Alumina particles with an initial moisture content of $5 \mathrm{~kg}$ per $\mathrm{kg}$ dry solid are dried in a batch dryer. For the first two hours, the measured drying rate is constant at $2 \mathrm{~kg} \mathrm{~m}^{-2} \mathrm{~h}^{-1}$. Thereafter, in the falling-rate period, the rate decreases linearly with the moisture content. The equilibrium moisture content is $0.05 \mathrm{~kg}$ per $\mathrm{kg}$ dry solid and the drying area of the particles is $0.5 \mathrm{~m}^{2}$ per $\mathrm{kg}$ dry solid. The total drying time, in $\mathrm{h}$, to reduce the moisture content to half its initial value is
- $4.13$
- $2.55$
- $3.22$
- $5.13$