Reaction of Methane with Copper Oxide in a Fluidized Bed T h e reaction of methane w i t h copper oxide was investigated in a fluidized T h e data indicate t h a t copper oxide readily oxidizes methane t o carbon and hydrogen w i t h high selectivity a t a temperature level of about 1700' F. were correlated on t h e assumption t h a t t h e limiting factor is t h e reaction dioxide and water vapor w i t h methane.
solid bed. monoxide T h e data of carbon
W. K. LEWIS, E. R. GILLILAND, A N D WILLIAM A. REED M A S S A C H U S S E T T S INSTITUTE OF TECHNOLOGY, C A M B R I D G E , M A S S .
I
NDUSTRIALLY, methane is converted t o carbon monoxide and hydrogen by partial oxidation with pure oxygen or by reforming with steam or carbon dioxide. The first involves the use of oxygen and the reforming reactions have high net heat requirements. Both problems can be eliminated by the partial oxidation of methane with a metal oxide which is reoxidized with air, I n utilizing the metal oxides for the oxidation of hydrocarbons t o carbon monoxide and methane there are two principal methods of operation. I n the first the ratio of the metal oxide to the hydrocarbon is adjusted so that only the stoichiometric amount of available oxygen needed for the reaction is utilized. I n this respect i t is similar t o the use of gaseous oxygen. The second method of operation involves choosing a metal oxide whose free energy relationships are such that even at equilibrium it cannot carry the oxidation significantly beyond the desired stage. The first of these methods may be termed stoichiometric control and the second equilibrium control. Preliminary work on a number of oxides showed that in general those metal oxides which were suitable for equilibrium control had undesirable characteristics from the viewpoint of rate of reaction and energy requirements. In most cases the rates of reaction were low and considerable cracking and carbon deposition were encountered. I n addition, these metal oxides have high heats of formation. While over all the reaction involves the oxidation of the hydrocarbon to hydrogen and carbon monoxide, which is moderately exothermic, in actual operation this energy is released in two stages: (1) oxidation of the hydrocarbon by the metal oxide and (2) reoxidation of the metal oxide. Metal oxides having a high heat of formation would give off a large quantity of heat during reoxidation and would absorb a large quantity of heat during the hydrocarbon oxidation stage. This would require some method of carrying the heat over from one stage to the other. I n the case of stoichiometric control, oxides having a lower affinity for oxygen could be employed; the energy relationships in such cases are much more favorable. Copper oxide is one of the most attractive in this category. It has Nc CH, co, a relatively low energy of formation, so t h a t the heat carry-over is small; i t is relatively
cheap; i t can be easily reoxidized by air; and i t is a very active oxidizing agent. However, its activity is so great that if a n excess is employed i t will give carbon dioxide and water instead of hydrogen and carbon monoxide. As a result of the preliminary studies copper oxide was chosen as the metal oxide to be evaluated in detail. I n order t o maintain
t t t d t co
1227
Hc
I
I
L
Figure 1.
Apparatus
1228
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 41, No. 6 the necessary stoichiometric control a fluidized bed was employed t o which the copper oxide and the methane could be fed continuously in the desired ratio. T h e oxidation of methane t o carbon monoxide and hydrogen requires relatively high temperatures. Equilibrium considerations are such that temperatures of the order 1800" t o 2000" F. are required t o obtain a reasonable conversion of the methane to carbon monoxide and hydrogen. I n preliminary tests i t was found t h a t pure coppcr ovide did not opera t e well because of a rapid growth in particle size. However, by depositing the copper oxide on supports such as silica gel, alumina gel, and kaolin, satisfactory operation wa8 obtained. APPARATUS AND PROCEDURE
3
3
>.
d
a
T h e apparatus employed in this investigation is shown in Figure 1. The reactor was a n electrically heated clear quartz tube P inch in inside diameter gage Tw-entg-two and 4 feet long.
C h r o m el-A l u m e 1 thermocpuples encased in quartz protection tubes $/'/le inch outside diameter were inserted through three side arms spaced a t 1foot intervals. R o t a m e t ers, maintained at approximately 860 mm. of mercury absolute by down&earn valves to avoid any large pressure corrections, metered the inlet gas t o the reactor. A small en-
June 1949
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
1229 GLASS WOOL
DETAfL Of
U
TU8C
ASBESTOS HELD QUARTZ ,TUBE
CAP/LLARY TUB/NG
FURNACE SLIDE SUPPORT d
;"ic
-
CHROMEL ALUMEL THERMOCOUPLE
U THfRMOMErER WATER JACICX.Fr
-
u Figure 2
closed variable-speed belt conveyer fed the solid powder from a reservoir tube into the gas stream which carried the powder into the reactor. From a n enlargement at the top of the reactor a solids overflow tube led to a closed solids receiver. The exit gas, and any solid it might carry, passed into a n external solid-gas separator, a cyclone, a filter, and a saturator, and out through a wet-test meter. The solids overflow tube as shown proved very helpful in evening out pressure variations in the column. Before i t was installed the overflow solid was carried by the exit gas into the first external solid-gas separator, so t h a t surges of solid because of slugging in the reactor served as momentary blocks in the exit line and magnified the pressure variations. The principle of the overflow tube as finally adopted is common in industrial fluidization units. Some difficulty was encountered in carrying the solids from the feeder into the reactor with the entering methane, because of the low volume of the cold inlet methane. It was found t h a t by having the solid fall down a steep angle into an upward moving gas stream the solids could be carried satisfactorily. The feeder as drawn works well on powders such as silica gel with 50% greater than 200 mesh. With powders through 200 mesh difficulty was encountered with sticking in the reservoir tube, which produced a n uneven layer of solid on the belt, or with clogging in the funnel section of the feeder. The procedure in making a methane oxidation run involved analyzing the solid to be used for the percentage of available copper oxide by reduction with hydrogen. The temperature of the reactor was adjusted to the desired value and the methane rate was set to give the desired oxygen-carbon ratio for the solid feed rate employed. As the solid was introduced it was necessary t o increase the heat supply to the reactor to compensate for the heat of reaction and the sensible heat of the inlet stream. A number of reforming runs in which methane reacted with carbon dioxide or steam in the presence of the fluidized powder were made in the same apparatus. I n this case the solids feed line and the solid overflow tubes were plugged off, The solid was introduced into the reactor from the top and
INDUSTRIAL AND ENGINEERING CHEMISTRY
1230
Q. D
L
-
I
Vol. 41, No. 6 was initially fluidized with nitrogen; and after satisfactory conditions had been obtained the nitrogen was replaced by the desired gas mixture. The gas samples were analyzed in a standard Fisher laboratory model analyzer with mercury as a confining liquid. The carbon dioxide was absorbed in potassium hydroxide, the unsaturates in Lusorbent, and oxygen by chromous chloride. Hydrogen and carbon monoxide were determined by combustion, followed by absorption of the resulting carbon dioxide in potassium hydroxide, and methane by slow combustion. Nitrogen was ordinarily determined by difference, but was checked a t intervals by ahsorbing the remaining oxygen. The solid samples mere analyzed for oxidizing power, expressed as per cent copper oxide, by measuring the hydrogen oxidized volumetrically. This method was adopted instead of the conventional titration methods because i t measured the oxidizing power of the solid under conditions very nearly approximating those in the reactor. When titration methods were tried it was found t h a t all the copper was not dissolved from the silica gel, so there was the possibility t h a t the oxidizing power measured in such a manner would differ from the oxidizing power in the reactor. Figure 2 shou s tb drawing of the apparatus used in the solids analysis. To make a n analysis the quartz U-tube was weighed, filled with sample, and weighed again. Glass wool plugs were placed in each leg t o be sure the solid would remain in the portion of the tube t o be heated later. The U-tube was connected with rubber tubing t o the buret and gas holder. About 100 cc. of nitrogen passed through the U-tube and flushed out any oxygen. The furnace was then slid around the U-tube and the temperature allowed to become constant. During the heating up time the excess nitrogen was alIowed to expand into the measuring buret. When the temperature had become constant, at about 1200" F., this excess nitrogen was thrown away and hydrogen was drawn into the buret. After its volume, temperature, and pressure were recorded, the hydrogen 15 as passed through the U-tube. During the first pass no gas was observed flowing into the gas holder until volumc very nearly equal t o thp volume representing the final oxidizing power of the oxide had been passed into the tube. After thiee or four passes, a t 5 t o 10 minutes per pass, a fairly constant volume was obtained and this was recorded.
1231
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1949
2+ % +
q x?
2l-
5 i= 0
Lu
d z 0 v)
i= 0
E TO00 TEMP.
OF.
(AVERAGE)
Figure 3. Reaction of Methane w i t h Fluidized 15% Copper Oxide on Silica Gel 0. 0.
1200
=
Runs WR21-WR25, O / C 1.0. CHI rate liter per minute Runs WR13, 14, and 15, OIC 1.2 to 1.3. rate C 0.7 ilter per minute
=
1600
1800
Figure 4. Reaction of Methane w i t h Fluidized 15% Copper Oxide on Silica Gel
= 1.0
Runs WR21 to WR25
CHd
When a reduced solid was to be analyzed, care was taken to keep air from coming in contact with it. The U-tube was flushed with nitrogen and weighed. The solid sample in a flask was then transferred under a stream of nitrogen into the U-tube. The tube was weighed again, packed with glass wool, and quickly attached t o the buret and gas holder, both of which were previously flushed with nitrogen. One hundred cubic centimeters of nitrogen were then passed through the U-tube and the analysis was concluded as described before. Preparation of Solids. For the fluidized runs, with the exception of two runs where physical mixtures were employed, the copper oxide was deposited on the carrier powder by mixing the carrier with a solution of C.P. copper nitrate and decomposing this first over Meker burners and finally, when only traces of nitrogen oxide were coming off, in a muffle furnace at temperatures from 1000" t o 1500" F. The carriers were ground to near the desired size befork the copper oxide was deposited. Little particle change occurred during the deposition of the copper oxide, so t h a t only a small amount of grinding with a mortar and pestle mas necessary in order t o adjust the size to approximately 50% 100 to 200 mesh, and 5070 through 200 mesh.
'
OO
0.2 E
RESULTS
Figure 5.
The results of the continuous runs made with methane and copper oxide deposited on silica gel (batch 1) are given in Table I. The fraction of methane decomposed is plotted against temperature in Figure 3 for the runs in this series where the methane and solids rates were held essentially constant t o give a molal oxygencarbon ratio in the product gas near 1.0, and for three runs in which the molal oxygen-carbon ratio was between 1.2 and 1.3. For the former runs the fraction selectivity, Hz CO/Hz CO HzO Cot, is plotted against temperature in Figure 4. Table I1 gives the results of related runs with methane and copper oxide with various carriers. T h e results of the runs in which methane was reformed with carbon dioxide and steam are given in Tables I11 and IV, respectively. The effect of the partial pressure of the various diluents on the fraction of methane decomposed in the methanecarbon dioxide runs is shown in Figure 5. Runs with Copper Oxide. Runs with Methane and 15% Copper Oxide Deposited on Silica Gel (Batch 1). The data taken during the runs with methane and 15% copper oxide deposited on silica gel (batch 1) given in Table I are sufficient for direct calculation of each gas and solid rate in and out of the reactor, with the
+
1400
TEMP OE(AVERAGE)
+
+
+
0. N2 dilution.
0.4
0.6
C8
D I L U E M , ATM.
Effect of Diluents A.
CO dilution.
H1 dilution
exception of the water and the free carbon out, which were calculated by material balances. A comparison of the carbon coming into the reactor as methane and t h a t going out as carbon dioxide, carbon monoxide, and unreacted methane shows there was no significant amount of carbon deposited but t h a t the inlet carbon was usually higher than the exit carbon by about 2%. It is possible t h a t about 2% of the total carbon was actually deposited as free carbon, b u t this value lies within the accuracy of the data, and could, for example, be explained by an error of about the same percentage in either the methane or the d r y exit gas rate. Inasmuch as the carbon deposition was not significant there were two possible methods of calculating the water formed. First, the difference between the hydrogen in as methane and the hydrogen out as hydrogen and the unreacted methane in the dry exit gas is equal t o the water formed. Secondly, assuming no carbon deposition, the molal ratio of hydrogen to carbon in the exit gas must be 2 t o 1 as it is in the inlet methane, and the total hydrogen out is therefore twice the total carbon out. The water
1232
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. 6
1) it is evident that roughly the first half of the runs were made with rather randomly (Catalyst, 100 grams fluidized C u on silica gel, b a t c h 1 solid) varying conditions. During Run No. WRH1-1 WRH1-2 WRHl-3 WRH1-4 WRH1-5 this peiiod the operating proce1500 1490 1385 1350 1380 1540 1530 dure was being perfected and 1430 1430 1430 1515 1505 1410 1400 1400 Bot t o in the general field as being ex0.437 0.508 0,575 0.466 0.410 Inlet C H n rate liters/min. a t i o 0 F 1 a t m . 0.587 0,501 0.542 0.396 0.418 plored. But in the runs from ~ n i eH t ~O rate,' equivalent in liters/;l;in. a t 700 F., 1 atm. 1.15 1,19 0.94 0.750 0.70 Dry exit gas rate, liters/xnin. a t 70° F., 1 a t m . WR21 through WR25 (sample Exit gas composition, % 2) the molal oxygen-carbon 4.5 5,s 6.2 6.2 6.3 coz 0.1 0.2 0.3 0.2 0.4 0 2 ratio was held as constant as 59.0 57.7 37.3 40.1 37. d Hz 12.8 13.5 4.7 5.2 4.5 possible a t a value slightly co 20.0 21 9 49.2 46.0 49.0 C-H4 2.3 greater than 1.0 by fixing the 2.2 2.3 2.4 hz (difference) __..- 2 . 2 -100 I O 100.0 100.0 100.0 100.0 methane rate a t about 1 liter 61.6 58.5 38.9 40.8 38.3 per minute and the solid rate yo H? calculated (4 X % C O -t 3 X % C O ) -2.6 -0.8 -1.6 -0.8 -0.7 % K2 calculated - % FIz b y analysis a t about 23 grams per minute, Basis, 100 moles dry exit gas as analyzed and the temperature \va3 varied 39.9 38.6 60.1 57.5 59.7 Atoms C 99.0 101.5 135.7 132.1 135.5 hfoles Hg from 1115' to 1635' F. For 24.6 22.9 17.7 18.2 17.7 Atoms 0 these runs the fraction decomHZ in = lOO(2 X 0 . 9 6 CH4 rate + Hz0 rate) 124.1 123 9 175.0 174.5 169.4 1. posed and the selectivity are d r y exit gas rate plotted in Figures 3 and 4. i\t 24.9 22.6 39.3 39.3 37.3 H 2 0 = Hz in - Hz out the low temperatures about ('2 X 0 , 9 6 CH4 r a t e i Hz0 rate) 1 3 1 . 3 121 . o 179.0 1 7 5 . 2 1 7 3 . 0 2. H2in C out 0 . 9 6 CHa rate one foul th of the niethane was 32.3 19.5 43.3 39.7 40.9 H 2 0 = H2in - H z o u t oxidized, principally to carbon 41.0 36.5 56.3 58.7 59.7 1. C in = 100 X 0.96 CH4 rate/dry exit gas rate dioxide and water. As the 38.6 60,l 39.9 59.7 57,o 2. C out in exit gas temperature increased the frac51.2 42.1 57.6 55.8 56.7 1. 0 in = 100 HzO rate in/dry exit gas rate tion of methane decomposed 49.5 45.5 57.0 57.0 55.5 2. 0 out = 0 out in dry gas + HzO b y (1) 1.40 increased rapidly, as did the 1.02 0.98 0.94 1.02 0jC ratio in 0.45 0.16 0.48 0.20 0.18 Fraction C l h decomposed carbon monoxide and hydrogen (Cod (Hq)/(CO) (IIzO) content of the gas, so that a t 0.9 1.1 1.3 1.3 1.3 For exit gas 1.0 1.2 1.0 1.3 1.3 At equilibrium the highest temperature for (PCOPHZ) 2 / ( P C 0 2 P C H 4 ) this series, 1636" F., 9070 of 0.3 0.4 0,008 0.005 0,005 For exit gas the methane was decomposed 35 250 250 35 35 For equilibrium with a selectivity of 94%. 0.5 0.6 0.6 0.4 0.4 Gas velocity a t top of reactor, ft./sec. The values for runs WR13, 14, and 1.5 are also plotted, although the molal oxygenis then the total hydrogen out in the exit gas minus t'he hydrogen carbon ratios for these runs are 1.29, 1 1 7 , and 1.20, respectively, out' in the dry exit gas. The first' method assumes that no hydroand the methane and solids rates are only about two thirds those gen remains in the reactor or goes out with the solid, and depends of the former series. At the highest temperature of 1690" F. in on both the inlet gas rate and the dry exit gas rate as well as the KRl5, the portion of methane decomposed increased to 97% with gas composition. The second method is independent of the gas rate and depends only on the composition of t,he dry exit gas and a selectivity of 90%. For both series the curves are Keaded upt'he absence of any free carbon. The discrepancy between the two Tvard and equilibrium has not been reached, so that even better values of water calculated is twice the difference between the conversion would be expected a t higher temperatures. carbon in and the carbon out, or about 4 n:oles per 100 moles of An inspection of the analyses of the inlet and exit solids shows dry exit gas; the v?-at.erby the hydrogen balance is the higher. the oxygen removal from the solid is nearly complete even in the runs a t lowest temperature. The percentage of copper oxide in The water as calculated by the hydiogen balance is arbitrarily the exit solid as determined by aiialysis decreases from 1.0 to 0.451, used in succeeding calculations, although it is not certain which as the temperature is increased from 1115' to 1635" F. Oxygen method gives the more nearly correct results. balances show that these percentages are probably high, as there Having calculated the water in the exit gas it was possible to is generally about 5% more oxygen leaving in the exit gas than is check the amount of oxygen transferred from the oxide to the exit being removed from the oxide. Thus the reduction of the copper gas. The amount of oxygen removed from the oxide was directly oxide is rapid and fairly complete even a t temperatures as low as available from the measured solids rate and the inlet and exit 1115'F. solid composition. The quantity leaving in the exit gas was calThe comparison between the actual ratios of (COz)(I-I2)/ culated by adding the various amounts leaving as water, carbon (CO)(HzO) in the exit gas and theequilibrium valuesshovvsthe exit dioxide, and carbon monoxide. The discrepancy between these gases are near equilibrium as far as the water gas reaction is contwo values varied from 0 to 10%. The oxygen rate calculated cerned over the whole range of temperatures. On the average, from the exit rate and composition is taken as being most nearly the actual values of the ratio are about 50% greater than the correct and is therefore used in calculating the molal oxygenequilibrium constants a t the corresponding temperatures, but carbon ratio in the exit gas. this is a small factor in view of the sensitivity of the constant The fraction of methane decomposed was calculated on the to relatively small changes in the gas composition. A possible basis of the analysis of dry exit gas alone. The selectivity, explanation is that steam reacts slightly faster with methane Ha CO/Ha CO -f- CO2 HzO, was calculated to give an over the catalyst than carbon dioxide does, but the small disindication of the fraction of the decomposed methane which has crepancy hardly justifies such a conclusion. converted to the desired carbon monoxide and hydrogen. The A comparison between the ratios ( P c o P E , ) ~ / ( P c H , P c o , ) for ratios (COZ)(HX)/(CO)(H~O) and ( P c o P H ~ ) ~ / P c H ~for P cthe o ~ exit the exit gas and the equilibrium values shows that the regas are compared to the corresponding equilibrium values. forming reactions have hardly started a t the lower temperaFrom the results (Table I) of the steady-state portion of the ture, and that a t the highest temperature the actual ratio is only continuous runs using 15% copper oxide and 85% silica gel (batch Table IV.
Reforming Methane w i t h Water
~
c _
I
+
+
+
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1949
one thirtieth t h a t at equilibrium. As the product of the equilibrium constants for the methane-carbon dioxide and the water gas reactions is equal t o the equilibrium constant for the methane-steam reaction, it is evident that this latter reaction also cannot be a t equilibrium. One may conclude from the high temperature coefficient of the over-all reaction, CHd CuO CO 2H2 Cu, under the conditions encountered t h a t gaseous diffusion is not the ratedetermining factor, as the rate would then vary approximately as the square root of the absolute temperature. Instead, the marked temperature dependence indicates chemical reaction to be controlling. Different steps in the over-all reaction, such as the reduction of the copper oxide, could well be controlled by diffusion, but if such is the case, the rates for these steps are so fast t h a t the over-all reaction rate is not appreciably affected. Whatever the mechanism of the over-all reaction, i t is obvious that carbon dioxide and water are important intermediates. I n the low temperature runs the major portion of the oxygen from t h e oxide appears in the exit gas as carbon dioxide and water and only a trace as carbon monoxide and hydrogen. The formation of any large amount of carbon monoxide and hydrogen would therefore have to proceed via carbon dioxide and water. I n the runs at higher temperatures the fraction of the oxygen appearing as carbon monoxide increases, but if the reaction were carried to equilibrium some additional carbon monoxide and hydrogen would be formed from carbon dioxide and water reacting with methane. Because of the relative rapidity with which copper oxide oxidizes hydrogen and carbon monoxide as compared to methane, it seems likely t h a t if the primary reaction were CH4 CuO + CO 2Hz Cu, this would be immediately followed by the reactionsCO CuOCO, Cu and H, CuO +HzO Cu. Hence i t is probable t h a t the first phase of the reaction results in t h e formation of carbon dioxide and water. The exact manner i n which these are formed is only a matter of speculation for any number of intermediates such as methanediol, methanol, formaldehyde, carbon monoxide, and water could be involved. The second phase of the over-all reaction would then be the reforming reactions. At low temperatures the rate of reduction of copper oxide with methane under the conditions in the reactor is very fast compared to the reforming reactions. I n run WR22 93% of the oxide was reduced whereas only traces of carbon monoxide and hydrogen were formed. All the phenomena noted concerning the reaction of methanecopper oxide on silica gel can be explained satisfactorily on the basis of a fast reduction of the copper oxide with the production of carbon dioxide and water, which in turn react with the remaining methane in the rate-determining step controlled by chemical reaction, t o form carbon monoxide and hydrogen. Carbon formation is not appreciable even in runs whose molal oxygen-carbon ratios were considerably less than 1.0. For example, a t a temperature of 1520’ F. if the oxygen-carbon ratio drops t o 0.95, at equilibrium 7% of the methane would be cracked t o carbon. However, in all the runs the (CO)2/CO2 ratio in the exit gas is less than that a t equilibrium with carbon, so t h a t any carbon deposition would have t o come from methane cracking and not from the reaction 2COC COP. Instead, under conditions in the runs this reaction would tend t o go in the opposite direction and keep carbon from being deposited. Some discussion of the fluidizing conditions is necessary for a complete appraisal of the results. As the reactor was only 1 inch in diameter, some “slugging” could not be avoided. Slugging, in contrast t o a uniform dispersal of the solid in the gas, consists of the formation of a number of slugs of solid. The slugs are separated by gas spaces, and pass upwards a t rates comparable t o t h a t of t h e gas stream. Obviously the contact between the gas and solid is not as good under such circumstances as when there is no slugging, but as a slug of solid moves upward i t con-
+
+
+ +
*
+
+
+
+
+
+
+
1233
tinually loses solid from the bottom, so that the solid in the slug is essentially poured through the gas space below it. I n an average run with copper on silica gel the slugging was sufficient t o cause a pressure variation a t the bottom of the column of about 0.5 em. of mercury out of a total pressure of about 4 em. when read on a n undamped mercury manometer. OTHER METHANE-COPPER OXIDE Ru~is. I n Table I1 the results are given of various methane-copper oxide runs in which the following solids were used as oxidants. 1. 15% CuO on silica gel, batch 2 2. 10% CuO depositedon silica gel of 20% 3. 700 prams of 10% CuO on silica gel plus 100 grams N~O on silica gei 4. A physical mixture of 25% CuO, 75% alumina gel 5. A physical mixture of 15% CuO, 85% silica gel 6. 10% CuO on Filtrol clay 7. 10% CuO on periclase 8. 10% CuO on powdered kaolin brick 9. 10% CuO on fused magnesia IO. 10%CuO on alumina gel
A comparison of the different carriers shows t h a t silica gel is far superior to all others tried. Thus, at approximately 1550’ F. the fraction of methane decomposed over 15% copper oxide on silica gel is 0.7, whereas i t is only about 0.25 for 10% copper on periclase and 10% copper oxide on fused magnesia, 0.30 for 10% copper oxide on powdered kaolin brick, and 0.47 for 10% copper oxide on alumina gel. The smaller fractions decomposed do not seem t o be caused by the rate of reduction of t h e copper oxide, as this is nearly complete in all cases, but rather by the reforming reaction’s not being catalyzed by copper on the other carriers. Runs WRQ, 10, and 11 were made with physical mixtures of ground, rod form, copper oxide, and silica gel or alumina gel in the hope of determining whether or not the deposition of the copper oxide on the carrier was necessary to have a highly catalytic solid. Unfortunately, agglomeration of the copper particles into clumps 0.125 inch or greater in diameter made satisfactory fluidization impossible. The deposition thus becomes a necessity for good fluidization. Because nickel is an excellent reforming catalyst, it was hoped that the addition of some nickel t o copper oxide on silica gel would increase the conversion of methane. Accordingly, runs WRN 1 and WRN2 were made with a mixture containing 7 parts of 10% copper oxide on silica gel and 1 part of 20% nickel oxide on silica gel. A comparison with WRl2, made with 10% copper oxide on silica gel from the same batch as t h a t used in runs W R N l and 2, shows t h a t the conversion is only slightly improved with the nickel. FLUIDIZATION CHARACTERISTICS WITH VARIOUS CARRIERS. The fluidization obtained with each carrier was observed a t room temperature in a glass duplicate of the reactor. The main difficulty encountered was the slugging, which varied with the gas velocity and the carrier employed. Silica gel, 50% 100 t o 200 mesh, 50% below 200 mesh, gave the least amount of slugging of the carriers tried, and by adjusting the velocity t o around 1 foot per second the slugging could be minimized. Various percentages of copper oxide deposited on silica gel were fluidized and it was found t h a t the good fluidization characteristic of silica gel alone was maintained with 15% copper oxide-85% silica gel, but a t 20% copper oxide and above the fluidization decreased in quality. With Filtrol clay below 200 mesh the fluidization was excellent a t velocities of around 0.5 t o 1foot per second; b u t a t 1500 F. the Filtrol clay with copper oxide deposited on it became sticky and stopped fluidizing altogether. The powdered kaolin brick 100 mesh t o below 200 mesh flowed very well when pouring but slugged very badly when fluidized in the dummy reactor. The slugging was reduced by grinding the mixture t o practically all below 200 mesh, but i t was still far from equal to silica gel. Periclase and fused magnesia, mostly under 200 mesh, were somewhat better than the kaolin powder, but still not equal t o O
INDUSTRIAL AND ENGINEERING CHEMISTRY
1234 Table V.
Reforming Methane w i t h Carbon Dioxide
( D a t a of hlains. R u n No.
Catalyst, C u o n silica gel, batch 2 solid) 9 11
Temperature, F. W t of catalyst grams InlAt gas rate, l;ters/min. a t 70° F., 1 a t m .
D r y exit gas rate, liters/min. a t 70' F..
Basis: 100 moles of dry exit gas as a n a lyzed, a t o m s C
Hn
0
( 2 X % C H I in) inlet gas r a t e , d r y exit ga? r a t e = Hz in ( I ) H2 o u t 2 X % CH4in2. Hzin = C out (% C H a i n % COzin) HzO o u t = 1% in ( 2 ) Hz o u t H20 if water gas equilibrium was satisfied
( Hz0 o u t H2in
=
-
-
in
=
C out
(5%
CHd in % CO2 in) inlet gas r a t e dry exit gas r a t e
15
1470 150 0.665
1580 150 0,876
1595 150 0,911
61.7 46.1 2.2 100 0
48.0 34.1 17.9 __
26 3 50 3 21.4 __ 100.0 1.36
1.11
73.2 73.2 64 6
) 80.6 7.4
78'9 5,7 8.6 76.4 75.2
100 0
1.44
47.5 59.13 37.8
50.8 26.4 55.6
58.4 -1.2
37.9 11.6
55.5
36.6 10.2 4.5
-4.1
0.5 60.0
47.5
52.6 50.8
in = (2 X % COz in) inlet gas r a t e 71, 41.5 67.3 dry exit gas r a t e 0 o u t [HzO b y (111 or zero 72.0 37.6 67.1 0.94 0.63 1.28 O / C in 0.37 0.65 0.87 Fraction of CHa decomposed O/C outa 0.965 0.802 1.178 Based o n assumption t h a t Hz0 o u t satisfied water gas equilibrium, a n d n o free 02 out. Q
silica gel. The marked difference with these was the much greater density, so that the pressure drop across the fluidizer was more than double the value when silica gel 'ryas being fluidized. The variations in the quality of fluidization obtained with the various solids must have a considerable effect on the rates of reaction compared in the previous section, but the differences noted there are so great that only a small fraction of them can be attributed to the variations in fluidization, particularly as the contact is sufficient to give almost complete reduction of the copper oxide. TEXPERATURE RESISTAWE OF SOLIDS.Some particle growth occurred with the first batch of 15% copper oxide on silica gel. This behavior was substantiated by the appearance of a similar growth in a second batch of the powder. The groTryth continued to appear when the copper oxide content was reduced to 12%. None of the other carriers used, Filtrol excepted, gave an?; evidence of particle growth, although they were not used a sufficient number of times t o prove that no substantial particle growth would take place with prolonged use. Reforming Methane with Carbon Dioxide over Copper on Silica Gel. Three types of runs were made in the study of the methanecarbon dioxide reaction over the reduced copper on silica gel powder from run TVR25. Holding the molal ratio of methane to carbon dioxide constant at 1.0, runs were made (1) with increasing nitrogen dilution, (2) with increasing carbon monoxide dilution, and (3) xvith increasing hydrogen dilution. The methane and carbon dioxide rates were reduced as the diluent rate was increased, so as to maintain the exit gas rate approximately constant. One hundred ,grams of the powder were used as a catalyst, for this was sufficient for good fluidization yet not enough to produce excessive carry-over during
Vol. 41, No. 6
a series of runs. The reactor temperature was set a t 1500° F., as this gave conversions with the above conditions of around 50(r, and thereby put all the important gas components in the range where the accuracy of the gas analysis was good. The results were calculated in much the same manner as in the methane-copper oxide runs. The main differences in calculation were: the molal hydrogen-carbon ratio of the inlet gas was no longer fixed a t 2 to 1, and the inlet oxygen was all in the gas phase and was known with the same degree of accuracy as that of the other components. T h e x a t e r \\*as again calculated as the difference between the total hydrogen coming into the reactor and the total hydrogen leaving in the dry exit gas. The hydrogen in was calculated (1) from the inlet rates and analyses, and (2) from the carbon out times the inlet ratio of hydrogen to carbon. The first method gives a value dependent on both inlet and exit rates, while the second is dependent on t,he inlet rates for the calculation of the inlet ratio of hydrogen to carbon, the exit gas analysis, and the asiumption t h a t no free carbon was formed. This latter assumption is good in view of the close agreement between the total carbon in and the total carbon out in the dry exit gas. The average percentage difference is about 1.5, and neither method gave consistently higher results. The agreement of the values for water was also good; the average difference was about 1 mole of water per 100 moles of dry exit gas. again neither value was consistently higher than the other. The average discrepancy between t'he total oxygen out and the oxygen in, using t'he water calculated by the first method, was about 2% of the values. A preliminary examination of the results shows that chemical reaction rather than gaseous diffusion is the rate-determining process in the reaction of methane with carbon dioxide. The primary evidence in this direct,ion is the high temperature coefficient of the reaction. The reforming runs in Table I V were all made near 1500" F., so t h a t the temperature dependence is not too obvious, but runs in which the only known variation was a slight temperature difference showed considerably different amounts or methane decomposition. For example, in runs WRR5-1 and 5-6 there was about 10 O F . increase in the average temperature and the fraction of methane decomposed increased from 0.46 to 0.54. Conclusive evidence is furnished by the data of ?\lains (2) 011 methane reforming with carbon dioxide under similar conditions H e found t h a t the fraction of methane decomposed increased from 0.36 t o 0.82 when the temperature increased from 1470" to 1580" F. (Table V). Finally, the copper oxide-methane runs which are correlated on the basis of the reforming reaction controlling have a high energy of activation of 48,000 calories p r y gram mole. Because t,he rate of reaction would vary approximately as the square root of the absolute temperature if gaseoub diffusion were controlling, rather than exponentially, chemical reaction must, be the limiting step. Further evidence that gaseous diffusion is not the rate-detwmining process comes from the change in the fract,ion of metharir decomposed as the type of dilution is changed. A plot of metharii, decomposed for the WRR5 runs versus the inlet partial pressurr of the diluent, in Figure 5, shows that as the inlet partial pressurc' of either nit,rogen or carbon monoxide rises from 0 to 0.7 atmosphere the fraction of methane decomposed increases from 0.6 t o 0.7. Hydrogen dilution, on the other hand, decreases the fraction of methane decomposed to about 0.25 as t'he inlet partial pressure of hydrogen rises t o 0.7 atmosphere. If gaseous diffusion were controlling, the rate of reaction would be greater with hydrogen dilution than with nitrogen or carbon monoxide dilution, because the diffusion coefficients of gases through hydrogen are generally about 3.5 times those in nitrogen or carbon monoxide. For example, the diffusion coefficient in hydrogen a t 0" C . and 1 atmosphere is 0.76 sq. cm. per second, but only 0.22 in carbon monoxide or nitrogen (3). As just the opposite effect. on the reaction rates was observed, the reaction must not be controlled by gaseous diffusion.
1235
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1949
The satisfactory correlation of the WRR runs by Equation 2 is not by itself evidence that pCH4 should not appear in the denominator. Because the inlet molal carbon dioxide to methane ratio was 1.0, p o = PCH* a t all points in the reactor (assuming no carbon deposition). Accordingly, the same correlation could have been obtained for these runs with either po or ~ C orH both ~ in -dn = PCH~(~IPCO~ +kZpHnO) the denominator of the rate equation. Evidence in favor of PO (1) dw (1 a p c ~ , bpcoz 4-CPH~O dpH, 4-e p c o ~ P N ~ ) ' in the denominator is available from the data of Mains (Table V). Qualitatively, his reforming runs with methane and carbon diwhere n = methane rate, w = weight of fluidized catalyst, p = oxide showed a much greater conversion of the carbon dioxide partial pressure, and kl, kz, a, b, c, d, e, f = constants. when methane was in excess than of methane when carbon diIntegration of such an equation required a relation between oxide was in excess. This would indicate that p~ should be in the each of the partial pressures and n, and some assumptions were denominator, so that i t would demonstrate some retarding action necessary before the relations became definite. First, the type when in excess. of flow of gas had to be assumed. Recent experiments by Kennel Quantitative evidence is afforded by the calculation of rate con( I ) and others indicate that gaseous mixing in a laboratory scale stants for his runs 11 and 15, in which the oxygen-carbon ratios fluidization unit is small, and piston flow of the gas was assumed. were 0.8 and 1.2 and the temperatures 1580" and 1595" F., reIn view of the small diameter and the relatively great length of the spectively. The k's, calculated on the assumption that a and b remained constant a t 24 and 8 irrespective of temperature, are 3.8 reactor, this seems justified. Secondly, some assumption was and 4.6. When these rate constants are reduced t o 1500' F. on necessary concerning the extent of the secondary reaction, COS the assumption that k has a temperature dependence correspondHz CO H20, for the four material balances alone are not ing to an activation energy of 48,000 gram calories per gram mole sufficient t o solve for five of the six partial pressures in terms of they become 1.6 and 1.7. T h a t the IC's calculated from his results are somewhat lower than the WRR series is substantiated by the sixth (methane). The additional assumption made was that Mains' run 9 made a t 1470" F., which gives a k of 1.2 when cprthe water gas equilibrium was satisfied a t all points in the reactor. rected to 1500" F. As he used a portion of the 15% copper.oxide In the reforming runs where the exit water was large enough that on silica gel batch 2 as catalyst rather than batch 1 used in the it could be calculated with reasonable accuracy, the ratios of WRR series, the differences in k's can easily be explained on the basis of a difference in catalytic activity. If in Mains' runs 11 and (COz)(Hz)/(CO)(H*O) a t the exit are on the average only 20% 15, pCH4were substituted in the denominator of the rate equation higher than the water gas equilibrium constant. On the other for p o , the k's corrected t o 1500" F. would have varied widely a t 4 hand, the reforming reactions were far from coming to equiand 0.8, respectively. This demonstrates that pa is necessary in librium, so i t seems likely that the secondary reaction is fast comthe denominator, and that a satisfactory correlation is possible pared to reforming reaction and that the water gas equilibrium without pCHa. was close to being satisfied throughout the reactor. The only That the rate constant is fairly insensitive to the gas rate is exceptions are the hydrogen dilution runs where the inlet gas conshown by run WRR5-7. The gas rate in this run was about onetained high carbon dioxide and high hydrogen, so that considerhalf the rate in the other runs, so the fluidization characteristics able shifting had t o occur before the water gas equilibrium could could have been changed considerably, yet the rate constant is be satisfied. With these two assumptions i t was possible to calcunot out of line with the others. The k for WRR5-7 is 3.11, which late all the partial pressures in terms of n, the methane rate. lies between the IC's for the other undiluted runs and is only 10% The data for the methane-carbon dioxide runs were well corhigher than the average for the WRR5 series. related by the equation Reforming with Methane-Steam over Copper on Silica Gel. The results of methane-steam runs WRH1-1 to 5 , listed in Table dn bCH4pO - = (2) IV, were calculated in essentially the same manner as the reformdw (1 -k 24pa ~ P H , ) ~ ing runs with carbon dioxide, but an additional check for carbon deposition was possible. The only reactions by which hydrogen When n is expressed in liters of methane per minute a t 70" F. can be produced in significant quantities without cracking of the and 1 atmosphere, w in grams of fluidized catalyst, and p in atmethane may be expressed by the equations mospheres, the average of the k's [corrected to 1500' F. assuming
Because chemical reaction is controlling, the rate-determining factors must be those relating t o conditions a t the solid surface. One equation which has been most successful in describing this type of phenomena is that of Langmuir. A Langmuir type rate equation was therefore tried, the form of which was
+
+
+
+
+
+
-
+
an Arrhenius activation energy of 48,000 gram calories per gram mole) was 2.85, with an average deviation of 6.6% and a maximum deviation of 17%. The term pa is the sum of pcoz and p a a o . The values of the coefficients of PO and PH%were determined by minimizing the deviations in the k's calculated for various values of the coefficients. A ratio of 3 to 1 for the coefficients was fairly critical, but the actual magnitude could vary a t least 20y0 without seriously affecting the correlation obtained. There can be no question but what Equation 2 gives a correlation well within the accuracy of the data. Errors in temperature alone can account for the 6.6% average deviation in the constants, for this means an average error in temperatureof only 6' F. The maximum deviation of 17% could be caused by a 15' F. error in the temperature. Varying temperature gradients over the length of the reactor could easily account for the temperature errors. Inasmuch as the heat requirements for the heats of reaction changed from run to run, the controlling resistances had to be changed frequently. This could undoubtedly have caused changes in the temperature gradients sufficient t o make the effective temperature as much as 15 F. different from the average of the three temperatures indicated by the thermocouple. In addition, errors in gas rates and gas analyses could account for a 10% error in the rate constants. O
+ HzO +3Hz + CO CHa + 2HzO +4Hz + COz CHI
+
The hydrogen by reforming is therefore 3 X CO 4 X COz. The values calculated in this manner are sensitive t o errors in either carbon dioxide or carbon monoxide, but they are dependent only on the exit gas analysis. On the whole the material balances check well for the W R H l runs. The rate constants of the W R H l runs on the basis of Equation 2, listed in Table VI, are in line with the corresponding constants
Table VI. BY equation
Tabulation of k for WRHl Series
*
- dw
PCH4
Run No. Av. WRH1-I 0.429 WRH1-2 0.408 WRH1-3 0.424 WRH1-4 0.332 WRHl-5 0.285 5 Corrected t o 1500°
=
PH2O
Av.
kpCHppO
(1
+ 24po f pC02,
Av. 0 . 3 7 8 0.022 0 . 3 9 2 0.022 0 . 3 8 3 0.022 0 . 3 4 0 0,019 0.387 0.023 F. on basis of A
8pHz)2,
PHz,
Av.
=
- An
0.135 0.079 0 . 1 4 2 0.072 0 . 1 3 4 0.090 0.236 0.226 0.237 0 . 1 9 0 48,000 gram
-
pco2 + Tsv.,
F. 6 k i d 1403 0 . 6 5 2 . 1 1403 0 . 6 4 2 . 1 1408 0 . 7 6 2 . 3 1508 3 . 2 3 2 . 9 0 1518 3 . 1 2 2 . 5 2 cal./gram mole
INDUSTRIAL AND ENGINEERING CHEMISTRY
1236
I r [ j
t i I
Vol. 41, No. 6
__
I
I
I
,
I
I
I
I
,
I
-tl
I
I
10.0 \
I
I
\
B
i
1
R
e
U
I
I -"* a /= k'S
I
\
i
i
FOR RUNS -CH4 AND 15% CUO-ON-
t
i
I
I
a.
Runs WR21-WR26, O / C 0. R u n s w h e r e O/C > 1
\
f0,OOO
/o,ooo OKELVfN
= 1.0
V.
0.
Runs w h e r e O / C < 1 WRR5 series, CHI COz
+
i n the reforming runs with carbon dioxide. The k ' s for runs WRH1-4 and 1-5 made a t average temperatures of 1508" and 1518"F. are 3.23 and 3.12, respectively. Corrected t o 1500" F., assuming the apparent activat'ion energy is 48,000 gram calories per gram mole, they are 2.90 and 2.60. The average of these two values is 2.75 compared to the average of 2.85 for the niethanecarbon dioxide series. This close agreement is sufficient t o justify the assumption t h a t the part,ial pressures of carbon dioxide and steam are additive, for, within the accuracy of the data, k is independent of the ratio of carbon dioxide t o water. The rate constants for runs WRHl-1, 2, and 3, made near 1400 O F. are 2.1, 2.1, and 2.3 when corrected to 1500 O F . on the basisof an apparent activation energy of 48,000 gram calories per gram mole. This could conceivably indicate a higher act,ivation energy t'han the m e assumed, but the lower k's could easily be caused by tempcrature errors, since the over-all temperat,ure difference y a s about' 50" F. The gas velocity was only two-thirds that in the last runs 4 and 5, so that the bottom thermocouple which was low prohably should have been weighted more heavily than the others. T h u s one may conclude that Equation 2 correlates satisfactorily both types of reforming runs over copper deposited on silica gel, batch 1 solid. Correlation of Runs with Methane and Copper Oxide Deposited on Silica Gel. As predicted by qualitative considerations, the methane-copper oxide runs may be correlated very well by the rate equation for the reforming reactions.
..
Figure 7.
e
~
Rate Constant for Reforming Runs
W RR5 series, CHa 4-COz (av. k ) WRRZ-WRR4 series, CHd COa (av. h )
0. WRH1 series, CHI
4- HzO
+
were then produced by the reforming reactions. Thus if no was the methane rate to the reactor and O/C the ratio of the atoms of oxygen in the exit gas to the atoms of carbon in as methane, the methane rate after reduction of the copper oxide, nI, vias taken as n,(l - 1/40/C)l and the carbon dioxide and steam rates as ' / m o O / C and l/?noO/C. The assumptions were again made t h a t the water gas equilibrium was satisfied a t all points in the reactor and that there was piston flow of the gas. Finally, i t was assumed t h a t the rate constant, IC, was the only factor in the rate equation which varied with temperature, SO t h a t the variations in the rate constant with temperature could only give a n -4rrhenius activation energy. The rate constants were then evaluated by integrating Equation 2 from n* to ne, where ne r a s calculated from the dry exit gas analysis and rate. The rate constants calculated for the JTR runs with methane and batch 1 solid are listed in Table 1-11and plotted on a log scale against reciprocal degrees Kelvin in Figure 6. From this plot it is evident that Equation 2 gives a n excellent straight-line correlation over the entire temperature range, 1100" to 1690" F. Also a t 1500" F. the rate constants are in excellent agreement with the constants for the reforming runs made at 1500O F . , so the assumption that the reforming reactions are controlling is v-ell justified. I n Figure 6, a straight line has been drawn through the circled points representing runs WR21 through WR25 in which the molal oxygen-carbon ratios were near 1.0 and methane flow rate was essentially constant near 1 liter per minute. The equation of the straight line is I n k = - - -477700 -
I n order to apply Equation 2 it was assumed that the copper oxide entering the bottom of the unit was immediately reduced t o its final state by oxidizing a portion of the inlet methane to carbon dioxide and water, and that the carbon monoxide and hydrogen
\:
I
OKELVfN
Figure 6. Rate Constant in Langmuir Type Equation for Reaction of Methane w i t h Copper Oxide
I
RT
+ 23.26
(3)
NThere R = 1.98 and T = degrees Kelvin. The maximum deviation of the circled points from this line, with the exception of the
INDUSTRIAL AND ENGfNEERING CHEMISTRY
June 1949
Table V I I .
Gas Temp. W, Sample A v . , lo? Grams No. F. K. Solid C WR1 1168 11.06 27cia 12 0.91 WR4 1.40 4 9 . 4 3 206 1450 WR4 6b 1.15 1463 9.41 206 WR5 8 . 9 8 200Q 0 . 8 9 7 1547 WR5 0.97 1520 9.09 8b 200a WR13 1.29 1590 4 200a 8 79 WR 14 1,17 1453 3 9.42 200a WR 15 1.20 8.39 1687 3 200a 1.29 8.73 WRlG 2 1600 205 1.32 WR 17 9.18 205 1 1500 WR18 1.21 9.17 190 1503 2 WR2 1 9.93 233 1353 1 1,08 WR21 1.15 9.91 233 1337 2 KR22 1.06 269 1098 11.58 1 1475 1.04 WR23 1 199 9.31 1.04 WR23 1488 2 199 9.25 1555 WR24 195 1,08 8.94 1 WR24 I563 195 1.06 8.90 2 WR25 199 1.01 8.62 1 1628 WR25 2 1630 199 0.99 8.61 l,l5 WR25 1635 199 8.59 3 WR25 1640 4 1,13 199 8.57 a Estimated values. b Inlet gas contained about 35% N2, 65% CH4
9
k
SO.
k =
$1"
24
+*dr
%
The fraction of the methane which was initially oxidized by the copper oxide was completely oxidized to carbon dioxide and water. The carbon monoxide and hydrogen were produced by reforming between the carbon dioxide and water formed during the initial oxidation and the methane t h a t was unoxidized. The rates of the reforming reactions were expressed by the: equation used in the correlation of the reforming runs.
-
The success with which the rung could be correlated on this basis is perhaps the best evidence that the assumptions are substantially correct.
Rate Constants
Run
In kcalcd =
0.062 1.9 2.2 4.0 3.3 8.3 1.6 18 6.9 3.2 3.2 0.57 0.62 0. eo9 2.4 2.8 5,9 6.5 14.1 11.9 14.0 15 3
-
47,100 K.
1237
Deviation 77 0.035 6 1.8 16 1.9 -26 5.4 - 17 4.0 -2 8.5 16 1.9 22 - 18 - 27 9.4 0 3.2 -3 3.3 6 0.54 9 0.57 18 0.011 0 2.4 0 2.8 0 5.9 2 6.4 14 12.4 -6 12.7 4 13.4 14.1 9 keslod.
+ 23.26
PCHa o
point at 1098" F. where the amount of reforming was so low that i t could be in error by a t least SO%, represents a 14% error in the k indicated by the line, and the average deviation only a 4% error. I n run WR22 a t 1098' F. the deviation is 18%. This agreement is exceptional in view of the large temperature coefficient of the reaction and the possible errors in the temperature. A 10% error in k can be accounted for by an error of less than 10" F. in temperature, and the average temperatures of the runs were obtained by averaging temperatures which spread over about 60" F. Evidently any error introduced in the averaging of the temperature was fairly consistent. The squares on the plot represent the runs with oxygen-carbon ratios greater than 1, and the triangle runs with oxygen-carbon ratios less than 1. The line drawn through the circled points represents all the runs satisfactorily, and within the limits investigated the additional variables such as oxygen-carbon ratio, methane flow rate, and nitrogen dilution seem t o have no particular effect on the rate constants. The average deviation of these additional k's from the line is 16% and the maximum is 80%. Because the maximum error occurs at a relatively low temperature, 1170' F., it represents only a 40' F. error in temperature. I n Figure 7 the k's for the reforming runs are plotted against reciprocal degrees Kelvin, along with the line representing the average k's for the copper oxide-methane runs. All the points plotted fall slightly below the line, but the agreement is good. Introducing the expression for k , Equation 3, into Equation 2, one obtains -47,700
(4) This equation correlates both the reforming runs and the 'methane-copper oxide runs satisfactorily over the entire range of temperatures and conversions encountered. An attempt was made t o correlate the data with a Freundlich type of equation. Reasonable correlation was obtained but i t was not as satisfactory as Equation 4. SUMMARY
The correlation of the WR runs in which methane was partially oxidized in a continuous fluidized unit with copper oxide deposited on silica gel have been correlated on the assumptions that: Chemical reaction, not diffusion, was the controlling factor. The rate of reduction of the copper oxide was fast compared to the rate-controlling process.
CONCLUSIONS
The reaction between methane and copper oxide deposited on silica gel, when stoichiometrically controlled to give a molal oxygen-carbon ratio of 1.0 or slightly greater, is an effective means of producing carbon monoxide and hydrogen in a molal ratio of about 1 to 2. Reaction rates are high enough so t h a t there is no reason for temperatures t o be far in excess of those required by equilibrium. I n a laboratory fluidization unit with a 4-foot bed a t 1640 O F . and 1 atmosphere, 94% methane decomposition was obtained with a selectivity of 92%. Higher conversions could be obtained with a higher temperature or a greater bed depth. At low pressures a fluidized process is well suited for carrying out the stoichiometrically controlled reaction between methane and copper oxide. Even on a laboratory scale stoichiometric control in a fluidized system affords a simple and effective means of controlling the degree of methane oxidation; on an industrial scale control would be even easier. The stoichiometrically controlled methane-copper oxide reaction may be regarded as proceeding in two steps: a n initial step (possibly consisting of a series of steps) which results in the rapid reduction of copper and the simultaneous complete oxidation of part of the methane, and a second step which is ratecontrolling and consists of the reforming of the remaining methane with carbon dioxide and water produced by the first step t o form carbon monoxide and hydrogen. The rates of the reforming reactions over copper on silica gel are controlled by chemical reaction and not by gaseous diffusion. The main evidence for this conclusion is the high temperature coefficient of the reaction rates, and the retardation of the reactions by hydrogen dilution while nitrogen and carbon monoxide dilution produce no apparent retardation. Equation 2 expresses the reaction rates well within the accuracy of the data. The temperature dependence of k m a y be expressed by an Arrhenius activation energy of 48,000 gram calories per gram mole. LITERATURE C I T E D
(1) Kennel, W., S. M. thesis in chemical engineering, Massachusetts Institute of Technology, 1947. (2) Mains, D. M., B.S. thesis in chemical engineering, Massachusetts Institute of Technology, 1947. (3) Sherwood, T. K., "Absorption and Extraction," New York. McGraw-Hill Book Co., 1937. RECEIVFD ,January 3 . 1949.
