Felix C. Leinweber

Chromatographic performance of monolithic and particulate stationary phases. Hydrodynamics and adsorption capacity.

Monolithic chromatographic support structures offer, as compared to the conventional particulate materials, a unique combination of high bed permeability, optimized solute transport to and from the active surface sites and a high loading capacity by the introduction of hierarchical order in the interconnected pore network and the possibility to independently manipulate the contributing sets of pores. While basic principles governing flow resistance, axial dispersion and adsorption capacity are remaining identical, and a similarity to particulate systems can be well recognized on that basis, a direct comparison of sphere geometry with monolithic structures is less obvious due, not least, to the complex shape of theskeleton domain. We present here a simple, widely applicable, phenomenological approach for treating single-phase incompressible flow through structures having a continuous, rigid solid phase. It relies on the determination of equivalent particle (sphere) dimensions which characterize the corresponding behaviour in a particulate, i.e. discontinuous bed. Equivalence is then obtained by dimensionless scaling of macroscopic fluid dynamical behaviour, hydraulic permeability and hydrodynamic dispersion in both types of materials, without needing a direct geometrical translation of their constituent units. Differences in adsorption capacity between particulate and monolithic stationary phases show that the silica-based monoliths with a bimodal pore size distribution provide, due to the high total porosity of the material of more than 90%, comparable maximum loading capacities with respect to random-close packings of completely porous spheres.

Fluid dynamics in monolithic adsorbents: Phenomenological approach to equivalent particle dimensions

Due to the complex, often sponge-like structure of monolithic adsorbents it is difficult to define appropriate constituent units that characterize the hydrodynamics of the material, or to determine relevant shape and size distribution factors comparable to those for spherical particles in (particulate) fixed beds. Based on a phenomenological analysis of the friction factor (Reynolds number relation and the longitudinal dispersivity – Peclet number dependence for random sphere packings) we derive characteristic lengths (i.e., equivalent particle dimensions) for a monolith with regard to its hydraulic permeability and dispersion originating in stagnant zones. Equivalence to the hydrodynamic behavior in “reference” sphere packings is established by dimensionless scaling of the respective data for the monolithic structure. This phenomenological approach, which is simply based on liquid flow and stagnation in a porous medium, can successfully relate hydrodynamic properties of the monolith to that of particulate beds.