A new derivation of the Taylor‐Aris Theory of solute dispersion in a capillary

The transport of a dilute solute under viscous motion in a straight capillary is described at three distinct space‐time scales. These are the kinetic, fluid mechanical, and Taylorian scales. The transition from one scale to the next higher scale is shown to be a consequence of the central limit theorem (CLT) of probability theory. The Taylor‐Aris dispersion equation is derived by an application of a recently proved CLT for Markov processes. An alternative computation of the dispersion coefficient is given using a ‘differential equation averaging’ approach. A third method, which also applies to solute transport in porous media, is illustrated for an approximate computation of the dispersion coefficient.