WO1988002036A1

WO1988002036A1 - Gas separation process

Info

Publication number: WO1988002036A1
Application number: PCT/US1987/001383
Authority: WO
Inventors: Richard M. Laine; Daryl L. Roberts
Original assignee: Sri International
Priority date: 1986-09-22
Filing date: 1987-06-15
Publication date: 1988-03-24

Abstract

A process for separating oxygen from a mixture of gases containing oxygen. The mixture of gases is passed through a solution of a solvent containing in solution an electrolyte, an organometallic complex oxygen carrier in which the metal is a polyvalent transition metal at a lower valence and an axial base to adsorb oxygen, this oxygen is released from the carrier by electrochemical oxidation of the carrier to raise the valence of the metal component of the carrier, the electrical input being sufficient only to oxidize only about one tenth to one half the amount required to electrochemically oxidize all of the metal of the carrier and release a part of the adsorbed oxygen. The remainder of the oxygen is released by action of the oxidized portion of the metal on the unoxidized carrier holding adsorbed oxygen.

Description

GAS SEPARATION PROCESS

Technical Field

Oxygen has been separated from gas mixtures containing oxygen by contacting such mixtures with organometallic complexes commonly termed "oxygen carriers". During the contact oxygen is bound to the carrier complexes. After all or a substantial part of the capacity of the carrier complex to bind oxygen has been exhausted the carrier complex is removed from further contact with the feed gas and the bound oxygen is separated from the carrier. In the past this separation has been made either by raising the temperature of the carrier containing bound oxygen which causes release of the oxygen from the carrier or by introducing the carrier containing bound oxygen into ^•a zone in which the pressure above the carrier is substantially below atmospheric pressure and this pressure reduction causes release of the bound oxygen. After release of the bound oxygen from the carrier the carrier may be returned to further contact with the feed gas to repeat the binding of oxygen to the carrier.

Background of the Invention

Recently it has been found that release of bound oxygen from the carrier can be made by subjecting the carrier containing bound oxygen to electrochemical oxidation in which the metal constituent of the carrier is oxidized to a higher valence level with concurrent release of the bound oxygen.

Gagne et al patent 4,475,994 describes release of bound oxygen from an organometallic complex containing bound oxygen by electrochemical oxidation of the complex containing bound oxygen. The specific description involves the use of a cobalt complex carrier, i.e., LxCo(II)ⁿ⁺ in which L is an organic ligand, x is the number of ligands associated with the cobalt ion and n is the total charge of the complex. A solution of the complex carrier is contacted with gaseous mixture containing oxygen and the following reaction occurs

0₂ + LxCo(II)ⁿ⁺ → LxCo(II)0₂ ⁿ⁺ (1)

The oxygen carrier containing bound oxygen is contacted with the anode of an electrochemical cell where the following reaction occurs

LxCθ(II)0₂ ⁿ⁺ — LxCo(III) ⁽ⁿ⁺l⁾+ + le + 0₂ (2) The released oxygen is then removed and the LxCo(III) ⁽ⁿ⁺¹)⁺ is contacted with the cathode where the following reaction occurs _le + Lχ_Co(III₎ Cn+1)+ _→ Lχ_Co₍II)ⁿ.⁺ (₃) and the product, LxCo(II)ⁿ⁺ returns to contact with the gaseous mixture containing oxygen and reaction (1) above occurs.

In the Gagne et al process the entire release of bound oxygen occurs at the anode pursuant to equation (2) above.

Brief Description of the Invention

The present invention is based on the finding that the process described by Gagne et al can be greatly improved by deliberately limiting the extent to which oxygen release from the carrier is obtained by electrochemical oxidation and by obtaining release of the remaining bound oxygen by chemical oxidation in which the oxidized carrier, freed of the bound oxygen, oxidizes the remainder of the carrier containing bound oxygen and effects release of the remaining bound oxygen.

If the entire oxygen release is made by a electro¬ chemical oxidation, the electrical input required is determined by the concentration of the LxCo(II)0₂ ⁿ⁺ in the solution and the flow rate of the solution into contact with the anode. If the concentration of LxCo(II)0₂ ⁿ⁺ is indicated by C (mols per liter) the flow rate by F (liters per minute) then C x F is the number of mols per minute contacting the anode of the electrochemical cell. If C x F equals one mol per minute than one Faraday per minute would be required to oxidize the Co(II) to Co(III) and release the bound oxygen.

Pursuant to the present invention only a portion, usually 20 to 60%, of oxygen release is achieved by electrochemical oxidation of the LxCo(II)0₂ ⁿ⁺ to LxCo(III) ⁽ⁿ⁺¹⁾⁺ + 0₂ and the remainder of the oxygen is released by chemical oxidation of LxCo(II)0₂ ⁿ⁺ by the LxCo(III) (^{n+1) +} produced during the electrochemical oxidation. This has a two-fold effect on the economics of the process, first electric consumption is reduced by about 50% or more and electrode size and cost are also reduced, the electrode cost being a very significant cost factor in a large plant for producing oxygen.

Detailed Description of the Invention

In the practice of the invention a solution capable of removing oxygen from gaseous mixtures containing oxygen, e.g., air, is employed. The solution contains a polyvalent metal complex- oxygen carrier, an electrolyte and a. solvent. Optionally, but preferably an axial base is made a component of the solution.

Polyvalent metal complex oxygen carriers are well- known in the art and have been extensively described in the literature.

Niederhoffer, Timmons and Martell in Chemical Reviews 1984, Vol. 84, No. 2, beginning at Page 137, set forth an extensive review of the literature relating to metal complexes which reversibly bind dioxygen (0₂) , chemically identify a great many complexes and provide equilibrium constants for the reactions of polyvalent metal complexes with oxygen in organic solvent. The numerous polyvalent metal complexes set out in Tables XXXIIID, XXXIIIE and XXXIIIF, which appear on pages 179 through 185 of the publication are suitable for use as oxygen carriers in the process of the invention.

Kimura et al in Journal of the American Chemical Society. 1984, Vol. 106, pp 5497-5505, describe a number of nickel complexes which are oxygen carriers and are suitable for use as such in the present invention.

Schiff base complexes of the following two formulas and their analogs have been found effective oxygen carriers for use in the process of the invention. COMPLEX I

COMPLEX II

OCH- OCH-

Analogs of these two compounds include those in which the polyvalent metal is a transition metal, preferably iron, nickel, manganese, rhodium, copper and ruthenium instead of cobalt, and in which the oxygen atoms are replaced by another element or group such as sulfur or NH₂.

The electrolyte component of the solution may be any electrolyte which is soluble in the solvent employed and which is chemically compatible with the solvent and with the oxygen carrier complex. Quaternary ammonium salts, such as tetrabutyl ammonium fluoborate, tetrabutyl ammonium chloride and other tetraalkyl salts of inorganic acids are suitable electrolytes. Quaternary phosphonium salts are also suitable electrolytes. When the solvent employed has the capacity to dissolve a reasonable amount of water then electrolytes such as sodium chloride and the like which may not be soluble in the solvent without the water being present may be employed with the result that the range of electrolytes which it is feasible to employ is greatly extended.

The solvents employed are organic solvents, preferable polar organic solvents, such as dimethlyfor- mamide, N-methylpyrrolidone, dimethylsulphox^'ide and generally lactones, lactams, amides, amines and the like. The essential property requirements of the solvent are that it be capable of dissolving the metal complex oxygen carriers in amount to provide concentrations of at least 0.01 molar and up to much higher concentrations, such as 5 molar, and that it also be capable of dissolving the electrolyte employed in amount sufficient to provide a high level of electrical conductivity to the total solution, and further that it be chemically compatible with both the oxygen carrier and with the electrolyte employed.

As indicated above, an axial base may be incorporated in the solution. The axial base cooperates with the oxygen carrier component to assist the reversible binding of oxygen to the carrier. Suitable axial bases include pyrazine, 4-methyl- pyridine, N-methylimidazole, dimethyl amino pyridine, 4,4' bipyr.idine and the .like. The axial base component is employed the quantity employed in amount such that the mol ratio of axial base to oxygen carrier is in the range 0.5:1 to 2:1.

A suitable solution would have the following composition:

155 ml - N-methyl pyrrolidone 0.3 molar - Tetrabutyl ammonium tetrafluoborate 0.025 molar - organometallic complex

(N,N'-bis(salicylidene) Ethylene diamin cobalt 0.040 molar - pyrazine Two embodiments of the invention are shown in the appended drawings, Figures 1 and 2, which are flow diagrams. Operation of the invention pursuant to each of the flow diagrams is described.

Referring now to Fig. 1 of the drawings, vessel 1, an absorber, is at start-up partly filled with the above-described suitable solution, similarly vessel 3 and hold tank 4 are partly filled with that solution and electrolytic cell 2 is completely filled with solution. Electrolytic cell 2 contains an anode compartment and a cathode compartment separated by a semipermeable membrane 22.

When operation is initiated, air is pumped through line 5 into the bottom portion of vessel 1, rises through the solution where oxygen is absorbed by the solution and air having a reduced oxygen content is released to the atmosphere through line 6.

Solution containing bound oxygen is withdrawn from vessel 1 via line 7 and the withdrawn solution is split into two streams. One stream passes through line 8 into the anode compartment of the electrolytic cell 2 where it contacts anode 9 and is completely oxidized electrochemically as shown in Equation 2 above. The oxidized solution containing released oxygen gas is passed through line 10 into vessel 3. Oxygen is withdrawn from vessel 3 via 11, passed through separator 12 where suspended droplets of solution are separated and passed through line 13 into hold Tank 4. Product oxygen is withdrawn from separator 12 through line 14. Liquid is withdrawn from vessel 3 and passed via line 23 into hold tank 4.

The other split stream, being more than one-half of the liquid effluent from vessel 1, passes through line 15 into chemical reaction vessel 16. At the same time solution is withdrawn from hold tank 4 and passed into vessel 16 via line 17.

In vessel 16 carrier containing bound oxygen (LxCo(II)0₂ ⁿ ) which was formed in vessel 1 comes contact with and reacts with oxidized carrier (LxCo(III) ^ⁿ ' ) earlier produced at anode 9, pursuant to

LxCo(III) (ⁿ⁺¹⁾⁺ + LxCo(II)0₂ ⁿ⁺ —

LxCθ(II)ⁿ⁺ + LxCθ(III) ⁽ⁿ⁺¹⁾⁺ + 0₂ (4)

Oxygen produced by this reaction is withdrawn from Vessel 16 as product through line 18. Liquid is withdrawn from the lower portion of vessel 16 and passed via line 19 into the cathode compartment of the electrolytic cell where it contacts cathode 20. At cathode 20 contained LxCo(III) ^ⁿ ' is reduced to LxCo(II)ⁿ and passed via line 21 into vessel 1 where it is again, used to bind oxygen contained in air.

The embodiment of the invention shown in Fig. 2 differs from that of Fig. 1 in that the liquid effluent from vessel 1 is not split into two streams. Rather, in the embodiment of Fig. 2, the entire liquid effluent from vessel 1 is passed into the electrochemical cell where it is subjected to controlled oxidation so that less than about one-half of the contained LxCo(II)0₂ ⁿ is oxidized to form LxCo(III) ⁽ⁿ ' + 0 . The oxygen is recovered as product and the liquid containing LxCo(III) ⁽ⁿ⁺¹⁾⁺ and LxCo(II)0₂ ⁺ is sent to a hold tank where the remaining oxygen is released by the reaction

LxCo(III) ⁽n⁺l⁾+ + LxCo(II)0₂ ⁺ → LxCo(II)ⁿ⁺ + LxCo(III) (ⁿ⁺¹⁾⁺ + 0₂

The oxygen is removed as product (Fig. 2, line 24) and the liquid is passed into the cathode compartment of the cell where the LxCo(III) (ⁿ⁺¹⁾⁺ is reduced to LxCo(II)ⁿ⁺. In operation pursuant to Fig. 2 the liquid exiting vessel 1 is continuously monitored to determine its concentration (C) (Hewlett-Packard UV Spectrophotometer #8450A) and the flow rate (F) into the electrochemical cell is measured. The product of F and C gives the number of mols of LxCo(II)0₂ ⁿ⁺ passing into the electrochemical cell per unit time. One Faraday per mol is required for complete oxidation of LxCo(II)0 ⁿ⁺ to release all of the bound oxygen so the current across the cell is adjusted so that the electrical input to the cell is held well below about one Faraday per mol of LxCo(II)0₂ ⁿ⁺. This input provides electrochemical release of only a portion of the contained oxygen and this oxygen is withdrawn via Fig. 2, line 14. The remainder of the oxygen is released by chemical oxidation as above described, and is withdrawn via Fig. 2, line 24.

The following examples are illustrative of the invention. Example 1

A solution of the organometallic compound CoSalPn, a schiff base cobalt complex having the formula

CoSalPn was prepared at concentration J of 0.025 mole/liter in a solvent, N-methylpyrrolidone, and with an_. axial base, methyl imidazole, present in a concentration of 0.025 mole/liter and with an electrolyte, tetrabutyl ammonium tetrafluoborate, in a concentration of 0.3 mole/liter.

A 55 ml volume of this solution was placed in a closed beaker having a total volume of 110 ml. This beaker was equipped with a reference electrode, a cathode, and an anode. The beaker was held at 268°K. Air was bubbled through the solution to saturate the solution with oxygen. Then, a 50 μl sample was drawn from the gas phase of the closed beaker for analysis by gas chromatography. The oxygen concentration in the gas space was 20.6%, normal atmospheric concentration.

Current was then passed through the cell; the anode was held at 0.3V with respect to saturated calomel electrode (SCE) . The oxygen concentration in the gas phase was monitored by gas chromatography, and the total current passed through the solution was measured with a coulometer. After a period of time, the gas phase oxygen concentration had increased to 23%, and there were 1.4 coulombs of electricity consumed. The ratio of oxygen molecules in the gas phase to the number of electrons consumed was 3.7.

The total oxygen release raised the oxygen concentration in the gas space from 20.6% to 23% i.e., 2.4%. The volume of the gas space was 55cc. The amount of oxygen release was therefore 55 x .024 = 1.32cc. 1.4 coulombs of electricity were consumed. The 1.4 coulombs would be 1.4/96500 mols of electrons and would release 1.4/96500 mols of oxygen via the reaction LxCo(II)ⁿ⁺0₂ -_* LxCo(III)ⁿ⁺¹ + le + 0₂. The volume of oxygen released by the 1.4 coulombs would be 1.4/96500 x 22,400 cc = 0.325 cc. The total oxygen released was 1.32 cc. The remainder of the^" oxygen was released by chemical reaction of LxCo(III)ⁿ⁺¹ with LxCo(II)ⁿ⁺0₂. The mechanism of reaction in this system is not fully understood but the experimental results are. The electrical input, 1.4 coulombs could produce only 0.325 cc. of 0₂ but the total oxygen released is 1.32 cc. about 4 times the amount that is produced by the electrical input.

Example 2

A solution of CoSalPn, 0.025 mole/liter in n-methyl pyrrollidone with axial base 4- pyrrolidinopyridine at concentration of 0.0375 mole/liter, and with electrolyte concentration of 0.3M BU4NBF₄. A similar electrolysis experiment was performed in a closed beaker. The anode potential was held at 0.7V SCE. The gas phase reached 30.6% oxygen, and total electricity consumed was 13.7 coulombs. The ratio of oxygen molecules released into the gas phase to electrons consumed was 1.7.

A number of runs paralleling the above examples were made using different organometallic oxygen carrier complexes and axial bases were made in which the oxygen release was greater than could be accounted for by action of the coulomb input. The symbol ER has been used to denote the ratio of total oxygen release to oxygen release to oxygen release due to coulomb input alone. ER values for 17 runs averaged 1.56 and ranged up to 3.7.

ER values vary for different oxygen carrier - axial base combinations. As noted above Schiff base cobalt complexes are very effective carriers. A number of these Schiff base complexes are listed at pages 149 and 150 of the Neiderhoffer et al. paper. Chem. Rev., 1984, 187-203. The nomenclature is difficult but understanding is helped by the glossary beginning at page 174.

Operation pursuant to the flow pattern described in either Fig. 1 or Fig. 2 accomplishes complete release of bound oxygen from the carrier. Part of the release is accomplished by electrochemical oxidation of the carrier complex containing bound oxygen which releases oxygen from the carrier and oxidizes the metal of the carrier complex to a higher valence. The carrier complex having its metal at a higher valence then chemically oxidizes the remaining carrier complex containing bound oxygen to complete the oxygen release.

Claims

1. A process for separating oxygen from gas mixtures containing oxygen which comprises:

Step 1. forming a solution consisting essentially of an organic solvent, an electrolyte and an organometallic complex oxygen carrier in which the metal is polyvalent and at a lower valence, the solvent, electrolyte and organometallic complex being chemically compatible,

Step 2. providing an adsorber vessel and filling it with the solution of step 1.

Step 3. providing a closed vessel having a central vertical divider separating said vessel into two compartments, said divider being a permeable membrane, an electrode disposed near the bottom of each of said compartments and a cell so connected to the electrodes that one becomes a cathode and the other an anode, said membrane being permeable in the sense that it prevents flow of said solution through it from either compartment to the other but provides liquid electrolytic communication between the compartments,

Step 4. introducing said solution into both compartments of said closed vessel.

Step 5. passing an oxygen containing gas into the adsorber vessel at a point near its bottom to cause oxygen absorption by said oxygen carrier and withdrawing gas depleted in oxygen from the top of the adsorber vessel.

Step 6. withdrawing liquid solution containing oxygen bound to the carrier from the adsorber vessel and passing it into the bottom of the anode compartment of said closed vessel to cause oxidation of the metal component of the oxygen carrier and release of the bound oxygen,

Step 7. determining the concentration of oxygen carrier containing absorbed oxygen in the solution entering the anode compartment and the strength of the current across the electrochemical cell and adjusting the residence time of the solution entering the anode compartment in that compartment so that about one tenth to about one- half of the metal component is electrochemically oxidized with concurrent release and recovery of oxygen.

Step 8. withdrawing the partially oxidized material from the anode compartment and passing it into a holding vessel.

Step 9. withdrawing oxygen gas from the top of the holding vessel and passing liquid solution from the holding vessel into the cathode compart¬ ment of the cell to cause reduction of the metal of the oxygen carrier to a lower valence and passing reduced oxygen carrier from the cathode compartment into the adsorber vessel.

2. The process of Claim 1 wherein the solution of step 1 also contains an axial base in amount such that the ratio of its molar concentration in the solution to that of the oxygen carrier is in the range about 0.5:2 to 2:1.

3. The method of releasing oxygen from an organometallic oxygen carrier having its metal at a lower valence and having oxygen bound to the carrier which comprises electrochemically oxidizing from about 1/10 to about one-half of the metal of the carrier to a higher valence with concurrent release and recovery of oxygen and then maintaining the resultant mixture in a holding vessel to cause further release and recovery of oxygen by reaction of the remaining carrier containing bound oxygen with the oxidized portion of the metal of the carrier and recovering the oxygen.

4. In a process in which an electrochemical cell is used in the separation of oxygen from a mixture of gases containing oxygen which comprises the steps of

1) intimately contacting said mixture with a solution of a transition metal complex of the formula LxCo(II)ⁿ⁺, wherein L is a ligand, x is the number of ligands associated with the cobalt ion, II represents the cobalt ion valence and n is the total charge of the complex, to bind oxygen to the complex forming LxCo(lI) 0 ⁿ⁺;

2) transporting the product of step 1 to the anode of an electrochemical cell where the Co(II) of the complex is oxidized to Co(III) and the bound oxygen is released for recovery, the improvement which comprises; a) transporting no more than one-half of the product of step (1) to the anode for oxidation of Co(II) to Co(III) with concurrent release of oxygen; b) recovering the released oxygen and passing the Co(III) complex into a reaction zone; c) transporting the remainder of the product of Step 1 into the reaction zone and intimately mixing it with the Co(III) complex to oxidize LxCo(II)0₂ ⁿ⁺ to LxCo(III) ⁽n+^l)+ and release oxygen; d) recovering oxygen from the reaction zone and transporting the LxCo(III)^n÷1 to the cathode where it is reduced to LxCo(II)ⁿ⁺.