An experimental solar tent dryer under natural convection was used to study thin layer
drying kinetics of amaranth (
Amaranthus cruentus
) grains. Drying of grains in the
dryer was carried out on a drying rack having two layers; top and bottom. The
ambient temperature and relative humidity ranged from 22.6-30.4°C and 25-52%,
respectively, while the inside temperature and relative humidity in the solar dryer
ranged from 31.2-54.7°C and 22-34%, respectively. Freshly harvested amaranth
grains with an average moisture content of 64% were dried under the solar tent dryer
for seven hours to a final moisture content of 7% (dry basis). A non-linear regression
analysis was used to evaluate six thin layer drying models (viz., Newton, Page,
Modified Page, Henderson & Pabis, Logarithmic and Wang & Singh) for amaranth
grains. The models were compared using coefficient of determination (R
2), root mean
square error (RMSE), reduced chi-square (χ
2) and prediction performance (η
p) in
order to determine the one that best described thin layer drying of amaranth grains.
The results show that the Page model satisfactorily described the drying of amaranth
grains with R
2 of 0.9980, χ
2 of 0.00016 and RMSE of 0.01175 for bottom layer and R
2
of 0.9996, χ
2 of 0.00003 and RMSE of 0.00550 for top layer of the drying rack. Based
on a ± 5% residual error interval, the Page model attained the highest prediction
performance (η
p = 80%) when drying the grains in both layers of the dryer. This
shows that there was a good agreement between the predicted and experimental
moisture changes during solar drying of amaranth grains under natural convection.
The transport of water during dehydration was described by applying the Fick's
diffusion model and the effective moisture diffusivity for solar tent drying of
amaranth grains was found to be 5.88 x 10
-12 m
2s
-1 at the bottom layer and 6.20 x 10
-12
m
2s
-1 at the top layer. High temperatures developed at the top layer of the dryer led to
high effective moisture diffusivity and this showed that temperature strongly
influences the mechanism of moisture removal from the grains.
