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International Conference on Science and Innovated Engineering (I-COSINE) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 536 (2019) 012042 doi:10.1088/1757-899X/536/1/012042

A Computational Fluid Dynamic Comparative Study on CO2

Adsorption Performance using Activated Carbon and Zeolite
in a Fixed Bed Reactor

N Sylvia1, R Mutia2, Malasari1, R Dewi1, Y Bindar3, and Yunardi4

1
Chemical Engineering Department, Malikussaleh University, Lhokseumawe,
Indonesia
2
Postgraduate Program, Chemical Engineering Department, Syiah Kuala University,
Banda Aceh, Indonesia
3
Department of Chemical Engineering, Faculty of Industrial Technology, Bandung
Institute of Technology, Bandung, Indonesia
4
Chemical Engineering Department, Syiah Kuala University, Banda Aceh, Indonesia

Email: yunardi@unsyiah.ac.id

Abstract. The increasing emission of carbon dioxide to the atmosphere from various sources
has become an issue of great concern all over the world due to its significant contribution to
climate change. Carbon capture and storage are commonly recognized as the major approaches
to prevent carbon dioxide from entering the atmosphere. A number of CO2 removal
technologies have been reported, including absorption, adsorption, membrane separation, and
microalgal fixation. In this study, a Computational Fluid Dynamics (CFD) study was
performed to investigate the performance of two adsorbents, coconut fiber activated carbon and
zeolite 13X in removing CO2 from a continuous gas stream in a fixed bed adsorption column.
A CFD code ANSYS R18.2 was used to investigate the influence of flow rate and bed height
on the CO2 removal efficiency and adsorption capacity by varying the inlet feed velocity and
bed heights. The results of the simulation showed that the highest CO 2 removal efficiency of
63.13 percent was observed when the gas flowed at a rate of 50 cm3/minute to the column
filled with the activated carbon adsorbent of 10 cm in height. While in the zeolite adsorbent
13X, the highest CO2 removal efficiency of 57.86 percent was also seen when the gas flowed at
a rate of 50 cm3/minute at the bed height of 10 cm.

1. Introduction
Indonesia is currently facing serious energy problems due to the high dependence on fossil fuels. In
fact, the country has a huge potential for renewable resources originated from agricultural wastes for
energy generation. It is estimated that the country is capable of producing a potential bioenergy of 50
GW, nevertheless, only less than 2 GW has been utilized up to now. The Indonesian Science Institute
has suggested using biogas to reduce the dependence on fuel import. Such consideration was based on
its availability, low capital and operational costs, renewable sources and environmentally friendly
energy. Either it is produced at industrial or smaller scale, biogas generally consists of CH 4 (50-75%),

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International Conference on Science and Innovated Engineering (I-COSINE) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 536 (2019) 012042 doi:10.1088/1757-899X/536/1/012042

CO2 (25-50%), and another minor number of gases. Its calorific value ranges from 17,900-25,000
kJ/m3; for a comparison, natural gas (LNG) has a calorific value of 37,300 kJ/m3 [1]. It is clearly seen
from the biogas composition that its main drawback is its high content of CO2 which significantly
reduces its calorific values. Therefore, for the purpose of utilizing biogas as the source of energy,
removal of CO2 prior to utilization is required.
There are a number of methodologies available for removing CO2 from biogas. Conventional
technologies for removing CO2 include water scrubbing, chemical and physical scrubbing,
membranes and Pressure swing adsorption [2]. New methodologies have been also introduced for the
use of improving the quality of biogas such as CO2 liquefaction and separation, amine absorption [3]
and cryogenic distillation [4]. Although such new development technologies provide better
performance, its implementation on a small scale and in the developing countries is still not beneficial
at the present time. Therefore, adsorption-based technologies would suit the need for implementation
in the developing countries due to its low energy required for regeneration, easy to operate, low-
pressure drop, and easier to scale down. With regard to the adsorbents, porous inorganic materials
such as zeolite 13X, zeolite 4A, bentonite, molecular sieve, and activated carbon have been utilized for
this purpose [5, 6, 7, 8,9,10,11,12]. Each adsorbent has its own advantages and disadvantages,
however, activated carbon is easy to prepare from different sources of agricultural biomass which is
abundance in every part of Indonesia. The same case was also applied to zeolite and bentonite which
are abundant in the local region.
CFD has been used to study the flow phenomena involving momentum, heat and mass transfer for
the purpose of design and optimization of process equipment. Such a method is a suitable tool to be
used when the process performance is dictated by fluid dynamics. In relation to the present study,
CFD is utilized to study the adsorption phenomena and adsorbent performance on the removal of CO2
from a mixture of CH4-CO2 [13,14] Two type of adsorbents, activated carbon and zeolite 13X, were
employed in the modelling of fixed-bed adsorption column by varying the height of the adsorbent bed
and the gas flow rate entering the column. Equilibrium characteristics of the CO2 adsorption process
were evaluated by test the data with various isotherm models.

2. Materials and Method

The present study modeled the adsorption column for separating of CO2 from a gas mixture
containing CH4 and CO2, using coconut fiber activated carbon and zeolite type 13X as adsorbents.
Each adsorbent was having the same particle diameter of 0.0029 m, while the bed porosity (ε) was set
0.39 for activated carbon and 0.43 for zeolite 13 X, respectively. Table 1 presented the geometry of
the adsorption column used in this study, while the modeling design of the adsorption column is
shown in Figure 1 [14]. All stages in the simulation, including pre-processing, processing and post-
processing were performed using the Fluent Ansys R18.2. The feed flow rates were varied by 50
cm3/minute, 100 cm3/minute 150 cm3/minute 200 cm3/minute and 250 cm3/minute [14] while the bed
height was varied with 6 cm, 8 cm, and 10 cm. Observations were taken on the removal efficiency,
adsorption capacity and adsorption isothermal.

Table 1 Geometry of the adsorption column

Geometry Height Diameter
Adsorbent 50 cm 5 cm
Filter - 5 cm

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International Conference on Science and Innovated Engineering (I-COSINE) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 536 (2019) 012042 doi:10.1088/1757-899X/536/1/012042

Figure 1. Geometry for the adsorption column being studied

3. Results and Discussion

The Computational Fluid Dynamics (CFD) study on the simulation adsorption process of CO2 removal
was done by varying the flow rate and column bed height for each adsorbent being used. Table 2
presented results on CO2 removal efficiency and adsorption capacity due to a variation of flow rate
and adsorbent bed height. The discussion of these results is presented in the next sub-sections.

Table 2. Results on CO2 removal efficiency and adsorption capacity due to a variation of flow rate
and adsorbent bed height
CO2 Removal Efficiency Adsorption Capacity
Flow Rate (%) (mg/gr)
RUN Bed Height (cm)
(cm3/min) Activated Activated
Zeolite Zeolite
Carbon Carbon
1 6 57,66 50,21 10898,14 6079,64
2 50 8 61,01 53,82 6446,13 3643,53
3 10 63,13 57,86 3614,14 2491,81
4 6 55,60 47,18 21033,27 11431,92
5 100 8 58,93 50,43 12550,55 6880,09
6 10 61,02 55,54 8074,48 4767,34
7 6 53,44 45,12 30333,87 16408,37
8 150 8 57,12 47,06 18314,2 9662,89
9 10 58,18 51,85 11780,52 6747,03
10 6 51,75 41,34 41194,89 19062,49
11 8 56,02 44,13 23136,78 11077,18
200
12 10 56,67 47,25 15814,86 9842,38
13 6 48,49 41,21 46573,06 25672,01
14 250 8 54,97 41,47 27956,2 13048,70
15 10 55,5 43,24 19490,6 11885,53

3.1. The Effect of Bed Height to the CO2 Removal Efficiency

From Table 2, it can be seen that the highest CO2 removal efficiency obtained from the use of
activated carbon and zeolite adsorbents were 63.13% and 57.86%, respectively, which occurred at a

3
International Conference on Science and Innovated Engineering (I-COSINE) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 536 (2019) 012042 doi:10.1088/1757-899X/536/1/012042

flow rate of 50 cm3/minute and with a bed height of 10 cm. Meanwhile, the lowest CO2 removal
efficiency for activated carbon and zeolite adsorbents were 48.49% and 41.21%, respectively, which
occurred at a flow rate of 250 cm3/min and with a bed height of 6 cm. The results indicated that
activated carbon was having a higher capability to absorb CO2 compared to that of zeolite 13X. The
CFD results of this study are in line with experimental results obtained by Chue et al [15] and Das et al
[16], demonstrated that the pore surface area of activated carbon is much higher than that of zeolite
13X. Consequently, when activated carbon is used as an adsorbent, it can purify more CO 2 from a gas
mixture that other adsorbents of lower pore surface area.
Figure 2 illustrated the relationship between flow rate and CO2 removal efficiency at different bed
heights for each type of adsorbent. This figure suggested that the CO2 adsorption is much higher in
the activated carbon adsorbent and the adsorption rate increases with the increase of the bed height.
However, the removal efficiency decreases as the flow rate of the feed to the column increased [17]. In
other words, the influence of flow rate on the adsorption capacity is the greater the feed flow rate, the
smaller the adsorption rate will be. When the greater CO2 flow rate entering the column, the less
contact time between CO2 and adsorbent will be, Consequently, the percentage of adsorption will also
be smaller. Figures 3 to 5 show the contours of the influence of the flow rate on adsorption rate using
zeolite 13X adsorbent, while Figure 6 to 8 shows the contours of the influence of the flow rate on the
adsorption rate using activated carbon of coconut fiber adsorbent.

activated carbon-zeolite
Figure 2. The effect of adsorbent bed height on CO2 removal efficiency.

From Figure 3 to 8, it can be seen that there is a color difference in terms of velocity in the
adsorption column. The redder in color illustrated the higher the flow rate, while the bluer in color
indicated the lower the flow rate entering the adsorption column. The results clearly showed that at
the higher bed the velocity of the gas in the column decreases, due to the increase of the resistance to
the flow allowing more CO2 to be retained on the surface of the adsorbent.

4
International Conference on Science and Innovated Engineering (I-COSINE) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 536 (2019) 012042 doi:10.1088/1757-899X/536/1/012042

(a) (b) (c)

Figure 3. A contour of fluid flow rate at 50 cm3/minute (a) bed height of 6 cm, (b) bed height of 8 cm,
and (c) bed height of 10 cm

(a) (b) (c)

Figure 4. A contour of fluid flow rate of 100 cm3/min (a) bed height of 6 cm, (b) bed height of 8
cm, and (c) bed height of 10 cm.

(a) (b) (c)

Figure 5. A contour of fluid flow rate of 150 cm3/min (a) bed height of 6 cm, (b) bed height of 8 cm,
and (c) bed height of 10 cm

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International Conference on Science and Innovated Engineering (I-COSINE) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 536 (2019) 012042 doi:10.1088/1757-899X/536/1/012042

(a) (b) (c)

Figure 6. A contour of fluid flow rate of 50 cm3/minute (a) bed height of 6 cm, (b) bed height of 8
cm, and (c) bed height of 10 cm

(a) (b) (c)

Figure 7. A contour of fluid flow rate of 100 cm3/min (a) bed height of 6 cm, (b) bed height of 8 cm,
and (c) bed height of 10 cm.

(a) (b) (c)

3
Figure 8. A contour of fluid flow rate of 150 cm /min (a) bed height of 6 cm, (b) bed height of 8 cm,
and (c) bed height of 10 cm

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International Conference on Science and Innovated Engineering (I-COSINE) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 536 (2019) 012042 doi:10.1088/1757-899X/536/1/012042

From Figure 3 to 8, it can be seen that there is a color difference in terms of velocity in the
adsorption column. The redder in color illustrated the higher the flow rate, while the bluer in color
indicated the lower the flow rate entering the adsorption column. The results clearly showed that at
the higher bed the velocity of the gas in the column decreases, due to the increase of the resistance to
the flow allowing more CO2 to be retained on the surface of the adsorbent.

3.2. The Effect of Flow Rate on Adsorption Capacity

Figure 9 depicted the relationship between flow rate and adsorption capacity in both columns. It is
seen that the highest adsorption capacity of 46,573.06 mg/g occurred in the column filled with
activated carbon adsorbent made of coconut fiber at a flow rate of 250 cm3/min and bed height of 6
cm. Meanwhile, the lowest adsorption capacity of 2491.81 mg/g occurred at the column filled with the
adsorbent of zeolite 13X at a flow rate of 50 cm3/minute and bed height of 10 cm. It shows that the
activated carbon adsorbent is better when it is used for CO2 purification process compared to that of
zeolite 13X. In this case, it can be seen that the greater the flow rate, the higher the adsorption
capacity, indicating that the higher the flow rate, the more CO2 molecules will come into contact with
the pore surface of the adsorbent so that the adsorption capacity increases [18, 19] as marked by the
red color in the lighter part of the porous zone.

activated carbon-zeolite
Figure 9. The correlation between flow rate and adsorption capacity in both columns

3.3. Isothermal Adsorption

Figures 10 and 11 showed the adsorption isotherm of carbon dioxide with different adsorbents,
respectively. The adsorption data were fitted with standard isotherm models, including those of
Langmuir and Freundlich. With regard to adsorption isotherm of carbon dioxide in activated carbon,
the Langmuir model seems to give a better fit compared to that of Freundlich. the equation and
linearization results are also shown in table 3.
The linearity of Freundlich isotherm adsorption is higher than Langmuir isotherm. It shows the
process of CO2 adsorption with an activated carbon of coconut fiber and zeolite 13X adsorbents is in a
multilayer process. The adsorption process in activated carbon of coconut fiber occurs because it has
CO2 adsorbing properties through N2 [20, 21, 22]. Besides, activated carbon has a physical structure
with a very hard granular shape making it has the potential to adsorb gas, while the adsorption process
in zeolite 13X occurs because the zeolite 13X has a very small and uniform pore size, high Si/Al
content, and Na content as a minor element of the zeolite.

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International Conference on Science and Innovated Engineering (I-COSINE) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 536 (2019) 012042 doi:10.1088/1757-899X/536/1/012042

Figure 10. Adsorption isotherm of carbon dioxide in activated carbon adsorbent

Figure 1. Adsorption isotherm of carbon dioxide in activated carbon-zeolite 13X

Table 3. Parameter isotherm model via linearized technique for pressure 1atm and temperature 25°C

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International Conference on Science and Innovated Engineering (I-COSINE) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 536 (2019) 012042 doi:10.1088/1757-899X/536/1/012042

4. Conclusion
The higher CO2 removal efficiency and CO2 adsorption capacity were produced by the adsorbent of
activated carbon made of coconut fiber than that of zeolite 13X. It shows that the relationship between
adsorption rate and capacity are inversely proportional. The process of CO2 adsorption using activated
carbon and zeolite 13X is in multilayer one. Further research is still required to investigate the
optimization of operating parameters with the aim at obtaining higher removal efficiency.

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IOP Conf. Series: Materials Science and Engineering 536 (2019) 012042 doi:10.1088/1757-899X/536/1/012042

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