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The development of concentration boundary layer (CBL) inside the hollow fibers, based on the experimental data from membrane-based solvent extraction of thallium (III) from chloride-containing acidic solutions with butyl acetate, was investigated. The experiments have been performed using three laboratory made contactors, consisting of hydrophobic polypropylene or polyvinylidene fluoride fibers of different length. The feed solution flowed inside the fibers, while the organic phase was pumped counter-currently at the module shell side. The influence of aqueous flow rate on the mean mass transfer coefficient in the aqueous phase was investigated. The tube-side Sherwood number was proportional to the Graetz number raised to powers ranging between 0.4 and 0.81. A model for the determination of CBL thickness and entry length was developed and used for the numerical analysis of the process. The entry length, i.e. the axial distance from the fiber inlet at which CBL reaches the axis of the fibers, was directly proportional to the aqueous phase flow rate and inversely proportional to the number of fibers in a module.

Membrane based solvent extraction has been effectively used for the separation of thallium (III) from chloride-containing acidic solutions with butyl-acetate. The existence of concentration boundary layer (CBL) in the aqueous phase inside the hollow fibers is the main limiting factor in Tl(III) transport through the interfacial area. Both E1 E2 E3 0.006 0.025 0.057 0.076 0.107 0.177 0.012 0.047 0.106 0.142 0.201 0.331 0.012 0.046 0.104 0.138 0.196 0.323 Le, m the mean mass-transfer coefficient in the aqueous phase and the axial distance from the fiber inlet at which CBL reaches the axis of the fibers, increases with increasing the aqueous flow rate.