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Problem 1 – Diffusion through a Stagnant Aquatic Phase
A chloroform spill sinks to the bottom of a puddle of water, forming a layer 1.5 cm thick, and
covered with 1.0 cm of water. Compute the time it will take for the chloroform to completely
evaporate. Assume the water is stagnant; the air above the water is humid, so the water itself
does not evaporate; the air is well mixed above the water, and contains a negligible concentration
of chloroform. Use a chloroform diffusivity in water of 8.19×10-6 cm
2
/s.
Problem 2 – Film Theory: Determination of Evaporative Losses
A pan of water and a pan of toluene are placed outdoors exposed to the air. After 24 hours 0.780
cm of the water and 3.687 cm of the toluene are found to have evaporated. Use these results to
compute the:
a. average mass transfer coefficient (MTC) for both contaminants
b. alpha coefficient for extrapolating MTC from the diffusion coefficient for both liquids, and
c. MTC and evaporation rate (cm/d) for methylene chloride.
The following diffusivity and vapor pressure values are given:
Parameter Water Toluene Methylene Chloride
D, cm2
/s 0.239 0.084 0.1036
Po
, mm Hg 17.5 28.1 349
Problem 3 – Fractional Mass Transfer Resistance in a Pond-Air System
Consider the mass transfer of methylene chloride dissolved in a pond into the atmosphere. What
fraction of the total resistance to mass transfer will be in the liquid phase in calm day when a
zero wind speed can be assumed?
Problem 4 – Soil Vapor Extraction of TCE
Estimate the mass transfer coefficient for the evaporation of trichloroethylene adsorbed on soil.
The soil has a mean particle size of 0.5 mm, a porosity of 0.35. Assume that air is drawn through
the soil at a superficial velocity of 10.0 cm/s.
Problem 5 – Evaporation Losses from a Flat Plate into Dry Air
Estimate the mass transfer coefficient and evaporation rate for spills of water and of benzene in a
5.0 mile/h wind.
Problem 6 – Mass Transfer Rate in Diffused Aeration
100 liters per minute of air is bubbled into a tank of 20 liters of water with a dissolved oxygen
concentration of 0.0. Assume that Cs for oxygen is 9.2 mg/L and KLa is 7.0 hr-1
.
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