Selective Methanation of Carbon Monoxide Amirali Rehmat' and Sarabjit S. Randhava Institute of Gas Technology, Chicago, 111. 60616 The selective methanation of carbon monoxide in a gas mixture containing hydrogen, carbon dioxide, and carboh monoxide (-3000
ppm) was investigated within a tem-
perature range of 125" t o 300°C using several catalysts. The object was t o decrease the concentration of carbon monoxide t o about 50 t o 100 ppm without considerably affecting the concentration of other components.
Ruthenium was the most effective
carbon monoxide methanation catalyst, followed closely b y the Raney nickel-type catalyst. Catalysts prepared b y precipitating nickel on the supports were unsatisfactory.
Economical grades of hydrogen suitable for commercial use are normally made by steam-reforming of hydrocarbons. followed by a shift reaction. The main problem associated with this process is the presence of impurities (CO?. CH,,and CO) in amounts that render the hydrogen unsuitable for specific purposes-e.g., its use in lowtemperature fuel cell systems, where CO is the major culprit. It is very difficult to remove or substantially decrease the amount of CO in the H? to acceptable levels. Two techniques are known, methanation and the selective oxidation of CO; both are unsatisfactory in the presence of large amounts of COJ. During the methanation process CO as well as CO? methanates rapidly, consuming a large amount of H2. This makes it impossible to control the temperature of the system. Since methanation is a highly exothermic reaction. The selective oxidation of CO in the presence of large quantities of C is highly unfeasible. Other possible techniques for the p fication of such a gas are available, but they are complex and expensive. Consequently, a considerable amount of research effort has been devoted to finding a suitable process t o remove or substantially decrease the CO levels in the gas mixtures mentioned. Baker et al (1963) suggested a unique process using a ruthenium catalyst, making it possible to reduce the CO concentration levels in the outlet gases to 10 to 20 ppm. This involves the selective methanation of the CO in the gas mixture under such conditions that C O Lremains virtually unreacted. Besides Ru, several othet catalysts are suitable for selective CO methanation. These are considerably cheaper and may be equally effective. Consequently, the purpose of this investigation was to examine some of these highly recommended commercial catalysts to determine economic feasibility for this process. To whom correspondence should be sent. 512
Ind. Eng. Chern. Prod. Res. Develop., Vol. 9, No. 4, 1970
Experimental
A diagram of the apparatus is shown in Figure 1. Detailed descriptions are given by Randhava et al. (1969). , A typical gaseous mixture contained -3050 ppm of CO, 8 0 7 H?,and 20"r CO?. The premixed gas was supplied by The Matheson Co. The details of the catalysts examined are given in Table 1.
Results
In selective methanation. CO is preferentially methanated or consumed without any noticeable methanation of C O , and its attendant consumption of H,. We were primarily concerned in investigating the above process within the temperature range of 125" to 3OO"C, because this temperature range ensured a negligible amount of reverse shift reaction and, a t the same time, vigorous CO methanation. The suppression of the reverse shift and C 0 2 methanation reactions were critically important, since these two reactions would increase CO and decrease H, levels. Table I. Composition of Catalysts Investigated Catalyst
Name
A B
Ruthenium I G T Raney nickel (twice leached)
C
I G T Raney nickel
D
Harshaw nickel
E
C.C.I. nickel
F
Girdler G-65
Composition
0 5', Ru on n-alumina
3 5 ' ~Si 5 ' r A1 60'r A1.0 1.3H 20 3 0 ' ~Xi l B r r A1 52'c Al?Oi.3H?O 5BCcNI on kieselguhr 47cr nickel oxide on alumina 32'; Ni on a-alumina
FLOWMETER
FEED GAS 3050 p , p m C o p 2 0 % coz B A L A N C E Hz
-
B
REACTOR
ZERO GAS 2 0 % co, 80 % H 2
C A L ALY S T
i
i
-VENT
ANALYZER MU LT I P 01N T THERMCOUPLE
SPAN G A S 5000 B A L A N C E H2
Figure 1. Schematic diagram of methanation apparatus
For all the catalysts studied, the CO concentration in the effluent decreased continuously until a temperature between 200" and 250" C was reached. Beyond this point, analysis showed a continuous increase in the outlet CO concentrations, much more rapid for the nickel catalysts than for ruthenium. This increase was so rapid for the nickel catalysts that a crossover point occurred between 250" and 300°C. Beyond this point more CO was produced, but a t a smaller space velocity. The catalysts were compared by measuring their CO conversion rates. The test variables were temperature and space velocity; the entire series was conducted a t atmospheric pressure. No definite decrease in activity with time was observed for any of the catalysts. Specific runs, repeated before and after a set of experimental data, showed excellent reproducibility within the margin of experimental error.
Typical experimental data are given in Figures 2 to 7 . Each data point was obtained under steady-state conditions. The CO concentration as a function of temperature and space velocity is presented separately for each catalyst. Catalysts A, B, and C are generally the most active over the entire range of conditions investigated. Catalyst B is more active than A for all the space velocities in the temperature range from 125" to 225'C, beyond which A becomes progressively more effective as a CO methanation catalyst. With catalyst A, it was possible to lower the concentration of CO in the effluent to about 50 ppm, with B t o about 60 ppm, and with C to about 100 ppmall a t the same space velocity of 9000 SCF per cu foot, catalyst-hour, but a t slightly different temperatures. Catalysts D, E, and F were unsatisfactory for selective 3 0 0 0 ~-
T T
-
.
_
.
-
1 - 7 -
T - 7 1
i 1
a
SCF/cu f t cot - h r
E 2000 a a
z 0
1000
CO - H 2 - C 0 2
, '125
3050ppm
I50
80
20
I
l
175
l
1
200 225 TEMPERATURE,
250
275
300
O F
Figure 2. Selective methanation of CO using ruthenium catalyst A
TEMPERATURE
~
O C
Figure 3. Selective methanation of CO using IGT Raney nickel catalyst B
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 4, 1970
5 13
I
'
I
'
I
1
1
I
1
Id
z
2500
s ,
13.500
9,000
CO
- H Z -COz
3050ppm
I25
150
200
175
225
250
80
,
20
275
1
300
TEMPERATURE,"C
Figure 7. Selective methanation of CO using Girdler G-65 catalyst F
-
0 125
150
175
200
225
250
275
300
T E M P E R A T U R E , 'C
Figure 4. Selective methanation of CO using IGT Raney nickel catalyst C
CO methanation. The latter two were especially poor, their maximum conversion not exceeding l'icr under any experimental conditions. For the Raney nickel-type catalysts, B and C. the peak conversions of CO occur in a very narrow band of temperatures, setting them apart from the rest of the catalysts, which exhibited a relatively broad band around the region of peak conversions. Discussion
To remove CO, the preferable methanation reaction between carbon monoxide and hydrogen is
CO
+ 3H? 2 CH, + H j 0
(1) However, in a mixture of gases containing H?. CO?, and CO. other undesirable reactions may also take place. They include
CO? + 4H2 2 CH, CO? + H?
Figure 5 . Selective methanation of CO using Harshaw nickel catalyst D
CO
+ 2H?O
(2)
+ H?O
(3)
Reaction 2 is undesirable because it produces methane and consumes H?.Reaction 3 is clearly objectionable because it results in the production of more CO. I t is important to limit the reaction to the selective methanation of CO. Within the temperature range we used, Reaction 1 seems to predominate up to a certain temperature, after which Reaction 3 becomes dominant, as can be seen from the increase in the outlet CO concentration. Table I1 represents the temperatures beyond which the reverse shift reaction (Equation 3) manifests itself. If also includes the minimum amount of CO obtainable with the catalysts investigated.
E d d
-.
W W
SCF/cu.ft. SCF/cu 11 c a t 3 550 0 0 1
Table II. Temperature at Which Reverse Shift Reaction Becomes Apparent vs. Minimum Amount of CO Produced
- hr
r -36,000
3
i
CO concentration in feed = 3050 ppm Space velocity = 9000 SCF/cu ft catalyst-hour
LL iL W
5
3000
Catalyst
0
U
A 2500
€3 125
150
I75
200
225
250
275
300
TEMPERATURE, OC
Figure 6. Selective methanation of CO using C.C.I. nickel catalyst E
514
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 4,1970
C D E F
Temp.,
250 200 225 225 200 250
C
CO. ppm
50 60 100 930 2590 2750
I n an earlier work (Randhava et a/.,1969). the authors studied the CO methanation separately, using a gas containing 3450 ppm of CO in H ? . The methanation Reaction 1 went almost to completion a t about 2 7 3 " C , a t a space velocity of 9000 SCF per cu foot catalyst-hour. Therefore, in the absence of Reaction 3 the methanation reaction at higher temperatures should go almost to completion, if the gas concentration remains within 3500 ppm of CO. The fact that we see an increase in CO concentration proves that Reaction 3 predominates above the temperatures indicated in Table 11. I t is possible that a t all the temperatures investigated the CO methanation reaction takes place along with the reverse shift reaction. The latter is less noticeable up to the temperatures indicated in Table 11, but much more apparent thereafter. With the nickel catalysts, the reverse shift reaction becomes predominant and a crossover takes place after -223" C. This is not the case with the ruthenium catalyst within the temperature range considered here. This can be explained by postulating that the nickel catalysts are powerful shift catalysts beyond 223" C. whereas ruthenium, besides being a moderate shift catalyst, is still a powerful methanating catalyst beyond 260" C. Baker et a1 (1963) in their investigations with ruthenium found that the temperature a t which the minimum CO concentration occurred in the effluent was between 150" and 220-C, at a space velocity of 500 to 2000 SCF per cu foot catalyst-hour. The temperature is higher in this investigation because we used much higher space velocities (9000 to 36,000 SCF per cu foot catalyst-hour). In this investigation. the ruthenium catalyst was found to be more suitable for selective methanation. Fischer and Tropsch (1930) suggested that the tendency of metals to yield methane was a direct function of their affinity for hydrogen. Recent data tend to disprove this hypothesis. McKee (1965) reported that the extent of hydrogen adsorption on ruthenium is increased in the presence of CO, suggesting that direct interaction is taking place on the catalyst surface. On the other platinum group metals, CO. chemisorbs so strongly that it displaces HLfrom the chemisorbed layer, decreasing the catalytic activity of the metal. The coefficient of adsorption of a catalyst can be arranged (Bond, 1962): B H < Bco < B c o (-230.C). When CO and C'O? are present in H:,the CO (the more strongly adsorbed) reacts first (CO methanation reaction). Carbon dioxide begins to react rapidly when CO is used up a t -225" C (shift reaction). Even though ruthenium is good for the selective methanation of CO,
the catalyst is so expensive that its commercial utilization is prohibitive. Fortunately, our investigation showed that some nickel catalysts perform almost as well as the ruthenium catalyst. This, coupled with the low cost of the nickel catalysts, makes them far more feasible for this process. We have examined two categories of nickel catalysts commercially available catalyst with varying amounts of nickel and the I G T Raney nickel catalyst with two different nickel levels. The I G T Raney nickel catalyst is a Ni-A1-A1,0,.3H10 alloy containing between 30 and 35'; nickel. I t is prepared by taking a nickel-aluminum alloy and leaching a substantial amount of aluminum with NaOH solution. This yields fine particles of catalyst which have a very large exposed surface area of nickel bonded to a skeletal base of Al-A1?0>. 3 H 2 0 . The optimum performance of this Raney nickel as a fixed-bed selective methanation reaction catalyst depends largely on the extent of aluminum conversion during caustic treatment; a catalyst containing nickel yields a final preparation of high activity. This can be seen by comparing the results of two Raney nickel catalysts with nickel contents of 30 and 3 F ; . All three catalysts where the nickel had been precipitated on the support displayed low activities for selective methanation. probably because of the lower amount of exposed surface area. Thus, even though their nickel contents were similar to or much higher than the Raney nickel-type catalysts, the utilization of this type of catalysts cannot be suggested for the successful selective methanation of CO. literature Cited
Baker, B. S.. Huebler. J., Linden, H . R., Meek, J., "Process for Selective Removal of Carbon Monoxide from Mixture of Gases," U. S.Patent Application Case 63,349 (Oct. 1, 1963). Bond, G. C., "Catalysis by Metals," p. 84, Academic Press, New York, 1962. Fischer, F., Tropsch, H., Gesammelte Abh. Kennfnis Kohle 10. 494 (1930). McKee, D. W., J . Catal. 8, 240-9 (1967). Randhava. S.S.,Rehmat, Amirali. Camara, E. H., Ind. Eng. Chem. Process Des. Develop. 8, 482 (1969). RECEIVED for review November 3. 1969 ACCEPTED September 14, 1970 Financial support received through the Institute of Gas Technology Basic Research Program.
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