Adsorbed natural gas (ANG) by Granular Activated Carbon (GAC) has been widely investigated as an alternative to CNG and LNG technologies for storage and transportation of natural gas. In the present work, a theoretical and experimental study was conducted to accurately determine the amount of adsorption of CH4 by GAC. To carry out the experiments, the volumetric method was used up to 4 MPa at constant temperature of 298 K. Also, various adsorption isotherm models were used to model the experimental data collected from the experiments. The accuracy of the results              obtained from the adsorption isotherm models was compared and the values for the regressed parameters were reported. The results shows that the amount of CH4 adsorbed is dependent on the physical characteristics of activated carbons. Moreover, the Toth, two term TVFM and Sips isotherm models show good agreement with the experimental data

A. Aleghafouri

A. Mohajeri

M. Asghari

M. Mahdyarfar

M. Mohsen-Nia

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55
Journal of Petroleum Science and Technology, Vol.1, No.1, 2011, 55-59
Experimental and Theoretical Study on the 
CH4 Adsorption by Granular and Microporous 
Activated Carbon
A. Aleghafouri1, M. Mahdyarfar2٭, M. Mohsen-Nia1, A. Mohajeri2 and M. Asghari1 
1- Chemical Engineering Group, Department of Engineering, University of Kashan, Kashan, Iran
2- Gas Division, Research Institute of Petroleum Industry (R.I.P.I.), Tehran, Iran
mahdyarfarm@ripi.ir
Abstract
Adsorbed natural gas (ANG) by Granular Activated Carbon (GAC) has been widely investigated 
as an alternative to CNG and LNG technologies for storage and transportation of natural gas. In 
the present work, a theoretical and experimental study was conducted to accurately determine the 
amount of adsorption of CH4 by GAC. To carry out the experiments, the volumetric method was 
used up to 4 MPa at constant temperature of 298 K. Also, various adsorption isotherm models were 
used to model the experimental data collected from the experiments. The accuracy of the results 
obtained from the adsorption isotherm models was compared and the values for the regressed pa-
rameters were reported. The results shows that the amount of CH4 adsorbed is dependent on the 
physical characteristics of activated carbons. Moreover, the Toth, two term TVFM and Sips isotherm 
models show good agreement with the experimental data. 
Key words: Adsorbed Natural Gas, Activated Carbon, Adsorption Isotherm
Introduction
One of the main drawbacks of natural gas (NG) as a fuel 
is its low density. For example, 1 L of NG at normal 
temperature and pressure (NTP) will yield 0.04 MJ on 
combustion while 1 L of gasoline will yield 34.8 MJ 
[1]. Whereas natural gas is a cheap and readily available 
source of fuel, its suitability for a particular application 
depends on the ability to store an adequate amount of 
it. This is a particularly acute problem in the design of 
gas driven vehicles. Storage is also a problem in the gas 
supply industry. NG needs to be stored at periods of low 
demand to meet peak requirements and it needs to be 
transported in containers by road, rail, and sea to regions 
not connected by pipeline. The principle constituent 
of NG is methane, which has a critical temperature of 
191 K. Thus, NG cannot be liquefied by pressure alone. 
Liquefied natural gas (LNG) is usualy stored as a boiling 
liquid at about 113 K in a cryogenic tank at a pressure 
of about 1 atm (0.103 MPa). Alternatively, the gas is 
stored as a compressed supercritical fluid known as 
compressed natural gas (CNG) at room temperature and 
about 200 MPa. LNG has a density of about 600 times 
greater than that at NTP (denoted 600 v/v) while CNG is 
about 220 v/v [1].
ANG by GAC has been widely investigated as an 
alternative to CNG and LNG technologies for storage 
and transportation of natural gases [2-8]. The physical 
characteristics of adsorbent, i.e., activated carbon can 
significantly afect the amount of gas adsorbed and 
desorbed by GAC. Extensive investigations have been 
directed to study such efect [9−11].
     In this study, theoretical and experimental investiga-
tions were performed to determine the adsorption of CH4 
by commercial microporous GACs up to 4 MPa at 298 
K and to study different adsorption isotherm models to 
correlate the adsorption data. 
Experimental 
Materials
Two commercial granular microporous activated carbons 
were used in this study: 
      The sample AC1 was purchased from Silcarbon Com-
pany and sample AC2 was supplied by Norit Company. 
It should be mentioned that except for the work of Mo-
hammadi et al. [12] in which a study of CH4 adsorption at  
pressures lower than 1 MPa has been carried out, experi-
mental and theoretical studies on the CH4 adsorption up to 
4 MPa by these adsorbents have not been previously done. 
56 Aleghafouri et al Journal of Petroleum Science and Technology
Table 1 contains the physical specifications of tow 
samples obtained by applying N2 adsorption using 
ASAP2010 (Micromeritics Co.) apparatus. 
CH4 Adsorption measurement
To obtain the adsorption isotherms of CH4 on activated 
carbons a volumetric based apparatus was used in this 
study. A schematic diagram of the apparatus is shown 
in Fig. 1. This set up also has a pressure transducer to 
measure the equilibrium pressure with the accuracy of 
±1 mbar. The apparatus has previously been tested for 
leak absence. Prior to measurement, the samples are de-
gassed at 300 oC for approximately 2 h at low pressure.
   To determine the amount of CH4 adsorbed on the 
activated carbon, at first stage, the adsorption cel is 
filed up with the degassed AC, the dead volume inside 
the cell is then determined with helium injection from 
the reference cel, which has been previously filed with 
helium gas up to the initial pressure and the adsorption 
cell reaches the thermodynamic equilibrium condition 
by opening valve 2. The total volume of the reference 
and adsorption cells and connecting lines is assumed to 
be V2, where helium can be penetrated into and it can be 
calculated using P1 , the initial pressure of the reference 
cell, P2 the equilibrium pressure of the whole adsorption 
system (the reference cell and the adsorption cell), and 
V1, the volume between valves 1 and 2. Therefore, it is 
possible to determine the dead volume (Vvoid) inside the 
adsorption section (between valves 2 and 3) using the 
following relation:
 Vvoid = V2-V1                                                                 (1)
     To inject CH4, the reference cel is filed with CH4 at 
a known pressure. Then, gas from the reference cell is 
injected to the adsorption cell by opening the connecting 
Table 1- Physical specification of AC samples used
Sample
 BET surface 
area(m2/gr)
 Micropores 
volume (cm3/gr)
 Average pore 
diameter (Ả)
AC1 1170 0.48 10.3
AC2 1117 0.45 11.5
Figure 1- Schematic diagram of the volumetric based apparatus used to CH4 adsorption measurements
valve. Having reached equilibrium, the connecting valve 
between the adsorption and reference cells is closed 
and this procedure is repeated for further pressures of 
adsorption isotherm. To determine the amount of CH4 
adsorbed at any equilibrium pressure, the following 
equation was employed:
 P1,i V1=Z1,i n1,i RT                                                           (2)
 P2,i V2=Z2,i n2,i RT                                                           (3)
 P2,i-1 Vvoid=Z2,i-1 nvoid,i RT                                                 (4)
     Where P1,i and P2,i are the initial pressure of the refer-
ence cell and the equilibrium pressure of the adsorption 
and reference cels for a defined point (for example i) of 
CH4 adsorption isotherm, respectively, and n1,i, n2,i and 
nvoid represent the mole numbers of CH4 before and after 
equilibrium in the V1,V2 and Vvoid volumes, respectively. 
Also, Z is the compressibility-factor of CH4, which can 
be determined at a defined pressure using the simple 
form of the virial expansion [13]:
                                                                                     (5)
In which
                                                                                      (6)
Where
                                                    (7)
                                                   (8)
    Finally, the mole numbers of CH4 adsorbed on the 
activated carbon at this pressure is determined as shown 
below:
 nadsorbed,i=n1,i +nvoid,i-1 -n2,i                                               (9)
     Therefore, the amount of adsorption (the mass ratio 
of adsorbed CH4 (g) to activated carbon (g)) at a defined 
pressure is calculated by dividing the weight of adsorbed 
CH4 (nadsorbed ×16.034) by that of the activated carbons 
filed in the adsorption cel.
57Experimental and Theoretical Study on the CH4 ...Vol. 1, No. 1
Modeling
To reduce the experimental data, different adsorption 
isotherm models were used. The adsorption isotherm 
models used in this study are rendered in Table 2. As 
observed, each adsorption isotherm model is based on 
its own physical and theoretical concept and contains 
several adjustable parameters. The details of the as-
sumptions made for each adsorption isotherm models 
are given elsewhere [14-16].
    The parameters for the adsorption isotherm models 
were obtained by the least square fit of the adsorption 
models to the experimental data collected in this study 
using the Sigma plot ver. 11.0 software. It is noteworthy 
that the coefficient of determinations (R2adj) was used to 
Table 2- Different isotherm models
Isotherm model Equation
Langmuir
Freundlich
Sips
Toth
Dubinin - Radushkevich (DR)
Two-Term TVFM
Where A = RT ln (Ps /P), P: operation pressure, Ps: saturation pressure
obtain the regressed parameters of the models.
Results and discussion
To determine the CH4 adsorption isotherms, the experi-
mental data obtained were used at twelve equilibrium 
pressures up to nearly 4 MPa and temperature of 298 
K for each GAC sample. Figure 2 shows the adsorption  
isotherms of CH4 for the GAC samples used in this study. 
It can be seen that amounts of CH4 adsorbed increases 
by pressure rising, but the increasing trend is not sig-
nificant at higher pressures. Also, the results show that 
sample AC1 has the higher adsorption in comparison  
with sample AC2, and this is because of the higher 
micropore volume and BET surface area.
Figure 2- CH4 adsorption isotherms for the GAC samples
58 Aleghafouri et al Journal of Petroleum Science and Technology
To compare samples used in this study with wood 
based granulated carbon used by ADVANTICA 
technology, the values of STP volume of CH4 adsorbed 
per volume of adsorbent (v/v) at 2.45 MPa and room 
temperature together with enhancement factor (ANG/
CNG storage) are presented in table 3 [17]. It can be 
seen that the values for the samples used in this study are 
better than those of the sample used by ADVANTICA 
technology. Also, it should be mentioned that although 
v/v values for the sample used in this study at 3.5 MPa 
and room temperature are not high (75 v/v for AC1 and 
55 v/v for AC2), the results reported from research in 
this area show that the target value of 150 v/v deliver-
able at 3.5 MPa and room temperature make the system 
commercially attractive [18] is still hardly reached.
     Table 4 shows the obtained results including adjust-
able parameters and R2adj adjusting six isotherm models, 
presented in Table 2, with the experimental data of CH4 
adsorption isotherms obtained from two GAC samples.
      As observed, it is concluded that the Toth, two 
term TVFM and Sips isotherms show better agree-
ment with the experimental data for two GAC samples 
in comparison with the other isotherms, whereas the 
Freundlich equation shows inferior results compared 
with the experimental data.
        The accuracy of the results obtained from the two-term 
TVFM isotherm model in comparison with the other 
isotherm models having the same theoretical concept 
(such as DR) is related to the presumptions considered 
for this model. As seen in Table 2, two-term TVFM 
equation contains four adjustable parameters, i.e., W01, 
E1, W02, and E2. For the homogeneous microporous 
structure adsorbents W02 = 0. The first term of the two­
term TVFM model represents the adsorption of CH4 onto 
micropores and the second term justifies the adsorption 
onto super-micropores. Therefore, the use of DR equation 
with one term cannot give superior results in the 
comparison of experimental data.
Conclusions
An experimental study was carried out to determine 
CH4 adsorption capacities of two different types of 
GAC up to 4 MPa at 298 K and the results obtained are 
comparable with wood based granulated carbon used 
by ADVANTICA technology. Also, various adsorption 
isotherm models were used to model the experimental 
data obtained from those two GAC samples. The accu-
racy of the results obtained from the adsorption isotherm 
models was compared and the values for the regressed 
parameters were reported. The results showed that the 
physical characteristics of activated carbons such as 
BET surface area, micropore volume, and pore size 
Table 3- Comparison of ANG capacity at 2.45 MPa and room temperature
Sample CH4 Adsorbed (v/v) ANG/CNG storage
AC1 61.1 2.83
AC2 40.4 1.87
Sample used by ADVANTICA 32.4 1.50
Table 4- Data reduction results
Isotherm model
AC1
Parameters                                      R2adj          
AC2  
Parameters                                   R2adj
Langmuir
  W0 = 8.852                                 o.990
   b = 8×10-4
  W0 = 7.876                              o.999
   b = 5×10-4
Freundlich
  K = 0.151                                    o.992
   n = 2.144
  K = 0.047                                 o.982
   n = 1.751
Sips
  W0 = 13.567                                o.998
   b = 0.003
   n = 1.417
  W0 = 7.549                               o.999
   b = 0.0004
   n = 0.964
Toth
  W0 = 20.029                                  o.999
   b = 0.001
   t = 0.419
  W0 = 7.639                                o.999
   b = 0.0005
   t = 1.042
Dubinin - Radushkevich (DR)
  W0 = 7.558                                     o.991
   E = 7473.874
  W0 = 5.890                                o.997
   E = 6484.589
Two-Term TVFM
  W01 = 2.541                                    o.999
   E1 = 2531.671
  W02 = 6.814
   E2 = 8033.606
  W01 = 3.058                               o.999
   E1 = 1595.040
  W02 = 5.739
   E2 = 6597.726
59Experimental and Theoretical Study on the CH4 ...Vol. 1, No. 1
distribution have significant efects on the amount of 
CH4 adsorbed and used in this study are better than those 
used in ADVANTICA technology. Also, it is concluded 
that the Toth, two-term TVFM and Sips isotherms 
show better agreement with the experimental data in 
comparison with the other isotherms.
References
[1] Cracknell R.F., Gordon P. & Gubbins K.E., “Influ­
ence of pore geometry on the design of microporous 
materials for methane storage”, J. Phys. Chem., Vol. 97, 
No.2, pp. 494-499, 1993.
[2] Brunauer S., Emmett P.H. & Teller E., “Adsorption 
of gases in multimolecular layers”, J. Am. Chem. Soc., 
Vol. 60, No. 2, pp. 309-319, 1938.
[3] Lu X.C., Li F.C. & Watson A.T., “Adsorption mea­
surements in Devonian shales”, Fuel, Vol. 74, No. 4, pp. 
599-603, 1995.
[4] Inomata K., Kanatawa K., Urabe Y., Hosono H. 
& Araki T., “Natural gas storage in activated carbon 
pelets without a binder”,  Carbon, Vol. 40, No. 1, pp. 
87-93, 2002.
[5] Diamond W.P. & Schattel S.J., “Measuring the gas 
content of coal: a review”, Int. J. Coal. Geology, Vol. 35, 
Nos. 1-4, pp. 311-331, 1998.
[6] Jiang S.Y., Zollweg J.A. & Gubbins K.E., “Theory of 
Adsorption of Trace Components”, J. Phys. Chem., Vol. 
98, No. 9, pp. 2403-2411, 1994.
[7] Vasiliev L.L., Yanonchik L.E., Mishkinis D.A. & 
Rabetsky M.I., “Adsorbed natural gas storage and 
transportation vessels”, Int. J. Therm. Sci., Vol. 39, Nos. 
9-11, pp. 1047-1055, 2000.
[8] Zhang S.Y., Talu O. & Hayhunt D.T., “High­pres­
sure adsorption of methane in zeolites NaX, MgX, CaX, 
SrX and BaX”, J. Phys. Chem., Vol. 95, No. 4, pp. 1722-
1726, 1991.
[9] Bastos-Neto M., Canabrava D.V., Torres A.E.B. & 
Rodriguez-Castellon E., “Efects of textural and surface 
characteristics of microporous activated carbons on the 
methane adsorption capacity at high pressures”, Appl. 
Surf. Sci., Vol. 253, No. 13, pp. 5721-5725, 2007.
[10] Shao X., Wang W. & Zhang X., “Experimental 
measurements and computer simulation of methane 
adsorption on activated carbon fibers”, Carbon, Vol. 45, 
No. 1, pp. 188-195, 2007.
[11] Quinn D.F. & Macdonald J.A., “Natural gas 
storage”, Carbon, Vol. 30, No. 7, pp. 1097-1103, 1992.
[12] Mohammadi T., Abedinzadegan Abdi M., Parvin-
pour A. & Mahdyarfar M., “Prediction of active carbon 
and carbon molecular sieve’s adsorption behavior with 
the adsorption models”, 7th Chemical Engineering Con-
ference, Tehran University, Tehran, Iran, 2002. 
[13] Smith J.M., Van Ness H.V. & Abbott M.M., Intro-
duction to chemical engineering thermodynamics, 6th 
ed., Ch. 3, pp. 97-98, McGraw-Hill Companies, Inc., 
2001.
[14] Toth J., Adsorption, Theory, Modeling, and Anal-
ysis, Vol. 107, Ch. 1, pp. 58-66, Marcel Dekker, Inc., 
2001.
[15] Do D.D., Adsorption Analysis: Equilibria and 
Kinetics, Vol. 2, Ch. 3, pp. 49-77, Imperial College 
Press, 1998.
[16] Dubinin M.M., “Fundamentals of the theory of ad­
sorption in micropores of carbon adsorbents: Charac-
teristics of their adsorption properties and microporous 
structures”, Carbon, Vol. 27, No. 3, pp. 457-467, 1989.
[17] Dvorak K., Hodrien R.C., Judd R., Bates D., 
Allen M. & Ingram J., Internal Advantica Technology 
Co. Report, 2003. 
[18] Atlanta Gas Light Adsorbent Research Group 
(AGLARG). Report to US Department of Energy, 
Contract 466590, 1997.


Experimental and Theoretical Study on the CH4 Adsorption by Granular and Microporous Activated Carbon

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Experimental and Theoretical Study on the CH4 Adsorption by Granular and Microporous Activated Carbon

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