Chemical Engineering Journal 173 (2011) 72–79

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Chemical Engineering Journal

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CO2 capture on polyethylenimine-impregnated hydrophobic mesoporous silica:

Experimental and kinetic modeling
Aliakbar Heydari-Gorji, Abdelhamid Sayari ∗
Department of Chemistry, University of Ottawa, Ottawa, Ontario, K1N 6N5 Canada

a r t i c l e i n f o a b s t r a c t

Article history: CO2 adsorption measurements for polyethylenimine (PEI)-impregnated pore-expanded MCM-41 were
Received 11 April 2011 conducted by gravimetry to investigate the effects of (i) amine loading, (ii) CO2 partial pressure, (iii)
Received in revised form 6 July 2011 adsorption and desorption temperatures. Amine impregnation was conducted on ethanol-extracted pore-
Accepted 20 July 2011
expanded MCM-41, referred to as PME which is a mesoporous silica whose internal surface is laden by
a layer of cetyltrimethylammonium cations (CTMA). The well-dispersed PEI inside the PME hydropho-
Keywords:
bic channels exhibited a CO2 adsorption capacity at 75 ◦ C as high as 206 mg/g for 55 wt.% PEI loading.
CO2 removal
Moreover, the current PEI-impregnated PME materials had high CO2 adsorption efficiency (g CO2 /g PEI)
Pore-expanded MCM-41
CO2 adsorption
than any other PEI-containing adsorbent reported in the literature. In contrast to most PEI-impregnated
Polyethylenimine materials reported in the literature, which because of diffusion resistance showed little or no CO2 adsorp-
Hydrophobic mesoporous silica tion at room temperature, the PEI-impregnated PME material showed high potential for CO2 removal at
ambient temperature. Also a new adsorption kinetic model was proposed to describe the adsorption of
CO2 over amine-impregnated materials. The model was found to be in good agreement with experimen-
tal data under a wide range of conditions including different PEI loadings, CO2 pressures and adsorption
temperatures.
© 2011 Elsevier B.V. All rights reserved.

1. Introduction Compared to other traditional separation technologies, adsorp-

tion is an economically attractive candidate to complement or
The greenhouse gas (GHG) emission in the world increased sig- substitute absorption-based processes due to its lower energy
nificantly; mostly due to the energy industry and the transportation requirements and also easy operation and maintenance [3].
sector [1]. Fossil fuels, which are believed to be the main contributor In recent years, extensive research activities have been focused
to GHGs, are likely to remain a major source of energy for the fore- on the development of technologies based on adsorption of CO2
seeable future. Therefore, separation, and ultimately sequestration on oxide surfaces [4], zeolites [5], activated carbons [6], metal-
of CO2 in fossil-fuel power plants stack gas and other industrial organic frameworks [7] and amine-supported materials [8–30].
gases is required to mitigate the impact of GHGs on the world Two approaches have been used to prepare amine-supported
climate. The removal of CO2 is also an important step in the treat- adsorbents, namely surface grafting of aminosilanes [8–14], and
ment of different gas streams for various applications, like natural physical impregnation of amine-containing species [15–27] over
gas sweetening, hydrogen purification, integrated coal gasification large surface area materials. The impregnation technique gained
combined-cycle plants and closed-circuit breathing systems [2]. wide acceptance because it is simple, low cost and affords
Among the strategies for CO2 separation, gas absorption using high amine loadings. Although materials impregnated with rel-
alkanolamine solutions is the most common technology used for atively low molecular weight amine-containing species such as
large scale operations. However, this method suffers a number of diethanolamine [16] and tetraethylenepentamine [20,24] exhib-
drawbacks such as high energy consumption for amine regener- ited interesting equilibrium and kinetic properties, they were
ation, amine loss via evaporation and oxidative degradation and unstable and showed a steady decrease in adsorption capacity.
limited amine concentration in the aqueous phase due to exten- Much more efforts were focused on impregnation of heavier species
sive corrosion. It is thus highly desirable to develop less energy with higher content of amine groups, namely polyethylenimine
intensive alternatives like adsorption and membrane separation. (PEI) on large surface area mesoporous materials. With regard
to PEI-supported adsorbents, the pore size and pore volume of
the support were found to play an important role [18–21,24].
∗ Corresponding author. Tel.: +1 613 562 5483; fax: +1 613 562 5170. Son et al. [18] achieved different CO2 adsorption capacities using
E-mail address: abdel.sayari@uottawa.ca (A. Sayari). PEI-impregnated on a variety of ordered mesoporous silica with dif-

1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cej.2011.07.038
 A. Heydari-Gorji, A. Sayari / Chemical Engineering Journal 173 (2011) 72–79 73

Fig. 1. Schematic representation of the synthesis of PME.

ferent textural properties and reported that at 50 wt.% PEI loading, The kinetic behavior of an adsorbent is of great significance
KIT-6 silica with 6 nm pores adsorbed 135 mg/g in a stream of pure for industrial applications. Analysis of the adsorption kinetics
CO2 at 75 ◦ C versus 111 mg/g when using MCM-41 with 2.8 nm pore allows determination of the residence time required for comple-
diameter and the same amine loading. This group also reported that tion of the adsorption process. This is a prerequisite to determine
larger pore silica monolith impregnated with 55 and 65 wt.% PEI the performance of fixed-bed or any other flow-through pro-
exhibited CO2 adsorption capacities of 160 and 210 mg/g, respec- cess. Some attempts have been made to formulate a general
tively under the same adsorption conditions [19]. Moreover, Chen expression describing the kinetics of sorption on solid surfaces
et al. [20] used hexagonal mesoporous silica (HMS) as support, for liquid/gas–solid phase sorption systems [35–39]. To the best
which exhibits a three-dimensional wormhole-like pore system. of our knowledge, no model have been proposed to describe the
Using pure CO2 , they achieved a sorption capacity of 184 mg/g at kinetics of CO2 adsorption using amine-modified mesoporous sil-
75 ◦ C in the presence of 60 wt.% PEI loaded HMS, indicating that ica although our group demonstrated that Avarami equation can
in addition to intrinsic pore size and volume, the pore structure properly describe the adsorption of CO2 on triamine-grafted PE-
may play an additional role. Using mesoporous hollow particles MCM-41 [39]. Thus, in addition to investigating the adsorptive
as a support, Qi et al. [24] reported an adsorption capacity of 231 properties of PEI-impregnated PME, the objective of this work was
and 250 mg/g for pure CO2 at 75 ◦ C for 75 and 83.3 wt.% PEI load- to propose a general model to describe the kinetic of CO2 adsorption
ing, respectively. However, despite the improvement in adsorption on such materials.
capacity via proper selection of support, most of these materi-
als showed optimum temperature at about 75 ◦ C with little or no 2. Experimental
adsorption at room temperature because of limited mass transfer
[19–22,24]. 2.1. Materials
To address the negative effect of diffusion resistance, Olah
et al. [29] decreased the viscosity of supported PEI by addition All chemicals were obtained from Sigma–Aldrich and used as
of polyols, such as low molecular weight PEG (MW: 400). Addi- supplied. The detailed preparation of PE-MCM-41 and its structural
tion of PEG improved the desorption properties of PEI-based characteristics may be found elsewhere [33]. Briefly, MCM-41 type
materials. An alternative route to enhanced CO2 adsorption is silica was synthesized at 80 ◦ C using cetyltrimethylammonium
via improved PEI dispersion, leading to easier accessibility of bromide (CTAB) as structure-directing agent and a 25% solution
adsorption sites. This may be achieved using large pore sup- of tetramethylammonium hydroxide (TMAOH) in water for pH
ports with a layer of highly hydrophobic species. As reported adjustment. Pore expansion was achieved via hydrothermal treat-
elsewhere [31,32], as-synthesized pore-expanded MCM-41 silica ment of as-synthesized MCM-41 at 120 ◦ C for 72 h using DMDA as
(PE-MCM-41) exhibits large channels (up to 20 nm) containing swelling agent. The as-synthesized PE-MCM-41 was washed with
a layer of cetyltrimethylammonium surfactant cations (CTMA), ethanol to selectively remove the pore-expanding agent, giving rise
with the ammonium groups interacting with the pore walls sur- to PME. A schematic representation of PME synthesis is shown in
face, as well as the swelling agent dimethyldecylamine, (DMDA). Fig. 1.
Selective extraction of DMDA by organic solvents such as ethanol, Polyethylenimine was introduced into the mesoporous silica
affords a highly porous material with a layer of CTMA cations supports by wet impregnation method. In this procedure, the
on the channels surface. This material is referred to as PME, required amount of PEI (Aldrich, average Mn ∼423) was dissolved in
whereas as-calcined PE-MCM-41 is designated as PMC. In an earlier methanol under stirring for about 15 min before adding the meso-
work [27], we reported that under otherwise the same condi- porous silica support. The weight ratio of methanol to silica was
tions, PEI-impregnated PME exhibited much higher CO2 uptake 8:1. The resultant slurry was continuously stirred overnight, then
than PEI-loaded PMC at all temperatures. For instance, at optimum the solvent completely evaporated at 60 ◦ C under reduced pressure
temperature, i.e. 75 ◦ C, the CO2 uptake for the PME-based adsorbent (700 mmHg). The prepared adsorbent was denoted as PME-PEI(x),
with 55% PEI was 2.3 times higher than its PMC counterpart. More- where x represents the weight percent of PEI in the adsorbent.
over, in addition to the enhanced CO2 uptake, the CO2 adsorption
was much faster. The objective of this work was to further investi- 2.2. Characterization of materials
gate the effects of PEI loading, CO2 partial pressure in the feed, and
adsorption and desorption temperatures on PEI-impregnated PME All materials were characterized by nitrogen
materials. adsorption–desorption at −196 ◦ C using a Micromeritics ASAP
74 A. Heydari-Gorji, A. Sayari / Chemical Engineering Journal 173 (2011) 72–79

2020 automated volumetric instrument. The specific surface area 180

(SBET ) was determined using the BET method in 0.05–0.2 relative
1000
pressure range, and the pore size distribution (PSD) was calculated 160
a
using the KJS (Kruk, Jaroniec, Sayari) approach [34]. The pore

Amount Adsorbed (cm3 STP/g)

140

Amount Adsorbed (cm3 STP/g)

diameter (DP ) corresponds to the maximum of the PSD and the
800
total pore volume was calculated from the amount adsorbed at a
120
relative pressure of about 0.99. The PEI loading was verified using
thermogravimetric analysis (Q500 model from TA Instruments).
600 100
Within experimental errors (±1%), the measured PEI content was
identical to the nominal loading. b 80

400
60
2.3. Adsorption measurements

40
A thermogravimetric analyzer connected to a gas delivery man- 200 c
ifold was used for CO2 adsorption measurements. High purity CO2 ,
20
5% and 10% CO2 in N2 were used at atmospheric pressure for the d
adsorption runs and ultra high purity N2 (99.995%) was used as 0 0
purge gas during CO2 desorption. A sample was pre-treated in 0 0.2 0.4 0.6 0.8 1
flowing N2 at 100 ◦ C, then cooled to the desired adsorption tem- P/P0
perature and exposed to flowing CO2 (90 mL/min) for 180 min. The
Fig. 2. Nitrogen adsorption–desorption isotherms for (a) PME, (b) PME-PEI(20), (c)
CO2 adsorption capacity and uptake in mg CO2 /g adsorbent were PME-PEI(30) and (d) PME-PEI(50).
calculated from the weight gain of the sample during adsorption.
In a separate experiment using He as purge gas, the N2 sorption
power of the driving force and mth power of the adsorption time
capacity for the current adsorbents was found to be negligible.
as follows:
∂qt
3. Adsorption kinetic model = kn t m−1 (qe − qt )n (4)
∂t
where kn , n and m are the model constants. The value of n reflects
Currently, the literature provides a large number of expressions
the pseudo-order of reaction with respect to driving force.
describing the kinetics of sorption on solid surfaces for liquid–solid
For n = 1, Eq. (4) reduces to Avrami equation; for m = 1 and n = 1
systems; but much less information is available to describe the gas
or 2, Eq. (4) reduces to pseudo-first order or pseudo-second order
adsorption kinetic on functionalized mesoporous materials.
kinetic models, respectively.
The pseudo-first order equation of Lagergren, which was first
Assuming  to be the fraction of surface sites which are occupied
proposed to describe the sorption of oxalic and malonic acids onto
by adsorbed gas, i.e.  = qt /qe , then Eq. (4) becomes
charcoal is generally expressed as follows [35]:
∂ n
= kn t m−1 qn−1
e (1 − ) (5)
∂qt ∂t
= k(qe − qt ) (1)
∂t The integrated form of Eq. (4) can be written as follows:
where qe and qt are the sorption capacity at equilibrium and at 1
qt = qe −  1/(n−1) (6)
time t, respectively and k is the rate constant.
((n − 1)kn /m)t m + (1/qn−1
e )
If the rate of sorption follows a second order mechanism, the
pseudo-second order sorption kinetic model is expressed by Eq. The least squares criterion was used to determine the model’s
(2): parameters. To determine the adequacy of model, the coefficient of
determination (R2 ) and the average absolute percentage deviations
∂qt according to Eq. (7) indicated the fit between the experimental and
= k(qe − qt )2 (2)
∂t calculated data:
N  
A fractional order kinetic model (Eq. (3)) was recently developed ((qeexp − qepred ))/qeexp )
i=1
based on Avrami’s kinetic model for particle nucleation [38], and ABPD% = × 100 (7)
N
was applied to describe the adsorption of CO2 on triamine-grafted
where ABPD(%) is the average absolute percentage deviations and
pore-expanded MCM-41 [39].
N is the total number of experimental points.
∂qt
= kn t n−1 (qe − qt ) (3) 4. Results and discussion
∂t

Serna-Guerrero et al. [39] showed that the pseudo-first and 4.1. Material characterization
pseudo-second order kinetic models presented some limitations
describing the CO2 adsorption on triamine-grafted PE-MCM-41 Fig. 2 shows the nitrogen adsorption–desorption isotherms for
and reported that the best fit to a kinetic model was obtained PME, PME-PEI(20), PME-PEI(30) and PME-PEI(50) materials. The
using Avrami’s equation. The kinetic of CO2 adsorption on amine- adsorption isotherms corresponded to type IV according to the
modified adsorbents consists of several steps such as film diffusion, IUPAC classification, which is characteristic of mesoporous materi-
intraparticle diffusion and adsorbate reaction (interaction) with als. Under the conditions used, the pore size of PME was 11.4 nm.
active sites (physisorption or chemisorption). In this work, we pro- Table 1 shows the surface area and pore volume of the selected
pose a general kinetic model to describe the adsorption rate of CO2 materials. The diminishing surface area and pore volume of PME
using chemical sorptions with amine active sites as function of time. upon PEI impregnation provide strong indication that PEI is loaded
In our model, the adsorption rate is directly proportional to the nth within the channels.
 A. Heydari-Gorji, A. Sayari / Chemical Engineering Journal 173 (2011) 72–79 75

Table 1 24
Structural properties of materials.

Materials SBET (m2 /g) Vp (cm3 /g) 20 75 ºC

PME 570 1.59
PME-PEI(20) 269 0.68

CO2 Uptake (wt%)

16
PME-PEI(30) 16.7 0.04
PME-PEI(50) <1 <0.005
12

4.2. CO2 adsorption measurements 25 ºC

8

4.2.1. Effect of adsorption temperature

Fig. 3 shows the CO2 uptake of PEI-containing PME adsorbents 4
after 180 min exposure to pure CO2 at different temperatures. As
seen, the CO2 uptake for materials with 40 wt.% PEI loading or 0
higher, i.e. PME-PEI(40,50,55) increased with increasing tempera- 0 10 20 30 40 50 60
ture up to about 75 ◦ C. The behavior of highly loaded adsorbents at PEI Loading (wt%)
temperatures below 75 ◦ C is associated with a diffusion-controlled
process, consistent with the fact that the pores have been filled Fig. 4. CO2 uptake for PME at different PEI loadings for pure CO2 at 25 and 75 ◦ C
with PEI, thus limiting the accessibility of amine sites. Although after 180 min exposure.

the actual adsorption is exothermic, as the temperature increased

up to 75 ◦ C, the CO2 uptake increased as a result of diminished dif- At 75 ◦ C, the CO2 uptake increased with increasing PEI loading and
fusion resistance. Beyond 75 ◦ C, CO2 diffusion became faster than reached a value as high as 206 mg CO2 /g of adsorbent for PME-
CO2 adsorption, the equilibrium was achieved and the adsorption PEI(55). It is demonstrated that all the adsorption data after 30 min
capacity decreased as expected. Further increase of the temper- exposure at 75 ◦ C correspond to thermodynamic equilibrium (vide
ature to 100 ◦ C led to diminishing uptake. As for materials with infra). Thus, at 75 ◦ C the CO2 capacity increases with PEI loading.
lower amine content, i.e. PME-PEI(20,30), the adsorption capac- On the contrary, at 25 ◦ C the CO2 uptake shows more complex
ity decreased with increasing temperature as expected based on behavior with respect to PEI loading (Fig. 4). This is associated with
the exothermic nature of the reaction of CO2 with amine. Here, the interplay between increased diffusion resistance and increased
PEI is more dispersed, and exhibits more readily accessible adsorp- number of adsorption sites at higher PEI loadings. Based on Fig. 4,
tion sites, thus CO2 diffusion does not play such an important role, it is inferred that up to a PEI content of 30 wt.% (0.43 g/gsupport ), the
even at low temperature. Therefore, it is not surprising that under CO2 uptake at 25 ◦ C is more strongly dependent on the amount of
mild conditions, e.g. 25 ◦ C, materials with low PEI content outper- amine groups than on the mass transfer resistance; thus it increases
form their highly loaded counterparts. Consistent with the above with PEI loading. Beyond 30 wt.%, the CO2 uptake decreased despite
statements, it was observed that in the presence of dry pure CO2 , higher PEI loadings, because of the predominant effect of increased
particularly at low temperatures, the adsorption process for PME- diffusion resistance. Notice that as the PEI content increased from
PEI(30) was much faster than PME-PEI(50) (vide infra). It is inferred 50 to 55 wt.%, the uptake increased slightly, possibly because of the
that at low temperature, adsorbent with low PEI loading may be occurrence of some PEI deposition on the outer surface.
more advantageous for CO2 capture than heavily loaded adsorbents Fig. 5 shows that in terms of CO2 adsorption efficiency, i.e. CO2
because of lesser diffusion resistance and higher adsorption rate. uptake per gram of PEI, at the optimum temperature 75 ◦ C, the
current PEI-impregnated PME adsorbents outperform all other PEI-
4.2.2. Effect of PEI loading containing materials reported in the literature, regardless of the PEI
Fig. 4 shows the CO2 uptake for PME-PEI(x) as a function of PEI loading. The superior behavior of PME-PEI(x) adsorbents is most
content, upon exposure to pure CO2 for 180 min at 25 ◦ C and 75 ◦ C. likely associated with the enhanced PEI dispersion due to the occur-
rence of a surface layer of hydrophobic long chain alkyl groups.

25
4.2.3. Effect of CO2 partial pressure
To evaluate the performance of the current adsorbents in the
20 presence of dilute CO2 streams, we investigated the adsorption of
5% CO2 in N2 . As seen in Fig. 6, the evolution of CO2 uptake for
CO2 Uptake (wt%)

PME-PEI(30) in the presence of 5% and 100% CO2 exhibits sim-

a
15 ilar temperature-dependent behaviors, although the uptake was
b smaller at lower CO2 partial pressure. Fig. 6 shows the temperature
dependence of the CO2 uptake is strongly affected by amine loading.
c
10 The behavior of CO2 uptake versus temperature at low (30%) and
high (55%) PEI loading is associated with two conflicting properties,
d
namely the thermodynamic adsorption capacity and the diffusion
5 e resistance, which both increase at decreasing temperature or at
increasing PEI loading. Thus, due to the dominant role of diffusion
limitation at high loading and low temperature, PME-PEI(55) exhib-
0
20 40 60 80 100 120 ited not only lower CO2 uptake than PME-PEI(30) at temperature
below ca. 50 ◦ C, but also increasing uptake up to 75 ◦ C, because the
Temperature (°C) effect of decreasing resistance was more important than the effect
of decreasing equilibrium capacity. Beyond 75 ◦ C, the CO2 capac-
Fig. 3. CO2 uptake for (a) PME-PEI(55), (b) PME-PEI(50), (c) PME-PEI(40), (d) PME-
PEI(30) and (e) PME-PEI(20) at different temperatures after 180 min exposure to ity decreased in-line with thermodynamic behavior. In contrast,
pure CO2 . PME-PEI(30) exhibited decreasing uptake over the whole range of
76 A. Heydari-Gorji, A. Sayari / Chemical Engineering Journal 173 (2011) 72–79

22 o
Adsorption at 75 C Desorption
20 3
o
75 C
18 o
85 C
o
16 2 90 C

CO2 Uptake (wt%)

o
100 C
14
1
12

10
0
8

6
-1
60 80 100 120 140 160 180
4

-2
0 30 60 90 120 150 180
Time (min)

Fig. 7. Comparison of PME-PEI(50) regeneration at different desorption tempera-

tures.

led to reduced driving force for CO2 , thus to low uptake for low CO2
Fig. 5. Literature survey on CO2 adsorption performance of PEI-containing meso- partial pressure streams and high adsorption temperature.
porous silica adsorbents at 75 ◦ C (highest uptake reported in each reference has been
considered here).
4.2.4. Desorption behavior
For practical applications, the adsorbent should not only possess
temperature, because of the diminished role of diffusion resistance a high adsorption capacity for CO2 , but should also display a long-
at low PEI loading. term stable cyclic adsorption–desorption performance. Desorption
At low temperatures, the qualitative behavior of both adsor- temperature is the most influential parameter on the working
bents was little affected by the gas phase CO2 concentration. adsorption capacity and adsorbent stability during cyclic opera-
Nonetheless, when CO2 adsorption is dominated by diffusion resis- tions. As a general rule, the minimum temperature at which the
tance, i.e. low temperature and high PEI loading, the CO2 uptake was adsorbent is completely regenerated as fast as possible while main-
limited by amine accessibility rather than CO2 availability. Thus, taining the integrity of the material is desired. Therefore, a series
the CO2 uptake was independent from the gas phase composition. of experiments were performed on PME-PEI(50) using desorption
In contrast, at high temperatures where the diffusion resistance is temperatures in the range of 75–100 ◦ C to evaluate the desorption
diminished, the uptake of both adsorbents at 5% CO2 is considerably behavior. The experiments were performed as follows: First, the
lower than that of pure CO2 . This capacity reduction was attributed adsorbent was pre-treated in flowing N2 at 100 ◦ C for 100 min, then
to diminished adsorption and promoted desorption of CO2 at high cooled to 75 ◦ C and exposed to dry pure CO2 for 60 min. The gas was
temperature and low CO2 partial pressure. In fact, when a mixture then switched to N2 (120 mL/min) and the desorption temperature
of CO2 and N2 is brought to the amine-containing surface, there is adjusted to the desired value. The experimental results are shown
an accumulation of N2 (non-adsorbing species) and a depletion of in Fig. 7. At 75 ◦ C, the adsorbent was not completely regenerated
CO2 (adsorbing species) in the boundary layer adjacent to the PEI due to the strong interaction of amine and CO2 and high diffusion
surface, causing a concentration gradient buildup in this film. This barrier. With increasing regeneration temperature to 85 ◦ C, most
of the adsorbed CO2 (about 98.3%) was released within 30 min. At
90 and 100 ◦ C, the regeneration was complete after 60 and 35 min
20 respectively, but at the end of the desorption stage about 0.45 and
1.1 wt.% of organic content (PEI + CTMA) was lost, respectively due
to evaporation. Adsorbents with lower amine loading were regen-
a
15 erated faster or at lower temperature due to lower diffusion barrier
CO 2 Uptake (wt%)

(not shown).

4.3. Kinetic modeling

10

b Fig. 8 shows that the CO2 uptake vs. time of PME-PEI(30) and
PME-PEI(50) in presence of pure CO2 at different temperatures, and
5
c the corresponding profiles generated using the present model are
in very good agreement. Notice that the uptake data in the first
d
few seconds are understandably not reliable. In fact, when the gas
0 is switched from pure N2 to CO2 stream, some time is required for
0 20 40 60 80 100 120
the CO2 to actually remove N2 in the dead volume before contacting
Temperature (°°C) the adsorbent. Table 2 shows the values of the kinetic constants and
the characteristic parameters of the present model, along with the
Fig. 6. CO2 uptake for (a) PME-PEI(55), (b) PME-PEI(30) using pure CO2 ; and (c)
PME-PEI(55), (d) PME-PEI(30) using 5% CO2 in N2 at different temperatures after associated coefficient of determinations (R2 ) and average absolute
180 min exposure. percentage deviations (ABPD). The values of the extremely high R2
 A. Heydari-Gorji, A. Sayari / Chemical Engineering Journal 173 (2011) 72–79 77

Table 2
Values of the kinetic model parameters.

Adsorbent Adsorption temp. (◦ C) n m kn (gn−1 /minm cgn−1) qe (cg/g) Err% R2

Pure CO2
PME-PEI(15) 75 2.00 1.86 12.3 4.98 0.27 0.976
PME-PEI(30) 25 1.08 0.47 0.20 11.6 0.41 0.998
50 1.74 0.53 0.09 11.5 0.05 1.000
75 2.00 1.54 1.24 10.3 0.26 0.993
PME-PEI(40) 75 1.85 1.25 0.66 14.2 0.12 0.993
PME-PEI(50) 50 1.45 0.34 0.003 20.3 0.50 0.999
75 1.80 0.96 0.23 18.4 0.19 0.995
PME-PEI(55) 75 1.78 0.69 0.134 20.8 0.26 0.994
10% CO2 in N2
PME-PEI(30) 75 1.38 0.98 1.42 7.13 0.11 0.998
PME-PEI(55) 75 1.30 0.61 0.18 15.5 0.51 0.990
5% CO2 in N2
PME-PEI(30) 75 1.20 0.88 0.74 6.31 0.21 0.999
PME-PEI(55) 75 1.20 0.68 0.30 13.1 0.43 0.991

cg stands for centigram.

and low ABPD(%), indicate strong agreement of experimental and With increasing adsorption temperature, the kinetics of adsorp-
model within the range of temperature and PEI loading considered. tion became faster and consequently the adsorption capacity was
As seen in Fig. 8, the CO2 adsorption over PME-PEI(30) is increased sharply in the first minutes of the process. For instance, at
much faster than PME-PEI(50), particularly at low temperature. 75 ◦ C, all adsorbents reached ca. 98% of their equilibrium capacity
within first 30 min of CO2 exposure (Fig. 9).
Fig. 9 shows the time dependence of CO2 uptake at 75 ◦ C for
22 PME-PEI(x) at different loadings in the presence of pure CO2 . The
20 PME-PEI(50) CO2 uptake increased rapidly to more than 75% of the maximum
75 °C
uptake within the first minute of adsorption before reaching a
18 constant equilibrium value. At increasing PEI loading, the uptake
16 increased where as the rate (slope) of CO2 adsorption decreased.
As seen in Fig. 10, at low CO2 partial pressure the uptake was
14
CO2 Uptake (wt%)

smaller and slower due to smaller driving force and to film diffusion
12 limitation, respectively.
50 °C The parameters n and m in Eq. (4), which reflect the effect of the
10
driving force (number of unoccupied sites) and diffusion resistance,
8 respectively were affected by adsorption temperature, CO2 partial
pressure and PEI loading. Large values of n indicate that the rate
6
of adsorption is strongly dependent on the driving force. Besides,
4 n presents the pseudo-order of reaction of CO2 with amine active
2
sites. As shown in Table 2, n increases with increasing adsorption
temperature and CO2 partial pressure. At PEI loading up to 30 wt.%,
0 the value of n was 2 at 75 ◦ C, while at lower adsorption temperature
0.01 0.1 1 10 100 1000
Time (min)
22
12
PME-PEI(30) 20
PME-PEI(55)
10 18
75 °C
PME-PEI(50)
50 °C 16
CO2 Uptake (wt%)

8
CO2 Uptake (wt%)

14
25 °C PME-PEI(40)
12
6
10
PME-PEI(30)
4 8

2 4 PME-PEI(15)
2
0
0.01 0.1 1 10 100 1000 0
Time (min) 0.01 0.1 1 10 100 1000
Time (min)
Fig. 8. Comparison of dynamic pure CO2 uptake on PME-PEI(50) and PME-PEI(30)
for 180 min adsorption at different temperatures (solid lines: model, dashed lines: Fig. 9. CO2 uptake vs. time of PME-PEI(x) for different amine loading after 180 min
experimental data). adsorption at 75 ◦ C (solid lines: model, dashed lines: experimental data).
78 A. Heydari-Gorji, A. Sayari / Chemical Engineering Journal 173 (2011) 72–79

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