Methane Storage within Dry and Wet Active Carbons - American

Energy & Fuels 2003, 17, 1283-1291

1283

Methane Storage within Dry and Wet Active Carbons: A Comparative Study A. Perrin, A. Celzard,* J. F. Mareˆche´, and G. Furdin Laboratoire de Chimie du Solide Mine´ ral, Universite´ Henri Poincare´ sNancy I, UMR CNRS 7555, BP 239, 54506 Vandoeuvre-le` s-Nancy, France Received March 20, 2003. Revised Manuscript Received June 26, 2003

Methane storage capacities on dry and water-wetted active carbon powders are compared. Sorption isotherms of methane at temperatures of 2°C and pressures up to 8 MPa are constructed for four carbonaceous materials. Three of these materials originate from the same precursor (coconut shell), are physically activated at various burnoffs, and are mainly microporous; the fourth material is a highly mesoporous, chemically activated pinewood carbon. In the dry state, these adsorbents exhibit classical Langmuirian behavior. Wetting the materials with a constant water/carbon weight ratio of ∼1 leads to isotherms that are now characterized by a marked step. The latter occurs near the expected formation pressure of methane hydrates, thus supporting their occurrence within the porous materials. The amount of gas stored at the highest pressures investigated then ranges from 180 to 230 volumes at standard temperature and pressure (STP) per unit volume of storage vessel (V/V), depending on the material, whereas only 110-160 V/V are obtained with dry carbons (at 2°C, 8 MPa). Hence, wetting the carbonaceous adsorbents improves the methane storage capacities, thus confirming recent works. The results are discussed on the basis of the known pore texture of each adsorbent, and stoichiometries of the formed hydrates are calculated. Considerations about adsorption and desorption kinetics, and pore size distributions, are also developed.

Introduction Natural gas, which primarily consists of methane, is a valuable alternative fuel. It has two major advantages, in comparison to gasoline: namely, it has a lower cost and is clean burning. However, because of its very low density, storing the greatest amounts of methane in a given limited volume is a real challenge for its application to both gas transportation and gas-powered vehicles. The latter indeed requires, as far as possible, light, small, and diversely shaped tanks. Consequently, for at least a decade, much effort has been made to produce materials that could adsorb the greatest number of volumes of gas at standard temperature and pressure (STP) per unit volume of storage vessel (V/V) at moderate pressures.1(and refs therein) Past research has shown that two Japanese patents2,3 that involved methane sorption on active carbons claimed incredibly high storage capacities; at least twice the commonly accepted target4 of 150 V/V deliverable at 3.5 MPa and room temperature was claimed. Such exceptional performance was attributed to low amounts of preadsorbed water, which leads to the occurrence of methane clath* Author to whom correspondence should be addressed. E-mail: [email protected]. (1) Cook, T. L.; Komodromos, C.; Quinn, D. F. Adsorbent Storage for Natural Gas Vehicles. In Carbon Materials for Advanced Technologies; Burchell, T. D., Ed.; Pergamon: Amsterdam and New York, 1999; pp 269-302. (2) Kaneko, K.; Maedo, Y.; Okui, T. Japanese Patent No. JP 1996 0037526 (for Tokyo Gas), 1996. (3) Tange, K. Japanese Patent No. JP 2000 161595 (for Toyota Motor Corp.), 2000. (4) Atlanta Gas Light Adsorbent Research Group (AGLARG), Report to U.S. Department of Energy, Contract No. 466590, 1997.

rate hydrates within the pore volumes of the adsorbents. It was, indeed, supposed that methane could be stored at higher densities in such clathrates than while adsorbed in a supercritical state. Unfortunately, these results could not be reproduced, and only very few (and even unfavorable) effects of the adsorbent wetting could be observed.5 Very recently, an enhancement of methane storage on active carbon, preadsorbed with water, was reported by Zhou et al.6 whose work used much more realistic conditions than those stated in the Japanese patents: higher water content, higher pressures, and lower temperatures. Under such conditions, reasonably large capacities, ∼200 V/V at 10 MPa and 2 °C, were observed. In the present work, such results could be reproduced and even improved. The paper is organized as follows. The experimental device and the materials tested are first described, while storage isotherms are subsequently given and discussed. Next, considerations about the formation kinetics, stoichiometry, and stability of hydrates are developed. Mesopore size distributions are also calculated from the hydrate dissociation data of two adsorbents. Benefits and disadvantages of storing methane through hydrate formation are finally discussed in the conclusion. Experimental Section Materials. Four active carbons, whose main characteristics are gathered in Table 1, were investigated. All were produced and supplied by Pica France. The NC series is derived from (5) Zhou, L.; Li, M.; Sun, Y.; Zhou, Y. Carbon 2001, 39, 773-776. (6) Zhou, L.; Sun, Y.; Zhou, Y. AIChE J. 2002, 48, 2412-2416.

10.1021/ef030067i CCC: $25.00 © 2003 American Chemical Society Published on Web 07/19/2003

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Table 1: Pore Texture (in the Dry State) of the Carbonaceous Adsorbents carbon

BET surface area, SBET (m2/g)

NC58 NC86 NC120 Picazine

1000 1587 2031 1967

Pore Volume (cm3/g) mesopores, Vm micropores, Vµ 0.405 0.570 0.814 0.65

0.049 0.088 0.141 0.72

Vµ + Vm

Packing Density (g/cm3) dry adsorbent, ddry wet adsorbent, dwet

0.454 0.658 0.955 1.37

0.4 0.34 0.29 0.16

1.27 0.98 0.85 0.54

Storage Isotherms Description of the Isotherms. The sorption isotherms at 2 °C for dry carbons, which are displayed in Figure 2, are expressed in units of moles of stored methane per kilogram of dry adsorbent. According to the IUPAC classification,8 type I curves are obtained. Such results are, indeed, always observed for methane that has been adsorbed on active carbons above its critical point (Tc ) 191 K). The data points may be accurately fitted by the Freundlich isotherm equation, which applies to adsorption on energetically heterogeneous surfaces:9

v ) AP1/n eq Figure 1. Volumetric apparatus for building the methane storage isotherms. Legend is as follows: 1, temperature probe; 2, vacuum pumps; 3, methane cylinder (+99.9% purity); 4, pressure transducer; 5, air-conditioned glovebox (25 °C); 6, storage cell; 7, water-glycol thermostatic control; and 8, calibrated vessel. coconut shell chars that were activated with steam at several burnoffs. Thus, the corresponding carbons are mainly microporous. Such adsorbents were chosen for the present study because of their availability, and also because water adsorption has already been studied on very similar materials.7 Picazine originates from pinewood and was chemically activated with orthophosphoric acid, which leads to a highly mesoporous material with a non-negligible micropore volume (see Table 1). The adsorbents were ground in such a way that the grain size was in the range of 100-200 µm. These carbon powders were introduced into a high-pressure vessel that was made of stainless steel, to which a cooling jacket was fitted. The resulting packing densities are listed in Table 1. The adsorbents could be wetted in situ, in such a way that the ratio of the water weight to that of the dry carbon was ∼1. The resulting packing densities are also listed in Table 1. The dry materials were outgassed at 200 °C overnight; however, to avoid the evaporation of water, the wetted materials were not subjected to such a treatment. A thermostatic device then allowed the temperature to be maintained at 2 ( 0.1 °C. Volumetric Apparatus. The storage isotherms were constructed according to the classical volumetric method, using a pressure transducer (Honeywell), for a pressure range of 0-40 MPa with an accuracy of (0.1%. Figure 1 shows a schematic view of the device. The sorption of methane was achieved on samples with a volume of ∼24 cm3. For each adsorbent, the methane uptake was measured point to point by discontinuous introduction of the adsorbate into the sample holder, up to pressures of ∼8 MPa. All the measurements were corrected from the compression of gaseous methane outside of the vessel, i.e., in the parts of the device free of adsorbent. At the end of each isotherm, the maximum amount thus stored was also accurately checked by weighing the vessel before and after the methane was allowed to be released at room temperature and normal pressure. (7) Cossarutto, L.; Zimny, T.; Kaczmarczyk, J.; Siemieniewska, T.; Bimer, J.; Weber, J. V. Carbon 2001, 39, 2339-2346.

(1)

in which v is the volume of adsorbed methane at the equilibrium pressure Peq, A is a constant, and n is a parameter that is linked to a characteristic energy Q0 such that

n)

Q0 qst ) RT RT(ln A - ln v)

(2)

where qst is the isosteric heat of adsorption, R is the gas constant, and T is the temperature. The values of Q0 ) nRT of each dry material at 2 °C are listed in Table 2. Such energies, which total a few kilojoules per mole, are typical of those of physisorption, whatever the substrate and the adsorbate.9 In addition, adsorption isotherms were obtained with dry carbons at 20 °C; again, eq 1 could be accurately fitted to the data points, and the corresponding values of Q0 were also listed in Table 2. Q0 is derived by fitting the Freundlich equation (eq 1) to the first part of the isotherm; therefore, its value is a weighted average over the pore size distribution. Thus, the smaller the pores, the greater the value of Q0 (indeed, Q0 decreases from adsorbent NC58 to adsorbent NC120, which corresponds to more and more activated carbons, i.e., to increasingly wider pores; see Table 2). Q0 is constant in the studied temperature range of 2-20 °C. According to Yang,10 finding temperature-independent values of Q0 means that the energetic heterogeneity of the surface is rather low. This conclusion is consistent with the fact that the carbonaceous adsorbents that have been investigated here are known to possess almost no surface functional groups.7 Finally, note that, because of the sharp slope of the isotherms at Peq ≈ 0, the amount of methane deliverable at 0.1 MPa is much lower (ca. 10 V/V less) than the total amount of methane stored. The latter is given in Table 3 for each adsorbent. (8) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603-619. (9) Rudzinski, W.; Everett, D. H. Adsorption of Gases on Heterogeneous Surfaces; Academic Press: London, 1992. (10) Yang, C.-H. J. Colloid Interface Sci. 1998, 208, 379-387.

Methane Storage within Dry and Wet Active Carbons

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Figure 2. Methane storage isotherms at 2 °C for the four dry carbonaceous materials whose main characteristics are listed in Table 1. The methane uptake, plotted as a function of the equilibrium pressure Peq, is expressed in moles stored per kilogram of dry adsorbent. Table 2: Characteristic Energies Derived from Application of the Freundlich Isotherm Equations (eqs 1 and 2) for Both Dry and Wetted Active Carbons Characteristic Energy, Q0 (kJ/mol) dry, 2 °C dry, 20 °C wet, 2 °C

carbon NC58 NC86 NC120 Picazine a

4.8 4.2 3.7 3.5

4.6 4.2 3.8 3.2

1.8 2.2 2.3 2.1

formation pressure of hydrates, Pfa (MPa)

width of pores in which hydrates occur at Pf and 2 °C, w (nm)

4.0 3.5 3.49 3.7

130 333 344 205

At 2 °C within the pores (with width w) of wetted materials. Table 3: Storage Capacitiesa of the Studied Carbonaceous Materials, Either Dry or Wetted by Water Dry Adsorbents

Wet Adsorbents

carbon

mol/kgdry

mol/kgdry × ddry

V/V

mol/kgdry

mol/kgwet

mol/kgwet × dwet

V/V

NC58 NC86 NC120 Picazine

12.8 21 23.1 40

5.1 7.1 6.7 6.4

107 158 121 164

10.5 16.5 21.9 35.7

5.8 9.2 10.9 17.4

7.4 9.0 9.2 9.4

177 216 223 227

a

Amount of methane stored at 8 MPa and 2 °C, expressed both in units of mol/(kg of adsorbent) and in units of V/V.

Figure 3 shows the methane storage isotherms that have been obtained at 2 °C with wetted carbons. Just as in the work of Zhou et al.,6 type I behavior is again exhibited by each curve at pressures lower than a critical value at which a step occurs. The first parts of the plot then correspond to a classical adsorption that is, however, hindered by the presence of water. Indeed, the latter is expected to induce two possible phenomena; namely, micropore filling and micropore inaccessibility. Both are present in adsorbent NC58; because it may be readily calculated, given the amount of sorbed water and the amount of methane stored under the form of hydrate (see below), ∼60% of the micropore volume has been determined to be filled with water. Such a calculation is based on the stoichiometry of the main clathrate hydrate structure: the SI phase (8 CH4, 46 H2O; discussion of this point is given later in the text). This result may also be checked within a few percent, using the amount of methane stored at 3 MPa, i.e., before the hydrates are formed. Thus, the micropores within adsorbent NC58 are not only partially filled, but are also not fully accessible, because some pathways are blocked

Figure 3. Methane storage isotherms at 2 °C for the same four carbonaceous materials mentioned in Figure 2, wetted with a water/carbon weight ratio of ∼1. The methane uptake is expressed in moles stored per kilogram of wet adsorbent.


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capillary pressure ∆P reads

∆P )

4σlh cos θ w

(3)

where σlh is the surface tension between liquid water and the hydrate phases onto which water spreads with a contact angle θ that is assumed to be zero (indeed, in such a model, a thin film of liquid water is assumed to remain between the pore walls and the hydrate phase). According to Smith et al.,12 σlh ) 0.0267 J/m2. The formation pressure, Pf, then becomes

Pf ) Pff + ∆P

Figure 4. Effect of wetting on the storage isotherms of adsorbent NC86. The methane uptake is expressed in STP volume of stored gas per volume of storage container (V/V). Data corresponding to compressed natural gas (CNG) are given for comparison.

(4)

where Pff is the formation pressure of hydrates in free water, such that Pff ) 3.18 MPa.14 Thus, for instance, it may be calculated that a formation pressure of 4 MPa at 2 °C corresponds to the occurrence of methane hydrates within macropores with a mean diameter of 130 nm. The Pf values were derived by fitting the empirical equation

by water that is present in the other types of pores (mesopores and macropores). In regard to the other carbons, whose pore volumes were not saturated by water, micropore inaccessibility should be the main phenomenon that leads to low amounts of adsorbed methane. Thus, the amounts of methane stored in the pressure range of 0-4 MPa are much less than those established for dry carbons (see Figure 3). Wetting the adsorbents is thus unfavorable, as far as moderate pressures are concerned. Moreover, the first part of the isotherm may still be fitted by eq 1, which leads to smaller values of Q0, as listed in Table 2. Such low adsorption energies are attributed to the unavailability of the smallest pores, because of the presence of water. Indeed, reducing the amount of accessible micropores, which have the highest adsorption energies, hence reduces, on average, the values obtained for Q0. Finally, note that the difference between the adsorption energies that correspond to dry and wet carbons decreases from adsorbent NC58 to Picazine (see Table 2). Such a finding is consistent with the fact that the volumetric amount of water within the wetted carbons decreases from adsorbent NC58 to Picazine (because, at a constant water/carbon weight ratio, the pore volume increases from adsorbent NC58 to Picazine; see Table 1). At a pressure of ∼4 MPa, the methane uptake increases strongly, thus crossing the isotherms of the dry carbons; Figure 4 shows, for example, the direct comparison between dry and wetted adsorbent NC86. Such a crossover phenomenon may be attributed to the formation of hydrates within the widest pores, i.e., within those for which the formation pressure is the lowest. Indeed, because of capillary effects, reducing the pore size increases the formation pressure and vice versa.11 Similar behaviors were observed within silica gels, which led to formation pressures that were greater than those in free water at the same temperature.12,13 Indeed, in a cylindrical pore that has a width w, the

to the data points that correspond to the second part of the isotherms shown in Figure 3. Equation 5, which includes two constants B and C, indeed accounts for the shape of such stepwise curves. The values of Pf thus derived are given in Table 2 for each adsorbent. The corresponding pore diameters were calculated according to eqs 3 and 4 and are also listed in Table 2. The hydrates should appear first in the macropores (i.e., such that w > 50 nm)8 and then are formed in smaller pores as the applied pressure increases. Moreover, eqs 3 and 4 show that hydrates are formed in mesopores (2 nm < w < 50 nm)8 at pressures greater than ∼5.3 MPa, assuming that water is still liquid in such pores. Thus, the microporosity does not seem to have a role in trapping methane via hydrate formation. Finally, note that the observation of decreasing formation pressures from adsorbent NC58 to adsorbent NC120 and, correspondingly, larger pore widths is in agreement with the fact that these materials were increasingly activated from the same precursor. Thus, adsorbent NC120 has, on average, larger pores and a wider pore size distribution than adsorbent NC58. Increasing the pressure further induces the formation of hydrates within smaller pores. Hence, the slope of the vertical part of the step should be linked both to the size distribution of the macropores and mesopores and to their initial filling with water. Moreover, some authors have already derived pore size distributions within porous matrixes from hydrate dissociation isotherms.12 The last part of the storage isotherms, i.e., at equilibrium pressures in the range of 5-8 MPa, exhibits either a plateau (for adsorbent NC58) or a positive slope (for the other adsorbents). Such differences are attributed to the different relative amounts of water that are initially wetting the carbons, which leads to the

(11) Clarke, M. A.; Pooladi-Darvish, M.; Bishnoi, P. R. Ind. Eng. Chem. Res. 1999, 38, 2485-2490. (12) Smith, D. H.; Wilder, J. W.; Seshadri, K. AIChE J. 2002, 48, 393-400.

(13) Uchida, T.; Ebinuma, T.; Ishizaki, T. J. Phys. Chem. B 1999, 103, 3659-3662. (14) Sloan, E. D. Clathrate Hydrates of Natural Gases, 2nd ed.; Marcel Dekker: New York, 1997.

v ) B[1 - e-C(P-Pf)]

(5)



Figure 5. Plots of the data shown in Figures 2 and 3, with the methane uptake expressed in units of V/V instead of mol/(kg of adsorbent).

saturation of the pore space of adsorbent NC58, whereas the pore volumes of the other adsorbents were not completely filled. Deriving pore size distributions from hydrate dissociation data supports the aforementioned assumptions concerning the last part of the isotherms. Indeed, it is shown further in the text that a plateau occurs when no more methane can be stored (because additional hydrate cannot be formed and gaseous methane cannot be compressed, because no pore volume is available), whereas a positive slope is related to unfilled pore volumes (i.e., additional hydrate cannot be formed because there is a lack of water, but methane can be compressed in the remaining pore volumes). Consequently, storage isotherms should be fully understood quantitatively if the different adsorbents are identically saturated with water. The corresponding experiments will be achieved soon. Amounts of Methane Stored. Expressing the stored amounts of methane in units of V/V (recall that V/V represents the STP volume of gas stored per unit volume of the storage vessel) is a convenient way of displaying isotherms. Indeed, such data may thus be compared with the target value of 150 V/V deliverable, which was suggested earlier by the Atlanta Gas Light Adsorbent Reseach Group (AGLARG) for application to naturalgas-powered vehicles.4 Figure 5 gathers the same data as in Figures 2 and 3, but expressed in units of V/V. The isotherms are much closer to each other than when they are expressed in units of mol/kg, whatever the adsorbent. Moreover, the curves that correspond to the wet materials are very similar. Such a striking feature originates from the various packing densities of the carbons, whose variations from one material to another compensate, more or less, those of the intrinsic capacities of the adsorbents. Thus, a carbon that is activated at a high burnoff level will present good storage capacity but a low packing density. Hence, the final amount of methane (expressed in units of V/V) stored in a vessel

full of such an adsorbent will not necessarily be greater than that obtained with a less-activated carbon. This is, indeed, an old problem in methane storage science, in that the product of capacity multiplied by packing density is rather constant15 (see Table 3). Getting the better adsorbent consequently results from a subtle compromise between high intrinsic capacities and high packing densities. The amount of methane stored (in units of V/V) that is attained at 8 MPa and 2 °C are listed in Table 3 for each material, either dry or wet. It can be observed that the target value of 150 V/V is easily and widely exceeded with wetted carbons, which is the obvious advantage of clathrate hydrate formation. In addition, given the shape of the storage isotherms of such wetted adsorbents (see Figure 5), the stored amounts are almost identical to the deliverable amounts (dry materials usually do not allow the methane to be completely released, as discussed in the previous subsection). However, as will be recalled in the conclusion of the present work, such performances require high pressures (and, hence, a storage tank that has very thick walls and maybe a cylindrical shape), low temperatures, and a weight of water that is as high as that of the carbon. Consequently, the tank should be much heavier than that required for more classical adsorption at 3.5 MPa and room temperature. Formation Kinetics, Stoichiometry, and Stability of Hydrates Kinetics. In addition to the fact that very different amounts of stored methane are found in dry and wet carbons at a given pressure, another major difference concerns the adsorption kinetics. Although a maximum (15) Perrin, A. M.Sc. Dissertation Book, University of Nancy I, Vandoeuvre-le`s-Nancy, France, 2001.


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Figure 6. Application of eq 7, accounting for the kinetics of methane hydrate formation within the pores of Picazine, for several final equilibrium pressures.

of a few tens of minutes is usually sufficient for dry adsorbents to reach equilibrium, several days are required for wet carbons, as far as pressures that are higher than Pf are concerned. However, such large times are typical of hydrate formation in porous media, such as those observed within mesoporous silica glasses that have mean pore diameters within the range of 10-50 nm.13 In addition, on average, equilibrium times increase as the pressure increases, which suggests diminishing accessibilities of water-filled pores to methane, because of the presence of clathrate hydrates that have already formed. With a good accuracy, and for periods as long as several days, the time dependence of the amount of methane stored at a given pressure follows a first-order kinetic law:

dV ) -ktR dt

(where R ) 1)

(6)

which can also be written as

ln

V ≈ kt Veq

(7)

Figure 6 thus shows the relevance of eq 7 for the case of Picazine. In eq 7, V is the STP volume of stored methane (measured as a function of time), Veq is the adsorbed amount at equilibrium, t is the time, and k is a constant that includes the kinetics of hydrate formation and both the mass- and heat-transfer terms. In addition, k should also be dependent both on the adsorbent and on the pressure; however, the values obtained (in the range of 10-3-10-4 min-1) are typical of that usually found in other porous systems.16 Gathering all our data that involves adsorption kinetics at pressures greater than Pf, the following relationships (16) Kono, H. O.; Narasimhan, S.; Song, F.; Smith, D. H. Powder Technol. 2002, 122, 239-246.

were derived between k and the equilibrium pressure Peq (given in units of MPa):

- ln k ≈ 5.11 + 0.23Peq

(for NC-type adsorbents) (8)

and

- ln k ≈ 7.82 + 0.06Peq

(for Picazine)

(9)

(see Figure 7). Thus, as already stated previously, hydrate formation becomes increasingly more difficult and slower with pressure, because methane must diffuse throughout a pore network that is partially, but increasingly, occupied by clathrates. The other reason for such a phenomenon is that water molecules, which must diffuse and reorganize to form new clathrates, are not very mobile, because of hydrogen bonding.14,17 Finally, note that such long adsorption kinetics are only observed above the hydrate formation pressure, whereas equilibria are quickly attained at pressures below Pf. This is another piece of evidence for a classical physisorption mechanism that is followed by clathrate occurrence at a critical pressure Pf. Stoichiometry. In the largest pores of the carbonaceous adsorbents, preadsorbed water should behave as a bulk liquid and, hence, the classical so-called SI clathrate structure is expected. In such a structure, it is well-known that the stoichiometry of the hydrate is 8 CH4, 46 H2O.14 (and refs therein) According to Miyawaki et al.,18 the SI phase is maintained within pores >10 nm in size. However, for narrower pores, which induce stronger potential fields, methane enrichment of the clathrate is likely. Indeed, methane first adsorbs onto (17) Shpakov, V. P.; Tse, J. S.; Tulk, C. A.; Kvamme, B.; Belosludov, V. R. Chem. Phys. Lett. 1998, 282, 107-114. (18) Miyawaki, J.; Kanda, T.; Suzuki, T.; Okui, T.; Meda, Y.; Kaneko, K. J. Phys. Chem. B 1998, 102, 2187-2192.



Figure 7. Dependence of the kinetic constants k on the equilibrium pressure Peq, for Picazine and the NC series of adsorbents.

carbon walls, and then water molecules come and build half and/or complete cages around CH4 units, depending on the pore width. In the smallest pores that still allow hydrate formation, extreme stoichiometries, such as CH4, 2 H2O, may be obtained.18 Recall that, in Figure 3, the step height of the adsorbent NC58 isotherm corresponds to 7.8 mol CH4 stored per kg of water at 8 MPa. Assuming that water is completely consumed by clathrate formation, the calculated composition of the hydrate is 5.8 CH4, 46 H2O. Hence, either several cages of water molecules are empty within the SI structure or some water is trapped in pores within which clathrate formation is not possible. The latter interpretation is believed to be the most convincing, given that water present in the micropores was invoked to account for both the poor physisorption and the low Q0 values that are observed for wetted carbons. Moreover, according to the recent work of Seo et al.19 that involved wellcharacterized mesoporous silica gels, no stoichiometry that was different from that of the SI structure could be evidenced, whatever the mean mesopore size. Calculating the hydrate compositions for the other adsorbents on the basis of the step heights leads to higher methane stoichiometries: 8.9 CH4, 46 H2O; 11.4 CH4, 46 H2O; and 18.6 CH4, 46 H2O for the adsorbents NC86, NC120, and Picazine, respectively. Thus, adsorbent NC86 leads to a hydrate composition that is similar to that of the SI structure, whereas that which is found for Picazine resembles the extreme stoichiometry CH4, 2 H2O, as suggested by Miyawaki et al.18 Such results could be explained by the wider pore size distribution of these carbons, thus allowing hydrate formation both over wider pressure ranges and within smaller pores than in adsorbent NC58. However, at the maximum pressure applied (∼8 MPa), eqs 3 and 4 show that the smallest pores in which hydrates may be formed are 22 nm wide. Such a width is ∼20 times larger than what is required to form the nanohydrates for which extreme (19) Seo, Y.; Lee, H.; Uchida, T. Langmuir 2002, 18, 9164-9170.

stoichiometries CH4, 2 H2O may be expected, according to the model structure of Miyawaki et al.18 Hence, the aforementioned rich stoichiometries should not correspond to pure hydrates, but rather to an average composition that includes hydrates plus compressed methane within the pores. Such a statement will now be supported by the pore size distribution, which can be derived from the available hydrate dissociation data. Stability. The methane stored within the dry carbons is released very quickly, as is that which is delivered from wetted adsorbents, because of the low metastability of clathtrate hydrates. Indeed, concerning the latter, the pressure measured in the sample holder vessel strongly and rapidly increases as the temperature increases. Figure 8 thus shows the thermal dependence of the equilibrium pressure over two wet adsorbents that were kept in their vessel after the construction of their isotherm. Dissociation temperatures of ∼7 °C are evidenced. Because of a strong hysteresis between the formation and decomposition of hydrates,6,14,20 such values were indeed expected to be slightly less than the formation temperature at the same pressure, i.e., 11.4 °C at 8 MPa.14 Finally, note that, in contrast with the formation of hydrates, the dissociation of the latter is so rapid that no simple kinetic studies could be performed. The data of Figure 8 may be used for determining the pore size distribution, on the basis of both the work of Smith et al.12 and the recent paper of Anderson et al.21 Theoretically, for clathrate hydrates that are crystallized in a given porous medium, the most-stable hydrates are those which are present in the largest pores. In other words, the hydrates included in the widest pores exhibit the highest dissociation temperatures. Hence, correlation of the clathrate dissociation data to the pore size is possible. The Gibbs-Thomson equation (20) Perrin, A.; Celzard, A.; Mareˆche´, J. F.; Furdin, G., unpublished results, 2002. (21) Anderson, R.; Llamedo, M.; Tohidi, B.; Burgass, R. W. J. Phys. Chem. B 2003, 107, 3500-3506.


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Figure 8. Temperature dependence of the equilibrium pressure Peq, measured over two wetted adsorbents after construction of their storage isotherms (i.e., up to pressures of ∼8 MPa). The dissociation pressures at ∼7 °C are shown on the plot.

was adapted to the case of hydrate dissociation, which leads to the following relationship:21

∆Td,pore 2σlh cos θ )Td,bulk Fs∆Hd,sw

(10)

In eq 10, ∆Td,pore is the difference between the pore dissociation temperature (Td,pore) and the bulk dissociation temperature (Td,bulk) at any given pressure, Fs is the density of the stoichiometric solid hydrate (914 kg/ m3), and ∆Hd,s (54.2 kJ/mol) is the latent heat of dissociation. The variables σlh, θ (which is equal to 0°), and w have the same meanings as mentioned previously. For each pressure P, the quantity ∆Td,pore(P) ) Td,bulk(P) - Td,pore(P) is measured using the values of Td,bulk(P) calculated by Sloan14 and the Td,pore(P) data of Figure 8. The pore size distribution is then derived as follows. Heating the porous media filled by clathrates of structure SI from a temperature Ti to Ti+1 induces the dissociation of hydrates included in pores that have diameters within a range of wi to wi+1, with the latter being calculated from eq 10. The methane thus released provides an increase of pressure from Pi to Pi+1. Applying the equation of state of methane allows one to calculate the variation of moles of gas produced per weight of sample (∆ui,i+1). Hence, knowing both ∆ui,i+1 and the number of CH4 molecules contained in the elementary cell γcell (having the volume Vcell in the SI structure), the corresponding pore volume ∆Vi,i+1 initially filled by the hydrate is deduced according to

∆Vi,i+1 )

VcellNA ∆ui,i+1 γcell

(11)

where NA is the Avogadro constant. However, eq 11 is only an estimate of the incremental pore volume, because the amount of the water still present on the pore walls after dissociation of the clathrates is not considered. Consequently, only the shape of the pore size distribution is discussed in the following, irrespective of the absolute pore volumes, which cannot be accurately measured with such a method.12

Figure 9. Pore size distributions calculated from the hydrate dissociation data of Figure 8 (filled symbols) and derived from N2 adsorption isotherms using the BJH method (open symbols), for adsorbents (a) NC58 and (b) NC86.

The resulting normalized pore size distributions are plotted in Figure 9 for both adsorbents NC58 and NC86. These curves are compared with what can be obtained from independent measurements, namely, by applying


the Barrett-Joyner-Halenda (BJH) method22 to nitrogen adsorption isotherms constructed at a temperature of 77 K with an automatic Carlo Erba Sorptomatic apparatus. Even if the pore volumes that correspond to pore widths of >50 nm are questionable when calculated from the BJH method,22 the agreement between both distributions is correct in the case of adsorbent NC58 (see Figure 9a). In our opinion, this finding indicates that the hydrate within this latter material was occupying the entire available pore space. Conversely, pore volumes measured from the hydrate dissociation data of adsorbent NC86 are smaller than those derived from nitrogen adsorption (see Figure 9b). This should mean that some volume was not occupied by hydrates, because the pore space was not fully saturated by water, as suggested earlier in this paper. Thus, free volumes could be occupied by gaseous methane which, under compression, can lead to positive slopes at the high-pressure parts of some storage isotherms observed in Figure 3. Conclusions In this work, the sorption of methane on a family of either rather microporous or rather mesoporous active carbons was investigated, both in the dry and in the wet states. The possibility of storing large amounts of methanesas much as 227 volumes at standard temperature and pressure (STP) per unit volume of storage vessel (V/V) at 8 MPa and 2 °Csvia clathrate hydrate formation has been confirmed. The isotherms thus obtained could be divided in two parts: one that accounts for a classical physisorption below ∼4 MPa, and another that involves hydrate formation at higher pressures. The limit between these two regimes corresponds to the formation pressure (Pf) of clathrates, which is dependent on the adsorbents at a given temperature. Below Pf, the amount of stored methane is much lower in wet carbons than in dry carbons. Consequently, wetting the adsorbents has favorable effects only at high pressures and rather low temperatures. This is somewhat harmful for application to both natural-gas-powered vehicles and gas transportation, for which high values of methane storage at moderate pressure and room temperature are strongly desirable. In addition, the weight of the storage tank should be (22) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373-380.


considerably enhanced, not only by the requirement that high pressures are to be used, but also because of the weight of the amount of added water. Finally, given the hydrate formation kinetics that have been discussed in this work (several days were needed to reach equilibrium), the filling of the tank is expected to be an extremely long process. However, the present paper only represents the early beginning of a wide field of interesting studies. Indeed, a huge amount of work remains to be done to improve and better understand methane storage in carbonaceous materials; the nature of the adsorbents should be more accurately investigated (pore texture, impurities, surface functional groups), the isotherms should be built at various temperatures, and thermodynamic and kinetic parameters should be derived. Simple calculations have shown that hydrates occur first in macropores and then in mesopores at higher pressures. Consequently, methane storage capacities could be still further improved, because the promising results obtained in this study were found with mainly microporous materials. Now, microporosity seems to be useless for clathrate formation. Thus, future studies should focus on mesoporous and macroporous materials. Moreover, several wetting conditions should be tested, including not only different amounts of pure water but also different types of additives that are already known to help the formation of hydrates at lower pressures than usually observed in free water.23 Soon, the same adsorbents will be saturated with water and studied under the same conditions. By conducting this research, we intend to correlate the stored amounts of methane trapped in clathrate hydrates with mesopore and macropore volumes. Moreover, the stoichiometries of these phases will be calculated more accurately. Acknowledgment. The authors are indebted to Gaz de France for having brought their attention on methane storage within wet carbonaceous adsorbents, and for providing several useful references cited in the present work. Pica France is thanked for supplying the active carbons. EF030067I (23) Mooijer-van den Heuvel, M. M.; Peters, C. J.; de Swaans Arons, J. Fluid Phase Equilib. 2000, 172, 73-91.

Methane Storage within Dry and Wet Active Carbons - American

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