Analysis and modeling of heat and mass transfer and adsorption isotherms of the aqueous extracts vacuum contact drying

Abstract

Vacuum contact drying is the most merit drying technology to ensure the quality and throughput of traditional Chinese medicine aqueous extracts. To study the heat and mass transfer of colloidal semi-solid porous media during vacuum contact drying, the sharp interface formulation was established to simulate the thin layer (4 mm) Lonicera japonica Thunb. aqueous extracts vacuum drying at high-temperature (110 and 120 ℃) and low-pressure (10, 15 and 20 kPa). And mathematical models were used to investigated drying kinetics and adsorption isotherms. Results show that the sharp interface model can predict final temperature and moisture ratio with the error is less than 20 %. The models of viscosity variation, specific heat capacity, heat capacity, effective thermal conductivity and heat transfer coefficient of boiling under different conditions were investigated to provide reference for subsequent research. When the vacuum drying superheat is between 50 ℃ and 75 ℃, the boiling heat transfer coefficient is among 6 to 9 W/ (m2·K). The Page is the most suitable semi-empirical model to describe the water migration law of the aqueous extracts. The adsorption isotherms of the extract powder were investigated by experiment and the Peleg is the most suitable model.