A wastewater stream of $0.038 \mathrm{~m}^{3} / \mathrm{s}$, containing $10 \mathrm{ppm}$ (by weight) of benzene, is to be stripped with air in a packed column operating at $298 \mathrm{~K}$ and 2 atm to reduce the benzene concentration to $0.005 \mathrm{ppm}$. The packing specified is $50-\mathrm{mm}$ plastic Pall rings. The airflow rate to be used is five times the minimum. Henry's law constant for benzene in water at this temperature is $0.6 \mathrm{kPa} \cdot \mathrm{m}^{3} / \mathrm{mol}$ (Davis and Cornwell, 1998). Calculate the tower diameter if the gas-pressure drop is not to exceed $250 \mathrm{~Pa} / \mathrm{m}$ of packed height. Estimate the corresponding mass-transfer coefficients. The diffusivity of benzene vapor in air at $298 \mathrm{~K}$ and $1 \mathrm{~atm}$ is $0.096 \mathrm{~cm}^{2} / \mathrm{s}$; the diffusivity of liquid benzene in water at infinite dilution at $298 \mathrm{~K}$ is $1.02 \times 10^{-5} \mathrm{~cm}^{2} / \mathrm{s}$ (Cussler, 1997).