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Journal of Engineering Science and Technology Vol. 10, No.11 (2015) 1394 - 1403 © School of Engineering, Taylor’s University
1394
AN INVESTIGATION ON THE OXYGEN AND NITROGEN SEPARATION FROM AIR USING
CARBONACEOUS ADSORBENTS
M. DELAVAR1, N. NABIAN
2,*
1Faculty of Chemical Engineering, Babol University of Technology, Babol, Iran 2University of Science and Technology of Mazandaran, Behshahr, Iran
*Corresponding Author: [email protected]
Abstract
Adsorption equilibria of pure oxygen and nitrogen were investigated in two
different carbonaceous adsorbents such as activated carbons (ACs) and Multi-
walled carbon nanotubes (MWCNTs). To improve the surface characteristics of MWCNTs for adsorption operations, the chemical pretreatment operations were
performed. The equilibrium adsorption of oxygen and nitrogen on adsorbents
were measured at different pressure values ranged from 0 to 50 bar at different
temperatures. Due to the different surface structures of these adsorbents, the
selectivity was different. A comparison between the separation factors of
MWCNTs and ACs showed that oxygen was adsorbed on MWCNTs better than nitrogen. However, the lowest separation factor obtained was 2.7 for oxygen
and nitrogen adsorption on chemically pretreated MWCNTs demonstrating that
the chemically pretreated MWCNTs enhanced the preferential adsorption of
oxygen to nitrogen.
Keywords: Adsorption, Separation factor, MWCNTs, ACs, Pretreatment, Uptake amount.
1. Introduction
Cryogenic distillation and pressure swing adsorption (PSA) are two conventional
methods for separation of oxygen and nitrogen from outdoor air. The first work for
separating desirable gases from air was performed in Europe in the 19th century and
several years later, in the 1970s, the PSA technique was developed. The latter
technique does not liquefy air to produce oxygen and nitrogen and instead, uses
adsorbents for air separation at normal temperatures. Comparing between these two
methods confirms that PSA is more economical than cryogenic distillation method
An Investigation on the Oxygen and Nitrogen Separation from Air Using . . . . 1395
Journal of Engineering Science and Technology November 2015, Vol. 10(11)
in small or medium size air separation plants [1]. By improving the adsorption
processes and enhancing the characteristics of adsorbents, PSA process is
considered to be useful even in large-quantity productions [1-4].
Air separation by adsorption of oxygen is desirable and several researches are
being performed to develop adsorbents that adsorb oxygen preferentially to nitrogen
to use in industrial adsorption systems. Zeolites and carbon molecular sieves are
usually proposed as adsorbents for gas separation and adsorption processes [2, 5, 6].
To design a PSA system, it is crucial to have the adsorption isotherm information [7].
Carbon nanotubes(CNTs) as a new generation of carbonaceous materials, are
very interesting as porous media [8, 9] due to their unique characteristics for gas
adsorption and separation processes. After the discovery of CNTs [10], many
researchers investigated synthesis, treatment, and physical properties of CNTs
[11-13]. Although there are numerous works dedicated to the synthesis of CNTs,
a limited number of studies investigated oxygen and nitrogen adsorption on CNTs
[14-17]. Yulong et al. investigated the methane adsorption on MWCNTs and
achieved to optimal value of 11.7% of mass storage capacity at room temperature
and the pressure of 10.5 MPa [14]. Delavar et al. studied the equilibria and
kinetics of natural gas adsorption on CNTs and indicated these adsorbents are
potential materials for natural gas uptake [15]. In other works done by this group,
the effect of chemical treatment on natural gas adsorption by multi-walled carbon
nanotubes has been investigated. They report increased adsorption capacity of
MWCNTs for natural gas adsorption [17].
There is the lack of knowledge about oxygen and nitrogen adsorption on
MWCNTs. Also, little research has been done on the enhancement of adsorption
capacity of CNTs material by chemical operation and their application on air
separation. The main purpose of this study was to investigate the adsorption
isotherms of oxygen and nitrogen at different operating conditions as well as the
effect of chemical treatment operation on adsorption characteristics of MWCNTs.
Furthermore, the separation factor of oxygen and nitrogen from air and the
selective adsorption of various adsorbents were studied.
2. Materials and Methods
The Multi-Walled Carbon Nanotubes (MWCNTs) used in this work were
synthesized by Chemical Vapor Deposition (CVD) method. The purity of
MWCNTs was more than 95%. The Coal-based, extruded Activated Carbons
(ACs) (2mm diameter tubes) were used in this study. Oxygen and nitrogen with
purity of 99.99% were purchased from Technical Gas Services, UAE.
The adsorbents were characterized with respect to their wall thickness, diameter
and length by Scanning Electron Microscopy (SEM) and Transmission Electron
Microscopy (TEM). The physical properties of adsorbent surfaces such as specific
surface area (SSA), mean pore diameter and total pore volume were measured by the
Brunauer, Emmett and Teller (BET) technique employing N2 adsorption isotherm at
77 ˚K to determine surface area, average pore diameter and pore volume.
Before performing the chemical treatments, the nanotubes were sonicated in a
proportion of 50 mg MWCNTs in 500 mL ethanol for 40 min to eliminate the
amorphous phase. Then, resulting solution was filtered and dried for 2 hours in a
1396 M. Delavar and N. Nabian
Journal of Engineering Science and Technology November 2015, Vol. 10(11)
vacuum oven at 90 °C. The acidic treatment of MWCNTs in the presence of a
concentrated nitric acid solution (70%) was carried out. To this end, the prepared
MWCNTs were doused in 200 ml concentrated nitric acid 63% for 2 hours under
sonication at 30˚C. Then the acidic concentrated solution was refluxed for 4 hours
at 90˚C. Resulting solution was adequately filtered and resulting CNTs washed
with deionized water until the pH of the filtered water reached 7, then dried at
100°C for 8 hours in a vacuum oven [10, 12, 13]. After drying, the samples were
taken out of the filters and applied as adsorbents in oxygen and nitrogen
adsorption experiments. Surface characteristics of the pristine untreated
MWCNTs as well as chemically pretreated MWCNTs sample used as adsorbent
were determined. Table 1 shows the physical properties of pristine and chemically
pretreated MWCNTs used in this study. For the ACs, the specific surface area and
the apparent density were determined to be 822 m2/g and 0.66 g/cm
3, respectively.
Table 1. Specifications of MWCNTs before
and after chemical treatment obtained from BET analysis.
Properties Pristine MWCNTs Treated MWCNTs
Specific surface area (m2/g) 293.86 346.69
Mean pore diameter (nm) 4.67 3.68
Total pore volume (cm3/g) 0.62 0.69
Adsorption measurement
The adsorption experiments for oxygen and nitrogen were performed using
volumetric technique at pressures ranging from 0 to 50 bar at different
temperatures. The adsorbents were heated at 393˚K for 5 hours to remove
undesirable gases. The amount of adsorbed gases was calculated using material
balance at equilibrium conditions. The nonideality of gas phase was corrected
using the SRK equation. The material balance equation for calculation of
adsorbed amount is shown below:
nMzRT
PV
zRT
PV
zRT
PV
zRT
PV
ALAL
+
+
=
+
2211
(1)
where P, T and V are pressure, temperature and volume, respectively; R is the gas
constant; M is the adsorbent mass; z is the gas compressibility factor (obtained
from SRK state equation); n is the adsorbed amount; 1 and 2 represent for the
state before and after the adsorption equilibrium, respectively.
By calculating the adsorbed amount of oxygen and nitrogen on
carbonaceous adsorbents at different equilibrium conditions, the adsorption
isotherms were obtained.
3. Results and Discussion
3.1. Characterization
Figure 1 shows the TEM image of MWCNTs and SEM images of MWCNTs and
activated carbons, respectively. The TEM analysis was used to determine the
An Investigation on the Oxygen and Nitrogen Separation from Air Using . . . . 1397
Journal of Engineering Science and Technology November 2015, Vol. 10(11)
characteristics of Pristine MWCNTs such as multiwall structure, inner and outer
diameter and length. The MWCNTs were 15-20 nm in outer diameter, about 4 nm
in inner diameter and about 30 µm in length.
(A)
(B)
(C)
Fig. 1. Transmission electron microscopy of MWCNTs (A),
Scanning electron microscopy of MWCNTs (B) and ACs (C).
1398 M. Delavar and N. Nabian
Journal of Engineering Science and Technology November 2015, Vol. 10(11)
Figure 2 shows the SEM images of MWCNTs before and after chemical
pretreatment. The structural characteristics of the MWCNTs have been changed
considerably after acidic treatment and ultrasonicating in the concentrated
solution. The acidic treatment caused break down in nanotube structure (reducing
its length) and created defects in its network. The shortening of MWCNTs length
can be clearly seen in Fig. 2. Similar observations were reported using SEM
images by other researchers [18]. According to the values of Table 1, the acidic
treatment led to considerable increase in the specific area and decrease in mean
pore diameters. The formation of cavities and defects in MWCNs structure led to
increasing the surface area by opening of the closed ends as it can be observed in
Fig. 2. The major change obtained in chemically pretreated MWCNTs structure
promoted its capacity for oxygen and nitrogen storage, compared to pristine
MWCNTs. Optimum values of oxygen and nitrogen adsorption capacity for
chemically pretreated MWCNTs were estimated to be 30 mmol/g and 28 mmol/g
at 283.15˚K and 50 bar, respectively. The overall results show that the amounts of
oxygen and nitrogen uptake on chemically pretreated MWCNTs were obviously
higher than the ones on ACs and pristine MWCNTs.
(A)
(B)
Fig. 2. Scanning electron microscopy (SEM) images of MWCNTs
before (A) and after (B) chemical pretreatment.
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Journal of Engineering Science and Technology November 2015, Vol. 10(11)
3.2. Equilibrium isotherms
Figure 3 shows the adsorption isotherms of pure oxygen and nitrogen on ACs,
MWCNTs and pretreated MWCNTs, respectively. As shown, the MWCNTs
adsorbents (pristine and pretreated) represent superior adsorption capacity for
oxygen and nitrogen compared to ACs. This high-adsorption capacity is
concluded from the unique surface properties of MWCNTs such as highly
uniform pore size and high pore volume. Since the average size of oxygen and
nitrogen molecules is 0.28 nm and 0.37 nm, respectively, they can be easily
loaded within the MWCNTs with uniform pore size of about 4 nm, while the ACs
contain a full range of micropores that most of total pores available in ACs are too
large for latter gases molecules.
(A)
(B)
1400 M. Delavar and N. Nabian
Journal of Engineering Science and Technology November 2015, Vol. 10(11)
(C)
Fig. 3. Adsorption isotherms of oxygen and nitrogen on
ACs (A), MWCNTs (B) and pretreated MWCNTs (C).
All the isotherms show that the amounts of oxygen and nitrogen uptake
increase with an increased in the pressure. It can be seen that oxygen
preferentially adsorbed in all adsorbents. Due to the chemically inert nature of
the graphite surface, nonpolar and weakly polar molecules can be absorbed by
carbonaceous material more strongly than other materials. The amount of
adsorbed materials per unit weight of adsorbents can be explained as adsorption
capacity. The adsorption capacity is the most important economical factor in
adsorption processes and selectivity is the significant parameter in selective
separation processes [19, 20], such as oxygen and nitrogen separation from air
which was investigated in this study. As shown in Table 2, the selectivity of
oxygen to nitrogen at 1bar and 298.15 K for activated carbons, MWCNTs and
chemically pretreated MWCNTs were about 1.1, 1.3 and 1.45, respectively. As
the molar compositions of oxygen and nitrogen in air are almost 0.21 and 0.79
at 298.15 K, respectively, the separation factor (SF) of oxygen to nitrogen
can be calculated by dividing the amount of adsorbed nitrogen and oxygen
by different adsorbents at 0.79 bar for nitrogen and 0.21 bar for oxygen. It is
given by:
bar 0.21at oxygen adsorbed ofamount
bar 0.79at nitrogen adsorbed ofamount =SF (2)
As the ratio of the molar composition of nitrogen to oxygen in air is about
3.762, the separation factors higher than this value will result in no separation of
nitrogen and oxygen from air. However, at separation factors lower than 3.762,
oxygen will be adsorbed on adsorbents more selectively than nitrogen. As
presented in Table 3, the calculated separation factor of activated carbons,
MWCNTs and chemically pretreated MWCNTs were determined to be 3.65, 3.05
and 2.7, respectively. Therefore chemically pretreated MWCNTs were the best
An Investigation on the Oxygen and Nitrogen Separation from Air Using . . . . 1401
Journal of Engineering Science and Technology November 2015, Vol. 10(11)
adsorbent for oxygen and nitrogen separation from air among the examined
adsorbents in this study because of their lowest separation factor [19].
Table 2. Selectivity of oxygen to nitrogen (O2/N2)
for different adsorbents at 298.15 K and 1 bar.
Adsorbent Adsorbed
Oxygen (mmol/g)
Adsorbed
Nitrogen
(mmol/g)
Selectivity
(O2/N2)
Activated
Carbons 0.16 0.14 1.1
MWCNTs 0.71 0.55 1.3
Pretreated
MWCNTs 0.95 0.65 1.45
Table 3. Required data for calculation of separation factor at 298.15 K.
Adsorbent
Adsorption of
Nitrogen
at 0.79 bar
(mmol/g)
Adsorption of
oxygen
at 0.21 bar
(mmol/g)
Separation
Factor
Activated
Carbons 0.11 0.03 3.65
MWCNTs 0.49 0.16 3.05
Pretreated
MWCNTs 0.54 0.2 2.7
4. Conclusions
To evaluate an air separation process, three carbon-based adsorbents (ACs,
MWCNTs and pretreated MWCNTs) were selected. Oxygen and nitrogen
adsorption isotherms for aforementioned adsorbents revealed that their separation
factors were different. By increasing the pressure and decreasing the temperature,
the amounts of the oxygen and nitrogen adsorption enhanced for all adsorbents. The
results confirm that chemical pretreatment operation enhances the selectivity of
MWCNTs. Therefore, the pretreated MWCNTs are the most suitable adsorbent for
air separation due to their porous structure and surface characteristics compared to
ACs and pristine MWCNTs.
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