J . Phys. Chem. 1993, 97, 494-499
494
Influence of Pore Geometry on the Design of Microporous Materials for Methane Storage Roger F. Cracknell, Peter Gordon, and Keith E. Gubbins' School of Chemical Engineering, Cornell University, Ithaca, New York, 14853-5201 Received: August 25, 1992
The advantages of storing methane by adsorption in microporous materials are briefly reviewed, and the merits of currently available zeolites and microporous carbons are discussed. Grand canonical ensemble Monte-Carlo computer simulations of methane in slit pores (to model porous carbons) and cylindrical pores (to model zeolites) were carried out to determine the best geometry and the optimal pore size for storing the maximum amount of methane at a given storage pressure. At 274 K,the optimal material is a porous carbon of pore size sufficient to contain two adsorbed layers of methane. At 500 psi (3.4 MPa), the energy density of such a material at 274 K is only a quarter that of gasoline. Our results suggest that an optimal zeolitic material would be a less useful material for adsorptive storage of methane than an optimal porous carbon.
TABLE I: Methane Storage Capacity at 298 K for Various
1. 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 0.04MJ on combustion while 1 L of gasoline will yield 34.8 MJ. While natural gas is a cheap and readily available source of fuel, its suitability for a particular application depends on being able to store an adequate amount of it. This is a particularly acute problem in the design of gasdriven 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, and so NG cannot be liquefied by pressure alone. Liquefied natural gas (LNG) is usually 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 about 600 times greater than it would at NTP (denoted 600 v/v) while CNG is about 220 v/v. Adsorption in a porous material offers the possibility of storing methane at high density while maintaining moderate physical conditions for the bulk phase, and the search for a suitablematerial is currently an active area of research. There are a number of factorswhich are important in the design of a suitable microporous material; first, the micropores of the material should be such that the amount adsorbed minus the amount retained when the methane is released (usually at 1 atm) should be a maximum. Second, the microporosity (fraction of micropore volume) should be a maximum; Le. space taken by the atoms of the microporous material and the space wasted by poor packing of crystallites should both be minimized. There have been a number of experimental studies of the feasibility of using already existing materials for methane storage. Among studies of zeolites are those by Ding et al.,' Zhang et a1.,2 and Chkadze et aL3 and for carbons, the work of Quinn and These data are summarized in Table 1. In this work we have adopted grams per liter as the unit of the amount of methane adsorbed per volume of adsorbate, but this is simply related to the uptake in v/v according to uptake (v/v) = uptake (g/L) X 1.50 (1) The corresponding values for bulk methane are shown by way of a comparison. A microporous adsorbent is only worth using if it provides a significantly higher methane density than that for the bulk fluid at the same pressure. The porous carbons currently 0022-3654/58/2091-0494S04.00/0
Mate-
io Crams Adsorbed per Liter of M e w
amt of methane adsorbed (g/L) 0.5 1 5 lit.
material PVDC (carbon from solid) AX2 1 27%Saran159/73% AX21 composite ZSM-5 (Si/AI = 49.8)b NaX (Si/Al = 1.3)b bulk methane
MPa
MPa
ref
carbon
MPa 30.5
82.1
105.6
6
carbon carbon
12.6 40.1
27.4 57.8
82.1 102.0
6 7
zeolite zeolite
52.1 47.3 3.3
65.2
59.6
67.0 111.0 37.0
1 2
type
6.6
g/L can be converted to v/v by multiplying by 1.50. Figures based on uptake in g/g; the values given in g/L assume perfect packing of crystallite particles.
available are still some way from being ideal. For AX21 packed into a container, for example, 85% of the space in the container is nonmicroporous volume? due mostly to poor packing of the crystallites. Thus, there is considerable scope for progress to be made in the design of porous carbons with a higher proportion of micropores. Several authors have used theoretical or simulation methods to estimate methane storage for idealized model materials. Tan and Gubbinsg used a combination of grand canonical MonteCarlo computer simulation (GCMC) and nonlocal density functional theory to study methane adsorption in model porous carbons for a wide range of pore sizes for various supercritical temperatures. The pores were modeled as slits with walls composed of an infinite number of stacked graphitic layers; it was found that there is an optimum pore size that maximizes the excess adsorption (i.e. the density in the pore minus the bulk density) for a given temperature and pressure. Matranga, Myers, and Glandtg (henceforward denoted MMG) have determined that a slit width of 11.4A is optimal for a system with a storage pressure of 3.4 MPa (500psi) and an exhaustion pressure (the lowest pressure in the desorption cycle) of 1 atm (0.103MPa) at ambient temperature. Their model of a porous carbon assumes that adjacent slits are divided by a single layer of graphite; such a situation represents a theoretical upper limit in the fraction of microporous volume of a material to total volume. MMG calculated from GCMC that even such an ideal material would deliver only 139 g/L of methane (209 v/v) under the specified conditions, thus 1 L of adsorbed methane will yield only 8.6 MJ, which compares very unfavorably with the value for gasoline given above. Bojan et a1.I0 have suggested that slit pores may not be a realistic representation of porous carbons and that triangular Ca 1993 American Chemical Society
Microporous Materials for Methane Storage shaped pores formed by the intersection of graphitic layers better represent existingmaterials. Unfortunately,though, such a model appears to give a lower methane storage capacity, and the MMG slit model represents the optimal theoretical arrangement of graphite layers in a porous carbon used for adsorptive storage of methane. To produce vehicles driven by methane with a comparable range to gasoline-driven ones, it may be necessary to use higher pressures or lower temperatures than previously suggested. Therefore it is necessary to investigate how much methane can be stored under various physical conditions. Moreover, the conditions under which methane might be stored by adsorption for serving peak needs in the gas supply industry and for surface transportation would not necessarily be the same as for a methane driven vehicle. In section 3 we present adsorption isotherms for methane in model porous carbons calculated from GCMC for different pore sizes at 213 and 274 K. The figures shown in Table I suggest that zeolites have adsorption capacities which are comparable with those for porous carbons. In section 4 we seek to answer thequestion as to whether zeolites are potentially better materials than porous carbons for adsorptive storage of methane, and we present adsorption isotherms for model zeolites ofvarious pore size and shape at 213 and 274 K. In section 5 we present results for the isosteric heat of adsorption of methane on carbons and discuss the relevance of this data to the practical questions of methane storage. We conclude with a summary of our results.
2. Method
2.1. Model for Methane. Methane-methane interactions were represented by a Lennard-Jones (1 2-6)potential, cut and shifted at 2.5~~. Thus
The Journal of Physical Chemistry, Vol. 97, No. 2, 1993 495
0
METHANE
-H
b
-
/ IAPHITE
Figure 1. Model of microporous carbon.
u
4 dr'
where the full Lennard-Jones 12-6potential is given by
4ff' = 4eff[(.fl/r)12 - (uff/r)61
(3) The major advantage of the cut and shifted potential is that it removes the need to consider long-range interactions, thus considerably speeding the simulation. The actual parameters used in (2) were UR = 3.81 A and cm/k = 148.12 K.ll At supercritical temperatures and for small pores the effect of using a cut and shifted potential was found to be negligible. By carrying out simulations using much larger cutoffs, we found that the errors introduced by using the cutoff of 2 . 5 were ~ ~ typically less than 0.3%. which is well within statistical uncertainties for this type of simulation. This potential is also used in calculating the properties of bulk methane used in this work. 2.2. Model for Microporom Carbon. We have adopted the model described by Matranga, Myers, and Glandt whereby adjacent pores are divided by a single graphitic layer. Nitrogen isotherms of AX21 would suggest a surface area of 2500 m2/g using the BETequati~n.~ In comparison, a single infinitegraphite layer would have a surface area of 2620 m2/g. It is well-known however that theBETequationcannot bestrictlyapplied to porous materials and may overestimate the surface area by as much as 40-50%.12 Even if one were to assume that the BET areas were in error by more than 100%. then one would predict that adjacent pores in AX21 were separated by no more than three graphite layers. Thus, the MMG model is physically reasonable for current carbons as well as representing a theoretical material with the lowest possible volume taken by the carbon atoms. Details of the surface structure (i.e. the positions of the carbon atoms) were omitted from the model since they are unimportant at supercriticaltemperatures? A schematicdiagram of the model pores is shown in Figure 1; His the distance between the centers of the carbon atoms in the opposing graphite layers. The
-
WALL
u
F i i 2. Model of a zeolite pore.
interaction of a methane molecule with a single wall is given by" (4)
which isderived from integratinga Lennard-Jones(1 2-6)potential between an adsorbate particle and an element of the graphite layer over an infinite plane. plZD is the number of carbon atoms per unit area of graphite. Actual values used in (4)were derived from those used by Tan and Gubbins*and were 4 k = 64.4K, f78f = 3.60 A,and plZDu,f3= 4.963. The total potential a r i m from the sum of the potential from both walls. The external potential was tabulated and values during the simulation were calculated by linear interpolation. The table contained lo00 points. The simulation box was usually 1Ourr long in the x and y directions, but we determined that for very low adsorbate loadings, a box dimension of 20q was required to get good statistics for the ensemble average of the configurationalenergy. Periodic boundary conditions were applied in the x and y directions. 2.3. Model for Zeolites. Our model zeolite has a onedimensional channel, surrounded by an infinite continuum of a Lennard-Jones solid. One-dimensional channels exist in some aluminosilicates, for example zeolite L, and also the alumino-
4%
The Journal of Physical Chemistry, Vol. 97, No. 2, 1993
phosphates AlP04-5, AlPO4-11, AlP04-8, and VPI-5, and although the model most closely represents such materials, it is possible to draw conclusionsfrom it about zeolites in general. We employed two model geometries for pore shape, a cylinder and a hexagonal prism. The external potential at r can be computed by integrating the Lennard-Jones 12-6 interaction between an adsorbate molecule and an element of volume located at r'in the wall over the whole volume of the wall
For a cylinder this integration can be performed analytically and the result has been given by Peterson et al.I3 In our work, computed values of the potential were tabulated as lo00 points in a lookup table, and the external potential was calculated during the simulation by interpolating between points in the table. For a pore of hexagonal cross section, the integral was calculated numerically and stored in a two-dimensional lookup table. The potential was calculated during the simulation by interpolation between these stored grid points. Although, from symmetry, only the potential in I / 12thof the pore cross-sectional area needed to be tabulated, the lookup table comprised 179 700 elements. The parameters used in the simulation were usr = 3.23 A, csf/k = 153.36 K, and psuB$= 2.20. usrandcSfarederived from Kiselev's parameters for the wall adsorbate interaction between argon and silicalitel4 (ZSM-5 with Si/Al = a),and psus$is taken from the effective density of oxygen atoms in the aluminophosphate VPI5.15 Themodel pores were lOurrin length and periodic boundary conditions were applied to the ends of the pore. 2.4. Simulation Details. The simulations were run under the formalism of the grand canonical ensemble in which the chemical potential, the volume of the simulation box, and the temperature remain constant throughout the simulation. The method used was identical to that described for simulating bulk fluids in the book by Allen and Tildesley.16 For a given temperature, pore size and chemical potential, the simulations yield an ensembleaveraged number of particlesin the pore ( N ). GCMC simulations of bulk methane (using the model described in section 2.1) were also carried out, enabling the chemical potential to be related to bulk pressure. The loading in the pore may thereby be plotted against bulk pressure and a simulated adsorption isotherm obtained. Unlike the previous work of Tan and Gubbins, density functional theory was not used for comparison, since it is known that for cylindrical pores the results are incorrect in the onedimensional limit.'' Runs were carried out on a DECstation 5000 in the School of Chemical Engineering at Cornel1 University. The runs were allowed to equilibrate for 1 million configurations and averages were taken over a further 2 million configurations. The time for a run depended on the number of particles in the simulation and varied from 5 min at low bulk pressures to several hours for a fully loaded pore.
3. R d t s for Cubon Pores Adsorption isothermswere obtained for three model pore sizes, H/un = 2,3, and 4. Here H is defined to be the distance between the centers of carbon atoms in the first layer of solid atoms in the two walls of the pore. These pore sizes correspond to being able to fit one, two, and three adsorbed layers of methane into the pore, Tan and Gubbinss having shown that the excess adsorption exhibits maxima in these cases. H / u n = 3 (1 1.43 A) corresponds to the pore size used by Matranga, Myers, and Glandt. Simulations were run at two temperatures, 213.3 and 274 K. The results for the lower temperature are shown in Figure 3 and for the higher one in Figure 4. Also shown in the figures is the curve for the bulk fluid; the isotherms have been taken to a pressure where they cross the bulk curve. Clearly, at pressures higher than this, it would be more economical to store methane as a bulk fluid.
Cracknell et al. 0 d, N
P/MPa Figure 3. Adsorption isotherm at 213.3 K from the simulationof methane ; H = 3 a ~ 0, ; H = 4u~; on model carbon adsorbents: H = 2 0 ~ A, dashed line, bulk fluid.
..
P/MPa Figure 4. Adsorption isotherm at 274 K from the simulation of methane on model carbon adsorbents: W, H = 20m; A, H = 3ua; 0, H = 4 u ~ ; dashed line, bulk fluid.
TABLE Ik Amount of Methane Delivered at 1 atm (Le. Amount Adsorbed at Storage ResMlre Minus A " t Adsorbed at 1 atm) for Vuiole Ston e Prescnues and Pore Sizes at 274 IS in Carbon Pow in Unfb of Cram of Methane per Liter of Material amt delivered at 1 atm (0.103 MPa) storage Pressure (MPal H ,l .m. = 2 HIUK . , .. = 3 Hlua .. = 4 bulk methane I
~
0.5 1 3.4 (=500 psi) 5 10
40.6 54.5 70.2 75.7 80.8
55.7 105.4 166.0 182.2 199.6
27.3 56.8 130.5 152.8 188.8
2.8 7.5 26.5 37.2 78.3
The density shown is the mass of adsorbed methane per unit volume of adsorbed material, assuming the model for the carbon previously described. As one would expect, the smallest pore fdls at the lowest pressure and, for a given temperature and pore size, pore loading decreases with temperature. It is clear that the optimal pore size depends on the temperature, storage pressure, and exhaustion pressure of the operating cycle. If the most important criterion is the total amount of methane which can be stored, then it can be easily seen in both Figures 3 and 4 that at low pressures a pore of width H/up = 2 would hold the most methane, while pores of width H/un = 3 and H/UN= 4 would hold more at higher pressures. Table I1 shows the results for an operating cycle at 274 K and an exhaustion pressure of 1 atm. AporeofwidthH/u~=3 appears tobeoptimalatthistemperature for a range of storage pressures, in agreement with the results of Matranga, Myers, and Glandt. For this optimal pore size with astoragepressureof50psi (3.4MPa)andanexhaustionpreasure of 1 atm, we estimate that 166 g/L of methane (3249 v/v) can be obtained at 274 K. This is about 20% higher than the value
The Journal of Physical Chemistry, Vol. 97, No. 2, 1993 497
Microporous Materials for Methane Storage
8
n 0
io-'
----I
10'
P/MPa
Adsorptionisothermsat213.3Kfromthesimulationofmethane onmodelzeoliticadsorbents: O,R=0.9urr;A,R= l.Sarr;O,R= 1.7506 0 , R = 2.15 0 6 A, R = 2.806 unfilled symbols, cylindrical pores; filled symbols, pores of hexagonal crm section; dashed line, bulk fluid. -5.
Figure 7. Comparison of adsorption isotherms for methane in cylindrical and hexagonal pore zeolite adsorbentsat 21 3.3 K from simulation. Pores of radii R = 1 . 5 0 and ~ R = 1 . 7 5 0 are ~ shown. Unfilled symbols are for cylindrical pores; filled symbols are for pores of hexagonal crm section.
I I
s-l
n
... 0
-7
04-
lo-'
I I
0
10'
P/MPa
0.0
I I
I
0.4
I
0.8
1
1.2
1
1.6
r/%
Figure 6. Adsorption isothermsat 274 K from the simulationof methane on model zeoliticadsorbents: 0,R 5 0 . 9 0 ~A, , R = 1.5ufi 0,R = 1.750rr; 0 , R = 2.1506 A, R = 2.806 unfilled symbols, cylindrical pores; filled symbols, pores of hexagonal cross section; dashed line, bulk fluid.
Figure 8. Density profile in a hexagonal prismatic pore of radius R = 1.750~,T = 213.3 K, P = 31 MPa. The solid line is a profile along a line from the pore center into the comer; the dashed line is for a line from the center normal to the pore wall.
calculated by Matranga, Myers, and Glandt at 296 K. At 213.3 K,if the same storage and exhaustion pressures are used, a pore size of H / U H= 3 is no longer optimal, since it can be seen from Figure 4 that a significant amount of methane is retained at 1 atm (0.103 MPa). Thus, for a storage pressure of 500 psi (3.4 MPa) at 213.3 K, 198 g/L would be released at atmospheric pressure from the material of pore size H/un = 4 compared with 157,2g/LforH/un= 3.0. Althoughagreateramount ofmethane could be delivered at 213.3 K for a given storage, it is not clear that the cooling of the methane tank would make it economically viable.
prism, R is defined by equating the cross-sectional area to T R ~ . The R values for which the hexagon is better than the cylinder depend on the packing arrangement within the pore. A comparison of cylindrical and hexagonal pores is shown in Figure 7 for R / U R= 1.5 and 1.75. Figure 8 shows a density profile in the hexagonal prism pore for R = 1 . 7 5 ~ ~The . solid line shows the density variation along a line from the center of the pore to a comer, the dashed line shows how the density varies along a line through the pore center but normal to one of the sides. It is clear that the particles are either located in the corners or in the center of the pore, at this pore radius. This represents a situation of optimal packing and is the reason why slightly more methane can be packed in a hexagonal pore of this size as compared to a cylindrical one. For a slightly smaller pore (R = 1 . 5 in~ Figure ~ 7) efficient packing is not possible in the hexagonal pore. Once again the choice of an optimal pore size depends on the criteria wed. If the maximum amount of methane which can be stored is the most important criterion,regardlessof the exhaustion pressure, then the smallest pore used in this study, R = 0.9ufican contain the highest methane density for pressures up to 7 MPa (from Figure 5). At 213.3 K between 7 and 10 MPa, the R = 1 . 5 pore ~ ~ has the highest density and, for higher pressures still, the R = 1.75~11pore holds the highest density. For T = 274 K , the R = 0.9011 pore has the highest density up to pressures of 10 MPa and the R = 1.5~11pore has the highest density until the bulk line is crossed. It is interesting to note that the R = 1.75ua pore holds significantly less methane for all pressures considered here than it does at 213.3 K,indicating that the packing into the hexagonal prismatic pore is thermally disrupted.
4. Results for zeolites
Results for model zeolites at the same temperatures (213.3 and 274 K)are shown in Figures 5 and 6. The density of methane is the mass of CH4 per unit volume of porous material. This is calculated by assuming that pore cross-sectional area (as defined by the locus of centers of the wall atoms) divided by unit cell area is constant for our model materials. The ratio is taken from VPI-S.*s The results obtained in this study are found to be in good agreement with simulationsof methane adsorption in spccific aluminophosphates in which the oxygen atoms in the pore walls are modeled explicitly.18 Isotherms wcro simulated for both cylindrical and hexagonal prismatic pores. For the results given in Figures 5 and 6, the isotherm shown for a given pore size is either the cylinder or the hexagon, depending on which adsorbed most. The R values shown for the cylindrical pores are the distance from the pore center to thecenter of the first layer of wall atoms, while for the hexagonal
Cracknell et al.
498 The Journal of Physical Chemistry, Vol. 97, No. 2, 1993
TABLE IIk Amount of Methane Delivered at 1 atm for Various Storage Pressures (Le. h u n t Adsorbed at Storage Pressure Minus Amount Adsorbed at 1 atm) and Pore S i z e s at 274 K in Zeolites in Units of Cram of Methane Adsorbed per Liter of Microporous Material storage pressure (MP4 0.5 1 3.4 ( 5 0 0 psi) 5 10
amt delivered at 1 atm (0.103 MPa)
R=
R=
R=
0 . 9 ~ ~1 . 5 ~ ~1.75~rr 17.1 21.6 25.1 28.7 30.5
15.74 21.74 53.1 57.34 63.44
10.1 20.3 46.0 51.4 62.2
R= 2.15~rr 7.6 15.6 34.2 49.3 62.2
Re 2 . 8 ~ ~ 5.3 11.1 25.0 41.9 58.6
If one again takes into consideration the amount of methane which is desorbed at 1 atm, Table 111shows the amount released at this pressure for various pores at various storage pressures. The smallest pore (R= 0.96) tends to retain a significant amount of methane at 1 atm, and hence the larger pores are better at all pressures. R = 1.50urr generally appears to be the best pore size for all the quoted storage pressures at this temperature although we note that the hexagonal prismatic pore R/un = 1.75 is better at 213.3 K. At 274 K, for a storage pressure of 500 psi and an exhaustion pressure of 1 atm, the best model zeolite will yield only 53.1 g/L (79.65 v/v), which is much lower than the corresponding value of 166 g/L for the best theoretical carbon. One could criticize this comparison on a number of grounds. The first is that the calculations for carbon pores were based on the structure of an optimal theoretical material which does not currently exist, whereas the porosity of our model zeolites was assumed from a structure that does exist (i.e. VPI-5). From an understanding of zeolite structures, it is impossible that adjacent pores could be separated by a single layer of atoms since zeolites are made up of secondary building units (SBUs).I9 In VPI-5, adjacent pores are, at the closest points, separated by the atoms that make a single 4-ring, which is the smallest known SBU.We conclude, therefore, that our zeolite model is closeto being optimal in any case. Our model for zeolites has straight channels with no interconnection. It therefore does not consider the interconnecting "zigzag" channels in ZSM-5 and other pentad aluminosilicates nor does it account for the cages in zeolite X. Differencesbetween results for our model zeolite and what is observed in Table I can thereby be explained. The differences, although significant, do not change our conclusion that an optimal zeolitic material is likely to be inferior to an optimal porous carbon as a medium for the adsorptive storage of methane. Whether such an optimal carbon can ever be produced in reality is a separate question. 5. Heat of Adsorption
Matranga, Myers, and Glandtg make the important point that in a real application of adsorptive storage of methane, the filling will not necessarily be carried out under isothermal conditions. The heat evolved during the adsorption process would cause the temperature to rise, and consequently, less methane would be stored at the storage pressure. Similarly, desorption would cause cooling, which would not be a problem where the methane was being released slowly but might cause difficultiesif fast exhaustion was required. Examination of the isosteric heat of adsorption is the best way of studying the heat evolved during the adsorption process. The isosteric heat at the limit of zero coverage is calculated from"
where u&) is the external potential exerted on an adsorbed molecule at r. The integrals in (6) were calculated for model
'\ 1.6
I
I
2.4
,
I
3.2
4.0
4.8
H/uu Figure 9. Isosteric heat at zero coverage, q&, plotted against pore size for model porous carbons: solid line, 213.3 K dashed line, 274 K.
"I
c
I
3
\
\
1
3
\ A
1(
\
01
1b-*
16-I
lb.
P/MPa
lbl
YO*
Figure 10. Isosteric heat vs bulk methane pressure for a porous carbon of width H J q r = 3 at 274 K.
porous carbons by numerical integration using (4). In Figure 9, we show this function for various pore sizes at the two temperatures shown. The initial heat evolved rises to a maximum at around H/un = 2; this corresponds to the situation where the adsorbed methane is in the potential minima of both walls. At this point 4 0 s ~is greater for the lower temperature, since at the higher temperature the well depth in units of kT is lower. While the negative of the differential internal energy of adsorption at zero loading (the second term on the right-hand side of (6)) is always greater at lower temperature, this is not true for the negative of the differential enthalpy of adsorption because of the kT term in (6) and this is the reason why the two curves cross in Figure 9. In Figure 10 we have plotted the isosteric heat of adsorption for various pore loadings of methane in a pore of width H / q= 3; qST was calculated from
where U and U,,are the average configurational energy per methane in the bulk and gas phases, respectively, and Z, is the isothermal compressibility of bulk methane at temperature T. Values of Us, PsV,, and 2,were taken from GCMC simulations of bulk methane. The validity of (7) has been demonstrated by Woads et aL20 The results shown in Figure 10 are in good agreement with similar calculationsat 296 K by Matranga,Myers, and Glandt, although our data have been calculated at sufficiently high loadings/pressures to observe a maximum in qST at P LY 8 MPa. qST then decreasts on further raising of the pressure. We note that qST is the differential heat of adsorption, and so the total heat released during the adsorption process can be found by integrating qST with respect to loading.
Microporous Materials for Methane Storage
d coaehriolm, Our results suggest that an optimal porous carbon is a more suitable material for adsorptive storageof methane thanan optimal zeolite. The best pore size to use depends on the operating conditions of the system. At 274 K for an exhaustion pressure of 1 atm, a carbon of pore width H/un= 3 and a cylindrical zeolite of radius R/un = 1.5 are optimal in their class for a wide range of storage pressures. For a storage pressure of 3.4 MPa (500 psi) at 274 K our model slit carbon pore yields 166 g/L compared to 53.1 g/L for the zeolite. Reduction in temperature does allow a greater amount of methane to be adsorptively stored for a given pressure, although it is not clear from an cconomic standpoint that the cooling would be viable. Although porous carbonsarc potentially more useful than zeolites, the ones currently available are probably not optimal. Whatever the future progress in design of porous carbons, the maximum density of energy that can be r e l d on combustion (in MJ/L) of adsorptively stored methane is unlikely to be comparable to gasoline. This work is part of a program to study fluids in microporous materials. We plan to extend the study to the investigation of methane adsorption in alumina-pillared clays and other novel materials such as graphitic tubules.21
A c k n o w ~ t This . work was funded by the Gas Research Institute (Contract 5086-260-1254). P.G. wishes to thank the Materials Science Center at Come11 for providing support under the Research Experience for Undergraduates program. We also wish to thank Dr. Alan Chaffee of BHP for providing useful assistance.
Refertms and Nota (1) Ding. T. F.; Ozawa, S.; Yamazaki, T.; Watanuki, 1.; Ogino, Y. Langmuir 1988,4, 392.
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