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In a solvent regeneration process, a gas is used to strip a solute from a liquid in a countercurrent packed tower operating under isothermal condition. Pure gas is used in this stripping operation. All solutions are dilute and Henry’s law, $y^*=mx$, is applicable. Here, $y^*$ is the mole fraction of the solute in the gas phase in equilibrium with the liquid phase of solute mole fraction $x$, and $m$ is the Henry’s law constant.

Let $x_1$ be the mole fraction of the solute in the leaving liquid, and $x_2$ be the mole fraction of the solute in the entering liquid.

When the value of the ratio of the liquid-to-gas molor flow rates is equal to $m$, the overall liquid phase Number of Transfer Units, $\text{NTU}_{OL}$ is given by

- $\dfrac{x_2-x_1}{x_1} \\$
- $\dfrac{x_2+x_1}{x_2-x_1} \\$
- $\ln \left( \dfrac{x_2}{x_1} \right) \\$
- $\ln \left( \dfrac{x_2+x_1}{x_2-x_1} \right)$