The “soil venting” shown in the figure (next page) is used to treat soil contaminated with volatile, toxic liquids. In the present situation, the porous soil particles are saturated with liquid TCE, a common industrial solvent. The contaminated soil is dug up at the waste site and loaded into a rectangular trough. The soil consists of porous mineral particles with an average diameter of 3.0 mm, loosely compacted into a packed bed with a void fraction 0.50. Air is introduced at the bottom of the trough through a distributor and flows upward around the soil particles. Liquid TCE satiating the pores of the soil particle evaporate into constant source for mass transfer, at least until 80% of the volatile TCE soaked within the soil is removed. Under these conditions, the transfer of TCE from the soil particle to the air stream is limited by convective mass transfer across the gas film surrounding the soil particles. The mass flow rate of air per unit cross section of the empty bed is 0.10 kg/m² s. The process is carried out at 293 K. At this temperature, the vapor pressure of TCE is P_A =  58 mm Hg. The molecular diffusion coefficient of TCE vapor in air is given in Example 3 of this chapter.
a. What is the gas–film mass-transfer coefficient for TCE vapor in air?
b. At what position in the bed will the TCE vapor in the air stream reach 99.9% of its saturated vapor pressure? In your solution, you may want to consider a material balance on TCE in the gas phase within a differential volume element of the bed. Assume the convective mass-transfer resistances associated with air flowing over the top surface of the bed are negligible and that there is no pressure drop of the gas stream through the bed so that the total system pressure remains constant at 1.0 atm.

Transcribed Image Text:

Air blower Air + TCE vapors diluted to atmosphere TCE contaminated packed soil bed Air flow distributor (0.1 kg air/m². sec) Z=L Raised trough for soil venting Z=0