MARION L. BROWN, Jr., and ALBERT W. GREEN Mississippi Chemical Corp., Yazoo City, Miss. GUNTHER COHN and HOLGER C. ANDERSEN Engelhard Industries, Inc., Newark, N. J.
Purifying Hydrogen b y .
..
Selective Oxidation of Carbon Monoxide I
INDUSTRIAL
METHODS (6, 8, 73, 75) for removing carbon monoxide from impure hydrogen require either substantial equipment investment or high operating and maintenance costs. However, a more economical process is suggested by the catalyzed reaction (3, 4, 9) :
co + ' / z
0 2
.-f
co2
(1)
I n an ammonia synthesis-gas plant, the number of vessels and hence capital investment may be reduced. Hydrogen lost by reaction with excess oxygen is compensated for by the gain in nitrogen from air added for the oxidation. Thus, a plant operating with selective oxidation of carbon monoxide has an advantage over a methanation plant where hydrogen is consumed without compensation. Also, unlike methanation via Reaction 2 where carbon monoxide is converted into an inert compound which is retained in the gas stream, the carbon dioxide formed is removable by absorption processes. The process was investigated on both a laboratory and pilot plant scale, using platinum metal catalysts. I n both instances, conditions were found for efficient carbon monoxide removal. Hydrogen manufactured by producer gas, water gas, or hydrocarbon-reforming processes invariably contains substantial amounts of carbon monoxide. Even after preliminary purification by the catalytic water-gas shift reaction, the carbon monoxide content is typically about 2 to 4%. Hydrogen for use in ammonia synthesis must be of high purity with respect to oxygen-containing compounds such as carbon monoxide. The ammonia industry considers 10 p.p.m. carbon monoxide as the upper limit. As little as 20 p.p.m. carbon monoxide is estimated to decrease the rate constant by 15 to 2070 (7) and to cause a permanent efficiency loss. Methods for removing carbon monoxide from impure hydrogen have been reviewed by Schmidt ( 7 7). Industrially, purification is carried out by absorption in cuprous salts (6), liquid nitrogen wash ( 8 , 75), or catalytic conversion of carbon monoxide to methane (73, 75) : CO
+ 3Hz
--+
CH4
+ Hz0
(2)
Nickel catalysts or ammonia synthesis catalysts may be used; in the precious metals group, ruthenium is particularly effective ( 70). However, these methods all require substantial equipment investment, and for the noncatalytic methods, substantial operating and maintenance costs. I n the reaction proposed (Equation l), excess oxygen reacts with hydrogen : Hz '/zOz --* HzO (3) Hydrogen lost by Reaction 3 is compensated for by the gain in nitrogen from air added for the oxidation. The carbon dioxide formed is readily removable by absorption processes. In contrast with methanation via Reaction 2, the carbon compound is thus completely removed from the gas stream, rather than converted to an inert compound which is retained in the stream. Retention of the carbon as methane in the stream accomplishes removal of the poison, but decreases efficiency because
+
plant capacity must be used to handle inerts. Since ammonia synthesis is carried out at pressures from 5000 to 15,000 p.s.i., compression costs are appreciable. Eventually, also, purging is required to rid the ammonia synthesis system of accumulated methane, and nitrogen and hydrogen are both lost. Laboratory and pilot plant studies indicate that selective catalytic oxidation of carbon monoxide could be applied to ammonia synthesis gas plants in several ways, although only two methods are discussed here. Proposed single- and two-stage oxidation units are illustrated along with the process now used by the Mississippi Chemical Corp. I t is beyond the scope of this article to present economic comparisons of the processes illustrated, but considerable savings in investment and operating costs appear possible by using the proposed single- and two-stage selective oxidation processes.
This selective oxidation process can reduce the number of vessels and possibly capital investment needed in an ammonia synthesis gas plant. Further advantages might be realized by incorporating the process in existing plants. Up to about 2.570 of carbon monoxide in hydrogencontaining gases can be oxidized catalytically to carbon dioxide with air or oxygen. Platinum, ruthenium, and rhodium are effective at temperatures from 250" to 320" F., especially platinum which requires 0.5 to 2 moles of oxygen per mole of carbon monoxide. At space velocities up to 14,000standard cubic feet per hour per cubic foot, the effluent gas contains less than IO p.p.m. of either carbon monoxide or oxygen. In a pilot plant study, this process was applied to ammonia synthesis gas, using a supported platinum catalyst on reformer gas at pressures up to 12 p.s.i. O n a large scale, temperature must be held within the selective range b y steam dilution. O n carbon dioxide-free gas, about 270 of carbon monoxide could be reduced to less than IO p.p.m.; with 16% carbon dioxide in the feed, carbon monoxide was reduced to a level of 40 to 80 p.p.m. VOL. 52, NO. 10
OCTOBER 1960
841
NATURAL G A S . STEAM 8 AlR
NATURAL GAS, STEAM B AlR
-+---+-
I
1
SYNTHESIS Q A S TO AMMONIA P L A N T
r
GAS COOLER
CO2
METHANATOR
ABSORBER
COOLER
This synthesis gas process, now used b y the Mississippi Chemical Corp., can b e simplified b y using selective catalytic oxidation of carbon monoxide
Selectivity a n d Temperature
For the selective oxidation method, laboratory and pilot plant results showed that below a certain temperature, complete reaction of oxygen does not occur, and above a certain temperature, carbon monoxide removal is incomplete, even though all the oxygen reacts. Selective carbon monoxide removal is therefore possible only within a temperature range which is designated as the selectivity zone. Breakdown of selectivity above a particular temperature may result from the tendency of carbon dioxide to be reduced by the reverse water-gas shift reaction : COz
+
H2 =
CO
+ HzO
(4)
Below 320' F., platinum metal catalysts are not effective in promoting this reaction.
wi?-
u
(Using Equation 4 : stream contains 1% COz, 96% H2, and 3% H20)
F.
co, P.P.M.
200
71
300
390 1300 2000
400 500
There is a practical upper limit to the concentration of carbon monoxide which is removable, because the exothermic reactions 1 and 3 produce a considerable temperature rise in the gas stream. For
842
ABSORBER
COOLER
OXlDAllON REACTOR
The proposed two-stage oxidation unit. Two major process vessels can b e eliminated and two carbon monoxide reactors can replace the second-stage carbon monoxide converter and methanator which are large compared to the oxidation reactors. Gas leaving the top of the saturator-heater i s saturated It i s heated with water vapor a t 200" F. and 16 p.s.i. to 250' F., fed t o the first stage oxidation reactor, cooled, passed to the carbon dioxide absorber, heated again, fed to the second stage oxidation reactor, and then passed to another carbon dioxide absorber. The effluent gas is ready for ammonia synthesis
removing 176 carbon monoxide (and simultaneously, 3Oj, hydrogen) with 2% oxygen, for example, a temperature rise of approximately 630' F. would take place in either a hydrogen or a 3 to 1 hydrogen-nitrogen stream. One way of maintaining the catalyst temperature within the selectivity zone is to dilute the stream with steam. I n older selective oxidation processes, catalysts consisting of oxides of copper, chromium, iron, nickel, and other base metals, and various mixtures and promoters were suggested. Space velocities in the range 500 to 1500 volumes of gas per hour per volume of catalyst have been described. The patent literature generally implies complete removal of carbon monoxide, but specific examples indicate residual values of 300 to 500 p.p.m, (5). No commercial plant appears to have used these older processes.
Laboratory Experiments. Commercial hydrogen, carbon monoxide, and oxygen were metered through calibrated capillary flowmeters, and the mixture passed through a glass vessel containing the catalyst. The vessel was contained in an electric furnace or oil bath controlled by a Variac. Absence of carbon monoxide in the catalyst effluent was determined indirectly by its inhibiting effect on a palladium catalyst for the hydrogen-oxygen reaction. For this purpose, electrolytic hydrogen and oxygen were added to the effluent stream, and the mixture passed into a Deoxo indicator (7) which indicates the temper-
INDUSTRIAL A N D ENOINEERING CHEMISTRY
I
I
SYNTHESIS GAS TO AMMONIA P L A N T
Expetimental Equilibrium Calculations for Carbon Monoxide Production
,
ature increase across a hydrogen-oxygen recombination catalyst. A small concentration of carbon monoxide causes a reversible drop in the temperature increment for a fixed oxygen addition; moderate carbon monoxide concentrations inhibit the hydrogen-oxygen reaction completely. Sensitivity of estimating carbon monoxide by this means was approximately 10 p.p.m. Selective oxidation a t atmospheric pressure was examined as a function of hydrogen content of the stream, carbon monoxide content, oxygen-carbon monoxide ratio, catalyst composition, space velocity, and temperature. For supported platinum catalyst, when oxygen was not added, only a small amount of carbon monoxide was removed, presumably by methanation (Reaction 2). Therefore, when oxygen is added, the nearly complete carbon monoxide removals are attributable largely to Reaction 1. The selective removal was feasibly only within a certain temperature zone, below which oxygen reaction falls off, and above which selectivity fails. Below 266' F., and above 320' F . , required oxygen-carbon monoxide ratio increases noticeably. Within the selectivity zone this ratio was about 2 to 1. Precious metal catalysts other than the 0.1% platinum-aluminum oxide indicated the same effective temperature zone ; thus, these temperature limits are generally applicable. But. some flexibility can be conferred on these limits by varying the oxygen concentration. Supported platinum, rhodium, platinum-rhodium, and ruthenium catalysts promoted selective oxidation. Palla-
CARBON MONOXIDE OXIDATION GAS FROM COP ABSORBER
SYNTHESIS GAS TO AMMONIA PLANT
cEy
METHAN ATOR
Lg&-J
1
E!
v
0
A
+ 2
The selective oxidation process can b e further simplified by using a single-stage unit which is the same as the twostage method through the first carbon dioxide absorber. Equipment from that point is modified as shown here to eliminate the satutator, second-stage carbon monoxide converter and heat exchangers, heater, gas cooler, and second carbon dioxide absorber. All of these items are replaced with only the first-stage oxidation reactor
0
0
c V
.-?!
Q'
a
.-+ 0
At low carbon monoxide levels, platinum and ruthenium catalysts are similar, but rhodium is somewhat less selective. At high carbon monoxide levels, supported platinum is clearly superior
P
a
I
0 .5
v
0" 2
dium, iridium, and ruthenium-rhodium were either not or only slightly effective. I n the effective group, platinum and ruthenium seemed especially selective, requiring low oxygen-carbon monoxide ratios a t low influent carbon monoxide levels. There was no significant difference between results with straight hydrogen and those with 7 to 93 hydrogen-nitrogen compositions which span ammonia synthesis gas mixture. A 0.1% ruthenium catalyst gave 248' F., this fresh unusual results-at catalyst caused some carbon monoxide removal a t a oxygen to carbon monoxide ratio of 3.2. If, after a short exposure to carbon monoxide-hydrogen, without oxygen, the catalyst was brought to 266' F., it then failed to remove carbon monoxide upon oxygen addition. The effect of this pre-poisoning could be nullified by omitting carbon monoxide from the stream a t the elevated temperature, after which carbon monoxide could be re-admitted and removed by oxygen addition. This sequence could be repeated as often as desired. Similar effects were not found with the 0.50j, ruthenium catalyst which, therefore, is suitable for mixed gas treatment. Pilot plant operations over extended periods, as described subsequently, showed a similar but more gradual poisoning effect with platinum catalyst. Means for overcoming this inhibition were readily developed. Pilot Plant Operation and Equipment. A pilot unit was built for further testing of this oxidation process on gas taken from Mississippi Chemical Corp. ammonia synthesis gas plant ( 2 , 72, 73)
0.6
located a t Yazoo City, Miss. The gas was reformed gas consisting, typically, on a dry basis, of 17y0 carbon dioxide, 1.7% carbon monoxide, 0.3% methane, 61Y0 hydrogen, and 20% nitrogen. Evaluations were made on a scale large enough to be potentially adiabatic. Therefore, provision was made for both steam addition and cooling of the catalyst beds. The reactor, consisting of a 9-inch square carbon steel shell, 5 feet 9 inches high, was divided into five equal sections into which catalyst was loaded. The top four sections contained finned-tube cooling coil bundles for water circulation to remove heat of reaction. Approximately 100 pounds of catalyst was charged to the react0r--17~/~pounds into each of the top four sections and 29 pounds into the bottom section. Catalyst was loaded so that the cooling coils were completely imbedded in the catalyst. Three-inch spaces were left between each catalyst bed. Flow of gas and air to the reactor was indicated separately by rotameters. Steam was added through a valve containing a per cent-open scale. By using a series of steam-to-gas ratio analyses, the valve was calibrated to deliver a specific quantity of steam a t a given pressure.
I %
co
1.5 by VOl.
2
2.5
Mixtures of gas, air, and steam were preheated in a steam-jacketed pipe heat exchanger. Water at flows up to 6 gallons a minute, temperature between 215' and 275' F. and a t 50 p.s.i.g. was pumped to the cooling coil bundles. Compositions ol inlet and outlet gas were determined by Orsat analysis. Exit gas was analyzed for carbon monoxide content with a Mine Safety Appliance Co. carbon monoxide detector, and occasionally by a mass spectrometer and a n infrared spectrometer. Infrared spectrometer analyses checked those of the carbon monoxide detector within 5 p.p.m. Steam-to-gas ratio analyses were made by determining the increase in weight of the sulfuric acid when the steam-gas mixture was passed through the acid. Oxygen content of the reactor exit gas appeared always to be less than 10 p.p.m. based on the method of Winslow and Liebhafsky (74). The catalyst consisted of l / ~ by inch cylindrical pellets coated with 0.3% platinum by weight. Bulk density of the catalyst was 56 pounds per cubic foot. Catalyst bed temperatures were brought to about 270" F. by passing 275' F. water through the cooling coils imbedded in the catalyst. Air a t VOL. 52, NO. 10
0
OCTOBER 1960
843
Pilot Plant Data on Selective Oxidation of Carbon Monoxide (Press., about 10 p.s.i.g.) Points on Illustrated Process A B C D Gas compn.,Qyo Table I.
c02
co
CHI HP
Ns Hz0 02/CO*
Steam/gas ratioC Space veloc., S C F H / C F catalyst Reactor inlet gas temp., F. Catalyst temp., O F. T1
10.2 1.0 0.2 36.7 12.1 39.8 2.0 1.0 3700 185
0.033 2800 230
200 255 275 285 310 3 10 300 300 293 283 40-80
250 270 350 320 300 320 355 400 350 350 To 300
16.5 1.6 0.3 59.0 19.4 3.2
1.5
0.0 2.0 0.4 72.6 23.8 1.2 1.4 0.012 1800 230
240 260 290 300 500 T6 540 T7 300 T8 258 T9 340 T10 293 C O in effluent, p.p.m.d 200 to 4000 a Before air or st~eamaddition. O n dry basis of the air-gas mixture. cluding steam in gas before air or steam was added. Dry basis.
approximately 270’ F. was then passed over the catalyst for 10 minures. Steam flow was then introduced a t the desired rate and the air flow was shut off. Then, after a few minutes of purging the unit with steam, air, and gas were simultaneously introduced to the reactor at the desired rates. Temperature and flow of the water to the cooling coil bundles were then adjusted to maintain the desired catalyst temperatures. At the completion of a test run, the gas and air were simultaneously shut off but steam flow was continued. Thus, gas alone was never in contact with the catalyst and the possibility of the gas inhibiting the catalyst was eliminated. About 2 hours was required to line out the reactor completely. After the line-out period, test runs of from 3 hours to several weeks were made. The oxygen to carbon monoxide ratio of the feed mixture was varied from 0.5 : 1 to 2.5 : 1, steam-to-gas ratios from 0 : 1 to 2.5 : 1, catalyst temperatures from 120’ to 600’ F. Feed gas compositions were also varied. Early test runs with the pilot unit were unsuccessful due in part to the methods of starting u p and shutting down the unit. When a nelv charge of catalyst was first tested, the results were excellent-less than 10 p.p.m. of unconverted carbon monoxide remained in the exit gas from the reactor. However, as more test runs were made, the catalyst was temporarily deactivated by- contact with the process gas alone. The catalyst could be reactivated by passing hot air over it. Such deactivation could be avoided by using the procedures previously outlined.
844
0.0 2.0 0.4 72.6 23.8 1.2 2.3 1.5 11,000-14,000 265 270 370 325 280 255 242 241 240 239 256
