Time for Drying in the 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 25.4 mm. Only the top surface was exposed. The drying rate during the constant-rate period was R = 2.05 kg H2O/h · m2 (0.42 lbm H2O/h · ft 2). The ratio LS /A used was 24.4 kg dry solid/m2 exposed surface (5.0 lbm dry solid/ft 2). The initial free moisture was X1 = 0.55 and the critical moisture content XC = 0.22 kg free moisture/kg dry solid.

Calculate the time to dry a batch of this material from X1 = 0.45 to X2 = 0.30 using the same drying conditions but a thickness of 50.8 mm, with drying from the top and bottom surfaces. (Hint: First calculate LS /A for this new case.)