OXYGEN PRODUCTION BY MOLTEN ALKALI METAL SALTS

Abstract: A continuous process for separating oxygen from air by means of a reversible chemical reaction wherein air is reacted with an oxygen acceptor which is subsequently decomposed to yield the product oxygen, and then recycled. The only energy input required is a conventional combustible fuel, and one object of the process is to reduce the amount of fuel energy required for oxygen production. The oxygen acceptor is a molten solution of alkali metal salt comprised predominantly of sodium nitrate and potassium nitrate. An efficient process is achieved with this salt by conducting oxygen absorption in multiple countercurrent stages. The compression energy requirement is minimized by combining isothermal and adiabatic compression, and the recovery of compression energy is maximized by processing the exhaust in the sequence combustion, partial expansion, heat exchange, and completion of expansion. Salt-to-salt heat exchange between reaction regions minimizes the pumping requirements.

Description: Description Oxygen Production by • Molten Alkali Metal Salts Technical Field This invention relates to processes wherein oxygen is produced by a decomposition type chemical reaction, and then the oxygen deficient composition, termed an "oxygen acceptor", is regenerated for recycle by reaction with air or other oxygen containing gas mixture. ' ' Background Art This application is a coπtinuation-in-part of U.S. application 7999^5 filed on May Z , 1977 by the above applicant. Research leading to the invention disclosed herein ωas funded in part by a grant from the U.S. Government. The prior art of this field αf invention includes U.S. Patents 3856923, 3579292, 2*t18^02, and 3121611. Numerous oxygen acceptors have been identified, including BaD, IMa 2 π0 3 , CuClgt SrO, and Hg. The attribute of chemical air separation processes as a class is that the high pressure air which undergoes reaction and thereby loses part αf its oxygen is still at high pressure after the reaction. Therefore it can be expanded through a turboexpander, recovering most or all of its compression energy. In contrast, in cyrogenic processes the air must be cooled and then depressurizεd to develop the desired cooling effect, thereby expending the compression energy. The most efficient liquefaction processes consume electrical energy at the relatively high rate of .29 kLJhr (1 M3) per kg 0 2 produced, which is equivalent to a thermal input of 22,8 kcal/mole 0 2 or 1300 BTU/lb 0„, given 33% conversion efficiency. Nevertheless, the liquefaction process has been superior to prior art chemical separation processes for various reasons. Most prior art processes have involved either an acceptor or an oxidized acceptor or both which are present OMPI in the solid state. This has made circulation αf the acceptor composition difficult, and therefore most processes have been batch mode. Most batch made processes have involved large pressure differences between the oxidation and decompositio parts of the cycle, and therefore have suffered from excessive vent and purge losses. Some processes have attempted to • minimize this pressure difference by conducting the decomposition reaction at a higher temperature than the oxidation reaction. This imposes a large heat requirement: not only does the sensible heat of the acceptor have to be furnished, but the full endothermic heat requirement of the deco pαsitiαn reaction must also be supplied. Those two heat requirements substantially exceed the 22.8 kcal/ mole needed for liquefaction. Another problem encountered in prior art processes is that the equilibrium p0„ of the acceptor " composition withdrawn from the absorber has always necessarily been less than or equal to the p0„ of the exhaust gas withdrawn from the absorber. This is because oxygen can only be transferred from a gas to :aπ acceptor if the p0_ of the gas is higher than that σf the acceptor. Therefore the larger the fraction of oxygen extracted from the supply air, the lower the exhaust p0 2 becomes, and hence the lower the equilibrium 0 2 of the oxidized acceptor, making decomposition more difficult and inefficient. Other problems with the prior art processes are that some of them produce unacceptable amounts αf impurities in the product gas, e.g. chlorine or mercury, . and some experience an unacceptable loss rate of the acceptor, due to chemical breakdown, chemical iπactivatioπ, or other possible causes. The background art which describes the chemical behavior of molten alkali metal nitrate and nitrite salts consists αf numerous articles from technical journals, including the following: Bartholomew.}. R.F., The Journal of Physical Chemistry, 70, 1966, pages 3442-3446; and Freeman, E. S., The Journal of Physical Chemistry, 60, 1956, pages 1487-1493, published in Eastoπ, Pennsylvania by the Mack Printing Co. Several characteristics of the reaction between those salts and oxygen have been noted. High levels αf nitrogen oxides 'have been invariably noted, as high as several percent, which is several orders of magnitude above environmental limits. The reaction kinetics have been quantified, showing a relatively high activation energy. At higher temperatures the salt decomposes to the solid oxide. Disclosure of Invention The invention claimed is an oxygen acceptor composition which is liquid and remains liquid when reacted with air at process conditions, and which does not introduce undesirable impurities in the product oxygen. The invention also includes the disclosure of a process which takes advantage αf the unique characteristics of the new oxygen acceptor, thereby substantially reducing the amount of energy necessary to produce oxygen. The oxygen acceptor is a molten mixture αf alkali metal salts, The salt anioπ composition includes SO to 94% nitrate, 4 to 25% nitrite, and 2 to 25% combined peroxides and super- oxides, where % signifies molar percent. The cation composition includes sodium and potassium in any proportions, preferably between 30% and 70% sodium, balance potassium. Important features αf the process are as follows. First, the reaction between the salt and air is conducted in multiple countercurrent stages, thereby enabling the salt p0 2 to approach that of the incoming air, and greatly exceed that of the exhaust air. The net result αf the countercurrent multi- staged absorption is that for a given decompαstion pressure and level of oxygen extraction, it is possible to greatly reduce the absorption pressure, thereby markedly improving overall process efficiency. Secondly the exhaust air is processed as follows: combusted with fuel; partially expanded; heat exchanged against incoming air, and then fully expanded. This sequence recovers maximum mechanical energy from the exhaust while still supplying air preheat requirements, owing tα the heat being applied to the process at the hottest possible temperature. Thirdly, it is advantageous tα have the absorption reaction and decomposition reaction in heat exchange relationship. This has the effect of substantially reducing salt pumping • requirements and also minimizing the temperature difference between the two reactions. Finally, the total amount of - OREA OMPI cα pression energy required for given process conditions can be minimized by using an isothermal compressor (i.e. one with stage coaling) fallowed by an adiabatic compressor, where each compressor furnishes at least one fourth of the total compression power. By using the above identified salt composition as the oxygen acceptor in a process incorporating the above features, with the approximate operating conditions subsequently tα be disclosed, it is possible to generate oxygen with a thermal energy input of 1.8 MJ/kg 0 2 , which is only 60% αf the liquefaction plant thermal energy requirement. Brief Description αf Drawings Figure 1 is a flowsheet illustrating the sequence αf major process steps which yield a high process-efficiency, including conventional absorption and decomposition towers. Figure 2 illustrates an alternative configuration for the absorption and decomposition towers wherein heat exchange is advantageously effected between the respective reactions. Figure 3 illustrates the ther odynamic behavior of the oxygen acceptor, and particularly the dependence αf its equilibrium p0 2 on salt concentration as well as on temperature. Best Mode for Carrying Out the Invention The best mode for carrying out this invention requires that a very specific salt composition be used as the αxygeπ acceptor. That composition has been found to be widely useful for producing oxygen in a variety of processes. The best process for producing oxygen using the above composition will vary with input conditions and process objectives, for example whether or not air is already available at high pressure, whether high temperature waste heat is available, and whether it is desired tα cogεπεratε mechanical or electrical power or nitrogen in addition tα αxygeπ. For the simple case αf a stand-alone process in which it is desired to produce only oxygen starting with ambient air and using a standard fuel as essentially the only energy input, a process will be disclosed which takes full advantage of the above composition, .producing oxygen very efficiently. Thus the best mode discussion will disclose the essential aspects αf the salt composition, the processing steps and apparatus for one process objective, and the best operating conditions for that same process. Salt Composition The following reaction stαichiometrics are present in the salt, where M signifies an alkali metal cation, i.e. either sodium or potassium: ( 1 ) MND 2 (1) + 36D 2 (g) = MN0 3 (1) (2) M 2 0 2 (1) + 0 2 (g) = 2M0 2 (1) (3) MN0 2 (1) + M0 2 (1) = MN0 3 (1) + )SM 2 0 2 (1) (4) 2 0 2 (1) + C0 2 (g) = 2 C0 3 (1) + έ0 2 (g) (5) M 2 0 2 U) + H 2 0(g) = 2 MOH(l ) + H ) 2 (g) (6) 2MN0 2 (1) = M 2 0 2 (1) + N 2 (g) + 0 2 (g) (7) N 2 (g) + 1.50 2 (g) =• N Q ( g) + N0 2 (g) The first reaction is most descriptive of the major exchange of oxygen between the salt phase. As oxygen reacts with the salt, the nitrate concentration increases and the nitrite concentration decreases. These species approximate ideal solution behavior, i.e. their activity is essentially equal tα their mole fraction. The equilibrium constant expression for reaction (1) is: ΔB K p = a(MN0 2 ) ( P°2 = ~ This expression shows that the equilibrium p0„ αf this oxygen acceptor is not only a function αf temperature, as is the case with all other oxygen acceptors, but it is also a function of the amount αf oxygen reacted with the salt, i.e. αf the changing activities of the nitrates and nitrites. Figure 3 illustrates that functionality - the equilibrium pC (in atmospheres) is seen to vary at constant temperature (degrees Kelvin) as the salt nitrate fraction varies, where the nitrate fraction signifies the ratio a(MN0 3 ) a(MN0 3 ) + a(MN0 2 ) Figure 3 was derived from experimental data,- and applies to cation compositions of between 30% and 70% sodium, balance potassium. Pure salt data is found in the references, and varies only slightly from the values shown in Figure 3. The above dependence of p0 ? on salt concentration is the reason a marked advantage is obtained from multistage counter- current absorption, ϋihen p0„ does not depend on the oxygen loading of the oxygen acceptor, as with most prior art acceptors, then the acceptor pCL is relatively constant throughout the absorber, and at a value which necessarily must be lower than the p0„ of the exhaust oxygen depleted air. iith the current salt and a countercurrent reaction however, the ther αdynamic limitations are that the incoming (oxygen deficient) salt must have a p0 ? lower than the exhaust oxygen depleted air, and also the exiting (oxygen loaded) salt must have a pO lower than the - ^ incoming air. Accordingly the exit salt pQ 2 can be much greater than the exhaust air pD_ The alkali metal peroxides and superoxides also undergo an exchange of oxygen with the gas phase, thereby contributing to the net production of oxygen, as shown in reaction (2). However their presence has been found to be much more vital than the small reaction (2) contribution would indicate, for two reasoπsj It has been found that when only trace concentrations of superoxides and peroxides are present, all gases withdrawn from the molten salt have excessively high concentrations αf nitrogen oxides, as high as several percent. However when the combined peroxide and superoxide concentration is 2% αr greater, the nitrogen oxides are reduced to close to their equilibrium values, i.e. in the low ppm range. Reaction (6) shows that the salt pN„ varies inversely with the oxide concentration; it is hypothesized that the lowest energy path for the salt to relieve the high nitrogen tension associated with low oxide concentration is via NO generation, which would account for the high NO levels in those circumstances. Secondly, a combined peroxide an.d superoxide concentration greater than 2% has been found tα markedly improve reaction kinetics. For example pure * . αxygeπ was bubbled through a mixture of pure nitrate and nitrite salt virtually without effect. Then the bubbling was repeated at the- same temperature after.adding 2% peroxides plus superoxides to the melt. The αxygeπ reacted rapidly and almost quantitatively with the second melt. This observation suggests the hypothesis- that -the -actual reaction mechanism which leads to the overall reaction (1) is probably primarily a combination of reactions (2) and (3). However irrespective of any hypothetical explanations, a critical minimum concentration of approximately- 2% combined peroxides and superoxides has been found to yield advantageous results, and should be maintained. The upper limit on combined peroxide and superoxide con¬ centration is not as critical. However high concentrations will reduce the purity level αf the prαduct oxygen by producing nitrogen (e.g. reaction (6))„ and also are more corrosive, hence a nominal upper limit of 25% is cited, where it is normally preferable tα remain close to the lower limit. The nitrate and nitrite concentrations provide the bulk of the oxygen carrying capability αf the salt, hence it is desirable to maximize their combined concentration, i.e. to minimize the concentration αf inert salt diluents or impurities. The impurities of greatest concern are carbonates and hydroxides, which will form from the C0„ and H„0 naturally present in air according to reactions (4) and (5). In order tα limit the carbonate and hydroxide concentration to less than approximately 10% at steady state, the C0„ and HpO concentrations in the supply air must be reduced to 6ppb and 1ppm respectively. This requirement for a high degree of purification of C0 ? and H 3 is shared by other chemical air separation processes and also by air liquefaction processes, and standard techniques are available to achieve it: molecular sieves, BaO guard reactors, etc. The ratio of nitrate to nitrite concentration in the salt corresponds to the equilibrium pE values encountered at process conditions, and ranges roughly from 4 tα 25. In other words, the anion composition αf the salt should include from 50 tα 94% nitrate and from 4 to 25% nitrite. The. cation composition αf the salt is an important means by which the anion composition can be maintained within desired OMPI raπges at steady state conditions. The free energy of formation of a given salt εonstitueπt typically varies with choice of cation. Hence varying the cation..mixture changes the -ΛG~, i.e. the stability, σf each constituent, and the concentration αf each varies with its stability. The temperature range αf intere far this oxygen acceptor is approximately 800K to 1100K; at the low end it is limited by reaction kinetics whereas at the high end it is limited both by corrosion and by a tendency of the salt to decompose' to solid oxide. Idith potassium cation, the nitrate is most stable, followed by nitrite, superoxide, peroxide, and then oxide. Hence in potassium salt the superoxide is the preponderant species of oxide, and its concentration typically ranges from 1 to 10%. With sodium the peroxide is much more stable " : more so than the nitrite, and at higher temperatures even more so than the. nitrate. Thus pure sodium salts typically have peroxide concentrations in the range αf 15 tα 75% at equilibrium, depending on process conditions. The main effect αf mixing sodium and potassium cations is to reduce the peroxide concentration while leaving the relative concentrations αf nitrate, nitrite, and superoxide relatively unchanged. This has several advantageous effects, including lower melting paint, lower combined total αf superoxide plus peroxide, which minimizes N„ and corrosion, and causes the concentrations of peroxide and superoxide tα be the same order of magnitude. Also the salt cost is lower than with pure pαt " assium cations. Uiith lithium cations the oxide is significantly more stable than any other species. It is so stable that the liquid oxide activity reaches the point at. which pure solid oxide condenses out of solution, and hence the melt slowly irreversibly decomposes to solid oxide. Fortunately with both sodium and potassium .cations the liquid oxide activity is low enough to preclude this condensation. However this limits the lithium cation capacity of the salt to no more than approximately 15%. In summary, a salt cation composition of 30 to 7-0% sodium, balance potassium, is preferred. However, compositions outside this range, even including same lithium, will also be found useful. O Process Disclosure (Best Made for Carrying Out the Invention) Referring to Figure 1, air is compressed tα an intermediate . pressure in isothermal compressor 1 (i.e. a compressor incorpαr- ating means far air cooling), purified of COr, and H„0 in a conventional purification apparatus 2, and then adiabatically compressed to the absorption pressure in compressor 3. This compression combination has been found tα yield the most efficient overall result. Strictly isothermal compression takes less mechanical energy but requires too much additional thermal energy from the heat exchanger; conversely, strictly adiabatic compression requires much greater mechanical energy, which the thermal savings in the heat exchanger are not sufficient to offset. By combining isothermal and adiabatic compression in that order, at least a 5% net energy savings is realized in the overall process ralative tα using either type αf compression alone, provided each compressor carries at least 25% of the total compression duty. The pressurized purified air is heated in heat exchanger 4 and then reacted with the oxygen acceptor in absorption tower 5. This tower can be any configuration in which multistaged countercurrent contact is obtained between the salt and air, for example packed tower, bubble cap or sieve tray tower, . etc. The exhaust air, substantially depleted in oxygen content, then enters a combustion chamber .6 and it is reacted with a fuel such as distillate, natural gas, or fuel gas. This raises its temperature and further depletes its free oxygen content. Then it is partially expanded in turbo-expander 7. The degree of expansion is controlled to regulate the turbine exhaust tem- perature, which in turn controls the temperature αf the air entering the absorption tower. After the partially expanded exhaust is heat exchanged against the incoming air in heat exchanger.4, the expansion to ambient. pressure is completed in turbo-expander 8. The above exhaust treatment sequence ensures that heat is added to the process at the hottest possible temperature, which improves efficiency. Also, the hottest temperature in heat exchanger 4 is substantially below maximum process temperature . Oxygen loaded salt from absorption tower 5 is reduced in pressure and then sprayed or routed into decomposition tower 9. This tower can also be any of several conventional configurations including a packed tower or even a simple single stage pot type reactor. The decomposition ' reaction is slower than the absorption reaction. Also the gas volume handled in the decomposition tower is larger due tα its lower pressure. Both factors con¬ tribute to the appreciably larger size of the decomposition tower relative tα the absorption tower. The oxygen depleted salt withdrawn from the decomposition tower is increased in pressure and recycled to absorption tower 5 by pump 10. The product oxygen is withdrawn from the decomposition tower over¬ head, heat exchanged against the supply air, further coaled as appropriate, and then compressed in evacuation compressor 11. In the absorption and desorption apparatus just described, the eπdothεrrnic heat of the decomposition reaction is supplied by the sensible heat of the salt. For each 1% of nitrate which decomposes tα nitrite plus oxygen, the salt is cooled by approximately 9 C. As decomposition temperature decreases, the decomposition pressure decreases markedly. In order to avoid pressure difficulties with the decomposition reaction, its temperature should not be more than 50K below the absorption reaction temperature. Therefore in the above apparatus the - * amount of salt which can be reacted per cycle is limited tα approximately 5%, which in turn requires a relatively large amount of salt pumping. It has been found that the required amount of salt pumping can be reduced, and hence the overall process efficiency can be increased, if the process is con- figured such that at least part αf the decomposition reaction • heat requirement is supplied from the exothermic heat released by the absorption reaction. That is, the absorption reaction salt and decomposition reaction salt are configured in heat exchange relationship, thereby minimizing the temperature difference between the two by the heat flow between them. An apparatus which accomplishes' this result is illustrated in Figure 2. Absorption is conducted in one or more cylinders 21 containing appropriate packing tα effect efficient countercurrent contact. A sleeve 22 encloses the cylinders and serves as a container for salt undergoing the desorptiαπ reaction. Thus heat exchange -is- effected through the- all -of -cylinder- 21, which may be finned or otherwise modified to improve heat transfer. Salt from t.ie absorption reaction is depressurized and sprayed into- optional decomposition tower 23 r . hich cools it and releases some αf the product αxygeπ. Then it is routed tα sleeve 22, where it picks up heat and cools the absorption reaction, thereby releasing additional oxygen. Finally it is increased in pressure and recycled back to the absorption cylinder 21 by pump 24. The pump can advantageously be a vertical submerged pump which avoids sealing difficulties, or. other known high temperature designs, including the blow case pump. The entire absorption-decomposition apparatus illustrated in Figure 2 can advantageously be substituted for the simpler apparatus without heat exchange in Figure 1. Operating Conditions The best mode for producing oxygen using the above dis¬ closed salt composition in the above disclosed process sequence uill be achieved by adhering to the following operating conditions. The temperature αf the absorption reaction should be in the range αf 900-1000K, and that αf the decαmpostiαπ reaction in the range 850-975H. Overall process efficiency is strongly enhanced by decreasing the absorption reaction pressure and by increasing the fraction of oxygen which is extracted from the incoming air. The latter parameter is limited by the amount αf αxygen which must be left in the exhaust tα provide the stoichiα- mεtric requirements far combustion with fuel. Combustion requires on the order of 15% of the inlet supply of oxygen, and hence the fraction αf αxygen extracted should range from 0.7 to 0.9. For a given fractional β extraction, lower absorption pressures require lower decomposition pressures. For fractional Or, extraction in the above range, the absorption pressure will be between 16 and 50 times the decomposition pressure. The minimum practicable decomposition pressure is determined by size?, of required equipment, rate of salt loss by evaporation or eπtraiπ eπt, and power required for the vacuum compressor, and OMPI WIPO is on the order of 0.1 atmosphere. Normally decomposition pressures in the range of 0.1 tα 3 atmospheres will be used, typically 0.4 atmosphere. This results in absorption pressures of 5 to 30 atmospheres, typically 10. The reaction temperatures " and pressures strongly affect the change in oxygen loading of the salt between the two reactions, which in turn is related tα the salt circulation rate. The amount of salt reacted per cycle falls in the range of 2% to 14% (molar percent), and the corresponding salt circulation rate is betweεπ 15 and 100 males of salt circulated per mole of αxygeπ produced. Example operating conditions for thε flaw sheet of Figure 1 are presented in the fallowing table. In the table, the tεm- peraturε and pressurε of the process stream exiting thε indicated component are tabulated. Also shown are thε powεr ratings and efficiencies αf the rotating machinery. The power ratings reflect steady state production of oxygen at a rate of 300 metric tons per day. Temperatures are in degrεes Kelvin, with degrees Fahrenheit indicated in parentheses. Pressures are in atmos¬ pheres, and power is in kilowatts, either mechanical or thermal as appropriate. Thε first compressor 1 has sufficient cooling to approximate a 333 K isothermal compressor, and compresses the air tα 5.13 atm. The adiabatic compressor 3 further compresses the air tα 11 atm., as it adiabatically heats up to 435 K. The res- pective power ratings are 3.734 kϋJ and 1947klιl, that is the adia¬ batic compressor supplies 34.3% of thε total compression power. Thε air is then heatεd tα 900 K in heat exchaπgεr 4, which has a thεrmal duty rating of 8874 kliJ. Than thε air is introducεd into absorption towεr 5 in countercurrent contact with circulating salt. The absorption tower in thε εxamplε has four theoretical plates.. The 0 2 of the incoming air is 2.31 atm., and it exits at a total pressurε of 10.7 atm and a p0 2 which has been reducεd to 0.6 atm., at 931 K. Thε salt εntεrs- thε absorption tower at a nitrate fraction αf 0.86 and a pO ™ of 0.5 atm. In the successive stages, its nitrats fraction iπcrεasεs to 0.89 and its. p0 2 increases in each stage to 0.86, 1.35, 1.91, and finally 2.31 atm., the exit concentration. The changing salt composition as it adiabatically traverses the absorption tower O Compoπent .Pressure .Temperature, Power.Efficiency Compressor 1 5.13 333 (140) 3734 .8 Compressor 3 11 - 435 (323) 19-47 .8 Heat exchanger supply 10.9 900 (1160) 8874 Absorption tower gas 10.7 931 (1215) Combustor 10.5 1278 (1840) 5455 Turbo-expander 7 3.22 967 (1280) 4881 .85 Heat exchanger exhaust 3.22 491 (423) ' Turbo-expander 8 1 1860 .85 Decomposition tower gas 0.35 927 (1209) Evacuation compressor 333 (140) 394 .8 Absorption tower salt 10.9 953 (1256) Decomposition tower salt 0.35 927 (1209) Pump 11 928 (1210) 673 is plotted on Figure 3, and indicated by trianglεs. Thus the p0„ of the ' salt exiting the absorber, 1.91, is much greater than the 0.6 p0„ of the oxygen depleted air withdrawn from the absorber, and approaches that αf thε incoming air. Aftεr " heating the exhaust to 1278 K by combustion with fuel at a heating value rate of 5455 kid (677 BTU/lb 0 2 ), the hot ' exhaust is expanded to 3.22 atm. and 967 K. This pressurε and corres¬ ponding temperature provide the rεquirεd air preheat temperature of 900 K. Then thε residual compression energy is recovered in turbo-expander 8, another 1860 kid. The possibility will be noted of mounting compressor 3 and expander 8 on a single shaft/ reduction gear complex, and similarly for compressor 1 and expander 7. A small motor generator gearεd to εach co plεx could supply εlectric power for pump and evacuation compressor operation, and also furnish startup power. The gas pressure in the dεsόrber is 0.35 atm., which is the supply pressure to both thε pump 10 and thε evacuation compressor 11. The dεsorber salt in decomposition tower 9 only is decrεasεd tα a p0 2 of 0.5 however, due tα kinetic limitations, and the tradeoff against the rεquirεd size of and residence time in the decomposition reactor. Overall, the energy required for αxygen production ia the process of the above example is less than 55% of the thermal enεrgy rεquirεd tα run a αdεrn liquefaction plant having a 33% thermal to mechanical conversion efficiεπcy. Othεr operatin cαnditiαns within the disclosed operating ranges will be found to yield even further improvements in energy efficiency. For example, thε dashed line loop on Figure 3 reflects thε salt composition locus for an absorption-decomposition cycle using the heat exchange apparatus of Figure 2, and the same endpoint pressures as εxamplε 1. The major change is that salt loading cycles betwεεn 86% nitrate fraction and 92.5% nitrate fraction, vice between 86% and 89%. This reduces pump power by slightly more than half, and yields about a 6% further reduction in total process εnεrgy requirement.

Claims: 1. A continuous procεss for sεparating αxygeπ from air com¬ prising reacting an oxygen acceptor with air in an absorption reaction, separately decomposing thε oxidizεd oxygen acceptor tα yield oxygen and regenerated Dxygeπ acceptor, and recycling the oxygen acceptor, characterized in that the oxygen acceptor is a molten solution of alkali metal salt comprised of a cation ' composition of sodium and potassium cations and an anion composition αf * 50 to 94%.nitrate, 4 to 25% nitrite, and 2 tα 25% combined peroxide and superoxide. 2. Thε process of claim 1 wherεiπ thε absorption reaction between thε molten alkali metal salt solution and air is conducted in multiple, countercurrent stages, whereby the p0„ αf the salt withdrawn from the absorption reaction is greater than the p0„ of the oxygen depleted air withdrawn from the absorption reaction. 3. The procεss of claim 2 comprising the additional steps of combusting a fuel with the oxygen depleted air, thereby yielding a hot combustion gas; partially expanding the hot combustion gas-to recovεr mεchanical energy in a turbo-expandεr; hεat exchanging thε partially expanded com¬ bustion gas against the air supplied to the absorption, reaction,; and completing the expansion of the cσoled partially expanded combustion gas to recover additional mechanical energy. 4. The procεss αf claim 3 comprising thε additional step of transferring heat from the salt undergoing the absorption reaction to the salt undergoing the decomposition reaction. 5. The process of claim 2 comprising thε additional step of compressing the air to absorption reaction pressurε using a cooled compressor first followed by an adiabatic com¬ pressor, wherein each compressor supplies at least 25% of the total compression power. ( OMPI 6. Thε prαcεss of claim 2 whεrεin thε decomposition reaction is conductεd in at least two sequεntial stagεs. 7. Thε procεss of claim 1, 2, 3, or 4 whεrεin the absorption reaction tempεraturε is in the range 875-1000 5 thε absorption rεactioπ pressure is in the range 5-30 atm; the decomposition reaction temperature is in the range 850-975 K; the dεcompositioπ rεaction pressure is in thε raπgε 0.1-3 at thε fraction of 0 2 εxtractεd from air is in thε ragπe 0.7-0. 10 the ratio of absorption pressure tα decomposition pressure is in the range 16-50; the salt, molar circulation per mole αf αxygeπ produced is 15-100 moles/mole; ' and thε cation composition is in the range αf 30 to 70% "15 potassium, balancε-sodium. OM A, IP

Inventors: ERICKSON D
Applicants: ERICKSON D
Application year: 1978
Main classification: C01B13
IPC codes: C01B13