$SO_2$ from air is absorbed by pure water in a counter current packed column operating at constant pressure. The compositions and the flow rates of the streams are shown in the figure.
In addition, use the following data and assumptions
- Column operates under isothermal conditions
- At the operating temperature of the column, $y^*=40x$, where $y^*$ is the mole fraction of $SO_2$ in the gas that is in equilibrium with water containing $SO_2$ at a mole fraction of $x$
- Solution is a dilute and the operating line is linear
- Negligible amount of water evaporates
The number of transfer units (NTU) for this column is