3 rd International Workshop on Process Tomography (IWPT-3), Tokyo, Japan 1 3D characterisation of the structure of activated carbon packed beds using X-ray microtomography M. C. Almazán-Almazán 1 , A. Léonard 2 , J. López-Garzón 1 , J. Abdullah 3 , P. Marchot 2 , S. Blacher 2 1 Department of Inorganic Chemistry, University of Granada, Fuentenueva s/n, Granada, Spain 2 Laboratory of Chemical Engineering, University of Liège, Sart-Tilman, Belgium – A.Leonard@ulg.ac.be 3 Centre for Computed Tomography and Industrial Imaging, Malaysian Nuclear Agency, Bangi, 43000 Kajang, Malaysia ABSTRACT When using carbons grains as adsorbent, the 3D bed pore structure can have a large impact on the operation of the filter, through the modification of the transport properties inside the bed. In order to gain insight into the relation between morphology and transport properties, X-ray microtomography coupled with image analysis was used to characterize the 3D porous structure of activated carbons packed beds. Two different tube-to-particle ratios were investigated by changing the filter diameter. Polydispersed commercial granular activated carbon with mean particle size close to 1 mm was used. Specifically dedicated image analysis algorithms were developed to determine the total porosity, the pore size distribution, and the radial porosity profiles. For both filter diameters, the total porosity measured by image analysis was very close to the value determined ‘physically’ knowing the carbon mass, its bulk density and the dimension of the filter The comparison of the pore size distributions indicates that void sizes are almost normally distributed around only one maximum for the large filter, while the distribution is larger and presents a more complex shape in the small filter. The radial porosity profiles showed an increase of the porosity from the center of the filter to the wall accompanyied with an oscillatory behaviour at the small scale. A deeper analysis based on the power spectrum indicated a periodic behaviour in the large filter, with a characteristic length matching well with the carbon particle size: the carbon grains are uniformly packed layer by layer in the bed. In the small filter, oscillations were found to be not periodic suggesting an uneven packing of grains. Keywords X-ray microtomography, activated carbon, 3D packing structure, void distribution 1 INTRODUCTION Activated carbon packed bed filters are commonly used to remove contaminants from gas streams, e.g. for flue gas treatment or respiratory protection. When the gas is forced through the filter, the pollutant is adsorbed on the activated carbon. The filter progressively saturates from the top to the bottom until breakthrough occurs, corresponding to the maximum life time of the filter. The so-called breakthrough curve is the plot of the vapour/gas concentration at the outlet of the bed, relative to the actual concentration at the inlet, as a function of time. If separation only results from physisorption and if the experiments are performed under constant inlet vapour concentration and superficial velocity, the shape and the width of the obtained breakthrough curve gives an indication of the bed efficiency: a sharper curve being an indication of a more efficient removal process. The shape of the breakthrough curve reflects how the concentration front moves through the bed, which depends roughly on two factors: the texture of the adsorbent at the nanometric scale and the macroscopic transport process in the bed. The textural characteristics of the adsorbent such as its pore volume, specific surface area and pore size distribution, can be accurately determined, e.g. from the analysis of the nitrogen adsorption isotherms at 77 K. On the other hand, the size and geometry of the bed and the adsorbent particles, which determine the transport properties within the bed, can be chosen at the time of the filter design. However, once the adsorbent is chosen in function of its texture and the filter is designed, other factors can influence the movement of the concentration front, making it more dispersive (i.e. a less sharp breakthrough curve). Indeed, when adsorbent particles are randomly packed in a bed, the void fraction in the bed and the spatial distribution of the adsorbent particles in the bed, which are determining factors for the transport properties, can only be calculated approximately. Moreover, wall effects can lead to a maldistribution of adsorbent particles (Suzuki et al. 2008) which can greatly modify the shape of the concentration front inside the bed. Hence, the possibility to characterize the 3D structure of activated carbon beds by X-ray microtomography will greatly help in understanding of