Diffusion Coefficients of CO
2
in Ionic Liquids Estimated by
Gravimetry
Cristian Moya,
†
Jose Palomar,*
,†
Maria Gonzalez-Miquel,
‡
Jorge Bedia,
†
and Francisco Rodriguez
‡
†
Departamento de Química Física Aplicada (Secció n de Ingeniería Química), Universidad Autó noma de Madrid, Cantoblanco, 28049
Madrid, Spain
‡
Departamento de Ingeniería Química, Universidad Complutense de Madrid, 28040 Madrid, Spain
* S Supporting Information
ABSTRACT: The estimation of diffusion coefficients of CO
2
in ionic liquids by gravimetry is analyzed with the aim of
establishing a measurement method that provides consistent values of diffusivity. Absorption kinetic curves of CO
2
in three
common ILs were measured at different temperatures (293-323 K) and pressures (1-20 atm) by using a high pressure sorption
analyzer with magnetic suspension balance operating in dynamic mode. A mass diffusion model widely used in the literature was
applied to estimate effective diffusion coefficients for CO
2
-IL systems from time-dependent absorption data. The measuring
conditions (IL mass, dimension of sample container, gas flow) in the dynamic absorption experiments were modified to verify the
assumptions of the diffusion model. Obtained results were compared to available data. In addition, the suitability of theoretical
methods commonly used for estimating diffusion coefficients of CO
2
in ILs was analyzed, in order to select a computational
approach for preliminary selection of ILs with favorable transport properties for CO
2
capture.
1. INTRODUCTION
The potential application of ionic liquids (ILs) in gas separation
processes is receiving increasing attention, mainly due to the
particular advantages of ILs as absorbents: high and tailorable
absorption capacity, very low vapor pressure, wide liquid
window, and adequate thermal stability. In particular, ILs are
being intensively investigated as absorbents for CO
2
capture in
order to develop novel technologies of capture, which
overcome the drawbacks associated with the amine-based
systems.
1-9
The design of the separation processincluding the
equipment sizingfor CO
2
capture by ILs implies a deep
knowledge of both thermodynamics and kinetics of the
absorption phenomena. Various experimental techniques
(gravimetric balance method, quartz crystal microbalance
method, isochoric saturation method, bubble point method,
and others) have been applied to measure the solubility of CO
2
in ILs.
10
Among them, the gravimetry has been proved to be an
adequate technique to accurately determine the equilibrium
isotherms of CO
2
absorption in ILs.
11-17
Indeed, systematic
experimental information about the thermodynamics of CO
2
physical absorption in ILs has been reported, taking into
account both the operating conditions and the IL na-
ture.
13,18-26
Experimental evidence indicate that the anion
structure mainly determine the CO
2
absorption capacity of the
IL solvent, observing that highly fluorinated anions increase the
physical solubility of CO
2
in ILs. As expected, significant effects
of the absorption temperature and pressure on the CO
2
solubility in ILs were reported. Comparatively, however, the
available studies focused on characterizing the absorption rate
of CO
2
in IL-based systems are scarce.
11,12,16,17,27-33
Shifflet
and Yokozeki estimated the diffusion coefficients of CO
2
in
[bmim][PF
6
] and [bmim][BF
4
] from thermogravimetric time-
dependent absorption data by using a simple diffusion model.
11
Chen et al.
14
defined an absorption rate parameter to
characterize the kinetics of CO
2
in ILs through gravimetric
analysis. Diffusion coefficients of CO
2
in ILs have also
estimated by the semi-infinitive volume approach,
27
the lag-
time technique,
28
the transient thin-liquid method,
29,30
FTIR
measurements,
31
and the high-pressure quartz spring method.
33
Recently, our group carried out gravimetric measurements by a
magnetic suspension balance to analyze the influence of both
the anion and the cation on the values of diffusion coefficients
of CO
2
in ILs.
16,17
It was concluded that the structural features
of ILs, as well as the operating temperature, have significant
influence on the mass transfer rate of physical absorption of
CO
2
in ILs, whereas pressure (or solute concentration) seems
to play a minor rule. In addition, it was evident by the kinetics
and the thermodynamics of CO
2
absorption may follows
opposite trends depending on the anion or cation structure for,
respectively, common-cation or common-anion IL series,
16,17
which must be considered to select the most favorable solvent
for CO
2
capture.
In general, reported experimental equilibrium data for CO
2
-
IL systems from gravimetric measurements at different
temperatures and pressures show adequate level of consistency
and reproducibility. In contrast, the analysis of the available
diffusion coefficient values of CO
2
in IL solvents reveals higher
data dispersion depending on the measurement method. The
main aim of this work is setting up a procedure to obtain
reproducible coefficient diffusion values of CO
2
in ILs from a
gravimetric balance method. For this purpose, time-dependent
absorption experiments were carried by using a high pressure
Received: May 12, 2014
Revised: July 18, 2014
Accepted: August 8, 2014
Published: August 8, 2014
Article
pubs.acs.org/IECR
© 2014 American Chemical Society 13782 dx.doi.org/10.1021/ie501925d | Ind. Eng. Chem. Res. 2014, 53, 13782-13789