The use of an ionic liquid in a Karr reciprocating plate extraction column
Highlights
▸ Ionic liquids can be used in commercial solvent extraction equipment. ▸ Models to describe the mass transfer performance with an ionic liquids are proposed. ▸ The hydrodynamics follow theories derived for conventional organic/aqueous systems.
Introduction
Solvent extraction is widely used on a commercial scale in the mining, pharmaceutical and chemical industries. The use of volatile organic solvents in this process is becoming more of a concern as a result of occupational health, safety and impact on the environment. Room temperature ionic liquids are an alternative to volatile organics, as they have lower volatility and are non flammable and can be designed to have low water solubility for biphasic extraction systems.
Much work has been published on the development of new ionic liquids for separation of a range of pharmaceutical, natural products and simple organic compounds, but little or no reports on how these fluids will perform in common industrial equipment. However, some studies on the use of ionic liquids for the extraction of aromatics, in pilot scale rotating disc columns have been reported (Meindersma et al., 2007, Meindersma et al., 2012, Onink et al., 2010). These studies indicate that ionic liquids have the potential to be of great benefit in process technology applications.
This study examines the performance of an ionic liquid as an extractant for phenol in commercial equipment, in this case a Karr reciprocating plate extraction column. The use of ionic liquids for phenol and phenolic based compounds is well documented (Khachatryan et al., 2005, Inoue et al., 2007, Fan et al., 2008, Mohanty et al., 2010). The aim is to determine if the models and correlations used to predict the performance of this type of equipment can be used for systems based on ionic liquids.
We have previously described dispersed phase holdup (Yung et al., 2012, Yung et al., in press) and the drop size distribution (Yung et al., 2012, Yung et al., in press) for an ionic liquid system in a Karr reciprocating plate extraction column. These studies found that this system conforms to the standard slip velocity relationships and that the holdup can be predicted using existing correlations for dispersed phase holdup in Karr reciprocating plate columns.
Previous work by Yung et al., 2012, Yung et al., in press has shown that despite the fact that the ionic liquid is more dense and of higher viscosity the hydrodynamics and drop size of the Karr column could be predicted from existing correlations for the system of 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and water. Drop size distributions were measured for the [bmim][PF6]/water system for a range of dispersed and continuous phase velocities and pulsation frequencies. The study (Yung et al., 2012, Yung et al., in press) showed that maintaining the dispersed phase flowrate constant (Vd = 0.18 cm/s), whilst varying the continuous phase flowrate, Vc = 0.17, 0.29, 0.42, 0.54 cm/s resulted in approximately 60% of the drops to fall into the 0.1–0.3 mm size category. The drop size was dependent on the agitation rate and only weakly dependent on the flowrate of the phases and the direction of mass transfer. It was found that the Aravamudan and Baird (1999) d32 model was appropriate for the prediction of drop sizes in this IL/water system if the dissipation rate variables were in terms of per unit volume rather than per unit mass.
This work examines the ability of an axial dispersion model with correlations developed for more traditional organic aqueous systems to predict the performance of a Karr column extracting phenol from an aqueous phase using [bmim][PF6].
Section snippets
Models used
Stella et al., 2002, Stella et al., 2006, Stella et al., 2008 investigated the performance of a 50 mm Karr reciprocating plate extraction column extracting phenol from water into a TBP, kerosene organic phase. In that work the mass transfer performance was modelled using a backflow model for the continuous phase and plug flow in the dispersed phase.
Axial dispersion
In the backflow model, the column is treated as a sequence of stages where each stage is well mixed and backmixing occurs by entrainment of the two phases (i.e. entrainment of the continuous phase in the dispersed one and of the dispersed phase in the continuous one) between the stages (Sleicher, 1960). The backmixing is signified by the term α, which is the ratio of backmixed to net forward interstage flow and is constant for all stages. The product of the volumetric mass transfer coefficient
Mass transfer coefficient
In the mass transfer investigation by Shen et al. (1985), n-butyric acid was used as the solute in a water (continuous)/kerosene (dispersed) system, conducted in a 50 mm diameter reciprocating plate column. Both directions of mass transfer were studied, where the stainless steel plate stack was wetted by the continuous phase. It was found that the mass transfer performance was more efficient from c → d than the opposite direction as indicated by the lower height of transfer unit (Hox) obtained for
The Karr column
The use of [bmim][PF6] as the extracting solvent was investigated using a 50 mm inner diameter Karr column as described by Stella et al., 2006, Stella et al., 2008 and Yung et al., 2012, Yung et al., in press. The specifications of the column used in this study are given in a previous study (Yung et al., 2012, Yung et al., in press) and are listed in Table 1.
The column was constructed from precision-bore 50 mm. i.d. Pyrex tube and consisted of 25 evenly spaced open-type perforated plates mounted
Results and discussion
Initial analysis involved the use of the mass transfer coefficient correlation obtained by the Stella (2002) to determine if it would also be applicable to the IL system. Stella (2002) examined various mass transfer coefficient models such as Harikrishnan et al. (1994), Steiner et al. (1986) and Venkatanarasaiah and Varma (1998) and found that, with a slight modification to the Harikrishnan et al. (1994) model, the correlation fitted best to the mass transfer work on the
Mass transfer model
The mass transfer performance was examined by determining the solute concentrations of the dispersed and continuous phases exiting the column. It was first necessary to establish the equilibrium distribution of the phenol in the two phases. The equilibrium concentration of x-phase () was obtained from the following nonlinear expression:
A linear equilibrium relationship for the continuous to dispersed phase (c → d) mass transfer in the column where A, B and n are 0, 15.574 and 1,
Conclusion
In this study an ionic liquid has been shown to be able to extract a solute from a waste solution in a Karr reciprocating plate column. The correlations developed for organic aqueous dispersion in these columns for axial dispersion, mass transfer coefficient in conjunction with holdup, drop size and physical properties can be used to predict the performance of the columns.
Acknowledgements
This work was made possible from the financial support of the Australian Research Council (ARC), GlaxoSmithKline Australia Ltd. (GSK) and Particulate Fluids Processing Centre (PFPC).
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