublimation and contributed towards developing a means by which freeze-drying patterns could be predicted. The results were helpful in providing guidelines during the development of the novel atmospheric fluidized-bed technique.