Calculation of Phase Equilibria ofIonic Liquids and CO2 using GC-NLF EoS

Korea University

Chemical & Biological Engineering

Kim, Yong-soo

Thermodynamics & Properties Laboratory

What Are Ionic Liquids?

Organic salts composed of cations and anions

Usually liquid state in near room temp.

Reprinted from Seddon et al., Pure Appl. Chem., 72, 2275-2287, 2000

Thermodynamics & Properties Laboratory

Characteristics of Ionic Liquids

Negligible vapor pressure– No fugitive emissions

Wide liquidus range : 300 to 400℃

Good sovents – Dissolving both polar and nonpolar species

Modification of physical and chemical properties– The substituents on the cations (alkyl groups)

– The choice of anion

Thermodynamics & Properties Laboratory

Applications of Ionic Liquids Ⅰ

Reactions– Hydrogenation, hydroformylation, isomerization,

alkylation, Diels-Alder reaction, etc…– Good or better reaction rates and selectivities

Electrolyte/fuel cells– Wide electrochemical window, high conductivity,

and a low dielectric constant

Lubricants– High thermal stability and large liquidus range

Thermodynamics & Properties Laboratory

Applications of Ionic Liquids Ⅱ

Combining of Ionic liquids and SCF CO2

– Recovery of organic solute from ionic liquids using SCF CO2

No contamination in SCF phase

– Continuous catalytic processes using SCR CO2

as mobile phase

Liquid-Liquid extraction

Gas separations– The solubility of CO2 in [bmim][PF6] is about

0.6-0.7 mole fraction at 100 bar.

Thermodynamics & Properties Laboratory

Motive

Studies on ionic liquids– Focused on the synthesis of ionic liquids and

the application of catalytic reaction as solvents

– Very few measurements and no modeling of the phase equilibria relating ionic liquids

This work– Modeling of phase equilibria for ionic liquids

and carbon dioxide

– Application of group-contribution NLF theory

Thermodynamics & Properties Laboratory

NLF-HB Theory

Nonrandom Lattice Fluid Hydrogen BondingTheory– NLF EOS by You et al. [1994 a, b]

– Expansion to associating system using Veytsman statistics[1990] by Yeom et al. [1999]

– A normalization of Veytsman statistics by Lee et al. [2001]

– Application of amino acids using extended Veytsman statistics by Park et al. [2002, 2003]

Thermodynamics & Properties Laboratory

GC-NLF Theory

Calculations of parameters for group contribution– Connectivity relation :

– Group size parameters :

– Group energy parameters :

)1(2)2( Gq

Gq

Gq lzrzq −+−=

=

=G

j

Gjiji rvr

1

= =

= =

= =

== G

k

G

l

Gl

Gkjlik

G

k

G

l

Gkl

Gl

GkjlikG

k

G

l

Gkl

Gjl

Gikij

qqvv

qqvv

1 1

1 1

1 1

εεθθε

Thermodynamics & Properties Laboratory

GC-NLF Theory

The temperature dependency of size and energy parameters proposed by Kehiaian

– Group size parameters

– Group energy parameters

( )( )ooGjo

Gj

Gj

Gj TTTTTcTThwr −++−+= ln)(

( )( )ooGijo

Gij

Gij

Gij TTTTTdTTba −++−+= ln)(ε

Thermodynamics & Properties Laboratory

Groups of Ionic Liquids

Cations Anions

– [BF4]- , [PF6]

- , [(CF3SO2)2N]-

NNCH2 CH3

CH2CH2

CH3

+

Thermodynamics & Properties Laboratory

Determination of Group Parameters

Methyl and ethyl group : from Kang’s data

Ionic liquids : Imidazolium and anions– Liquid density

– Assuming that its vapor pressure is less than 10-5 bar.

Carbon dioxide– Using PVT data

– Conversion of values of You et al. [1994]

Carbon dioxide and other groups– VLE data

Thermodynamics & Properties Laboratory

Group size parameters

Thermodynamics & Properties Laboratory

Group interaction parameters Ⅰ

Thermodynamics & Properties Laboratory

Group interaction parameters Ⅱ

Thermodynamics & Properties Laboratory

Result – Density of Ionic Liquids

The comparions of experimental and calculated densitiesof ionic liquids at various temperatures

Temperature [K]

290 300 310 320 330 340 350

Den

sity

of i

onic

liqu

ids [

g/cm

3 ]

0.6

0.8

1.0

1.2

1.4

[bmim][PF6] (Gu et al., 2002)[c8mim][PF6] (Gu et al., 2002)[emim][BF4] (Noda et al., 2001)[c8mim][BF4] (Gu et al., 2002)GC-NLF EoS Experimental accuracy ± 0.60 %

Average deviation with experimental data is ± 0.55 %

Thermodynamics & Properties Laboratory

Result – Low Pressure for [bmim][BF4]

Average deviation with experimental data is ± 2.6 %

Experimental accuracy ± 1.0 % in mole fraction

The solubility of carbon dioxide in [bmim][BF4] [Husson-Borg et al., 2003]

Pressure [bar]

0.0 0.2 0.4 0.6 0.8 1.0

Mol

e fr

actio

n of

CO

2 in

[bm

im][

BF 4]

0.000

0.005

0.010

0.015

303.7 K313.9 K324.1 K 334.4 K344.5 KGC-NLF EoS

Thermodynamics & Properties Laboratory

Result – Medium Pressure for [bmim][PF6]

The solubility of carbon dioxide in [bmim][PF6] [Anthony et al. 2002]

Pressure (bar)

0 2 4 6 8 10 12

Mol

e fr

actio

n of

CO

2 in

[bm

im][

PF6]

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35283.15 K298.15 K323.15 KCalculation at 283.15 KCalculation at 298.15 KCalculation at 313.15 K

Average deviation with experimental data is ±0.0079 in mole fraction.

Thermodynamics & Properties Laboratory

Result – High Pressure for [bmim][PF6]

The solubility of carbon dioxide in [bmim][PF6] [Kamps et al. 2003]

Pressure / bar

0 20 40 60 80 100

CO

2 mol

e fr

actio

n in

[bm

im][

PF6]

0.0

0.1

0.2

0.3

0.4

0.5

0.6

293.15 K313.15 K333.15 K353.15 K373.15 K393.15 KGC-NLF EoS

Average deviation with experimental data is ±0.017 in mole fraction.

Thermodynamics & Properties Laboratory

Result – Length of Alkyl Chains

The solubility of carbon dioxide in [Cxmim][BF4] at 298.15 K

Pressure (bar)

0 2 4 6 8 10

Mol

e fr

actio

n of

CO

2 in

[Cxm

im][

BF 4]

0.00

0.05

0.10

0.15

0.20

[c6mim][BF4][emim][BF4]GC-NLF EoS The solubility of CO2 increase with

length of alkyl chain substituted on imidazolium ring.

Thermodynamics & Properties Laboratory

Conclusions

GC-NLF equation of state was found to be effective to predict the phase equilibria of carbon dioxide and ionic liquids.

– For wide range of pressure and temperature

– The effect of the length of alkyl chain in imidazolium ring on the solubility of carbon dioxide in ionic liquids