Three very different techniques for measuring the catalyst wetted fraction -or wetting efficiency-in trickle bed reactors have been carried out and compared. The first one based on pressure drop data performed successively in single gas then liquid phase, then in gas-liquid phase is experimentally very simple but did not result in meaningful wetting efficiency estimation. The other ones, based on RTD analysis and on dye adsorption on wetted surfaces, gave similar results, and were used to investigate different parameters: liquid velocity and gas molecular weight and pressure by RTD, liquid-solid interaction or affinity (contact angle) and particle shape and diameter by dye fixation. The main operating parameter is the liquid velocity while the effect of gas flow is very weak. Higher solid-liquid affinity (heptane versus water) improves wetting efficiency only at very low liquid velocity (<2•10 -3 m/s).

in Wiley InterScience (www.interscience.wiley.com).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 Â 10 À3 -10 À2 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 Â 10 À3 m/s.

The wetting efficiency of liquid trickle flow over a fixed bed reactor has been measured for a wide range of parameters including operating conditions, bed structure and physico-chemistry of liquid/solid phases. This data bank has been used to develop a new correlation for averaged wetting efficiency based on five different non-dimensional numbers. Finally liquid/solid mass transfer has been determined in partial wetting conditions to analyse what are the respective effects of wetting and liquid/gas flow turbulence. These effects appear to be separated: wetting being acting on liquid/solid interfacial area while the liquid/solid mass transfer coefficient is mainly connected to flow turbulence through the interstitial liquid velocity. A correlation has been proposed for liquid/solid mass transfer coefficient at very low liquid flow rate.

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