Abstract
Local and average partial wetting efficiencies in trickle‐beds have been directly measured by image processing and PIV analysis. Two different setups have been implemented: a vertical monolayer of pellets where the wetting structure and stability and also local velocity gradient are characterized by PIV, and a fixed bed to analyze (by photography) the axial evolution of wetting over bed cross section after a transient injection of dye. Porous alumina spheres and different liquids have been used to examine the effect of liquid–solid affinity; two liquid distributors, and bed prewetting were investigated at low liquid superficial velocities (0.5 × 10^−3^–10^−2^ m/s). Contrary to usual techniques (excepting MRI), not only average wetting efficiencies are derived from the colorimetric tracing, but also local features: axial evolution of wetting, size, and locations of dry zones, distributions of wetting efficiency at a particle scale. All those local data are important to improve reactor models assuming uniform pellet wetting. The effect of liquid–solid affinity is predominant in the case of the monolayer of beads, and, contrary to usual assumption, is still significant for the real trickle bed in the low range of wetting efficiency, corresponding to liquid superficial velocity lower than 2 × 10^−3^ m/s. © 2007 American Institute of Chemical Engineers AIChE J, 2007