Adsorption of Methane on Fuller's Earth W. T. GRANQUIST Floridin Company, Warren, P a . T h e adsorption of methane on crude and extruded fuller's earth has been investigated at the temperature of boiling methane. Adsorption isotherms at other temperatures over the range 112' to 123' K. have been calcvlated by the method of Polanyi from the experimental isotherm. Isobaric and isosteric plots have been obtained by cross-plotting the Polanyi isotherms, and the isosteric heat of adsorption has been determined. The results of this work have been compared with earlier adsorption work involving nitrogen on the same clay at liquid nitrogen temperatures.
T
HE storage of natural gas forms a major problem of utility companies, particularly in regions where suitable underground strata are not available for such a use. The most practical method yet devised consists of liquefaction, storage, and regasification of the natural gas, but the hazards of the process have been clearly illustrated.
,SAMPLE
TUBE
lished by Spangler ( I d ) . The present paper gives the results of the fundamental investigation of the adsorption of methane on fuller's earth; these results form the basis for the pilot plant investigation which is under way at the present time. EXPERIMENTAL PROCEDURE
The gas adsorption apparatus described by Emmett and Brunauer ( 5 ) ,by Emmett (IO),and recently by Barr and Anhorn ( 1 ) in collaboration with L. 0. Joyner, was used, with some modifications, in this investigation. T h e main innovation was a bath designed t o permit operation a t the boiling point of methane without creating any undue hazards due to accumulation of gaseous material. This bath is illustrated in Figure 1. Temperatures were measured using a methane vapor pressure thermometer, filled with Phillips research grade methane. Methane of 99.0% purity obtained from the hfatheson Company was used for the adsorbate. T h e liquid for the constant temperature bath was conveniently obtained from the refrigeration installation in the pilot plant previously mentioned; this liquid boiled a t - 161 C., for the most part. T h e adsorbant samples were prepared by outgassing at 200" and ca. 10- mm. of mercury for 1 hour. The adsorption was carried out in the usual manner, rare being taken that equilibrium was reached at each point on the isotherm. A more complete description of the technique employed is given by Granquist and Amero (7'). The manometer was equipped with a zero-indirating device to assist in maintaining the free space a t a constant value during a given run. This free space was determined by compressing helium into the space and obtaining a series of pressure-volume relationships a t the temperature of the adsorption; these data could be converted to the desired volume. T h e adsorption calculations were carried out as usual, and corrections were made for the volume of gas compressed into the free space. All volumes were based on standard temperatures and pressure. O
VENT
TABLEI. EXPERIMENTALLY DETERMINEDADSORPTION ISOTHERMS FOR METHANE Run 1 Vol.,
Crude sample, T -161.0° C. THERMOS
u Figure 1.
Extruded sample, T = -161.0' C.
Constant Temperature Bath
J. F. Pritchard and Company and Floridin Company have thoroughly investigated the possibility of storing natural gas on fuller's earth, a hydrous aluminum magnesium silicate mineral ( 8 )occurring extensively in Gadsden County, Fla. Results have been obtained which indicnte that such a method can compare favorably with liquefaction from an economic standpoint, and a t the same time eliminate most of the hazards attending that process. A survey of t,he economics, engineering, and safety features of the proposed methane storage system has been pub2572
P/Po 0.143 0.284 0.438 0.577 0.644 0.718 0.814 0.8ij9 0,884 0.928 0.960 0.986 1.0 0.139 0,272 0.414 0.536 0.617 0.664 0.774 0.823 0.845 0.882 0.915 0,928 0.954 0.960 0.974 0,978 0.978 0.986 0.998 1.0
ml./g.
19.2 22.8 26.6 31.4 38.1 43.4 62.7 74.8 83.0 107.0 131.3 158.9 175.0 22.4 26.5 30.8 36.2 42.4 48.0 62.7 74.6 82.8 100.7 130.9 158.0 183.8 213.5 230.0 240.0 259.0 269.0 287.0 297.5
Run 2 Vol..
P/Po 0.138 0.276 0.427 0.569 0.665 0.717
rnl./g.
0.808
20.0 23.4 27.0 31.8 38.2 44.1 61.5
0.144 0.280 0.422 0.551 0.647 0.702 0 . 805 0.839 0.860 0.886 0.909 0.925 0.944 0.958 0.969 0.989
23.0 27.0 31.3 36.1 41.9 47.1 65.2 76.5 86.1 109.5 134.0 160.5 186.0 214.0 240 5 268.0 ~
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
December 19SO
2573 -
PLOTTED FROM ISOTHERM DATA
-
-
-
T - 1610 “C. CLAY OUTGASSED AT 2 0 0 O F : FOR I HR AT 1 6 5 ~ ~
-
i
w
I2
P/ Po
Figure 2.
9
Methane Adsorption Capacity
Figure 3. Characteristic Polanyi Potential Curve for Adsorption of Methane on Fuller’s Earth
Crude and extruded fuiler’e earth
RESULTS
Figure 2 and Table I present the experimentally determined adsorption isotherms for methane on extruded and crude fuller’s earth. The nature of the curves indicates a small increase in surface area and a large increase in pore volume upon extrusion; this is similar to the effect noted in earlier work with nitrogen (7). The isotherm for methane on extruded fuller’s earth shown in Figure 2 was used to calculate the isotherms at other temperatures shown in Figure 4. The method of Polanyi was employed for this purpose. Polanyi assumed that the adsorbant particle is surrounded by layers of the adsorbate, each layer being associated with a certain “adsorption potential,” e, and containing a certain volume of the adsorbed phase, rp. He defined the adsorption potential as an isothermal work of compression represented by ei
=
each e is associated with a given q, it should be possible to plot a curve such that e = f(rp). It is assumed that the adsorption potential does not change with temperature, dec/dT = 0, and therefore, dppi/dT = 0, and thus the curve, = f(9), is independent of temperature; Polanyi termed this the characteristic curve. It is therefore possible to determine an isotherm, plot
I l l 1
l
l
i
l
l
l
l
I
l
1
I l l
ISOTHERM AT -161 ‘C. ACTUAL EXP DATA OTHER ISOTHERMS CALCULATED BY METHOD OF POLANYI
I
a 260
1
ti 240
VdP
6,
where c i is the potential a t a point where the denAity of the adsorbate is bi, 6* is the density in the gas phase, and V = M / 6 , where M is the molecular weight of the adsorbate. To evaluate this integral it is necessary to assume that the adsorbed phase and the gas phase obey the same equation of state. Because
8 m
160
n
6 120
TABLE 11. TOTAL CORRECTION FACTOR, .f T,O K.
T/Tea
112 1.005 113 1.013 114 1.022 115 1.031 116 1.040 117 1 049 119 1.068 123 1.102 a Temperature at boiling point.
Pz = 100 mm. Hg
Pz = 1000
1.07 1.07 1.06 1.06 1.05 1.05 1.04 1.03
1.09 1.09 1 .os 1.08 1.07 1.07 1.06 1.05
mm. Hg
2
100
d?
80
:1‘I 20 --
XK) ‘
I’
400 /
600
800
’I
Id00
’ ’ 1200
la00
-
PRESSURE M M HG
Figure 4.
Isothermal Behavior of Methane Adsorption on Extruded Fuller’s Earth
2574
INDUSTRIAL AND ENGINEERING CHEMISTRY
Val. 42, No. 12
and heating the clay through the sin:dI temperature range indicated. Brunauer ( 3 )in his discussion of the heat of adsorption inclutlw a thermodynamic derivation trf a n equation srialogous t o t h e Clausius-Clapeyron equation
which iri its integrated form gives the familiar
The differential form of the equation can be written
' Figure 5.
112
I
1/4
'
118
I\b TK '
'
BP
A plot of In P against 1/T should be a straight line over small changes of P and T , and evaluation of the slope should give the the isosteric heat of adsorption. Figure 6 presents the isosteres obt,ained by cross-plotting the isotherms from Figure 4; these were then replotted according to the method just described, and a good linear relationship was obtained. The slopes were determined and the heat of adsorption was c:ilculat,cd; the average value obtained was 2138 caloriw pcr mole. ILO
, 1
124
DISCUSSION O F RESULTS
Constant Pressure Behavior of Methane on Fuller's Earth
the characteristic curve, and then calculate the isotherms at temperatures other than the experidental one. A complete discussion is given b y Brunauer ( 3 ) ; the above information was abstracted from this reference. Practically,
It is of interest to compare thew data with those previously reported ( 7 ) €or the adsorption of nitrogen on the same clay samples. The isotherms shown in Figure 2 were used for this comparison. A plot, \vas made intlicating the variation of P / V ( P o - P ) with P / P , for both crude and extruded clay according to the standard Brunauer-Emmett-Teller method of dctermining the volume of gas for a monolayer ( I O ) . The plot was linear and t8he slope of the line W A R evaluated; V m values obtained
Po
dP = RT In P,/P, p,
90
and Qt
= X/bT
where 8~ is the density of the liquid a t temperature 2'. conveniently,
More
T,
- fss A
where 68 is the density at the boiling point, X is the weight of adsorbed vapor, and f is a correction factor for compressibility and expansion with temperature. Figure 3 is the characteristic curve obtained from the experimental data; this was the basis for the isotherms in Figure 4 for temperatures other than 112' K. The actual mechanics of the calculation of a given isotherm proceeded in the following manner. Suitable values for P,/P, were chosen and the corresponding values of €6 were calculated. The proper values for 'pi were thcn selected from the characteristic curve and 5 was calculated aftcr selection of a proper f for the temperature and pressure being considered. The value of x was then converted to gas volume a t standard conditions. Values off were taken from Table V (Chapter V, 3). Table I1 lists the exact values of f used for each isotherm, including the change in j due t o pressure. f is a combination correction for compressibility and temperature expansion. The isobars shown in Figure 5 were simply cross-plotted from the isotherms; these curves are important from an operational standpoint, because plant operation would be a t constant pressure and adsorption and desorption would be accomplished by cooling
W
ul
84
a a
300
20
IC!'
' ' '
Figure 6.
/I2
/I4
1
'
116 T "K
118
'
I!?,
'
152
'
Adsorption Isosteres of Methane on Extruded Fuller's Earth
12!4
December 19W
INDUSTRIAL A N D ENGINEERING CHEMISTRY
2575
To do this they used the relation
Vrn (N2) X 16.2 = area
)sag(-
6.2
a 8
\
5.6
52
80
” 8.2 ” ” ”84” ’
8.6
\ 8.8
WO]
9.0
I;x 103 Figure 5 . Adsorption Isosteres of Methane on Extruded Fuller’s Earth
were 15.9 ml. for the crude sample and 18.5 ml. for the extruded sample. By selecting R suitable valuc for the area of the methane molecule, it should be possible to obtain methane surface areas corresponding to these two monomolecular layers. Emmett and Brunauer (6) calculated a number of molecular areas usirtg the equat,ion Area = ( 4 ) (0.866) (M/4
T h i c~Iculationwas carried out in this case and methane areas of 26.6 sq. A. for adsorption on extruded material and 27.8 sq. A. for crude fuller’s earth were obtained; an average value of 26.5 sq. A. should be satiifactory for calculating surface area for fuller’s earth from methane adsorption data. When this work was started, a calculation was made predicting the volume of methane that would be adsorbed a t saturation. Assunling a gas (S.T.P.)-liquid_(boiling point a t 760 mm.) ratio of 592 for methane d s e d on a density of 0.424 gram per cc. ( 4 )at the boiling point, a gas-liquid ratio=of 642 for nitrogen based on a density of 0.804 gram per cc. (9) at the boiling point, and a pore volume of 0.541 ml. obtained by nitrogen adsorption for the extruded sample, it was predicted that approximately 325 ml. of methane would be adsorbed at saturation. Thd experimental methane adsorption capacity was 298 ml.; this is equivalent to a pore volume of 0.505 ml., representing good agreement with the nitrogen value. ’ The considerable difference between the Vm values for methane and nitrogen adsorption, and th_e cloRe total adsorption and pore volume results for the two gases seem contradictory. It probably would be more correct to assume that the molecular area calculated from the density approximated the correct value and that the Vm obtained did not represent complete coverage of the adsorbant surface, than to,assume a very large molecular adsorption area for the methane molecule, only to find that it requires 16 monomolecular layers ( V s / V m ) of this large molecule to fill the same amount of pore spacc as 12 layers of nitrogen molecules. The heat of adsorption obtained from the isosteric data was approximately the same as the heat of liquefaction (qlmataric = 2138, qr, = 2200 calories per mole, 9), as would be expected for adsorption in layers other than the first. The experimental data did not extend to low enough P/P,values to obtain heats a t adsorption levels lower than one monomolecular layer; thus no deductions can be made as to isotherm type on this basis, although these curves appear to be Type 11. For design purposes, the heat involved in this process c m be considered equal to the heat of liquefaction.
42‘
where M = molecular weight, A = Avogadro’s nunher, and 1) is the density in either the liquid or the solid state. They obtained a value of 15.0 sq. A. for methane packed as in the solid state and 18.1 sq. A. for packing as in the liquid state. However, the density used in calculating the liquidpacked area was 0.3916 gram per cc. at -140” C. Because this adsorption was carried out a t approximately the boiling point of methane, the value of the area was recalculated using a density of 0.424 gram pee cc. ( 4 ) . An area of 17.2 sq. A. was obtained for the methane molecule under these conditions. This wts then wed to compute the area of the sample, using the V m values mentioned above, with resulting areas of 86.2 and 74.1 square meters per gram for extruded and crude fuller’s earth, respectively. Thew are to be compared with areas of 128 and 120 square meters per gram obtained for the same samples using nitrogen as the adsorbate; in the latter two instances, the corresponding Vm values were 29.4 and 27.4 ml. and the area of the nitrogen molecule was assumed to be 16.2 eq. A. (8). If it is assumed that the nitrogen surface area is correct, and that the proper value for the methane molecular area has been selected, it is apparent that a monomolecular layer of methane represents about 65% surface coverage. Nay and Morrison ( 1 2 ) have presented a study of molecular adsorption areas of hydro-. carbons on charcoal, in which they assumed the nitrogen surface area to be correct and have selected molecular areas for the hydrocarbons studied that would result in the same surface area.
ACKNOWLEDGMENT
The author wishes to acknowledge the assistanee of Anthony Rock, R. B. Dietsch, and C. H. Edwards of the Floridin Company laborator in carrying out the experimental work and the able help of L. B. 6hristie in preparing the curves. The did and encouragement of P. H. Emmett and R. G. Capell of Melloii Institute and C. V. Spangler and W. W. Bodle of J. F. Pritchard and Company are also greatly appreciated. LITERATURE CITED
(1) Barr, W. E.,and Anhorn, V. J., “Scientific Glass Blowing,” Pittsburgh, Pa., Instruments Publishing Co.. 1949. (2) Bradley, W. F., Am. Mineral., 25, 405 (1940). (3) Brunauer, S.,“Adsorption of Gases and Vapors,” Vol. I, Piinoeton, N. J., Princeton University Press, 1943. (4) Egloff, G., “Physical Constants of Hydrocarbons,” Vol. I, New York, Reinhold Publishing Corp., 1939. (5) Emmett, P. H., and Riunauer, S., J . Am. Chem. Soc., 56, 35 (1934). (6) Ibid., 59, 1558 (1937). (7) Granqqist, W. T., and Amero, R. C., Zbid.,70, 3265 (1948). (8) Harris, B. L., and Emmett, P. H., J. Phus. & CoZZoid Chem., 53, 811 (1949). (9) Hodgman, C. D., ed., ”Handbook of Chemistry and Physics,” 29th ed., Cleveland, Ohio, Chemical Rubber Publishing Co., 1945. (10) Kraemer, E.O.,.ed., “Advances in Colloid Science,” Vol. I, New York, Intersoience Publishers. 1942. (11) Nay, M. A., and Morrison, J. L., Can. J. Research, 27B, 205 (1949). (12) Spangler, C.V.,Oil Gus J.,47, 94 (May 5, 1949). RECEIVED .July 18,1949.
