An open well contains water contaminated with volatile benzene at the bottom of the well, with dimensions shown in the figure on page 525. The concentration of dissolved benzene in the water is 156 g/m³, and remains constant. The system is isothermal at 25ºC. We are interested in determining the emission of benzene, a carcinogen, into the atmosphere from the well.

a. Define the source, sink, and system boundary for all of the species undergoing mass transfer. State three reasonable assumptions that describe the mass-transfer process.
b. State reasonable boundary conditions, and specify their numerical values with units, for all of the species undergoing mass transfer.
c. What are the maximum emission rates (in mole/day) of benzene and water vapor from the well? What is cumulative benzene emission (in grams) over a period of 30 days? Is it significant?

Potentially useful data: The molecular diffusion coefficient of benzene in dissolved water is 1.1 × 10^-5 cm²/s at 25°C, and the molecular diffusion coefficient of benzene vapor in air is 0.093 cm²/s at 1.0 atm and 25°C. The Henry€™s law constant for benzene partitioning into water is H = 4.84 × 10^-3 m³ · atm/mole at 25°C. The vapor pressure of liquid water is 0.0317 bar (0.031 atm) at 25°C, and the density is 1000 kg/m³ at 25°C. The vapor pressure of benzene is 0.13 atm at 25°C. The humidity of the air flowing over the well hole is 40% of relative saturation at 25°C. The molecular weight of benzene is 78 g/mole.

Transcribed Image Text:

Flowing air 25°C, 40% RH Well 0.1 m diameter 3.0 m length Liquid water contaminated with benzene