A 1.75-cm diameter naphthalene mothball is suspended in an air stream at 280 K, 1.0 atm, and constant velocity of 1.4 m/s. Solid naphthalene exerts a vapor pressure of 2.8 Pa at 280 K. Consequently, the naphthalene very slowly sublimes into the passing air stream, with the rate limited by convective mass transfer. The density of solid naphthalene is 1.14 g/cm³,
and the molecular weight is 128 g/mole.
a. What is the initial evaporation rate of naphthalene mothball?
b. How long will it take for the mothball to shrink to half of its original diameter? Remember: as the mothball shrinks, its diameter is decreasing, which in turn affects the Reynolds and Sherwood numbers.