GATE 2022 CH – Question 33
A wet solid containing 20% (w/w) moisture (based on mass of bone-dry solid) is dried in a tray-dryer. The critical moisture content of the solid is 10% (w/w). The drying rate (kg m$^{-2}$ s$^{-1}$) is constant for the first 4 hours, and then decreases linearly to half the initial value in the next 1 hour. At the end of 5 hours of drying, the percentage moisture content of the solid is _________% (w/w) (rounded off to one decimal place).
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Correct answer: 8.06 to 8.14
Explanation
In the constant rate period the moisture falls from 20% to the critical 10% in 4 h, which is 2.5% per hour at the initial rate. In the following hour the rate falls linearly (with time) to half, so its average is 0.75 of the initial rate: the moisture lost is $2.5 \times 0.75 = 1.875$%. The final moisture content is $10 - 1.875 = 8.1\%$.