A multiscale modelling study on the sense and nonsense of thermal conductivity enhancement of liquid chromatography packings and other potential solutions for viscous heating effects.

We report on a numerical study of the thermal conductivity and temperature distribution in analytical packed bed and monolithic HPLC columns to assess the feasibility of a number of potential solutions to the viscous heating problem that would normally impede high efficiency separations when moving to extreme operating pressures (e.g., 2500 bar). Computational fluid dynamic (CFD) simulations were employed to study heat transfer on three hierarchical levels of the column: meso‑pore level, through-pore level and column level. At the first level, realistic values were determined for the conductivity of the porous zone (kpz), depending on the internal structure of the porous zone and the mobile phase used (acetonitrile, water or a mixture of both). These kpz-values were in turn used at the second level to determine realistic values for the effective conductivity of the bed (keff). It was shown that the presence of a solid core only has a minor effect on the packed bed conductivity. Using highly conducting materials as core material can be expected to maximally lead to a 60% increase in bed conductivity. Contrarily, in monolithic beds, the presence of a core material would form one continuous phase of highly conducting material, thus greatly enhancing the conductivity of the bed. At the third level, the temperature field in the entire column (bed and column housing) was resolved for three typical boundary conditions: isothermal, adiabatic and still-air oven. The effect of different physical properties (inlet pressure, mobile phase composition, bed conductivity, wall conductivity and column ID) on these temperature fields was investigated. It was shown that, theoretically, besides 1 mm ID columns also "core-shell monoliths" can provide a solution to viscous heating (by increasing the bed conductivity). Other possible solutions are proposed and discussed.