Determination of mass transfer coefficient for continuous removal of cadmium by emulsion liquid membrane in a modified rotating disc contactor

• Overall mass transfer coefficient (Koc) in a MRDC for an ELM system was calculated.
• Mass transfer resistance at the external interface of drops (1/Kint) was estimated.
• Effects of type and concentration of surfactant on the 1/Kint was investigated.
• Influences of different variables on mass transfer rate of Cd removal were studied.
• Empirical correlations were established for the coefficients of Koc and Kint.

Based on our earlier proposed model, the mass transfer characteristics in a modified rotating disc contactor (MRDC) for continuous removal of cadmium by emulsion liquid membrane (ELM) were investigated. The overall mass transfer coefficient (Koc) in the water and oil layers at the external interface of emulsion drop was calculated using the model. Unlike the classical two-film theory, the mass transfer resistance in the interfacial film (1/Kint) between the water and oil layers could not be ignored due to the effect of surfactant. As a result, the overall mass transfer resistance (1/Koc) was composed of the mass transfer resistances in the water layer (1/kM), the interfacial film (1/Kint), and the oil layer (1/kE). The 1/Kint increased significantly with the molecular weight and the concentration of the surfactant. The effects of different experimental conditions on the mass transfer rate of Cd removal were also studied, respectively. The results showed that the increase in rotating speed and paddle width could increase the turbulence which led to the increase in the Koc, the mass transfer coefficient (Kint) in the interfacial film, and specific interfacial area (a). Flow ratio mainly affected the a, while total flow markedly influenced the Koc and the Kint. The variables of extractant concentration, pH in feed solution, and sulfuric acid concentration in internal phase primarily influenced the partition equilibriums between the phases. Finally, dimensionless correlations were calculated to estimate the Koc and the Kint with the AAREs of 14.2% and 13.3%, respectively.

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