Nonuniform local flow inside randomly porous media of gas–solid packed beds of low aspect ratios ranging from 1.5 to 5 was investigated by three-dimensional modeling and near-infrared tomography. These beds are known to demonstrate heterogeneous mixing and uneven distributions of mass and heat. The effects of the confining wall on flow dynamics were found nonlinear, particularly for aspect ratios lower than 3. High velocities were mainly observed in regions near the wall of aspect ratio value of 1.5 and those of values higher than 3, owing to high local porosities in these zones. Mass dispersion characterized both by experimental near-infrared imaging and by particle tracking showed discrepancies with literature models, particularly for aspect ratios lower than 3. Uncertainties were more significant with the radial dispersion due to bed size limits. Beyond this value, the wall affected more the axial dispersion, confirming the nonlinear impact of the wall on global hydrodynamics.

Nic an tSaoir, M. (2012). Three dimensional water vapour visualization in porous packing by near-infrared diffuse transmittance tomography. Industrial and Engineering Chemistry Research,[189][190] ABSTRACT: This work presents a procedure based on two-dimensional and three-dimensional spatially resolved near-infrared imaging to observe temperature and composition maps in gas−solid packed beds subjected to effects of aspect ratio and nonisothermal conditions. The technique was applied to the water vapor flow in a fluidized bed and a packed bed reactor and confirmed uneven water vapor flow channeling and temperature distributions in the core packed bed and in the vicinity of the wall due to flow maldistribution. In addition, the heat uptake and local cross-mixing were experimentally ascertained.

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a b s t r a c tNear-infrared (NIR) imaging was used to observe water vapour flow in a gas-solid fluidized bed reactor. The technique consisted of a broadband light, an optical filter with a bandwidth centred on strong water vapour absorptions, a Vidicon NIR camera, a nozzle from which an optically active mixture of gas and water vapour was trans-illuminated by an NIR beam and collected data of transmittance were normalized to actual optical path. The procedure was applied to a thin fluidized bed reactor with a low aspect ratio of tube to particle diameters (D t /d p ) in order to validate the wall effect on flow dynamics and mass transfer during the reduction of ceria-silica by hydrogen. High concentrations of water vapour emerged in the vicinity of the wall when the bed was operated at pseudo-static conditions but disappeared when the bed was run at minimum bubbling conditions. This result shows the capability of optical methods with affordable costs to 2D imaging opaque packed bed by using a spatially resolved probe located at the exit, which is of great benefit for in situ visualization of anisotropic concentrations in packed beds under industrially relevant conditions and thus for elucidation of the underlying reaction mechanism and diffusion interactions.Crown

This work presents a procedure based on spatially-resolved near-infrared imaging, in order to observe temperature and composition maps in gas-solid packed beds subjected to effects of aspect ratio and non-isothermal conditions. The technique was applied to the water vapour flow in a packed bed adsorber of low aspect ratio, filled with silica gel, using a tuneable diode laser, focal planar array detector and tomographic reconstruction. The 2D projected images from parallel scanning permitted data to be retrieved from the packing and above the packing sections of 12.0 Â 12.0 Â 18.2 mm 3 at a volume-resolution of 0.15 Â 0.15 Â 0.026 mm 3 and a time-resolution of less than 3 min. The technique revealed uneven temperature and composition maps in the core packed bed and in the vicinity of the wall due to flow maldistribution. In addition, the heat uptake from the packed bed and local crossmixing were experimentally ascertained by local profiles of the water vapour composition and temperature under various aspect ratios and feed flow rates. The relative deviations in temperature and compositions were 11.1% and 9.3%, respectively. The deviation in composition, which covers the packing and above the packing sections, was slightly higher than the deviation of 8% obtained up-todate but was limited to the exit of a packed bed adsorber.