CA1103900A

CA1103900A - Process for the catalytic oxidation of carbon monoxide

Info

Publication number: CA1103900A
Application number: CA287,329A
Authority: CA
Inventors: Johann G.E. Cohn; Asmund A. Boyum; Otto J. Adlhart
Original assignee: Engelhard Minerals and Chemicals Corp
Current assignee: Engelhard Minerals and Chemicals Corp
Priority date: 1976-10-07
Filing date: 1977-09-23
Publication date: 1981-06-30
Also published as: GB1555826A; JPS5353596A; AU2941577A; FR2367016A1; AU515630B2; DE2745012A1

Abstract

The catalytic selective oxidation of carbon monoxide in a gaseous mixture containing hydrogen and water vapor and being essentially saturated with respect to its water vapor content, is improved by adjusting the gaseous mixture such that the gaseous mixture is unsaturated with respect to its water vapor content prior to contacting said gaseous mixture with a catalyst, for the selective oxidation of the carbon monoxide in said gaseous mixture. Further, desirably, during the catalytic selective oxidation operation the gaseous mixture and the resulting catalytically oxidized gaseous mixture is maintained unsaturated with respect to its water vapor content.

Description

39~0 This invention relates to an improved process for the ¦l :
catalytic selective oxidation of carbon monoxide in the presence of hydrogen, particularly the selective oxidation of carbon monoxide in a gaseous mixture containing a ma~or amoun of hydrogen. Such gaseous mixtures are to be found in ammoni plants and may be exemplified by an ammonia synthesis gas produced by the catalytic steam reforming and shift conver-sion of a normally gaseous hydrocarbon, such as natural gas, e.g. methane. It is pointed out, however, that in other 10 chemical processes there are produced gaseous mixtures con-taining hydrogen and carbon ~onoxide and the practices of thi invention are applicable to the treatment or processing of such gaseous mixtures for the selective oxidation of the carbon monoxide therein.
In the discussion hereinafter emphasis is placed on the applicability of the practices of this invention with respect to the preparation and treatment of ammonia synthesis gas, i.e. a gaseous mîxture consisting essentially of nitrogen and hydrogen in substantially stoichiometric amounts suitable for 20 the production of ammonia and being saturated with respect to its water vapor content. In the preparation of ammonia synthesis gas hydrocarbons are reacted with steam and air ove suitable catalysts to product a gas which, after removal of the carbon oxides therein, consists essentially of,hydrogen and nitrogen. More specifically, in the production of ammoni

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synthesis gas from a normally gaseous hydrocarbon, e.g.
methane, methane is catalytically steam reformed followe~ by shift conversion, usually carried out in two stages, to produce a gaseous effluent which is saturated with water vapo and which contains primarily nitrogen, hydrogen, and carbon dioxide together with minor amounts of carbon monoxide. In such a gaseous effluent the carbon monoxide content is usuall in the range from about 0.1 to about 2.0% by volume. This carbon monoxide content r although small, must be further reduced, preferably below 10 ppm, in order to avoid depressin the activity of the catalyst employed for the manufacture oi ammonia. It is conventional practice to remove the bulk of the carbon dioxide and then to pass such a gas through a methanator or methanation reactor which converts carbon monoxide and residual carbon dioxide therein to methane. In this operation hydrogen is consumed which necessarily reduces the hydrogen availab7e for the production of ammonia, thereby ad~ersely affecting the yield of ammonia produced. Further, ` although the methane produced during the methanation reaction does not have an adverse affect on the ammonia synthesis catalyst, the resulting produced methane acts as an inert gas in the ammonia synthesis reaction or synthesis reaction 1QP
and must be purged periodically with consequent loss of ammonia production. It is desirable therefore to produce an ammonia synthesis gas having a minimal carbon oxide (carbon monoxide and/or carbon dioxide) content as well as a minimum meth-ne content.

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11~3~ ' The catalytic synthesis of ammonla from normally gaseous hydro-carbons, such as methane and natural gas, is highly developed, see particularly the article entitled "Check List for ~igh Press~re Reforming"
by Quartulli, Hydrocarbon Processing, pages 151-162, April, 1965, the articie entitled "Questions and Answers on Today's ~mmonia Plants" by Bressler and James, Chemical Engineering, pages 109-118, June 1965, and U.S. Patent No. 3,132,010 ~1964). In the above-referenced publications, particularly the Quartulli article, there are disclosed and illustrated ammonia synthesis plants wherein an ammonia synthesis gas derived by the steam reforming and shift conversion of natural gas is produced, the produced ammonia synthesis gas consisting essentially of nitgogen and hydrogen being substantially saturated with water and wherein the carbon dioxide content of the ammonia synthesis gas is removed by a carbon dioxide absorber and the resulting carbon dioxide-stripped synthesis gas then subjected to methanation reaction for the substantially complete conversion of the carbon oxides in the treated gas to inert methane before the thus treated synthesis gas is passed to the catalytic converter for the conversion of nitrogen and hydrogen in the synehesis gas to ammonia.

It is known to catalytically selectively oxidize carbon monoxide to carbon dioxide. By selective oxidation of carbon monoxide is meant the reaction of the carbon monoxide with oxygen to form carbon dioxide, even in the presence of large or major amounts of hydrogen. During the selective oxldation of carbon monoxide present in an ammonia synthesis gas whlch contains, in addition to nitrogen, a substantial or major amount of hydrogen, ;~ the oxidation of hydrogen, during the selective oxidation of the carbon ;~ monoxide therein, is to be minimized since the hydrogen in the ammonia ~ ~ synthesis gas was produced for the intended purpose o~ catalytically reacting , , .
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with the nitrogen therein to produce the desired product ammonla. Ideally, the selective oxidation of the carbon monoxide in ammonia synthesis gas should be carried out such that only the carbon monoxide in the ammonia synthesls gas is oxidized.
In U.S. Patents 3,216,782 and 3,216,783 there are disclosed processes for the selective oxidation of carbon monoxide employing precious metal catalysts, such as a platinum-containing catalyst, a ruthenium-con-taining catalyst and a rhenium-containing catalyst. In these patents the operating temperature for the catalytic selective oxidation of carbon monoxide is disclosed to be in the range about 100-200 C with oxygen present or introduced in the carbon monoxide-containing gas stream being in the range 1:1 to 3:1, expressed as the molar ratio 02:CO. Further, U.S. Patent 3,631,073 describes a process for the catalytic oxidation of carbon monoxide at a relatively low temperature in the range of 20-100 C, the molar ratio of oxygen to carbon monoxide in the carbon monoxide-containing gaseous mixture being in the range from about 0.5:1 to about 3:1. Additionally, it is mentioned that U.S. Patent 3,631,073 is directed to resolving the serious problem of catalyst deterioration with respect to the use of preclous metal ; catalysts at lower temperatures for the selective oxidation of carbon monoxide, specifically the problem of deactivation of such catalysts after a relatively brief period of time when employed in the selective oxidation of carbon monoxide.
- Accordingly, it is an object of this invention to provide an improved process for the selective oxidation of carbon monoxide in the , presence of gaseous hydrogen.

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It is another object of thls lnventlon to provlde an lmproved process for the removal of carbon monoxide by selective oxidation from carbon monoxlde-contalning ammonia syrlthesis gas such as might be produced by the catalytic steam reforming and shift conversion of normally gaseous hydrocarbons, e.g. natural gas or methane, for the production of ammonia synthesis gas for catalytic conversion for the production of ammonia.
It is another object of this invention to provide an improved process for the handling of water saturated carbon monoxide-containing gases, such gases also containing a substantial or major amount of hydrogen, for the selective oxidation of the carbon monoxide therein.

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How these and other objects of this invention are accomplished will become apparent in the light of the accompanying disclosure. In at least one embodiment of the practices of this invention at least one of the foregoing objects will be achieved.
It has been discovered that the deactivation of precious metal catalysts employed for the selective oxidation of carbon monoxide in gaseous mixtures containing hydrogen, such as a carbon monoxide-containing ammonia synthesis gas, is related to the presence in such gases of water vapor. More parti-cularly, in accordance with this invention, it has been discovered that the employment of a precious metal catalyst for the selective oxidation of carbon monoxide in a gaseous mixture containing the same together with hydrogen, such as a carbon monoxide-containing ammonia synthesis gas, is improved by adjusting or altering the water vapor content of the carbor monoxide-containing gaseous mixture such that the carbon ; monoxide-containing mixture is unsaturated with respect to it water vapor content prior to contacting said catalyst with ` said gaseous mixture. Additionally, in the selective oxida-~; tion of carbon monoxide in a carbon monoxide-containing gaseous mixture employing a precious metal catalyst as the selective oxidation catalyst for carbon monoxide, improved results are obtained not only by altering or adjusting the carbon monoxide-containing gaseous mixture to be unsaturated with respect to its water vapor content but also by main-¦ talniog the c bon m~noxide-oontaining gaseous mixtur-, .
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unsaturated witn respect to its water vapor content during the catalytic selective oxidation operation and also by main-taining the resulting treated gases after having.undergone selective oxidation operation but still in contact or exposed to the precious metal catalyst such that the gases in contact with the catalyst are unsaturated with respect to water vapor content.
. Described in greater detail; the subject invention consists of an improved process for the selective oxidation of carbon monoxide in a carbon monoxide-containing gaseous mixture which also contains other gases, such as oxygen, carbon dioxide and particularly nitrogen and hydrogen, in the propor-tions and amounts usually found in an ammonia synthesis feed gas. In the practices of the invention the relative humidity of the carbon monoxide-containing gas undergoing catalytic ::
selective oxidation of the carbon monoxide therein, is reduced below 10~%, i.e. the carbon monoxide-containing gas:.is adjusted or treated such that the water vapor content of the gas is below saturation content~ ~
As a specific embodiment, the present in.vention provides an improvement in a process for the selective oxidation of carbon monoxide in a ~aseous mixture comprising carbon monoxide, hydrogen and water vapor, carbon monoxide comprising from about 0.1 to about 2 volume percent of the gaseous mixture and hydrogen com-prising a major amount of the gaseous mixture, and wherein the gaseous mixture is passed at a temperature in the range of about 20 C to about 60C into initial contact with a precious metal : . catalyst effective for the selective oxidation of carbon monoxide to carbon dioxide in the prescence of hydrogen and in the presence, of oxygen, the oxygen being present in an amount in the range of 0,5 to 3 mols oxygen per mol.of the carbon monoxide. This `improvement comprise5 adjusting the water vapor content of the gaseous mixture such that the relative humidity of the gaseous ¢~
~ - 8 -mixture is within the range of 35 to 95 percent prior to the initial contact of the gaseous mixture with the $atalyst by saturating the gaseous mixture with water vapor at a tem-perature in the range of 5C to 20 C lower than the temperature of the initial contact with the catalyst, and heating the resulting gaseous mixture prior to contact with the catalyst to the initial catalyst contact temperature.
Various techniques are useful in the practices of this invention for altering or adjusting the saturation of the carbon monoxide-containing gas such that the carbon monoxide-containing gas is unsaturated with respect to its water vapor content. In the pr.eferred embodiment of the practices of this invention a water saturated carbon monoxide-containing ammonia synthesis feed gas, as might be obtained, employed or ~ . . : -, . .

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produced in an ammonia plant based on the steam reforming and shift conversion of natural gas, is heated or superheated to at least 1 degree Centigrade above the dew point or satura-tion temperature of the carbon monoxide-containing gas so that the resulting carbon monoxide-containing gas is unsatu-rated with respect to its water vapor content. The thus-superheated carbon monoxide-containing gas, now unsaturated with respect to its water vapor content, is then passed into contact with the precious metal catalyst for the selective oxidation of the carbon monoxide therein. Desirably, also, during the selective oxidation operation the resulting reacted gases, now having undergone selective oxidation and having a reduced carbon monoxide content, are maintained unsaturated with respect to water vapor content during the . 15 selective oxidation operation. In the above-mentioned selective oxidation operation oxygen, either as oxygen or as air, if not already present in the carbon monoxide gas undergoing selective oxidation, is added to the carbon monoxide-containing gas such that oxygen is present along ~ 20 with the carbon monoxide in the gaseous mixture undergoing ;~ selective oxidation, preferably an amount of oxygen stoichiometrically sufficient to convert all of the carbon monoxide in the gaseous mixture to carbon dioxide.
Other techniques might be employed in accordance with the practices of this invention for adjusting or alterin~
the water vapor content of the carbon monoxide containing gas : to a value below saturation. For example, the water saturatec carbon monoxide-containing gas may be treated with liquid andjor solid drying agents, such as silica gel, or activated `:
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alumina, to remove at least a portion of the water vapor content of the gas so as to reduce the water vapor content of the resulting treated gas below its saturation value, i.e.
to a relative humidity below 100%.
Another technique in accordance witll the practices of this invention for reducing the water vapor content of the -carbon monoxide-containing gas below saturation level prior to contact with the precious metal cat~lyst involves cooling the water saturated carbon monoxide-containing qas to a temperature below its dew point or saturation temperature, e.g. at least l~C, such as in the range 2-20C below dew point or saturation temperature of the gas, removing the resulting condensed water and then reheating the resulting cooled carbon monoxide-containing gas, now having a reduced water vapor content, to a temperature above its saturation -temperature or dew point so that the resulting reheated gas is now unsaturated with respect to its water vapor content.
Still another technique in accordance with the practices of this invention for reducing the relative humidity or water ; 20 content of the water saturated carbon monoxide-containing gas to a value below saturation involves the addition of another gas to the water saturated carbon monoxide-containing gas such that the resulting gaseous admixture is unsaturated with respect to its water vapor content and thereupon passing the resulting unsaturated gaseous admixture to contact with the ` precious metal catalyst for the selective oxidation of the carbon monoxide thereln. A suitable gas for addition to the . . . . . . . . . . .

carbon monoxide-containing gas to reduce its ~later vapor content below saturation includes oxygen or air or a mixture of nitrogen and oxygen and/or hydrogen, all such gases desirably being unsaturated with respect to water vapor content and desirably at substantially the same temperature ac the carbon monoxide-containing gas so that the resultins gaseous admixture is unsaturated with respect to its water vapor content.
In the practices of this invention the selective oxida-tion of carbon monoxide might be carried out by means of any nurl~er of suitable catalytic materials effective for the selective oxidation of carbon monoxide, particularly the selective oxidation of carbon monoxide in the presence of hydrogen. Suitable catalytic materials include precious metal-containing catalytic materials similar to those for the oxidation of carbon monoxide in internal combustion engine effluents. Of the precious metal-containing catalysts, the platinum metal-containing catalysts are preferred. Mention is made of U.S. Patent 3,631,073, referred to hereinabove, wherein the selective oxidation of carbon monoxide is carried out at atmospheric pressure and a temperature in the range ; 120-160C or at an elevated or superatmospheric pressure in the range 100-200 psig and a temperature in the range 60-120~C. When the selective oxidation operation is carried out under the aforesaid conditions it is preferred to employ a supported platinum-containing catalyst but/ as mentioned in U.S. Patent 3,216,783, other catalysts, such as ruthenium and rhodium-containing catalysts might also be usefully employed.

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9(~0 Platinum metal catalysts are useful for the selective oxidation of carbon monoxide and desirably are promoted by incorporating therein minor amounts of oxides of metals selected from the group consisting of manganese, iron, cobalt, nickel and mixtures thereof. These promoted precious metal catalysts, e.g. platinum-containing catalysts, are particu-larly useful at higher pressures and, indeed, such catalysts are capable of effecting the selective oxidation of carbon monoxide in the presence of hydrogen at lower temperatures than has been possible heretofore with other catalysts.
Accordingly, the selective oxidation operation in accordance with this invention can be carried out at relatively low temperatures, such as a temperature in the range 20-60C, e.g.
at 25-~0C, tllereby taking advantage of improved selectivity with respect to the oxidation of carbon monoxide and higher conversion, both of which are obtainable when relatively low temperatures are employed in the selective oxidation operation .
For example, in accordance with the practices of this inven- ¦
tion it is preerred to process the carbon monoxide-containing gas undergoing selective oxidation to contact the precious metal catalyst at a temperature i.n the range 20-60C since, as indicated hereinabove, a lower feed gas temperature is preferred to a higher feed gas temperature, e.g. above about 60~ioocc. It is pointed out, however, when the practice of this invention is integrated with or employe~ in an ammonia synthesis plant, the pressure of the carbon monoxide-containi g ammonia synthesis gas stream to be treated governs the pressure used in carrying out the selective oxidation of the llC39UO

carbon monoxide therein. The reactor temperature would be selected by considering not only the preferred reaction conditions but also the process temperatures in the ammonia plant upstream and downstream of the selective oxidation reactor.
The preferred catalyst for carrying out the selective oxidation of the carbon monoxide in accordance with this invention would employ one or more platinum group metals, e.g.
platinum, rhodium and/or ruthenium on a suitable support material, such as alumina, silica, silica gel or clay. The precious metal ma~ing up the catalyst would be in the range from about 0.01 to about 5~ by weight on the total catalyst including the support, preferably in the range 0.01-2% by weight. The preparation and availability of such catalysts are well known.
The following examples are illustrative o. the practices of this invention:

Example I

feed gas containing 97.5 mol ~ H2, 2.1 mol % N2, 0.14 mol % 2' 0.14 mol % CO, and water-saturated (about 0.12 mol % H20) was passed at a space velocity of 10,000 V/V-hr over a promoted catalyst containing 0.3 wt.% Pt at a pressure of 2~.2 atmospheres and 26C. Conversion of CO to C2 dropped rapidly as shown in the following table:

Outlet Time on Stream CO % CO
hrs ppm Conversion _ _ 1.4 99.9 64.3 61.6 95.6 97.~ 131.6 90.6 192 201.6 g5.6 ' . : . , . ., . :

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Of particular importance is the increase in CO at the catalyst outlet, since carbon monoxide should be as low as possible.
It may be speculated that the deactivation of the catalys by water saturated gas streams is related to competition between water and carbon monoxide for the active sites Oll the catalyst. In addition to water entering with the feed gas, water is formed by the oxidation of hydrogen. While it is undesirable for economic reasons for hydrogen to be oxidized, i.e. carbon monoxide should be oxidized with a high deyree of selectivity, some of the hydrogen present does oxidize to water. The reaction, however, is highly selective for carbon monoxide and the amount of water produced by the oxidation of hydrogen in the gas stream undergoing treatment is not large.
The effect of increased water content is compensated for by the heat o~ reaction which warms the reacted gas stream.
The effect of reducing the moisture content of the feed gas stream is quite dramatic and maintenance of the catalyst is sharply improved as the moisture content of the gas is reduced. Example II illustrates the substantial improvement in performance when operating at reduced humidity (below lO0~) or with the gas unsaturated with respect to water content in accordance with this invention.

Example II
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~ A water-saturated gas containing 0.1 mol ~ CO, 0.055 mol ;~ 25 % 2' and the balance hydrogen was passed at a space velocity ;~; of lO,000 V/V-hr over a bed of promoted catalyst comprising 0.5 wt.% Pt on alumina at a pressure of 28.2 atmospheres and ~` a temperature of 23C. After about 16 hours on-stream the C0 content at the catalyst outlet ~ose to 500 ppm. Thereafter, the inlet gas was heated with the following results:
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` 1103900 Inlet Gas % E~2O Outlet CO
C Saturation ppm -27 7~.8 2~0 33 55.8 23 Example II illustrates that 4C superheat of the feed gas improves the performance of the catalyst whereas with 10C superheat, a remarkable improvement was obtained and which can be maintained, as illustrated in Exam~le III.

Exam~le III

;A gas which was water-saturated at 47C containing 0.203S mol ~ CO, 0.14 mol % 2~ 19.3 mol % Co2, 0.2 mol ~ A, -20.46 mol % N2, and 59.85 mol ~ H2 was passed over a promoted catalyst with 0.5 wt. % Pt at a space velocity of 19,200 VJV-hr, and a pressure of 21.4 atmospheres. The gas was superheated above the saturation temperature and the~
temperature at the middle of the catalyst bed measured over a period of time.
It should be noted here that since the oxidation of ~carbon monoxide is highly exothermic, a substantial increase in temperature occurs across the catal~st bed. A loss of catalyst performance is indicated by a drop in such a ~temperature increase.
:: -Temperature Increase 25~ Time on-stream Midbed - Inlet C
hrs 3C superheat11C superheat 0 1~ 17

4 176 17 28 ~1.5 17 ~ ~ -15-: ~ `
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It will be seen that ~xample III supports the results illustrated in ~xample II in that increasing superheat enable the catalyst to retain its performance. In this example superheat of 3C is less than desirable and not preferred in order to prevent a gradual loss of activity of the catalyst, as indicated by the slow reduction in temperature difference between the inlet and the middle of the catalyst bed. With 11C superheat, no change of the temperature differential occurred, indicating that the catalyst has retained its activity.
In addition to preventing the deactivation of catalyst it appears that there is an improvement in the average conversion with increased superheat (reduced humidity), as illustrated in Example IV.

Example IV

In the test of Example III the average conversion of carbon monoxide for the two conditions of superheat shown was measured as follows:

Reactor Superheat Average Co at Average CO
Inlet, C Creactor outlet ppm conversion, %

50 3 100 96.5 58 11 45 98.4 It should be noted that since the gas was water-saturated at 47C in Examples III and IV the absolute quantity of moisture remained the same when the temperature was increased to ~; decrease the relative humidity.

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~ ~103~U0 While t;le data of ~xample IV indicate that conversion of carbon monoxide increases as the su~erheat o~ the feed qas stream is increased. this result would aP~ear contrarv to the general principle mentioned earlier, namely, that increasing temperature is detrimental to selectivity and overall conversion. The following data show that when the gas is dry (about 0-1% humidity), the selectivity and conver-stion of carbon monoxide improves with decreasing temperature.

Example V

Reactor Inlet Conversion C CO,96 Conditions for Example V are similar to those for Examples III and IV, namely 21.~ atmospheres pressure, space velocity 10,000 V/V hr, CO concentration at the reactor inlet 0.25 mol %, the remainder of the gas composition as in Example III
The data show that operating at lower temperatures would be preferred to obtain good selectivity and high conversion of CO when the gas is dry and moisture had no significant eEfect on the reaction. Adding superheat according to the present invention raises the feed gas temperature, which should resul ~in a decrease in conversion but, as shown in Example IV, the conversion is increased. It appears that relative humidity o the feed gas is a stronger variable than temperature, since the data of Example IV show that conversion of CO increases when one might have predicted a decrease.

~, In a preferred embodiment of the invention, as applicd to ammonia synthesis and as has been indicated in the fore-going examples, the feed gas temperature is reduced to thé
lowest level economically feasible after shift conversion.
Following the temperature adjustment, any water condensed during the cooling step is removed, leaving the feed gas in a water-saturated condition. Thereafter, the feed gas is super-heated above its saturation temperature, generally in the range of 1 to 20C and more particularly in the range of 5 to lO~C. After superheating, the relative humidity of the feed gas would be in the range of about 95 to 35~, more particu-larly in the range of about 80 to 60~. The desired amount of oxygen or air would be introduced into the feed gas, which typically would be in the range of about 0.5 to about 3 mols oxygen per mol of carbon monoxide, preferably in the range of about 0.5 to about 1Ø It may be noted that, since oxygen which does not react with carbon monoxide will consume hydro-gen, the amount of oxygen is carefully controlled and actually may be less than the amount needed to react with carbon ~ monoxide (0.5:1) under some conditions. The conditioned feed gas is then passed over an effective catalyst to selectively oxidize carbon monoxide, while minimizing the oxidation of hydrogen. Subsequent to the selective oxidation of carbon monoxide, it is typical for the gas stream to be passed to a carbon dioxide removal system which removes substantially all of the carbon dioxide from the gas stream. After carbon dioxide removal, the gas stream typically is passed to a methanation step to complete the removal of carbon monoxide and resldual carbon dioxide by conversion to methane thereby preparing it for the ammonia synthesis reaction. After havinc I oxidized carbon monoxide in the process of the invention, carbon monoxide requiring methanation is much reduced and additional ammonia can be produced.

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Although particularly useful in the preparation or treatment of ammonia synthesis gas, the process of the inven-tion is also useful in other processes where minor amounts of carbon monoxide must be removed. One such situation is the steam reforming of hydrocarbons to produce hydrogen for use ir fuel cells. In such a process carbon dioxide need not be scrubbed out of the gas but carbon monoxide which has a detrimental effect on the fuel cell, should be removed from the gas. The process of the invention may be applied to react carbon monoxide selectively in the presence of hydrogen to form carbon dioxide by adjusting the relative water vapor content of the gas after shift conversion to belcw 100% of saturation, adding a suitable amount of oxygen or air and passing the resulting gas mixture over a catalyst effective for selective oxidation of the carbon monoxide therein.
Although in the preferred embodiment superheating of the gas stream is used to adjust or alter the relative humidity of the feed gas, alternative embodiments or techniques, as already described, may be used as desired. In one alternativ ; 20 embodiment, gas dryers may be used, for examole, fixed beds of activated alumina, silica gel or other desiacants to dry all or a portion of the feed gas stream to adjust the humidit thereof below saturation. Liquid drying facilities may also be applied to accomplish the same result. In one typical useful embodiment of the practices of this invention a slip or side stream is taken from the main feed gas stream and dried with a desiccant to produce a-substantially water-free gas stream, which is then reintroduced into the main feed gas stream to provide a gas stream havins a relative humidity below 100% or saturation.

In another alternative embodiment, refrigeration equip-ment is used to cool the gas stream below ambient temperature in order to remove a substantially greater amount of moisture than is possible with usual or typical cooling facilities S which reject heat to water or air at essentially ambient temperature. An advantage for such a technique is that a lower temperature is available for the oxidation step than when cooling to ambient temperature is used. After cooling, condensed water is removed and the gas superheated to reduce ~ humidity in a similar fashion to that described in connec-tion with the preferred embodiment.
In still another alternative embodiment a gas stream from an extraneous source, e.g. 2 or N2 from an oxygen plant or air li~uefaction plant and substantially free of water is introduced into the feed gas stream in order to adjust the water content. Thereafter, if necessary, oxygen or air is added to the carbon monoxide-containing feed gas and the resulting gas mixture passed over a catalyst to selectively oxidize the carbon monoxide therein.
In summary, in the practice of this invention it is an essential feature thereof that the carbon monoxide-containing ; gas stream and a sufficient amount of oxy~en for the conver-~;~ sion of the carbon monoxide therein to carbon dioxide, is treated or adjusted such that its water vapor content is below saturation. Thereupon the water-unsaturated carbon monoxide-containing gas stream is contacted with a precious metal catalyst effective for the selective oxidation of the carbon monoxide therein to carbon dioxide. Although it is preferred to carry out the catalytic selective oxidation of the carbon monoxide at a relatively low temperature in the range 20-60C higher temperatures might be employed, such as a temperature in the range 100-200~C, and higher. Similarly, the selective oxidation of the carbon monoxide in the gas undergoing treatment may be carried out at any suitable pressure, ambient, superatmospheric or subatmospheric pressure, such as a pressure in,the range from about 0.5 psia up to about 400 psig or higher. In the practice of this invention it is usually preferred to treat the carbon monoxide _ containing gas stream for the selective oxidation of the carbon monoxide therein at the temperature and pressure substantially existing in the gaseous stream prior to adjust-ment of the water vapor content therein below saturation.
The practice of this invention is particularly applicable to the treatment of a carbon monoxide-containing gas stream wherein the carbon monoxide is present therein in a minor amount, such as in the range 0.1-2% by volume, although a wider range would be operable. The practice of this inven-tion is particularly applicable to the treatment of a carbon monoxide-containing gas stream which contains a major amount of other gases, such as hydrogen or a mixture of hydrogen and nitrogen, in the molar proportions 3:1, as in an ammonia synthesis gas.
Other embodiments will be apprent to those s~illed in the art in view of the disclosure made herein and which are considered to be within the scope of the invetion as claimed hereafter.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a process for the selective oxidation of carbon monoxide in a gaseous mixture comprising carbon monoxide, hydrogen and water vapor, carbon monoxide comprising from about 0.1 to about 2 volume percent of said gaseous mixture and hydrogen comprising a major amount of said gaseous mixture, and wherein said gaseous mixture is passed at a temperature in the range of about 20 C to about 60 C into initial contact with a precious metal catalyst effective for the selective oxidation of carbon monoxide to carbon dioxide in the presence of hydrogen and in the presence of oxygen, said oxygen being present in an amount in the range of 0.5 to 3 mols oxygen per mol of said carbon monoxide, the improvement which comprises adjusting the water vapor content of said gaseous mixture is within the range of 35 to 95 percent prior to said initial contact of said gaseous mixture with said catalyst by saturating said gaseous mixture with water vapor at a temperature in the range of 5°C to 20°C lower than the temperature of said initial contact with said catalyst, and heating the resulting gaseous mixture prior to contact with said catalyst to said initial catalyst contact temperature.

2. A process in accordance with claim 1 wherein said catalyst is a platinum containing catalyst.

3. A process in accordance with claim 2 wherein said platinum-containing catalyst is promoted with a minor effective amount of an oxide of a metal selected from the group consisting of manganese, iron, cobalt, nickel and mixtures thereof.

4. A process in accordance with claim 1 wherein said gaseous mixture also includes nitrogen and carbon dioxide.

5. A process in accordance with claim 1 wherein said initial reaction temperature is within the range of 20°C to 40°C.

6. A process in accordance with claim 5 wherein said oxygen is present in an amount within the range of 0.5 to 1 mol oxygen per mol carbon monoxide.

7. A process in accordance with claim 5 wherein said saturation tem-perature is in the range of 5°C to 10°C below the temperature of said initial contact with the catalyst.

8. A process in accordance with claim 5 wherein the relative humidity of said gaseous mixture is within the range of 60 to 80 percent.