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Water vapor adsorption by activated carbon: a modification to the isotherm model of Do and Do
Letters to the editor / Carbon 39 (2001) 1421 – 1446
1437
Water vapor adsorption by activated carbon: a modification to the isotherm model of Do and Do a, a b Marcel Neitsch *, Wolfgang Heschel , Matthias Suckow a
¨ Bergakademie Freiberg, Institut f ur ¨ Energieverfahrenstechnik und Chemieingenieurwesen, Reiche Zeche, Technische Universitat 09596 Freiberg, Germany b Fachhochschule Lausitz, Fachbereich Chemieingenieurwesen /Verfahrenstechnik, Großenhainer Str. 57, 01968 Senftenberg, Germany Received 29 January 2001; accepted 25 March 2001 Keywords: A. Activated carbon; C. Modeling; D. Adsorption properties
Recently Do and Do [1] presented a model for water vapor adsorption by activated carbon based on new insights into the underlying mechanism. Kaneko [2] stated that the steep rise in the adsorption isotherm of water is not caused by capillary condensation but by formation of water clusters. These clusters were formed at water molecules (secondary sites) adsorbed at functional surface groups (primary sites) which are located at the entrance of the micropores [1], desorbs from these secondary sites and adsorbs into the micropores. Do and Do proposed the following isotherm equation on the assumption that five water molecules form one cluster called a pentamer:
O O
O
n
n
Kf ip ir Km p ri i51 i56 q 5 qf ]]]] 1 qmS ]]]]] n n n 1 1 Kf p ir Km p ri 1 p ri25 i51
O O
i56
(1)
i56
where q is the water vapor loading in [g g 21 ], pr is the relative pressure, qf is the functional group concentration in [g g 21 ], Kf is the equilibrium constant for chemisorption and H-bonding on primary sites, qmS is the saturation concentration in the micropores in [g g 21 ] and Km is the micropore equilibrium constant. The first term on the right hand side of this equation describes the adsorption on the primary sites and is of BET type. The second term refers to the desorption from the secondary sites and the adsorption of the pentamer into the micropores. So the equation describes a micropore filling mechanism. From the mathematical point of view one can show, that this equation can be simplified to:
O O n
Kf ip ir Km p r6 i51 ]]]] ]]] q 5 qf 1 qmS n Km p 6r 1 pr 1 1 Kf p ir
(2)
i51
* Corresponding author. Tel.: 149-3731-394-516; fax: 1493731-394-555. E-mail address: [email protected] (M. Neitsch).
Our approach is to generalize this model in such a way that the size of the water clusters is variable. This assumption is based on several investigations which can be found in the literature. Iiyama et al. [3] stated that the water clusters, which are formed in ACF, have a size of two to five water molecules per cluster. One should note, that the pore width of ACF is very narrow and so the size of the clusters could be restricted by the pore width. Vartapetyan and Voloshchuk [4] proposed that the clusters on the surfaces of nonporous carbon adsorbents can reach a maximum size of 150 molecules per site. They also stated, that this cluster size is influenced by the distance between primary sites. Molecular simulations [5] confirmed that the cluster size depends on the density and the distribution of these sites and they also predicted a dependence between cluster size and pore width. So we have modified the model of Do with regard to the cluster size m which is now an averaged value and not a constant:
O O
sm 11d
Kf ip ir Km p rm 11 i51 ]]]] q 5 qf ]]]] 1 q mS m 11 m 11 s d Km p r 1 pr 1 1 Kf p ri
(3)
i51
This model is called the CIMF isotherm (cluster formation induced micropore filling). We tested both isotherm equations for several activated carbons with different amounts of primary adsorption sites. The results for two of these carbons are shown in Fig. 1. A comparison between the parameters of the isotherm equation of Do and the model proposed in this paper is given in Table 1. One can see, that the model of Do and Do can only describe the isotherm of the GAC (granulated activated carbon) which have a higher amount of primary adsorption sites. The cluster size computed by the CIMF model was only slightly above 5. So the results for the GAC are comparable. For the BAC (bead-shaped activated carbon) with a small amount of primary sites it can be seen, that the model of Do fails. The modified equation describes the experimental adsorption isotherm very well. One can see from Table 1 that the computed cluster size for the BAC is
0008-6223 / 01 / $ – see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 01 )00077-X
Letters to the editor / Carbon 39 (2001) 1421 – 1446
1438 Table 1 Fitted parameters
GAC (SCXII)
BAC (SaR-0)
Do and Do
CIMF
Do and Do
CIMF
qf [g g 21 ] Kf qmS [g g 21 ] Km m
0.010 45 0.344 27.9 5 (constant)
0.012 28 0.325 58.7 6.0 (variable)
0.003 10 0.513 10.7 5 (constant)
0.004 15 0.415 3004.7 16.0 (variable)
r2
0.998
0.999
0.979
1.000
a high concentration of these sites. In this way the clusters formed on the primary sites can consist of more than 5 molecules. So the model proposed in this paper can be used for an exact modeling of type IV and type V isotherms, but it has to be proofed for highly oxidized activated carbons and ACF in the future.
References
Fig. 1. Water adsorption isotherms. Fitting of the models to experimental data.
much higher than 5. This seems to be clear if one supposes that clusters can increase up to an amount where they spatially interact as proposed in Ref. [4]. Consequently, in carbons with a small density of primary adsorption sites the cluster size should become greater then in carbons with
[1] Do DD, Do HD. A model for water adsorption in activated carbon. Carbon 2000;38(5):767–73. [2] Iiyama T, Ruike M, Kaneko K. Structural mechanism of water adsorption in hydrophobic micropores from in situ small angle X-ray scattering. Chem Phys Lett 2000;331(5 / 6):359–64. [3] Kaneko K. Specific intermolecular structures of gases confined in carbon nanospaces. Carbon 2000;38(2):287–303. [4] Vartapetyan RSh, Voloshchuk AM. The mechanism of the adsorption of water molecules on carbon adsorbents. Russ Chem Rev 1995;64(11):985–1001. ¨ [5] Muller EA, Rull LF, Vega LF, Gubbins KE. Adsorption of water on activated carbons: a molecular simulation study. J Phys Chem 1996;100(4):1189–96.
An attempt to prepare carbon nanotubes by the spinning of microcapsules D. Hulicova a , F. Sato a , K. Okabe a , M. Koishi b , A. Oya a , * b
a Faculty of Technology, Gunma University, Kiryu, Gunma 376 -8515, Japan Faculty of Industrial Science and Technology, Science University of Tokyo, Tomino 102 -1, Oshamanbe, Yamakoshi, Hokkaido 049 -3514, Japan
Received 2 March 2001; accepted 25 March 2001 Keywords: A. Carbon nanotubes; B. Carbonization; C. Scanning electron microscopy; D. Texture
*Corresponding author. Tel.: 181-277-30-1350; fax: 181277-30-1353. E-mail address: [email protected] (A. Oya).
Carbon nanotubes have great potential for use in technological applications, and therefore the development of a mass-production method has high priority. Many methods
0008-6223 / 01 / $ – see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 01 )00078-1
1437
Water vapor adsorption by activated carbon: a modification to the isotherm model of Do and Do a, a b Marcel Neitsch *, Wolfgang Heschel , Matthias Suckow a
¨ Bergakademie Freiberg, Institut f ur ¨ Energieverfahrenstechnik und Chemieingenieurwesen, Reiche Zeche, Technische Universitat 09596 Freiberg, Germany b Fachhochschule Lausitz, Fachbereich Chemieingenieurwesen /Verfahrenstechnik, Großenhainer Str. 57, 01968 Senftenberg, Germany Received 29 January 2001; accepted 25 March 2001 Keywords: A. Activated carbon; C. Modeling; D. Adsorption properties
Recently Do and Do [1] presented a model for water vapor adsorption by activated carbon based on new insights into the underlying mechanism. Kaneko [2] stated that the steep rise in the adsorption isotherm of water is not caused by capillary condensation but by formation of water clusters. These clusters were formed at water molecules (secondary sites) adsorbed at functional surface groups (primary sites) which are located at the entrance of the micropores [1], desorbs from these secondary sites and adsorbs into the micropores. Do and Do proposed the following isotherm equation on the assumption that five water molecules form one cluster called a pentamer:
O O
O
n
n
Kf ip ir Km p ri i51 i56 q 5 qf ]]]] 1 qmS ]]]]] n n n 1 1 Kf p ir Km p ri 1 p ri25 i51
O O
i56
(1)
i56
where q is the water vapor loading in [g g 21 ], pr is the relative pressure, qf is the functional group concentration in [g g 21 ], Kf is the equilibrium constant for chemisorption and H-bonding on primary sites, qmS is the saturation concentration in the micropores in [g g 21 ] and Km is the micropore equilibrium constant. The first term on the right hand side of this equation describes the adsorption on the primary sites and is of BET type. The second term refers to the desorption from the secondary sites and the adsorption of the pentamer into the micropores. So the equation describes a micropore filling mechanism. From the mathematical point of view one can show, that this equation can be simplified to:
O O n
Kf ip ir Km p r6 i51 ]]]] ]]] q 5 qf 1 qmS n Km p 6r 1 pr 1 1 Kf p ir
(2)
i51
* Corresponding author. Tel.: 149-3731-394-516; fax: 1493731-394-555. E-mail address: [email protected] (M. Neitsch).
Our approach is to generalize this model in such a way that the size of the water clusters is variable. This assumption is based on several investigations which can be found in the literature. Iiyama et al. [3] stated that the water clusters, which are formed in ACF, have a size of two to five water molecules per cluster. One should note, that the pore width of ACF is very narrow and so the size of the clusters could be restricted by the pore width. Vartapetyan and Voloshchuk [4] proposed that the clusters on the surfaces of nonporous carbon adsorbents can reach a maximum size of 150 molecules per site. They also stated, that this cluster size is influenced by the distance between primary sites. Molecular simulations [5] confirmed that the cluster size depends on the density and the distribution of these sites and they also predicted a dependence between cluster size and pore width. So we have modified the model of Do with regard to the cluster size m which is now an averaged value and not a constant:
O O
sm 11d
Kf ip ir Km p rm 11 i51 ]]]] q 5 qf ]]]] 1 q mS m 11 m 11 s d Km p r 1 pr 1 1 Kf p ri
(3)
i51
This model is called the CIMF isotherm (cluster formation induced micropore filling). We tested both isotherm equations for several activated carbons with different amounts of primary adsorption sites. The results for two of these carbons are shown in Fig. 1. A comparison between the parameters of the isotherm equation of Do and the model proposed in this paper is given in Table 1. One can see, that the model of Do and Do can only describe the isotherm of the GAC (granulated activated carbon) which have a higher amount of primary adsorption sites. The cluster size computed by the CIMF model was only slightly above 5. So the results for the GAC are comparable. For the BAC (bead-shaped activated carbon) with a small amount of primary sites it can be seen, that the model of Do fails. The modified equation describes the experimental adsorption isotherm very well. One can see from Table 1 that the computed cluster size for the BAC is
0008-6223 / 01 / $ – see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 01 )00077-X
Letters to the editor / Carbon 39 (2001) 1421 – 1446
1438 Table 1 Fitted parameters
GAC (SCXII)
BAC (SaR-0)
Do and Do
CIMF
Do and Do
CIMF
qf [g g 21 ] Kf qmS [g g 21 ] Km m
0.010 45 0.344 27.9 5 (constant)
0.012 28 0.325 58.7 6.0 (variable)
0.003 10 0.513 10.7 5 (constant)
0.004 15 0.415 3004.7 16.0 (variable)
r2
0.998
0.999
0.979
1.000
a high concentration of these sites. In this way the clusters formed on the primary sites can consist of more than 5 molecules. So the model proposed in this paper can be used for an exact modeling of type IV and type V isotherms, but it has to be proofed for highly oxidized activated carbons and ACF in the future.
References
Fig. 1. Water adsorption isotherms. Fitting of the models to experimental data.
much higher than 5. This seems to be clear if one supposes that clusters can increase up to an amount where they spatially interact as proposed in Ref. [4]. Consequently, in carbons with a small density of primary adsorption sites the cluster size should become greater then in carbons with
[1] Do DD, Do HD. A model for water adsorption in activated carbon. Carbon 2000;38(5):767–73. [2] Iiyama T, Ruike M, Kaneko K. Structural mechanism of water adsorption in hydrophobic micropores from in situ small angle X-ray scattering. Chem Phys Lett 2000;331(5 / 6):359–64. [3] Kaneko K. Specific intermolecular structures of gases confined in carbon nanospaces. Carbon 2000;38(2):287–303. [4] Vartapetyan RSh, Voloshchuk AM. The mechanism of the adsorption of water molecules on carbon adsorbents. Russ Chem Rev 1995;64(11):985–1001. ¨ [5] Muller EA, Rull LF, Vega LF, Gubbins KE. Adsorption of water on activated carbons: a molecular simulation study. J Phys Chem 1996;100(4):1189–96.
An attempt to prepare carbon nanotubes by the spinning of microcapsules D. Hulicova a , F. Sato a , K. Okabe a , M. Koishi b , A. Oya a , * b
a Faculty of Technology, Gunma University, Kiryu, Gunma 376 -8515, Japan Faculty of Industrial Science and Technology, Science University of Tokyo, Tomino 102 -1, Oshamanbe, Yamakoshi, Hokkaido 049 -3514, Japan
Received 2 March 2001; accepted 25 March 2001 Keywords: A. Carbon nanotubes; B. Carbonization; C. Scanning electron microscopy; D. Texture
*Corresponding author. Tel.: 181-277-30-1350; fax: 181277-30-1353. E-mail address: [email protected] (A. Oya).
Carbon nanotubes have great potential for use in technological applications, and therefore the development of a mass-production method has high priority. Many methods
0008-6223 / 01 / $ – see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 01 )00078-1