an investigation on the oxygen and nitrogen separation …jestec.taylors.edu.my/vol 10 issue 11...

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


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



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).



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.



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)



(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



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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