Mass transfer variation within a packed bed and its relation to liquid distribution

Abstract

The limiting current method is applied to characterize the local mass transfer in the axial, radial and angular directions throughout a packed bed of Pall rings. This is expressed in terms of the particle-to-fluid mass transfer coefficient of ferricyanide ions obtained from the limiting currents measured during its reduction to ferrocyanide at different locations in the bed. Particular focus is placed on the variability of the mass transfer coefficient with position in terms of the quantity MT VAR, equivalent to the normalized deviation of the function Sh/ Sc 0.33. The results confirm those reported by other researchers using more traditional liquid collection and tracer methods. Mass transfer coefficients throughout the bed closely follow normal distributions for the range of Re values investigated. When a multipoint distributor is used, MT VAR is already low at the lowest flow rate ( Re=95) considered and remains unchanged with any further increase in flow rate. In the case of a single-point distributor, on the other hand, MT VAR decreases with increasing flow rate, but always remains significantly higher than that observed with the multipoint distributor. Moreover, the MT VAR value depends on whether irrigated or full flow conditions prevail and on the cross-sectional area of the sampling electrodes, suggesting that local mass transfer variations are caused by local liquid velocity variations once the Pall rings are sufficiently wetted.