Membrane Gas Separation

Part III 
Membrane Separation of CO 
2
from 
Gas Streams 

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Membrane Gas Separation Edited by Yuri Yampolskii and Benny Freeman
© 2010 John Wiley & Sons, Ltd
10 
Ionic Liquid Membranes for Carbon 
Dioxide Separation
Christina R. Myers , David R. Luebke , Henry W. Pennline , Jeffery B. Ilconich
and Shan Wickramanayake
P.O. Box 10940, Pittsburgh, PA 15236 - 0940, USA
10.1
Introduction 
A new industrial revolution is occurring around the world. Rapid improvements in equip-
ment, control, and process confi guration are being made in an attempt to minimize raw 
material consumption and waste production while improving effi ciency. ‘ Environmentally 
friendly ’ and ‘ green ’ have become the buzz words for the new millennium. This growing 
environmental awareness represents not just a shift in public opinion but a global recogni-
tion that sustainable practices have become imperative. 
The focus of much of the new awareness has turned towards examining greenhouse 
gas emissions and their impact on global climate. Greenhouse gases include carbon 
dioxide (CO 
2
), methane (CH 
4
), nitrous oxide (N 
2
O), ozone (O 
3
), hydrofl uorocarbons 
(HFCs), perfl uorocarbons (PFCs), and sulfur hexafl uoride (SF 
6
) [1] . Among these gases, 
CO 
2
, CH 
4
, and N 
2
O have become the focal point of efforts to limit climate change since 
these gases are released in large quantities and have signifi cant global warming potential 
(GWP), a measure of the effect of a species on climate based on its ability to absorb 
infrared radiation, the particular location of that absorbance on the spectrum, and atmos-
pheric lifetime. A breakdown of emissions based on their relative climate impacts is 
shown in Figure 10.1 . Reducing emissions of CO 
2
from energy production is particularly 
important because those emissions are the largest and fastest growing contributor to 
climate change. In the United States, emissions rose 21.8% during the period 1990 – 2007, 
and has likewise increased in other developing economies [1] .

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186
Membrane Gas Separation
As the reality of future limits on CO 
2
emissions and the low readiness level of renew-
able energy technologies have become apparent, many businesses, academics, and gov-
ernment agencies have taken an aggressive approach in developing technologies addressing 
CO 
2
capture and sequestration. The majority of these technologies focus on fossil energy 
power generation systems since approximately 83% of CO 
2
emissions are energy related 
[2] and capture from stationary point sources is a more attainable goal than capture from 
mobile sources. 
The power generation systems currently of greatest research interest are those based 
on combustion and gasifi cation of coal since energy from coal is responsible for more 
than half of electricity production in the United States [3,4] . Research issues associated 
with CO 
2
capture from these systems include minimization of parasitic load, reduction in 
the cost of oxygen production, application of capture technology to existing system 
designs, integration with other power system components, scale - up, and reduction of 
capital costs [5] . 
Pulverized coal combustion (PCC) technology, which generates the majority of elec-
tricity in the U.S. [3] , has been used for more than 100 years with steady improvement 
in effi ciency and reduced pollution. Coal is combusted with air in a boiler to produce 
high - pressure steam. The steam drives a steam turbine coupled with a generator producing 
electricity. Particulates are captured from the fl ue gases exiting the boiler and sub
-
sequently the acid gases are removed by selective catalytic reduction and wet lime scrub-
bing. The fl ue gas, which is saturated with moisture after the fl ue gas desulfurization unit, 
contains approximately 10 – 15% CO 
2
at atmospheric pressure that is then emitted through 
a stack. Capturing CO 
2
from these systems presents a particular challenge. The low pres-
sure, dilute concentration, and a high volume of gas lead to a low driving force for CO 
2
separation. Compressing the large gas volume requires a large parasitic load, and trace 
impurities in the fl ue gas limit the capture technologies that may be used. 
HFCs, PFCs, SF
6
2.2%
Nitrous Oxide
5.4%
Methane
8.6%
Other CO
2
1.5%
CO
2
form
Energy
82.3%
Figure 10.1 United States greenhouse gas emissions (equivalent global warming basis). 
Reproduced with permission from the National Energy Technology Laboratory (NETL), US 
Department of Energy. Data taken from Energy Information Administration, Emissions of 
Greenhouse Gases in the United States 2006 (Washington D.C., November 2007)

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Ionic Liquid Membranes for Carbon Dioxide Separation
187
In the integrated gasifi cation combined cycle (IGCC) power generation, oxygen and 
steam react with coal in a gasifi er under pressure producing synthesis gas (syngas), a 
mixture containing CO, CO 
2
, H 
2
, H 
2
O and contaminants. The syngas exits the gasifi er at 
high temperature and pressure, meaning that there is an innate advantage for CO 
2
capture 
from IGCC processes compared to conventional pulverized coal combustion systems 
since compression requirements to reach sequestration pressures is appreciably less. 
Contaminants and particulates are removed from the syngas, and the gas is then fed to a 
combustion turbine/generator to produce electricity. The combination of a combustion 
turbine/generator, a heat recovery steam generator, and a steam turbine/generator, known 
as a combined cycle, is more effi cient than conventional systems, allowing for higher 
effi ciency and lower operating costs. Of the two energy production technologies, the 
IGCC process has the potential for higher effi ciency but at increased capital costs. 
Process effi ciency can be improved in IGCC systems by taking advantage of potential 
benefi ts associated with integration of CO 
2
capture and the water gas shift (WGS) reac-
tion. The WGS reaction is exothermic and equilibrium limited under the process 
conditions.
CO H O
H
CO
kJ mol
+
↔
+
= −
2
2
2
298
41
;
Δ
H
Conventional WGS reactors take advantage of Le Chatelier ’ s principle [6] by cooling the 
reacting gases to 533 K and adding steam to increase conversion and produce a shifted 
synthesis gas containing approximately 30% CO 
2
and less than 1% CO. 
Le Chatelier ’ s principle is the basis for the WGS reaction ’ s sensitivity to temperature 
and its tendency to shift towards the products side as the temperature decreases. The 
principle can alternatively be utilized by selectively removing one of the reaction 
products, CO 
2
or H 
2
. Under these conditions only stoichiometric steam and cooling to the 
separation temperature would be required, allowing for an increase in process effi ciency 
as depicted in Figure 10.2 .
Currently CO 
2
can be removed from the IGCC system at 313 K using Selexol [7] or at 
even lower temperatures with Rectisol [8] . Such substantial cooling to accommodate the 
low operating temperatures of conventional capture solvents followed by reheating to the