A very thin polymeric coating of thickness 0.1 mm uniformly coats a rectangular surface. The rectangular surface has a length of 20 cm and a width of 10 cm. the coating contains a solvent that must be evaporated away from the coating in order to cure the coating. Initially, there is 0.001 mole of solvent per cm3 of coating loaded in the coating. A heated plate just beneath the surface maintains the coating at a uniform temperature of 408C, and the vapor pressure exerted by the solvent is 0.05 atm at 408C. Air gently flows parallel to the surface at a velocity of 5.0 cm/s. The surrounding air at 1.0 atm total system pressure and 208C represents an ‘‘infinite sink’’ for mass transfer. You may neglect any molecular diffusion of the solvent through the very thin polymeric film and focus only on the convection aspects of the problem. The diffusion coefficient of species in air at 208C is 0:1 cm2=s. a. Determine the Reynolds, Schmidt, and Sherwood numbers associated with this process. b. What is the film mass-transfer coefficient, ky, (mole fraction based driving force) associated with this process? c. How long will it take for the solvent to completely evaporate from the coating?
A very thin polymeric coating of thickness 0.1 mm
uniformly coats a rectangular surface. The rectangular surface
has a length of 20 cm and a width of 10 cm. the coating contains a
solvent that must be evaporated away from the coating in order to
cure the coating. Initially, there is 0.001 mole of solvent per cm3
of coating loaded in the coating. A heated plate just beneath the
surface maintains the coating at a uniform temperature of 408C,
and the vapor pressure exerted by the solvent is 0.05 atm at 408C.
Air gently flows parallel to the surface at a velocity of 5.0 cm/s.
The surrounding air at 1.0 atm total system pressure and 208C
represents an ‘‘infinite sink’’ for
any molecular diffusion of the solvent through the very thin
polymeric film and focus only on the convection aspects of the
problem. The diffusion coefficient of species in air at 208C is
0:1 cm2=s.
a. Determine the Reynolds, Schmidt, and Sherwood numbers
associated with this process.
b. What is the film mass-transfer coefficient, ky, (mole fraction
based driving force) associated with this process?
c. How long will it take for the solvent to completely evaporate
from the coating?
30.2 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 1:013 105 Pa. The naphthalene remains at 290 K; at this
temperature, the diffusivity of naphthalene in air is 5:61
106 m2/s and its vapor pressure is 26 Pa. Determine
a. the value of the mass-transfer coefficient at a point 0.3 m
downstream from the leading edge.
b. the moles of naphthalene per hour lost from the section of
the plate 0.5–0.75 m downstream from the leading edge.
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