FE $304-$ Unit Operations in Food Engineering

Practice $8$

Questions from Christie J$.$ Geankoplis, Transport Processes and Separation Process

Principles

$9\mathrm{.}6-1.$ Time for Drying in Constant$-$Rate Period. A batch of wet solid was dried on a tray dryer using constant drying conditions and a thickness of material on the tray of $25\mathrm{.}4mm.$ Only the top surface was exposed. The drying rate during the constant$-$rate period was $R=2\mathrm{.}05kgH{H}_2\frac{O}{h}.m^2(0\mathrm{.}42I{b}_m{H}_2\frac{O}{h}.f{t}^2).$ The ratio $\frac{L_s}{A}$ used was $24\mathrm{.}4kg$ dry solid $\frac{?}{m^2}$ exposed surface dry solid $/f{t}^2).$ The initial free moisture was $x_{\text{I}}=0\mathrm{.}55$ and the critical moisture content $x_c=0\mathrm{.}22kg$ free moisture $\frac{?}{kg}$ dry solid.

Calculate the time to dry a batch of this material from $x_1=0\mathrm{.}45$ to $x_2=0\mathrm{.}30$ using the same drying conditions but a thickness of $50\mathrm{.}8mm,$ with drying from the top and bottom surfaces. $($Hint: First calculate $\frac{L_s}{A}$ for this new case.$)$

Ans. $t=1\mathrm{.}785h$