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Vol. 22, No. 10
Adsorption of Methane and Hydrogen on Charcoal at High Pressure' Per K. Frolich2 and A. White DEPARTMENT O F CHEMICAL ENGINEERING. MASSACHUSETTS INSTITUTE
OB
TECHNOLOGY, C A M B R I D G E , MASS,
Adsorption isotherms of methane and of hydrogen HE problem of separatThe reservoir was first calion charcoal have been determined over the pressure ing methane and hybrated with the gas to be range of 1 to about 150 atmospheres at temperatures drogen was recently s t u d i e d . This was accomof 25" and 100" C. for hydrogen. The amount of gas encountered in this laboraplished by filling it with gas adsorbed increased markedly with pressure, approachtory. The mixture of the two from the storage cylinder to ing a saturation value at 80-100 atmospheres. A t about 200 atmospheres, closgases was available a t a pres25" C. the maximum adsorption is approximately 12 sure of approximately 200 ating valve A , and bleeding the and 4 liters per 100 grams of charcoal for methane and mospheres and, after removal gas slowly through valve B t o hydrogen, respectively. of most of the methane, the the wet meter. P r e s s u r e Experiments with mixtures of methane and hydrohydrogen was to be used in a readings on the reservoir and gen under pressure at 25" C. have shown that charcoal process operating at t h a t volume readings on the wet adsorbs methane preferentially to the practically compressure. Owing to the high meter were taken simultaneplete exclusion of hydrogen. The amount of methane cost of c o m p r e s s i o n , the oiisly until the pressure in the adsorbed is approximately equivalent to that which best solution would therereservoir dropped to atmoswould be adsorbed if the gas were present alone at a fore be to carry out the pheric. These d a t a w e r e pressure equal to its partial pressure in the mixture. separation without r el e a s plotted and used later to deing the pressure. A m o n g termine the volume of gas the methods of accomplishing separation, the removal of bled into the adsorption chamber from the rcservo'r. the methane from the cycle gas by adsorption presented In starting a run the calibrated reservoir was again filled itself as having interesting possibilities. with gas to 200 atmospheres. The adsorber was charged The use of adsorption on charcoal for rerovering volatile with 27 grams of activated charcoal3 of bulk density 0.47 solvents such as benzene ( 5 ) and for producing natural-gas and actual density 1.8j4and evacuated completely. Gas gasoline (.$) has long been practiced. Although the success- from the reservoir was then bled into the adsorber through ful applicat'on of this principle to the stripping of wet natural valve C until the pressure on the reservoir dropped 100 gas is due to the selective adsorption of the higher hydro- pounds per square inch (6.8 atmospheres). Valve C was carbons. it is well known that the adsorptive capacity of closed and after about a minute, the time necessary to arrive charcoal for methane is small at atmospheric pressure (1). a t a constant pressure in the adsorber, pressure readings It therefore remained to determine whether pressure would on both adsorber and reservoir were recorded. This operation was repeated; that is, successive portions of the gas were introduced from the reservoir into the adsorber with a pressure drop of 100 pounds per square inch (6.8 atmospheres) on the reservoir in each case, until the pressure on the adsorber equaled that on the reservoir. In most of the experiments the reservoir dropped from 200 to about 120 atmospheres, while the adsorption chamber increased
T
Figure 1-Apparatus
for D e t e r m i n a t i o n of Adsorption I s o t h e r m s at H i g h Pressure
materially increase the adsorption of methane and favor its retention in preference to hydrogen to such an extent as to make the method of any practical value to the present problem. T o this end a study was made of the adsorption of methane and hydrogen, individually and in admixture, on charcoal a t high pressures. Adsorption Isotherms of Hydrogen and Methane on Charcoal
The apparatus used to determine the adsorption isotherms a t elevated pressures is shown in Figure 1. It consisted of a 5-liter gas storage cylinder, a 200-cc. reservoir, a 60-cc. adsorption chamber with carefully calibrated gages, valves, etc., a s indicated. Received August 29, 1930. 2 Present address, Standard Oil Development Co., P. 0. Box 276, Elizabeth, N. J. 1
Figure 2-Adsorption
I s o t h e r m s for M e t h a n e on Charcoal
from 1 to about 120 atmospheres. From these data and the calibration data previously obtained on the reservoir, the a Charcoal from the Carbide and Carbon Chemicals Corp., used as received. 4 Determined by evacuating a weighed quantity of charcoal and measuring its real volume by kerosene displacement.
INDUSTRIAL A N D ENGINEERTNG CHEMISTRY
October, 1930
total volume of gas in the adsorber at the various pressures was figured and plotted on rectangular coordinate paper with pressure as the abscissa and volume as the ordinate. T o calculate the amount of gas adsorbed on the charcoal, the volume of gas in the free space had to be determined. This was accomplished by replacing the charcoal in the adsorber with an equivalent volume of copper shot (127 grams, density 8.98) and going through the procedure outlined above. These two sets of data were plotted on the same paper and the difference in gas volume in the adsorption chamber with charcoal and with copper shot for any one pressure v a s considered the volume of gas adsorbed on the charcoal a t that pressure. -4
n d
,483a~ur.- PecaaucE
/N
RrmoaPMtma
$Y J.
Figure 3-Adsorption
Isotherms for Hydrogen on Charcoal
During the experiments and calibrations the temperature of the reservoir was maintained constant a t 25" C. and that of the adsorber at any desired value. The isotherms for rnethane a t 25', 50", loo", and 180" C. and for hydrogen a t 25" and 100' C. are given in Figures 2 and 3, respectively. hfost of these curves were checked by duplicate experiments. It will be seen that the adsorptive capacity increases markedly with pressure, approaching a saturation point a t 80-100 atmospheres. At 25" C. this saturation value is 12 and 4 liters per 100 grams of charcoal for methane and hydrogen, respectively. It is significant that a t 2.5" C. the amount of methane adsorbed a t any one pressure is 2.5 to 3 times that of hydrogen a t the same pressure, while a t 100' C'. methane and hydrogen are adsorbed roughly in the ratio of 9: 1. The adsorption of the two gases a t these pressures does not follow the Freundlich adsorption isotherm ( A ) , although the curves are represented as straight lines when plotted according t o the Langmuir equation ( 2 ) . McBain and Britton have recently shown that the same relations hold for the adsorption of nitrogen, nitrous oxide, and ethylene over the lower end of the pressure range studied in these experiments ( 2 ) . Figure 4 s h o w that the rate of decrease in adsorptive capacity with temperature is very nearly the same for both methane and hydrogen over the range studied. I n the neighborhood of 150" C. the adsorption of hydrogen a t 120 atmospheres is negligible, indicating that methane can be separated from hydrogen in this temperature region by simply passing the mixture of the two gases over activated charcoal. Preferential Adsorption of Methane from Mixtures of Methane and Hydrogen under Pressure
I n studying the adsorption of methane-hydrogen mixtures Methane-hydrogen mixtures of known composition were passed a t a rate of about 30 liters per hour over the charcoal in the adsorber a t the desired pressure until the analysis of the gas leaving the adsorber was the same as that entering. The apparatus employed was essentially the same as that shown in Figure 1, with the exception that the adsorber was of aklout 120 cc. a dynamic method was employed.
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volume containing 52 grams of charcoal and that the calibrated reservoir was eliminated from the set-up. When equilibrium had been established, as indicated by exit analyses, the gas-storage cylinder was shut off and the gas from the adsorber was bled out into a calibrated holder by releasing the pressure followed by heating of the chamber to 400" C. to remove the last traces of the adsorbed gas. From the volume of the gas and its analysis were calculated the amounts of hydrogen and methane present in the chamber a t equilibrium. The charcoal in the chamber was replaced by an equivalent volume of copper shot and the gas mixture was bled into the reactor to the pressure employed in the charcoal experiment. This gas was then released and measured to determine the volume of mixture in the free space. Since, in the above experiments with charcoal, a t the time of dynamic equilibrium the inlet and exit analyses were the same, it follows that the composition of the gas in the free space was that of the original mixture. Therefore, the volume and composition of the gas in the free space a t equilibrium being known, the amounts of hydrogen and methane in the free space could be determined and, by subtracting these values from the total hydrogen and methane in the adsorber as obtained in the experiment with charcoal, the values for the hydrogen and methane adsorbed were obtained.
Figure 4-Effect
of Temperature o n Adsorption of Methane a n d Hydrogen o n Charcoal
Figure 5 presents graphically the results of four experiments a t 25" C. with methane-hydrogen mixtures of different compositions a t several pressures. On this plot are presented the methane and hydrogen isotherms together with points indicating the observed adsorption from the various mixtures. The points are somewhat scattered, indicating a high percentage of error. This, however, is not surprising, considering the indirect method employed in obtaining the results. It will be noted that in no case, not even at the highest partial pressures, was any hydrogen adsorbed on the charcoal, and that the methane adsorbed was approximately the same as that which would have been adsorbed had it been used alone a t its partial pressure in the mixture. It appears, therefore, that over the range of pressure and gas composition investigated methane is adsorbed by charcoal to practically the complete exclusion of hydrogen. Miscellaneous Observations
X series of experiments made to study the adsorption of mixtures of hydrogen and methane a t atmospheric pressure showed that in all cases hydrogen was adsorbed along with the methane. Owing to the small amounts adsorbed, the results of this work, as well as of determinations made of the adsorption isotherms for the two gases separately
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at subatmospheric pressures, are not presented here. However, when seen in connection with the data obtained a t high pressure. these results bring out the interesting point that pressure favors the preferential adsorption of methane on charcoal.
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some hydrogen is initially adsorbed, particularly toward the exit end of the chamber, where the gas has already been deprived of a substantial amount of its methane content by contact with the charcoal, and that a certain time is required for this adsorbed hydrogen to be displaced by methane. This interchange of the two gases must take place very rapidly, however, because equilibrium was always established in the time required for flushing out the gas contained in the free space of the reactor, no matter how high the gas velocity. Conclusions
. .
Pap r m P . e o 5 ~/N~ A T ~ O J P M ~ K ~
Figure 5-Data on Adsorption of M e t h a n e a n d Hydrogen f r o m Mixtures of Two Gases a t 25O C.
It was further observed that the methane adsorbed a t higher pressures could be recovered by merely releasing the pressure. The total amount of gas adsorbed could be practically completely recovered from the charcoal by heating to 400" C. a t atmospheric pressure. If the charcoal was heated to a temperature higher than 400" C. in hydrogen a t atmospheric pressure, it took up this gas quite readily and failed to liberate it even when heated to 600' C. Charcoal treated in this manner adsorbed less hydrogen than fresh charcoal, but its capacity for adsorption of methane was unimpaired. On the other hand, no such effect could be detected on heating the charcoal in the presence of methane. These observations, therefore, show that activated charcoal acts somewhat like a highly unsaturated hydrocarbon in that i t i capable of binding hydrogen permanently at high temperatures apparently by chemical interaction. The time required for equilibrium to be reached in the high-pressure flow experiments with mixtures of methane and hydrogen is exceedingly short. It may be expected that
The results of these experiments show that pressure materially increases the specific adsorption of methane and hydrogen on activated charcoal, the maximum amount of methane adsorbed being about 12 liters per 100 grams of charcoal a t 25' C. and 80 atmospheres or above. The corresponding value for hydrogen is 4 liters per 100 grams of charcoal. Pressure also favors the preferential adsorption of methane from mixtures of the two gases to almost complete exclusion of hydrogen. In a flow system designed to remove the bulk of the methane from such mixtures, equilibrium is established so rapidly that the controlling factor is the time required for flushing out the gas in the free space of the charcoal adsorber. It is conceivable that this method might be employed for separation of methane and hydrogen on a commercial scale, particularly where the enriched hydrogen is to be used a t high pressure. Acknowledgment
This method of separating methane from hydrogen was suggested by W. K. Lewis, who also took an active interest in directing the experimental work. The data reported here represent part of the results obtained in an investigation conducted by the Research Laboratory of Applied Chemistry, and the writers are indebted to several members of the laboratory staff for their participation in the work. Literature Cited (1) (2) (3) (4)
International Critical Tables, Vol. 111, p. 250. McBain and Britton, J . A m . Chenr. Soc., 62, 2198 (1930). Oberfell and Alden, "Natural Gasoline," p. 294 (1924). Walker, Lewis, and McAdams, "Principles of Chemical Engineering," p. 645 (1927). (5) Walker, Lewis, and hIcAdams, Zbid., p 715.
Specific Heats of Gases at High Pressures 111-Results
for Nitrogen to 150" C. and 700 Atmospheres' B. H. Mackeyz and Norman W. Krase DEPARTMENT OF CHEMISTRY, UNIVERSITY OF ILLINOIS, URBAXA, ILL.
Methods of measurement and calculation of the specific heats of gases as a function of temperature and pressure have been developed, and the results for nitrogen up to 150" C. and 700 atmospheres are presented. Below 100" C. and above 400 atmospheres the effect of pressure on the heat capacity of nitrogen is small, the isothermals ex-
hibiting a pronounced tendency to flatten out parallel to the pressure axis. At higher temperatures this tendency is not so marked. Data at higher temperatures are needed and this laboratory contemplates continuing the work in this direction on nitrogen and other gases.
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I
N TWO previous papers ( I , @ the authors have described apparatus and methods of measurement and calculation of specific heats a t constant pressure a t both ordinary and elevated temperature. The present paper summarizes the 1
3
Received September 2, 1930. Present address, Dupont Ammonia Corp., Charleston, W , Va.
results for nitrogen to 150" C. and 700 atmospheres pressure. For a complete understanding of the method and measurements reference to the preceding papers is suggested. Since the accuracy of each specific heat measurement depends on so many individual measurements, it is necessary to know the probable error in each and the corresponding
