2.9b. Mass transfer from a flat plate.
A 1-m square thin plate of solid naphthalene is oriented parallel to a stream of air flowing at 20 m/s. The air is at 310 K and 101.3 kPa. The naphthalene remains at 290 K; at this temperature the vapor pressure of naphthalene is 26 Pa. Estimate the moles of naphthalene lost from the plate per hour, if the end effects can be ignored.

Solution
2.10b. Mass transfer from a flat plate.
A thin plate of solid salt, NaCl, measuring 15 by 15 cm, is to be dragged through seawater at a velocity of 0.6 m/s. The 291 K seawater has a salt concentration of 0.0309 g/cm3. Estimate the rate at which the salt goes into solution if the edge effects can be ignored. Assume that the kinematic viscosity at the average liquid film conditions is 1.02 ´ 10Ð6 m2/s, and the diffusivity is 1.25 ´ 10Ð9 m2/s. The solubility of NaCl in water at 291 K is 0.35 g/cm3, and the density of the saturated solution is 1.22 g/cm3 (Perry and Chilton, 1973) .

Solution
Laminar flow
At the bulk of the solution, point 2:
At the interface, point 1:
2.11b. Mass transfer from a flat liquid surface.
During the experiment described in Problem 2.3, the air velocity was measured at 6 m/s, parallel to the longest side of the pan. Estimate the mass-transfer coefficient predicted by equation (2-28) or (2-29) and compare it to the value measured experimentally. Notice that, due to the high volatility of acetone, the average acetone concentration in the gas film is relatively high. Therefore, properties such as density and viscosity should be estimated carefully. The following data for acetone might be needed: Tc = 508.1 K, Pc = 47.0 bar, M = 58, Vc = 209 cm3/mol, Zc = 0.232 (Reid, et al., 1987).

Solution
Average film properties:
Estimate the viscosity of the mixture from Lucas Method
Estimate the diffusivity from the Wilke-Lee equation
2.12b. Evaporation of a drop of water falling in air.
Repeat Example 2.9 for a drop of water which is originally 2 mm in diameter.

Solution