A mathematical model of drying processes

Abstract

A drying model is proposed which may be used to describe drying behavior of hygroscopic and non-hygroscopic materials. The constant rate, the first falling rate and the second falling rate periods in drying are addressed separately. The concept of ‘bound water conductivity’ is introduced. Movement of bound water and its contribution to moisture transfer within hygroscopic materials are discussed. The bound water conductivity is found to be affected by moisture content as well as desorption isotherms of the drying material. The major internal moisture transfer mechanisms are considered to be capillary flow of free water in the wet region and movement of bound water and vapor transfer in the sorption region. The convective heat and mass transfer coefficients are assumed to vary with the surface free water content in the first falling rate period. Three systems with different hygroscopic properties, wool, brick and corn kernels, are chosen to evaluate the validity of this model. The moving finite element method is used to solve the differential equations numerically. The predicted drying curves and the temperature and moisture distributions compare favorably with reported experimental results.