#1

1. A flat plate coated with a volatile organic compound (VOC) is exposed to a parallel flow of atmospheric air at 20C, with a free stream velocity (i.e., upstream uniform velocity) of 5 m/s. The length of the flat plate in the direction of the flow is 0.5 m. The saturation concentration of the VOC in air at these conditions is 1.85 x 10 mol/cm', and its diffusivity in air under these conditions is 0.068 cm/s. The density of the VOC is 1.21 g/cm and its molecular weight is 140.2 g/mol. Assume the viscosity of the air under these conditions to be 0.155 cm/s. Determine (A) the mass transfer coefficient, km for this system, and (B) the time it would take for the average thickness of the VOC layer to decrease by 1 mm under these conditions. (Hint: though the system deals with diffusion of a solute through a gas instead of a liquid, the solution is similar. Be sure to select the correct equation that models this system.) (Ans: km = 0.464 cm; t = 11.63 days)