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Energy & Fuels 2006, 20, 766-770
Influence of Degassing Temperature on the Performance of Carbon Molecular Sieves for Separations Involving O2, N2, CO2, and CH4 Isabel P. P. Cansado,* Manuela Ribeiro Carrott, and Peter J. M. Carrott Centro de Quı´mica de EÄ Vora and Departamento de Quı´mica, UniVersidade de EÄ Vora, Cole´ gio Luı´s Anto´ nio Verney, 7000-671 EÄ Vora, Portugal ReceiVed NoVember 11, 2005. ReVised Manuscript ReceiVed January 20, 2006
The effect of thermal pretreatment on the diffusion of O2, N2, CO2, and CH4 at 298 K in the commercial carbon molecular sieve Takeda 3A was studied. The results indicate that pore mouth barrier controls nitrogen transport. For oxygen and carbon dioxide, however, two mechanisms are present. Pore mouth barrier control determines the transport at lower temperature degassing, and micropore diffusion is present with a high temperature degassing. When the degassing temperature is increased, the adsorption as a function of contact time is almost constant for O2 and CO2, is almost null for CH4, and increases significantly for N2 between 373 and 653 K. The rates of diffusion also increase with increasing degassing temperature, but more appreciably for N2, which results in a significant decrease in selectivity for O2/N2 and CO2/N2 separations. These separations are therefore more efficient for Takeda 3A if it is only subjected to temperatures lower than 373 K. On the other hand, for CO2/CH4, the separation is more efficient if the Takeda 3A sample is first submitted to a degassing temperature around 673 K.
Introduction Carbon molecular sieves (CMS) are carbonaceous materials with pores of molecular dimensions that provide a relatively high adsorption capacity and kinetic selectivity for various adsorptives. The molecular sieve properties of an adsorbent may be caused by the small dimensions of the micropores themselves or by the reduced dimensions of the entrances into the micropores.1 Many researchers have studied the adsorption and transport of gases in carbon molecular sieves.2-29 However, it * Corresponding author. E-mail:
[email protected]. (1) Dubinin, M. M.; Stoeckli, H. F. J. Colloid Interface Sci. 1980, 75, 34-42. (2) Rutherford, S. W.; Do, D. D. Langmuir 2000, 16, 7245-7254. (3) Bae, Y. S.; Lee, C. H. Carbon 2005, 43, 95-107. (4) Villar-Rodil, S.; Denoyel, R.; Rouquerol, J.; Martı´nez-Alonso, A.; Tasco´n, J. M. D. Chem. Mater. 2002, 14, 4328-4333. (5) Qinglin, H.; Farooq, S.; Karimi, I. A. Langmuir 2003, 19, 57227734. (6) Qinglin, H.; Farooq, S.; Karimi, I. A. AIChE J. 2004, 50, 351-367. (7) Qinglin, H.; Sundaram, S. M.; Farooq, S. Langmuir 2003, 19, 393405. (8) Rutherford, S. W.; Coons, J. E. J. Colloid Interface Sci. 2005, 284, 432-439. (9) Chagger, H. K.; Ndaji, F. E.; Sykes, M. L.; Thomas, K. M. Carbon 1995, 10, 1405-1411. (10) Cansado, I. P. P. Ph.D. Thesis, University of EÄ vora, EÄ vora, Portugal, 2003. (11) Rutherford, S. W.; Coons, J. E. Carbon 2003, 41, 405-411. (12) Carrott, P. J. M.; Ribeiro Carrott, M. M. L.; Cansado, I. P. P. Appl. Surf. Sci., in press. (13) Moreira, R. F. P. M.; Jose´, H. J.; Rodrigues, A. E. Carbon 2001, 39, 2269-2276. (14) Rutherford, S. W.; Nguyen, C.; Coons, J. E.; Do, D. D. Langmuir 2003, 19, 8335-8342. (15) Ding, L. P.; Yuan, Y. X.; Farooq, S.; Bhatia, S. K. Langmuir 2005, 21, 674-681. (16) Reid, C. R.; Thomas, K. M. J. Phys. Chem. B 2001, 105, 1061910629. (17) Rutherford, S. W.; Coons, J. E. Langmuir 2004, 20, 8681-8687. (18) Bello, G.; Garcia, R.; Arriaga, R.; Sepu´lveda-Escribano, A.; Rodrı´guez-Reinoso, F. Microporous Mesoporous Mater. 2002, 56, 139145.
is clear that the mechanism of gas transport and the effect of CMS pretreatments have not yet been fully clarified.7 A number of kinetic models for diffusion of molecules into carbon molecular sieves have been used. The gas uptake into CMS may be considered as a pseudo-first-order mass transfer between the gas phase and the carbon adsorption sites. In terms of a pseudo-first-order rate, the linear driving force (LDF) model is described by the following equation:4,14,16
V ) 1 - exp(-kt) Veq
(1)
where V is the volume uptake at time t, Veq is the volume uptake at equilibrium, and k is a kinetic constant. When the gas uptake is controlled by the micropore diffusion, the fractional uptake is expressed by the Fick law and the most usual procedure for evaluating molecular sieve properties involves the solution to the equation:4,8,12,21 (19) Ruthven, Douglas M. Chem. Eng. Sci. 1992, 47, 4305-4308. (20) Singh-Ghosal, A.; Koros, W. J. J. Membr. Sci. 2000, 174, 177188. (21) Crittenden, B.; Thomas, W. J. Adsorption Technology and Design; Butterworth-Heinemann: Oxford, 1998. (22) Cazorla-Amoro´s D.; Alcaniz-Monge, J.; De la Casa-Lillo, M. A.; Linares-Solano, A. Langmuir 1998, 14, 4589-4596. (23) Schalles, D. G.; Danner, R. P. AIChE Symp. Ser. 1988, 83-89. (24) Lozano-Castello´, D.; Cazorla-Amoro´s, D.; Linares-Solano, A. Carbon 2004, 42, 1231-1236. (25) Valente Nabais, J. M.; Carrott, P. J. M.; Ribeiro Carrott, M. M. L.; Padre-Eterno, A. M.; Mene´ndez, J. A.; Dominguez, A. A.; Ortiz, L. Carbon, in press. (26) Stoeckli, F.; Slasli, A.; Hugi-Cleary, D.; Guillot, A. Microporous Mesoporous Mater. 2002, 51, 197-202. (27) De La Casa-Lillo, M. A.; Alcaniz-Monge, J.; Raymundo-Pinero, E.; Cazorla-Amoro´s, D.; Linares-Solano, A. Carbon 1998, 36, 1353-1360. (28) Villar-Rodil, S.; Navarrete, R.; Denoyel, R.; Alibiniak, A.; Paredes, J. I.; Martı´nez-Alonso A.; Tasco´n, J. M. D. Microporous Mesoporous Mater. 2005, 77, 109-118. (29) Lagorsse, S.; Magalha˜es, F. D.; Mendes, A. J. Membr. Sci. 2004, 241, 275-287.
10.1021/ef050374j CCC: $33.50 © 2006 American Chemical Society Published on Web 02/21/2006
Degassing Temperature and the Performance of CMS
V Veq
)1-
6 π
D/r2
∞
∑ 2 n)1
[ ( 1
D exp - n2π2t 2 n r2
)]
Energy & Fuels, Vol. 20, No. 2, 2006 767
(2)
s-1
where the units of are if t is expressed in seconds. The most common approach has been to apply eq 2 at long contact times, corresponding to uptakes greater than about 60-70%. In this case, eq 2 reduces to:8,9,21
(
V D ) 1 - C exp - 2π2t Veq r
)
(3)
where C ) 6/π2 and the effective diffusion coefficient, D/r2, can then be calculated from the slope of a plot of ln(1 - V/Veq) versus t, which has an intercept close to -0.5 if Fick’s law is followed.8 Comparing eqs 1 and 3, it can be seen that the constant k of the LDF model is essentially comparable to Dπ2/ r2 in the Fickian diffusion model and the intercept of the ln(1 - V/Veq) versus t plot is zero for LDF kinetics. Alternatively, eq 2 can be applied at short contact times, corresponding to uptakes less than 30% of the equilibrium capacity, in which case it reduces to:8,21
( )
D V ) 6 2t Veq πr
0.5
(4)
and the effective diffusion coefficient, D/r2, can then be calculated from the slope of a plot of V/Veq versus xt. The major problem with this equation is the lower accuracy of the experimental data obtained at the initial part. However, some authors found that the diffusion coefficient obtained from eq 4 is accurate if applicable only for values V/Veq < 0.25.9 In a previous article,12 we compared diffusion coefficients found in the literature, in particular for Takeda 3A for O2, N2, CO2, and CH4. In general, for all gases, the effective diffusion coefficients increased with temperature, but a considerable dispersion was confirmed between results from different authors.2-9 We suggested that this dispersion could be due to differences in the precise experimental procedures, or in the fixed pressure used to determine the kinetic uptake curves, or even to differences in the precise microstructure of the samples.12 Hence, the primary objective of the present study was to investigate the influence of degassing and stabilization or regeneration temperature on the molecular sieve properties, diffusion coefficients, and selectivity of carbon materials whose molecular sieve effect is based on the constrictions present at the micropore entrance. This study was motivated not only by our previous work but also by certain concerns expressed by other authors. For instance, Bae and Lee reported weight loss and pore enlargement on CMS when this was regenerated at high temperature (548 K) in air.3 Qinglin et al. also refer to probable pore enlargement, affecting the uptake of oxygen and nitrogen, when the sample regeneration takes place in the presence of oxygen.7 On the other hand, Dubinin and Stoeckli stated that, if the sieve properties on a CMS were caused by narrow micropores, a small activation at 1223 K with CO2 had no effect, but if the sieve properties were caused by reduction at the micropore entrances, a moderate activation in the same conditions promoted the loss of the molecular sieve properties.1 Unfortunately, the sample degassing or stabilization or regeneration conditions applied between two experimental kinetic runs are frequently not specified in published work. Some of the data that is available and refers to sample degassing or regeneration temperatures over the range 333-653 K is given in Table 1. Another difficulty, which can
Table 1. Diffusion Coefficients for O2 and N2 on CMS Takeda 3A and Bergbau Forschung Determined at Various Temperatures Using Different Experimental Conditions degassing temp (K)
adsorption temp (K)
D/r2 (O2) (s-1)
D/r2 (N2) (s-1)
selectivity D/r2 (O2)/D/r2 (N2)
not specified2 3638,9,14 4735-7
293 293 267-273 302 302 273 298 300 298
1.2 × 10-3 1.3 × 10-3 4.8 × 10-3 1.3 × 10-2 6.8 × 10-3 2.4 × 10-4 1.3 × 10-2 3.5 × 10-3 2.5 × 10-4
6.7 × 10-5 6.7 × 10-5 1.3 × 10-4 2.8 × 10-4 4.3 × 10-4 3.5 × 10-6 9.8 × 10-4 1.1 × 10-4 4.7 × 10-5
17.9a 19.4a 36.8a I 46.1a II 15.9b 68.6b 14.4a 38.3b 5.3b
52319 5234,28 52319 62313 a
Takeda 3A sample. b Bergbau Forschung sample.
sometimes make it difficult to compare published work among different authors, is apparent from the work of Qinglin et al. who reported different results from two different samples of Takeda 3A (Takeda 3A I and Takeda 3A II).5-7 This could explain, for example, the difference between the micropore volumes found in the literature for the “supposed same” material, Takeda 3A, for which values of 0.15,10,30,31 0.162,3 0.166,11 and 0.2022,24 cm3 g-1 have been reported. Experimental Section The adsorbent used in this study was a commercial carbon molecular sieve from Takeda designated Takeda 3A. Takeda 3A is a CMS with ink bottle micropores generated through treatments of the micrograins via carbon deposition, which coats the entrances to the micropores and creates a barrier at the pore mouth.14 Takeda 3A was previously characterized by us by means of nitrogen adsorption at 77 K, carbon dioxide adsorption at 273 K, and methanol adsorption at 298 K, and analysis of the carbon dioxide and methanol isotherms, by application of the DR equation, indicated that this sample had a micropore volume of 0.15 cm3 g-1 and a mean pore size around 0.57 nm.10,30,31 In this work, the molecular sieving properties were evaluated by measuring single gas uptakes at 298 K as a function of time at a contact pressure of approximately 1 bar in a manometric-type apparatus. The sample temperature was controlled by means of a continuous flow of a water-antifreeze mixture between an insulated jacket around the sample flask and a Grant LTD20 thermostat bath. The flow was maintained constant by means of a Cole Palmer peristaltic pump. Prior to each experimental run, the sample was degassed at different temperatures (298-653 K) during almost 5 h until the pressure was less than 10-4 mbar. The gases used and their respective molecular diameters were CO2 (0.330 nm), O2 (0.346 nm), and N2 (0.364 nm), which are linear, and CH4 (0.380 nm), which is tetrahedral.12
Results and Discussion Evaluation of Diffusion Mechanism. Figures 1 and 2 show the uptake curves for O2/N2 and CH4/CO2, respectively, obtained on Takeda 3A, at 298 K, when the sample was degassed between each run at different temperatures. It can be seen that the uptakes of the smaller molecules, CO2 and O2, were not significantly altered by the degassing temperature, while the N2 uptake was similar when the sample was degassed at 298 or 373 K, but when the sample was degassed at 473 or 653 K, an increased uptake over the time scale of the experiments was clearly evident. This phenomenon could be explained by a loss of surface groups at the pore entrance and through a polarity (30) Carrott, P. J. M.; Ribeiro Carrott, M. M. L.; Cansado, I. P. P. Stud. Surf. Sci. Catal. 2000, 128, 323-331. (31) Carrott, P. J. M.; Ribeiro Carrott, M. M. L.; Cansado, I. P. P. Carbon 2001, 39, 193-200.
768 Energy & Fuels, Vol. 20, No. 2, 2006
Cansado et al.
Figure 1. Uptake curves for O2 and N2 at 298 K on Takeda 3A degassed at different temperatures. N2 - open symbols. O2 - filled symbols.
Figure 3. Variation of ln(1 - V/Veq) versus time for the adsorption of O2 and N2 at 298 K on Takeda 3A degassed at different temperatures. N2 - open symbols. O2 - filled symbols.
Figure 2. Uptake curves for CO2 and CH4 at 298 K on Takeda 3A degassed at different temperatures. CO2 - filled symbols. CH4 - open symbols.
Figure 4. Variation of ln(1 - V/Veq) versus time for the adsorption of CO2 at 298 K on Takeda 3A degassed at different temperatures.
decrease, which reduces the interaction between nitrogen and surface groups. On the other hand, the extent of these alterations was not sufficient to influence the accessibility of a larger CH4 molecule, as it was found that with CH4 the degassing temperature had no effect on the uptake which was always close to zero during the measurement time. Bae and Lee reported that for Takeda 3A the times to reach equilibrium for the same gases were different and CH4, in particular, took almost 40 h to reach equilibrium.3 It is also evident from the figures that as the degassing temperature increased the uptake of all adsorptives took place at lower contact time. In particular, the uptake of N2 and CO2 was significantly enhanced at short contact times ( CO2 > N2 > CH4, which agrees with the results obtained by Bae and Lee3 and others.12 The variation in the kinetic selectivity given in the last two columns of Tables 4 and 5 agrees with that of the uptake selectivity in most respects. Thus, for O2/N2 separation, the selectivity was much better in the low uptake region and, in either region, decreased significantly when the degassing temperature was increased above 373 K. For CO2/N2 separation, there is also a significant decrease in selectivity after higher
Table 4. Effective Diffusion Coefficients at 298 K, Obtained with Eq 3 at the High Uptake Region and Corresponding Kinetic Selectivities for CO2/N2 and O2/N2 on Takeda 3A D/r2 (s-1)
kinetic selectivity
degassing temp (K)
CO2
O2
N2
298 373 473 653
8.2 × 10-4 8.1 × 10-4 1.9 × 10-3 2.6 × 10-3
1.5 × 10-3 2.5 × 10-3 3.9 × 10-3 6.7 × 10-3
6.4 × 10-5 9.9 × 10-5 2.9 × 10-4 6.1 × 10-4
D/r2
)/D/r2
(CO2
(N2)
D/r2 (O2)/D/r2 (N2)
12.8 8.2 6.5 4.3
23.4 25.3 13.4 11.0
Table 5. Effective Diffusion Coefficients at 298 K, Obtained with Eq 4 at the Lower Uptake Region and Corresponding Kinetic Selectivities for CO2/N2 and O2/N2 on Takeda 3A D/r2 (s-1) degassing temp (K) 298 373 473 653
CO2 10-4
4.0 × 4.3 × 10-4 1.3 × 10-3 2.2 × 10-3
O2 10-3
1.6 × 1.6 × 10-3 2.0 × 10-3 3.3 × 10-3
kinetic selectivity N2 10-5
1.3 × 2.0 × 10-5 9.1 × 10-5 2.9 × 10-4
D/r2 (CO2)/D/r2 (N2)
D/r2 (O2)/D/r2 (N2)
30.8 21.5 14.3 5.4
125.8 82.2 21.7 11.4
770 Energy & Fuels, Vol. 20, No. 2, 2006
temperature degassing, and the difference between the low and high uptake regions is somewhat less significant (compared to O2/N2), although the effect is not quite as noteworthy as that found with the uptake selectivity. The most obvious difference between the variation in uptake and kinetic selectivity is that in the former case the O2/N2 selectivity is always less than the CO2/N2 selectivity, whereas the opposite is observed with the kinetic selectivity. The much higher CO2/N2 uptake selectivities arise because the reduced temperature of adsorption is much lower for CO2 (T/Tc ) 0.98, 1.93, 2.36, and 1.57 for CO2, O2, N2, and CH4), and the uptake at any specified pressure is therefore significantly greater. Conclusions The equilibrium and kinetics of adsorption at 298 K for oxygen, nitrogen, carbon dioxide, and methane in a Takeda 3A sample have been investigated in a range of degassing or regeneration temperatures from 298 to 653 K, and the results clearly show that thermal treatment can have a highly significant effect on the separation performance of CMS. It was found that oxygen and carbon dioxide uptakes show a transition from a barrier resistance model with lower temper-
Cansado et al.
ature degassing to a diffusion control model at higher temperature degassing. With nitrogen, the adsorption data fit an LDF model, which is consistent with barrier resistance at the entrance of the micropore. With methane, the uptake was almost insignificant in all experiments. The results presented here show that Takeda 3A can be used for the O2/N2 and CO2/N2 separations, but that these are only efficient if the sample is never submitted to temperatures higher than 373 K. Higher temperatures of sample degassing or regeneration results in a loss of selectivity for O2/N2 and CO2/ N2. On the other hand, for the purification of gas mixtures containing CO2/CH4, the adsorption of CO2 is significantly enhanced when the sample is submitted to higher temperatures, and the CMS can therefore be pretreated at temperatures up to at least 673 K without impairing the separation performance. Acknowledgment. We are grateful to the Fundac¸ a˜o para a Cieˆncia e a Tecnologia (Portugal) and the Fundo Europeu para o Desenvolvimento Regional (FEDER) for financial support, and to Takeda (Japan) for providing the Takeda 3A sample. EF050374J
