Please cite this article in press as: Hemmati, A., et al., Mass transfer coefficients in a Kühni extraction column. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.07.011 ARTICLE IN PRESS CHERD-1644; No. of Pages 8 chemical engineering research and design x x x ( 2 0 1 4 ) xxx–xxx Contents lists available at ScienceDirect Chemical Engineering Research and Design j ourna l h omepage: www.elsevier.com/locate/cherd Mass transfer coefficients in a Kühni extraction column Alireza Hemmati a , Meisam Torab-Mostaedi b,∗ , Mehdi Asadollahzadeh b a Department of Chemical Engineering, Faculty of Engineering, South Tehran Branch, Islamic Azad University, Tehran, Iran b Nuclear Fuel Cycle Research School, Nuclear Science and Technology Research Institute, P.O. Box 11365-8486, Tehran, Iran a b s t r a c t Mass transfer performance has been presented for a 117 diameter Kühni extraction column using axial diffusion model for two different liquid–liquid systems. The influence of operating variables including the rotor speed as well as the continuous and dispersed phase flowrates on the volumetric overall mass transfer coefficients is investigated. Effective diffusivity is substituted for molecular diffusivity in the Gröber equation for prediction of dispersed phase overall mass transfer coefficients. The enhancement factor is determined experimentally and there from an empir- ical correlation is derived for prediction of enhancement factor in terms of Reynolds number and dispersed phase holdup. The prediction of overall mass transfer coefficients from the presented equation is in good agreement with experimental data. © 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Kühni extraction column; Mass transfer coefficient; Axial diffusion model; Effective diffusivity; Enhance- ment factor 1. Introduction Liquid–liquid extraction is an important separation processes encoun- tered in many industrial processes such as chemical, pharmaceutical, environmental, oil, food, nuclear, and hydrometallurgical industries for product purification and/or raw material recovery. Mechanically agi- tated extraction columns are widely used in industry. For this type of extractor, there are two different methods of phase agitation: rotary agitation by discs, turbines etc. and agitation by pulsators. The Kühni column is one type of extraction column in which the necessary inter- face for mass transfer is achieved with turbines placed one above the other compartments. In designing extraction columns, an important step is to determine the column height to obtain a given extraction efficiency. Therefore, the design of an extraction column warrants the availability of reliable correlations for prediction of mass transfer coefficients. The predic- tion of mass transfer coefficients in an extraction column is subject to considerable uncertainty. In typical equipment drops may undergo coalescence and splitting, distortion and erratic motion, all affected ∗ Corresponding author. Tel. +98 2188221117. E-mail address: mmostaedi@aeoi.org.ir (M. Torab-Mostaedi). by packings, agitators or baffles. Moreover, the counter-current extrac- tion columns mostly suffer from axial mixing of one or both phases, which reduces process efficiency. Two distinct types of models are in use, namely, axial diffusion model, which assumes turbulent axial diffusion of solute superimposed on plug flow of the phase under con- sideration (Pratt and Stevens, 1992), and the backflow model, which assumes well-mixed non-ideal stages between which backflow occurs (Pratt and Stevens, 1992). The state of the art for the design and anal- ysis of an extraction column is to use the axial diffusion or backflow model, where one parameter accounts for all deviations from the ideal plug flow behavior (Bart et al., 2008). Although the of Kühni column has been widespread for a range of important separation processes, the need for more reliable methods for predicting the performance and scale-up of such columns continues to be a matter of significant importance. In this study, the volumetric overall mass transfer coefficients of a pilot scale Kühni column have been measured. The effects of operational variables including rotor speed, and the continuous and dispersed phase velocities on mass transfer performance are http://dx.doi.org/10.1016/j.cherd.2014.07.011 0263-8762/© 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.