Liquid–solid mass transfer distributions in trickle bed reactors

•Sub-particle scale bifurcation of area specific liquid–solid mass transfer.•Extend of stagnancy highly dependent on prewetting procedure.•No relation between static and residual holdup found.•Static LS mass transfer liquid velocity dependent and higher than prior reports.

Micro-electrodes with exposed areas ranging from 5 to 30 mm2 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 liquid–solid mass transfer coefficients were found to be higher than those obtained from tracer response analyses on porous particles.