PARTIAL COMBUSTI METHANE WITH OXYGEN RICHARD MUNGEN A N D M. B. KRATZER Stanolind Oil and Gas Co., Tulsa, Okla.
T h e partial combustion of methane with oxygen at 300 pounds per square inch gage has been studied on a pilot plant scale as part of the program to develop a synthetic fuels process. The independent variables given primary attention w-ere the ratio of oxygen to carbon in the feed, the preheat temperatures of the feed stream, and the space velocity. The reaction is assumed to proceed in two steps-a primary reaction when carbon dioxide and water are the main products of the reaction of methane and oxygen, followed by reforming of methane with carbon dioxide and water to carbon monoxide and hydrogen. The reforming seaction appears to be the rate-controlling step in the process. Equilibrium was not attained under the conditions eniployed. The calculated final reaction temperatures ranged from 2000" to 2500" F. The partial combustion of methane with oxygen under assumed commercial conditions was shown to be a practical method of supplying €eed gas to the hydrocarbon synthesis process.
brick lining. The gas and oxygen were fed to the top of the chamber through a stainless-steel, water-cooled burner. Steam, when required, was fed into the gas stream ahead of the preheater. The burner was located a t the top of the chamber to minimize fouling of the burner ports by carbon or dislodged refractory. Two combustion chambers of the same design were built, one being kept on stand-by, to avoid interrupting concurrent investigations of hydrocarbon synthesis. Auxiliary equipment included two gas-fired preheaters, a 4000cubic foot gas holder for natural gas storage, 100,000-cubic foot oxygen storage, and instrumentation for the measurement and control of feed gas flows. The product gas was cooled in a waterjacketed transfer line. This was followed by direct contact Rith cooling water in a packed tower, which also served to scrub carbon from the product gas. The carbon 15 as removed from the water in a plate and frame filter press. NET PRODUCT PREHEATER
WATER
02 STORAGE
REMOVAL TOWER
HE hydrocarbon synthesis process produces hydrocarbons
FILTER PRESS
T
and oxygenated chemicals from hydrogen and carbon monoxide. For the production of such a feed gas from natural gas, two methods are generally considered: (1) reforming with steam and carbon dioxide, and (2) partial combustion with oxygen. With the development of PI ocesses for the large scale, lowcost production of oxygen, the partial combustion process has become economically competitive with or superior to reforming. The first commercial hydrocarbon synthesis plant in the United States utilizes the partial-combustion process to produce synthesis gas. In a commercial partial-combustion unit, efficient utilization of both oxygen and carbon is important, the former because it influences the size of the oxygen plant required, and the latter because it is a factor in the raw material costs over the life of the plant. Another important design consideration is the avoidance of carbon formation, which may seriously affect the operability of the process. The major independent variables of the process are: the ratio of oxygen to carbon feed, the preheat temperatures of the feed streams, the space velocity, the reactor pressure, and the reactor design. Montgomery and Weinberger (a) and Mayland and Hays ( 1 ) have analyzed the partial-combustion process from the standpoint of thermodynamics. However, few if any actual operating data have been presented to date. This paper presents the results of pilot plant studies of the partial-combustion process. These were limited to investigations of the first three of the above variables. EXPERIMENTAL EQUIPMENT
A flow diagram of the partial combustion system is shown in Figure 1. The combustion chamber was 10 inches in internal diameter and 6.5 feet long and consisted of a steel shell with refractory
C03LING
KNOCK OUT DRUM
COMBUSTW CHAMBER PRODUCT
n
u -7
STEAM
H0U)ER
' 2COMPRESSOR
Figure 1.
Pilot Plant Unit for Partial Combustion of Methane and Oxygen
A number of temperatures pertinent to the operation of the system, such as the feed gas preheat and cooling water temperatures, were measured by thermocouples. Attempts to measure the final temperature in the combustion zone by thermocouples and a radiation pyrometer were unsatisfactory. EXPERIMENTAL PROCEDURE
Following minor changes in burner design, reliable production operation of the combustion chambers was attained. The startup procedure was simple and trouble-free. The chamber was heated a t atmospheric pressure to a temperature of 1500' F. with a gas-air burner inserted through the gas exit port. After this burner was removed and the product gas transfer line was replaced, small gas and oxygen flows were started through the regular burner. Combustion started smoothly a t the hot chamber wall. The flow rates and pressure were then gradually increased to their required values. During production runs, the operating variables were adjusted as required to produce synthesis gas of the desired specifications. This involved, primarily,
2782
INDUSTRIAL A N D ENGINEERING CHEMISTRY
December 1951
varying the steam feed rate to adjust the hydrogen-carbon monoxide ratio in the product gas, and varying the oxygen feed rate to hold the residual methane content a t a low value. Material balances were conducted over 8-hour periods, during which operating conditions were held as steady as possible. Composite gas samples were collected and analyzed by Orsat procedures, and water and carbon yields were weighed. Gas flow rates were obtained from calibrated orifice meters. The material balance calculations were based on the inlet natural gas flow and composition. The hydrocarbon content was converted to an equivalent GI basis. The volume of product gas was computed by a carbon balance. The water production was then adjusted to satisfy a hydrogen balance, using the corrected product gas flow. Finally, the inlet oxygen flow was corrected to force an oxygen balance using the corrected gas and water production rates. Several other material-balance calculation procedures were tested. As these led to poorer correlations than the above, only the data obtained using the above method are presented in this report. Direct measurements of combustion chamber temperature were unsatisfactory. A Chromel-Alumel thermocouple employed in the earliest runs gave erratic readings. A platinum and platinum-rhodium thermocouple used subsequently gave consistent readings for a short period of time, but these were still below calculated temperatures. An optical pyrometer proved unsatisfactory because of clouding of the quartz window with soot. No attempt was made to measure temperatures in the later work 'which constitutes the basis of this report. Instead, final reaction temperatures were calculated by means of heat balance, using a standard correction for heat loss through the reactor wall and from the water-cooled burner. The experimental program on partial combustion was influenced by the gas requirements for the synthesis unit. Thus, the range of operating variables was limited by the requirements of providing a specified type and amount of product gas for the synthesis studies, The data used as the basis of this report are from the runs in which the operating conditions were similar to those applicable to commercial operation. Two combustion chamber runs, however, were conducted for the primary purpose of investigating variables of the partial combustion process. In one run the ratio of oxygen to methane feed wm varied a t about 1850 cubic feet per hour per cubic foot space velocity, and in the other run the ratio was varied at about one half this space velocity. Most of the data included in this report were obtained using a feed gas containing about 16% nitrogen. RESULTS
8
The results of the experimental work are presented in Figures They are summarized below, according to the variables studied. Ratio of Oxygen to Carbon Feed. The ratio of oxygen t o hydrocarbon, expressed as equivalent methane fed (oxygen-c, 2 to 10 and Tables I to VI.
2783 -
I.
Table
Period No. 23-2A 23-2B 23-2C 23-3A 23-3B 23-3C 23-48 23-4B 23-40
Effect
of Ratio of Oxygen to Carbon Feed
%
WC1 Feed Ratio 0.641 0.621 0.638 0.620 0.678 0,605 0.531 0,541 0.568
Carbon Efficiency 2.54 2.49 2.46 2.44 2.36 2.43 2.18 2.24 2.27
CI Converted 97.7 95.5 95.9 94.6 90.8 93.2 83.3 85.4 87.5
Gas Period Feed Preheat, No. F. 24-3 1186 24-4 1175 24-5 1184 24-6 1177 25-3 1170 25-5 1162 25-6 1162 26-5 1166 6-3 1159 27-5 1164 6 -4 1154 28-7 930 9-4 959 28-1 1 948 9-2 963
Carbon Formation, % of Inlet C 0.035 0.0
0.012 0.003 0.020 0.011 0.240 0 200 0.094
Operating Conditions 1.1 carbon number feed gas 267 lb./sq. inch gage Reactor plus burner heat loss, 65,000 B.t.u./hour Space velocity, 1900 cu. feet Ci/hour/cu. foot
1200° F. gas preheat 600" F. oxygen preheat 16% Ne in feed gas 7 . 4 % Nz in oxygen
Table
Oxygen Efficiency 3 96 4.00 3.86 3.94 4 08 4.01 4.10 4 14 4 00
Calcd. Exit Temp., O F. 2500 2635 2680 2460 2515 2490 2475 2465 2300
II.
Effect of Preheat Temperatures
0z/C1 Feed
7&onCI
Ratio 0.595 0.589 0.622 0.572 0.611 0.680 0.553 0.598 0.601 0 556 0.585 0.610 0.599 0.629 0.637
verted 90.4 91 .o 9.1, 9 86.8 91.6 89.2 84.6 88.6 89.1 84.5 88.1 88.3 88.0 89.7 92.9
Calqd.
Carbon Efficiency 2.35 2.39 2.38 2.27 2.36 2.36 2.19 2.27 2.30 2.17 2.30 2.24 2.24 2.26 2.34
Oxygen Efficiency 3.95 4.06 3.83 3.97 3.86 4.07 3.96 3.80 3.82 3.90 3.93 3.67 3.75 3.60 3.67
TExit ynp 2430 2445 2550 2360 2525 2300 2320 2560 2620 2470 2630 2750 2530 3040 2590
Operating Conditions 600' F. oxygen preheat
250 lb./sq. inch gage Space velocity, 740 to 960 cu. feet Cl/hour/ou. foot 1.1 carbon number feed gas Reactor plus burner heat loss, 65,000 B.t.u./hour 16% nitrogen in feed gas
ratio), was varied while other operating conditions were maintained as nearly constant as possible. The effect of oxygen-C1 ratio on the percentage conversion of inlet carbon (per cent C1 conversion) is shown in Figure 2. The C1 conversion increased with increasing oxygen-CI ratio. The conversion of oxygen has always been observed to be complete. The calculated final reaction temperature, shown in Figure 3, also increased with increasing oxygen-CI ratio. In Figures 4 and 5, the moles of hydrogen plus carbon monoxide produced per atom of carbon and per mole of oxygen fed are plotted against the oxygen-C1 feed ratio. These two expressions are subsequently referred to
100
'
CALCULATED' FROM
0
HEAT BALANCE
^^^^
96
z
0
e
2
92
0
v-
Z
68 1200 'E GAS PREHEAT 600 OF. 02 PREHEAT 16 % Ne IN GAS 93 % 02 PURITY 267 LB,/SO. IN. GAGE
84
c
2000
1200 E' GAS PREHEAT 600 OF. 02 PREHEAT 16 % N2 IN GAS 9 3 % 02 PURITY 267 LB/SP. IN.GAGE
/
I
~~
.50
.54
.58 02/C,
Fiaure 9.
.62
FEED
.66
.70
.50
RATIO
Effect of Oxvaen-Ct Ratio on C, Conversion
.54
0, / G I
Figure 3.
.62
.58 FEED
.66
.70
RATIO
Effect of O x y g e n - 6 Ratio on Final TemDerature
INDUSTRIAL A N D ENGINEERING CHEMISTRY
2784 Table Period KO. 23-2.4 23-2B 23-2c 23-3.4 23-3B 23-30 23-4.4 23-4B 23-40 24-3 24-4 24-5 24-6 25-6 26-3 26-5 27-5 25-3 6-4 6-3
AtomsC1 Feed/ Hour 17.74 17.56 17.89 17.75 18.13 17.68 17.70 17.49 18.03 8.55 8.35 8.41 9.00 8.49 8.01 8.33 8.52 8.64 8 46 8.44
Period
KO.
6-3 6-4 24-3 24-4 24-5 24-6 25-3 25-5 25-6 26-5 27-5 26-4
26-4 26-7 26-8 27-3 27-4 27-6 28-4
% CI Converted
Carbon Efficiency
Oxygen Efficiency
Calcd. Exit Temp.
I ,
O F .
///{
EQUILIBRIUM
~
0
.
"- 2.4
2 +
OBSERVED
I
'
2.2
1900 CF C, /HR/CF EM) E ' GAS PFIEHEAT 600 OF: 02 PREHEAT 16 S. NZ IN GAS 9 3 % 0, PURITY !267 LB./SQ. IN GAGE
u
I
2.0
I
I Operating Conditions 1.1carbon number feed gas 250 lb./sq. inch gage Reactor plus burner heat loss, 65,000 B.t.u./hour
Operating Conditions 267 lb./sq. inch wage Reactor plus bu&r heat loss, 65,000 B.t.u./hour Space velocity, 1900 cu. feet Cl/hour/cu. foot
V. Observed Effects of Hydrocarbon Feed Composition Oz/Ci Feed Ratio
1200' F. gas preheat 600" F. pxygen preheat 16% P;z in gas
% Ci
Converted 89.1 88.1 90.4 91.0 91.9 86.8 91.6 89.2 84.5 88.5 84.5 93.1 92.5 92.6 93.2 84.5 86.8 86.0 94.4
Carbon Efficiency
Oxygen Efficiency 3.92 3.93 3.95 4.06 3.83 3.97 3.86 4.07 3.96 3.80 3.90 4.21 3.68 3.82 3.77 4.19 4.07 4.21 3.72
Calcd. Exit Temp,, O F. 2620 2630 2430 2445 2550 2360 2525
2300 2320 2560 2470 2260 2610 2680 2680 2410 2680 2470 2760
I
,
I
I
IV. Effect of Hydrocarbon Feed Composition on Equilibrium OF Partial Combustion Process
Feed Gas CarbonSo. Atoms C/ Mole H C 1.07 1.07 1.08 1.09 1.10 1.07 1.09 1.06 1.10 1.08 1.11 1.19 1.18 1.19 1.19 1.19 1.18 1.18 1.17
I I
2.8
2.6
120O3 F. gas preheat GOOo F. oxygen preheat 16% N1 in gas 7.47& Xz in oxygen
fable
Effect of Space Velocity
OdCi Feed Ratio
1200° F. gas preheat 60O0 F. oxygen preheat 16% in feed gas
Table
111.
Vol. 43, No. 12
.54
30
.58
O2 /GI
Figure 4.
Effect
.62
FEED
.66
10
RATIO
OF Oxygen-C1 Ratio on Carbon Efficiency
as the carbon and oxygen efficiency of the process. In Figure 6, the hydrogen-carbon monoxide ratio in the product gas is shown to be virt'ually independent of the oxygen-C1 feed ratio. In each of these figures, a corresponding curve based on the attainment of thermodynamic equilibrium has been included. The experimental data for Figures 2 to 6 are summarized in Table I. Preheat. The effect of preheat temperature of natural gas is shown in Figure 7 and Table 11. A lower temperature resulted in lower conversion of carbon a t otherwise constant conditions. Exaniination of Table I1 shows that lower carbon and oxygen efficiencies occurred a t lower preheat temperatures. Space Velocities. The effect of space velocity, expressed as standard cubic feet of hydrocarbon (as methane) feed/(hour) (cubic foot of reactor volume) is shown in Figure 8 and Table 111. An increase in space velocit,y increased conversion as well as carbon and oxygen efficiency. Feed Gas Composition. HEAVY H Y D R O C I R B O N S . The effect of variations in the hydrocarbon composition of natural gas feed is shown in Figure 9 and Table V. An increase in the percentage of heavy hydrocarbons resulted in greater carbon conversion. Complete analyses of the feed gas for the runs reported are not available. A mass spectrometer was installed after the work reported here mas complete and feed gas samples taken over a 3day period m-ere analyzed. These data, included in Table V,
1900 CF
1200 'E
~
600
Operating Conditions 250 to 300 lb./sq. inch gage Reactor plus burner heat loas, 65,000 B.t.u./hour Space velocity, 740 to 960 cu. feet Cl/hour/cu. foot
OF
GI /HR/CF GAS PREHEAT 02 PREHEAT
Feed Gas Analyses (Comparable t o runs 6-3 and 6-4) Sample
KO. 1
2 3 4
5
; 8 9
Feed Gas Carbon KO. Atoms C / l I o l e HC 1.15 1.10 1.09 1.11 1.09 1.09 1.10 1.10 1.16
Average Corrected to 100% hydrocarbon basis
C1 90.5 91.1 90.2 91.7 90.7 90.5 90.5 90.9 91.8
Mole Per Cent Cz Ca 5.77 1.81 5.50 1.80 5.54 1 78 5.74 1.63 5,29 1.64 5.36 1.73 5.47 1.95 5.41 1.73 6.66 1.48
CI' 0.66 0.64 0.64 0.59 0.61 0.61 0.64 0.63 0.60
90.9
5.52
1.73
0.62
92.03
5.59
1.75
0.63
H?/C 1.90 1.90 1.90 1.90 1.90 1.90 1.90 1.90 1.91 50
Figure 5.
.54
.5B
.62
.66
70
Effect of Oxygen-C1 Ratio on Oxygen EFFiciency
.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
December 1951 Table Period
No.
OdCi Feed Ratio
VI.
Effect of Steam and Nitrogen Addition
HzO/Ci Feed Ratio
Carbon Efficiency
%o:!
verted A.
Oxygen Effioiency
STDAM ADDITION 2.87 2.08 3.21 2.34
Hn/CO Product [Gas Ratio
Calcd. Exit Temp.,
and the water gas reaction:
HzO
F.
+ CO = Hz + COz
(3)
The equilibrium expressions for any two of the above reactions, together with the material-balance equations for carbon, hydrogen, and oxygen, are Operating Conditions required to determine the composition of the gases 250 lb./sq. inch gage 60' F. oxygen preheat in the system a t equilibrium. Space velocity 620 t o 550 cu. feet Cl/hour/cu. foot 0% Nz in feed gas Reactor DIUS burner heat loss, 65.000 B.t.u./hour 95% 0 2 uuritv In constructing the equilibrium curves for i .ls o&on number feed gas Figures 2 to 6, allowance w&s made for the heat Gas Calcd. loss for the burner and combustion chamber. Oa/Ci N d C i Exit PreOx gen Thus, they are on a basis comparable with the Period heat, Preteat, Feed Feed Carbon Oxygen Temp., No. a F. F. Ratio Ratio verted E5ciency Efficiency F. experimental results. However, the hydrocarbon B. NITROQEN ADDITION feed was assumed to be pure methane rather than 2.34 3.84 2730 92.8 0.610 0 60 9-6 970 a mixture containing the equivalent of about 10% 92.6 2.32 3.81 2680 0.609 0.252 600 1162 26-7 ethane which was actually used. Operating Conditions Ratio of Oxygen to Carbon Feed. The observed Space velocity. 830 CU. feet Cl/hour/cu. foot 1,18carbon number feed gas CI conversion, shown in Figure 2, Jvas always Reactor plus burner heat loss, 65,000 B.t.u./hour 250 lb./sq. inch gage No steam addition below the corresponding equilibrium value, although both exhibited similar trends with oxygenC1 ratio. As equilibrium conversion was not obindicate that although the carbon number varied 3t3% the tained, it was concluded that the partial combustion of methane hydrogen-carbon ratio was essentially unchanged. and oxygen is rate-controlled. NITROGEN ADDITION. No experimental data are available in which the nitrogen content of the feed was varied independently of other variables. STEAMADDITION. The effect of steam addition is shown by the data in Table VI. The purpose and effect of steam addition to the feed were primarily to raise the hydrogen-carbon monoxide ratio of the product gas. In addition, it resulted in lower conversion and efficienciesa t otherwise constant conditions. Carbon Formation. The effect of oxygen-C1 ratio on carbon 750-950 CF Cl/HWCF formation is shown i,n Figure 10. Carbon formation was in600 'E O2 PREHEAT significant a t oxygen-CI feed ratios above 0.58, at the conditions a I2OOoF: GAS PREHEAT 0 950'F: GAS PREHEAT employed. Below this value, carbon increased rapidly with I decreasing oxygen-C1 ratio. .XI .54 .58 .62 .66 .70 32-2 32-4
0.725 0.729
1.99 0.40
92.0 98.5
2.61 1.89
2200 2600
802
O
O2
DiSCUSSlON OF RESULTS
Figure
When carbon and oxygen are assumed to be absent from the final mixture, equilibrium in the product gas may be expressed in terms of the reforming reactions: CHd
+ HZO
i=
GO
+ 3H2
(1)
1900 CF CI I H R I C F 1200 'E GAS PREHEAT 600 *E 02 PREHEAT 1 6 % N2 IN GAS
50
34
.58 0,
Figure
/ GI
.66
.62 FEED
.m
RATIO
6. Effect of Oxygen-CI Ratio on Hydrogen-Carbon Monoxide Ratio
/ CI
FEED RATIO
7. Effect of Preheat Temperature on CI Conversion
In practice, complete oxygen conversion was always obtained, and the yields of water and carbon dioxide were greater than those predicted by equilibrium. These facts indicate that the partial combustion process does not take place in the single step CHa
+ '/zOz
= GO
+ 2Hz
(4)
as incomplete conversion according to this equation would result in both residual methane and residual oxygen, without the formation of carbon dioxide and water. This suggests a reaction mechanism of two stages, in which the first step is the rapid consumption of all the oxygen and only a portion of the methane, producing large amounts of water or carbon dioxide, or both. This is followed by a second, and rate-controlling step, which comprises reforming of the residual methane with carbon dioxide or water, according to Equations 1 and 2. Part of the sensible heat liberated by the highly exothermic first-stage reaction is absorbed by the endothermic reforming reactions. The over-all reaction' remains, however, strongly exothermic. An important consequence of this reaction mechanism is that high conversions of methane will be favored by high temperature from the standpoint of both reaction rate and equilibrium. In addition, failure to obtain equilibrium conversions will result in low yields of hydrogen plus carbon monoxide, since a portion of the methane converted will appear as undesirable water and carbon dioxide. I Figure 3 illustrates one result of the postulated reaction mech-
Vol. 43, No. 12
INDUSTRIAL AND ENGINEERING CHEMISTRY
2786
PI I I 50
54
O2
Figure 8.
1
1I
/ GI
0 750-950 CF C~;HR/CF
.62
.58
FEED
.66
I
.70
RATIO
Effect of Space Velocity on CI Conversion
anism. The calculated reaction temperature was above equilibrium because the endothermic reforming reactions under the experimental conditions employed had not proceeded to completion. The carbon efficiency, shown in Figure 4,was belon equilibrium but exhibited an increase similar to the carbon efficiency at equilibrium with increasing oxygen-C1 feed ratio. At oaygenC1 ratios higher than those studied, both the equilibrium and observed carbon efficiency would begin to decrease as an increasing amount of methane was converted to carbon dioxide and water. The observed oxygen efficiency also was less than equilibrium and decreased with increasing oxygen-C1ratio (Figure 5). The equilibrium value of oxygen efficiency also decreases with increasing oxygen-C, ratio, At ratios lower than those studied, however, oxygen efficiency begins to decrease. This occurs when insufficient oxygen is supplied to furnish the high temperatures necessary for high conversion of water, carbon dioxide, and residual methane to hydrogen plus carbon monoxide. The basic significance of the plots shown in Figures 4 and 5 is that the optimum oxygen-CI feed ratio to be employed, a t otherwise fixed operating conditions, will be between the points of maximum carbon efficiency and maximum oxygen efficiency. The exact location of the optimum is dependent upon economic factors such as the relative costs of oxygen and natural gas. The fact that the observed efficiencies were significantly below theoretical again reflects the fact that water and carbon dioxide are produced in excess of their equilibrium amounts. The experimental results obtained are limited not only to the particular set of operating conditions, but also to the combustion chamber design actually utilized. For example, if conversion were increased by providing greater contact time without greater heat loss per
mole of reactants, it would be expected that the carbon and oxygen efficiencies would be improved and the optimum oxygenCI ratio lowered. Any factor that improves the approach toward reforming equilibrium will increase both the oxygen and carbon efficiency of the process by reducing the final concentration of water and carbon dioxide which appear in excessive amounts when equilibrium is not att'ained. The observed hydrogen-carbon monoxide ratio of the product gas, shorn in Figure 6, was lower than the calculated equilibrium value. Part of this difference was due to the presence of hydrocarbons heavier than methane in the natural gas used in the experimental work. The carbon number of the hydrocarbon feed, provided only paraffins are present, exercises a direct influence on the hydrogen-carbon monoxide ratio of the product gas because t'he hydrogen-carbon ratio of the parafFin hydrocarbons decreases viith increasing chain length. There was no apparent trend of the observed hydrogen-carbon monoxide ratio with oxygen-C1 feed ratio. An explanation of the behavior of the observed hydrogen-carbon monoxide ratio is complicated by the fact that the water gas shift reaction H20
+ CO = H? -t CO2
(3)
may occur in the product gas stream during cooling. That this does occur is suggested by the fact that the observed ratio of the water gas shift components corresponds to equilibrium a t a temperature lower than the calculated final temperature. As water gas shifting during cooling would tend to increase the hydrogen-carbon monoxide ratio, the ratio before cooling a t combustion chamber exit conditions must have been lovier than t,he equilibrium ratio, because after the shifting the hydrogen-carbon monoxide ratio was still below the water gas equilibrium value. On the basis of these data, it is concluded that reforming equilibria (Equations 1 and 2) were not attained under the conditions used. Preheat Temperature. As shown in Figure 7 , an increase in preheat t,emperature of natural gas increased Ct conversion. This effect is in accord with the suggested reaction mechanism, as the reforming reactions are endothermic and are favored by higher temperature from the standpoint of both equilibrium and reaction rate. Any change, such as improved insulation or increased oxygen preheat, which raises the temperature level of the combustion chamber, should have a similar effect on conversion. Space Velocity. The increase in conversion which accompanied an increase in space velocity has been attributed to an increase in final reaction temperature which more than off set the effect of decreased residence time. As these experiments were performed in a fixed reactor, the over-all heat loss per unit of reactant fed was greater a t the lower space velocity. This would result in a higher final t,emperature for the higher space velocity. The twofold change in space velocity represented in the data of
1900 CF C, /HR/CF 1200 'E GAS PREHEAT 600 'E 02 PREHEAT
1
.
I
.50
.54
O2
Figure 9.
.62
.58
I GI
FEE0
.66
.io
Effect of Hydrocarbon Feed Composition on Conversion
L
.50
RATIO
CI
.54
O2
Figure 10.
62
.58
/ Cl
FEED
.66
.70
RATIO
Effect of Oxygen-C1 Ratio on Carbon Formation
December 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 8 corresponds to a difference of about 150' F. in calculated final temperature. It seems likely that this could account for the observed higher conversion a t the greater space velocity. It is recognixed that the conversion would not continue to increase indefinitely as space velocity is increased. With further increases in space velocity, the effect on final reaction temperature would become smaller as the heat loss became a negligible fraction of the total heat release, The residence time, on the other hand, would continue to decline inversely with increasing space velocity and ultimately result in lower conversion. Feed Gas Composition. HEAVY HYDROCARBONS. The equilibrium calculation for Figures 2 to 6 were based on a hydrocarbon feed consisting entirely of methane. The effect of including 10% ethane, based on total hydrocarbon, on the equilibrium values for several variables is shown in Table IV. The presence of the heavier hydrocarbon increased the final reaction temperature and the equilibrium carbon conversion. The heavy hydrocarbons decreased the carbon efficiency and the oxygen efficiency slightly, and significantly lowered the hydrogen-carbon monoxide ratio. The observed effects of changes in the hydrocarbon composition of the feed, shown in Figure 9 and Table V, were similar to those predicted from equilibrium-that is, higher over-all conversion but poorer efficiencies at higher concentrations of heavy hydrocarbons. The higher carbon conversions obtainable with heavy hydrocarbons are due primarily to the higher reaction temperatures, which result from the fact that the heat of combustion per atom of carbon increases with increasing chain length. The poorer carbon and oxygen efficiencies result from the lower hydrogen content of the heavier hydrocarbons. NITROGENADDITION.When nitrogen is included in the reactor feed a t conditions of constant preheat temperature and total pressure, it reduces the final reaction temperature and lowers reactant concentrations. The results of two tests, in which the nitrogen content of the feed was varied, are tabulated in Table VI. Approximately the same conversion and yields were obtained in each period, despite a higher gas and oxygen preheat during the period of nitrogen addition. -4s the effect of higher preheat alone is to increase conversion, it is apparent that the addition of nitrogen in this case suppressed the reaction rate. This occurred primarily because of the reduction in final temperature which accompanied the nitrogen addition. STEAM ADDITION. The addition of steam to the reacting system permits variation and control of the hydrogen-carbon monoxide ratio of the product gas, by affecting the course of the water gas shift reaction Equation 3 It also is a reactant in the reforming reaction (l), and should, therefore, shift the equilibrium to increase methane conversion. Actually, methane conversion is reduced because the addition of steam a t preheat temperature suppresses final reaction temperatures and reduces methane concentrations, both of which reduce the reaction rate. The data in Table VI show that a t water41 ratios of 2 to 1 and a t the preheat temperatures used, CI conversion was reduced. Carbon Formation. When the oxygen-C1 ratio was decreased below a critical value a t fixed operating conditions, elemental carbon appeared in the equilibrium product of the system. The
2781
equilibrium between carbon and the components of the gas can be expressed by the equations:
c + co, = 2CO
(4)
C
(5)
+ 2H2 = CHI
Either of the above equations, in combination with any two of Equations 1 to 3 and the three elemental material-balance equations, is sufficient to determine whether carbon will be present in the system at equilibrium. In Figures 2 to 6, the critical value of oxygen-CI ratio for carbon deposition is a t the lower limit of the range, 0.48. Figure 10 shows, however, that appreciable carbon formation did occur a t 0xygen-G ratios as high as 0.58. In these experiments, where equilibrium was not obtained, carbon should not be present according to Equation 4 because the ratio of carbon dioxide to carbon monoxide was higher than equilibrium. Carbon could be present according to Equation 5 because the ratio of hydrogen to methane was lower than equilibrium. The actual presence of carbon a t the combustion chamber exit would, therefore, depend on the relative rates of these carbon-forming reactions. However, because of the temperature dependence of the equilibria of Reactions 4 and 5 , carbon formation according to Equation 4 will become thermodynamically possible as the product gas is cooled, while becoming impossible by Equation 5. Thus, carbon formation may occur in the product gas cooler if the gas is cooled too slowly through a temperature range where rate of formation is high. As the carbon production was measured after cooling of product gas, it is not known to what extent it was formed in the combustion chamber and to what extent in the transfer line. In any event, a portion of the carbon formed was deposited on the cooling surface of the transfer line where it impaired the heat transfer coefficient. No significant deposits of carbon were found in the combustion chamber following any of the runs reported. The partial combustion units have been operated without interruption for peripds up to 2000 hours. The oxygen-C1 ratio is considered to be the most important single independent variable. It is concluded that the partial combustion of methane and oxygen is a practical method of supplying synthesis gas to a hydrocarbon synthesis process. CONCLUSIONS
The partial combustion of methane and oxygen is rate-controlled. Reforming equilibria Equations 1 and 2 were not attained under the conditions used. The partial combustion of methane and oxygen is a practical method of supplying synthesis gas to a hydrocarbon synthesis process. LITERATURE CITED
(1) Mayland, B. J., and Hays, G. E., Chem. Eng. Progress, 45, 452 (1949). (2) Montgomery, C. W., and Weinberger, E. B., IND.EN@. CHEM.,40,
601-7 (1948). RECEIVED May 21, 1951.
