October 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
K
= equilibrium constant, dimensionless
QB
= volume of band solution run into column, cc. of liquid eluted from that was added to =
Q
r = Re =
u V
= =
z
= =
2 y
=
Y =
column during run (measured by subtracting voids volume from volume of eluent), cc. 1/K particle Reynolda number dimensionless group, c/c,. volumetric flow rate, cc. of liquid leaving column/sec. dimensionless group, ka/V total resin in column. mea. dimensionless group, 'kc,Q]V dimensionless group, k c o Q ~ / V
2195
LITERATURE CITED
(1) Brinkley, s. R., Jr., S. Bur. Mines, Rept. 3172, 1951. (2) Epstein, P., and Goldstein, D, J., M.S. thesis, in chemical engineering, M.I.T., 1953. (3) Gilliland, E. R., and Baddour, R. F., IND. ENG.CHEM.,45, 330
u.
(1953). (4) Goldstein, S., Proc. Roy. SOC.(London),A219, 151-85 (1953). ( 5 ) Hiester, N. K., and Vermeulen, T., J . Chem. Phys., 16, 1087 (1948). (6) Thomas, H. C., J . Am. Chem. Soc., 66, 1664 (1944).
RECEIVED for review January 30,
1964.
ACCEPTED June 16, 1954.
Reactivity of Deposited Carbon E. R. GILLILAND AND PETER HARRIOTTI Massachusetts Institute of Technology, Cambridge, Mass.
REVIOUS studies of the oxidation of carbon with steam, P o x , , wen, and carbon dioxide have shown that the chemical reactivity of carbon under given reaction conditions, defined as the atoms gasified per minute per atom of solid carbon, depends both on the type of carbon and on the specific surface (area per unit weight), which is a measure of the relative number of carbon atoms exposed to the reacting gases. An extremely reactive type is carbon that is deposited on a porous carrier so that nearly all of the carbon atoms are on the surface or accessible to the reacting gases. Such carbon deposits often form on catalysts that are exposed to hydrocarbons or carbon monoxide at high temperatures. Dart (6) and Hagerbaumer and Lee (11) studied the regeneration of commercial cracking catalysts with air. Their results showed that the deposited carbons were several thousand times as reactive as coke or coal. From the literature (6, 12, 1.4, 16,S I , 32), the high reactivity of soot deposits to air, carbon dioxide, steam, and even hydrogen, can be inferred, but few quantitative results can be obtained. The authors investigated the reactivity of deposited carbons by studying the deposition and regeneration of such carbons from solid catalysts. In each run, a carbon deposit was formed on a catalyst, and the deposit was partially gasified, with emphasis on the gasification rate measurements. Most of the data presented are for the reaction of hydrogen with carbon deposited on a nickel-silica gel catalyst. The runs with hydrogen covered temperatures from 800' to 1400' F. and were mostly at atmospheric pressure. Some runs were made with catalysts other than nickel, and in a few runs the carbon was gasified with steam, oxygen, or carbon dioxide. EQUIPMENT AND PROCEDURE
The experimental runs were made in a batch fluidized reactor, 2.5 inches in diameter and 12inches high (Figure 1). The reactor was heated with exkrnal electrical windings, Reacting gases passed in order through a metering orifice, a preheating section, a supporting screen, about 6 inches of fluidized solids, a disengaging section, a cyclone, a filter, a condenser, and a gas eample line. For a few equilibrium runs the gases were recycled with a diaphragm pump. The gas flow rates corresponded to superficial velocities of 0.2 to 0.5 foot per second in the reactor for both regular and equilibrium rum. Axial traverses generally showed a constant temperature from the screen to the top of the fluidized bed. Furthermore, calculations indicated that there was neg1 Present address, Chemical Engineering Department, Cornell UniTrersity, Ithaoa, N. Y .
ligible temperature difference between the gas and the catalyst particles and between the center and the surface of the particles, even a t the most rapid reaction rates obtained. Sampling. Gas samples were taken by displacing a sodium sulfate-sulfuric acid solution and were analyzed by conventional Orsat techniques. Solid samples were taken by evacuating the sample flask and quickly opening the stopcock to the reactor to suck out a slug of gas and solids. The flask was flushed with nitrogen before sampling, and the sample was kept under nitrogen until it was analyzed. Some samples were reactive enough to decrease in carbon content in air at room temperature, and accidental exposure of the samples to air while they were still warm is one explanation for some analyses which were obviously too low. The solid samples were analyzed €or carbon and occasionally for hydrogen by burning with oxygen in a combustion train (Figure 2 ) . A sample was placed in one half of the Vycor U-tube; in the other half was 2 grams of copper oxide-silica gel catalyst to ensure complete combustion to water and carbon dioxide. Water was collected with Dehydrite (magnesium perchlorate), and carbon dioxide was collected with Ascarite (potassium hydroxide on asbestos). A furnace on sliding supports was placed around the U-tube to heat it to 1200' F. Combustion was usually complete in 15 minutes. The combustion train was also operated a t 700' F. to estimate the reactivity of the carbons to oxygen. At 700' F. the rate of combustion was slow enough to be conveniently determined by periodic weighings of the bulb of Ascarite. Catalyst. The nickel catalyst was prepared by soaking 28 to 200 mesh silica gel (Davison Co.) in a solution of nickel nitrate and heating the solid to decompose the nitrate. The catalyst was reduced with hydrogen in the reactor at the start of the run to give a catalyst with 15% nickel, The original area of the silica gel was over 500 square meters per gram. Surface areas for the used catalyst were not determined, but a rough estimate was made from room temperature adsorption tests. For sample K-2 No. 1 (4.4% carbon), a t 25" C. ethane isotherm showed 0.27 mg.-mole adsorbed per gram at 600 mm. of mercury; a comparison with isotherms for silica gel and Columbia G charcoal (16) indicated that the area was probably in the range 100 to 200 square meters per gram. A 10% copper catalyst was prepared in the same way. Also used was a commercial catalyst of the Fischer-Tropsch type which contained 28% cobalt, 5% thoria, and 67% silica and had an area of 300 square meters. Procedure. The procedure for a typical run was as follows: A charge (200 t o 600 grams) of catalyst was made through the top of the reactor. The reactor was heated to 1100" F. Any carbon was oxidized with air (15 minutes). Catalyst was reduced with hydrogen a t 1100" F. (15 minutes). (The amount of water collected in the condenser indicated that reduction was nearly complete in less than 10 minutes at 1100' F.)
2196
INDUSTRIAL AND ENGINEERING CHEMISTRY
Reactor was swept with nitrogen (4 minutes). Methane was passed in to deposit carbon (15 minutes), while gas samples were taken a t 3- t o 5-minute intervals. Reactor was swept with nitrogen (6 minutes), while a 20-gram solid sample was taken and the temperature was adjusted. Hydrogen was passed in (30 minutes), while gas samples were taken a t 5- to 10-minute intervals. Nitrogen was passed in and a final solid sample was taken. Electrolytic hydrogen (99.9%), prepurified nitrogen (99.9%), and standard cylinder grades of other gases were used.
NE
Figure 1. Fluidized Reactor
The m i g h t of catalyst, temperature, gas flow rates, gas analyses, and solid analyses comprised the basic data. The carbon deposition and gasification rates were calculated from the inlet flow rate and ratios of gases in the product, neglecting any adsorption. The amount of carbon present at any time was generally determined by integration of the deposition and gasification rates, with the solid analyses providing a check on the total amounts of carbon deposited and gasified. Carbon balances for some of the methane decomposition runs (where nitrogen was present) gave satisfactory checks on the deposition rates. For carbon monoxide decomposition, integration of the deposition rates resulted in values for the weight of carbon formed that were 0 to 20% higher than values obtained from solid analyses and integration of the gasification rates. I n a few runs, the reactivities were based solely on successive solid samples.
data point8 would tend to be low. The difference in slopes for the 1100" F. runs is a result of different partial pressures of methane in the feed gas and different flow rates The net deposition rate increased with partial pressure of methane and with flow rate, but no general correlation w it11 partial pressures was attempted. The deposition rate nas not noticeably affected by flow rate a t low (0 to 10%) hydrogen concentrations, in runs 6 - 2 and G-4 the flow rate was cut about in half for the last two samples, and the per cent hydrogen increased to give a net rate that was in line with previous samples (rate B I O T Y Idecreasing ~ with increasing per cent carbon). The apparent activation energy for decomposition of methane on nickel-silica gel was estimated from runs G-7, 12, 13, 24, and 25, which covered a range of 700" to 1100" F. The calculation was based on the average rates over the range 0 to 0.4% carbon, corrected by assuming the rate proportional to the methane partial pressure. The value obtained of 42,000 B.t u. per pound-mole (23,000 calories per gram-mole) i s about the same as the 20,900 calories per gram-mole reported by Wright ( 3 2 ) for a nickel catalyst a t 200" to 500" F. If diffusion into the pores of the catalyst were slow enough to limit the reaction, carbon would deposit first near the outside of the particle, and either a longer diffusion path or plugged pores would cause the deposition rate to fall more rapidly with increasing per cent carbon. Calculations folloaing the ThieleWheeler approach (IO) showed that for the maximum deposition rate measured, over 99% of the catalyst surface would be effective with pores 10 A. in diameter. Diffusion would not be a controlling factor in either deposition or gasification unless the pore entrances were nearly plugged by carbon deposits several atoms thick.
VYCOR U-TUBE
FURNACE ON SLIDING SUPPORTS
& -
SAYPLE
/
A __
SULFURIC ACID
DEPOSITIOY OF CARBON
Deposition rates €or some typical runs are presented in Figures 3, 4, and 5. The deposition rates are given in millipound-atoms of carbon per minute per pound of catalyst (which is proportional to the rate of increase of per cent carbon on the catalyst). Deposition conditions for all runs are given in Table I. The rates for Figures 3 and 4 were corrected to common temperatures using an activation energy of 42,000 B.t.u. per pound-mole. Figure 3 shows that a t or below l l U O o F. the rate of methane decomposition on nickel-silica gel catalyst decreases nearly linearly with increasing per cent carbon on the catalyst. This indicates that the rate of methane decomposition on carbon is much less than that on nickel, and it suggests that a t a value of about 2.0% carbon, which corresponds to two carbon atoms for every three nickel atoms, most of the nickel surface may he covered with a carbon layer one atom thick. The linearity of some of the plots is probably a result of compensating deviations. The nickel surface is undoubtedly heterogeneous (9, SO). The more active sites tend to be covered with carbon first, and the rate of decomposition of the remaining surlace would be less than the initial rate. Also, the fraction of methane decomposed decreased during a run, and since no correction was made for this changing gas composition the first
Vol. 46, No. 10
BULB
Figure 2.
Equipment for Analysis of Solids
At 1400' F. the decomposition curves are of a different nature. Figure 4 s h o m that the carbon deposition rates with 1 to 1.5% carbon on the catalyst are actually less than those at 1100' F., though the initial rates are higher. The rapid decrease could be attributed to carbon atoms plugging the pore entrances or to a decreased nickel surface area because of sintering. I n either case, the nickel surface would have been covered or made inaccessible at a lower per cent carbon, and continued slow deposition of carbon probably gave thicker layers of carbon than were obtained at the lower temperatures. Carbon was also deposited by cracking carbon monoxide or butane a t 660' to 1050' F. The deposition rates decreased with time hut were still fairly high a t 2% carbon, and carbon deposits n-ith 4% carbon could readily be formed. This indicated that carbon monoxide and butane cracked a t an appreciable rate on a carbon surface as well as on the nickel and that these deposits were likely to have thicker layers of carbon than deposits formed from methane. A few typical decomposition curves for carbon monoxide are presented in Figure 5. The initial decomposition
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
October 1954
TABLE I. DEPOSITION CONDITIONS
-
Series G,J, K Ni-silica eel catalyst = Co-TbOvsilica gel catalyst Series 0 Series J = fixed bed runs
Run
Flow Rate. Lb.-Mole CataX loa/ Av. lyst (Min.)(Lb. Temp., Agea Catalyst) O F.
Methane in Feed Gas,
%
Total, Deposit Time Min.'
Final Carbon,
%
Methane-Nitrogen 6
G-la
0
0.88 0.52
1360
68
G-2 b
1
0.73 0.67 1.09 1.09 0.67 0.66
1390
97
G-3
2
1.38
1070
97
13
1.82
G-4 b
0
0.84 0.84 0.84 0.45 0.45
855
72
15
0.31
18.3
0.79 1.54
G-5
1
0.85
1400
72
15
1.65
G-6a G-Gb
2
0.85 0.6
1080 1055
72 72
3 50
0.5 1.84
G-7 G-8 G-9 G-10
3 0 2 3
1.25 0.63 0.51 0.57
1080 1085 1050 1060
72 72 72 50
15 15 3 6
1.8 1.3 0.32 0.6
G-12 b
5
0.59 0.37 0.44
800
50
61
0.62
G-13 G-14 G-15 G-16 G-17
4 5 6 7 7
0.46 0.57 0.56 0.58 0.52
1005 1105 1085 I090 1090
50 50 50 50 50
15 30 6 30 30
0.76 1.7 0.5 1.7 1.7
G-21a G-21b G-21c
7
0.62 0.62 0.62
1090 1095 1100
97 97 97
4 4 4
0.63 0.76 0.8
G-23a G-23b G-23c G-24 G-25 G-26 G-27
3 4 5 6 7 8 9
0.60 0.60 0.63 0.66 0.61 0.63 0.61
1090 1105 1110 850 700 1005 1000
97 97 97 97 97 97 97
2 2 2 15 133 15 15
0.18 0.21 0.19 0.39 0.37 1.2 1.2
Butane H-la H-lb
0
0.41 0.36
850 860
2 14.5
0.43 1.82
H-2 H-G H-7 H-9
1 6 7 6
0.45 0.34 0.36 0.34
920 880 875 880
53 2 2 2
0.60
60 30
0.38 1.5
16 33 6 6 8 40 1.2
2.25 4.4 1.25 1.7 2.2 1.3 0.43
4.24 0.61
0.51
Methane-Nitrogen J- 1 5-2
0 0
0.30 0.25
K- 1
5 6 6 0
0.47 0.53 0.51 0.64
2 3
0.72 0.61
810 1090
60 50
Carbon Monoxide K-2 K-6 0-1 0-2 0-3 0-4
1
0.60
1050 890 920 905 845 660 835
Tumber of times catalyst had been oxidized and reduced (usually equals number of previous runs). b Successive flow rates correspond to consecutive samples or t o oonsecutive data points on Figure 3.
rates were limited by the approach to equilibrium, since up to 80% of the carbon monoxide decomposed. The more favorable equilibrium made the net rate of decomposition greater at 890" than a t 1050" F. GASIFICATION WITH HYDROGEN
From 10 to SO% of the carbon deposit was gasified with hydrogen, the reaction producing methane with only occasional traces (less than 1%) of higher hydrocarbons.
2197
The reactivities for some typical runs are plotted against the fraction gasified in Figures 6, 7, and 8. The reactivities for these plots were corrected to a temperature of 900' or 1100" F. using an apparent activation energy of 65,000 B.t.u. per pound-moleusually a 10" to 20' F. correction. Reactivities for other runs are presented in Tables I1 and 111. The hydrogen flow rates were about 0.5 to 1.0 millipound-mole per pound of catalyst per minute, and the exit gas contained 1 to 15% methane. Tests a t l l O O o and 1400' F., in which the exit gas had less than 10% methane, showed no effect of flow rate on gasification rate.
o
0.4
0.a
t.z
1.6
2.0
2.4
PER CENT CARBON
Figure 3. Decomposition of Methane on NickelSilica Gel Catalyst
With only one exception, the runs showed a decrease in reactivity with increasing fraction gasified, the reactivity generally falling to half its initial value before 50% gasified. To test the possibility that the decrease in reactivity was an age effect, some carbon deposits were held in nitrogen before being gasified. For carbons formed from methane a t 1100' F., the reactivity of light deposits (formed in 2 t o 6 minutes) was not significantly greater when the deposits were gasified immediately than when gasified after being held as long as 70 minutes a t 1100" F. However, the initial reactivity of carbon deposited from butane did decrease on aging to as low as one third the value obtained with nearly immediate gasification, as shown in Table 111. The decrease in this case was probably caused by desorption of hydrocarbons from the deposit. The aged samples still showed a decrease in reactivity with fraction gasified, and the decrease probably means that there is a large variation in the reactivity of the deposited carbon. Since the more reactive carbon tends to gasify first, the average reactivity of the remaining carbon decreases as gasification proceeds. Some of the variation in reactivity is a result of the heterogeneity of the nickel surface, but even if carbon were deposited on a uniform nickel surface, variation in reactivity would be expected. The hydrocarbon fragments, CH, CH,, CHI, which may be present would form methane sooner than C fragments. However, combustion analyses and material balances showed only small amounts of hydrogen in the carbon deposits. Carbons deposited from methane had compositions from CHO.' to CHo.S. The light
INDUSTRIAL AND ENGINEERING CHEMISTRY
2198
Vol. 46, No. 10
was taken as an average over the range 25 t'o 50% gasified. In several other runs the temperature was suddenly changed by as much as 100" F., and the results indicated that t,he apparent activation energy for all the carbons v a s roughly the same, 65,000 i 10,000 B.t.u. per pound-mole. Effect of Initial Per Cent Carbon. With carbon deposited from methane at 1100" F., several tests showed that the reactivity of light carbon deposits (about 0 . 5 % carbon) was five to 10 times greater than that for deposits vhich contained 1.5 t,o 1.8% carbon. There was no significant variation in reactivity from 0 . 2 to 0.670 carbon. Figure 9 s h o w the effects of initial per cent carbon on the reactivity and gasification rate. With carbon from methane, the maximum gasificat'ion rate was for deposits with 0.5 to 1.070 carbon; with carbon from butane, runs 0.4
-
0.2
3
3 2
v 2s
0.1
0.08 0.06
T
-4
0.04
0
2 PER CENT CARBON
Figure 4.
Decomposition of RIethane o n NiclcelSilica Gel Catalyst at 1400" F.
'
0.02
1 0.01 - 1 depositsrhad relatively more hydrogen than the heavj- deposits, and the carbons deposited from butane had somewhat more hydrogen than carbons from methane. (Empirical compositions from CH06 to CH1.0 have been reported for carbon deposits on cracking catalysts.) Changes in the relative number of carboncarbon bonds or changes in the size of the carbon particles seem the most likely explanation for a wide variation in reactivity.
PER CENT CAREON
Figure 5. Decomposition of Carbon Monoxide on Kickel-Silica Gel (Series K) and Cobalt-Silica Gel (Series 0)
To determine the effects of other variables, reactivities were compared a t the Bame fraction gasified, usually a t less than 30% gasified. Apparent Activation Energy. The apparent activation energy for reaction 1 is about 65,000 B.t.u. per pound-mole (36,000 calories per gram-mole), a figure based primarily on the runs presented in Figure 8. These runs had deposits of 1.7% carbon formed under the same conditions, but the carbons were gasified a t IOOO", 1115", and 1220" F. The value of the activation energy
0
0.2
0.4
0.6
0.8
1.0
FRACTtON GASIFIED
Figure 6. Reactivity to Hydrogen of Carbons Deposited from Methane Carbons deposited and gasified at 1100O F.
with deposits of 0.4, 1.8, and 4,2% carbon showed a maximum gasification rate for the l,Syodeposit. For carbon deposited from carbm monoxide, the heavy deposit of Run K-2, 3, 4 (4.4% carbon) was the least reactive, and Figure 7 indicates that half of this deposit was a residue which was nearly inert to hydrogen. (Runs K-3 and K-4 were continuations of K-2 a t successively higher temperatures from 900" to 1250" F.) By contrast, in a series of three runs made with light carbon deposits, G-21a, b, c, during which the catalyst was not regenerated between runs, half to three fourths of the carbon m-as gasified in each run rsithout evidence that an inert residue was accumulating. Effect of Pressure. The fluid bed reactor could not be operated a t high pressures. Two runs were made in a fixed bed reactor, built by Hipkin ( l 7 ) ,and they showed that high pressures were not much of an advantage for hydrogen gasification of the carbons. The hydrogen flov rates, measured a t standard temperature and pressure (S.T.P.), were the same as for fluid bed runs, but no fluid bed runs had quite the same deposition conditions as the fixed bed runs, and accurate comparisons of reactivity could not be made. In run 5-2, carbon gasified a t 30 atmospheres pressure had about the same reactivity as carbon gasified at 1 atmosphere pressure in run G-8. However, run G-8 had a lower initial per cent carbon (1.3 versus 1.5%) and a deposition rate twice as great (Figure 3 ) , and both conditions tend to increase the reactivitv. A comparison between 5-2 and G-14, for which the deposition rates were more nearly equal, shows that the reactivity for 5-2 was 2.5 times that for (2-14,
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1954
TABLE TI. REACTIVITY TO HYDROGEN
G- 1 G-2 G-4 G-5 G-6a G-7 G-9 G-10 G-12 G-13
G-21o G-23a G-23b G-23c G-24
1400 1400 1415 1435 89 0 89 5 1370 1210 1095 Ill5 1050 1085 810 795 1010 1015 1000 1025 1110 1110 1095 1095 1120 915 920 800 800 1000
0,053 0.022 0.021 0.024 0.060 0.031 0.023 0.005 0.29 0.084 0.16 0.24 0.021 0.011 0.22 0.053 0.028 0.024 0.096
deposits of 0.5% carbon); the very slow rate of deposition made studies a t lower temperature impractical. Carbon deposited from carbon monoxide showed the same trend, but there were not enough data for a detailed comparison.
0.12 0.02 0.07 0.15 0.13 0.23 0.04 0.06 0.35 0.14 0.43 0.3 0.09 0.22 0.64 0.25 0.40 0.53 0.69
0-0,55 0 . 1 1 (av.) 0.25 0.17 0.25 0.17 0.25 0.16 0.06 0.026 0.12 0.010 0.04 0.015 G-25 0.07 0,006 0.04 G-26 0.021 0.13 0.015 G-27 b 0.07 1000 0.035 H-la 905 0.22 0.15 0.06 0.036 H-lb 845 0.031 0.06 H-2 930 0.0017 0.10 755 J-1C 0.12 800 0.015 0.16 800 0.011 810 0.010 0.19 0-2 830 0.11 0.039 0.016 0.22 825 0.07 0-4 800 0.025 a Atom of carbon gasified/(min.) (atom solid carbon remaining). b Gas/fied with 37% hydrogen. C Gasified a t 15 atmospheres.
TABLE ITT. EFFECT OF DEPOSITION TEMPERATURE (Carbons deposited from methane on nickel oatalyst) Deposition Temp., Reactivity Relstive
F.
1400 1100 1000 850 800 700
H-6 H-7
Age ofa Deposit , Min. 1.5 3.0 5.3 6.8 9.8 121 123 126
Reactivity a t 900° F. 0.092 0.052 0.074 0.029 0.020 0.043 0.034 0.019
0.04 1
1.3 3 6 10
The rate of carbon deposition for some of the l l O O o and 1400" F. runs was varied by changing the flow rate and the per cent methane in the feed gas. Increased deposition rates definitely gave more reactive carbon deposits, as shown by Table V . The wide range of reactivities shown in Figure 6 is the result of different deposition rates as well as difrerences in initial per cent carbon.
TABLEV. EFFECTOF RATE OF DEPOSITION ON REACTIVITY TO HYDROGEN Deposition Rate ata
Initial g% Carbon 1.82
TABLE 111. AGISGTESTSON CARBON DEPOSITED FROM BUTANE Run H-9
ON
REACTIVITY TO HYDROGES
Run
Fraot. of Original Deposit Gasified 0.13 0.24 0.14 0.23 0.30 0.07 0.15 0.22
2199
1.8
1.84
Q
b
1% Carbon, Mo!e X 103,'
Deposition Temp.,
(Min.)(Lb. Catalyst)
F.
Reactivityb a t 20% Gasified
1070
1080
1055
From Figures 3 and 4, corrected t o l l O O o or 1400' F. Atom of oarbon gasified/(min.)(atom solid carbon remaining) at l l O O o
F. for runs G-3,7,6b, 14 and a t 1400' F. for runs G-2, 5.
0.1
Measured from halfway through deposition period (2 minutes). Reactivity t o hydrogen, atom of carbon gasified/(min.) (atom solid carbon remaining). a b
but some of this difference could be attributed to the use of fresher catalyst and the lower initial per cent carbon for run 5-2. By allowing for the probable effects of different deposition conditions, the reactivity a t 30 atmospheres was estimated to be twice that a t 1 atmosphere for carbon deposited and gasified a t 1100" F. There was no significant difference between the reactivity a t 1 atmosphere (G-12) and a t 15 atmospheres (J-1) for carbon deposited and gasified a t 800" F. In run G-26 a mixture of 37% hydrogen and 63% nitrogen was used to gasify the carbon. The reactivity was 0.53 times the reactivity for run G-27, a closely controlled comparison run a t 1 atmosphere. At low pressures, the rate may be proportional to (pH%)". Temperature and Rate of Deposition. Decreasing the temperature of deposition greatly increased the reactivity of the deposited carbon. By correcting the measured reactivities to a common temperature and comparing deposits of about the same per cent carbon, approximate values of relative reactivity were calculated for carbon deposited from methane on nickel (Table IV). The carbons deposited at 700' to 800" F. were the most reactive (initial reactivity about 2% per minute a t 800" F. for
E
T
0.001
di
2
Q"
0.0001
0
0.1
0.2
0.3
0.4
0.5
0.6
FRACTION GASWIED
Figure 7. Reactivity to Hydrogen of Carbons Deposited from Carbon Monoxide
-
INDUSTRIAL AND ENGINEERING CHEMISTRY
2200
0IO
o oa 0 06
P' 0 01 0 00.5 0 006
0 004
0 002
0 001 0
02
04
06
O B
IO
FRAC TtON GASIFlED
Figure 8. Effect of Gasification Temperature on Reactivity to H j drogeii
Other workers (1.5) have shown that higher deposition temperatures gave larger and less reactive particles of carbon black. I n this work, high temperatares and loa- rates of deposition led to less reactive carbon deposits, probably because mole carboncarbon bonds rrere formed, and this could be conqidered a change in the size of the groups of carbon atoms. However, the change in reactivity from 700" to 1100" F. probably does not represent merely a change in specific surface. These deposits primarily were only 1 atom thick according to the interpretation given the deposition rate curves. Some of the decrease in reactivity from l l O O o to 1400' F. can be attributed to a decrease in specific surface with formation of thicker deposits. The effect of deposition temperature from TOO" to 1100" F. would be e w n greater than noted in Table IV if allotwince were made for the slomr deposition rates a t low temperatures.
TABLEVI. EFFECT OF SOURCEOF CARBOX ox REACTIVITY TO
Source of Carbon Butane Methane Butane Methane Carbon monoxide Butane Carbon monoxide Methane a Atoms of carbon b Fresh catalyst.
HYDROGEN
Fract. of ReacOriginal tiuitya Deposit Initial 70 Run Carbon a t 900' F. Gasified 0.43 0,14 H-lab 0.22 0.31 0.037 0.22 G-4b 0.01 0.08 0.13 H.8 0.39 0.006 0.13 G-24 4.4 0.011 0.06 K-2 0,018 0.06 4.2 H-2 0.12 1.25 0.07 K-6 0.39 0,007 0.12 G-24 gasified/(min.) (atom solid carbon remaining:).
Source of Carbon and Type of Catalyst. The reactivities of carbons deposited from different sources under similar conditions are compared in Table VI. Two comparisons show carbon deposited from butane a t 900' F. to be four to 10 times as reactive as carbon deposited from methane. Much of this difference could be the result of a higher proportion of easily desorbed
Vol. 46, No. 10
hydrocarbon fragments in the carbon formed from butane. Carbons formed from carbon monoxide were about as reactive as those from butane. The rates of deposition in butane and carbon monoxide runs were three to 10 times higher than in methane runs. These results suggest the oossibility that, except for the easily removed hydrocarbons or hydrocarbon fragments which make up part of some deposits, carbons deposited at the same temperature and rate from different sources have about the same reactivity. There wap some decrectse in activity of the nickel catalvst with use. Fresh catalyst was used for only six runs, the other runs used catalyst that had been regenerated and reduced again froin one to 10 times. Carbons deposited on fresh catalyst a t 900" E'. in runs H-la and G-4 were as much as four times as reactive as carbons deposited on used catalysts in runs H-6 and G-24. For runs G-1 and G-8, made with fresh catalyst at 1400" and l l O O o F., the carbon deposits were estimated to be about 1.5 times ne reactive ae similar deposits on used catalyst. There mas little further change in catalyst activity after it had been used once, and in determining the effects of other variables, comparisons were generally made between used catalyst runs or between fresh catalyst runs. Carbons deposited from carbon monoxide on the GASIFICATION RATE ATOMS GASIFIED / MIN. cobalt-thoria-silica catalyst (series 0) were about as rcactive as a light carb o n d e p o s i t on the n i c k e 1- si 1i c a catalvst (run X-6). The reactivity deINITlAl PER CENT CARRDN creased with fraclion gasified, but Figure 9. Effect of Initial Per Cent the reactivity for Carbon on Reactivity and Initial Gasification Rate series 0 runs xaB about the same for deposits of 0.4 t o 2.2y0 carbon, in contrast to the much IoTver reactivity found for heavy deposits on the nickel catalyst. GASIFICATION BY STEAX, CARBQN DIOXIDE, AND OXYGEN
Carbon deposited from butane in run H-lb vias partially gasified with hydrogen and then reacted with steam. The steam was mixed with nitrogen so that its partial pressure was 0.5 atmosphere. The reactivity t o steam mas about the same as ieactivity to hydrogen. Reaction with Hydrogen Temp.,
R
Reaction with Steam Ttmp.,
F F. R F 0.06 855 0.030 0.23 where R = reactivity in atoms of carbon gasified per (minute) (atom of solid carbon remaining) and F = fraction of original deposit gasified. O
F.
845
0.036
With steam gasification, the exit gas on a dry basis contained 2.67, hydrogen, 3.0% carbon dioxide, 0.6% carbon monoxide, and 2.8% methane. The fraction of carbon gasified that appeared as methane was much higher than that reported in the gasification of other carbons with steam. Some carbons deposited from methane a t 1400' F. were partially gasified xyith hydrogen and then reacted with carbon dioxide. At 1400" F., the reactivity to carbon dioxide (Table VII) was about 10 times the reactivity to hydrogen. In aeveral runs with copper catalyst (data not included) the methane decomposition rates were about one fifth a9 great and the carbon deposits about one fourth to one half as reactive toward carbon dioxide as in nickel catalyst runs.
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1954
2201
TABLE VIH. REACTIVITY TO CARBON DIOXIDE Brant. .... of Original Exit Gas Composition, % ReacDeposit Temp., Run COa co tivitya Gasified O F. G-1 32.8 41.3 0.50 0.30 1360 62.0 7.7 0.14 0.55 1370 66.8 2.7 0.17 0.75 1380 G-2 36.2 38.2 0.26 0.30 1370 0.054 0.59 1375 64.3 5.7 E Atom of carbon gasified/(min.) (atom solid carbon remaining). ~
~~
IO
TABLE VIII. REACTIVITY TO OXYGEN Sample G-4, No. 1
Initial % Carbon 0.31
G-5, No. 1 G-10, No. 1
1.65 0.6
H-2, No. 1
4.2
Fract. of Original Deposit Gasified 0.2 0.45 0.54 0.2 0.2 0.3 0.1 0.2 0.2 0.3 0.3 remaining).
Reactivitya a t 700' F. 0.051 0.037 0.017 0.003 0,009
0.006 0.020 0.016 H-I, N o . 2 0.02 1.65 0.015 1.0 0.015 (6) Atom of carbon gasified/(min.) (atom solid carbon
The rate of combustion of some samples was measured with the solid analysis equipment. The oxygen flow rate was about 1 cc. per second (S.T.P.) and the sample size 2 grams. Typical results are given in Table VIII. The reactivity to oxygen a t 700" F. was about 20 times the reactivity to hydrogen (extrapolated from 900" F.) and was the same order of magnitude as an extrapolation of values reported for regeneration of cracking catalysts (Or). The reactivity to oxygen decreased with increasing fraction gafiified and with increasing deposition temperature, but these effects were not so pronounced as for the reactivity to hydrogen. The diff'erences between the reactivities t o hydrogen, steam, carbon dioxide, and oxygen were much less than values reported a t higher temperatures for carbons such as coke and coal. EQUILIBRIUM IN CARBON-HYDROGEN-METHANE SYSTEiM
The carbon-hydrogen-methane equilibrium has been studied several times in the last 50 years with both graphite and amorphous carbons (1, 6, 4, 8, 18-11, 94, %7, 69, SI). Some of the early results are presented again by Browning and Emmett ( 2 ) along with their values for carbon from iron carbide Their values are presented in Figure 10 along with values calculated from the thermodynamic properties of hydrogen, methane, and P-graphite (d5), which are considered more reliable than any of the measured equilibrium values. The measured Kp)s do indicate that the amorphous carbons are less stable (higher K , for methane formation) than graphite, but the experimental errors were, in general, too great to afford quantitative comparison between the different types of carbon. Problems of Equilibrium Studies. Further invebtigations of the carbon-hydrogen-methane equilibrium, particularly for deposited carbons, were made. There are three problems in getting reliable equilibrium data for this system that have not been generally considered. I. NATUREOF AMORPHOUSCARBON.The type of carbon used may change during the experiments as the original carbon is gasified and methane cracks to deposit fresh carbon. When nickel catalysts are mixed n-ith graphite, as was done by some investigators (4, 19, d l ), the more reactive amorphous carbon deposited on the nickel influenced the equilibrium more than the unreactive graphite. A carbon may be considered amorphous for any of four reasons: 1. Its structure differs from that of diamond or graphite. Some alternate structures based on hexagons of carbon atoms have been proposed and discussed by Riley (66).
l .C
XP
0.t
00l
0001 16
I 4
I2
l 0
08
06
IO00 / OK.
Figure 10. Carbon-Hydrogen-RIethane Equilibrium
2. It has impuritiep, generally hydrogen, bonded to some of the carbon atoms. The presence of hydrogen in the original carbon sample should not affect the equilibrium concentrations of hydrogen and methane as long as carbon is the stable solid phase. Some hydrocarbon fragments may be present, but only in small amounts as intermediates. Wright and Taylor (Sd), who studied the exchange reaction between methane and methane-& (CD4) on nickel, estimated the relative abundance of the species C, CH, CHZ,and CHa to be 100, 10, 1, and 0.1 a t 200" C. 3. The particles of carbon are so small that they have appreciable free surface energy. It seems likely that carbon particles with fewer than 100 atoms would have a t least 1000 calories per gram-atom of surface energy. The larger particles of carbon tend to grow by carbon deposition in the same fashion that large crystals in a slurry grow a t the expense of small ones. 4. The carbon is present as an adsorbed film on a catalyst. The adsorption forces tend t o replace carbon to carbon bonds and offset the surface energy of the thin carbon layer, but as long as the catalyst-carbon bonds are weaker than the carbon-carbon bonds, the adsorbed carbon will have greater free energy than graphite. 11. INCOMPLETE SURFACE COVERAGE.When catalysts are used, the equilibrium values measured will depend on what fraction of the catalyst surface is covered by carbon. Consistent values will not be obtained until appreciable amounts of carbon are present on the surface. If a methane-hydrogen mixture previously a t equilibrium with a carbon deposited on nickel is brought in contact with a fresh nickel surface, the methane will decompose until there is enough carbon on the surface so that the rate of formation of methane from this carbon equals the rate of methane decomposition. This effect was demonstrated by Schultz and Emmett ( 7 ) , who passed water vapor over metallic cobalt and noted that the first portion of evolved gas contained hydrogen
INDUSTRIAL AND ENGINEERING CHEMISTRY
2202
TABLE Ix. EQUILIBRIUM I N CARBON-HYDROGEX-hfETHANE Run G-19 G-19 G-20 G-20 G-20 G-22 G-22 G-22 G-20 G-22 Average Average
Sample No. 2
Approach from CHa Side Temp., O F. 1.29 1115
KP
...
3
1.31
1
2 3
2.2
1
2 3
,..
4
... ...
0.62
... ... ...
1.18
1126 1115
i3k5 (i29' C.)
1.30
1'1'20 (604" C.)
(6) Day, J.
...
1116
...
, . .
0.19
0.22 0.26
l i l 0 (599' C.) 1320 1310 1316
235 (1936). (7) E m m e t t , P . H . , a n d
Esid. % Carbon on Catalyst 1.5 1.6 0.6 1.5
, . .
0.11
1'70 (799' C.)
0.22
1315 (713' C.)
E., IKD.ENG.
CHEXI.,28,
SYSTEN
Approach from Hz Side KP Temp., F.
...
0.21
...
4
.
Vol. 46, No. 10
Schultz, J. F., J . Am. Chem.
Soc.,
51, 3249
(1929).
(8) Fonda, G. R.. and Van
Aernem, H. N., IKD.ENG. CHEM., 14, 539 (1922). (9) F r a n k e n b u r g , Tir. G . , Komarewsky, V. I., and Rideal, E. K. (editors), Advances in Catalysis,
1.6
0.3 1.6 1.6 1.0 1.0
(IO) Ibid., 111, p. 280.
Hagerbaumer, W. A , and Lee, R., Trans. Am. Soc. N e c h . Engrs., 69, 780 (1947). Harris, G. PI., Nauture, 160, S71 (1947). Hofer, L. J. E., U. S. Bur. Mines, Rept. Invest.. 3770, 1944. Hutchinson, W.S.. 11.8.thesis, chemical engineering, M.I.T., (11)
in excess of the quantity expected for equilibrium. When sufficient oxygen had accumulated on the metal to form detectable amounts of oxide, the expected ratio of hydrogen to water vapor was obtained. If an appreciable fraction, say lo%, of the metal surface must be covered by carbon to obtain consistent equilibrium values for the carbon, a catalyst with an area of 500 square meters per gram would require a deposit of more than lyocarbon. Troesch ( S I ) , studying the methane equilibrium, probably had only a small fraction of nickel surface covered with carbon. In a typical run, the carbon deposited could cover only 1 square meter of the total surface available in the 2 grams of catalyst (perhaps 50 to 200 square meters). In a fexv runs in which he allowed carbon to accumulate on the surface, the measured per cent methane at equilibrium incieased with the amount of carbon present. 111. CARBIDEFORMATIOS. Another eriect to consider when metal catalysts are used is the possibility of carbide formation. Cobalt and nickel carbides are endothermic compounds a hich are stable only at high temperatures (800" to 1000" C.), but they can be formed from the metals and some carbon compounds at low temperatures (300" C.). Examination of literature data ( 3 , 13> 96, $6,28) indicates that little if an) carbide vould be formed in depositing carbon from either methane or carbon monoxide under the conditions of the experiments reported in this work. Experimental Results. Three equilibrium runs were made by recirculating hydrogen-methane mixtures through the reactor a t atmospheric pressure after some caibon had been deposited on the catalyst by cracking methane. The results are given in Table IX, and the average R,'s for each temperature are plotted in Figure 10. Equilibrlum was approached from both sides. I n run G-19 pure hydrogen and methane were used as starting gases. In runs G-20 and G-22 mixtures of methane and hydrogen close to the equilibrium composition were used. The direction in which the reaction Was proceeding was noted by measuring the molecular weight of the gas at 3-minute intervals with a weighing bulb. The gas composltion was gradually adjusted until the reaction started to go in the other direction, and then a sample was taken for Orsat analysis. The K,'s for the formation of methane were calculated from the Orsat analyses. The values of K , are 2.3 to 2.6 times those given for graphite (23). Contrary to expectations, the K, values did not show any significant change with the per cent carbon on the catalyst. The K,'s for the deposited carbon correspond to a free energy of 1500 calories per gram-atom greater than that for graphite. It is felt that this free energy is primarily a result of the large surface presented by the carbon, or the fact that nearly every carbon atom is on the surface and does not have all its valences satisfied. LITERATURE CITED
Berl, E., and Bemmann, R., 2. phyailz. Chem., A162, 71 (1932). (2) Browning, L. C., and Emmett, P. H., J . Am. Chem. Soc., 73,
(1)
581 (1951).
(3) I b i d . , 74, 1680 (1952). (4) Coward, H. F., and Wilson, S. P., J . Chem. Soc., 115, 1380 (1919).
( 5 ) Dart, J. C., Savage, R. T., and Kirkbride, C. G., Chem. Eng.
Prop., 102 (1949).
1949,
Jley, R., and Riley, H. S.,J . Chem. Soc.. 1948, p. 1362. Lewis, W. K., Gilliland, E. R., and associates, IND.ENG. CHEX., 42, 1326 (1950). Lexis, \Ti. K., Gilliland, E. R., and Hipkin, H., I b i d . , 45, 1697 (1953). >
,
&I., and Altmayer, V., Ber., 40, 2134 (1907). Pring, J. K,, J . Chem. Soc., 97, 499 (1910). Pring, J. Tu'.,and Fairlie, D. AI., I b i d . , 99, 1796 (1911); 101, 91 hIayer,
(1912).
Randall, RI., and Mohammad, A , IND.ENG.CHEM.,21, 1048 (1929).
Rilev. H. L.. J . Chem. Phvs.. 47. 565 (1950). Rosjini, F. D., and associates: Xatl. Bur. Standards, Circ. C461, 1946.
Scheffer, F. E. C., Dokkum, T., and A1, J., Rec. traw. chim., 45, 8 0 3 (1926).
Schmidt, V. J.. 2. anorg. u. allgem. Chem., 216, 85 (1933). Storch, H. H., "The Fischer-Tromch Synthesis," Wiles, New York, 1951.
Szabo, Z., J . Am. Chem. Soc., 72, 3497 (1950). Tebboth, J. A . , S O C .C h e n . I n d . ( L o n d o n ) , 67, 62 (1948) Travers, hI.. T ~ a n s Faraday . Soc., 34, 580 (1938). Troesch, A , J . C h i m Phys., 47, 148 (1950).
Ibid., p. 274. Wright, bI. >I., and Taylor, H. S., Can. J . Research, 27B, 303 (1949). RECEIVED for review Augusi 6, 1963. ACCEPTED June 1, 1954, Presented a t the meeting of the Gas and Fuels Division of the AMERICAN CHEMICAL SOCIETY a t Pittsburgh, Pa., April 1953.
Aggregation of Suspensions by Polyelectrolytes-Correction In the article on L'Aggregation of Guepensions by Polyelectrolytes" [IsD. EXG.CHEII., 46, 1485 (1954)] the third column of Table I should read:
[?I 0 38 0.19
ALAXS.MICHAELS
....... Toxicity of Various Refinery Materials to Fresh Water Fish-Correction I n the article on "Toxicity of Various Refinery Materials t o Fresh Water Fish" [Turnbull, Harry, D e h n n , J. G., and Weston, R. F., TND. EYG.CHEM, 46, 324 (1954)], the material referred to in the second paragraph below Table IV on page 329 should have been cupric chrome glurosnte.
