- Email: [email protected]
Catalyst degradation in high temperature methanation
Fuel Processing Technology, 5 (1981) 91--101 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
91
CATALYST DEGRADATION IN HIGH TEMPERATURE METHANATION
EREK J. EREKSON, EDWARD L. SUGHRUE and CALVIN H. BARTHOLOMEW*
Department of Chernical Engineering, Brigham Young University, Provo, Utah 84602 (U.S.A.) (Received June 25th, 1980, accepted January 12th, 1981.)
ABSTRACT The activity of nickel and nickel bimetallic methanation catalysts was investigated under conditions representative of an industrial high temperature methanator. All catalysts were observed to suffer significant losses in activity above 723 K as a result of sintering and carbon deposition. At temperatures exceeding 723 K Ni/AI,O 3 and Ni/NiAI204 catalysts are the most active while Ni/NiAI,O 4 and Ni--MoO3/A1203 catalysts are the most thermally stable, even in comparison with commercially available catalysts. The contributions of sintering and carbon deposition to catalyst degradation in the presence and absence of reactant steam are presented and discussed. In long term tests of Ni/AI:O 3 at 773 K, accumulated carbon resulted in plugging of the reactor and catastrophic failure after only 17 hours. Analysis by transmission electron microscopy of samples r u n at high temperature showed the presence of carbon filaments terminated by nickel crystallites.
INTRODUCTION
Catalytic processing of hydrogen deficient fuels is hindered by catalyst thermal degradation. While thermal degradation may have several causes, in the hydrogenation of carbon monoxide to methane (methanation) two major causes are, (i) sintering of the active phase and (ii) carbon deposition [1 -3]. In industrial reactors sintering and carbon deposition cannot be entirely eliminated, but these phenomena may be greatly reduced by proper choice of conditions and catalyst formulations. The present study was undertaken, (i) to determine the extent of sintering and carbon deposition in nickel methanation catalysts under high temperature reaction conditions, (ii) to observe the effects of thermal degradation on catalytic activity and catalyst life, and (iii) to discover reaction conditions and catalyst formulations which minimize the effects of high temperature degradation. EXPERIMENTAL
Catalysts chosen for this study included two commercial methanation *To whom inquiries should be directed.
0378-3820/81/0000--0000]$02.50
© 1981 Elsevier Scientific Publishing Company
1
A’,(‘, Zeolite
Ni-NiAl,O, Ni-Moo,
G87P MC-100
aDuring dry test.
Ni-M
A1,0, on monolith NW,0 AJO, 4
Al,“,
Ni-Pt
-%O,
A’, 0,
~_
Ni-Co
Ni
__-~-
_
.- _
.-._
--
.-__.
30% Ni Ni = 10% MOO, = 10% Ni = 30-40% Ni = 20-30%
162 47 119 179
209 53 138 131
15
9.8
1.3 0.2
800 820
803 816
-
-
138
-
820
6.3
138
163 82
185 58
84
124
79
157
211
117
Ni = co= Ni = Pt T 15%
10% 10% 15% 0.5% Ni
20
886 763 721 125
213 18 10.4 0.4 0.3
After steam test ___ Hydrogen uptake (rmoles/g)
Max. reactor temperaturea (K)
173 210 176 94
Hydrogen uptakes and carbon analyses before and after conversion versus temperature tests __After dry test Catalyst Metal Before test Support loading hydrogen uptake Hydrogen uptake % c (wt %) (nmoleslg) (wnoles/g)
TABLE
0.8 0.0
0.2 0.2
_
0.2
0.1
0.3
% C
93
catalysts and six nickel catalysts prepared in this laboratory [4]. The commercial catalysts were G-87P (Girdler) and MC-100 (Union Carbide). All the catalysts had high metal loadings (% 20 wt% ) and contained various high surface area support materials (see Table 1). The composition and properties of catalyst samples were determined b y three methods: (i) hydrogen chemisorption measurements of metal surface area before and after reactor testing [4], (ii) carbon analyses using a Perkin--Elmer Elemental Analyzer to determine the weight percent carbon in each catalyst after reactor testing, and (iii) electron microscopy [ 5--7 ] of selected samples after reactor testing. Two type~, of conversion vs. temperature tests were performed on each catalyst to measure activity and catalyst stability under industrial reactor conditions. In the first test (without steam) the conditions were 2500 kPa (350 psig), a space velocity of 30 000 h - ' , and a reactant mixture of 64% CH4, 16% At, 14% H2, 4% CO, and 2% CO2. This reactant composition and the reaction conditions were chosen to be similar to those of an industrial recycle methanator [8]. In the second test (with steam) the conditions were the same except that 29% steam was added to the reactant mixture. These tests were performed in a 25 mm diameter stainless steel reactor [4] with a thermocouple positioned in the catalyst bed. The normal catalyst charge was 10 cm 3 or a b o u t 5 g of 3.2 mm diameter pellets (bed depth of a b o u t 2--3 cm). Each test was started at 575 K. The temperature was increased at the rate of 1--2 K/minute until a decrease in methane production was observed by means of chromatographic flame ionization detection (HP-5834). The tests were designed to minimize temperature gradients through the reactor estimated to be 50--75 K. The thermocouple placed near the outlet of the bed provided an approximate indication of maximum bed temperature. Long term deactivation tests were performed on a powdered 20 wt.% nickel on alumina catalyst. The conditions for these were the same as the previously described tests except that the temperature was held at 723 or 773 K for the duration of the test. One test was performed at 773 K with 29% steam added to the reactants. For the long term tests 1 cm 3 (0.5 g) of powdered catalyst was loaded into a 13 mm diameter stainless steel reactor. RESULTS AND DISCUSSION
Conversion vs. temperature tests (without steam) were performed on all eight catalysts, and curves for five catalysts are shown in Fig.1. Ni--NiA1204, Ni--Mo/AI~O3 performed similarly to Ni/AI203 [5], although the curves are n o t shown here. The open square symbols correspond to the equilibrium conversion of the reactants calculated by the Edwards Thermochemical Program [9], which minimizes the free energy of a system given temperature, pressure and total chemical species. For all the catalysts, the conversion vs temperature curves rose rapidly between 573 and 673 K. Above 673 K the curves dropped o f f similarly to the equilibrium curve. However, none of the
94
~80 .~ 701
5o
84o 3O 20 10
0
I
I
I
I
I
I
I
I
I
573
623
673
723
773
823
B73
923
973
1023
Temperature [K) Fig. 1. Conversion of carbon m o n o x i d e versus temperature for high loading catalysts at 2500 kPa, space velocity = 30,000 h ~.1, and reaction mixture containing 64% CH4, 16% Ar, 14% H~, 4% CO, and 2% C02 (no steam). Q equilibrium, • Ni, ~ Ni--Co, o Ni--MoO3, • G-87P, • MC-100.
9O 80
X
~,5o ~ 4o c
0 ¢~3O
~ 2o lO
Temperature (K) Fig.2. Conversion of CO to CH 4 versus temperature for high loading cata]ysts at 2500 kPa, space velocity = 30,000 h -1, and reaction mixture containing 64% CH4, 16% Ar, 14% H2, 4% CO, and 2% CO2. • Ni, • Ni--MoO3, • G-87P.
catalysts achieved equilibrium conversion; although the Ni and Ni--Co catalysts nearly reached equilibrium at 673 K. The major products of reaction observed under these conditions were CH4 and CO2. The fraction of inlet CO converted to methane (methane production) followed about the same trend as CO conversion with temperature as indicated by representative curves in Fig.2. Apparently a large fraction (80--100%) of the CO was converted to methane over the range of temperature. In typical conversion vs. temperature tests performed with reactant
95 steam (Fig.3), the m a x i m u m CO conversion vs. temperature curve was shifted 20--50 K to the right compared with the tests conducted in the absence of steam, b u t the shapes of the curves were otherwise similar. However methane production was significantly less in the presence of steam; indeed, the fraction of inlet CO converted to CH4 decreased rapidly from about 60--70% at 573--623 K to zero at about 773 K (see Fig.4) presumably due to a larger contribution of the water-gas shift reaction. Above 773 K, react a n t methane was apparently steam reformed, accounting for the negative production of methane observed in Fig.4. In other words, a large excess of steam can be very detrimental to the efficient production of methane at high temperatures. Metal surface areas of Al203-supported nickel and cobalt or platinum p r o m o t e d nickel measured by hydrogen chemisorption generally decreased after the conversion vs. temperature test in the absence of steam and decreased even further after a test with steam (see Table 1). Metal surface areas of Ni--NiA1204, Ni--MoO3 and G-87P decreased after the conversion vs. temperature tests w i t h o u t steam. However, H2 uptakes increased after the test with steam for these three catalysts. The MC-100 performed differently from any of the other catalysts, as an increase in surface area was observed after the test w i t h o u t steam while a decrease occurred after the test with steam. The large increase in surface area after the test w i t h o u t steam may have resulted in part from more complete reduction of the nickel to the metallic state in this Ni/zeolite. The decrease in the presence of steam may be due to collapse o f the zeolite structure. Following conversions-temperature tests in the absence of steam, most of the catalysts were found to contain relatively large amounts of carbon (see 10o
0 0 2o lO 0
1 573
I 623
I 673
L ?23
I 773
I 823
I 873
[ 923
I 973
1023
Temperature ( K )
Fig.3. Conversion of carbon monoxide versus temperature for high loading catalysts at 2500 kPa, space velocity = 30,000 h-l, and reaction mixture containing 45% CH4, 11.5%
Ar, 10% H2, 3% CO, 1.5% CO2 and 29% H~O. ~ Ni, o G-87P, D MC-100.
96 lOO 80-
"
~
'
~
O 20e.-
"
~ I ~ ,
\\
Oi "
> -20
573
623
673
723
773
823
873
923
973
1023
Temperature (K) •
Fig.4. Conversion of CO to CH 4 versus temperature for high loading catalysts at 2500 kPa, space velocity = 30,000 h -1, and reaction mixture containing 45% CH4, 11.5% Ar, 10% H~, 3% CO, 1.5% CO2, and 29% H20. ~ Ni, o Ni--Pt, • Ni--M, D Ni--NiA1204. 100 A
CARBON
//
.~
/ ',
//
/
40
/ /
HYDROGEN
5731~ ~
/
',
OXYGEN
Fig.5. Ternary diagram showing carbon-forming regions at 573 and 873 K and 2 5 0 0 kPa • No H20 , • 29% H20.
Table 1). This observation was not unexpected since the reactant composition for these tests lies clearly in the carbon-forming region predicted by equilibrium thermodynamics (see Fig.5). Data for 20% Ni/A1203 indicate that the carbon content increased significantly with increasing maximum reactor temperature (see Table 1). Comparison of data obtained at the same final temperature (800--820 K) shows that Ni/MoO3 and Ni/NiA1204 involved the least carbon build-up. Indeed, the order of increasing carbon content for tests under similar temperature conditions was Ni -MOO3,
97 Ni/NiAI204, Ni--Pt, G-87PI and MC-100. Moreover, the Ni tested to 763 K contained more carbon than the Ni--Pt, Ni--NiA1204 and Ni/MoO3, which were terminated at higher temperatures. This shows that catalyst composition affects the deposition of carbon under reaction conditions. In previous tests of NiMoO3/A1203 at low pressure (1 atm) and low CO conversion it was observed that carbon was deposited at a faster rate than with Ni/Al2 03 catalyst [10]. In this study the conversions of CO over Ni--MoO3 ranged from 65-95%, the partial pressure of CO at the surface was undoubtedly lower, and the water content of the gas through the catalyst bed undoubtedly higher. Thus, the differences in behavior of Ni/A1203 and Ni--MoO3/A1203 under these two conditions suggest that carbon deposition is a complex phenomenon and that results obtained under one set of conditions cannot necessarily be extrapolated to different reaction conditions. The fact that relatively significant decreases in surface area occurred following the conversion--temperature tests (without steam), particularly for those catalysts containing significant amounts of carbon (the 20% Ni/AI203 and MC-100 excepted) suggests a possible connection between the two phenomena. Indeed, previous studies of carbon deposition [10--12] provide evidence that nickel crystallites are removed from the support at the end of growing filaments of carbon and ultimately encapsulated by carbon layers. These relatively inert encapsulating layers are considered to result in deactivation of the nickel crystaUite with respect to further adsorption and reaction [13, 14]. This could explain, at least in part, the loss of surface area by several of the catalysts after the "dry test". The encapsulation of crystallites in Ni/A1203 and MC-100 was apparently minimal, possibly because rates of carbon hydrogenation on these catalysts were sufficiently large to prevent accumulation on the metal surface. Of course, in the catalysts other than Ni/A1203 or MC-100, surface area may have also been lost by thermal degradation (i.e. sintering of the metal and collapse of the support). However, the tests in the presence of steam provided some additional insight on this particular question. After the steam tests all catalyst samples contained very little carbon. This result was expected, since the composition of the reacting mixture is thermodynamically below the carbon forming line (see Fig.5). Thus, for these samples sintering o~ the metal and collapse of the support were probably major causes of surface area loss. The presence of water vapor undoubtedly contributed to the loss of surface area, due to sintering under these conditions. However, two of the catalysts (Ni--NiA1204 and Ni--MoO3/A1203) lost little or no surface area during the test in the steam-containing reaction mixture, suggesting that these two catalysts are quite thermally stable and thus good candidates for high temperature methanation. Since two catalysts (Ni--MoO3/A1203, Ni/NiA1204) lost significantly more surface area in the test without steam compared with the one with steam, the major mode of deactivation in the absence of steam was undoubtedly due to carbon encapsulation of the metal surface.
98
0.9 -/I I 0.7
"~ 0.6
0.4 0.3 0.2 0.1 I
2
I
4
I
s
I
8
I
~o
I
~2
I
14
I
~6
J8
~
IO
2
I
22
24
Ti me (Hours)
Fig.6. Long-term deactivation curves for tests at 2500 kPa, and space velocity = 30,000 h -~ on 20% Ni on AI~O3. For the steam test the reaction mixture was 45% CH,, 11.5% Ar 10% H~, 3% CO, 1.5% CO2 and 29% H20. For the dry tests the mixture was 64% CH4, 16% Ar, 14% H2, 4% CO, and 2% CO2. -- -- -- Steam test at 773 K; dry tests at 723 K, o 773 K. Activity vs. time plots f or t he long term deactivation runs are shown in Fig.6. F o r the test at 773 K with steam no drop in activity was observed over 24 h, and after the test no carbon build-up was observed on t he catalyst. F o r th e tests at 723 and 773 K with dry reactants little decrease in activity (less than 10%) was observed. However, for the test at 773 K t he react o r plugged with carbon and t he run had to be terminated after 17 h. Various portions o f t he catalyst beds were analyzed for carbon c o n t e n t after the long ter m tests w i t h o u t steam (Fig.7). At 723 K a significant a m o u n t o f carbon was deposited (1.5 to 10.5%), while at 773 K massive carbon build-up occurred above t he bed as well as in t he entrance (upper portion) o f the bed (72%). The exit (lower portion) o f t he bed contained less carbon (36%) and th e quartz wool used f or supporting t he catalyst was stiU white. U n d o u b t e d l y , the small a m o u n t o f H20 p r o d u c e d from reaction helped to reduce carbon build-up on t he exit o f the bed and prevent carbon lay-down on th e quartz wool. Moreover, the decrease in CO concent rat i on across the bed should have resulted in a lower rate of carbon deposition at the exit than at the entrance [ 10, 14]. Nevertheless, t he massive carbon build-up above and at th e entrance t o t he bed plugged t he react or and forced the s h u t d o w n o f th e test. An electron micrograph of t he carbon from the lower p o r t i o n o f the bed is shown in Fig.8. The long filaments of carbon terminated by a nickel metal crystallite are similar t o those report ed [10--12] upon d eco mp o s itio n o f CO or h y d r o c a r b o n s on nickel. Proposed mechanisms for f o r m a t i o n o f these carbon filaments have been discussed in this previous literature [ 10--12 ].
99 CARBON DEPOSITION 124 Hour Testl 723 I{
Flow %Carbon
773 I(
1
Y~;arbon
71.4
-
Black
Quartz 1~47
Black
Wool ~atalyst Bed
1.5
White
"
~ -
~
71.8 70.8 69.3 .
36.2
White
Fig.7. Schematic diagram of carbon deposits after dry long-term tests.
Fig.8. Electron micrograph of carbon formed in the catalyst bed during the dry long-term test at 773 K.
100 CONCLUSIONS
1. Ni and Ni bimetallic catalysts generally lose 20 to 30% of their metal surface areas when tested above 800 K. Ni on NiA1204 and commercial G-87P lose 10 to 15% metal surface area due to carbon deposition, b u t not more than 10% due to sintering. NiMoO3/A1203 appears to lose relatively little surface area under the same conditions. 2. Carbon build-up on a catalyst is a function of temperature and gas composition in the catalyst bed. In a carburizing atmosphere, the extent of carbon deposited increases with increasing temperature and CO concentration. Reactant steam can be used to eliminate carbon deposition; however, it also contributes to thermal deactivation and at high temperatures (> 623 K) shifts product distribution in favor of CO2. 3. Catalyst composition also influences the rate and extent of carbon build-up. Ni--NiA1204, NiMoO3, and Ni--Pt deposit less carbon than Ni in tests involving equivalent reaction conditions. 4. Methanation on Ni/A1203 at high temperatures (e.g. 773 K), in otherwise typical industrial conditions and in the absence of steam, can result in massive deposits of carbon at the entrance of the bed which cause plugging of the reactor and catastrophic failure of the process. 5. Carbon filaments formed on Ni/A1203 under high temperature, high pressure reaction conditions in the methanation process are similar to those formed at low pressure in decomposition of CO and hydrocarbons on metals. To our knowledge this is the first observation that such filaments are formed during catalytic methanation. ACKNOWLEDGEMENTS
The authors gratefully acknowledge financial support from the Department of Energy (Contract EF-77-S-01-2729) and technical assistance from Gordon Weatherbee, Donald Mustard, Keven Mayo and John Watkins.
REFERENCES
I Greyson, M., (1956). In P.H. Emmett (Ed.), Methanation in Catalysis, Vol. IV., Rheinhold Pub. Corp., N e w York.' 2 Mills,G.A. and Steffgen, F.W., (1973). Cat. Reviews 8: 159. 3 Pedersen, K., Skov, A. and Rostrup-Nielsen, J.R., (1980). Prepr. A C S Div. Fuel Chem., 25 (2): 89. 4 Bartholomew, C.H., (1977). Alloy catalysts with monolith supports for methanation of coal-derived gases. D O E Final Technical Progress Report, FE-1790-9. 5 Bartholomew, C.H., (1978). Alloy catalysts with monolith supports for methanation of coal-derived gases. D O E Annual Technical Progress Report, FE-2729-4, Oct. 5. 6 Mustard, D.G. and Bartholomew, C.H., (1981). J. Catal., 67: 186. 7 Bartholomew, C.H., (1979). Alloy catalysts with monolith supports for methanation of coal-derived gases. D O E Quarterly Technical Progress Report, FE-2729-5, Jan. 5.
I01
8 Institute of Gas Technology, (1975). Pipeline gas from coal hydrogenation. ERDA Interim Report No. 2, FE-1221-144, pp. 7--76. 9 Selph, C., (1965). Generalized thermochemical equilibrium program for complex mixtures. Rocket Propulsion Laboratory, Edwards AFB, CA. 10 Gardner, D.C. and Bartholomew, C.H., (1981). I & EC Prod. Res. & Dev., 20: 80. 11 Rostrup-Nielsen, J. and Trimm, D.L., (1977). J. Catal. 48: 155. 12 Baker, R.T.K. and Harris, P.S., (1979). In: P.L. Walker, Jr. (Ed.), Chemistry and Physics of Carbon, Vol. 14. Marcel Dekker, New York, p. 83. 13 Bartholomew, C.H. and Weatherbee, G.D., (1980). Chem. Engr. Commun., 5: 125. 14 Moeller, A.D. and Bartholomew, C.H., (1980). Prep. ACS Div. Fuel Chem. 25 (2): 54. 15 Bartholomew, C.H., Pannell, R.B. and Fowler, R.W., (1977). ACS Div. Petr. Chem., 22 (4): 1331.
91
CATALYST DEGRADATION IN HIGH TEMPERATURE METHANATION
EREK J. EREKSON, EDWARD L. SUGHRUE and CALVIN H. BARTHOLOMEW*
Department of Chernical Engineering, Brigham Young University, Provo, Utah 84602 (U.S.A.) (Received June 25th, 1980, accepted January 12th, 1981.)
ABSTRACT The activity of nickel and nickel bimetallic methanation catalysts was investigated under conditions representative of an industrial high temperature methanator. All catalysts were observed to suffer significant losses in activity above 723 K as a result of sintering and carbon deposition. At temperatures exceeding 723 K Ni/AI,O 3 and Ni/NiAI204 catalysts are the most active while Ni/NiAI,O 4 and Ni--MoO3/A1203 catalysts are the most thermally stable, even in comparison with commercially available catalysts. The contributions of sintering and carbon deposition to catalyst degradation in the presence and absence of reactant steam are presented and discussed. In long term tests of Ni/AI:O 3 at 773 K, accumulated carbon resulted in plugging of the reactor and catastrophic failure after only 17 hours. Analysis by transmission electron microscopy of samples r u n at high temperature showed the presence of carbon filaments terminated by nickel crystallites.
INTRODUCTION
Catalytic processing of hydrogen deficient fuels is hindered by catalyst thermal degradation. While thermal degradation may have several causes, in the hydrogenation of carbon monoxide to methane (methanation) two major causes are, (i) sintering of the active phase and (ii) carbon deposition [1 -3]. In industrial reactors sintering and carbon deposition cannot be entirely eliminated, but these phenomena may be greatly reduced by proper choice of conditions and catalyst formulations. The present study was undertaken, (i) to determine the extent of sintering and carbon deposition in nickel methanation catalysts under high temperature reaction conditions, (ii) to observe the effects of thermal degradation on catalytic activity and catalyst life, and (iii) to discover reaction conditions and catalyst formulations which minimize the effects of high temperature degradation. EXPERIMENTAL
Catalysts chosen for this study included two commercial methanation *To whom inquiries should be directed.
0378-3820/81/0000--0000]$02.50
© 1981 Elsevier Scientific Publishing Company
1
A’,(‘, Zeolite
Ni-NiAl,O, Ni-Moo,
G87P MC-100
aDuring dry test.
Ni-M
A1,0, on monolith NW,0 AJO, 4
Al,“,
Ni-Pt
-%O,
A’, 0,
~_
Ni-Co
Ni
__-~-
_
.- _
.-._
--
.-__.
30% Ni Ni = 10% MOO, = 10% Ni = 30-40% Ni = 20-30%
162 47 119 179
209 53 138 131
15
9.8
1.3 0.2
800 820
803 816
-
-
138
-
820
6.3
138
163 82
185 58
84
124
79
157
211
117
Ni = co= Ni = Pt T 15%
10% 10% 15% 0.5% Ni
20
886 763 721 125
213 18 10.4 0.4 0.3
After steam test ___ Hydrogen uptake (rmoles/g)
Max. reactor temperaturea (K)
173 210 176 94
Hydrogen uptakes and carbon analyses before and after conversion versus temperature tests __After dry test Catalyst Metal Before test Support loading hydrogen uptake Hydrogen uptake % c (wt %) (nmoleslg) (wnoles/g)
TABLE
0.8 0.0
0.2 0.2
_
0.2
0.1
0.3
% C
93
catalysts and six nickel catalysts prepared in this laboratory [4]. The commercial catalysts were G-87P (Girdler) and MC-100 (Union Carbide). All the catalysts had high metal loadings (% 20 wt% ) and contained various high surface area support materials (see Table 1). The composition and properties of catalyst samples were determined b y three methods: (i) hydrogen chemisorption measurements of metal surface area before and after reactor testing [4], (ii) carbon analyses using a Perkin--Elmer Elemental Analyzer to determine the weight percent carbon in each catalyst after reactor testing, and (iii) electron microscopy [ 5--7 ] of selected samples after reactor testing. Two type~, of conversion vs. temperature tests were performed on each catalyst to measure activity and catalyst stability under industrial reactor conditions. In the first test (without steam) the conditions were 2500 kPa (350 psig), a space velocity of 30 000 h - ' , and a reactant mixture of 64% CH4, 16% At, 14% H2, 4% CO, and 2% CO2. This reactant composition and the reaction conditions were chosen to be similar to those of an industrial recycle methanator [8]. In the second test (with steam) the conditions were the same except that 29% steam was added to the reactant mixture. These tests were performed in a 25 mm diameter stainless steel reactor [4] with a thermocouple positioned in the catalyst bed. The normal catalyst charge was 10 cm 3 or a b o u t 5 g of 3.2 mm diameter pellets (bed depth of a b o u t 2--3 cm). Each test was started at 575 K. The temperature was increased at the rate of 1--2 K/minute until a decrease in methane production was observed by means of chromatographic flame ionization detection (HP-5834). The tests were designed to minimize temperature gradients through the reactor estimated to be 50--75 K. The thermocouple placed near the outlet of the bed provided an approximate indication of maximum bed temperature. Long term deactivation tests were performed on a powdered 20 wt.% nickel on alumina catalyst. The conditions for these were the same as the previously described tests except that the temperature was held at 723 or 773 K for the duration of the test. One test was performed at 773 K with 29% steam added to the reactants. For the long term tests 1 cm 3 (0.5 g) of powdered catalyst was loaded into a 13 mm diameter stainless steel reactor. RESULTS AND DISCUSSION
Conversion vs. temperature tests (without steam) were performed on all eight catalysts, and curves for five catalysts are shown in Fig.1. Ni--NiA1204, Ni--Mo/AI~O3 performed similarly to Ni/AI203 [5], although the curves are n o t shown here. The open square symbols correspond to the equilibrium conversion of the reactants calculated by the Edwards Thermochemical Program [9], which minimizes the free energy of a system given temperature, pressure and total chemical species. For all the catalysts, the conversion vs temperature curves rose rapidly between 573 and 673 K. Above 673 K the curves dropped o f f similarly to the equilibrium curve. However, none of the
94
~80 .~ 701
5o
84o 3O 20 10
0
I
I
I
I
I
I
I
I
I
573
623
673
723
773
823
B73
923
973
1023
Temperature [K) Fig. 1. Conversion of carbon m o n o x i d e versus temperature for high loading catalysts at 2500 kPa, space velocity = 30,000 h ~.1, and reaction mixture containing 64% CH4, 16% Ar, 14% H~, 4% CO, and 2% C02 (no steam). Q equilibrium, • Ni, ~ Ni--Co, o Ni--MoO3, • G-87P, • MC-100.
9O 80
X
~,5o ~ 4o c
0 ¢~3O
~ 2o lO
Temperature (K) Fig.2. Conversion of CO to CH 4 versus temperature for high loading cata]ysts at 2500 kPa, space velocity = 30,000 h -1, and reaction mixture containing 64% CH4, 16% Ar, 14% H2, 4% CO, and 2% CO2. • Ni, • Ni--MoO3, • G-87P.
catalysts achieved equilibrium conversion; although the Ni and Ni--Co catalysts nearly reached equilibrium at 673 K. The major products of reaction observed under these conditions were CH4 and CO2. The fraction of inlet CO converted to methane (methane production) followed about the same trend as CO conversion with temperature as indicated by representative curves in Fig.2. Apparently a large fraction (80--100%) of the CO was converted to methane over the range of temperature. In typical conversion vs. temperature tests performed with reactant
95 steam (Fig.3), the m a x i m u m CO conversion vs. temperature curve was shifted 20--50 K to the right compared with the tests conducted in the absence of steam, b u t the shapes of the curves were otherwise similar. However methane production was significantly less in the presence of steam; indeed, the fraction of inlet CO converted to CH4 decreased rapidly from about 60--70% at 573--623 K to zero at about 773 K (see Fig.4) presumably due to a larger contribution of the water-gas shift reaction. Above 773 K, react a n t methane was apparently steam reformed, accounting for the negative production of methane observed in Fig.4. In other words, a large excess of steam can be very detrimental to the efficient production of methane at high temperatures. Metal surface areas of Al203-supported nickel and cobalt or platinum p r o m o t e d nickel measured by hydrogen chemisorption generally decreased after the conversion vs. temperature test in the absence of steam and decreased even further after a test with steam (see Table 1). Metal surface areas of Ni--NiA1204, Ni--MoO3 and G-87P decreased after the conversion vs. temperature tests w i t h o u t steam. However, H2 uptakes increased after the test with steam for these three catalysts. The MC-100 performed differently from any of the other catalysts, as an increase in surface area was observed after the test w i t h o u t steam while a decrease occurred after the test with steam. The large increase in surface area after the test w i t h o u t steam may have resulted in part from more complete reduction of the nickel to the metallic state in this Ni/zeolite. The decrease in the presence of steam may be due to collapse o f the zeolite structure. Following conversions-temperature tests in the absence of steam, most of the catalysts were found to contain relatively large amounts of carbon (see 10o
0 0 2o lO 0
1 573
I 623
I 673
L ?23
I 773
I 823
I 873
[ 923
I 973
1023
Temperature ( K )
Fig.3. Conversion of carbon monoxide versus temperature for high loading catalysts at 2500 kPa, space velocity = 30,000 h-l, and reaction mixture containing 45% CH4, 11.5%
Ar, 10% H2, 3% CO, 1.5% CO2 and 29% H~O. ~ Ni, o G-87P, D MC-100.
96 lOO 80-
"
~
'
~
O 20e.-
"
~ I ~ ,
\\
Oi "
> -20
573
623
673
723
773
823
873
923
973
1023
Temperature (K) •
Fig.4. Conversion of CO to CH 4 versus temperature for high loading catalysts at 2500 kPa, space velocity = 30,000 h -1, and reaction mixture containing 45% CH4, 11.5% Ar, 10% H~, 3% CO, 1.5% CO2, and 29% H20. ~ Ni, o Ni--Pt, • Ni--M, D Ni--NiA1204. 100 A
CARBON
//
.~
/ ',
//
/
40
/ /
HYDROGEN
5731~ ~
/
',
OXYGEN
Fig.5. Ternary diagram showing carbon-forming regions at 573 and 873 K and 2 5 0 0 kPa • No H20 , • 29% H20.
Table 1). This observation was not unexpected since the reactant composition for these tests lies clearly in the carbon-forming region predicted by equilibrium thermodynamics (see Fig.5). Data for 20% Ni/A1203 indicate that the carbon content increased significantly with increasing maximum reactor temperature (see Table 1). Comparison of data obtained at the same final temperature (800--820 K) shows that Ni/MoO3 and Ni/NiA1204 involved the least carbon build-up. Indeed, the order of increasing carbon content for tests under similar temperature conditions was Ni -MOO3,
97 Ni/NiAI204, Ni--Pt, G-87PI and MC-100. Moreover, the Ni tested to 763 K contained more carbon than the Ni--Pt, Ni--NiA1204 and Ni/MoO3, which were terminated at higher temperatures. This shows that catalyst composition affects the deposition of carbon under reaction conditions. In previous tests of NiMoO3/A1203 at low pressure (1 atm) and low CO conversion it was observed that carbon was deposited at a faster rate than with Ni/Al2 03 catalyst [10]. In this study the conversions of CO over Ni--MoO3 ranged from 65-95%, the partial pressure of CO at the surface was undoubtedly lower, and the water content of the gas through the catalyst bed undoubtedly higher. Thus, the differences in behavior of Ni/A1203 and Ni--MoO3/A1203 under these two conditions suggest that carbon deposition is a complex phenomenon and that results obtained under one set of conditions cannot necessarily be extrapolated to different reaction conditions. The fact that relatively significant decreases in surface area occurred following the conversion--temperature tests (without steam), particularly for those catalysts containing significant amounts of carbon (the 20% Ni/AI203 and MC-100 excepted) suggests a possible connection between the two phenomena. Indeed, previous studies of carbon deposition [10--12] provide evidence that nickel crystallites are removed from the support at the end of growing filaments of carbon and ultimately encapsulated by carbon layers. These relatively inert encapsulating layers are considered to result in deactivation of the nickel crystaUite with respect to further adsorption and reaction [13, 14]. This could explain, at least in part, the loss of surface area by several of the catalysts after the "dry test". The encapsulation of crystallites in Ni/A1203 and MC-100 was apparently minimal, possibly because rates of carbon hydrogenation on these catalysts were sufficiently large to prevent accumulation on the metal surface. Of course, in the catalysts other than Ni/A1203 or MC-100, surface area may have also been lost by thermal degradation (i.e. sintering of the metal and collapse of the support). However, the tests in the presence of steam provided some additional insight on this particular question. After the steam tests all catalyst samples contained very little carbon. This result was expected, since the composition of the reacting mixture is thermodynamically below the carbon forming line (see Fig.5). Thus, for these samples sintering o~ the metal and collapse of the support were probably major causes of surface area loss. The presence of water vapor undoubtedly contributed to the loss of surface area, due to sintering under these conditions. However, two of the catalysts (Ni--NiA1204 and Ni--MoO3/A1203) lost little or no surface area during the test in the steam-containing reaction mixture, suggesting that these two catalysts are quite thermally stable and thus good candidates for high temperature methanation. Since two catalysts (Ni--MoO3/A1203, Ni/NiA1204) lost significantly more surface area in the test without steam compared with the one with steam, the major mode of deactivation in the absence of steam was undoubtedly due to carbon encapsulation of the metal surface.
98
0.9 -/I I 0.7
"~ 0.6
0.4 0.3 0.2 0.1 I
2
I
4
I
s
I
8
I
~o
I
~2
I
14
I
~6
J8
~
IO
2
I
22
24
Ti me (Hours)
Fig.6. Long-term deactivation curves for tests at 2500 kPa, and space velocity = 30,000 h -~ on 20% Ni on AI~O3. For the steam test the reaction mixture was 45% CH,, 11.5% Ar 10% H~, 3% CO, 1.5% CO2 and 29% H20. For the dry tests the mixture was 64% CH4, 16% Ar, 14% H2, 4% CO, and 2% CO2. -- -- -- Steam test at 773 K; dry tests at 723 K, o 773 K. Activity vs. time plots f or t he long term deactivation runs are shown in Fig.6. F o r the test at 773 K with steam no drop in activity was observed over 24 h, and after the test no carbon build-up was observed on t he catalyst. F o r th e tests at 723 and 773 K with dry reactants little decrease in activity (less than 10%) was observed. However, for the test at 773 K t he react o r plugged with carbon and t he run had to be terminated after 17 h. Various portions o f t he catalyst beds were analyzed for carbon c o n t e n t after the long ter m tests w i t h o u t steam (Fig.7). At 723 K a significant a m o u n t o f carbon was deposited (1.5 to 10.5%), while at 773 K massive carbon build-up occurred above t he bed as well as in t he entrance (upper portion) o f the bed (72%). The exit (lower portion) o f t he bed contained less carbon (36%) and th e quartz wool used f or supporting t he catalyst was stiU white. U n d o u b t e d l y , the small a m o u n t o f H20 p r o d u c e d from reaction helped to reduce carbon build-up on t he exit o f the bed and prevent carbon lay-down on th e quartz wool. Moreover, the decrease in CO concent rat i on across the bed should have resulted in a lower rate of carbon deposition at the exit than at the entrance [ 10, 14]. Nevertheless, t he massive carbon build-up above and at th e entrance t o t he bed plugged t he react or and forced the s h u t d o w n o f th e test. An electron micrograph of t he carbon from the lower p o r t i o n o f the bed is shown in Fig.8. The long filaments of carbon terminated by a nickel metal crystallite are similar t o those report ed [10--12] upon d eco mp o s itio n o f CO or h y d r o c a r b o n s on nickel. Proposed mechanisms for f o r m a t i o n o f these carbon filaments have been discussed in this previous literature [ 10--12 ].
99 CARBON DEPOSITION 124 Hour Testl 723 I{
Flow %Carbon
773 I(
1
Y~;arbon
71.4
-
Black
Quartz 1~47
Black
Wool ~atalyst Bed
1.5
White
"
~ -
~
71.8 70.8 69.3 .
36.2
White
Fig.7. Schematic diagram of carbon deposits after dry long-term tests.
Fig.8. Electron micrograph of carbon formed in the catalyst bed during the dry long-term test at 773 K.
100 CONCLUSIONS
1. Ni and Ni bimetallic catalysts generally lose 20 to 30% of their metal surface areas when tested above 800 K. Ni on NiA1204 and commercial G-87P lose 10 to 15% metal surface area due to carbon deposition, b u t not more than 10% due to sintering. NiMoO3/A1203 appears to lose relatively little surface area under the same conditions. 2. Carbon build-up on a catalyst is a function of temperature and gas composition in the catalyst bed. In a carburizing atmosphere, the extent of carbon deposited increases with increasing temperature and CO concentration. Reactant steam can be used to eliminate carbon deposition; however, it also contributes to thermal deactivation and at high temperatures (> 623 K) shifts product distribution in favor of CO2. 3. Catalyst composition also influences the rate and extent of carbon build-up. Ni--NiA1204, NiMoO3, and Ni--Pt deposit less carbon than Ni in tests involving equivalent reaction conditions. 4. Methanation on Ni/A1203 at high temperatures (e.g. 773 K), in otherwise typical industrial conditions and in the absence of steam, can result in massive deposits of carbon at the entrance of the bed which cause plugging of the reactor and catastrophic failure of the process. 5. Carbon filaments formed on Ni/A1203 under high temperature, high pressure reaction conditions in the methanation process are similar to those formed at low pressure in decomposition of CO and hydrocarbons on metals. To our knowledge this is the first observation that such filaments are formed during catalytic methanation. ACKNOWLEDGEMENTS
The authors gratefully acknowledge financial support from the Department of Energy (Contract EF-77-S-01-2729) and technical assistance from Gordon Weatherbee, Donald Mustard, Keven Mayo and John Watkins.
REFERENCES
I Greyson, M., (1956). In P.H. Emmett (Ed.), Methanation in Catalysis, Vol. IV., Rheinhold Pub. Corp., N e w York.' 2 Mills,G.A. and Steffgen, F.W., (1973). Cat. Reviews 8: 159. 3 Pedersen, K., Skov, A. and Rostrup-Nielsen, J.R., (1980). Prepr. A C S Div. Fuel Chem., 25 (2): 89. 4 Bartholomew, C.H., (1977). Alloy catalysts with monolith supports for methanation of coal-derived gases. D O E Final Technical Progress Report, FE-1790-9. 5 Bartholomew, C.H., (1978). Alloy catalysts with monolith supports for methanation of coal-derived gases. D O E Annual Technical Progress Report, FE-2729-4, Oct. 5. 6 Mustard, D.G. and Bartholomew, C.H., (1981). J. Catal., 67: 186. 7 Bartholomew, C.H., (1979). Alloy catalysts with monolith supports for methanation of coal-derived gases. D O E Quarterly Technical Progress Report, FE-2729-5, Jan. 5.
I01
8 Institute of Gas Technology, (1975). Pipeline gas from coal hydrogenation. ERDA Interim Report No. 2, FE-1221-144, pp. 7--76. 9 Selph, C., (1965). Generalized thermochemical equilibrium program for complex mixtures. Rocket Propulsion Laboratory, Edwards AFB, CA. 10 Gardner, D.C. and Bartholomew, C.H., (1981). I & EC Prod. Res. & Dev., 20: 80. 11 Rostrup-Nielsen, J. and Trimm, D.L., (1977). J. Catal. 48: 155. 12 Baker, R.T.K. and Harris, P.S., (1979). In: P.L. Walker, Jr. (Ed.), Chemistry and Physics of Carbon, Vol. 14. Marcel Dekker, New York, p. 83. 13 Bartholomew, C.H. and Weatherbee, G.D., (1980). Chem. Engr. Commun., 5: 125. 14 Moeller, A.D. and Bartholomew, C.H., (1980). Prep. ACS Div. Fuel Chem. 25 (2): 54. 15 Bartholomew, C.H., Pannell, R.B. and Fowler, R.W., (1977). ACS Div. Petr. Chem., 22 (4): 1331.