Executive Summary
Filtration properties are strongly dependent on the properties of the suspension, the
belonging fluid and particles. Size, shape, wetting behavior and other properties of the
particles affect the structure of the filter cake. Associated with them are resulting properties
of the filter cake, such as its pore size distribution and tortuosity, and associated process
parameters such as the filtration resistance.
The aim of this project was to investigate the influence of particle properties on the filter
cake structure and other relevant filtration properties. Using X-ray computed tomography,
the process-relevant structural effects of the filter cakes can be investigated nondestructively.
The main objective was to obtain local information from the inside of the
filter cake, i.e. to obtain spatially resolved data instead of integral parameters.
For this purpose, it was necessary to downscale a standard pressure Nutsch for direct use
in µCT in order to examine the filtration process and the filter cakes built up without change
during and after the process. The direct measurements in the µCT (in situ) were preceded
by preliminary investigations, which proved the transferability of the results from the in
situ experiment to the larger scale. The proof was provided by validation experiments in
the laboratory (Laboratory VDI Nutsch) and in the downscaled in situ Nutsch filter.
The proof of comparability was followed by analyses of filter cake built in the in situ cell
and measured in the µCT. After the measurement, a morphological analysis of the filter
cake structure followed. The image data sets of the tomography were denoised, smoothed,
sharpened and segmented by means of image processing before further analysis.
Commercially available software as well as specially developed code was used. In a first
step, simple parameters such as total porosity or particle size distribution of the scanned
filter cakes were determined and compared with results of standard measurement methods.
Detailed analysis of the tomographic data provided pore size distributions, the particle
coordination number, pore-to-pore relationships and the tortuosity of the filter cake. An
analysis of wet filter cakes allowed the measurement of the local contact angle distribution
along the wetting line of the hydraulic isolated liquid areas within the pore space.
Varying the wettability served as a parameter study. For this purpose hydrophobic coated
Al2O3 (alumina) and CaSiO3 (wollastonite) particles were filtered at different combinations
with the mother liquor. By changing the ethanol content of the otherwise aqueous
suspension, the wetting behavior could be precisely adjusted. The influence of changes in
the local contact angle within the cake structure could be demonstrated on many of the
investigated particle and filter cake properties. Laboratory tests of the integral parameters
validate the results.