A novel method to simultaneously measure liquid-solid-mass-transfer and external wetting efficiency was employed at column to particle ratios of 10, 6 and 3. Two prewetting procedures representing the upper (Kan) and lower (Levec) hysteresis branches were used.For a multipoint distributor on a Kan-prewetted bed wetting efficiency and the specific masstransfer coefficient were almost unaffected by column diameter. The multipoint distributor on a Levec-prewetted bed exhibited a decrease in specific mass transfer with decreasing column diameter. Pointsource experiments resulted in significantly lower wetting and mass-transfer measurements with an increasing trend with respect to decreasing column diameter. The results indicate that with proper distribution and prewetting, the effect of column diameter on averaged wetting and liquid-solid mass transfer is almost negligible, a powerful result considering the importance of these parameters on reaction experiments.

Dissolution as well as electrochemical techniques confirmed the existence of multiplicity. The commonly accepted upper multiplicity branch (achieved by Kan liquid prewetting) outperformed the lower branch (achieved by Levec prewetting) by as much as 1.6 times in Sherwood numbers. Although similar trends were observed for the two measurement techniques, the dissolution measurements were significantly lower than the electrochemical measurements. It was further shown that the multiplicity behavior of liquid-solid mass transfer is not linked solely to liquid hold-up and wetting efficiency variations, indicating major differences in flow structures between the multiplicity modes employed. In addition, a decrease in Sherwood numbers with bed depth was observed for both multiplicity modes.

Micro-electrodes with exposed areas ranging from 5-30 mm 2 were placed onto 4.5 mm alumina spheres and used for the sub-particle scale quantification of liquid-solid mass transfer. A novel electrochemical technique was applied where the external cathode wetting fraction and liquid-solid mass transfer were simultaneously quantified. Clear bifurcation of the area-specific liquid-solid mass transfer was observed, providing direct evidence of the two-wetted-zone theory previously inferred from tracer response analysis. The lower hysteresis branch (Levec prewetting) exhibited larger fractions of static wetted area compared to the upper branch (Kan prewetting). The static zones were not completely stagnant and the average static mass transfer rate increased with liquid superficial velocity. The spatial position of the static zones varied for a given packing configuration, while no relationship was found between the positioning of static and residual holdup. Static liquidsolid mass transfer coefficients were found to be higher than those obtained from tracer response analyses on porous particles.

The wetting efficiency and solid liquid mass transfer coefficient was measured separately and simultaneously using the electrochemical cell technique while varying the superficial liquid velocity and hydrodynamic state of the column by using different pre-wetting procedures.The wetting efficiency in columns with an aspect ratio -distance from distributor divided by column diameter -of up to 33, decreased with distance from the distributor regardless of the hydrodynamic state of the column. Although the solid liquid mass transfer coefficient also decreased with an increase in axial position, this decrease was much more significant where the pre-wetting procedure that resulted in a lower overall wetting efficiency was used. When a pre-wetting procedure that resulted in a higher average wetting efficiency was used, the liquid solid mass transfer coefficient was much less dependent on liquid velocity and only a relatively small decrease with axial position was observed. The results highlight the difference in surface renewal processes that is possible between columns operated at the same conditions but under different hydrodynamic states and emphasise the importance of hydrodynamic multiplicity when liquid redistribution is considered.