Open sun drying experiments in thin layers of crain-crain (CC), fever (FV) and bitter (BT) leaves grown in Abeokuta, Nigeria were conducted. The drying process took place in the falling rate period and no constant rate period was observed from the drying curves. Eight thin layer mathematical drying models were compared using the multiple determination coefficients (R2), reduced chi-square (χ2) and root mean square error (RMSE) between the observed and predicted moisture ratios. Accordingly, Midilli et al. model satisfactorily described the drying curves of the three leaves with R2 of 0.9980, χ2 of 2.0×10-4 and RMSE of 1.09×10-2 for CC leaves; R2 of 0.9999, χ2 of 2×10-6 and RMSE of 1.11×10-3 for FV leaves; and R2 of 0.9998, χ2 of 1.9×10-5 and RMSE of 3.3×10-3 for BT leaves. The effective diffusivity was found to be 52.91×10-10, 48.72×10-10 and 43.42×10-10 m2/s for CC, BT and FV leaves, respectively.
The drying behavior of fever leaves was investigated in a convective hot air dryer with forced convection at a fixed air velocity of 1.5m/s and drying air temperatures of 35, 45, 55 and 65 0 C. The constant rate period was absent and the drying curve took place in the falling rate period. Experimental data was fitted to six thin-layer drying models -Page, Modified page II, Henderson and Pabis, Modified Henderson and Pabis, Two term and Newton. The performances of the models were investigated by comparing the coefficient of determination (R2), reduced chi-square (÷2) and root mean square error (RMSE) between observed and predicted moisture ratios. The page model showed a good agreement with the data obtained while the effect of drying air temperature on the constants and coefficient of the page model was investigated. The effective diffusivity increases as temperature increases and range between 5.551 x 10-12 -3.379 x 10-11m 2 /s while the activation energy was estimated to be 80.78kJ/mol.
In this study, a laboratory convective hot air dryer was used for the thin layer drying of blanched yam slices and experimental moisture ratio was compared with Newton, Logarithmic, Henderson and Pabis, modified Henderson and Pabis, approximation of diffusion, modified page 1, two-term exponential, Verma et al. and Wang and Singh models. Among all the models, the approximation of diffusion model was found to satisfactorily describe the kinetics of air-drying of blanched yam slices. The increase in air temperature significantly reduced the drying time with no constant rate period but drying occurs in falling rate period. The effective diffusivity values varied between 7.62 · 10 )8 to 9.06 · 10 )8 m 2 s )1 and increased with increase in temperature. An Arrhenius relation with an activation energy value of 8.831 kJ mol )1 showed the effect of temperature on moisture diffusivity.
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