CFD analysis of facilitated transport in microextraction processes

A. Basauri, J. Gómez-Pastora, M. Fallanza, E. Bringas, I. Ortiz
University of Cantabria,
Spain

Keywords: microextraction, facilitated transport, two-phase liquid-liquid microfluidic systems, CFD modeling, hexavalent chromium separation

Summary:

In recent years, there has been a proliferation of applications of microfluidic devices in multiple fields of knowledge, including medicine, biology and chemistry due to the several advantages that this technology offers, i.e. high surface area to volume ratio, enhanced mass and heat transfer rates, etc. Microfluidics is emerging state-of-the-art technology that handles small volumes of liquids, being able to integrate different fluid and solid phases and supports. In this context, microprocesses modeling may contribute to take a step forward from theoretical training to achieve an application-oriented expertise. This work reports the mathematical modeling of microextraction processes based on the contact of two immiscible phases in Y-Y shaped microchannels. A microdevice of 2 mm long and 150 µm width has been used for the extraction of hexavalent chromium species from an aqueous to an organic phase composed of Shellsol D-70 as solvent and Alamine 336 as the selective extractant agent. Despite conventional solvent extraction has been thoroughly reviewed, this work studies the hydrodynamics, mass transfer and chemical reactions in multiphase microsystems providing valuable knowledge for the optimal design of microseparation processes based on facilitated transport. The detailed model herein presented takes into account the pH speciation dependence in the aqueous phase, mass transfer phenomena and the reaction mechanisms of the species present in the solution. Rheological fluids properties have been also considered to describe the system hydrodynamics as well as the interface formation (Fig 1 a)). For each species present in both phases, the model implements mass and momentum conservation equations and the equilibrium model approach describes the mass transfer across the interface between chromium species in both phases. The two phases in contact are separated by an interface and the unidirectional species mass transfer that only occurs from the aqueous to the organic phase, has been included in the Eulerian model. The model, which was solved with ANSYS FLUENT CFD software, combines the description of the fluids flow and accurately quantifies the species concentration in both phases as well as the extraction performance. The predictive model calculations have been compared to the results obtained in a set of experimental runs (Fig. 1 b)) depicting a satisfactory fitting with an error lower than 10% that evidences the accuracy of the model making it an ideal tool for the design of two-phase microextraction systems.