Che425: Problem set #1



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CHE425: Problem set #1
1. A beaker filled with an equimolar liquid mixture of ethyl alcohol and ethyl acetate evaporates at 0oC into still air at 101 kPa. All of the mass-transfer resistance is in the still air layer in the beaker. If each component evaporates independently determine the liquid composition when half the ethyl alcohol has evaporated. You can assume well-mixed liquid, ne negligible bulk flow effect, and Raoult’s law. The following data are available:





Vapor pressure (kPa) at 0oC

Diffusivity in air (m2/s)

Ethyl acetate (AC)

Ethyl alcohol (AL)



3.23

1.62


6.45×10-6

9.29×10-6


Therefore, the mole fractions in the well-mixed liquid when 50% of the AL has evaporated are,




2. An open tank, 10 ft in diameter, containing benzene at 25oC is exposed to air. Above the liquid surface is a stagnant air film 0.2 in. thick. If the pressure is 1 atm and the air temperature is 25oC, what is the loss of benzene in lb/day? The specific gravity of benzene at 60oF is 0.877. The concentration of benzene outside the film is negligible. For benzene, the vapor pressure at 25oC is 100 torr, and the diffusivity in air is 0.08 cm2/s.

Benzene loss rate = 9.14 x 10-7 (78.11)(72,930)(3600)(24)/454 = 991 lb/day



3) An insulated glass tube and condenser are mounted on a reboiler containing benzene and toluene. The condenser returns liquid reflux down the wall of the tube. At one point in the tube, the temperature ia 170oF, the vapor contains 30 mol% toluene, and the reflux contains 40 mol% toluene. The thickness of the stagnant vapor film is estimated to be 0.1 in. The molar latent heats of benzene and toluene are equal. Calculate the rate at which toluene and benzene are being interchanged by equimolar counter diffusion at this point in the tube in lbmol/hft2, assuming that the rate is controlled by mass transfer in the vapor phase. Gas diffusivity of toluene in benzene = 0.2 ft2/h. Pressure = 1 atm (in the tube). Vapor pressure of toluene at 170oF = 400 torr.

Benzene diffuses at the same rate in the opposite direction.



4.2 Air at 25oC and a dew-point temperature of 0oC flows past the open end of a vertical tube filled with water at 25oC. The tube has an inside diameter of 0.83 inch, and the liquid level is 0.5 inch below the top of the tube. The diffusivity of water in air at 25oC is 0.256 cm2/s. (a) How long will it take for the liquid level in the tube to drop 3 inches? (b) Use Matlab to plot the tube liquid level as a function of time for this period. Label the graph with your name using the Title command.
Pure water

0.5 in cm

t = 2,260 h

5.2 Two bulbs are connected by a tube, 0.002 m in diameter and 0.20 m long. Bulb 1 contains argon, and bulb 2 contains xenon. The pressure and temperature are maintained at 1 atm and 105oC. The diffusivity is 0.180 cm2/s. At a time when the argon mole fraction at end 1 of the tube is 0.75, and 0.20 at the other end, determine the: (a) Rates and directions of mass transfer of argon and xenon; (b) Transport velocity of each species; (c) Molar average velocity of the mixture.



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