Drying equations of Thai Hom Mali paddy by using hot air, carbon dioxide and nitrogen gases as drying media

The objective of this study was to develop a drying equation for predicting the thin layer drying kinetics of dried Thai Hom Mali paddy using different drying gases. Thai Hom Mali paddy cv. Khao Dok Mali 105 with initial moisture content of 32% dry basis was dried in a heat pump dryer at 0.4 m/s fixed superficial velocity, 60% fixed evaporator bypass air ratio, and varied drying temperatures of 40, 50, 60 and 70 °C using hot air, CO2 and N2 gases as drying media. Drying rate was not affected by drying gases but increased with drying temperatures. Moisture ratios, at any given time during the drying process, were compared among various models, namely, Newton, Page, Modified Page I, Henderson and Pabis, two-term, approximation of diffusion, and Midilli. The effect of drying air temperatures on the coefficients of the best moisture ratio model was determined by single step regression method. The R2 coefficient, root mean square error (RMSE) and chi-square (χ2) were criteria for selecting the best model. The study found that the Midilli model was the best model for describing the drying behavior of Thai Hom Mali paddy in every evaluated drying gas. It should be possible to predict the moisture content of a product with a generalized model that shows the effect of drying air temperature on the model constants and coefficients.

► Thai paddy had the similar drying behavior under various gases: hot air, N2 and CO2 in the range of 40–70 °C.
► The Midilli model was the best model for describing the drying behavior of paddy in each gas.
► The drying constant of Midilli model in the form of Arrhenius type model, is suitable for drying under hot air.
► The drying constant of Midilli model in the form of linear type model was selected for CO2 and N2 drying.