Determination of collected particle repartition in filter using synchrotron
X-ray microtomography coupled with a multiresolution method
M. Peralba1, A. Charvet1, S. Rolland du Roscoat2-3, P. Dumont1, J.F. Bloch1, P.
Mangin4
1
Laboratoire Génie des Procédés Papetiers (LGP2), CNRS UMR 5518, Grenoble-INP Pagora,
461 rue de la Papeterie BP65, 38402 Saint-Martin-d’Hères Cedex, France
2
Laboratoire Sols-Solides-Structures-Risques (3S-R),
Universitaire BP 53, 38041 Grenoble cedex 9, France
CNRS
UMR
5521,
Domaine
3
European Synchrotron Radiation Facility, Polygone Scientifique Louis Néel, 6 rue Jules
Horowitz, 38000 Grenoble, France
4
Centre Intégré en Pâtes et Papiers (CIPP), Université du Québec à Trois-Rivières, 3351
boul. Des Forges, Trois-Rivières, Québec, GA Canada
Abstract
The aim of this article is to present an experimental technique to study the clogging of fibrous filter with
mineral particles. Furthermore it will contribute to validate a filtration model extracted from literature. Indeed,
the efficiency of filter and the captured particles repartition in the thickness are commonly modelled.
However, they are validated using indirect or partial method. The considered filters are used to remove
particles larger than 0.1 micron. So, the size difference between fibre and particles implies to characterise a
clogged structure at different scales. In this paper, X-ray microtomography was used to study captured
particle localization into the filter. We coupled this technique with a multiresolution method which enables to
measure the same sample at different resolutions.
Keywords: Filtration - model – microtomography – fibrous structure – mineral particles
INTRODUCTION
Microtomography technique enables to obtain structural and quantitative information on a large variety of
materials. However the used scale has to be chosen and dedicated to the studied microstructure.
Concerning fibrous materials, the possibility to have a pixel size of the order of 1 micron opened new
perspectives to study their microstructures. Porosity parameters and fillers repartitions can be studied and
bring important information for their characterisation. The choice of the pixel size is obviously an important
parameter for microtomographic measurements as it must be dedicated to the studied structure and to the
studied physical properties. Nevertheless, for some materials several resolutions are needed to observe their
multiscale structure. In the presented study dedicated to filtration, the resolution has to be adapted to both
fibre and filler sizes. Multiresolution measurements can bring a solution to this problem. Indeed, this
technique enables to measure the same sample with different resolutions without resizing it. After a brief
presentation of the experimental method some results illustrate the ability of the proposed method to analyse
the 3D repartition of mineral particles into fibrous filters. Then results are compared with a filtration model
extracted from literature.
METHODS
The studied filter contains monosized polymer fibres which facilitates comparison with the chosen theoretical
model. They have been used to filter water containing TiO 2 particles with a diameter between 0.1 micron and
20 microns. Experiments consisted in a flow of water with constant TiO2 particle concentration through the
filter for various durations. The applied hydraulic pressure on the filter was constant due to an overflow on
the water column. The filter pressure drop was measured during experiments. The flow was measured by
collecting amount of water that passes through the filter. The mass balance was obtained by weighting TiO2
particles introduced in the system, collected by the medium and recovered in the liquid downstream of the
filter. This last mass was determined using an adapted filter crucible.
The clogged structures were imaged using synchrotron X-ray microtomography. This technique is based on
the attenuation of X-rays due to the sample [Baruchel, 2006]. Thanks to the new Multiresolution method,
these measurements for a given sample were made at 3 pixel sizes, namely 1.7 micron, 0.7 micron and 0.35
micron. Whereas the sample has the same size for each pixel size, the size of the measured volume is
dependant of the pixel size. By starting measurements at the bigger pixel size and modifying the resolution,
the camera was focused on the centre of sample. As a consequence, we imaged the volume at a pixel size
of 1.7 micron which corresponds to an imaged volume of 3.5 * 3.5 * sample thickness mm 3. Then we
zoomed to image the volume at 0.7 micron, which corresponds to an imaged volume of 1.4 * 1.4 * sample
thickness mm3. Finally, we zoomed to a pixel size of 0.35 micron, which corresponds to a volume of 0.7 * 0.7
* sample thickness mm3. The vertical position was then adjusted to observe the input zone. It may be noted
that a complementary visualisation of the global thickness may also be obtained.
Finally, as absorption coefficient of each compound is different from one to others, a threshold techniques
was used to ternarized images (black, grey and white for air, particles and fibres, respectively).
RESULTS
Each X-ray microtomography measurements have been reconstructed and ternarized. Figure 1 exemplifies
the result of the same part of one sample taken at 2 different resolutions.
a)
b)
c)
Figure 1: Reconstructed slides; a) clogged filter grey level image, resolution: 1.7 micron/pixel ; b) clogged filter grey
level image, resolution : 0.35 micron/pixel ; c) ternarised image, resolution: 0.35 micron/pixel ; ESRF, Grenoble,
France
By studying the Elementary Representative Volume [Rolland du Roscoat, 2007], we showed that a pixel size
at 1.7 micron is necessary to determine the porosity of these filters. This Elementary Representative Volume
is smaller for determination of collected particle repartition. We also checked that the obtained porosity
profiles were similar for all the resolutions: the images were cropped to represent the same volume and the
porosity profiles were calculated thanks to a Matlab® program. Then we compared visually the influence of
the resolution on particle detection. A resolution of 0.7 micron was necessary to study the collected particles.
The collected particle repartitions were also studied on the input side at a resolution of 0.35 micron.
Results were then compared to a model developed by Thomas [Thomas, 1999]. It considers 3 collecting
mechanisms: interception, brownian diffusion and inertial impaction. This model enables to calculate the
particle heterogeneous repartition in the filter thickness considering a series of elementary slices that are
assumed to be homogenously loaded. Particles are considered to be collected by fibres and also by deposed
particles whose forms dendrites. We can note that re-entrainment phenomena are not considered whereas it
can be important in liquid filtration. The inputs of the model are the flow, the particle concentration in the fluid,
the particle size distribution and the median size of fibre.
CONCLUSION
The spatial repartition of mineral particles was obtained on clogged filters using X-ray microtomography.
Complementary, the porosity profile was determined. The multiresolution technique allowed to access to
both porosity and number of collected particles on the same sample. These new information may be useful
to understand filtration mechanisms and improve modelling of the involved phenomena.
REFERENCES
[Baruchel, 2006] - Baruchel, J., J.-Y. Buffiere, P. Cloetens, M. DI Michiel, E. Ferrie, W. Ludwig, E. Maire and
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[Rolland du Roscoat, 2007] - Rolland du Roscoat, S., M. Decain, X. Thibault, C. Geindreau and J. F. Bloch
(2007). "Estimation of microstructural properties from synchrotron X-ray microtomography and determinaton
of the REV in paper materials." Acta materiala 55: pp. 2841-2850.
[Thomas, 1999] - Thomas, D., P. Contal, V. Renaudin, P. Penicot, D. Leclerc and J. Vendel (1999).
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