CA1300850C - Catalysts for absorptive air separation - Google Patents
Catalysts for absorptive air separationInfo
- Publication number
- CA1300850C CA1300850C CA000576459A CA576459A CA1300850C CA 1300850 C CA1300850 C CA 1300850C CA 000576459 A CA000576459 A CA 000576459A CA 576459 A CA576459 A CA 576459A CA 1300850 C CA1300850 C CA 1300850C
- Authority
- CA
- Canada
- Prior art keywords
- oxygen
- acceptor
- metal
- transition metal
- present
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
Classifications
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B13/00—Oxygen; Ozone; Oxides or hydroxides in general
- C01B13/02—Preparation of oxygen
- C01B13/0229—Purification or separation processes
- C01B13/0233—Chemical processing only
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B2210/00—Purification or separation of specific gases
- C01B2210/0043—Impurity removed
- C01B2210/0046—Nitrogen
Abstract
211-P-US03308 ABSTRACT The present invention is directed to catalysts for the absorptive sepa-ration of oxygen from oxygen-containing gas mixtures, such as air, using re-versible chemical reaction systems, such as, . The catalytic action is the result of the addition of transition metal oxides to the oxygen accepting system.
Description
- ~Q~
CATALYSTS FOR ABSORPTIVE AIR
SEPARATION
TECHNICAL_FIELD
The present 7nvention ls directed to the catalysis of the absorpt1ve separation:of oxygen ~rom an oxygen-conta~ning gas us7ng certa7n trans7-tion metal ox7des. More spec7fically the present 7nventlon is dlrected to the catalys7s of a nitrate-nltr7te system w7th transit~on metal oxides showlng effective redox chemistry.
BACKGROUN~ OF THE PRIOR ART
-It is known in the prior art to absorb o~yg~n from oxygen-contalning gases using var70us chemically bind7ng agents to extract ava71able oxygen : 10 from gas streams such as a7r. For 7nstance it 7s known to use bar7um oxide sodium manganese oxide stront7um oxlde mercury copper chloride praseodymlum or cerium oxldes chrome ox7des strontlum-chromlum oxides alkal7 metal n7trate-n7tr7tes and alkall metal perox7des to reversibly absorb oxygen from an oxygen-conta7n7ng ~lu7d.
Exemplary of the nitrate-n7tr7te oxygen absorption system 7s U.5.
Patent 4 13~ 766 ln whlch oxygen ~s extracted from air us7ng alkal7 met-al nitrate and nitrite molten salt 17quids. The patent addresses the : problem of decomposlt70n of:the molten salt 17quids to ox7des or super-oxldes. The patent further alludes to-the problem wlth the presence af 20 water and~carbon dlox7de in the feed alr to an absorptive separatlon of oxygen from a7r us7ng such a n7trate-n7trite sys~em as well as other : ~ known chem7~cal absorptive separat70ns of oxygen.
U.5.:Patent 4 287 17Q disclosed an 7mprovement ln nitrate-nitrlt~
` absorpt7ve chemical separation~:of~oxygen from a7r where~n addltional 5~trace~quantit7es of oxygen are removed~from an ~nitial separatlon ef-~:~: fluen~ us7ng an oxygen scavenger :such as manganese oxide. Other metall7c~oxygen scavengers :are mentioned such as copper iron nickel cobalt9 vanadium tin chromium lead and blsmuth oxldes. These ox7des are:not m7xed wlth the nitrate-nitrite bath but are contalned 7n the 30 separately operated closed-circuit scavenger subcycle.
::: : :
,.. ... .. ...
U.S. Patent 4,340,578 d7scloses yet another 7mprovement 7n oxygen separat70n from a7r streams us7ng a chem7cal absorpt7ve separatory agent, ~nclud~ng n~trate-n7tr7te molten salt baths, where7n such bath conta7ns an add7t~onal amount of peroxide and superoxide. It ls noted ~n th7s S patent that the nitrate-nitr1te mo1ten salt bath is suscept7ble to de-compos~t70n 7nto the respective metal oxide, perox7des and super oxides, whlch w711 have a detrlmental affect on salt concentrations, as we11 as corros~on of process equipment.
In U.S. Patent 4,529,577 it is noted that ox7de levels ~n a molten lG salt bath or oxygen absorptlve separation ~rom oxygen-containing gas, had previously been ma7ntalned in the 1 to 2% range ~n order to m7nim7ze cor-ros70n. The presently dlscussed patent teaches that these ox7des should be malnta~ned below 1 moleX, based upon sod7um perox7de, 7n order to avo7d extens7ve corros70n problems in such an overall process.
It can be seen that alkal7 metal ox7des 7ncluding perox7des and supero~ides used 7n chemical absorpt7ve separat~ons, such as the alkal7 metal n1trate-n7trite systems, have been known 7n the pr70r art to pro-v7de necessary catalys~s to that separatory process, but are known 7n the pr70r art to be deact~vated by water and car60n dfox7de, wh7ch are 2~ typ~cally found in alr, the most prevalent source of an oxygen-contain7ng gas from which oxygen would be separated. Add~t70nally, 7t has been noted that nitrogen dioxlde deactivates the alkal7 metal oxides present ln an alkal7 metal nltrate-nitr7te molten salt bath, desp7te the ut71~ty of nltrogen dlox7de to avo~d the decompos7tion of the nltrate-n7trite 2S system. Accordlngly, a problem exists 7n the prior art w7th the use of alkal~ metal oxides as catalysts for a nltrate-n7trite oxygen separatory system. Such alkal7 metal oxldes are typically removed from a cont7nu-ously operatlng process by react70n wlth the mater~als of constructlon of the process plant, react~on with the feed lmpur7tles presently ex~stlng ln untreated alr, such as water and carbon d~ox7de, and by vapor~zat~on of the alkal~ metal oxides at the h7gh temperatures of operat70n neces-sary for alkal7 metal nitrate-n7trite molten salt bath separatory sys-tems. Once the alkall metal ox7de levels 7n the nltrate-n7trite salt mixture are removed or reduced in concentration, th7s oxygen separatory reaction does not occur at commercially ~eas7ble or econom~c rates.
s~
In order to overcome th1s problem with alkali metal ox~de catalysts ln chemical absorptive separatory systems, such as the alkall meta1 nltrate-nitr~te system, 1t has been suggested to replen1sh the alkall metal ox1des continuously during the cont~nuous operat10n of the under-1ylng process. Alternatively, it has been suggested to generate add~-tlonal alkall metal ox1de spec~es in s1tu, presumably by the decsmpos1-t~on of the nitrate-nitrite system to the detr1ment of that systems concentratlon 1n the overall process.
Another teach1ng of the necessity of alkal1 metal ox~des for cat-alysis ~n such systems 1s d1sclosed (F. Par1ccia and P. C. Zambon~n, J. Phys. Chem. 78, 1693 [1974]). Such alkali metal ox1des are present ~n the form of 02-,02- and 2- (P- G. Zambonin, Electroanalytlca1 Chem. and Interface Electrochemistry, 45, 451 [1973]). Add1tlonally, 1t 1s also taught elsewhere 1n the pr~or art that water and carbon dlox~de lS react w~th the alkal~ metal ox~des in molten alkal~ nltrates, as set forth in (P. G. Zambon1n, Anal. Chem. 44, 763 ~1972]; P. G. Zambon~n, Anal. Chem. 43, 1571 ~1971~). The art has recognized that the removal of these alkal~ ~etal oxldes by any mechanism or theory, resu1ts 1n the slow k~net1cs of react10n for the oxygen uptake 1n a n~trate-nltr~te system, as set forth 1n (F. Pallm1sano, L. Sabbat1n1 and P. G. Zambonin, J. Chem.
Soc. Faraday Trans. 1, 80, 1029 [1984]; D. A. Nlssen and 0. E. Meeker, Inorg. Chem. 22, 716 C1983]). However, the art has recognized that such alkal1 metal ox1des can be generated 1n s~tu to replace those that are lost by var10us mechan1sms, but th~s resul~s 1n a decrease in the alkall metal nitrate-n1tr1te (C. M. Kramer, Z. A. Munln and K. H. Stern, H~gh Temp. Sci. 16, 257 ~1983]).
Transltlon metal oxides are known to have extenslve redox chem~stry ; (F. A. Cotton and G. Wllklnson, Advanced Inorgan k Chemlstry, 4th Ed.
30hn W~ley and Sons, Inc. New York, 1980), and in contrast to alkal~
metal ox1des, form less stable oxy-anions (K. ~. Stern, J. Chem Educa-tlon, 46, 645 tl969]). The redox potent1als of alkal~ metal nitr~te 1s known from (M. H. M~les and A. N. Fle~cher, J. Electrochem. Soc., 127, 1761 [1980]). The redox behav10r of various transition metal compounds in molten salts has also been discussed ~n the prior art (D. H. Kerridge 3S "Molten Salts as Nonaqueous Solvents", The Chemistry of Nonaqueous Sol-~3~8S~
vents, J.J. Lagowski, ed., Academic Press, New York, 1978, pages 269-329). Finally, it is known in the prior art that oxide formation from a nitrate-nitrite molten salt bath can be suppressed by the introduction of additional amounts of nitrogen dioxide (E. Plumat, A.
Labani and M. Ghodsi, J. Electrochem. Soc. 130, 2192 [1983]).
Ths present invention overcomes the problem of adequately catalyzing a chemical absorptive separation of oxygen from an oxygen-containing gas, wherein corrosion problems are minimized, water and carbon dioxide contamination do not constitute significant operational problems and an unexpected heightened activity is recognized to the benefit of the overall oxygen separatory process.
BRIEF SUMMARY OF THE INVENTION
In accordance with an embodiment of the present invention there is provided in a method for separating oxygen from an oxygen-containing gas comprising reacting ; an oxygen acceptor consisting essentially of a molten salt sol~ltion of alkali metal nitrates and nitrites with the gas in an absorption reaction to produce an oxygen depleted gas effluent, separately decomposing the oxidized acceptor to yield oxygen and regenerated oxygen acceptor, an improvement comprising adding to the oxygen acceptor a catalytic amount of a transition metal oxide selected from the group consisting of oxides of manganese, ruthenium, rhenium, osmium, rhodium, iridium and mixtures thereof.
Preferably, the method is performed in a continual manner, whereby the regenerated oxygen acceptor is recycled to react with gas in said absorption reaction.
Optimally, the ~ransition metal oxide is selected , . .
~. .
3L3~5~
-from the group consisting of NaRuO4, KMnO4, MnO2, RuO2, and mixtures thereof.
Preferably, the transition metal oxide is present in an amount in the range of approximately 0.2 to 3 mole %
of the oxygen acceptor.
In accordance with another embodiment of the present invention there is provided in a method for recovering oxygen or nitrogen from air comprising reacting an oxygen acceptor consisting essentially of a molten salt solution of alkali metal nitrates and nitrites with air in an absorption reaction to produce a nitrogen-rich effluent, separately decomposing the oxidized acceptor to yield oxygen and regenerated oxygen acceptor, an improvement comprising adding to the oxygen acceptor a catalytic amount of a transition metal oxide selected from the group consisting of oxides of manganese, ruthenium, rhenium, osmium, rhodium, iridium and mixtures thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph of the activity of the transition metal oxide species of the present invention (NaRuO4, KMnO4) as catalysts for the reversible reaction of oxygen with an alkali metal nitrate-nitrite molten salt bath, relative to the activity of alkali metal oxides of the prior art, for the same reaction.
FIG. 2 is a graph of relative activity of a catalyzed alkali metal nitrate-nitrite molten salt bath comparing low concentrations of the transition metal oxide catalyst of the present invention against high concentrations of the alkali metal oxide catalyst of the prior art.
FIG. 3 is a graph of the relative activity of a catalyzed alkali metal nitrate-nitrite molten salt bath :: :
~A ~
~ ", j, . ~ , when nitrog~n dioxide is administered to reduce the insitu production of alkali metal oxides.
FIG. 4 is a graph of relative activity of a catalyzed alkali metal nitrate-nitrite molten salt bath, showing how catalytic activity is affected by nitrogen dioxide, which inhibits alkali metal oxide insitu formation and carbon dioxide which selectively reacts with alkali metal oxides in contrast to the transition metal oxides of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides transition metal oxide catalysts for enhancing the reversible oxygen binding of various oxygen selective chemical absorbents for use in a continuous process, for the separation of oxygen from an oxygen containing gas, such as the recovery of oxygen from air to result in either an oxygen product, or two products, oxygen and nitrogen in commercially acceptable purities.
The chemical ab~orptive separatory systems of the present invention, which can be successfully and unexpectedly catalyzed with the addition of certain transition metal oxides, include known oxygen acceptor materials such as alkali metal nitrates and nitrites and alkali me.tal oxides, pernxides or superoxides. Oxygen segregation from oxygen-containing gas streams can be done in either a batch or continuous mode.
Typically, oxygen is separated from the oxygen-containing gas by a regenerative chemical process. The oxygen-containing gas is contacted with an oxygen acceptor, such as a molten solution of alkali metal nitrate and nitrite salts at elevated temperature and pressure, causing the oxygen to react with the nitrite, and thereby increasing the proportion of nitrate in the :~..;A
13(~ 0 - 6a -salt solution. The oxidized oxygen acceptor is separated from the oxygen-depleted air, and then the oxygen partial pressure is reduced by, for example, reducing the pressure or elevating the temperature or both, thereby causing the release of relatively pure oxygen, which is collected. The oxygen acceptor, restored to its approximate original composition, is recycled to the oxidation step. Since the oxygen acceptor remains in the liquid state throughout the cycle, both salt to salt heat exchange and salt circulation are facilitated, making possible a continuous process of high efficiency. Such a process is set forth in U.S. Patent 4,132,766, U.S.
Patent 4,287,170, U.S. Patent 4,340,578, and U.~. Patent 4,529,577. The subsequent discussion will utilize a preferred embodiment to describe the present invention, which is an alkali metal nitrate and nitrite molten salt solution utili~ed to extract oxygen from air, to result in an oxygen and/or nitrogen product, but it is understood that the description is applicable to the other systems described above and should not be deemed to be limited to the particular exemplary description of the preferred embodiment.
The transition metal oxides of the present invention include the oxides of manganese, ruthenium, rhenium, osmium, rhodium, iridium and mixtures thereof.
Specifically, the preferred transition metal oxides are NaRuO4, KMnO4, MnO2, RU02 and mixtures thereof. These transition metal oxides display appropriate redox chemistry for integration with the oxygen acceptors, most specifically, the alkali metal nitrate and nitrite systems. Although the present inventors do not wish to be held to any :::
~300 speclf~c theory as to why the transitlon metal oxldes ldentlfled above prov~de enhanced catalytic act~v1ty for regenerable oxygen acceptors, lt ls suggested that the interconvers~on of ox~des, peroxldes and super oxlde specles, g~ves a catalyzed oxygen-generatlng system. Such transl-tlon metal oxides possess redox chemlstry wh~ch have several lntereon-vertlble oxldatlon states withln the redox wlndows of the oxygen ac-ceptors, most notably the alkall metal nltrate and nltrites. Other transit~on metal oxldes outslde the group set forth above are not deemed to have aceeptable catalytle actlvlty, because they are ox1dlzed to thelr 10 hlghest oxidatlon states by the known oxygen acceptors and notably by the alkal~ metal nitrate and nltr~te solution, to stable ox~des whlch would not render sufflclent act~vlty to catalyze the oxygen absorptlon-desorp-t10n reaction favored hereln. Others of the transitlon metal oxldes not identifled above would decompose ~nto oxygen and the elemental metal upon 15 exposure to the oxygen acceptor. Some other transltlon metals may be un-acceptably radloactlve. Therefore, the transltlon metal oxldes ldent~fled above, ~n contrast to the remaln~ng transitlon metal ox~des llsted ~n the perlodlc table of the elements, are effect1ve catalysts, because they have an approprlate redox potentlal.
The transltlon metal oxldes of the present inventlon not only ex-hlblt enhanced catalytlc actlvlty, but may be expected to reduce the problem of corroslon wlth mater~als of constructlon of the overall separatory process, because of the freedom of removlng alkall and metal ox~des from the oxygen acceptor bath and the potent~al of runn~ng the reaction and partlcularly the desorption reactor at lower temperatures.
The trans~t~on metal o~lde catalysts of the present lnventlon also are not deactlvated by water and carbon dioxide ln the feed oxygen-conta~nlng gas or alr to the process, whlch is attract1ve ln that 1t ellmlnates the-necesslty for pretreatment of the feed to the process. Flnally, the translt~on metal ox1des of the present inventlon w~ll not be as eas~ly vaporized and lost from the system as the a1kall metal oxides of the prlor art.
Wlth reference to the preferred embodlment of the present 1nvent~on ln which the oxygen acceptor ~s a molten salt solutlon of alkall metal nltrate and nitr~te, the present lnvent~on w~ll be set forth whereln the ~L3~5t~
. , cycl~c oxygen absorptlon and desorptlon ~s predlcated on the followlng equat10n.
kl 1 (1) N03 ~ N02 + 2 2 k_l The feasib111ty of us~ng Equatlon 1, relles upon the thermodynamics and the klnet~cs of the reactlon, whlch have been shown to be strongly 10 dependent upon the amount of alkali metal oxldes ~n the solut~on. These alkall metal oxldes, whlch are present as oxides, peroxldes and super oxides, act as catalysts for Equat~on 1, through a redox mechan~sm. The prlor art has rel~ed upon the add~tlon of alkall metal oxides, such as Na202 or upon the lns~tu generat~on of oxlde specles by the thermal 15 decompositlon of the nltrate salt ltself, to ach~eve reasonable react10n rates. These oxldes, however, can be lost through a number of processes lncludlng vaporlzatlon and reactlon wlth the contalnment vessel. Also, lmpurltles ln alr, particularly carbon dioxlde and water react with the oxldes ln the solutlon by reactlon, such as Equaff on 2 and 3 below.
(2) C2 + o2 ~ C032 (3) H20 ~ o2 ~ 2 OH
W~thout the presence of such ox~des the k~netlcs of quatlon 1 are very slow and problematlc from a commerclal, economic perspective.
The present lnventlon overcomes the problems of the prlor art alka71 metal oxides by us~ng the alternative trans~tlon metal ox1des of th~ pres-ent ~nvent~on for catalyz~ng Equat10n 1, whereln such catalyt~c o~ides are not effected by alr lmpurlt~es, such as water and carbon d70x~de, and have b~en ~ound unexpectedly to ~mpart faster klnet~cs to the react~on shown ln Equatlon 1. The transitlon metal ox~des of the present lnven-t~on catalyze these reactlons by havlng the approprlate redox potential necessary to react in a revers~ble manner wlth the nltrate and nitrlte of :
, . .
130~
the molten salt bath, const~tuttng the oxygen-selectlve oxygen acceptor.
The transltion metal oxide catalyst of the present ~nvention also have unstable hydroxides and carbonates at the operatlng temperatures of the process, so that the presence of carbon dioxlde and water will not affect the kinet~cs of the catalyzed reaction, because the react~on of the tran-sit~on metal ox~des with such contaminants w~ll not remove the oxides on a permanent basls from the reaction system. Such reactlon temperature ~s preferably in the range of 450 to 6759C for the nitrate-nitrlte oxygen acceptor system.
The transition metal ox~des of the present ~nvent1On are ~ffect~ve catalysts because of their actlve redox chem~stry wh~ch results ~n the format1On of less stable oxy-anlons than alkali metal oxldes. The redox chemistry of the transltlon metal oxldes of the present ~nvention when used with an alkal~ metal nitrate-n~tr~te molten salt solut~on are found to be reversible, so that they can adequately catalyze the reaction shown in Equatlon l. These trans~tion metal oxides of the present invention are not overly ox~dizlng so that they are not lrrevers~bly reduced by the nitrite and nltrate half reactions, Equations 4 and 5, respectlvely below.
CATALYSTS FOR ABSORPTIVE AIR
SEPARATION
TECHNICAL_FIELD
The present 7nvention ls directed to the catalysis of the absorpt1ve separation:of oxygen ~rom an oxygen-conta~ning gas us7ng certa7n trans7-tion metal ox7des. More spec7fically the present 7nventlon is dlrected to the catalys7s of a nitrate-nltr7te system w7th transit~on metal oxides showlng effective redox chemistry.
BACKGROUN~ OF THE PRIOR ART
-It is known in the prior art to absorb o~yg~n from oxygen-contalning gases using var70us chemically bind7ng agents to extract ava71able oxygen : 10 from gas streams such as a7r. For 7nstance it 7s known to use bar7um oxide sodium manganese oxide stront7um oxlde mercury copper chloride praseodymlum or cerium oxldes chrome ox7des strontlum-chromlum oxides alkal7 metal n7trate-n7tr7tes and alkall metal perox7des to reversibly absorb oxygen from an oxygen-conta7n7ng ~lu7d.
Exemplary of the nitrate-n7tr7te oxygen absorption system 7s U.5.
Patent 4 13~ 766 ln whlch oxygen ~s extracted from air us7ng alkal7 met-al nitrate and nitrite molten salt 17quids. The patent addresses the : problem of decomposlt70n of:the molten salt 17quids to ox7des or super-oxldes. The patent further alludes to-the problem wlth the presence af 20 water and~carbon dlox7de in the feed alr to an absorptive separatlon of oxygen from a7r us7ng such a n7trate-n7trite sys~em as well as other : ~ known chem7~cal absorptive separat70ns of oxygen.
U.5.:Patent 4 287 17Q disclosed an 7mprovement ln nitrate-nitrlt~
` absorpt7ve chemical separation~:of~oxygen from a7r where~n addltional 5~trace~quantit7es of oxygen are removed~from an ~nitial separatlon ef-~:~: fluen~ us7ng an oxygen scavenger :such as manganese oxide. Other metall7c~oxygen scavengers :are mentioned such as copper iron nickel cobalt9 vanadium tin chromium lead and blsmuth oxldes. These ox7des are:not m7xed wlth the nitrate-nitrite bath but are contalned 7n the 30 separately operated closed-circuit scavenger subcycle.
::: : :
,.. ... .. ...
U.S. Patent 4,340,578 d7scloses yet another 7mprovement 7n oxygen separat70n from a7r streams us7ng a chem7cal absorpt7ve separatory agent, ~nclud~ng n~trate-n7tr7te molten salt baths, where7n such bath conta7ns an add7t~onal amount of peroxide and superoxide. It ls noted ~n th7s S patent that the nitrate-nitr1te mo1ten salt bath is suscept7ble to de-compos~t70n 7nto the respective metal oxide, perox7des and super oxides, whlch w711 have a detrlmental affect on salt concentrations, as we11 as corros~on of process equipment.
In U.S. Patent 4,529,577 it is noted that ox7de levels ~n a molten lG salt bath or oxygen absorptlve separation ~rom oxygen-containing gas, had previously been ma7ntalned in the 1 to 2% range ~n order to m7nim7ze cor-ros70n. The presently dlscussed patent teaches that these ox7des should be malnta~ned below 1 moleX, based upon sod7um perox7de, 7n order to avo7d extens7ve corros70n problems in such an overall process.
It can be seen that alkal7 metal ox7des 7ncluding perox7des and supero~ides used 7n chemical absorpt7ve separat~ons, such as the alkal7 metal n1trate-n7trite systems, have been known 7n the pr70r art to pro-v7de necessary catalys~s to that separatory process, but are known 7n the pr70r art to be deact~vated by water and car60n dfox7de, wh7ch are 2~ typ~cally found in alr, the most prevalent source of an oxygen-contain7ng gas from which oxygen would be separated. Add~t70nally, 7t has been noted that nitrogen dioxlde deactivates the alkal7 metal oxides present ln an alkal7 metal nltrate-nitr7te molten salt bath, desp7te the ut71~ty of nltrogen dlox7de to avo~d the decompos7tion of the nltrate-n7trite 2S system. Accordlngly, a problem exists 7n the prior art w7th the use of alkal~ metal oxides as catalysts for a nltrate-n7trite oxygen separatory system. Such alkal7 metal oxldes are typically removed from a cont7nu-ously operatlng process by react70n wlth the mater~als of constructlon of the process plant, react~on with the feed lmpur7tles presently ex~stlng ln untreated alr, such as water and carbon d~ox7de, and by vapor~zat~on of the alkal~ metal oxides at the h7gh temperatures of operat70n neces-sary for alkal7 metal nitrate-n7trite molten salt bath separatory sys-tems. Once the alkall metal ox7de levels 7n the nltrate-n7trite salt mixture are removed or reduced in concentration, th7s oxygen separatory reaction does not occur at commercially ~eas7ble or econom~c rates.
s~
In order to overcome th1s problem with alkali metal ox~de catalysts ln chemical absorptive separatory systems, such as the alkall meta1 nltrate-nitr~te system, 1t has been suggested to replen1sh the alkall metal ox1des continuously during the cont~nuous operat10n of the under-1ylng process. Alternatively, it has been suggested to generate add~-tlonal alkall metal ox1de spec~es in s1tu, presumably by the decsmpos1-t~on of the nitrate-nitrite system to the detr1ment of that systems concentratlon 1n the overall process.
Another teach1ng of the necessity of alkal1 metal ox~des for cat-alysis ~n such systems 1s d1sclosed (F. Par1ccia and P. C. Zambon~n, J. Phys. Chem. 78, 1693 [1974]). Such alkali metal ox1des are present ~n the form of 02-,02- and 2- (P- G. Zambonin, Electroanalytlca1 Chem. and Interface Electrochemistry, 45, 451 [1973]). Add1tlonally, 1t 1s also taught elsewhere 1n the pr~or art that water and carbon dlox~de lS react w~th the alkal~ metal ox~des in molten alkal~ nltrates, as set forth in (P. G. Zambon1n, Anal. Chem. 44, 763 ~1972]; P. G. Zambon~n, Anal. Chem. 43, 1571 ~1971~). The art has recognized that the removal of these alkal~ ~etal oxldes by any mechanism or theory, resu1ts 1n the slow k~net1cs of react10n for the oxygen uptake 1n a n~trate-nltr~te system, as set forth 1n (F. Pallm1sano, L. Sabbat1n1 and P. G. Zambonin, J. Chem.
Soc. Faraday Trans. 1, 80, 1029 [1984]; D. A. Nlssen and 0. E. Meeker, Inorg. Chem. 22, 716 C1983]). However, the art has recognized that such alkal1 metal ox1des can be generated 1n s~tu to replace those that are lost by var10us mechan1sms, but th~s resul~s 1n a decrease in the alkall metal nitrate-n1tr1te (C. M. Kramer, Z. A. Munln and K. H. Stern, H~gh Temp. Sci. 16, 257 ~1983]).
Transltlon metal oxides are known to have extenslve redox chem~stry ; (F. A. Cotton and G. Wllklnson, Advanced Inorgan k Chemlstry, 4th Ed.
30hn W~ley and Sons, Inc. New York, 1980), and in contrast to alkal~
metal ox1des, form less stable oxy-anions (K. ~. Stern, J. Chem Educa-tlon, 46, 645 tl969]). The redox potent1als of alkal~ metal nitr~te 1s known from (M. H. M~les and A. N. Fle~cher, J. Electrochem. Soc., 127, 1761 [1980]). The redox behav10r of various transition metal compounds in molten salts has also been discussed ~n the prior art (D. H. Kerridge 3S "Molten Salts as Nonaqueous Solvents", The Chemistry of Nonaqueous Sol-~3~8S~
vents, J.J. Lagowski, ed., Academic Press, New York, 1978, pages 269-329). Finally, it is known in the prior art that oxide formation from a nitrate-nitrite molten salt bath can be suppressed by the introduction of additional amounts of nitrogen dioxide (E. Plumat, A.
Labani and M. Ghodsi, J. Electrochem. Soc. 130, 2192 [1983]).
Ths present invention overcomes the problem of adequately catalyzing a chemical absorptive separation of oxygen from an oxygen-containing gas, wherein corrosion problems are minimized, water and carbon dioxide contamination do not constitute significant operational problems and an unexpected heightened activity is recognized to the benefit of the overall oxygen separatory process.
BRIEF SUMMARY OF THE INVENTION
In accordance with an embodiment of the present invention there is provided in a method for separating oxygen from an oxygen-containing gas comprising reacting ; an oxygen acceptor consisting essentially of a molten salt sol~ltion of alkali metal nitrates and nitrites with the gas in an absorption reaction to produce an oxygen depleted gas effluent, separately decomposing the oxidized acceptor to yield oxygen and regenerated oxygen acceptor, an improvement comprising adding to the oxygen acceptor a catalytic amount of a transition metal oxide selected from the group consisting of oxides of manganese, ruthenium, rhenium, osmium, rhodium, iridium and mixtures thereof.
Preferably, the method is performed in a continual manner, whereby the regenerated oxygen acceptor is recycled to react with gas in said absorption reaction.
Optimally, the ~ransition metal oxide is selected , . .
~. .
3L3~5~
-from the group consisting of NaRuO4, KMnO4, MnO2, RuO2, and mixtures thereof.
Preferably, the transition metal oxide is present in an amount in the range of approximately 0.2 to 3 mole %
of the oxygen acceptor.
In accordance with another embodiment of the present invention there is provided in a method for recovering oxygen or nitrogen from air comprising reacting an oxygen acceptor consisting essentially of a molten salt solution of alkali metal nitrates and nitrites with air in an absorption reaction to produce a nitrogen-rich effluent, separately decomposing the oxidized acceptor to yield oxygen and regenerated oxygen acceptor, an improvement comprising adding to the oxygen acceptor a catalytic amount of a transition metal oxide selected from the group consisting of oxides of manganese, ruthenium, rhenium, osmium, rhodium, iridium and mixtures thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph of the activity of the transition metal oxide species of the present invention (NaRuO4, KMnO4) as catalysts for the reversible reaction of oxygen with an alkali metal nitrate-nitrite molten salt bath, relative to the activity of alkali metal oxides of the prior art, for the same reaction.
FIG. 2 is a graph of relative activity of a catalyzed alkali metal nitrate-nitrite molten salt bath comparing low concentrations of the transition metal oxide catalyst of the present invention against high concentrations of the alkali metal oxide catalyst of the prior art.
FIG. 3 is a graph of the relative activity of a catalyzed alkali metal nitrate-nitrite molten salt bath :: :
~A ~
~ ", j, . ~ , when nitrog~n dioxide is administered to reduce the insitu production of alkali metal oxides.
FIG. 4 is a graph of relative activity of a catalyzed alkali metal nitrate-nitrite molten salt bath, showing how catalytic activity is affected by nitrogen dioxide, which inhibits alkali metal oxide insitu formation and carbon dioxide which selectively reacts with alkali metal oxides in contrast to the transition metal oxides of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides transition metal oxide catalysts for enhancing the reversible oxygen binding of various oxygen selective chemical absorbents for use in a continuous process, for the separation of oxygen from an oxygen containing gas, such as the recovery of oxygen from air to result in either an oxygen product, or two products, oxygen and nitrogen in commercially acceptable purities.
The chemical ab~orptive separatory systems of the present invention, which can be successfully and unexpectedly catalyzed with the addition of certain transition metal oxides, include known oxygen acceptor materials such as alkali metal nitrates and nitrites and alkali me.tal oxides, pernxides or superoxides. Oxygen segregation from oxygen-containing gas streams can be done in either a batch or continuous mode.
Typically, oxygen is separated from the oxygen-containing gas by a regenerative chemical process. The oxygen-containing gas is contacted with an oxygen acceptor, such as a molten solution of alkali metal nitrate and nitrite salts at elevated temperature and pressure, causing the oxygen to react with the nitrite, and thereby increasing the proportion of nitrate in the :~..;A
13(~ 0 - 6a -salt solution. The oxidized oxygen acceptor is separated from the oxygen-depleted air, and then the oxygen partial pressure is reduced by, for example, reducing the pressure or elevating the temperature or both, thereby causing the release of relatively pure oxygen, which is collected. The oxygen acceptor, restored to its approximate original composition, is recycled to the oxidation step. Since the oxygen acceptor remains in the liquid state throughout the cycle, both salt to salt heat exchange and salt circulation are facilitated, making possible a continuous process of high efficiency. Such a process is set forth in U.S. Patent 4,132,766, U.S.
Patent 4,287,170, U.S. Patent 4,340,578, and U.~. Patent 4,529,577. The subsequent discussion will utilize a preferred embodiment to describe the present invention, which is an alkali metal nitrate and nitrite molten salt solution utili~ed to extract oxygen from air, to result in an oxygen and/or nitrogen product, but it is understood that the description is applicable to the other systems described above and should not be deemed to be limited to the particular exemplary description of the preferred embodiment.
The transition metal oxides of the present invention include the oxides of manganese, ruthenium, rhenium, osmium, rhodium, iridium and mixtures thereof.
Specifically, the preferred transition metal oxides are NaRuO4, KMnO4, MnO2, RU02 and mixtures thereof. These transition metal oxides display appropriate redox chemistry for integration with the oxygen acceptors, most specifically, the alkali metal nitrate and nitrite systems. Although the present inventors do not wish to be held to any :::
~300 speclf~c theory as to why the transitlon metal oxldes ldentlfled above prov~de enhanced catalytic act~v1ty for regenerable oxygen acceptors, lt ls suggested that the interconvers~on of ox~des, peroxldes and super oxlde specles, g~ves a catalyzed oxygen-generatlng system. Such transl-tlon metal oxides possess redox chemlstry wh~ch have several lntereon-vertlble oxldatlon states withln the redox wlndows of the oxygen ac-ceptors, most notably the alkall metal nltrate and nltrites. Other transit~on metal oxldes outslde the group set forth above are not deemed to have aceeptable catalytle actlvlty, because they are ox1dlzed to thelr 10 hlghest oxidatlon states by the known oxygen acceptors and notably by the alkal~ metal nitrate and nltr~te solution, to stable ox~des whlch would not render sufflclent act~vlty to catalyze the oxygen absorptlon-desorp-t10n reaction favored hereln. Others of the transitlon metal oxldes not identifled above would decompose ~nto oxygen and the elemental metal upon 15 exposure to the oxygen acceptor. Some other transltlon metals may be un-acceptably radloactlve. Therefore, the transltlon metal oxldes ldent~fled above, ~n contrast to the remaln~ng transitlon metal ox~des llsted ~n the perlodlc table of the elements, are effect1ve catalysts, because they have an approprlate redox potentlal.
The transltlon metal oxldes of the present inventlon not only ex-hlblt enhanced catalytlc actlvlty, but may be expected to reduce the problem of corroslon wlth mater~als of constructlon of the overall separatory process, because of the freedom of removlng alkall and metal ox~des from the oxygen acceptor bath and the potent~al of runn~ng the reaction and partlcularly the desorption reactor at lower temperatures.
The trans~t~on metal o~lde catalysts of the present lnventlon also are not deactlvated by water and carbon dioxide ln the feed oxygen-conta~nlng gas or alr to the process, whlch is attract1ve ln that 1t ellmlnates the-necesslty for pretreatment of the feed to the process. Flnally, the translt~on metal ox1des of the present inventlon w~ll not be as eas~ly vaporized and lost from the system as the a1kall metal oxides of the prlor art.
Wlth reference to the preferred embodlment of the present 1nvent~on ln which the oxygen acceptor ~s a molten salt solutlon of alkall metal nltrate and nitr~te, the present lnvent~on w~ll be set forth whereln the ~L3~5t~
. , cycl~c oxygen absorptlon and desorptlon ~s predlcated on the followlng equat10n.
kl 1 (1) N03 ~ N02 + 2 2 k_l The feasib111ty of us~ng Equatlon 1, relles upon the thermodynamics and the klnet~cs of the reactlon, whlch have been shown to be strongly 10 dependent upon the amount of alkali metal oxldes ~n the solut~on. These alkall metal oxldes, whlch are present as oxides, peroxldes and super oxides, act as catalysts for Equat~on 1, through a redox mechan~sm. The prlor art has rel~ed upon the add~tlon of alkall metal oxides, such as Na202 or upon the lns~tu generat~on of oxlde specles by the thermal 15 decompositlon of the nltrate salt ltself, to ach~eve reasonable react10n rates. These oxldes, however, can be lost through a number of processes lncludlng vaporlzatlon and reactlon wlth the contalnment vessel. Also, lmpurltles ln alr, particularly carbon dioxlde and water react with the oxldes ln the solutlon by reactlon, such as Equaff on 2 and 3 below.
(2) C2 + o2 ~ C032 (3) H20 ~ o2 ~ 2 OH
W~thout the presence of such ox~des the k~netlcs of quatlon 1 are very slow and problematlc from a commerclal, economic perspective.
The present lnventlon overcomes the problems of the prlor art alka71 metal oxides by us~ng the alternative trans~tlon metal ox1des of th~ pres-ent ~nvent~on for catalyz~ng Equat10n 1, whereln such catalyt~c o~ides are not effected by alr lmpurlt~es, such as water and carbon d70x~de, and have b~en ~ound unexpectedly to ~mpart faster klnet~cs to the react~on shown ln Equatlon 1. The transitlon metal ox~des of the present lnven-t~on catalyze these reactlons by havlng the approprlate redox potential necessary to react in a revers~ble manner wlth the nltrate and nitrlte of :
, . .
130~
the molten salt bath, const~tuttng the oxygen-selectlve oxygen acceptor.
The transltion metal oxide catalyst of the present ~nvention also have unstable hydroxides and carbonates at the operatlng temperatures of the process, so that the presence of carbon dioxlde and water will not affect the kinet~cs of the catalyzed reaction, because the react~on of the tran-sit~on metal ox~des with such contaminants w~ll not remove the oxides on a permanent basls from the reaction system. Such reactlon temperature ~s preferably in the range of 450 to 6759C for the nitrate-nitrlte oxygen acceptor system.
The transition metal ox~des of the present ~nvent1On are ~ffect~ve catalysts because of their actlve redox chem~stry wh~ch results ~n the format1On of less stable oxy-anlons than alkali metal oxldes. The redox chemistry of the transltlon metal oxldes of the present ~nvention when used with an alkal~ metal nitrate-n~tr~te molten salt solut~on are found to be reversible, so that they can adequately catalyze the reaction shown in Equatlon l. These trans~tion metal oxides of the present invention are not overly ox~dizlng so that they are not lrrevers~bly reduced by the nitrite and nltrate half reactions, Equations 4 and 5, respectlvely below.
(4) N02 ~ N02 ~ le~
~5) N03 ~ N2 ~ 2 2 + le~
In add~t~on, the transitlon metal oxides of the present ~nvent~on are not 2S overly reducing so as to be lrrevers~bly ox~d~zed by the nltrate half re-actlon, Equation 6 below.
2~
(6) N03 ~ 2e~ ~ N02 + 0 The catalytlc activity o~ the select trans~t~on metal oxldes of the 3~ present lnventlon has been posit~vely ascertained by suppress~ng the ln situ formation of alkali metal ox~des from the alkal1 metal nitrate and nitrite molten salt bath so that the activ~ty of the alkall metal oxides of the prior art does not ~nterfere w1th the apparent measured catalyt k activlty of the translt1On metal oxides~ This is accomplished by intro-ducing n~trogen dioxide into the tests performed with the translt~on 5~) metal ox~des ln order to equlllbrate the alkall metal oxides by Equa-tlon 7.
(7~ 2N03 ~ 2N02 + 2 2 +
Tests were also performed to confirm the contlnued actlY~ty of the tran-s~t~on metal ox~de catalyst of the present inventlon in the presence of carbon diox~de and water, whlch would be ~nd~cat~ve of the~r performance in untreated alr as a feed to the overall process.
For testing purposes, lsothermal experlments at 600C were performed on a thermograv~metric (TGA) apparatus us~ng sod~um nitrate (Fisher ACS
reagent grade). Catalyt~c actlv~ty for Equation 1 was determined by switch~ng a test sample contalned ln a platinum pan between an oxygen-contain~ng and a non-oxygen-conta~n~ng atmosphere and measurlng the rate of oxygen uptake and release. Nltrogen diox~de, carbon d1Oxlde and water were used to suppress alkall metal oxlde catalysts. The transition metal oxides of the present lnvention belng tested for catalytic activity were mechanlcally mlxed with the sodium n~trate generally ~n concentratlons between 1 and 3 mole%.
As a base case, a 3 moleX sod~um peroxide ln sod~um nitrate sample was tested f~rst. The sod~um peroxide addltlon catalyzed Equatlon 1 effectlvely at 600-C. However, the catalytic act~vlty was greatly dl-m~nlshed w~th the introduction of nltrogen diox~de and was further de-creased wlth the add~t1On of carbon dioxlde. Table 1 llsts a number oftransit~on metal oxldes tested in the above manner for catalytic act~v~ty at 600C.
~::
, ~30085~) Catalyst Under Transltlon Metal Catalyst Under 1343 ppm NO2 +
Oxlde Added Mole ~ 1465 ppm N2 - 438 ppm C02 -Na~uO4 0.2 to 3.0 Yes Yes KMnO4 0.8 to 3.0 Yes Yes MnO2 3 Yes Yes V205 3 No No Fe203 2 No No Na2W04 3 No No Na2CrO4 a 3 -- NO(a) Ag2CrO4 2 No No Ru02 2.5 Yes yes(a~
(a) tested under 500 ppm of C~ only In addltion to the above experlments a 50/50 moleX (Na/K) N03 salt to whlch was added 0.54 mole~ KMnO4 has been demonstrated to be cata-~5 lytically actlve for Equatlon 1 at 650C under 1465 ppm of N02.
To quantify the above results, the height of the der~vatlve tracefrom the T~A was measured and was taken as a value for the relat~ve catalytlc actlvity of the catalyzed salt. The height of the derivative trace corresponds to the Q wt%/~t at the points o~ gas sw~tch~ng. The 3~ relatlve activlty of sodlum nitrate under 1465 ppm of nitrogen dloxlde without any added catalyst ~s taken to be 1. Table 2 below summarizes the relat~ve catalytlc activ~tles of salts with varlous candldate cat-alysts added when tested under nltrogen dloxlde and n1trogen dloxlde wlth carbon dlox~de.
~30(~350 .
Relative ActiYity Under Catalyst Added1465pem N02_1343ppm N02 t 438ppm C02_ None l(a) 0.57 3Z Na202 0.85 0.34 3% NaRuO4 12.0 10.6 1% NaRuO4 5-9 4-3 0.2~ NaRuO4 1.6 1.4 3X KMnO4 9.1 9.0 0.8% KMnO4 6.8 6.3 3.0% MnO2 10.5 8.3 2.5Z Ru02 6.7 (b) --(a~ Normallzat~on standard set to one.
(b) Shown to be active in 500 ppm C02.
Not tested ln N02/C02 mlxture.
.
Th~ relat~ve act~v~ty of a sod1um peroxlde catalyzed salt under nltrogen dlox~de ~s essentially the same regardless of s~arting sodium peroxlde ~ concentratlon because n~trogen d~ox~de converts sod~um peroxide to n~-trate. The relative activlty of sodium n~trate contalning an alternate ; catalyst ~s proport~onal to catalyst concentration.
The effect of water on the pr~or art catalysts and the catalyst of the present ~nvention were also examlned to see the effect of water on the comparatlve catalytlc act~vity of the present invention catalyst ~n comparison to those catalysts of the pr~or art. ~hen sodlum nltrate ls catalyzed w~th 3 moleX of sodlum peroxide and ls exposed to feed gas con-talnlng water (an amount near the dew point at room temperature) 1t has been ~ound by the present ~nventors to have a relative act~vity of only ~ 30 approxlmately 0.9 using as a reference the actlvlty o~ sodlum n1trate `~ ~under 1465 ppm of n~trogen dlox~de wherein the activity is 1Ø Uslng the same act~vity seale ~the present inventors exper~mentally catalyzed sod1um nltrate w~th 3 mole~ of various transitlon metal oxides of the present lnvention and exposed the catalyzed sodlum nitrate to nitrogen dioxlde carbon dioK~de and water w~th the resulting activ~ties given ~n Table 3 below.
.
, ,~ ,, , 13~08S0 .
Transitlon MetalRelative Act~vity Under Ox~de (3 mole %) N02 + C02 + H20 NaRuO4 a 10.0 KMnO4 a 11.7 MnO2 b 7.9 a 1151 ppm N02 ~ ~75 ppm C02 ~ HzO (dew point near room temperature) b 1343 ppm N02 ~ 438 ppm C02 + H20 (dew point near room temperature) .
As is apparent from a comparlson of Table 3 with the activ1ty of 0.9 experlmentally derived for sodium perox~de, the trans~tion metal oxides of the present ~nvention provlde enhanced act~v~ty, whlch is not dimin-lshed by known po1sons to the catalytic actlvity of the catalysts of the prior art. Th1s ~s exemplary of the unexpecte~ r~s-ults in the present invent10n for transltion metal ox~des selected from the above-identifled groups.
The benef1ts of the translt~on metal ox1des of the present lnvent10n over the prlor art ox1de catalyst will now be demonstrated 1n greater de-tall with reference to the drawlngs.
FIG 1 shows the effect of atmospher~c condit~ons on the activity of sodlum perox~de, sodlum ruthenium oxlde and potassium permanganate at 3 moleZ ln sod~um nltrate. Po~nt 1 of the graph ln FIG 1 corresponds to the lnlt~al actlvity of the salt under an oxygen/n~trogen sweep. Polnt 2 corresponds to the actlv~ty after one hour under th~s oxyycn/n1trogen 3~ sweep. As can be seen, the activ1tles of the salts under the response of the various catalysts ~ncrease with t1m~ under these conditions because of the product~on of add1t10nal catalyt~c oxides by Equat~on 7 above.
The oxygen/nitro~en sweep cont1nually removes the nitrogen dlox~de re-~ sultlng ln a build-up of o2~. In the ease of the trans~tlon metal ; 35 ox1de catalysts of the present 1nvent~on, th1s r~se ~n activ~ty ls also ~ due in part to catalyst d~ssolut10n ~n the melt or to catalyst act~vatlon :.
13~085~
reactions. It is signiflcant to note that at points 1 and 2 of the graph of FIG 1, the actlvit~es of the sodlum ruthenlum oxlde and the potass~um permanganate catalyzed salts are hlgher than the sod~um peroxlde cat-alyzed salt. At po1nt 3 ~n the graph of FIG 1, 1465 ppm of n~trogen dlox~de is added to the oxygen/nltrogen sweep. Th~s causes the react~on of Equatlon 7 above to shlft back to the left convertlng catalytlc oxlde to nltrate. The sodlum peroxlde catalyzed salt ls thus deactlvated s~nce the equ~librium level of o2 under these condltlons ls very low. In the transltlon metal oxlde catalyst contalnlng salts of the present ~n-10 vention, the non-transitlon metal oxldes, o2 /02t02, are converted to nitrate. However~ the transition metal oxldes of the present invention are unaffected w1th respect to thelr catalyt1c properties, and the salts remaln actlve for oxygen uptake. The introduction of nitrogen dioxlde causes the activltles of all salts to become constant with time since an lS equillbr~um level of oxide ls established by the reactlon of Equation 7, unlike points 1 and 2 where the nitrogen dloxide produced was contlnually removed. This lntroduction of nltrogen dioxide clearly demonstrates tha$
the transit~on metal ox~des of the present lnvention are true catalysts for the oxygen reverslblllty of sodlum nltrate, s~nce lt acts only to 20 convert non-trans~tion m~tal oxides back to nitrate without introducing add1tlonal chemlcal species. At point 4, 438 ppm of carbon dloxide and 1343 ppm of nltrogen dioxide are introduced into the oxygen/nitrogen sweep gas. This further reduces the activ~ty of the sodlum peroxide catalyzed salt by about one-half. The oxlde equillbrium level determlned by Equat7On 7 and Equatlon 2, above, ls lower than under nitrogen dioxide alone. The actlvity of the transition metal ox~de catalyst of the pres-ent inventlon in the experimental salts are not affected by the presence of carbon dloxlde. This is particularly slgnlflcant slnce now removal of carbon dloxide w111 not be requ~red ~n the gas separat~on processes using the present invention.
FIG 2 ls essentially the same as FIG 1, except that the concentra-tlons of sodium ruthen~um oxide and potassium permanganate have been reduced to 1.0 and 0.8 m~leX, respectlvely. The signiflcance of the plot of the graph ln FIG 2 ~s that at a concentrat~on of only about 1 moleX, the transit~on metal ox~de catalysts of the present inYent~on show h~gher actlvity than 3 moleX of sodlum peroxlde.
~30(~3S0 FIGs 3 and 4 show the effect of ~ncreaslng catalyst concentratlon upon the act~vlty of the sodlum nltrate salt bath ~n the presence of the oxygen/nltrogen sweep gas, h~hen varlous catalyst polsons dlscussed above are added to the sweep gas. Wlth regard to FIG 3, the graph shows that an uncatalyzed salt bath (O mole% catalyst) has very low ln~t~al actlv~ty.
Thls act~vlty does not ~mprove ~n the presence of elther nltrogen d~oxlde or w~th reference to FIG 4 ~n the presence of nltrogen d~ox~de and carbon dloxlde.
Accordingly, the transit~on metal ox~des of the present inventlon 1~ descr~bed above have varlous unexpected enhancements ln the catalys~s of the reverslble oxygen uptake and removal ~n a chem~cal absorpt1Ye process, partlcularly those contaln~ng alkall metal nitrate and n~trlte molten salt baths. Not all of the trans~t~on metal ox~des have thls act~v~ty. Only those transit~on metal ox~des ~dent~f~ed above wh~ch have 15 good redox chem~stry wlth regard to the varlous oxygen acceptor and oxy-gen donor systems ~dent~fled above have the approprlate catalyt1c actlv-~ty. These trans~t~on metal ox~de catalysts of the present lnvent10n have been shown to be more act~ve than the pr~or art sod1um peroxlde catalyst. Accord~ngly, th~s hlgher catalyt~c act~v~ty may lower the temperature of the process, most specif~cally in oxygen desorber unlts.
Thls lower temperature wlll reduce corrosion and nltrogen dloxlde l~vels. The translt~on metal oxlde catalysts of the present lnvent~on are not deact~vated by nltrogen dlox~de, carbon dloxide or water. Th~s w~ll ellminate the need for feed gas pretreatment, such as removal of water and carbon dioxide from a~r when separatlng oxygen from nitrogen.
The present ~nventlon has been descrlbed wlth reference to several preferred embod~ments. However, the scope of the ~nventlon should not be ; llm~ted to such preferred embodlments, but rather should be ascertalned from the cla~ms wh~ch follow.
~5) N03 ~ N2 ~ 2 2 + le~
In add~t~on, the transitlon metal oxides of the present ~nvent~on are not 2S overly reducing so as to be lrrevers~bly ox~d~zed by the nltrate half re-actlon, Equation 6 below.
2~
(6) N03 ~ 2e~ ~ N02 + 0 The catalytlc activity o~ the select trans~t~on metal oxldes of the 3~ present lnventlon has been posit~vely ascertained by suppress~ng the ln situ formation of alkali metal ox~des from the alkal1 metal nitrate and nitrite molten salt bath so that the activ~ty of the alkall metal oxides of the prior art does not ~nterfere w1th the apparent measured catalyt k activlty of the translt1On metal oxides~ This is accomplished by intro-ducing n~trogen dioxide into the tests performed with the translt~on 5~) metal ox~des ln order to equlllbrate the alkall metal oxides by Equa-tlon 7.
(7~ 2N03 ~ 2N02 + 2 2 +
Tests were also performed to confirm the contlnued actlY~ty of the tran-s~t~on metal ox~de catalyst of the present inventlon in the presence of carbon diox~de and water, whlch would be ~nd~cat~ve of the~r performance in untreated alr as a feed to the overall process.
For testing purposes, lsothermal experlments at 600C were performed on a thermograv~metric (TGA) apparatus us~ng sod~um nitrate (Fisher ACS
reagent grade). Catalyt~c actlv~ty for Equation 1 was determined by switch~ng a test sample contalned ln a platinum pan between an oxygen-contain~ng and a non-oxygen-conta~n~ng atmosphere and measurlng the rate of oxygen uptake and release. Nltrogen diox~de, carbon d1Oxlde and water were used to suppress alkall metal oxlde catalysts. The transition metal oxides of the present lnvention belng tested for catalytic activity were mechanlcally mlxed with the sodium n~trate generally ~n concentratlons between 1 and 3 mole%.
As a base case, a 3 moleX sod~um peroxide ln sod~um nitrate sample was tested f~rst. The sod~um peroxide addltlon catalyzed Equatlon 1 effectlvely at 600-C. However, the catalytic act~vlty was greatly dl-m~nlshed w~th the introduction of nltrogen diox~de and was further de-creased wlth the add~t1On of carbon dioxlde. Table 1 llsts a number oftransit~on metal oxldes tested in the above manner for catalytic act~v~ty at 600C.
~::
, ~30085~) Catalyst Under Transltlon Metal Catalyst Under 1343 ppm NO2 +
Oxlde Added Mole ~ 1465 ppm N2 - 438 ppm C02 -Na~uO4 0.2 to 3.0 Yes Yes KMnO4 0.8 to 3.0 Yes Yes MnO2 3 Yes Yes V205 3 No No Fe203 2 No No Na2W04 3 No No Na2CrO4 a 3 -- NO(a) Ag2CrO4 2 No No Ru02 2.5 Yes yes(a~
(a) tested under 500 ppm of C~ only In addltion to the above experlments a 50/50 moleX (Na/K) N03 salt to whlch was added 0.54 mole~ KMnO4 has been demonstrated to be cata-~5 lytically actlve for Equatlon 1 at 650C under 1465 ppm of N02.
To quantify the above results, the height of the der~vatlve tracefrom the T~A was measured and was taken as a value for the relat~ve catalytlc actlvity of the catalyzed salt. The height of the derivative trace corresponds to the Q wt%/~t at the points o~ gas sw~tch~ng. The 3~ relatlve activlty of sodlum nitrate under 1465 ppm of nitrogen dloxlde without any added catalyst ~s taken to be 1. Table 2 below summarizes the relat~ve catalytlc activ~tles of salts with varlous candldate cat-alysts added when tested under nltrogen dloxlde and n1trogen dloxlde wlth carbon dlox~de.
~30(~350 .
Relative ActiYity Under Catalyst Added1465pem N02_1343ppm N02 t 438ppm C02_ None l(a) 0.57 3Z Na202 0.85 0.34 3% NaRuO4 12.0 10.6 1% NaRuO4 5-9 4-3 0.2~ NaRuO4 1.6 1.4 3X KMnO4 9.1 9.0 0.8% KMnO4 6.8 6.3 3.0% MnO2 10.5 8.3 2.5Z Ru02 6.7 (b) --(a~ Normallzat~on standard set to one.
(b) Shown to be active in 500 ppm C02.
Not tested ln N02/C02 mlxture.
.
Th~ relat~ve act~v~ty of a sod1um peroxlde catalyzed salt under nltrogen dlox~de ~s essentially the same regardless of s~arting sodium peroxlde ~ concentratlon because n~trogen d~ox~de converts sod~um peroxide to n~-trate. The relative activlty of sodium n~trate contalning an alternate ; catalyst ~s proport~onal to catalyst concentration.
The effect of water on the pr~or art catalysts and the catalyst of the present ~nvention were also examlned to see the effect of water on the comparatlve catalytlc act~vity of the present invention catalyst ~n comparison to those catalysts of the pr~or art. ~hen sodlum nltrate ls catalyzed w~th 3 moleX of sodlum peroxide and ls exposed to feed gas con-talnlng water (an amount near the dew point at room temperature) 1t has been ~ound by the present ~nventors to have a relative act~vity of only ~ 30 approxlmately 0.9 using as a reference the actlvlty o~ sodlum n1trate `~ ~under 1465 ppm of n~trogen dlox~de wherein the activity is 1Ø Uslng the same act~vity seale ~the present inventors exper~mentally catalyzed sod1um nltrate w~th 3 mole~ of various transitlon metal oxides of the present lnvention and exposed the catalyzed sodlum nitrate to nitrogen dioxlde carbon dioK~de and water w~th the resulting activ~ties given ~n Table 3 below.
.
, ,~ ,, , 13~08S0 .
Transitlon MetalRelative Act~vity Under Ox~de (3 mole %) N02 + C02 + H20 NaRuO4 a 10.0 KMnO4 a 11.7 MnO2 b 7.9 a 1151 ppm N02 ~ ~75 ppm C02 ~ HzO (dew point near room temperature) b 1343 ppm N02 ~ 438 ppm C02 + H20 (dew point near room temperature) .
As is apparent from a comparlson of Table 3 with the activ1ty of 0.9 experlmentally derived for sodium perox~de, the trans~tion metal oxides of the present ~nvention provlde enhanced act~v~ty, whlch is not dimin-lshed by known po1sons to the catalytic actlvity of the catalysts of the prior art. Th1s ~s exemplary of the unexpecte~ r~s-ults in the present invent10n for transltion metal ox~des selected from the above-identifled groups.
The benef1ts of the translt~on metal ox1des of the present lnvent10n over the prlor art ox1de catalyst will now be demonstrated 1n greater de-tall with reference to the drawlngs.
FIG 1 shows the effect of atmospher~c condit~ons on the activity of sodlum perox~de, sodlum ruthenium oxlde and potassium permanganate at 3 moleZ ln sod~um nltrate. Po~nt 1 of the graph ln FIG 1 corresponds to the lnlt~al actlvity of the salt under an oxygen/n~trogen sweep. Polnt 2 corresponds to the actlv~ty after one hour under th~s oxyycn/n1trogen 3~ sweep. As can be seen, the activ1tles of the salts under the response of the various catalysts ~ncrease with t1m~ under these conditions because of the product~on of add1t10nal catalyt~c oxides by Equat~on 7 above.
The oxygen/nitro~en sweep cont1nually removes the nitrogen dlox~de re-~ sultlng ln a build-up of o2~. In the ease of the trans~tlon metal ; 35 ox1de catalysts of the present 1nvent~on, th1s r~se ~n activ~ty ls also ~ due in part to catalyst d~ssolut10n ~n the melt or to catalyst act~vatlon :.
13~085~
reactions. It is signiflcant to note that at points 1 and 2 of the graph of FIG 1, the actlvit~es of the sodlum ruthenlum oxlde and the potass~um permanganate catalyzed salts are hlgher than the sod~um peroxlde cat-alyzed salt. At po1nt 3 ~n the graph of FIG 1, 1465 ppm of n~trogen dlox~de is added to the oxygen/nltrogen sweep. Th~s causes the react~on of Equatlon 7 above to shlft back to the left convertlng catalytlc oxlde to nltrate. The sodlum peroxlde catalyzed salt ls thus deactlvated s~nce the equ~librium level of o2 under these condltlons ls very low. In the transltlon metal oxlde catalyst contalnlng salts of the present ~n-10 vention, the non-transitlon metal oxldes, o2 /02t02, are converted to nitrate. However~ the transition metal oxldes of the present invention are unaffected w1th respect to thelr catalyt1c properties, and the salts remaln actlve for oxygen uptake. The introduction of nitrogen dioxlde causes the activltles of all salts to become constant with time since an lS equillbr~um level of oxide ls established by the reactlon of Equation 7, unlike points 1 and 2 where the nitrogen dloxide produced was contlnually removed. This lntroduction of nltrogen dioxide clearly demonstrates tha$
the transit~on metal ox~des of the present lnvention are true catalysts for the oxygen reverslblllty of sodlum nltrate, s~nce lt acts only to 20 convert non-trans~tion m~tal oxides back to nitrate without introducing add1tlonal chemlcal species. At point 4, 438 ppm of carbon dloxide and 1343 ppm of nltrogen dioxide are introduced into the oxygen/nitrogen sweep gas. This further reduces the activ~ty of the sodlum peroxide catalyzed salt by about one-half. The oxlde equillbrium level determlned by Equat7On 7 and Equatlon 2, above, ls lower than under nitrogen dioxide alone. The actlvity of the transition metal ox~de catalyst of the pres-ent inventlon in the experimental salts are not affected by the presence of carbon dloxlde. This is particularly slgnlflcant slnce now removal of carbon dloxide w111 not be requ~red ~n the gas separat~on processes using the present invention.
FIG 2 ls essentially the same as FIG 1, except that the concentra-tlons of sodium ruthen~um oxide and potassium permanganate have been reduced to 1.0 and 0.8 m~leX, respectlvely. The signiflcance of the plot of the graph ln FIG 2 ~s that at a concentrat~on of only about 1 moleX, the transit~on metal ox~de catalysts of the present inYent~on show h~gher actlvity than 3 moleX of sodlum peroxlde.
~30(~3S0 FIGs 3 and 4 show the effect of ~ncreaslng catalyst concentratlon upon the act~vlty of the sodlum nltrate salt bath ~n the presence of the oxygen/nltrogen sweep gas, h~hen varlous catalyst polsons dlscussed above are added to the sweep gas. Wlth regard to FIG 3, the graph shows that an uncatalyzed salt bath (O mole% catalyst) has very low ln~t~al actlv~ty.
Thls act~vlty does not ~mprove ~n the presence of elther nltrogen d~oxlde or w~th reference to FIG 4 ~n the presence of nltrogen d~ox~de and carbon dloxlde.
Accordingly, the transit~on metal ox~des of the present inventlon 1~ descr~bed above have varlous unexpected enhancements ln the catalys~s of the reverslble oxygen uptake and removal ~n a chem~cal absorpt1Ye process, partlcularly those contaln~ng alkall metal nitrate and n~trlte molten salt baths. Not all of the trans~t~on metal ox~des have thls act~v~ty. Only those transit~on metal ox~des ~dent~f~ed above wh~ch have 15 good redox chem~stry wlth regard to the varlous oxygen acceptor and oxy-gen donor systems ~dent~fled above have the approprlate catalyt1c actlv-~ty. These trans~t~on metal ox~de catalysts of the present lnvent10n have been shown to be more act~ve than the pr~or art sod1um peroxlde catalyst. Accord~ngly, th~s hlgher catalyt~c act~v~ty may lower the temperature of the process, most specif~cally in oxygen desorber unlts.
Thls lower temperature wlll reduce corrosion and nltrogen dloxlde l~vels. The translt~on metal oxlde catalysts of the present lnvent~on are not deact~vated by nltrogen dlox~de, carbon dloxide or water. Th~s w~ll ellminate the need for feed gas pretreatment, such as removal of water and carbon dioxide from a~r when separatlng oxygen from nitrogen.
The present ~nventlon has been descrlbed wlth reference to several preferred embod~ments. However, the scope of the ~nventlon should not be ; llm~ted to such preferred embodlments, but rather should be ascertalned from the cla~ms wh~ch follow.
Claims (8)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In a method for separating oxygen from an oxygen-containing gas comprising reacting an oxygen acceptor consisting essentially of a molten salt solution of alkali metal nitrates and nitrites with said gas in an absorption reaction to produce an oxygen-depleted gas effluent, separately decomposing the oxidized acceptor to yield oxygen and regenerated oxygen acceptor, the improvement comprising adding to the oxygen acceptor a catalytic amount of a transition metal oxide selected from the group consisting of oxides of manganese, ruthenium, rhenium, osmium, rhodium, iridium and mixtures thereof.
2. The method of claim 1, wherein the absorption reaction is performed in a continual manner whereby the regenerated oxygen acceptor is recycled to react with gas in said absorption reaction.
3. The method of claim 1, wherein the transition metal oxide is selected from the group consisting of NaRuO4 , KMnO4, MnO2, RUO2 and mixtures thereof.
4. The method of claim 1, wherein the transition metal oxide is present in an amount in the range of approximately 0.2 to 3 mole % of the oxygen acceptor.
5. The method of claim 1, wherein the transition metal oxide is present in the oxygen acceptor in combination with alkali metal oxides.
6. In a method for recovering oxygen or nitrogen from air comprising reacting an oxygen acceptor consisting essentially of a molten salt solution of alkali metal nitrates and nitrites with air in an absorption reaction to produce a nitrogen-rich effluent, separately decomposing the oxidized acceptor to yield oxygen and regenerated oxygen acceptor, the improvement comprising adding to the oxygen acceptor a catalytic amount of a transition metal oxide selected from the group consisting of oxides of manganese, ruthenium, rhenium, osmium, rhodium, iridium and mixtures thereof.
7. The method of claim 6, wherein the transition metal oxide is selected from the group consisting of NaRuO4, KMnO4, MnO2, RuO2, and mixtures thereof.
8. The method of claim 6, wherein the transition metal oxide is present in the oxygen acceptor in combination with alkali metal oxides.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US07/094,637 US4800070A (en) | 1987-09-08 | 1987-09-08 | Catalysts for absorptive air separation |
| US94,637 | 1987-09-08 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1300850C true CA1300850C (en) | 1992-05-19 |
Family
ID=22246293
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000576459A Expired - Lifetime CA1300850C (en) | 1987-09-08 | 1988-09-02 | Catalysts for absorptive air separation |
Country Status (6)
| Country | Link |
|---|---|
| US (1) | US4800070A (en) |
| EP (1) | EP0306840B1 (en) |
| JP (1) | JPH01111701A (en) |
| AT (1) | ATE74109T1 (en) |
| CA (1) | CA1300850C (en) |
| DE (1) | DE3869524D1 (en) |
Families Citing this family (14)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CA1337722C (en) * | 1989-04-18 | 1995-12-12 | Madan Mohan Bhasin | Alkylene oxide catalysts having enhanced activity and/or stability |
| US4988488A (en) * | 1989-10-19 | 1991-01-29 | Air Products And Chemicals, Inc. | Iron aluminides and nickel aluminides as materials for chemical air separation |
| US4944934A (en) * | 1990-01-03 | 1990-07-31 | Air Products And Chemicals, Inc. | High temperature oxygen recovery with reversible chemical reaction |
| US5254216A (en) * | 1991-06-21 | 1993-10-19 | Texas Instruments Incorporated | Oxygen scavenging in a plasma reactor |
| US5556825A (en) * | 1995-01-06 | 1996-09-17 | Ford Motor Company | Automotive catalysts with improved oxygen storage and metal dispersion |
| AT412842B (en) * | 2003-07-24 | 2005-08-25 | Gheczy Rudolf Von | PROCESS FOR OPERATING A COMBUSTION PLANT |
| US20130251622A1 (en) * | 2011-12-19 | 2013-09-26 | Phillips 66 Company | Enhanced oxygen generation from molten salt |
| DE102013209658A1 (en) | 2013-05-24 | 2014-05-22 | Deutsches Zentrum für Luft- und Raumfahrt e.V. | Thermo-chemical production of oxygen from air involves heating metal oxide by (in)direct concentrated solar radiation, and oxidizing metal oxide at lower temperature and in air atmosphere by partial/complete reduction of solar radiation |
| EP3323471B1 (en) | 2016-11-18 | 2020-05-13 | Diehl Aviation Gilching GmbH | Device for generating oxygen |
| EP3323781B1 (en) | 2016-11-18 | 2019-08-14 | Diehl Aviation Gilching GmbH | Method for generating oxygen from compositions comprising ionic liquids |
| EP3323783B1 (en) * | 2016-11-18 | 2019-07-17 | Diehl Aviation Gilching GmbH | Use of ionic liquids in compositions for generating oxygen |
| EP3323780B1 (en) | 2016-11-18 | 2020-05-20 | Diehl Aviation Gilching GmbH | Compositions comprising ionic liquids and peroxides |
| EP3323470B1 (en) | 2016-11-18 | 2022-06-29 | Diehl Aviation Gilching GmbH | Oxygen generator using ionic liquids |
| EP3323779B1 (en) | 2016-11-18 | 2020-06-03 | Diehl Aviation Gilching GmbH | Method for generating oxygen using ionic liquids for decomposing peroxides |
Family Cites Families (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US70705A (en) * | 1867-11-12 | Improved mode of producing oxygen gas | ||
| US2004243A (en) * | 1928-01-17 | 1935-06-11 | Ig Farbenindustrie Ag | Method of and means for operating breathing apparatus |
| GB628572A (en) * | 1946-08-23 | 1949-08-31 | Standard Oil Dev Co | Improved process for the extraction of oxygen from air |
| US4340578A (en) * | 1977-05-24 | 1982-07-20 | Erickson Donald C | Oxygen production by molten alkali metal salts |
| US4132766A (en) * | 1977-05-24 | 1979-01-02 | Erickson Donald C | Separation of oxygen from gaseous mixtures with molten alkali metal salts |
| US4287170A (en) * | 1980-03-06 | 1981-09-01 | Erickson Donald C | Nitrogen and oxygen via chemical air separation |
| US4529577A (en) * | 1984-07-11 | 1985-07-16 | Air Products And Chemicals, Inc. | Oxygen/nitrogen production with molten alkali salts |
| US4565685A (en) * | 1984-09-14 | 1986-01-21 | Air Products And Chemicals, Inc. | Air separation with temperature and pressure swing |
-
1987
- 1987-09-08 US US07/094,637 patent/US4800070A/en not_active Expired - Fee Related
-
1988
- 1988-09-01 DE DE8888114294T patent/DE3869524D1/en not_active Expired - Lifetime
- 1988-09-01 AT AT88114294T patent/ATE74109T1/en not_active IP Right Cessation
- 1988-09-01 EP EP88114294A patent/EP0306840B1/en not_active Expired - Lifetime
- 1988-09-02 CA CA000576459A patent/CA1300850C/en not_active Expired - Lifetime
- 1988-09-08 JP JP63225527A patent/JPH01111701A/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| EP0306840A2 (en) | 1989-03-15 |
| EP0306840B1 (en) | 1992-03-25 |
| US4800070A (en) | 1989-01-24 |
| ATE74109T1 (en) | 1992-04-15 |
| EP0306840A3 (en) | 1989-08-23 |
| DE3869524D1 (en) | 1992-04-30 |
| JPH0531485B2 (en) | 1993-05-12 |
| JPH01111701A (en) | 1989-04-28 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| CA1300850C (en) | Catalysts for absorptive air separation | |
| CA2063123C (en) | Method for producing hydrogen peroxide | |
| US2582885A (en) | Method of removing free oxygen or free hydrogen from a gaseous medium | |
| US5384301A (en) | Catalyst for elemental sulfur recovery process | |
| EP0871535B1 (en) | Method for absorbing gaseous oxidisable constituents through a membrane | |
| US4743435A (en) | Method for cleaning exhaust gases | |
| CA2121300A1 (en) | Process for producing hydrogen peroxide | |
| US4012486A (en) | Process for reducing the total sulfur content of Claus off-gases | |
| US3260678A (en) | Catalyst composition, particularly for catalyzing oxidation of hydrogen chloride to chlorine | |
| US4277452A (en) | Carbon monoxide absorbing liquid | |
| CN115477282B (en) | Method for removing CO in high-purity hydrogen, purified hydrogen and application thereof | |
| AU649577B2 (en) | Process for the manufacture of hydrogen peroxide | |
| DE4305386A1 (en) | Catalyst and process for cleaning carbon dioxide | |
| JPH05111618A (en) | Removing method of carbon monoxide | |
| US4582950A (en) | Removal of acetylene and carbon monoxide from methane and ethane | |
| GB1498572A (en) | Preparation of active-carbon-supported oxidation catalyst | |
| US4224293A (en) | Removal of low concentrations of hydrogen from chlorine gas | |
| US3383164A (en) | Removal of carbon monoxide | |
| CA2110224A1 (en) | Method of desulfurization of town gas | |
| JPS5932169B2 (en) | Adsorbent and adsorption method for hydrogen sulfide-containing gas | |
| JPH09241009A (en) | Production of hydrogen peroxide | |
| CA1305307C (en) | Process for the removal of sulphur dioxide from gases | |
| JP3330412B2 (en) | Method for removing hydrogen from air or inert gas | |
| JPH0771615B2 (en) | Exhaust gas purification method | |
| RU2062145C1 (en) | Adsorbent and catalyst for fine purification of inert gases |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKLA | Lapsed | ||
| MKLA | Lapsed |
Effective date: 19951120 |