Deep-Bed Grain Drying - A Review of Particular Theories

Theories developed for explaining the drying phenomena in grain drying systems are referred to as particular grain drying theories. The objectives of this paper are to review various particular grain drying theories and to suggest directions for future research in this area. Grain drying models are classified as non-equilibrium, equilibrium, and logarithmic type. Four non-equilibrium models (stationary bed, cross-, concurrent-, and counter-flow) were reviewed. Under low temperature and low airflow conditions, drying in a stationary deep-bed occurs at near equilibrium. The derivation of an equilibrium grain drying model and its necessary assumptions were discussed. Logarithmic models were developed and applied by many researches mostly to low temperature drying in stationary deep-beds. A semi-empirical thin-layer equation is commonly used in the reported grain drying models. The inclusion of this equation is an important factor, that determines the cognitive and practical values of a grain drying model. Due to this equation, the discussed models lack the capability of explaining the drying process by the laws of diffusion and the mathematical theory of drying solids. The computer simulation of grain drying systems can provide information for design and improvement of grain drying systems.