A well-mixed open pond contains wastewater that is contaminated with a dilute concentration of dissolved methylene chloride. The pond is rectangular with dimensions of 500 m by 100 m, as shown in the figure (above right). Air at 27°C and 1.0 atm blows parallel to the surface of the pond with a bulk velocity of 7.5 mIs. At 20°C and 1.0 atm, for the gas phase (A = methylene chloride, B = air), the diffusion coefficient (D_AB) is 0.085 cm²/s, and kinematic viscosity (v_B) is 0.15 cm²/s. At 27°C, for the liquid phase, (A = methylene chloride, B = liquid water), the diffusion coefficient (D_AB) is 1.07 × 10^-5cm²/s, and the kinematic viscosity (v_B) is 0.010 cm²/s. 

a. At what position across the pond is the air flow no longer laminar? Would it be reasonable to assume that the mean film mass-transfer coefficient for methylene chloride in air is dominated by turbulent flow mass transfer?

b. As part of an engineering analysis to predict the emissions rate of methylene chloride (species A) from the pond, determine the average gas film mass-transfer coefficient associated with the mass-transfer methylene chloride from the liquid surface to the bulk air stream.

c. Compare the Schmidt number for methylene chloride in the gas phase vs. the liquid phase, and explain why the values are different.

Transcribed Image Text:

air flow (1.0 atm, 27°C) V = 7.5 m/s pond surface L2 = 100 m = 500 m