(239f) Asymmetric Framework for Modeling Liquid-Liquid Equilibrium Involving Ionic Liquids | AIChE

(239f) Asymmetric Framework for Modeling Liquid-Liquid Equilibrium Involving Ionic Liquids



            An expanding field of research involves a class
of tunable solvents known as room temperature ionic liquids (ILs).  ILs are
being investigated for a wide variety of reaction, separation and extraction
processes involving liquid-liquid phase behavior.  For instance, certain ILs
have been shown to selectively extract alcohols from fermentation broths and to
recover amino acids from aqueous media [1,2].  Since the number of possible systems
involving ILs is enormous, comprehensive coverage of ternary liquid-phase
behavior via experimental observation is impossible.  Therefore, it is
important to model the liquid-phase behavior of mixtures containing ILs.

 

            The modeling of liquid-liquid equilibrium (LLE) is
a problem that has been extensively studied; however, the macroscopic modeling
of LLE in systems involving ionic liquids has just begun.  Experimental binary
and ternary LLE data involving ILs can be correlated using conventional
excess Gibbs energy models.  However, the predictive capability of these models
in this context has not been widely studied.  The goal of this work is to
develop an approach, based on excess Gibbs energy models, that can be used to predict
ternary LLE from binary measurements and pure component data.  This is a
stringent test of the suitability of various models for describing LLE in
systems containing ILs.

 

            Previously, we have tested [3] the suitability
of various excess Gibbs energy models, including NRTL, UNIQUAC, and
electrolyte-NRTL (eNRTL) [4], for describing LLE in ternary systems containing
ILs.  In some cases, this provided a useful approach for predicting entire
ternary diagrams using parameters determined [5] from binary measurements. 
However, in other cases, these models proved inadequate.  In these previous
studies, a symmetric model was used; that is, the same excess Gibbs energy
models was used for all phases.  However, ILs may behave very differently in
different types of phases, and this symmetric approach does not account for
this.  For example, in an IL-rich phase, the IL may remain relatively
associated, but in a water-rich phase the IL may become almost entirely
dissociated.  Therefore, we present here an asymmetric modeling framework that
uses different models to describe different types of phases.  This allows different
phases to have different degrees of ionic dissociation.  As an example, we will
use the NRTL model for organic- and IL-rich phases, thus treating the IL ions
as associated in these phases, and the eNRTL model for aqueous phases, thus
treating the IL as completely dissociated in these phases.  Results will be
presented for several systems of interest, showing the extent to which this
approach can be used to predict LLE of ternary systems using parameters
determined from binary and pure component data.  Issues that arise in parameter
estimation and computation of phase equilibrium when using the asymmetric
framework will also be discussed.

 

1.         Fadeev, A.G., M. M. Meagher, ?Opportunities for ionic
liquids in recovery of biofuels.? Chem. Commun., 2001, 3, 295-296.

 

2.         Wang, J.J., Y.C. Pei, Y. Zhao, and Z.G. Hu, ?Recovery
of amino acids by imidazolium based ionic liquids from aqueous media.? Green
Chemistry, 2005, 7, 196-202.

 

3.         Simoni, L.D., Y. Lin, J.F. Brennecke, and M.A.
Stadtherr, ?Prediction of liquid-liquid equilibrium using excess Gibbs energy
models for mixtures containing ionic liquids.  In preparation, 2007.

 

4.         Chen, C.-C., H.I. Britt, J.F. Boston, and L.B.
Evans, Local composition model for excess Gibbs energy of electrolyte solutions. 
Part I:  Single solvent, single completely dissociated electrolyte systems.? AIChE
J., 1982, 28, 588-596.

 

5.         Simoni, L.D., Y. Lin, J.F. Brennecke, and M.A.
Stadtherr, ?Reliable computation of binary parameters in activity coefficient models
for liquid-liquid equilibrium. Fluid Phase Equilibria, in press, 2007.