Production of synthesis gas By Catalytic Decomposition of Methanol HERMAN S. SEELIG AND ROBERT F. MARSCHNER,
synthesis gas, regeneration would be necessary. This paper describes work on regeneration of the catalyst, and the construction and operation of a service unit to produce as much as 250 cubic feet per hour of synthesis gas.
METHANOL decomposition over a supported catalyst of copper and nickel oxide has been employed to produce moderate quantities of a synthesis gas having a hydrogen to carbon monoxide ratio of 2. The only by-products are traces of carbon dioxide and hydrocarbons, and carbon. The catalyst may be regenerated by periodically burning off the deposited carbon. A service unit to produce 250 to 300 cubic feet per hour of synthesis gas was constructed. Operation for 550 hours has not impaired the catalyst. The method is simple in operation, requires only readily available chemicals and equipment, and is highly recommended as a convenient means for generating both small and intermediate quantities of 2 to 1 synthesis gas.
Experimental Unit
The glass apparatus used in the small-scale preliminary work is shown in Figure 1. Methanol was forced into the boiler a t a constant rate by means of air pressure. The methanol vapor leaving the boiler was heated t o the desired temperature in the preheater, and then entered the catalyst chamber, which consisted of an insulated electrically heated 3.5-om. glass tube 30 cm. long, holding 200 ml. of catalyst. This was prepared as described by Eversole, using U.S.P. chemicals. After leaving the catalyst chamber the gases were cooled to about 15 C. in a water-cooled condenser. Condensed methanol was collected in a flask. A short packed tower removed the remaining methanol by water-scrubbing. The gas from the top of the scrubber passed through a wet-test meter, and gas samples were taken periodically.
I
N T H E experimental work on the synthesis of hydrocarbons
by reduction of carbon monoxide it became necessary t o have a moderate supply of reasonably pure hydrogen and carbon monoxide. Gas had been obtained by mixing hydrogen, from cylinders, with carbon monoxide obtained by decomposing formic acid over phosphoric acid a t 175" C. This method is suitable for preparing a feed gas having any ratio of hydrogen to carbon monoxide. However, to generate 250 cubic feet per hour of a feed gas having a hydrogen-carbon monoxide ratio of 2, about 30 gallons per day of 85% formic acid and 36 cylinders of hydrogen are required. A better method was therefore sought. Other methods considered were : partial combustion of hydrocarbon gases, re-forming of methane with carbon dioxide and steam, reduction of carbon dioxide with carbon, and catalytic decomposition of methanol. The f i s t two methods have been employed t o generate large quantities of carbon monoxide and hydrogen, and the third has undeveloped possibilities, but it was thought that the catalytic decomposition of methanol would be simple in operation and low in cost, and would require a minimum of development work for a service unit. The constant hydrogen-carbon monoxide ratio of the gas produced would restrict the utility of the method, but the ratio of 2 produced is satisfactory for present purposes. A catalyst and its use for decomposing methanol are described in a patent issued to Eversole
Operation. The unit was operated 24 hours a day. During the f i s t run the gas production rate fell off steadily for the first 90 hours, in spite of an increase in temperature from 350" t o 400" C. An attempt t o operate a t a lower temperature from 90 t o 115 hours resulted in a further decrease in gas production rata and a considerable decline in the amount of methanol converted. Thereafter the run was continued at increasing temperatures (450' C . and higher) but, after a brief increase in gas production, the rate again declined. After 300 hours, the catalyst chamber temperature was raised t o over 500" C. for a period of 6 hours in an attempt t o regenerate. Following this period the catalyst was put back on stream a t a maximum temperature of 460" C. After 15 hours on stream a t this temperature only 460 liters of gas had been produced and practically all the methanol was recovered a s condensate. A final run a t 510" C. was made, but the gas production rate was still considered unsatisfactory and the hydrogen110 AEMANOL BOILER
(1).
The catalyst consists of a Filtros-supported mixture of copper and nickel oxides. It is prepared by melting the nitrates of the metals, adding the support, and heating until the oxides are formed. The patent specification describes a catalyst consisting of 37 parts of cupric nitrate trihydrate, 15 parts of nickel nitrate'hexahydrate, and 50 parts of Filtros. The dehydrogenation temperature is 350 t o 400 C., and 100 to 200 volumes (S.T.P.) of methanol vapor pass over one volume of catalyst per hour. The oxides of the metals are reduced by the methanol that first passes over the catalyst. O
O
Standard Oil Company (Indiana), Whiting, Ind.
1
/THERMOMETER
CAP1 LLARY TUBING
$7
A !
-
Hg : : --.
PREH
._
Although no statements qre made in the patent regarding the life of the catalyst, it was found experimentally that the life was of the order of 200 hours. To be a practical method of generating
MET RES
IOL 01R
Figure 1.
583
Catalyst testing unit for catalytic decomposition of methanol
584
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 40, No. 4
these runs were considered important from an operational standpoint. When methanol vapor first contacts the catalyst oxides, there occurs a highly exothermic reaction forming water, and (if the temperature is high enough) apparently also carbon. When the maximum catalyst temperature was lowered for successive reduction periods, higher gas generation rates and lower operating temperatures were obtained. It was therefore concluded that successful operation of the catalyst depends to a considerable extent on the reduction, and that the temperature must be kept a t a minimum during this period. Regeneration is unnecessary more often than every 200 hours if high conversions are not essential. Since combustion of carbon is an exothermic process, care must be taken during regeneration to prevent overheating of the catalyst chambers. Data obtained toward the end of the second run indicated that higher space velocities, up t o 250, might give a product having a hydrogen-carbon monoxide ratio closer t o 2.0. The liquid condensate was slightly acid and contained about 1% of what appeared to be formic acid. If the condensate were to be recycled in the larger unit, corrosion problems might arise. From the preliminary runs it was concluded that a larger service unit based upon the Eversole catalyst was entirely feasible. Service Unit
As a result of the experimental work described above, a unit to produce at least 250 cubic feet per hour of hydrogen and carbon monoxide in a ratio of about 2.0 was designed and constructed. This unit is shown in Figure 3.
Figure 2.
CATALYST AGE (HOURS) Conversion of methanol to hydrogen and
carbon monoxide
carbon monoxide ratio was considerably above that observed initially. Before the catalyst was removed, regeneration was attempted by passing heated air over the catalyst. The temperature of the catalyst bed rose to well over 1000 ' C. and deformed the vessel. Evidently this excessive temperature was due to the large amounts of carbonaceous material deposited on the catalyst. On the basis of these results, a run was made with a new batch of the same catalyst but employing regeneration with air at frequent intervals, with the results shown in Table I and Figure 2. This run lasted 374 hours without appreciable change in the composition of the gas produced. The catalyst was regenerated by combustion and re-reduction three times. After each treatment it was found to be more active than during the previous period. Indications were that the catalyst may be regenerated a large number of times. Methanol conversion as calculated upon the assumption that the gas produced was all carbon monoxide and hydrogen was never complete and fell as low as 50%. Unconverted methanol was partly recovered when the partial pressure of methanol exceeded 90 mm. (the methanol vapor pressure a t condenser temperature) ; consequently the yields of gas based upon unconverted methanol were always above a minimum value, which wm about 70%. Only when no unconverted methanol condensed was the gas yield above this value. The gas leaving the scrubber was essentially free of methanol and averaged above 97% hydrogen plus carbon monoxide, and its hydrogen-carbon monoxide ratio averaged 2.17. The yield of synthesis gas based upon methanol actually converted was therefore nearly the theoretical. The byproducts were 0.2'$&carbon dioxide and 2% inerts including nitrogen and, as is shown in more detail below, gaseous hydrocarbons. General Observations. Several observations made during
Methanol (synthetic grade of 99.8YGpurity) is transferred from drums into a storage tank by a centrifugal pump. The feed is then pumped into the evaporator through a feed rotameter. The cold feed enters the top of the evaporator and flows over a weir down the sides of the evaporator for some preheating. The quantity of methanol in the evaporator is determined by the rate of evaporation, and this in turn depends on the feed rate and the heat transfer area. Those factors automatically adjust themselves, so that the feed and the quantity of methanol evaporating are equal. The evaporator operates at 5 to 10 pounds per square inch gage. From the evaporator, methanol in vapor form enters the Dowtherm jacketed preheat,er and is heated to the desired temperature, which is 320" to 345" C. From the preheater, the hot methanol vapor passes into the header a t the top of the reactors. From this point until the gas reaches the scrubber, all the equipment in contact with the gas is stainless steel. The reactors are 3-inch pipes 10 feet long with an annular 0.25-inch thermocouple well. An orifice plate is located a t the inlet of each reactor t o limit the flow to each reactor and to maintain a constant inlet rate independent of small variations in the pressure drop across the reactor and scrubber. Another orifice is located at the outlet of each reactor t o measure its gas production. Each reactor may be vented separately, so that any one
TABLEI.
CONVERSION OF METHANOLTO HYDROGEN ASD CARBON MONOXIDE
Run
Time Interval, Hours
34 5
42- 48 4891 91-114
6 7 8
114-138 138-166 166-213
9 10 11 12
213-257 257-304 304-351 351-374
(200-ml. catalyst testing unit) M ~ ~Space , Velocity, conCatalyst Vg/hour/Vc ysrTemp., Product sion, OC. Feed gas % Regeneration
(z::)
440 Regeneration 350 380 410 Regeneration 335 365 400 420
Analysis Hz CO CO
+
198 168 173
426 294 271
71.7 58.3 52.0
2.18 2.17 2.19
97.8 97.6 97.6
198 208 211
465 425 412
78.1 68.0 64.7
2.16 2.17 2.14
96.8 97.9 97.1
205 204 202 263
633 480 456 581
86.8 78.3 73.5 70.4
2.20 2.20 2.18 2.15
97.9 96.3 96.9 97.3
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1948
585
Figure 3. Flow diagram of feed gas preparation unit I
I
I
I
I
I
II I I
I
I
I
I
II
can be regenerated separately. Each reactor is provided with two 1500-watt heating coils, one fixed and one variable. The product gases and undecomposed methanol from the reactors are passed through a water-cooled condenser, to remove part of the undecomposed methanol from the gas stream. After leaving the condenser, the cooled gases enter the bottom of the scrubber, where the last traces of undecomposed methanol are removed. The scrubber employed is 7 feet of 6-inch pipe packed t o a height of 5 feet with 0.5-inch Berl saddles. I n order to prevent gas from escaping from the bottom of the scrubber, a liquid level controller is employed. From the top of the scrubber the gases pass through a product rotameter and into gas holders, or directly to a, compressor, whichever is desired.
Operation. The data summarizing the operation of the synthesis gas generation unit are given in Table 11. During these
TABLE 11. CONVERSION OF METHANOL TO HYDROGEN AND CARBON MONOXIDE
(5ervice unit. Time Interval, Hours
C a t J y s t , 21.5 pounds.
Temperature, 260-320' C.)
Space Velocity Vg/hour/Vc Product Feed gas Yieldb
Analysis Hz Hs
TJij
co+
Gas Production C m
Hour
ft.
2 3
11-26 26-51 51-79
195 195 195
279 272 253
76 78 70
2.18 2.14 2.15
97.9 96.9 96.4
103 100 94
1540 2510 2620
4 5
79-94 94-108 108-125
199 188 192
321 290 261
79 81 71
2.19 2.16 2.16
97.1 97.3 95.8
119 107 96
1780 1500 1630
200 192 189 191 191 191 (140)
282 238 259 308 265 256 234
72 72 72
2.18 2.19 2.17
97.9 97.8 97.7
10 11 12 13A
125-140 140-159 159-171 171-185 185-213 213-282 282-358
65 57 (68)
2:i4 2.16
98:5 98.2
104 88 96 114 98 95 87
1560 1670 1150 1590 2750 6540 6580
13B
358-379
(140)
253
(74)
..
..
93
1960
..
..
85 13660
Run5 1
6 7 8 9
14
Regeneration
Regeneration
Regeneration ' Regeneration
233
67
..
..
379-538 (140) (67) Regeneration 105 2100 97.3 284 15 (71) 2.21 538-558 (1401 a Runs 1 to 6 are continuous; unit shut down from 1 to 15 days between
other runs. b Based on methanol disappealvrnce (inoludes vapor lost in gas). 0 Regeneration partly ineffective because reduction temperature was too high.
runs only one reactor was on stream. It contained 21.5 pounds (0.37 cubic foot or 10.5 liters) of catalyst which filled 7 feet of the IO-foot reactor and produced about 150 cubic feet of feed for each gallon of methanol consumed. The temperature during the reduction was kept low except after the third regeneration period noted in Table 11. The space velocity in the early runs was 195 Vg/Vc/hour. I n the last four runs the space velocity was about 140 Vg/Vc/hour, resulting in higher methanol conversion. The hydrogen-carbon monoxide ratio seems to have increased only slightly as a result of this decrease in space velocity. The first two regenerations were carried out primarily to develop the procedure for regenerating. Subsequent runs bear out the indications from the small scale studies that regeneration is necessary approximately every 200 hours. Before regenerating the unit should be purged with inert gas. Regeneration is accomplished by unpreheated air, and the flow and heat on the reactors are adjusted to prevent the temperature from rising above 650" C. This temperature appears to be adequate. The time required to regenerate one reactor is about 3 hours. The catalyst was removed after 558 hours on stream; it was found to be in excellent condition and was therefore put back into service. An analysis of a sample of condensate from the water-cooled condenser showed no measurable increase in acidity over the feed, and the refractive indexes of the charge stock and the condensate were almost identical (722," 1.3287 us. 1.3283). On distillation 97.5% came over within 1O C. of the boiling point of pure methanol and the bottoms showed no acidity. Therefore this condensate was pumped back to the feed tank for re-use. Temperature control was simple, as the space velocity was high enough to ensure more feed than the unit could dehydrogenate. Under these conditions most of the heat input is consumed in decomposing the methanol; only a small fraction is used in heating the product gases and the undecomposed methanol. As the catalyst loses its activity, the temperature of the reactor slowly increases, showing that more of the heat is being used to heat the reactant and products and less t o decompose methanol. The slow increase in catalyst temperature is always accompanied by a decrease in gas production rate, all other variables being constant. On the basis of the 15 runs with the pilot plant unit,
INDUSTRIAL AND ENGINEERING CHEMISTRY
586
Vol. 40, No. 4
eter analysis shows that a minute but definite quantity of mcthTABLE111. G s s AXALYSISOF PRODUCT FROXI Gas GENERATOR ane and of hydrocarbons of higher molecular weight are actually Run 15 synthesized in the generator. This is presumably due to partial - -< ;\lass Average hydrogenation of the carbon monoxide in the presence of either Spectrometer Orsat Orsat the nickel catalyst itself or possibly iron contaminants. Hz 65.35 68.1 67.1
co
cor
Cila C2H6 C6H6 CIH8 C1H6 CaHio
H?/CO
33.74 0.30 0.11 0.21 0 03 0.16 0.05
30.8 0.3 , . . . , . . .
0.8
....
....
0.05 100.00
lhO.0 __ 2.21
1.93
30.9 0.2
.... .... .
1.8 , .
...,
....
100.0 -
2.17
a reactor temperature of 260 to 320 C. is adequate for the dehydrogenation reaction. Operating practice has been to regenerate when the catalyst temperature began to exceed the higher figure. Mass spectrometer and Orsat analyses run on a gas sample obtained during run 15 are presented in Table 111, together with a n averaged Orsat analysis for other runs. No explanation is offered for the discrepancy between the Orsat and mass spectrometer analysis, but it is believed that the Orsat values for hydrogen and carbon monoxide are the more reliable. Only a small fraction of the product gas, usually about 2%, consists of products other than hydrogen and carbon monoxide. The mass spectromO
Conclusions
No difficulties in the operation of the unit have been encountered. The unit can be started in 3 hours from a complete shutdown and shut down in about a half hour. By leaving partial heat on the reactor and preheater, the starting time may be halved. The method is considered satisfactory and is highly recommended for producing moderate quantities of reasonably high purity hydrogen, and carbon monoxide in a ratio of 2 .
O
Acknowledgment
The authors appreciate the important contributions which W. B. Plummer, E. L. D’Ouville, B. L. Evering, J. Zisson, and F. Kalina made to various phases of the work. Literature Cited
S.Patent 2,010,427 (assigned t o Carbide and Carbon Chemicals Corp., August 6, 1936).
( 1 ) Eversole, J. F., U.
R E C E I V EOctober D 4, 1947.
Production of water gas from pulverized coa A Continuous Process JOHN F. FOSTER Battelle Rlemorial Institute, Columbus, Ohio
M
ANY processes for the continuous production of water gas from pulverized coal have been proposed in the past, and descriptions of a number of these have been given by the Institute of Gas Technology in a comprehensive review of gas-making processes ( 2 ) . A common basis for all the proposals for the use of pulverized coal in the continuous production of water gas has been the belief that the conventional water-gas generator is subject to improvement on two counts. (1) The conventional generator requires solid fuel of large and relatively uniform size in order that the flow of air or steam may be evenly distributed through the fuel bed, and the bed may thus be maintained in good condition for gas making over long periods. The necessity for using an optimum size of coke or coal prevents the use of smaller, cheaper fuels. If a generator can be developed to utilize pulverized fuel, an attractive saving in fuel costs appears possible. (2) The production of water gas in a conventional generator is an intermittent process, in which air is first blown through the fuel bed to release by combustion, and to store, the heat necessary for the subsequent endothermic reaction between carbon and steam, and to raise the fuel bed temperature to a level a t which the reaction will proceed at a reasonable rate. Next, the air is cut off and steam is passed through the bed; in this period water gas is produced, and the temperature falls because of the heat absorbed by the reaction. As soon as the temperature reaches a minimum below which the reaction will not proceed at an eco-
nomical rate, the steam must be cut off and air again admitted to raise the temperature of the bed before further gas can be produced. If the heat for the carbon-steam reaction could be supplied through the walls of the reaction chamber, and the process thus made continuous, the necessity for precise control of the timing of the gas-making cycle would be avoided, and possibly labor and investment costs for a generator of given capacity would also be reduced. As pointed out by Barnes ( I ) , there have been two obstacles to the development of successful water-gas processes using indirect heating of the reaction chamber to maintain continuous gas production: (1) When refractory walls were used, the comparatively low thermal conductivity of the refractory limited the rate a t which heat could be supplied, and this in turn limited the capacity of the apparatus. (2) With metal walls, the maximum temperature which could be used without rapid deterioration of the metal was comparatively low, and the rate of gasification was correspondingly small. Barnes suggested that the then recent development of temperature-resistant alloys, capable of withstanding temperatures several hundred degrees higher than the alloys previously available, justified an investigation of the possibilities of gasification of pulverized coal by steam in an externally heated chamber constructed of one of these alloys. As a result of Barnes’ recommendations, the present investigalion was initiated under the sponsorship of Bituminous Coal
