A revisit of the concept of external film mass transfer resistance in the packed beds used in high-performance liquid chromatography

This work discusses the origins of the apparent contradiction between expected and predicted external film mass transfer resistance terms in chromatographic columns. The expected term was rigorously derived from the presumed thickness of the thin stagnant film of eluent surrounding the spherical particles contained in the column beds. The predicted terms were based on the commonly accepted models of Pfeffer, Wilson and Geankoplis, Kataoka, Sircar, and Lightfoot. The significant differences observed between the expected and predicted values of the Sherwood number are explained by the fact that the models of external mass transfer developed in chemical engineering account for mass transfer across the hydrodynamic boundary layer of eluent, the layer through which the eluent velocity continuously decreases from the constant fluid velocity in the center of the eluent stream, far away from the surface, to zero. In contrast, the expected term accounts for diffusion through the thin layer of eluent against the particles, in which the eluent velocity is actually zero. It is shown that the predicted mass transfer resistance term is accounted for by the trans-channel eddy diffusion term directly derived in the general theory of the dynamics of chromatography.

► Chemical engineers and chromatographers use different general rate models of chromatography.
► The actual thickness of the stagnant liquid film around the particles is negligible.
► Chemical engineers use the external mass transfer coefficient.
► Chromatographers use trans-channel eddy diffusion instead.