The change of pressure drop during large-scale chromatography of viscous samples

The variation of the inlet pressure during the injection and the elution of a band of a viscous compound, or pressure-drop profile, was calculated in overloaded elution chromatography, assuming a significant difference between the viscosities of the solvent and the pure solute. The viscosity of the mixture was calculated using the Grunberg-Nissan equation. By means of the numerical integration of the mass balance equation, the concentration profile along the length of the column was calculated. Then, using the instantaneous concentration profile to derive the local viscosity, hence the local pressure gradient, the pressure drop was determined by integrating the local pressure gradient along the column. The change of pressure drop calculated is significant only when the concentration of the injected sample is very high. If a dilute sample is injected, even large sample volumes will not really change the pressure drop. The maximum pressure drop observed in each case is practically independent of the absorption isotherm. The only determining factors are the solute concentration in the sample, the amount of sample injected, and the solute diffusivity.