Studies on the Drying of Agricultural Products (I)

Moisture Transfer between Hull and Hulled Rice during Drying Pause

Abstract

The authors studied moisture transfer mechanism from hulled rice to hull, and the relation between moisture contents of hull and drying pause time.
The results were as follows:
1. The driving force of moisture transfer during drying pause was mainly the difference in initial moisture contents between hull and unhulled rice.
2. The rate of moisture transfer increased proportionately with the increase of absolute temperatures and the increase of the difference in initial moisture contents between hull and unhulled rice.
3. At the initial stage of drying pause, nearly all of the moisture transfered in liquid from hulled rice to hull by capirally mechanism.
Then a small amount of moisture transfered to the hull by water vapour diffusion from unhulled rice through the gap between the two.
4. The maximum moisture content of hull during drying pause was expressed by the following equation.
M₀-Mci=e^8.36(Mhi-Mci/T)^0.475
5.0≤Mhi-Mci≤35.0, 293≤T≤313
where M₀ was the moisture content of the hull, Mci the inital moisture content of hulled rice at the drying pause.
5. The moisture content of hull during drying pause was expressed by the following equation.
Mui-Mcp/Mui-Mci
=5.03-9×10^-3(Mhi-Mci)-1.255×10^-2/θ^0.178
10≤θ≤180, 293≤T≤313
where Mcp was the moisture content of hull during drying pause, Mui was the initial moisture content of unhulled rice at the drying pause.
6. The relationship of moisture contents between hull and unhulled rice after drying was indicated by the following equations.
(a) Drying Temperature 60°C (air)
16.5≤Mu≤23.6 Mci=3.7e^0.0284M_u
23.6<Mu≤31.8 Mci=0.10e^0.0281M_u
(b) Drying Temperature 40°C (air)
16.6≤Mci≤24.0 Mci=3.8e^0.0243M_u
24.0<Mu=34.0 Mci=0.16e^0.158M_u
where Mu was the content of unhulled rice at the drying.