i n t e r n a t i o n a l j o u r n a l o f g r e e n h o u s e g a s c o n t r o l 1 ( 2 0 0 7 ) 1 1 – 1 8

CO2 capture by adsorption: Materials and processdevelopment

Alan L. Chaffee a,b,*, Gregory P. Knowles a,b, Zhijian Liang a,b, Jun Zhang a,c,Penny Xiao a,c, Paul A. Webley a,c

aCRC for Greenhouse Gas Technologies, Monash University, 3800 Victoria, Australiab School of Chemistry, Monash University, 3800 Victoria, AustraliacDepartment of Chemical Engineering, Monash University, 3800 Victoria, Australia

a r t i c l e i n f o

Article history:

Received 8 August 2006

Received in revised form

24 January 2007

Accepted 8 February 2007

Published on line 23 March 2007

Keywords:

CO2 separation

Vacuum swing adsorption (VSA)

Hybrid adsorbents

a b s t r a c t

Vacuum swing adsorptive (VSA) capture of CO2 from flue gas and related process streams is

a promising technology for greenhouse gas mitigation. Although early reports suggested

that VSA was problematic and expensive, through the application of more logical process

configurations that are appropriately coupled to the composition of the feed and product gas

streams, we can now refute this early assertion. Improved cycle designs coupled with tighter

temperature control are also helping to optimise performance for CO2 separation. Simulta-

neously, new adsorbent materials are being developed. These separate CO2 by selective

(acid-base) reaction with surface bound amine groups (chemisorption), rather than on the

basis of non-bonding interactions (physisorption). This report describes some of these

recent developments from our own laboratories and points to synergies that are anticipated

as a result of combining these improvements in adsorbent properties and VSA process

cycles.

# 2007 Elsevier Ltd. All rights reserved.

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1. Introduction

The ever increasing concentrations of the CO2 in the atmo-

sphere are requiring mankind to consider ways of controlling

emissions of this greenhouse gas to the atmosphere. Since the

vast majority of this CO2 is produced by the combustion of

fossil fuels and, simultaneously, the global energy demand is

escalating producing more CO2 it is important to find ways to

sequester it (IEA Greenhouse Gas R&D Programme, 2006).

Fossil fuel powered electricity generation plants are the

locations where a very large proportion of the anthropogeni-

cally produced CO2 is emitted to the atmosphere. Thus, it is at

such locations that the incorporation of CO2 sequestration

technology is likely to be most effective in a global sense.

Before CO2 can be sequestered it must be concentrated,

since the concentration of CO2 in the flue gas is typically only

* Corresponding author. Tel.: +61 3 9905 4626; fax: +61 3 9905 4597.E-mail address: Alan.Chaffee@sci.monash.edu.au (A.L. Chaffee).

1750-5836/$ – see front matter # 2007 Elsevier Ltd. All rights reserveddoi:10.1016/S1750-5836(07)00031-X

10–15%. Water vapour (8–12%), residual (unreacted) oxygen (2–

3%) and nitrogen account for the remainder. There are a

variety of approaches to CO2 separation from other flue gas

components, each with their pros and cons (Aaron and

Tsouris, 2005). Currently, the most widely adopted approach

uses solvents, for example aqueous solutions of monoetha-

nolamine, which selectively absorb (or solubilise) the CO2

around ambient conditions (40 8C). It is important to note that

solvents like aqueous methanolamine are basic and therefore

undergo a chemical reaction with the mildly acidic CO2, as it is

passed through the solution, to form dissolved carbamates

and bicarbonates. This provides the basis of the high

selectivity of the solvent for CO2 relative nitrogen.

CO2 is then recovered, as the solvent is ‘regenerated’, by

heating to temperatures well above 100 8C. Thus, there is a

substantial energy penalty associated with the regeneration.

.

i n t e r n a t i o n a l j o u r n a l o f g r e e n h o u s e g a s c o n t r o l 1 ( 2 0 0 7 ) 1 1 – 1 812

There are problems resulting from the corrosive nature of the

solvent, particularly in the presence of residual oxygen in flue

gas, which can attack carbon steel and result in amine loss.

Also, stable solvent by-products (salts) can accumulate as a

result of reaction with acid gas impurities (SOx and NOx) during

operation. However, a big attraction of the absorption

approach is that it is already in use commercially, albeit on

a very small scale relative to what would be required by a

power station producing hundreds or thousands of megawatts

of electricity.

Work in our laboratories has focussed on the use of solid

adsorbents (Knowles et al., 2005, 2006a,b; Liang et al., 2006). We

are developing novel solid adsorbents which possess the same

chemical functionality at the surface as the aforementioned

absorbents, so that high selectivity can be maintained. The

principal aim of materials development work has been to

prepare novel adsorbents that are insensitive to moisture and

capable of operation at elevated temperature.

The materials work has been coupled with process

development work focussed on the use of Vacuum Swing

Adsorption (VSA), a derivative of the better known Pressure

Swing Adsorption (PSA). An early study by the IEA Greenhouse

Gas R&D Programme (1992) indicated that PSA did not appear

promising on account of its energy intensity and since ‘the

requirements of CO2 capture from low pressure, high temperature

streams . . . pushes the envelope of required knowledge substantially

past that of usual practice’.

Since then, however, there have been many new develop-

ments in bed configurations, adsorbent packing arrange-

ments, cycle organisation and heat exchange assemblies that

can substantially reduced the energy intensity and, also, the

process volume and footprint required to achieve a given

production rate; PSA is now a widely accepted and commercial

technology for a number of applications (Diagne et al., 1995).

PSA cycles and conditions can be manipulated to meet a

variety of demand requirements, for example to provide high

purity or high recovery, or to minimise power requirements as

the situation demands (Gomes and Yee, 2002; Reynolds et al.,

2005). Although PSA technology for removal of trace amounts

of CO2 from air is well known, the cycles used for this purpose

are inappropriate for recovery of CO2 from stream which

contain >3% CO2 (Aaron and Tsouris, 2005). The development

of cycles for removal of bulk CO2 from flue gas is still in its

infancy (Zhang et al., 2005). Moreover VSA, where the product

CO2 is recovered at sub-ambient pressure is seen to be more

prospective for CO2 capture from flue gas (Webley et al., 2005).

2. Methodology

2.1. Materials development

Mesoporous siliceous substrates, such as HMS and SBA-15,

were prepared with organic templates using literature

methods. Organic groups containing basic N groups were

then covalently tethered to the surface of substrates under

anhydrous conditions in toluene to complete the synthesis of

the organic–inorganic hybrids. Fuller details have been

previously described (see Knowles et al., 2006a,b; Liang

et al., 2006).

Products were characterised by powder X-ray diffraction

(XRD, to determine pore spacing), nitrogen adsorption/

desorption (77 K, to determine pore volume and pore

diameter), thermogravimetric analysis (TGA, to determine

the organic loading on the siliceous substrate) and elemental

analysis (to confirm the N content of the product).

CO2 adsorption measurements were carried out using a

combined differential thermal analysis (DTA) and TGA

apparatus, after first treating samples in a stream of flowing

Ar (typically at 105 8C) until a constant weight was achieved.

After adjusting the furnace to the desired temperature, the gas

flow was switched to 90% CO2/10% Ar while simultaneously

monitoring the mass change and the heat flow. CO2 adsorption

capacities and heats of adsorption were calculated from the

DTA/TGA data. CO2 adsorption isotherms were determined

using an Intelligent Gravimetric Analyser (IGA).

2.2. Process development

A pilot scale VSA test unit has been constructed which

incorporates three insulated beds, each 1 m long with 7.8 cm

internal diameter, and each containing 3.6 kg of adsorbent. A

top manifold interconnects the beds and incorporates sepa-

rate waste and recycle valves for each bed; likewise, a bottom

manifold incorporates separate feed, purge and evacuation

valves. Simulated flue gas is prepared by mixing dried

compressed air with pure CO2. Product gas, rich in CO2, is

recovered into a product tank via a vacuum pump (<10 kPa).

The unit is equipped with several thermocouples, pressure

transducers and CO2 analysis points. Operation and data

collection are all integrated under microprocessor control to

provide maximum versatility of cycle design.

Our in-house simulator, MINSA (Monash Integrator for

Numerical Simulation of Adsorption) was simultaneously

used to evaluate new cycles under development. This

simulator has been previously described (Todd et al., 2003)

and is based on conservation of mass and energy within an

adsorption bed. The model has been validated against pilot

scale data for air separation (Todd et al., 2001). The model is

non-isothermal and incorporates features to simulate pres-

sure drop across adsorbent beds, switching and control valves

which accurately reflect the experimental PSA system. A

variety of isotherm and kinetic models are available to permit

accurate modelling of adsorption equilibrium and dynamics.

The adsorbent used in both the experiments and simula-

tions described here was 13X zeolite. Isotherms for CO2 and

nitrogen over a range of temperatures and pressures were

measured volumetrically and fitted to a dual-site Langmuir

equation.

Total power consumption was calculated by summing the

power consumed during individual feed, vacuum and purge

(when applicable) steps, as follows:

PfeedW

cycle

� �¼ 2:78� 10�4 k

k� 1

� �QfPf

hf

Pf

Patm

� �ðk�1Þ=k� 1

" #

PvacuumW

cycle

� �¼ 2:78� 10�4 k

k� 1

� �QfPf

hf

Patm

Pf

� �ðk�1Þ=k� 1

" #

Fig. 1 – Illustration depicting the cross-sectional morphology of mesoporous substrates (with pore diameter less than about

10 nm) onto which short organic hydrocarbon chains incorporating basic nitrogen groups are chemically bonded or

tethered.

i n t e r n a t i o n a l j o u r n a l o f g r e e n h o u s e g a s c o n t r o l 1 ( 2 0 0 7 ) 1 1 – 1 8 13

� � � � � �ðk�1Þ=k" #

PrecycleW

cycle¼ 2:78� 10�4 k

k� 1QfPf

hf

Patm

Pf� 1

where hf = 0.7 (blower/vacuum efficiency), k = 1.28 (CO2), or 1.4

(air), Qf = instantaneous feed flow after compression (m3/h),

Pf = bed pressure (kPa) and Patm = atmospheric pressure

(101.37 kPa).

For a power station, the energy penalty (CO2 capture) can be

calculated:

energy penalty ð%Þ ¼ 100�

power producedwithoutcapture

�power producedwithcapture

power producedwithoutcapture

3. Results and discussion

3.1. Materials development

The principal aim of materials development work has been to

prepare novel adsorbents that are insensitive to moisture and

capable of operation at above ambient temperature. We have

focussed on the development of inorganic–organic hybrid

Fig. 2 – CO2 adsorption on an inorganic–organic hybrid adsorbe

surface bound propylamines, middle image: zwitterionic ammo

bicarbonates.

adsorbents where the mesoporous inorganic substrate pro-

vides both substantial pore volumes and high surface area into

and onto which basic organic groups, can be incorporated

(Fig. 1) (Chaffee, 2005; Knowles et al., 2005, 2006a,b; Liang et al.,

2006). The mesoporous nature of the substrate permits good

diffusivity of organic reactants into the pore space and,

following functionalisation, good gas diffusion of adsorbate

gas molecules into and out of the structure (except when the

pores are blocked, as detailed below).

The amine groups react with the acidic CO2 molecules in

the absence of water to form surface bound ammonium

carbamates with an apparent stoichiometric limit of 1CO2

molecule for every 2 N atoms (Fig. 2). However, in the presence

of water, the adsorption capacity is sometimes improved

further, towards a theoretical limit of 1CO2 molecule for every

N atom, via the formation of bicarbonates after proton

exchange. Thus, the chemistry is analogous to that which

occurs by absorption in solution. The mechanism of adsorp-

tion involves chemical bond formation and is therefore quite

different to conventional adsorbents which operate according

to the principles of physisorption.

Physisorption occurs due to electrostatic forces and van der

Waals’ interactions, such that there is strong competition

nt, illustrating the formation of covalent bonds. Left image:

nium carbamate and right image: ammonium

Fig. 3 – Adsorption isotherms for a hybrid adsorbent at

various temperatures.

i n t e r n a t i o n a l j o u r n a l o f g r e e n h o u s e g a s c o n t r o l 1 ( 2 0 0 7 ) 1 1 – 1 814

between adsorbates like those in flue gas—CO2, N2 and H2O.

Indeed the polar water molecule is adsorbed very strongly and

must be removed from the feed gas stream as a separate step

to permit separation of CO2 and N2. The selectivity for

adsorption of CO2 over N2 is modest, but sufficient to achieve

separation.

Unlike conventional adsorbents, the N-containing hybrid

materials can provide high selectivity of CO2 over N2, since N2

does not interact by an acid-base reaction. Moreover, the

presence of H2O in the feed gas stream is not detrimental to

CO2 adsorption, but can assist the adsorption.

As a result of chemical bond formation, the measured heat

of adsorption, DHads (CO2), is higher (typically 60–90 kJ/mol)

and similar in magnitude to that of water. Nevertheless,

desorption is reversible and pressure dependent (no hyster-

esis). The isotherm slopes gently at low pressure such that the

‘working capacity’, D, defined as the difference between the

amount adsorbed at the high and low pressure of interest for

the VSA cycle, can be improved relative to conventional

adsorbents with similar absolute capacity at the VSA feed

pressure (Harlick and Tezel, 2004). An example isotherm for a

Fig. 4 – Strategies for increasing the incorpor

hybrid adsorbent is illustrated in Fig. 3, where it can be seen

that the working capacity, at 20 8C, is about 75% of its feed

pressure adsorption capacity at that temperature.

Since basic nitrogen (N) atoms are primarily responsible for

the adsorption phenomena, one strategy to improve the

capacity is to more completely fill the mesopores with N atoms

(see Fig. 4). The results for one series of structurally related

adsorbents for which this was attempted are illustrated in

Fig. 5. It can be seen that, as the pores were increasingly filled

by tethered nitrogen, the residual pore volume was reduced. In

the last two cases, designated RAS and RSC, the pores were

essentially blocked (zero pore volume). Simultaneously, the

CO2 adsorption capacity increased – but only up to a point.

Thus (at ambient temperature), when the pores were full

(blocked), CO2 could not effectively interact with the basic sites

located within the adsorbent.

When the adsorption experiments were repeated at

elevated temperature (75 8C) it can be seen that the CO2

adsorption on the three adsorbents with readily accessible

pores, was reduced. However, the two adsorbents which

exhibited almost no adsorption at ambient temperature now

exhibited appreciable adsorption. Indeed the adsorption for

sample RSC (‘blocked’ pores) was even higher than that

observed for sample RSH (with open pores). The observations

suggest that, at the modestly elevated temperature, the

mobility of the N-containing tethers within the ‘blocked’

adsorbent have become sufficiently mobile to facilitate CO2

diffusion into the mesoporous hybrid material.

Thus, it can be seen that this work is establishing structure–

property relationships that are assisting the development of

improved adsorbents for VSA based CO2 separation.

However, there are a still range of issues that need to be

addressed to establish the true viability of this approach. For

example, the effects of impurities in the gas feed stream will

need to be evaluated. It is considered that residual oxygen in

the feed gas, a problem for amine based solvent absorption

systems, will not be serious for VSA – since the amine groups

are covalently attached to the solid and solution phase

reactions cannot occur. The effect of acid gas impurities

(SOx and NOx) is less certain since they are likely to react with

surface bound amine groups. A variety of approaches to

address this issue are under consideration.

ation of N groups into the pore volume.

Fig. 5 – Relationship between pore volume (left) and CO2 adsorption capacity (right) for a series of structurally related

adsorbents, determined at both 20 8C and 75 8C.

Fig. 6 – Experimental CO2VSA Apparatus.

i n t e r n a t i o n a l j o u r n a l o f g r e e n h o u s e g a s c o n t r o l 1 ( 2 0 0 7 ) 1 1 – 1 8 15

Also, the precise effect of the higher DHads (CO2) in these

systems is incompletely resolved. Since heat is released when

adsorption occurs, an increase in temperature occurs locally.

The actual working capacity, D, will in ‘real system’ will be

somewhat reduced as a result. We are addressing this issue

through process modeling and further materials development

work, since the DHads (CO2) can be ‘tailored’ to a significant

extent by incorporating functional groups that have inher-

ently lower heats of adsorption (Knowles et al., 2006a).

3.2. Process development

The aim of the experimental process research has been to

develop and test new process cycles and conditions which will

minimise power and capital costs of CO2 capture systems.

Previous IEA studies (1992) relied on (early) experience with

landfill gas and extrapolated this to flue gas separation. In this

case the feed gas composition (mixture of CH4, CO2, H2O and

N2, with relatively high concentrations of CO2) is substantially

different and the feed gas stream was pressurized. However, it

is clear that pressurizing a large flue gas stream of which CO2 is

only 10–15% is expensive from an energy consumption point

of view. For this reason, our CO2 adsorptive capture process

uses a vacuum swing cycle (VSA). CO2 VSA has been studied

both commercially and through simulation in previous work

(Ko et al., 2005; Chou and Chen, 2004; Ishibashi et al., 1996;

Webley et al., 2005). We have recently constructed a pilot scale

VSA apparatus to study the capture of CO2 from CO2/N2 gas

streams (Zhang et al., 2005). The pilot plant (Fig. 6) is of

sufficient scale to ensure data credibility and to permit reliable

scale-up. The test results reported here are based on the use of

a conventional (physisorption type) adsorbent, 13X.

We have developed a range of improved cycles for CO2

separation. The sequence for an example 3-bed, 6-step cycle is

described by Fig. 7. With simulated flue gas (12% CO2, 88 % dry

air as a proxy for N2) as the feed and concentrated CO2 as the

product, the cycle incorporates a number of feed, repressur-

isation (Repr), pressure equalisation (PE) and product evacua-

tion (Evac) steps. The duration of each step within the overall

cycle can be independently adjusted, but is bound by the

constraint that the overall cycle time for the three beds must

be the same.

Table 1 summarises the performances achieved in the pilot

plant for both the 6-step cycle described and a similar 9-step

cycle (not detailed). The major difference is that the 9-step

cycle incorporates a product purge step which helps increase

the partial pressure of CO2 in the product producing bed at the

start of the evacuation step, thereby providing higher purity

product during the recovery step. It is clear that the absolute

power requirement and the associated energy penalty are

both substantially smaller than previously reported for PSA

capture of CO2 (IEA Greenhouse Gas R&D Programme, 1992). It

can be seen that a purity of over 90% can readily be achieved,

together with recoveries of 60–70%, for a 9-step cycle.

The operating temperature of pilot scale rig can be varied,

and the effect that this has on performance is illustrated in

Fig. 7 – Sequential description of a 6-step, 3-bed cycle for CO2 separation from simulated flue gas. Feed gas: 12% CO2, 88% dry

air; product gas: concentrated CO2; Pr PE: pressure equalisation (provider); Rec PE: pressure equalisation (receiver); Repr:

repressurisation.

Table 1 – Performance data for 6- and 9-step VSA cycles

Performances 6-step cycle without purge 9-step cycle with purge

Purity (%) 82–83 90–95

Recovery (%) 60–80 60–70

Power (kW/TPDca) 4–8 6–10

Energy penalty (%) 8–16 12–20

a Kilowatts per tonne per day carbon captured.

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Fig. 8. In this figure, we have changed the feed temperature

while keeping the vacuum pressure and feed pressures

constant at 0.04 and 1.3 bar, respectively. The individual step

times are held constant for a total cycle time of 10 min. It can

be seen that there is slightly beneficial effect on the purity of

the recovered product. However, this comes at the expense of

recovery and, less significantly, a slightly increased power

requirement (and energy penalty). The latter is predominantly

a result of the extra power required by the feed stream

Fig. 8 – Effect of feed gas temperature on CO2VSA process

performance (6-step cycle; feed gas: 12% CO2, 88% air; cycle

time: 10 min).

blowers, to cope with the expanded gas volume at the higher

temperatures (for the same throughput).

Using the process simulator, we have investigated the

effect of feed gas composition on process performance, for the

6-step cycle, as shown in Fig. 9. Again, the vacuum and feed

pressures have been held constant at 0.04 and 1.3 bar,

respectively. It can be seen that the purity and recovery both

increase modestly as feed concentration increases, but the

power drops rapidly. This is because of the ability to recover

Fig. 9 – Effect of feed concentration on CO2 VSA process

performance (6-step cycle; feed gas temperature: 45 8C;

cycle time: 10 min).

Fig. 10 – Effect of vacuum pressure on total specific power.

i n t e r n a t i o n a l j o u r n a l o f g r e e n h o u s e g a s c o n t r o l 1 ( 2 0 0 7 ) 1 1 – 1 8 17

the product at more modest vacuum levels while still

providing satisfactory purity and recovery performance.

Indeed, the ultimate vacuum pressure is the most

important process variable in determining the CO2 recovery

and production per cycle. (Purity is less sensitive to vacuum

level.) However, the relationship is not straightforward.

Although low vacuum pressure would suggest high power

requirements, the increased recovery partially compensates

resulting in an ‘‘optimal’’ vacuum level for minimum specific

power. Fig. 10 illustrates that for the studied case (zeolite 13X)

the optimum vacuum level is about 0.04 bar, but the optimum

will depend on the isotherm shape of the adsorbent under

consideration.

4. Conclusions

Over recent years, there has been rapid development of cycles

and of process understanding for the VSA system. The VSA

process is very versatile in terms of cycle design and its ability

to adapt to changes in feed gas conditions or other market

requirements (recovery, purity, energy consumption). This is a

strength of the approach relative to other options for CO2

capture.

There have been simultaneous developments of new

adsorbents with improved selectivity and working capacities

on a per cycle basis. These rapid developments in process and

materials continue to improve the prospects for large scale

adsorptive separation of CO2 from flue gas and related

streams.

Our future work will examine the use of advanced

materials in our pilot process, thus exploiting the synergy

between the materials and process development work. It will

also be necessary to evaluate the effect of impurities and water

on the performance of the VSA unit.

Acknowledgements

The authors acknowledge financial support for this work

from the Cooperative Research Centre for Greenhouse

Technologies (CO2CRC), which is established and supported

under the Australian Government’s Cooperative Research

Centres Program. Early parts of the research program were

funded under the Australia Research Council’s Discovery

Grant Scheme.

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