Interphase mass transfer involves the transport of a species from one phase (e.g. gas) to another (e.g. liquid). Whitman\'s **Two-Film Theory** assumes that laminar films exist on both sides of the interface, and equilibrium exists at the contact line.

Overall Mass Transfer Coefficient

The total transport resistance is the sum of gas-film and liquid-film resistances. The overall mass transfer coefficient based on gas concentrations \(K_Y\) is calculated as:

\[\frac{1}{K_Y} = \frac{1}{k_y} + \frac{m}{k_x}\]

Where \(k_y\) and \(k_x\) are local convective transfer coefficients, and \(m\) is the slope of the Henry\'s law equilibrium line: \(y_i = m x_i\).

The Lewis Number (\(Le\))

The Lewis number relates thermal diffusivity \(\alpha\) to mass diffusivity \(D\):

\[Le = \frac{\alpha}{D} = \frac{Sc}{Pr}\]

When \(Le \approx 1\) (e.g. air-water mixtures), thermal and mass boundary layers grow at the same rate.

References

  • Lewis, W. K., & Whitman, W. G. (1924). Principles of gas absorption. Industrial & Engineering Chemistry, 16(12), 1215-1220.