A sheet of cellulose acetate film containing 5.00wt% liquid acetone enters an adiabatic dryer where 90% of the acetone evaporates into a stream of dry air flowing over the film. The film enters the dryer at Tf1 = 35°C and leaves at Tf2 (°C). The air enters the dryer at Ta1 (°C) and 1.01 atm and exits the dryer at T = 49°C and 1 atm with a relative saturation of 40%. Cp may be taken to be 1.33 kJ/ (kg· °C) for dry film and 0.129 kJ/ (mol·°C) for liquid acetone. Make a reasonable assumption regarding the heat capacity of dry air. The heat of vaporization of acetone may be considered independent of temperature. Take a basis of 100 kg film fed to the dryer for the requested calculations.

(a) Estimate the feed ratio [liters dry air (STP)/kg dry film].

(b) Derive an expression for Tai in terms of the film temperature change, (Tf2 - 35), and use it to answer parts (c) and (d).

(c) Calculate the film temperature changes if the inlet air temperature is 120°C.

(d) Calculate the required value of Ta1 if the film temperature falls to 34°C, and the value if it rises to 36°C.

(e) If you solved parts (c) and (d) correctly, you found that even though the air temperature is consistently higher than the film temperature in the dryer, so that heat is always transferred from the air to the film, the film temperature can drop from the inlet to the outlet. How is this possible?