SOLID STATE OXYGEN COMPRESSOR

Abstract: A solid-state, electrochemical compressor for compressing oxygen and other diatomic, ionizable gases is disclosed. An oxygen ion transport membrane such as a ceramic metal oxide, for example, zirconia, which conducts oxygen ions when subjected to an electrical potential is a component of such a compressor.

Description: SOLID STATE OXYGEN COMPRESSOR Background of the Invention This invention relates to the electrochemical compression of ionizable gases capable of electrochemical transport through a solid electrolyte and especially to the compression of oxygen. Pure oxygen has numerous applications. For instance, oxygen is used in large quantities in enrichment of blast furnaces, chemical synthesis, oxy-acetylene welding, life support and other medical uses. U.S. commercial consumption exceeds 18 metric tons (20 million short tons) per year. Oxygen costs about 17.7 cents per cubic meter (5 cents per cubic foot) in small quantities, and about 10 cents per kilogram ($15/ton) in large quantities. Currently, 99% (percent) of such oxygen is prepared by liquefaction of air and about 1% (percent) by electrolysis. Efficient storage and transport requires that oxygen be prepared at high pressure by mechanical compression. Mechanical compression of pure oxygen is a dangerous and inefficient operation due to the large positive Joule Thomson coefficient of oxygen. The present invention offers an alternative to mechanical compression of oxygen. The present invention makes use of oxygen-ion conductive solids to prepare high pressure oxygen electrochemically. It has long been known that various solids such as zirconia, ceria, and bismuth-oxide will conduct oxygen ions when subjected to an electrical potential gradient across such solid oxides but will remain electronic insulators. Furthermore, some of these materials possess high strength. Oxygen-ion conductive solids are currently used in numerous practical devices for power generation, oxygen partial pressure measurement and oxygen separation. In U.S. Patent 4,725,346 an oxygen delivery device is dis¬ closed. The specification of that patent is incorporated herein by reference. An understanding of oxygen-ion transport in certain metal oxides may be facilitated by considering an ion-conductive solid that separates oxygen at two different partial pressures which has a porous electronic conductor that is in contact with opposed surfaces of such ionic conductor at the gas-solid interfaces. This situation is depicted schematically in FIG. 1, which is an electrochemical cell. The ion conductive solid is the electrolyte and the electronic conductors are the electrodes. The half reactions occurring at each interface are: 02 + 4e- —> 20 = Interface 1 20 = —> 0 2 + 4e ~ Interface 2 The net chemical potential at each interface is the sum of the chemical potentials of the individual species at that interface. The electrolyte will conduct oxygen in order to move the system toward a state of equilibrium. Equilibrium is achieved when the net chemical potential difference between reactants at one interface and products at the other interface, P, each times their stoichiometric coefficient, is zero. This is expressed mathematically by the following expression, ∑v.μ. = 0 2 "e,l + ^O ,1 " 2 "«,2 + "O ,2 The net chemical potential difference of oxygen between interfaces 1 and 2 can be affected by controlling the electronic potential difference of electrons between interfaces 1 and 2. The chemical potential of oxygen at a gas-solid interface is direct function of oxygen partial pressure in the gas. Therefore, the oxygen partial pressure difference in the gas phase at the two interfaces can be affected by controlling the electronic potential difference. Description of the Drawings FIG. 1 is a schematic illustration of oxygen transport through an oxygen-ion conductive electrolyte; FIG. 2 is a cross-sectional view of a solid state oxygen compressor illustrating one embodiment of the invention; FIG. 3 is a graphical illustration of the rate of oxygen pressure increase achieved by the compressor illustrated in FIG. 2; FIG. 4 is a cross-sectional view of a solid state compressor similar to that illustrated in FIG. 2 which includes a heater internal to the high pressure chamber; FIG. 5 is a graphical illustration of the rate of oxygen pressure increase achieved by the compressor illustrated in FIG. 4; FIG. 6 is a cross-sectional view of a solid state compressor similar to that illustrated in FIG. 2 in which dead volume has been minimized and illustrates another embodiment of the invention; FIG. 6A is an enlarged sectional view of the end of the solid state compressor shown in Figure 6; and FIG. 7 is graphical illustration of the rate of oxygen pressure increase achieved by the compressor illustrated in FIG. 6. Description of Invention An oxygen compressor utilizing a solid state oxygen ion transport membrane which transports oxygen ions when subjected to a voltage differential is disclosed. Particularly useful oxygen ion transport membranes are ceramic metal oxides such as zirconia, ceria, hafnia, bismuth oxide and the like. Such electrolytes are disclosed and described in U.S. Patent 4,725,346 infra, the description thereof being incorporated herein by reference. The present invention consists of an ion- conductive solid constructed in such a way as to create a mechanical barrier between a low pressure oxygen reservoir and a confined volume. The barrier and confined volume SUBSTITUTE SHEET -3a- must be capable of supporting high pressure gas (most particularly oxygen) . Each side of the ion-conductive s UBSTΓΓUTE SHEET -4- solid (electrolyte) is coated with an electronically conductive material (electrode) to form a substantially continuous coating that is either porous or pervious to oxygen, i.e. it has appreciable oxygen solubility and diffusivity. This material is applied in such a way that a uniform electronic potential can be maintained throughout each electrode layer and so that a uniform electronic potential difference can be maintained across the ion- conductive solid. The electronic potential is maintained by means of an electrical power supply and lead wires. This is greatly facilitated if the ion conductor is an electronic insulator. Transport of oxygen will occur from the reservoir to the confined volume. The rate of oxygen transport is directly proportional to the overall chemical potential difference between each interface. High partial pressures of oxygen can be achieved in the confined volume since transport of oxygen ions will occur until chemical potential difference generated by the oxygen between the low pressure reservoir and the high pressure gas in the confined volume exactly cancels the applied electromotive force (EMF) . Most oxygen-ion conductive solids are metal oxides, a class of ceramic materials, such as zirconia, hafnia, bismuth oxide and the like. The chief solid material that is used as an oxygen ion conductor is stabilized zirconia. Stabilized zirconia has a combination of properties that makes it well suited for application in practical devices. These include high oxygen ion conductivity, especially at temperatures greater than 800°C, high strength, and sinterability which facilitate ease of fabrication of useful shapes, such as tubes, cups, cylinders, open-ended cylinders, flat plates and the like. The essence of the art in this field of technology is net shape fabrication techniques, processing parameters, microstructure control, type and amounts of dopants to control conductivity, electrode materials, electrode -5- application techniques and processing parameters, electrode morphology and many other details of design. Example 1 An electrochemical oxygen compressor was fabricated as depicted in FIG. 2. A closed end stabilized zirconia (92 mole % (percent) Zr0_ - 4 mole % (percent) Y,0 3 - 4 mole % (percent) Yb-0.) tube 0.95 centimeters outer diameter x 0.67 centimeters inner diameter x 24.13 centimeters long (0.375 inches outer diameter x 0.266 inner diameter x 9.5 inches long) was used as the electrolyte. About 11.4 centimeters (4.5 inches) of the length of the zirconia tube was coated inside and out successively with lanthanum strontium manganite and silver to form the electrodes. A silver lead was coiled around the external electrode to form the external lead wire. Another silver wire was laid along the internal wall of the tube to form the internal lead wire. The open end of the zirconia tube was secured into the top flange 11 of an Inconel pressure vessel 12 (internal volume of 0.4 liters) using a compression fitting. The top flange 12 was bolted into a fixed flange 12a welded to the pressure vessel 12. The internal diameter of the zirconia tube was exposed to the atmosphere by a hole in the top flange into which the compression fitting was welded. A teflon ferrule was used to create the seal. The internal surface of the pressure vessel was lined with alumina insulation 13. A heater 14 was placed along the internal diameter of the zirconia tube. This heater was fabricated by coiling nickel-chrome wire around an 0.31 centimeters outer diameter (0.125 inches outer diameter) alumina tube 15 and coating with alumina cement. The alumina tube 15 also served as an air inlet and preheater. Air was pumped in through the alumina tube and out through the annulus between the alumina tube and the zirconia tube. The zirconia tube was heated to approximately 800°C. The air inside the pressure vessel reached 137,900 newtons per square meter -6- (twenty psig) due to heating. When the temperature reached 800°C a potential of one volt was applied across the electrolyte such that the outside surface of the zirconia tube was positive. Oxygen was conducted through the zirconia tube until a pressure of 344,750 newtons per square meter (fifty psig) was reached external to the zirconia tube. Pressure increase developed inside the vessel as a function of time is plotted in FIG. 3. Example 2 The oxygen compressor of Example 1 was modified by installing a heater that was external to the zirconia tube as depicted in FIG. 4. A sterling silver wire was coiled in contact with the internal diameter of the electrolyte tube to serve as the lead wire. The compressor was operated in the same manner as in example 1 except that the pressure caused by heating was released to atmospheric pressure before applying the potential across the electrolyte. The pressure inside the vessel was brought from 0 newtons per square meter (0 psig) to 1,172,000 newtons per square meter (170 psig) in approximately 570 minutes. The pressure increase developed as a function of time is plotted in Figure 5. Example 3 An electrochemical oxygen compressor was fabricated as depicted in FIG. 6. The design of the compressor was changed to minimize the volume of the design. As in the two previous examples, a closed end stabilized zirconia (92 mole % (percent) ZrO. - 4 mole % (percent) Y 2 0- - 4 mole % (percent) Yb O ) tube 0.95 centimeters outer diameter x 0.67 centimeters inner diameter x 24.13 centimeters long (0.375 inches outer diameter x 0.266 inches inner diameter x 9.5 inches long) was used as the electrolyte. The pressure vessel was des.igned so that the internal wall of the pressure vessel was close fitting to the zirconia tube. In this case a -7- furnace was placed around the outside of the pressure vessel in order to heat the electrolyte to 800°C. The bottom 19.1 centimeters (7.5 inches) of the zirconia tube was coated inside and out successively with lanthanum strontium anganite and silver to form the electrodes. Silver wires were coiled around the electrodes to act as lead wires. The zirconia tube was secured into the top flange of the pressure vessel (internal volume of 0.025 liters) using a compression fitting. The internal diameter of the zirconia tube was exposed to the atmosphere by a hole in the top flange into which the compression fitting was welded into. A teflon ferrule was used to create the seal. Air was pumped through an 0.31 centimeters outer diameter (0.125 inches outer diameter) alumina tube and out through the annulus between the alumina tube and zirconia tube. The zirconia tube was heated to 800°C. . Internal pressure due to heating was released at this point. A potential of one volt was applied across the electrolyte. The vessel was pressurized to 1,379,000 newtons per square meter (200 psi) in approximately 18 minutes. The pressure as a function of time is plotted in FIG. 7. In an oxygen compressor of the invention it is desirable to utilize an ion (oxygen ion) conducting electrolyte having excellent strength. Zirconia or hafnia are for this reason preferred electrolytes with zirconia especially preferred because of its strength and commercial availability. The zirconia should preferably be in the shape of an elongated cylinder. A cylindrical tube with one closed end and one open end is especially preferred. Another feature of the invention is that the high pressure region is external to the electrolyte. Thus, the pressure acts on the external surface of the tube radially toward the longitudinal axis of the tube. While it is common in metal systems to contain high pressure inside the smallest diameter member, the tendency -8- of ceramics to fail in tension militates against containing high pressure internally in a ceramic tube. Placing the high pressure region external to the zirconia may require a heavier walled metal container (shell) , however, this is more desirable than making the electrolyte with a very thick tube wall. Electrical resistance of the electrolyte increases with electrolyte wall thickness, thus diminishing the electrical efficiency of the compressor. Ceramics generally are much stronger in compression than in tension, thus a cylindrical electrolyte tube of a certain diameter and wall thickness will sustain a greater pressure on its external surface. External pressure puts the cylindrical wall under compression, thus taking advantage of the electrolytes compressive strength which is much greater than its tensile strength. A feature of the present solid state compressor is a low ratio of the pressure chamber volume to electrolyte area. The quantity of oxygen transported through a particular electrolyte is proportional to current for a given temperature. In oxygen compressors it is desired to achieve the operating pressure as rapidly as possible. Thus, providing a small annular space between the interior of the pressure shell and the exterior of the electrolyte causes a rapid buildup of pressure within the pressure chamber. The compressor illustrated in FIG. 6 is particularly effective in rapidly reaching a desired high- pressure output. The high pressure creates some back EMF which at a pressure of 13,790,000 newtons per square meter (2000 psi) amounts to about 115 millivolts. Thus, at an operating voltage of one volt or more the back EMF is minimal. -9- Although the present invention has been described in detail in terms of oxygen compression, the techniques and devices described herein may be utilized generally in the compression of ionizable diatomic gases capable of electrochemical ion transport through a solid electrolyte. Such other transportable gases include hydrogen, chlorine, fluorine and the like. Hydrogen may be readily ionized and transported through proton conductors such as barium cerate, hydrogen uranyl phosphate, strontium cerate, etc. Similarly, chlorine and fluorine gases may be readily ionized and transported through chlorine ion conductors such as SrCl -Al O and fluorine ion conductors such as LaF or PbF . These protons conductors may be readily substituted for the oxygen ion transporting electrolytes described hereinabove and illustrated in the attached drawings. Although water is generally not considered a gas, both oxygen and hydrogen may be extracted from water vapor. Under appropriate conditions, water vapors disassociates into hydrogen and oxygen. In the presence of an oxygen ion-transporting electrolyte, oxygen may be separated and using the techniques of the instant invention, readily compressed. For example, serial compression of oxygen and hydrogen may be readily accomplished. Water vapor at an elevated temperature may be introduced into an oxygen compressor of the type described herein to produce high pressure oxygen and a by- product water vapor stream rich in hydrogen. This by¬ product stream may then be introduced into a hydrogen compressor utilizing a proton conducting electrolyte to yield high pressure hydrogen and vent gas of water vapor. Alternatively, the byproduct stream from the oxygen compressor could be fed to a condenser to condense the water vapor, recover pure hydrogen which could be -10- mechanically compressed, especially after the hydrogen is passed through a drier to remove any residual moisture. Hydrogen compression by disassociation of hydrogen, either in pure form or in combination with other gases, is preferably done at temperatures in the range of 50° to about 1000°C for the following proton conductors: phosphate (50-100°C) , barium cerate (500-1000°C) . Electrodes suitably used in conjunction with such proton conductors are palladium, lanthanum strontium chromite, platinum, silver and copper, lanthanum strontium manganite. Although various electrodes may be utilized on the present invention such as porous platinum, porous or non-porous silver and similar metallic compositions, the preferred electrode system is composed of lanthanum strontium manganite (LSM) . Multiple layers are preferably applied until an electrode thickness of about 20 microns to about 200 microns is achieved. These LSM electrodes are especially adherent to the electrolyte, have a coefficient of thermal expansion similar to that of the electrolyte and are generally unaffected by the oxidizing condition to which the electrodes are subjected. Conductive ceramic electrodes other than LSM that are useful in the instant invention are lanthanum strontium chromite, strontium iron cobaltite and the like. The sheet resistance of LSM electrodes tends to be higher than that of metal electrodes such as silver. An effective manner of distributing current throughout the whole area of the LSM electrode is to use a metal wire mesh in intimate contact with the electrode. One technique useful in the instant invention is to form a cylinder of nickel or Inconel wire mesh then force an LSM coated tubular electrolyte into the interior of the mesh cylinder. The metal mesh may be heated to an elevated temperature to cause expansion of the mesh before it is put over the electrolyte. Upon cooling, the mesh achieves intimate contact between the electrode and electrolyte by -11- eans of a shrink-fit. An overcoat of electrode material such as silver or LSM is preferably added to ensure intimate secure contact between the mesh and the electrode. The internal electrode of a tubular electrolyte may be similarly structured by forcing a wire mesh cylinder into the electrolyte tube and then coating the mesh in a similar fashion to that done on the external electrode. Ceria, especially ceria stabilized with calcia, strontia or yttria, may be readily substituted for zirconia. Lanthanum strontium cobaltite is an especially effective electrode for use with ceria.

Claims: -12- Clai s We claim: 1. A solid state electrochemical compressor for compressing an ionizable diatomic gas comprising: a solid tubular electrolyte which conducts ions of said gas through the electrolyte when said electrolyte is subjected to an applied electromotive force; a low pressure gas inlet communicating with the interior of said tubular electrolyte; electrodes pervious and/or porous to said ionizable diatomic gas, said electrodes being intimately in contact with opposed surfaces of said electrolyte; a high pressure chamber external to said tubular electrolyte; a high pressure gas outlet communicating with said high pressure chamber; and pressure restriction means in said high pressure gas outlet. 2. The compressor of claim 1 wherein said gas is oxygen. 3. The compressor according to claim 2 wherein the ion conductive electrolyte contains zirconia. 4. The compressor of claim 1 according to claim 1, wherein the low pressure gas is air. 5. The compressor according to claim 1, wherein the low pressure gas is essentially pure oxygen. 6. The compressor according to claim 1, wherein the high pressure gas is essentially pure oxygen. -13- 7. The compressor according to claim 1, where the oxygen-ion conductive solid electrolyte contains at least one additive selected from any of the following: Hf0 2 , Ce0 2 , Bi 2 0 3 , Ti0 2 , Th0 2 , Y 2 0 3 , Yb 2 0 3 , Al^. 8. The compressor according to claim 1, where the oxygen ion electrolyte is in proximity to a heater capable of maintaining said electrolyte in a temperature range from about 500 to 1300°C. 9. The compressor according to claim 1, where the electrodes contain any of the following lanthanum strontium manganite, lanthanum-strontium chromite, platinum, silver, gold, nickel, Inconel, nickel oxide, palladium, copper. 10. The compressor according to claim 1, where the lead wires contain any of the following: silver, gold, palladium, platinum, Inconel, nichrome, nickel and copper.

Inventors: JOSHI ASHOK V, NACHLAS JESSE A, STUFFLE KEVIN, JOSHI, ASHOK, V., NACHLAS, JESSE, A., STUFFLE, KEVIN
Applicants: CERAMATEC INC, CERAMATEC, INC.
Application year: 1990
Main classification: C25B1
IPC codes: B01D53, C01B13