A wastewater stream flowing at the rate of 600 gpm (gallons per minute) and containing 10 ppm (parts per million by weight) benzene is to be stripped with air in a packed column operating at 25 °C and 2 atm absolute pressure to produce water containing 0.005 ppm benzene. The packing is 1-inch Berl saddles. The solubility of liquid benzene in water at 25 °C is 0.180 g per 100 g of water. An expert has suggested using a flow rate of air of 1,000 scfm (standard cubic feet per meter) of air (60°F, 1 atm), at which condition one should achieve for the mass transfer of benzene:
k_La = 0.067 s⁻¹ and k_Ga = 0.80 s⁻¹
Determine:
(a) The minimum air stripping rate in scfm. Is it less than the rate suggested by the expert? If not, use 1.4 times your minimum value.
(b) The stripping factor based on the air rate suggested by the expert.
(c) The number of transfer units, NOG, required.
(d) The overall mass-transfer coefficient, KGa, in units of mol/m³-s-kPa and s⁻¹. Which phase controls mass transfer?
(e) The volume of packing in cubic meters.