Detailed Insight inot Microscopic Phenomena Using 3D-Tomography Data to Develop a Better Model of Dead-End Filtration

Executive Summary
The wettability of the solid surface within the filter cake structure significantly determines its dewatering properties. Associated with the wettability, several properties of the filter cake change, such as its pore size distribution and tortuosity, and corresponding process parameters such as capillary entry pressure and residual saturation.
X-ray computed tomography allows non-destructive investigation of the process-relevant structural effects of the filter cakes, such as obtaining local information from within the filter cake, i.e., spatially resolved data instead of integral parameters.
The measurement of dewatering characteristics involved analyses of filter cakes built up in the in situ cell followed by dewatering and subsequent measurement in μCT. After the measurement, a morphological analysis of the filter cake structure includes the local parti-cle geometry as well as the inverse pore geometry. The analysis of the partially dewatered filter cakes allowed the measurement of the local contact angle distribution along the wet-ting line of the hydraulically isolated liquid regions within the pore space.
The variation of wettability served as a parameter study. For this purpose, hydrophobically coated Al2O3 (aluminum oxide) particles are suspended in different liquid solutions. By changing the ethanol content of the otherwise aqueous suspension, the wetting behavior could be specifically adjusted. The influence of changes in the local contact angle within the cake structure could be demonstrated for many of the particle and filter cake properties investigated, which resulted in a model approach to reproduce the dewatering properties.
For this purpose, the tetrahedron approach by Löwer et al. was applied, which provides a good approximation of the distribution of the capillary pressure as a function of the degree of saturation to laboratory values. Without knowledge of the capillary pressure curve of the particle system, the capillary pressure range between a fully saturated and a fully de-watered bulk structure can thus be estimated over a wide range of contact angles < 90°, but also > 90°.