PRODUCT AND PROCESS DEVELOPMENT
Catalyst for Producing Methane Hydrogen and Carbon M. D. SCHLESINGER, J. J. DEMETER,
AND
MURRAY GREYSON
Branch of Bifuminous Coal, Bureau o f Mines, Region V , Pitfsburgh, Pa.
studies on the catalytic conversion of synthesis
+ CO) to methane involved testing a large number
of catalysbs in fixed and fluidized beds for methanation activity and process life 12). These exneriments were made as Dart of a program directed toward the synthesis of high-B.t.u. fuel gas from coal. Raney alloys, if properly prepared, were active and long-lived catalysts for dry-bed methanation. Catalysts from Raney alloys, in addition to being highly reactive, permitted high synthesis-gas throughputs without excessive temperature rise, even in fixed beds. Bed temperatures normally limit the capacity of highly eyothermic reaction systems. This paper describes the method of catalyst preparation and presents the results of operation under synthesis conditions. Partly extracted Raney nickel alloy proved to be best catalyst
Raney nickel is well known as an active hydrogenation catalyst and many techniques for its preparation are described in the literature ( 1 , 3 ) . The first attempt, by this laboratory, at
SYNTHESIS G A S Figure 1 .
68
producing an active catalyst, was to oxidize the Raney alloy surface with steam at 650" to 700" C. for 6 hours. Following the oxidation, the material was reduced with hydrogen at 400" C. for 8 hours. Neither the nickel-aluminum nor the cobaltaluminum alloy, which was also tried, yielded an active catalyst by this method of preparation. The second attempt was to extract all of the aluminum from the nickel-aluminum alloy b) treating the material with a solution of sodium or potassium hydroxide. d fine powder unsuitable for dry-bed operation resulted rather than the desired skeleton particle. Six- to 8-mesh granules were used for fixed-bed experiments, and 80- to 230-mesh (U.S S.) particles for the fluidized-bed experiments. A Raney alloy containing 42% nickel and 587, aluminum was crushed to size and immersed in sodium hydroxide or potassium hydroxide to extract the aluminum. If a concentrated alkali solution was used, the catalyst was held in the solution for only a short time, then washed with water. An alternative procedure, the one finally adopted, was to use only enough alkali to extract 3 to 570 of the aluminum. In either case, the extracted material n-as washed until the wash a-ater gave a
Flow diagram of multiple-feed fluidized bed unit
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 1
PRODUCT AND PROCESS DEVELOPMENT neutral reaction with litmus or pH paper. The catalyst, while still wet, was charged to the reactor, and the water was removed by passing hot hydrogen through the catalyst bed. The cat* lyst was considered to be dry when, in fixed-bed tests, water ceased condensing in the liquid-recovery system; or when, in the fluidized bed, the motion of the differential pressure manometer indicated that the catalyst bed was Auidized. Synthesis was initiated by raising the pressure in the reactor to 300 pounds per square inch gage with hydrogen, establishing the fresh feed and recycle flow rates, and then admitting synthesis gas. Temperatures were increased gradually until conversion of the synthesis gas was essentially complete, I n the multiple-feed, fluidizedbed unit (Figure 1) further adjustments of the feed-gas distribution were made to maintain an acceptable temperature profile (a stable temperature distribution that could be controlled) along the catalyst bed. Fixed-Bed Experiments. An unextracted sample of 6- to 8-mesh Raney nickel-aIuminum alloy was steam-treated, reduced with hydrogen, and then charged to the small fixed-bed testing unit ( 2 ) . With 2.5 Hzto 1CO synthesis gas a t 300 pounds per square inch gage and a space velocity of 300 hr.-f [volumes of gas (NTP) per hour per volume of catalyst], no rtppreciable conversion occurred in the range 300" to 400" C. However, when the alloy was extracted partly with sodium hydroxide and tested a t the same pressure and space velocity, almost complete conversion of the synthesis gas occurred at 312' C. The product gas (free from water and carbon dioxide) contained 91.501, methane. Space velocities of fresh feed could be raised to 1500 hr,-I without producing runaway temperatures in the catalyst bed. The ability to withstand temperatures of 400" to 450" C. is unique for reduced nickel catalysts. A typical set of temperature profiles for a test of extracted Raney nickel is shown in Figure 2, where it is apparent that all of the reaction occurs within the first few inches of the catalyst bed. The run was terminated voluntarily after 339 hours.
4501
I
I
I
I
I
I
Prepared Raney alloy catalysts for converting synthesis gas
. . . give complete and rapid conversion over small surface area
. permit high throughputs . . . are easily and economically reactivated Fluidized-Bed Experiments. Synthesis gas ( 3Hz:1CO) was admitted to the dried, 80- to 230-mesh catalyst at a settled-bed space velocity of 7700 hr.+ and a temperature of 330" C. Gas conversion was essentially complete when the temperature reached 370" C. The data in Table I show the steady-state conditions during three periods that totaled more than 1100 hours of synthesis. No difficulties were experienced at any time with temperature control or fluidization. Representative temperature profiles taken with an axially located sliding thermocouple are shown in Figure 3. There was a temperature rise near the points of gas admission. I n the upper section of the reactor the conversion was complete and no additional heat was being liberated.
Table I.
Methanation of Synthesis Gas over Raney Nickel in Fluidized Bed (300 Ib./sq. inch gage) Fresh Regenerated Catalyst Catalyst First Seoond
Hours on condition Temperature O C. Space velocity hr. --I Recycle-fresh !eed ratio Fresh feed. Ha:CO Fresh feed distribution. std. C U . ft./hr. Bottom Middle TOP Linear velocity ft./sec. Conversion, Gas oontractiod, yo COz-free, % c
HZ
'492a 370-387 7700 1 2.94
472a 380-395 9300 0.4-0.7 2 94 .~
70 21 20 1.4 73.0
70 21 20 1.25 71.7
70 21 20 1.30 71.1
95.9 99.2 97.5 2.88 90
95.1 99.2 96.2 2.79 85
93.8 97.7 94.8 2.79 81
920 950 99.1 a 250 0
I
2
I
1
I
4 6 8 DISTANCE FROM TOP OF BED,INCHES
I IO
1 12
Figure 2. Effect of space velocity on temperature profile in fixed-bed experiment with extracted Raney catalyst
A similar series of tests was made with a Raney cobalt-aluminum alloy. The unextracted, steam-treated, and reduced alloy was inactive. The alkali-extracted alloy was very active; high conversions were obtained at space velocities as high as 2800 hr.-l. At high space velocities excessive deposition of carbon occurred, and the teats had to be terminated. The dry, carbon dioxide-free product gas contained 92.1 % methane and had a heating value over 950 B.t.u. per cubic foot (60' F., 30 inches of mercury, dry). January 1956
870 . 920 99.1
165a 388-394 10,000 0.8 2 93
860 890 98.0
Hour! until gas contraction became less than 69%.
b Terminated voluntary.
Contraction, CO1-free,
- fresh feed in-outlet gas (Cop-free) x 100. fresh feed in
0 -
The catalyst was discharged after 492 hours when the gas contraction (carbon dioxide-free) fell below 69%, a contraction that corresponds to a product gas of 850 B.t.u. per cubic foot. At 69% contraction the product gas contained appreciable amounts of unreacted synthesis gas of low heating value. The catalyst was subjected to another partial extraction with alkali and recharged to the reactor. When synthesis was initiated this time a t a space velocity of 9200 hr.-l the gas contraction, &g shown in Table I, wm slightly less than that obtained in the first test. After extraction and sampling, the catalyst volume was smaller but the feed rate was kept constant; hence the space velocity was higher in the tests with regenerated catalyst. This
INDUSTRIAL AND ENGINEERING CHEMISTRY
69
PRODUCT AND PROCESS DEVELOPMENT second period lasted for 472 hours, a t which time the contraction became less than 69%. The catalyst n-as again partly extracted with alkali and recharged to the unit. The twice-regenerated catalyst was still active. Although the experimental work was voluntarily discontinued after an additional 165 hours on stream, it was believed that the activity of the catalyst could have been maintained by successive extractions until the particles were too small to be successfully fluidized in the unit.
0
20 DISTANCE
40
60
BO
FROM BOTTOM O F REACTOR,INCHES
Figure 3. Representative temperature profiles of fluidized bed
Before termination of the experimental work the space velocities of fresh feed were increased to 16,000 and 17,500 hr.-1 to see if the system was operable at these conditions. Although the contraction fell slightly, no hot spots were observed in the catalyst bed, The maximum temperatures observed in the catalyst bed at these conditions were 400’ to 405” C. Conclusions
Synthesis in both fixed and fluidized beds with Raney alloys indicated that only a small amount of active nickel surface was required for complete conversion of the synthesis gas. Because so little nickel surface needs to be exposed, the active material remained well bonded to the parent alloy. Attrition losses rere, therefore, at a minimum and difficulties with fluidization, such as deaeration, did not occur. Rhen the surface nickel is no longer catalytically active, it can be removed and a fresh surface generated by extraction.
END
In this way it should be possible to reactivate a bed of this catalyst until the extraction process yields particles of such small size that it is impractical t o utilize them in the fluidized bed. The separated fines and spalled nickel could be used for further alloy preparation, thereby conserving the costliest element of the catalyst; thus only aluminum, sodium or potassium hydroxide, and water would be consumed. The methanation reaction proceeds very rapidy over this catalyst at 300 pounds per square inch gage, 300” to 400’ C., and space velocities of about 10,000 hr.-l in the fluidized bed. With a feed gas ratio of 3Hs:1C0, methane and water are the major products; only trace amounts of higher hydrocarbons are produced. Some carbon dioxide and unreacted feed gas also appear in the product gas. The raw product gas may be used as it comes from the system after the water has been separated, or the heating value further increased by removing part or all of the carbon dioxide. Composition of the feed gas is very important. Equilibrium calculations (a)indicate that when the ratio of hydrogen to carbon monoxide falls below about 2.6 it is possible to deposit carbon. On the other hand, when there is an excess of hydrogen the heating value of the product gas is reduced by the presence of unconverted hydrogen. As a result, the H2:CO ratio that is used must be evaluated carefully in terms of the heating value that is required and the degree of carbon deposition that can be tolerated. Acknowledgment
The authors wish to thank Manuel Abelson for conducting the fixed-bed tests that demonstrated the catalyst activity; A. J. Sharkey for supplying the mass spectrometer gas analyses; and R. B. Anderson, R. C. Corey, H. C. Howard, H. W. Wainwright, J. H. Field, and H. H. Storch for reviewing the manuscript. literature cited
(1) Adkins, H., “Reactions of Hydrogen,” Univ. Wisconsin Press, Madison, Wis., 1944. (2) Greyson, M., Demeter, J. J.,Schlesinger,11.D., Johnson, G. E., Jonakin, J., and Myers, J. W., Bur. Mines Rept. Invest. 5137 (July 1955). (3) Raney, hf., U. S. Patent 1,628,190 (May 1927). RECEIVED for review July 26, 1955.
ACCEPTEDOctober 28, 1955.
OF PRODUCT AND PROCESS D E V E L O P M E N T SECTION
I N D U S T R I A L AND E N G I N E E R I N G C H E M I S T R Y
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