Introduction

Mass transfer is the study of the transfer of a component in a homogeneous mixture from one phase to another coexisting phase, or within the same phase from a region of high concentration to low concentration.

Transfer of a component from one phase to the another is due to the difference of chemical potential (\(\mu\)) of a component in the coexisting phases.

The direction of mass transfer is from higher chemical potential to lower chemical potential. If \(\mu_i^\alpha > \mu_i^\beta\), component \(i\) transfers from \(\alpha\) to \(\beta\) phase.

Chemical potential (\(\mu\)) is a conceptual property. It can be expressed in terms of measurable properties such as mole fraction, concentration, pressure, vapor pressure etc.

Phase equilibrium (same \(T\) and \(P\) for the coexisting phases) determines an upper limit for mass transfer. At equilibrium, the chemical potential of a component is same in both the coexisting phases. \[\mu_i^\alpha = \mu_i^\beta\] For vapor-liquid equilibrium of ideal solutions, \[y_iP = x_iP_i^{\text{sat}} \tag*{Raoult's law}\] In a simpler way, equilibrium relationship can be expressed as \[y_i = m x_i\]