TECHNICAL REVIEW
Catalysts for Fischer-Tropsch and lsosynthesis Yatish T. Shah. Depflment of Chemical Engineering University of Pittsburgh, Pittsburgh. Pennsylvania 1526 1
Anthony J. Perrotta Gulf Reswrch and Development &mpny, Pinsburgh. Pennsylvania 15230
Introduction The search for practical new energy sources provides the stage for the examination of a set of catalytic reactions which can influence the supply of hydrocarbon fuels. Presently petroleum and natural gas supply about three-quarters of our energy needs. A substantial amount of the remainder is supplied by the burning of coal. Of the fossil fuels present in the United States, coal is the most ahundant. The amount of coal presently available in the United States is able to supply enough energy for about 300 years a t the present rate of energy consumption. Presently, coal can be used by directly burning low-sulfur types (principally western and some eastern coals) or by burning high-sulfur coal in combination with stack gas scrubbers to eliminate the sulfur oxides. Coal can also be hydrogenated to a liquid which can be further hydrotreated to eliminate sulfur as H2S before burning. Another route is the partial oxidation of coal to carbon monoxide which can, over various catalysts, be hydrogenated to form methane, higher hydrocarbons, alcohols, and other oxygenated species. This latter route is the subject of this paper. More specifically, i t relates the products from various catalytic systems that may be useful in the catalytic conversion of carbon monoxide derived from coal. The amount of work done by the German workers such as Franz Fischer, Hans Tropsch, Helmut Pichler, and staff members a t the Bureau of Mines, such as Robert Anderson, Ernst Cohn, Keith Hall, and many others, has been very extensive. This has resulted in book reviews of the many articles written on this topic. There are over 200 references alone in the chapter written by Professor Anderson. A separate literature and patent search was performed on isosynthesis. Hydrogenation of Carbon Monoxide The general reactions for the hydrogenation of carbon monoxide are given as: (2n + 1)Hz + n C 0 = CnHzn+2+ nH2O
(1)
+ 1)H2+ 2nCO = C,,Hzn+2 + nCOz
(2)
(n
2nH2
(n
2nH2 + nCO = C,Hz,
+ nH20
(3)
nH2 + 2nC0 = C,H2,,
+ nCO2
(4)
+ nCO = C,H2,+10H + ( n - 1)HzO
Yatish T. Shah is Associate Professor i n the Department of Chemical Engineering, Uniuersity of Pittsburgh, Pittsburgh, Pa. He receiued the B.S. degree from the Uniuersity o f Michigan and the M.S. and Ph.D. degrees from Massachusetts Institute of Technology. Before'joining the Uniuersity of Pittsburgh, he was associated with Abcor Inc., Cambridge, Mass. During the year 1970-1971, he was a visiting Bcholar at the Uniuersity of Cambridge, Cambridge, U.K. He presently teaches courses in reaction engineering and has research interests in reaction engineering and special types of transport phenomena.
Anthony J. P e r r o t t a receiued a B.S. degree in Ceramic Science from Penn State University i n 1960. He receiued the M.S. and Ph.D. degrees in Mineralogy and Crystallography from the University of Chicago in 1965. After completing two years as a Captain in the U.S. Army, he spent two years at the Union Carbide Research Institute. For the past six years, he has been doing catalyst research i n the Process Research Diuision of the Gulf Research and Deuelopment Company.
(5)
+ 1)Hz + (2n - 1)CO = C,H2,+10H + ( n - 1)C02 (6) Ind. Eng. Chem.. Prod. Res. Dev.. VoI. 15, No. 2. 1976
123
These reactions represent the formation of paraffins, monoolefins, and alcohols. The reactions show the evolution of water or carbon dioxide as the side product. Although the by-products change from water to carbon dioxide as the concentration of carbon monoxide relative to hydrogen is increased, the formation of the desired product still can be obtained. Since the synthesis reactions lead to a smaller number of moles of product, the equilibriuq conversion at any temperature increases with pressure. The influence of temperature and pressure on equilibrium conversion of a reaction of type 3 above is shown in Table I. The increase in pressure of up to 50 atm allows a temperature increase of about 175 OC in the formation of 1-decene, a typical synthesis of type 3. Generally, the reactions forming carbon dioxide rather than water have larger equilibrium constants (more negative .4Fovalues). The standard free energy changes per carbon atoms, AFOln, for reactions producing methane are more negative than for the reaction forming the higher hydrocarbons. In addition, the free energy of formation of elemental carbon is more negative than that for the higher hydrocarbons; therefore, the production of higher hydrocarbons for liquid fuels must depend on the selectivity of the catalyst. As shown in Table 11, different catalysts and promoters under changed temperature and pressure conditions produce varying products from the reaction between carbon monoxide and hydrogen (40). The Fischer-Tropsch synthesis process can be made generally selective by proper choice of operating conditions and catalyst. The status of the several Fischer-Tropsch processes through 1950 is shown in Table 111. The yield of C3+ product per cubic meter of synthesis gas shown in this table is very important since nearly 70% of the total cost of product from these processes using coal as the source of synthesis gas is the cost of producing purified synthesis gas. Forseeable improvements in the catalysts that would increase operability and decrease operating costs in these processes were ( 9 ) development of a catalyst for the fluidized iron process that will not catalyze the reaction 2CO COz C and that will not appreciably oxidize during the steady-state life of these catalysts and (34) the development of a more active and mechanically stable catalyst for the oil-powdered Fe catalyst slurry process to further reduce the yields of C1 + Cz (39). Further improvement can also be made in each of the processes by developing a catalyst which minimizes the shift reaction. I t has been suggested (40) that further selectivity in Fischer-Tropsch reactions can be attained by either poisoning acceptable catalysts with sulfur compounds or by selecting sulfides of less frequently used catalysts. Fujimara and his co-workers (36) report that the initial effect of small amounts of HzS is the increase in activity of nickelmanganese catalyst. This is confirmed by Herrington and Woodward (20) for cobalt-thoria-kieselguhr (100:18:100) catalysts. Hydrogen sulfide is mixed with synthesis gas in small measured batches. No hydrogen sulfide is eliminated in the off-gas during the course of the sulfur poisoning experiments. The results are shown in Table IV. The first additions of H2S cause a marked increase in the yield of liquid hydrocarbons at constant temperature. As sulfur addition continues there is a decrease in the yield of gaseous hydrocarbons. Total hydrocarbon yield increases until 8 mg of sulfur has been added to each gram of catalyst. This work suggests the advantage of stopping sulfidization a t a low level (1to 4 mg of sulfurlg of catalyst) to obtain benefits of increased liquid hydrocarbon yield. It also suggests that the catalyst might show the same behavior or presulfided to the same degree before introducing synthesis gas. Another
+
124
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 2, 1976
-
Table I. Influence of Temperature and Pressure on Equilibrium (6)Conversion in a Typical Synthesis Reaction: 20H, + lOC0 = C,,H,, + 10H,O (Conversion = 80%) Temp, C
P,atm abs
300 390 4 20 445 475
1 10 20 30 50
earlier reported use of sulfur-containing catalyst is the patent of Stewart (37). Sixty-nine percent conversion of CO was attained using a molybdenum disulfide catalyst alkalized with 2-3% KOH in a feed of 2H2 + CO a t 530 O F and 200 psig. Products from this synthesis are low boiling with 30% of the product Cs+ hydrocarbons and organic oxygenated compounds. Layne (22) describes a method for minimizing COZ formation during hydrogenation of CO to form hydrocarbons. COz resulting from shifting of the gaseous components is minimized by allowing the sulfur content of the iron catalyst to build to an optimum ratio and maintaining it at that level. Nickel Catalysts Nickel catalysts are usually prepared from nickel nitrate by precipitation of nickel hydroxide via the addition of a base such as an alkali hydroxide or carbonate. A support such as kieselguhr, (a porous silica formed from diatoms or alumina) is present as a dispersed phase upon which the nickel hydroxide is deposited. Upon heating, the nickel hydroxide is dehydrated to nickel oxide. In the presence of hydrogen at the synthesis gas temperatures, the nickel oxide is reduced to elemental nickel. In many of the catalytic systems, promoter oxides are present to increase the yields and selectivities. Such oxides as thoria are not reducible in the synthesis gas environment. The conversion of a synthesis gas of composition 2H2 and CO a t atmospheric pressure is shown in Table V. These catalysts are made by deposition of the components from solution on a kieselguhr support by addition of sodium carbonate. The data show that the amount of liquid hydrocarbon is dramatically increased by the addition of thorium oxide. The addition of another nonreducible oxide, alumina, does not give an increase in liquid hydrocarbon but, in fact, decreases the yield when added to either the pure nickel or the nickel-thoria catalysts. Such early comparative data are interesting since the nickel catalysts are known to be good methanation catalysts and as we shall see later, thorium oxide is a good isoparaffin catalyst in the isosynthesis. The addition of another metal, copper (as with alumina), decreases the yield of hydrocarbons. Since the ratio of thoria has not been reduced (only metal substitution) it is not clear why the copper should decrease the yield to such a large extent. The use of nickel catalysts for methanation has been discussed extensively by Mills and Steffgen (23). More recently Van Herwijnen (40) studied kinetics of the CO methanation over a nickel catalyst in the temperature range of 170-210 "C. Free1 (19) studied the chemisorption of hydrogen and carbon monoxide on Raney nickel. They found that prolonged heating of the catalyst in boiling water considerably reduces the nickel area effective for chemisorption. Most recently Dalla Beta ( 9 ) measured initial rate of CO hydrogenation over nickel and ruthenium catalysts. Cobalt Catalysts The cobalt on kieselguhr catalysts, as with the nickel catalysts, have been studied extensively with regard to sup-
It is well known that the hydrogenation of carbon monoxide can lead to various products such as methane, higher hydrocarbons, alcohols, and other oxygenated species depending upon the reaction conditions (i.e., temperature and pressure) and the nature of the catalysts. This paper relates the product variations resulting from various catalytic systems with specific emphasis on Fischer-Tropsch synthesis and isosynthesis. The effectiveness of iron, nickel, cobalt, and ruthenium catalysts for the Fischer-Tropsch synthesis is discussed. Some useful catalysts (e.g., thoria) for isosynthesis are evaluated. It is our hope that the present review will help to set guidelines for future work on the development of better catalysts for the Fischer-Tropsch synthesis and isosynthesis.
Table 11. Products of Reaction Between Carbon Monoxide and Hydrogen ( 3 5 ) Catalysts A. Methane synthesis
Promoters
Ru
C. Methanol synthesis
D. Higher alcohol synthesis E. Isosynthesis
Pressure, atm
1-30
Ru
150-250
100-1000
ZnO, Cu, Cr,O,, MnO Same as in C
200-400
100-1000
Chiefly methane Chiefly methane Parafinic and olefinic hydrocarbons up to waxes, plus small to large quantities of oxygenated products High molecular weight parafinic hydrocarbons Methanol
Alkali
300-450
100-400
Methanol and higher alcohols
Tho,, ZnO
K,O
4 0 0 -50 0
100-1000
100-200
100-200
Saturated branched hydrocarbons Oxygenated organic compounds
Fe, Co, Ni
+
1 1
A1’203
F. Oxosynthesisa a This
Product
250-500 250-500 150-350
Tho,, MgO Tho,, MgO, A1,0,, K,O
Ni
B. Fischer-Tropsch synthesis
Temp, C
CO, Fe
reaction involves hydrogen , carbon monoxide, and olefins.
Table 111. Characteristics of Various Fischer-Tropsch Processes ( 3 8 ) ~
Catalyst
Temp, “C
Pressure, atm
C,+, g/m3
C,+,Q kg/(m3)
Gasolineb
Dieselb
H.O. + wax
Watersoluble Stee1,c Motor chemi- tons/ octane calsb (bbl day) n0.d
Granular Catalyst, Externally Cooled, N o Gas Recycle 140 8 56 33 11 e 2.7 10 150 10 35 35 30 e 2.4 Fe 10 125 10 32 18 35 15 2.5 Granular Catalyst, Externally Cooled, Gas Recycle co 190-224 10 160 13 50 22 22 8 1.9 Fe 230 20 145 14 19 19 58 8 2.1 Fe 275 20 145 11 68 19 3 8 2.2 Powdered Catalyst, Oil Slurry, Gas Recycle Fe 250-275 20 170 20 25 30 31 4 1.2 Granular Catalyst, Internally Cooled, Gas Recycle Fe 240-2 80 20 170 58 58 10 24 8 0.7 Granular Catalyst, Hot-Gas Recycle 140 32 70 17 1 12 0.7 Fe 300-320 20 Fluidized Catalyst, Gas Recycle Fe 300-320 20 150 115 73 7 3 17 0.6 UKilogram of total product, excluding water, carbon dioxide, methane, ethane, and ethylene per volume of unit time. b Weight percent of oil plus water-soluble product. C In converter and its accessories only. d Bauxite T.F.L. added. every small, less than 1%.
co co
175-200 175-200 200-225
1
port, promoter metals and oxides, plus effect’of pressure on the synthesis gas reaction. Typical data shown in Table VI for the reaction of 2H2 and CO a t 1 atm over various cobalt catalysts give information on the value of C,+ hydrocarbons per cubic meter of synthesis gas. The thoria promoted catalysts are far better than just cobalt and kieselguhr catalysts as shown by the data comparing the two catalysts formed by precipitation using potassium carbonate solutions. It also appears that the base used has a profound effect on the hydrocarbon yield. The potassium carbonate precipitant gives a much greater yield than that experienced by using sodium carbonate. Ammonium carbonate gives catalysts that are much more active
Cetane no.
50 25
100 100
..
..
...
..
..
..
.. ..
..
..
74
78
75
50
76 ... reactor per treated, but no
than the sodium salt. In addition, the catalysts formed by use of the hydroxides of sodium or potassium were virtually inactive. The behavior of these catalysts has been determined but there is no reason given for the difference in activity or its lack thereof as in the case of the hydroxides. There is speculation that these differences “may be due” to the nature of the precipitated phases, basic carbonate or oxide, with respect to ease of removal of ion when washing the precipitate, changes in structure upon reduction, and interaction of the precipitant with the kieselguhr support. The effect of various concentrations of thoria on cobaltkieselguhr catalysts is shown in Table VII. The data indicate that the ratio of thoria required to maximize the Ind. Eng. Chem., Prod. Res.
Dev., Vol. 15, No. 2, 1976
125
Table IV. Effect of Hydrogen Sulfide on Yield (20, 36)
Amount of sulfur added. mg/g of catalyst
Ratio of hydrocarbon yield to corresponding yield before addition of sulfur Hvdrocarbons Condensed in off gas
Catalyst temp, “C
0.67 3.50 7.94 7.94 13.4 33.5
c*A 7 H 5 . 7 1.o
oil 1.0 2.3 2.1
183 183 183 183 19 5 195 207
0
1.0 0.8 0.5 1.8
2.0 1.5
2.1
1.2
Table V. Comparison of Nickel-Based Catalysts on Kieselguhr ( 1 5 ) :1 1. of 2H, + CO,/h/g of Ni, 1 atm
c,
Temp, Composition
“C
Ni
100Ni:15A1,03 100N:18Th0, 90Ni:lOCu: 18Th0, 100Ni:18Th0,: 15A1,0,
hydroDura- carbons Cs+, ti0n.h x 100 cm3/m3
220 220 180 188 182
42 40 42 93 90
4 Trace 63 8
8
-
70 22
-
6
Table VI. Effect of Composition and Mode of Preparation of Cobalt ( 1 6 ) Catalysis: 1 l./g of CO/h CO 100
100 100
100
Composition Tho, Kg 18 18 18
100 100 100 100
-
Precipitant (NH,),CO, Na,CO, K,CO, K,CO,
Temp, “C
C,+,
cm3/m3 121
200 210
61.5 142 47.5
195 210
Table VII. Influence of Thoria Content on Synthesis with CO-Kieselguhr (17) Catalysts: CO-Kieselguhr = 100:100; 11. of 2H, + CO Gas/h/g of CO at 1 atm Thoria per lOOC0 by wt 0
12 18 24 48
Temp, “ C
C, +, cm3/m3
210 185 185 185 185
47.5 102.5 115.5 17 0
amount of C5+ hydrocarbons is about 18 parts of thoria per 100 parts of CO. Above this concentration of thoria the amount of C5+ hydrocarbons rapidly declines below that even of the pure cobalt-kieselguhr catalyst. Comparison of these data for cobalt with those given earlier for the nickelkieselguhr-thoria catalysts shows the pure cobalt catalysts to be much more active than the pure nickel catalysts and almost as active as the thoria-promoted nickel catalyst. This is in keeping with the general accepted view of nickel being an excellent methanation catalyst. The interesting observation is that in absolute quantities, the cm3/m3 increase for the nickel-based catalyst is 59 cm3/m3 as compared to 68 cm3/m3 for the cobalt-based catalyst, both of course promoted with thoria. These values are fairly close, which may tend to suggest that the thoria on the kieselguhr is acting independently to give higher hydrocarbons with the difference in total yields being essentially due to the 126
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 2, 1976
higher activity of cobalt over that of nickel to produce higher hydrocarbons. Promoters other than thoria such as manganese, magnesium, copper, zinc, and aluminum were tested under similar conditions. The only promoter that increased the yield of C5+ hydrocarbons was manganese. The magnesium and zinc-promoted catalysts produced about 40-50 cm3/m3 of C5+ which is comparable to that of the pure cobalt-kieselguhr catalyst (47.5 cm3/m3). Copper and aluminum substantially decreased this yield. The manganese-promoted catalyst, although giving an increase, falls short of the thoria-promoted catalyst as its highest yield is about 68.5 cm3/m3 of synthesis gas (17). All of the data above are for Fischer-Tropsch synthesis at 1 atm which is considered to be the “normal pressure synthesis.” There has been found to be an effect of pressure on the Fischer-Tropsch synthesis and these pressure effects on product distribution have been put into two categories, “medium” and “high.” Medium pressure synthesis is conducted at pressures between 5 and 20 atm. Since the best results for generating Cg+ was from the cobalt-thoria-kieselguhr catalyst cited above, this catalyst was used in the pressure synthesis study. The results are given in Table VIII. As the pressure is increased to the medium pressure range, the ratio of total hydrocarbons is increased. The ratio of C5f is also significantly increased in the medium pressure range. As the pressure is increased to 50 atm and above, the amount of total hydrocarbons and also C5+ decreases. Therefore, the optimum range for producing C5+ is between 5 and 20 atm a t about 185 “C. In addition, the catalyst life is longer under these conditions as compared especially to higher pressure runs. The shorter catalyst life at the higher pressures is attributed to the removal of cobalt by the formation and loss of volatile carbonyls. In comparison, the cobalt catalysts give higher C5+ selectivity than nickel catalysts; the promoted catalysts give another increase in selectivity to Cs+; and finally, it has been recognized that more significant changes in selectivity are produced by increasing the pressure from atmospheric to 5 to 15 atm. In addition, the effectiveness of promoters in increasing the average molecular weight of mediumpressure synthesis was slight. Some reported work on the use of cobalt and mixtures of cobalt-nickel-iron catalysts for methanation is briefly reviewed by Mills and Steffgen ( 2 3 ) . More recently, Balaji Gupta (8) and Sastri (34) studied the interaction of hydrogen and carbon monoxide as well as of their mixtures in the ratio 1CO:lHZ and 1CO:2Hz in the temperature range 0-100 “C on a cobalt-thoria-kieselguhr catalyst. The analysis of the adsorbed phase suggested the formation of a -C(H)OH complex on the surface and the mutual enhancement of adsorption at higher temperatures. Sastri (34) also studied the adsorption of 1:l mixture of CO and Hz on a cobalt Fischer-Tropsch catalyst in the temperature range of 0-132 “C. The rates and amounts of adsorption were mutually enhanced indicating interactive adsorption.
Iron Catalysts Iron catalysts, unlike nickel and cobalt catalysts, tend to form oxides and carbides resulting in some degree of loss of activity and changes in selectivity. Additionally, during the formation of oxides or carbides the catalyst may disintegrate due to the large volume changes. In comparison to the cobalt-nickel catalysts, the iron catalysts performed very poorly in the normal pressure synthesis. The important discoveries made with iron catalysts have been in runs conducted in the medium-pressure synthesis.
Table IX. Activity in Medium-Pressure Synthesis ( 1 )
Table VIII. Products from Synthesis at Different Pressures with ( 1 8 ) CO-Tho,-Kieselguhr -_.-.___-
Hydrocarbon products, g/m3 Pressure, atm, gauge 0 1.5
Total hydrocarbons 155
5 15
181 I83 178
50 150
159 144
C,-C, 38 50
33 33 21 31
FP*+
c,
+
117 131 150
145 138 104
Fe2+,Fe3+ Fe3+
Chloride
Moderate High High
Moderate High Very low
Table X. Distribution of C3+ Hydrocarbons as Functions of Potassium ( 2 )Carbonate Content in Fe,O,-K,CO, Catalyst ( l H , + 1CO Gas at 15 atm, 235 C ) -~
C, + C, 20 10
% K,CO,
The iron catalysts were formed by dissolution of iron salts such as ferric nitrate in dilute nitric acid. The iron was precipitated from solution by use of hot sodium carbonate or potassium carbonate solution. Important points ( I ) in the formation of these catalysts are the following. (1)Catalysts formed from iron sulfates have poor activity probably due to residual sulfate. (2) Active catalysts could not be formed from ferric chloride but could be formed from mixtures of ferrous and ferric chloride. (3) A short period of heating a precipitate formed with iron nitrate is desirable after precipitation. (4) The kieselguhr was mixed with the wet precipitate and facilitated the filtration step. Qualitatively, the activities of various nitrate and chloride iron catalyst are given in Table IX. These correlations show the desirability of retaining a mixture of di- and trivalent iron ions in iron catalysts when using the chlorides for medium-pressure synthesis. In addition, it is interesting that ferric nit,rate is very active but the chloride is quite low in activity. This behavior is attributed to the retention of chloride ions by the phase (3-Fe203aH20 which is crystallized when using ferric chloride alone. The retention of the chloride ion is decreased when the ferrous ion is present because Fe:j04 is formed. In the case of the nitrates, either a Fez03 or a-Fe203"20 is formed and both preparations have a high activity. As with nickel and cobalt catalysts, promoters were used with iron catalysts. The first promoted catalysts were those with alkali ions, especially potassium ions. The alkali-promoted iron catalysts did not, affect the activity. The activity was essentially unaltered by the concentration of alkali or if the alkali were incorporated during the precipitation step or impregnated on the catalyst after precipitation with ammonia. Although the activity was unaffected by alkali content, the selectivity of iron catalysts was changed as shown in Table X. The distribution of hydrocarbons is markedly changed when the potassium carbonate is varied from 0 to 1%.A substantial increase in the wax concentration occurs with a decrease in both the LPG and the liquid hydrocarbons. Above the 1%level of potassium carbonate, a significant change is observed in the hydrocarbon distribution. In addition to alkali-promoted catalysts in the mediumpressure synthesis, some work was done on copper promot,ion of iron-kieselguhr-KzCO3 catalysts at atmospheric pressure. These data are shown in Table XI. The amount of C,+ a t atmospheric pressure with no copper promotion clearly shows the low yield of Cs+ with a potassium promoted Fe-kieselguhr catalyst. The introduction of copper does allow the synthesis to be operated a t atmospheric pressure. In fact, it is related that the above catalyst containing about 20 parts of Cu per 100 parts of Fe is equivalent for Cg+ yield to a catalyst containing no copper in the medium-pressure synthesis at suitable temperature in each case. There is no advantage to forming catalysts above about 10 parts of Cu, as above this concentration the yield of Cj+ remains essentially constant.
Nitrate
0 1 2
5
Hvdrocarbons Liquid hydrocarbons 65 48
8
50
9
49
Wax 13
42 42 42
Table XI. Influence of Copper on Fe-Kieselguhr-K,CO, (2H, + 1CO 1 atm (3) Yield of C , + , cm3/m3
Parts of Cu Der 100 Fe
0
0 5 17 25
48 60
62 64 58
35 50
Table XII. Bureau of Mines Tests of 100Fe:lOCu Catalysts Supported ( 4 ) on Kieselguhr (lH, + 1CO at 7.8 atm, 230 "C)
K,O/ lOOFe
0.44 0.87
Kiesel- Activity guhrl per g l00Fe of -Fe 0 100
141 189
Products, wt % CH,
C,
c3 +c,
c,+
5.1 18.9
4.4 11.9
9.3 26.6
81.2 42.6
Other nonreducible oxide promoters such as magnesium oxide, calcium oxide, and alumina were investigated in the medium-pressure synthesis. Only calcium oxide derived from dolomite apparently increased the activity and possibly increased the molecular weight of the product. The other oxides apparently were useful in maintaining the mechanical stability of the iron-based particles which, as mentioned earlier, tend to break apart due to the volume changes associated with phase transitions. Since the copper-promoted iron catalysts gave promising results at atmospheric pressure as related earlier, catalysts with 10 parts of Cu per 100 parts of Fe were tested in the medium-pressure synthesis. In addition, since kieselguhr gave beneficial effects with nickel and cobalt catalysts, the kieselguhr was tested as a support and compared to the unsupported catalyst. Some of these data are shown in Table XII. The ratio of potassium is increased in the run containing natural kieselguhr because the kieselguhr reacts with some of the potassium which then decreases the promotional effect of the Fe-Cu catalyst. The data show that the kieselguhr support gives an increase in activity per gram of iron. However, the distribution of hydrocarbons is shifted to lower molecular weights with the addition of the kieselguhr. The amount of Cg+ hydrocarbons is decreased to about one-half that received with the unsupported catalyst. These selected data indicate the problem of using the support to generate liquid fuels, and much more work was done on this problem. However, the maximum amount of Cg+ obtained was about 67%. This was obtained using a calcined kieselguhr with close to 2K20 per 100Fe. These Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 2, 1976
127
data certainly reflect the variation in production distribution as one moves from pure iron catalysts, to dual-promoted iron catalysts (K, Cu), and then finally to promoted iron catalysts on supports. Besides the presence of iron oxides and iron carbides mentioned earlier, the formation of iron nitrides is sometimes desirable. The iron nitrides often show greater activity, and in addition show a different selectivity than the reduced or carburized catalysts. The data shown in Table XI11 giving the temperature required to maintain a constant conversion shows the initial catalyst having an N/Fe ratio of only 0.032 to be significantly superior. The increase in the N/Fe ratio to 0.44 resulted in a large temperature decrease, and in addition had a "leveling out" effect on activity. The selectivity with the iron catalysts differs in that although the fraction of liquid produced is comparable to the other iron catalysts, the liquid produced from the nitrided catalysts contained larger concentrations of alcohols, aldehydes, and ketones. There are additional variations, of course, considering the amount of nitrogen, the associated promoters for both the nitrided catalysts, and the reduced catalysts which lead to an overlapping of product yields and distribution. There are even experimental data on intermediate carbonitrides of iron if one wants to consider an even broader experimental picture. In comparing the iron catalysts to the cobalt catalyst, the iron catalyst is not reduced as is cobalt and in addition, it appears, as just shown, that the presence of iron nitrides (or even carbides) may be desirable. Secondly, iron catalysts have poor activity and life in the atmospheric-pressure synthesis. Thirdly, the activity of iron catalysts and catalyst life increases with pressure up to about 30 atm. Selectivity varies with pressure as shown in Table XIV. Using a precipitated iron catalyst, the conversion of the CO decreased initially and then remained fairly constant. The yield of wax increased significantly up to about 10 atm accompanied with a decrease in the gasoline fraction. Although not mentioned earlier, olefins besides oxygenates are also formed in the medium-pressure synthesis. The olefinic content of the gasoline and diesel oil fractions remained essentially constant with pressure. The poisoning of iron Fischer-Tropsch catalysts by small concentrations of HzS in synthesis gas was studied by Karn (21). Reduced, fused iron oxide and reduced steel turning catalysts were tested with synthesis gas containing 6.9, 23, and 69 mg of sulfur as H2S per cubic meter, a t constant temperature of about 260 "C. For both catalysts the activity decreased linearly with the amount of sulfur fed to the catalyst until about 60% of the activity was lost and 0.2 and 0.3 mg of sulfur per gram of iron had been introduced. On further poisoning the activity decreased less rapidly with sulfur introduced. The activity of the turnings decreased steadily to zero when 1 to 2 mg of sulfur per gram of iron was introduced; however, the activity of the fused iron oxide catalysts approached a constant value of 5 to 10%. By increasing the temperature, the productivity of fused iron oxide catalysts was maintained constant for moderate periods of time. Dry and Oosthuizen (10) studied the correlation between catalyst surface basicity and hydrocarbon selectivity in the Fischer-Tropsch synthesis. Carbon dioxide chemisorption studies a t 0 "C were used to determine the relative basicities of alkali-promoted reduced magnetite catalysts. Catalysts promoted with oxides of Li, Na, K, Ca, and Ba were studied. For the equivalent amounts of alkali present the basicity of the reduced catalysts increased in the order of Ba, Li, Ca, Na, and K. Higher surface basicity was correlated with lower methane selectivity. Impregnating the catalyst with alkali after fusion rather than addition of the al128
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 2, 1976
Table XIII. Temperature-Time Relation Comparison for Reduced and Nitrided ( 7 ) Fe Catalysts (7.8 atm lH, + CO) Catalyst
Temp, C
Time, days
Reduced Reduced Reduced N/Fe = 0.032 N/Fe = 0.032 N/Fe = 0.032 N/Fe = 0.44 N/Fe = 0.44 N/Fe = 0.44
24 5 268 282 233 240 243 210 224 228
0 20 40 0 20 40 0 20 40
Table XIV. Selectivity as a Function of Pressure ( l H , + 1CO Gas) ( 5 )
-
Pressure, atm Conversion, CO % Yield of C,+, g/m3 Liquid + solids, wt % Gasoline Diesel oil Wax Olefin content. vol % Gasoline Diesel oil
1 95 90
3 70 98
5 75 86
10 75 118
20 75 120
57 24 19
32 30 38
30 25 45
25 20 55
22 22 56
68 41
64 48
63 49
62 47
63 46
Table XV. Synthesis with Ruthenium Catalysts at Various Pressures (2H, + 1CO Gas at 180 C ) Pressure, atm 1 50 100 1000
co %
conversion
Wax
Liquid
Gas
0 48 68 92
46 53 59
-
-
33 31 26
21 16 15
kali a t the fusion stage of preparation resulted in catalysts with higher basicities and consequently lower methane selectivity. Silica depressed surface basicity and resulted in increased methane production. Carbon dioxide appeared to chemisorb on the oxide promoters present on the surface of the reduced catalyst and possibly on the metallic iron surface. The BET surface areas of the catalysts decreased with increasing basicity of the alkali promoters. The heats of chemisorption on promoted iron surfaces and the role of alkali in Fischer-Tropsch synthesis were studied by Dry ( 1 1 ) . Promotion of reduced magnetite with KzO increased the heat of CO adsorption a t low coverages while it decreased the initial heats of hydrogen adsorption. The known influence of K20 promotion on both the activity and selectivity of the Fischer-Tropsch synthesis was explained in terms of the observed influence of K20 on the heats of adsorption of CO and of hydrogen on iron catalysts. Dry (12) also studied the rate of the Fischer-Tropsch reaction over triply promoted iron catalysts a t 240 "C in the pressure range of 10 to 20 bars with the synthesis gases of H2/CO ratio varying from 1 to 7. Water was the primary product while COz was a secondary product formed via the water gas shift reaction. The influence of small ratios of different gases or vapors on the rate of carbon monoxide decomposition to carbon over an iron carbide catalyst was studied by Dry ( 1 3 ) .Hydrocarbons, both saturated and unsaturated, had no effect on the rate. Hydrogen, water, acetic acid, ethyl alcohol, and methyl ethyl ketone enhanced the rate of the Boudouard reaction (Le., reaction to form carbon). In all cases, the increase in Boudouard reaction was postulated to be due to increased hydrogen adsorption and thereby enhanced carbon monoxide adsorption. Dry
Table XVI. Operating Conditions and Selectivity with 0.5% Ruthenium on Alumina (2.4 atm Pressure, 300 Hourly Space Velocity) Gas composition Temperature, C Conversion, % Distribution of hydrocarbons
3H, + 1CO 222 77
2H, + 1CO 225 66
100
c,
C,
c,-c.
90 80 70 60 50 40 30 20
l H , + 1CO 239 40
c, C,-C,
464 " C
>464"C
0
(14) also studied the influence of promoters on the decomposition of carbon monoxide over iron carbide a t 325 O C . Alkali promoters such as potassium and sodium carbonate increased the rate per square centimeter of iron area. When the alkalies were present as silicates (which are less basic) the intrinsic rates were lower than in the case of the carbonates but the overall rates were higher due to higher surface areas of the silica-promoted sampler. Structural promoters such as A1203, while having a considerable effect on the overall rate of the reaction due to their influence on the available iron area, had little influence on the rate of deposition per square centimeter of iron surface.
Ruthenium Catalysts Synthesis gas reactions a t high pressures were performed over platinum-group metal catalysts (24, 25, 26, 32, 33). Of these metals, ruthenium yielded high-molecular-weight paraffins. Rhodium catalysts produced good yields of oxygenated material and less wax than ruthenium. Osmium catalysts produced larger yields of gaseous hydrocarbons. Platinum, palladium, and iridium had low activities. It is interesting that, in contrast to the nickel, cobalt, or iron catalysts, the pure ruthenium catalyst was the best catalyst. Supports or promoters did not improve the behavior of the ruthenium catalyst. Unlike the iron catalysts, ruthenium is not oxidized or carburized under operating conditions. Although ruthenium carbonyl formation is favorable, it apparently does not affect the activity. The synthesis with ruthenium catalysts in a typical run gives the products shown in Table XV. The conversions were quite low a t pressures less than about 50 atm. As the pressure was increased, the amount of wax produced increased at the expense of the liquid and gaseous hydrocarbons. It should be noticed that the average molecular weight of the wax produced a t 1000 atm is about 23 000 and some fraction of the wax is polymethylene with molecular weights of 1000 000 or higher. However, the resin productivity is quite low. If the resin productivity is not low (0.12 g/g of Ru/h) then a direct route to high density polyethylene from synthesis gas was obtained. Thus, one moves from the production of methane, LPG, liquid fuels such as gasoline and diesel oil, to that of high molecular weight polymers. Ruthenium is much more expensive than the nickel, cobalt, and iron catalysts. In fact, the iron catalysts were used in preference to nickel and cobalt and the medium-pressure synthesis developed because of economic factors. An additional point is that ruthenium is readily poisoned by
sulfur in the form of hydrogen sulfide or organic sulfur such as thiophene and mercaptans. Ruthenium is more readily poisoned by sulfur than cobalt. These catalysts are all poisoned by sulfur in concentrations less than that needed to form the metal sulfides. Recently, Karn (21) studied hydrogenation of carbon monoxide on catalysts consisting of 0.5% ruthenium impregnated on alumina pellets. The molecular weight of the product decreased sharply with increasing H&O ratio of the feed gas. Large yields of hard wax were produced with H2 CO gas a t 21.4 atm and 220 OC. Some of the operating conditions and selectivity a t 21.4 atm pressure and 300 hourly space velocity found in this study are shown in Table XVI.
+
Isosynthesis The isosynthesis is interesting as compared to the other synthesis discussed here in that difficult reducible oxides are used as catalysts (27-30). Secondly, these catalysts, under proper conditions, give saturated, branched-chain, aliphatic hydrocarbons containing 4 to 8 carbon atoms. In fact, the reason for developing these catalysts was for the production of isobutane for high-octane gasoline. The oxide catalysts used were thorium oxide, aluminum oxide, tungstic oxide, uranium oxide, and zinc oxide. The thoria and alumina catalysts could be prepared by precipitation from nitrate solutions by adding a hot sodium carbonate solution. The other oxide catalysts were prepared by addition of alkali to their respective nitrate solutions except in the case of tungsten which was precipitated from Na2W04 solution by nitric acid addition. The most promising catalyst for the isosynthesis is thoria or promoted thoria catalysts. Unlike catalysts of the iron group, thoria is not poisoned by sulfur compounds. In addition, activity decline because of carbon deposition could be overcome by simply passing air over the catalyst to generate COz at the synthesis temperature. Some typical results for one-component catalysts a t various pressures are given in Table XVII. The most effective catalysts are the tetravalent oxides, thoria, ceria, and zirconia. Alumina and the other oxides are not quite as effective in producing isobutane. Other oxides such as chromia, lanthana, praseodynium oxide, magnesia, manganese oxide, titania, and berylia were tested but had relatively low activity. The most significant result is that found with the thoria catalysts. Thoria produces hydrocarbons from water gas (Hz, CO) with relatively high carbon number. In addition, Ind. Eng. Chern., Prod. Res. Dev., Vol. 1 5 , No. 2, 1976
129
Table XVIII. Results from ThO,-Al,O, Catalysts (300atm)
Table XVII. Isosynthesis Results over One-Component Catalysts (450 "C, Water Gas Conversion)
Catalyst
Water gas conversion % i-C, 30 150 300 %Hydroin C, atm atm atm carbons Cno. fraction ~~
Thoriaa Thoria Thoria Muminab Aluminab Aluminaa WZO, ZrOP ZrO, ZrO,
19 46 66 54 53 21
58
2.1 5.1 6.4 11.0 10.5 3.1 12.9 1.o
9
31
2.1 2.6 2.8 1.4 1.5 2.0 1.3
88
36
the so-called "gasol" hydrocarbons consisting of mainly Cs-Cd also contain large amounts of isobutane. In contrast, the trivalent oxide alumina produced mainly methane and carbon with small amounts of "gasol" and traces of isobutane. The most active divalent oxide, zinc oxide, produced no liquid hydrocarbons but mainly methane and alcohols. The general observations on the thoria catalyst showed that the best temperature region for isosynthesis lay between 375 and 475 OC. Alcohol formation predominated below 375 OC while methane and LPG were mainly obtained between 300-600 atm. Below 300 atm, gas conversion was small; above 600 atm, methane and dimethyl ether were obtained. As with the other catalytic systems, the effect of promoters and carriers was studied with the best oxide catalyst, thoria. Three important additives were tested: alkali added to increase the molecular weight of the product as found with iron catalysts; phosphoric acid to convert unsaturated hydrocarbons such as propane or n-butene to higher branched hydrocarbons, particularly to the dimers; zinc oxide and alumina to enhance the formation of alcohols and subsequently the dehydration of alcohols, respectively, for the formation of branched hydrocarbons. The summary of results for the addition of alkali in the form of potassium carbon showed quite simply that the activity of the thoria catalysts decreases as the alkali content increases. The addition of alumina to thoria is much more interesting. These results are shown in Table XVIII. The data show that the amount of isobutane is increased by the addition of alumina. Best results were obtained with catalysts that contained 20% alumina based on thorium oxide. Interestingly, catalysts prepared by separate precipitation and mixing of the wet precipitates produced the highest i-C4 yields. The amount of "gasol" increased with an increase in temperature but the C,+ fraction decreased. In addition, the i-C4 fraction also increased with temperature in each case. Although not shown in the table, the alcohol concentration also decreased with temperature. It should also be noted that these catalysts were prepared as the best single oxide catalysts were prepared, namely the thoria was precipitated from a nitrate solution with sodium carbonate while the aluminum oxide was precipitated from a sodium aluminate solution using sulfuric acid. The best thoria-alumina-zinc oxide catalysts gave lower yields of isobutane than the thoria-alumina catalysts at comparable conditions (i.e., 31.7 g vs. 60 g for best zinc-free Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 2, 1976
0 0
20 20 20 20
Simultaneous Simultaneous Separate Separate
-
Yields g/m3 T, " C Gasoline C,+
i-C,
450 475 450 475 450 475
23 27 47 55 60 69
79 92 79 94 112 113
42 40 21 18
34 26
59
3.5 2.1 82 3.5 2.3 u0,a 3.7 1.4 ZnOa 10 1.2 1.3 ZnO 44 10.0 1.1 Ce0,a 7 0.5 2.0 CeO, 10 1.0 2.4 81 a From nitrates. b From sodium aluminate (or tungstates).
130
Alumina Precipitation % of Tho, method
catalyst). However, the Cg+ hydrocarbon yield was somewhat larger than the thoria-alumina catalysts. Conclusions
The wealth of data generated in the Fischer-Tropsch and related synthesis has been immense. This type of experimental data has led to building a basis for both gas and liquid pilot plant processes for producing clean fuels from the gasification of coal. The study of the reviews given in this paper point to the large number of catalyst systems that have been tried and others yet to be tested with the hope of obtaining better catalysts. Presently, according to Pichler and Schulz (31), the synthesis of liquid hydrocarbons and other petrochemicals from coal is economically feasible today only in South Africa. The generation of more active and selective catalysts that are more resistant to aging may hopefully alter the economic picture. Literature Cited (1) Anderson, R. B., "Catalysis," Vol. IV, pp 120-123, P. Emmett, Ed., Reinhold, New York, N.Y., 1956. (2) Anderson, R. B., "Catalysis." Vol. IV. D 127, P. Emmett, Ed.. Reinhold, New York, N.Y.. 1956. (3) Anderson, R. B., "Catalysis," Vol. IV, p 133, P. Emmett, Ed., Reinhold, New York, N.Y., 1956. (4) Anderson, R. B., "Catalysis," Vol. IV. pp 136, 137, P. Emmett, Ed., Reinhold, New York, N.Y., 1956. (5) Anderson, R. B.. "Catalysis," Vol. IV, p 229, P. Emmett, Ed., Reinhold, New York, N.Y., 1956. (6) Anderson, R. E., "Catalysis," Vol. IV. p 5, P. Emmett, Ed., Reinhold. New York, N.Y., 1956. (7) Anderson, R. B., Shultz. J. F., Seligman. B., Hall, W. K.. Storch, H. H., J. Am. Chem. SOC.,72,3502 (1950). (8)Balaii GuDta. R.. Viswanathan, B., Sastri, M. V. C.. (Part I), J. Cats/., 26, 212 (1972): (9) Dalla Betta. R. A.. Piken, A. G., Shelef. M., J. Catab, 35, 54-60 (1974). (10) Dry, M. E., Oosthuizen, G. J.. J. Catal., 11, 18 (1968). (11) Dry. M. E., Shingles, T.. Boshoff. L. J., Oosthuizen, G. J.. J. Catal., 15, 190 (19691. (12)-D;y. M. E., Shingles, T., Boshoff. L. J., J. CataL, 25, 99 (1972). (13) Dry, M. E., Shingles, T., Boshoff, L. J.. Botha, C. S. Van H., J. Catal., 17, 347 (1970). (14) Dry, M. E., Shingles, T., Botha, C. S. Van H., J. Catal., 17, 341 (1970). (15) Fischer, F., Meyer, K., Brennst. Chem., 12, 225 (1931). (16) Fischer, F., Koch, H., Brennst. Chem., 13, 61-68 (1932). (17) Fischer, F., Koch, H., Brennst. Chem., 13, 61-68(1932). (18) Fischer, F., Pichler, H.. Brennst. Chem., 20, 41-48 (1939). (19) Freel, J., Robertson, S. D., Anderson, R. B., J. Catal., 18, 243 (1970). (20) Herington, E. F. G.. Woodward, L. A,, Trans. FaradaySoc., 35, 958-967 (1939). (21) Karn. F. S., Shultz, J. F. Kelly, R. E.. Anderson, R. B., lnd. Eng. Chem., Prod. Res. Dev., 2, 32 (1963). (22) Laynes, E. T. (to Hydrocarbon Research Inc.), US. Patent No. 2,446,426 (Aug 3, 1946). (23) Mills, G. A., Steffgen, F. W.. "Catalytic Methanation," pp 159-210, 1973. (24) Pichler, H.,Bellstedt, F., Erdoel Kohle. 26, 560-564 (1973). (25) Pichler, H., Firnheber, B., Brennst. Chem., 44, 33-37 (1963). (26) Pichler, H., Firnheber, B., Angew. Chem., 75, 166 (1963). (27) Pichler. H., Ziesecke, H.-H., "The Isosynthesis." translated by R. Brinkley and N. Golumbic, Bureau of Mines Bulletin 488 (1950). (28) Pichler. H., Ziesecke, K.-H., Brennst. Chem., 30, 13-22 (1949). (29) Pichler, H.. Ziesecke, K.-H., Titzenthaler, E., Brennst. Chem., 30, 333-347 (1949). (30) Pichler. H., Ziesecke, K.-H., Traeger, B., Brennst. Chem., 31, 361-374 (1950). (31) Pichler, H., Schulz. H., Chem. Eng. Tech., 42 (18). 1162-1174(1970). (32) Pichler, H., et al.. Makromol. Chem., 70, 12-22 (1964). (33) Pichler, H., et ai., Brennst. Chem., 44, 337-338 (1963). (34) Sastri. M. V. C., Gupta. R. B., Viswanathan, B., (Part Il), J. Catal.. 32, 325 (1974). (35) Storch, H. H., Golumbic, N., Anderson, R. E., "The Fischer-Tropsch and Related Synthesis." Wiley, New York. N.Y., 1951.
(36) Storch, H. H.. Golumbic, N., Anderson, R. B., "The Fischer-Tropsch and Related Synthesis," p 314, Wiley, New York, N.Y., 1951. (37) Storch, H. H., Golumbic, N., Anderson, R. B., "The Fischer-Tropsch and Related Synthesis," D 315, Wiley, New York, N.Y., 1951. (38) Storch, H. .H., Golumbic, N., Anderson, R. B., "The Fischer-Tropsch and Related Synthesis," p 437, Wiley, New York, N.Y., 1951. (39) Storch, H. H., Golumbic, N., Anderson, R. B., "The Flscher-Tropsch
and Related Synthesis," p 439, Wiley, New York, N.Y., 1951. (40) Van Herwijnen, T., Van Doesburg, H., DeJong, W. A., J. Cafal., 28, 391 (1973).
Received for reuiew November 19,1975 Accepted December 15,1975
Elimination of Excessive Carbon Formation during Catalytic Butene Dehydrogenation Harold E. Swlft,' Harold Beuther, and Raymond J. Rennard, Jr. Gulf Research 8 Development Company, Pittsburgh, Pennsylvania 15230
During the commercial production of butadiene by conventional dehydrogenation using the Dow Type B catalyst, a serious problem can arise which is the formation of large carbonaceous monolithic deposits in the catalyst bed which cause costly periodic shutdowns. By the use of magnetic studies it was found that a small amount of the nickel in this catalyst was converted to metallic nickel. Based on this finding, it was postulated that metallic nickel was responsible for the initiation and propagation of these carbonaceous deposits. By adding a compound to the feed capable of sulfiding metallic nickel as it formed, these carbonaceous deposits were eliminated, thus greatly improving the commercial performance of the process.
Introduction The catalytic dehydrogenation of butenes to 1,3-butadiene using superheated steam as a heating medium and diluent has been used extensively for years. The Dow Type B catalyst, which is chromium-promoted calcium nickel phosphate, has been one of the catalysts most extensively used for this process (Britton and Dietzler, 1948; Britton et al., 1951). With this catalyst in commercial operations, ultimate yields of 1,3-butadiene of 8648% have been obtained a t an n-butene conversion level of 40%. The process is cyclic, where n-butene and steam are passed over the catalyst followed by a regeneration cycle with air and steam to remove carbon and oxidize the catalyst. Besides butadiene and hydrogen, low concentrations of carbon dioxide, carbon monoxide, methane, ethane, ethylene, propane, and propylene are produced. After a period of time the concentration of hydrogen and carbon oxides in the reactor effluent can increase due to decomposition of butenes with a resultant gradual decrease in butadiene yield. This also creates an additional problem in the ease of butadiene purification. However, the major problem which can occur is the formation of hard carbonaceous deposits, called carbon mounds. A carbon mound is an area of the catalyst bed enclosed or cemented in a mass of carbon. After a period of time this carbon mound can grow to extend above the bed and impede vapor flow. Once carbon mounds are formed, extensive feed decomposition occurs, as if it were an autocatalytic effect. Thus, the efficiency of the process is greatly reduced and the reactor has to be shut down. These shutdowns can be frequent and of course very expensive due to labor and material costs and lost production time. During a shutdown the catalyst has to
be removed from the reactor and screened to remove carbon mound material and destroyed catalyst tablets. Makeup catalyst must be added and the reactor must be recharged. The time a t which a given lot of catalyst will degenerate or go "wild" cannot be predicted. Catalyst life for some batches has been as low as 50 to 100 lb of butadiene/ lb of catalyst and on rare occasions up to as much as 600 lb/lb. The average life in one large commercial plant over a period of several years has been about 300 lb/lb, which makes catalyst costs very high. Exactly how a carbon mound is initiated and propagates is not known. A condition which probably constitutes its initiation is the production of more carbon than can be removed during the regeneration cycle. Early in the research directed toward solving this problem it was suspected that the presence of metallic nickel contributed to the initiation of the carbon mound. Metallic nickel would be formed by a breakdown of the CaSNi(PO& lattice or from an uncombined nickel oxide phase in the catalyst (Afanasev et al., 1974). That the latter is possible is supported by the variation in performance from various catalyst batches. Any nickel coming from the lattice would be converted to nickel oxide on regeneration which would be easily reduced to metallic nickel with hydrocarbons a t elevated temperatures. I t is well known that nickel can be a potent catalyst for the complete decomposition of hydrocarbons into coke and hydrogen (Thomas, 1970). The large excess of steam suppresses hydrocarbon decomposition in combination with the correct components; however, with easily reducible nickel oxide this is not so. The carbon mound growth or propagation could then be the result of a region having a higher than normal temperaInd. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 2, 1976
131
