Powell (Trans. Inst. Chem. Eng. Lond., 13, 175 (1935); 18, 36 (1940)] evaporated water from the
outside of cylinders into an airstream flowing parallel to the axes of the cylinders. The air
temperature was 25°C, and the total pressure standard atmospheric. The results are given by
wl 23.17 x 10-%(ul)*
Pu ~ Ba
where w= water evaporated, g/cm?- s
P, = partial pressure of water in airstream, mmHg
Py = water-vapor pressure at surface temperature, mmHg
u = velocity of airstream, cm/s
? = length of cylinder, cm
(a) Transform the equation into the form_jp = ¥(Re,), where Re, is a Reynolds number based
on the cylinder length.
(b) Calculate the rate of sublimation from a cylinder of napthalene 0.075 m diam by 0.60 m
long (3 in diam by 24 in long) into a stream of pure carbon dioxide at a velocity of 6 m/s (20 ft/s) at
1 std atm, 100°C. The vapor pressure of naphthalene at the surface temperature may be taken as
1330 N/m? (10 mmHg) and the diffusivity in carbon dioxide as 5.15 & 107® m/s (0.0515 cm”/s) at
STP. Express the results as kilograms naphthalene evaporated per hour. Ans.: 0.386 kg/h.