Project ForgeA wet solid of 100 kg is dried from a moisture content of 40 wt% to 10 wt%. The critical moisture content is 15 wt% and the equilibrium moisture content is negligible. All moisture contents are on dry basis. The falling rate is considered to be linear. It takes 5 hours to dry the material in the constant rate period. The duration (in hours) of the falling rate period is _____
Explanation
For a linear falling-rate curve through the origin, t_falling/t_constant = [(Xc−X*)/(X0−Xc)]·ln[(Xc−X*)/(Xf−X*)]. With X0=0.40, Xc=0.15, Xf=0.10, X*=0: ratio=(0.15/0.25)×ln(0.15/0.10)=0.6×0.4055=0.2433. t_falling=5×0.2433≈1.22 hours.
Source: GATE CH 2014 (2014)
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