Conventional ‘single point’ spectroscopic techniques have been very
convenient in helping to understand underlying phenomena in gas-solid processes,
leading to concepts that rely on pseudo-homogeneous descriptions. These concepts,
however, are either not sufficiently valid at different spatial scales, or mature enough to
be able to describe local events since they use averaged profiles of concentration,
temperature and packing structure. Nevertheless, spatially-resolved optical techniques
are increasingly cited due to developments in tunable lasers, 2D array detectors and
communication technology. Optical techniques allow experiments to be performed nonintrusively
at high spatiotemporal resolution. The present review presents two
experimental procedures based on spatially-resolved near-infrared (NIR) imaging, in
order to observe temperature and concentration maps in gas-solid packed beds subjected
to effects of the entrance aspect ratio and non isothermal conditions. The first technique
was applied to a gas-solid fluidized bed reactor with a low aspect ratio of tube to
particle diameters (Dt/dp). The technique used NIR broadband light, interference optical
filters centred on absorbing and non-absorbing wavelengths of water vapour, a Vidicon
NIR camera and simple back-projection of collected images. The second technique was
applied to water vapour flow in a packed bed filled with a hydrophobic resin, using a
tunable diode laser, focal planar array detector and tomographic reconstruction
procedure. By “looking into” a thin fluidized bed, the proposed technique allowed
existing models of fixed bed reactors to be extended to pseudo-static bed operations.
The technique was applied to ceria-silica reduction in a fluidized bed reactor, where
radial profiles of water concentration allowed the distinction between surface and bulk
reduction regimes of ceria-silica packing. The tomography technique however, which
observes 3D spatially resolved imaging of temperature and water vapour concentration
in packed beds, revealed cold and hot spots, concentration maps and flow dynamics in
the core packed bed and in the vicinity of the wall. In addition, heat uptake from the
wall and mass transfer between and inside resin particles were found to be strongly
affected by local concentration, temperature and packing structure profiles.
Keywords: Near-infrared, visualization, adsorption, porous media, diffuse tomography, packed beds, reactors, 2D modeling, transient processes, water
vapour, fluid flow, spatial distribution, spectroscopy, catalyst reduction,
dispersion.
