Method and apparatus for cryogenic removal of impurities from oxygen gas

Abstract

A process and apparatus useful for producing purified oxygen gas using a cryogenic adsorption layer to remove impurities such as nitrogen-containing impurities (e.g., nitrogen (N2), nitrogen oxides, ammonia, amines), water, carbon dioxide, carbon monoxide, and hydrocarbons from oxygen gas are described.

Description

Technical Field

[0001] This specification relates to a process and apparatus useful for producing purified oxygen gas using a cryogenic adsorption layer to remove impurities such as nitrogen-containing impurities (e.g., nitrogen (N2) and nitrogen oxides), water, carbon dioxide, etc. from oxygen gas.

Background Art

[0002] Oxygen is a fundamental chemical element and in purified form is used in numerous commercial processes.

[0003] In one application, oxygen is a raw material for preparing ozone for commercial processes such as the manufacture of semiconductors and microelectronic devices. For these commercial applications, ozone must have extremely high purity, and very high purity oxygen is required for use in preparing ozone. To prepare ozone having a purity level useful for the manufacture of semiconductors and microelectronic devices, the oxygen source used to prepare ozone must be commercially available oxygen gas in a high purity level range. For these purposes, the feed oxygen gas for preparing ozone should have a purity of at least 99.9 percent oxygen, such as 99.995 percent oxygen designated as oxygen 4.5.

[0004] In the manufacturing processes of semiconductors and microelectronic devices, oxygen is also used to form oxide layers. Here too, the oxygen used to form the oxide layer must be of very high purity. Nitrogen as an impurity in the oxygen used to form the oxide layer can particularly damage the quality of the prepared oxide layer. When nitrogen is present as an impurity in the oxygen used to form the oxide film, nitrogen can form nitrides on the surface instead of the desired oxide. The surface nitride compound can change important physical or electrical properties of the deposited oxide film, for example, by changing the conductivity or resistivity of the film. The presence of nitrogen and nitrogen impurities in the oxygen gas used to form oxides on semiconductors and microelectronic devices is very carefully controlled and monitored.

[0005] Currently, most commercially available purified oxygen products are prepared by cryogenic distillation technology.

Summary of the Invention

[0006] Described are novel processes, systems, and apparatuses that can be used to produce purified oxygen gas by a cryogenic adsorption method for removing one or more impurities from oxygen gas. The impurities may be nitrogen-containing impurities such as nitrogen (N2) or nitrogen oxides, or may be other types of impurities contained in water, carbon dioxide, carbon monoxide, or oxygen gas. Using this process, impurities can be removed from oxygen gas that has already been processed and purified to have a very high level of purity, for example, a purity of at least 99.9 or more (less than 0.1 percent total impurities). Examples of the described methods can be useful for efficiently producing purified oxygen having a lower level of impurities than oxygen prepared by cryogenic distillation technology currently used commercially.

[0007] Exemplary methods and systems can be useful for removing a substantial amount of impurities initially present in gaseous oxygen (i.e., "oxygen gas"). The oxygen gas can have various impurities, perhaps in combination with one or more of hydrogen, argon, water, carbon dioxide, hydrocarbons, and carbon monoxide, including one or more nitrogen-containing impurities (e.g., N2, or nitrogen oxides, or both). The impurities are removed by adsorbing them onto the surface of an adsorption medium at cryogenic temperatures while preventing the oxygen from condensing during the cryogenic process. To prevent the oxygen from condensing, the system includes a temperature control system that monitors the temperature within the cryogenic adsorption layer and controls the temperature to a setpoint higher than the condensation temperature of the gaseous oxygen.

[0008] In one aspect, the present disclosure relates to a method of purifying oxygen gas to form purified oxygen gas. The method includes contacting the oxygen gas with an adsorption medium at a temperature below -100 degrees Celsius to adsorb nitrogen-containing impurities and optionally other impurities contained in the oxygen gas onto the surface of the adsorption medium without condensing the oxygen.

[0009] In another aspect, the present disclosure relates to a system for removing impurities from oxygen gas. The system includes an oxygen gas source and a cryogenic adsorption layer connected to the oxygen gas source, the cryogenic adsorption layer being maintained at a cryogenic temperature and containing adsorbent particles adapted to adsorb nitrogen-containing impurities contained in the oxygen gas without condensing the oxygen, and a temperature control system that controls the temperature to a setpoint higher than the condensation temperature of the gaseous oxygen.

Brief Description of the Drawings

[0010]

Figure 1

Modes for Carrying Out the Invention

[0011] A novel process and apparatus that can be used to produce purified oxygen gas by cryogenic adsorption are described. This method reduces the temperature of the oxygen gas to a cryogenic temperature at which oxygen does not condense, but impurities adsorb onto the surface of the adsorbent, thereby removing nitrogen-containing impurities such as nitrogen (N2) or nitrogen oxides (e.g., N2O, NO, NO2), as well as especially water, carbon monoxide, carbon dioxide, and hydrocarbons from the oxygen gas. This method can be used to remove gaseous impurities from oxygen gas that has already been processed and purified to have a very high level of purity, for example, a purity of at least 99.9 or higher.

[0012] The process and the apparatus used in the process are designed to remove impurities by adsorbing the impurities onto the surface of the adsorbent while simultaneously preventing oxygen from condensing. The apparatus includes a control system that prevents the cryogenic cooling system from reducing the temperature of the cryogenic adsorption process to a temperature low enough to condense oxygen. The control system can be programmed to include a "minimum temperature setpoint" (abbreviated as "setpoint"), which is a temperature that is approximately equal to or slightly higher than the condensation point of oxygen in the process at the processing pressure. When the control system detects a temperature equal to the minimum temperature setpoint, the cryogenic cooling system stops additional cooling and raises the temperature.

[0013] Oxygen gas containing various types and amounts of impurities is commercially available as a gaseous raw material. Examples of the specifications of oxygen gas as a raw material include, for example, the following. TIFF2025521969000002.tif56170

[0014] As shown in Table 1, examples of different classifications of oxygen gas with different purity levels and types of impurities are available as "Oxygen 4.5" having an oxygen purity level of at least 99.995 percent, "Oxygen 5.0" having an oxygen purity level of at least 99.999 percent, "Oxygen 5.6" having an oxygen purity level of at least 99.9996 percent, and "Oxygen 6.0" having an oxygen purity level of at least 99.9999 percent. These and other versions of purified oxygen gas may contain impurities including hydrogen (H2), nitrogen (N2), argon (Ar), water (H2O), carbon dioxide (CO2), hydrocarbons (C n H m ), or other types of molecular contaminants.

[0015] The oxygen gas for the processes described may have the levels and types of impurities presented, but may contain either different types of impurities, a higher level of any one of these impurities, or both. Examples of other impurities include, among others, nitrogen or nitrogen oxides (e.g., N2O, NO, NO2), ammonia (NH3), and amines. In this specification, nitrogen (N2), nitrogen oxides, ammonia, amines, and other nitrogen-containing compounds that are impurities in oxygen gas may be collectively referred to as "nitrogen-containing impurities". Also, the oxygen gas processed herein may contain any of these listed impurities in greater amounts, for example, up to 10, 20, or 30 ppm (e.g., less) of water.

[0016] The amount of impurities in oxygen gas may be described as a percentage or alternatively as ppm or ppb. The terms "ppm (parts per million)" and "ppb (parts per billion)" are used herein in accordance with their use in the chemical arts, including the art of manufacturing microelectronic and semiconductor devices. In this regard, one part per million ("1 ppm") is commonly used as a dimensionless measure of low-level (concentration) impurities in a gas and is expressed as milligrams per liter of fluid (mg / L), measuring the mass of the impurity per volume of the fluid. 1 ppm is equal to 1×10 -6 or 0.0001% of the total substance. One part per billion ("1 ppb") is equal to 1×10 -9 or 0.0000001% of the total substance.

[0017] As used herein, the term "oxygen gas" refers to the source or stream of oxygen gas used in the described process in which impurities are removed from the oxygen gas using a cryogenic adsorption step. In this specification, the term "oxygen gas" is used to refer to an oxygen-containing gas that is used in accordance with the described process and that enables various different forms of gas at different stages of the process. "Oxygen gas" includes the oxygen gas as a raw material (before any treatment), as well as one or more steps that are carried out prior to the cryogenic adsorption step, for example, in each case, during and after the step of converting hydrogen in the oxygen gas to water by catalysis, which is carried out prior to the step of purifying the oxygen gas using a cryogenic adsorption layer, and during the step of cooling or "pre-cooling" the oxygen gas, including the various forms that the oxygen gas assumes when it is being processed. After the oxygen gas has been treated by a cryogenic purification apparatus to reduce the amount of one or more impurities in the oxygen gas, the oxygen gas may be referred to as "purified oxygen" or "purified oxygen gas".

[0018] According to the present specification, the oxygen gas purified by the cryogenic adsorption step starts as an oxygen gas having a high level of purity but also containing a certain level of impurities that can be removed by the cryogenic adsorption step. In the cryogenic adsorption step, the oxygen gas containing impurities is flowed and brought into contact with the adsorbent at a cryogenic temperature such as less than -100 degrees Celsius. The impurities contained in the oxygen gas are adsorbed onto the surface of the adsorbent and thus removed from the oxygen gas stream.

[0019] The oxygen gas is flowed into contact with a solid adsorbent material, and the impurities present in the oxygen gas are attracted to and adsorbed onto the surface of the adsorbent to remove the impurities from the oxygen gas stream. A significant amount of the oxygen gas in the stream is not adsorbed onto the surface of the adsorbent and flows through the adsorbent surface. Also, according to the described method, when the oxygen gas flows through the adsorbent surface, the oxygen does not condense.

[0020] To prevent the oxygen in the oxygen gas stream from condensing on the adsorbent during the cryogenic adsorption step, the temperature of the oxygen, the temperature of the adsorbent, or both are maintained at a temperature higher than the temperature at which gaseous oxygen condenses at the pressure experienced during the cryogenic adsorption step.

[0021] In an exemplary system, the cryogenic adsorption layer includes a temperature control system including one or more temperature sensors and an electronic (computerized) control device that electronically controls the temperature of the cryogenic adsorption process at a temperature higher than the condensation temperature of the oxygen flowing through the cryogenic adsorption layer. For example, the temperature control system can be used to control the temperature of the oxygen gas so that it does not fall below a preset minimum level, i.e., a "setpoint" of -180 degrees Celsius, -185 degrees Celsius, or -190 degrees Celsius.

[0022] The exemplary temperature control system can include at least a computerized hardware processor and a memory operably connected to the processor. The memory can store instructions to be executed by the processor. At least one temperature sensor operably connected to the processor is also provided to measure the temperature of one or more of oxygen, adsorbent, or other components of the cryogenic adsorption process. Examples of temperature sensors can include thermistors, negative temperature coefficient sensors, infrared temperature sensors, thermocouples, or any other known type of temperature sensor that can be utilized to measure temperatures in the cryogenic temperature range. Optionally, in various exemplary systems herein, the temperature control system can include, as a computer processor, any form of microprocessor, such as a process logic controller (PLC controller) incorporated into an application specific integrated circuit (ASIC), etc.

[0023] The processor is adapted to execute instructions stored by the memory such that it can perform various temperature control functions or methods, including establishing and monitoring the temperature of the system against a minimum temperature setpoint, which is the minimum allowable temperature of the adsorption layer and the oxygen passing through the adsorption layer. In various embodiments, the temperature control system can use the temperature sensor to dynamically (continuously or at various intervals) measure the temperature of the adsorption layer and, optionally, output a signal to one or more components of the cooling system such that further cooling of the cryogenic adsorption layer or its components is stopped, configured to prevent the cryogenic adsorption layer from reaching a temperature below a minimum temperature threshold or "setpoint".

[0024] The cryogenic adsorption step is a step of bringing an oxygen gas stream into contact with a solid adsorption medium at cryogenic temperatures to adsorb impurities in the oxygen gas onto the surface of the adsorption medium. According to this specification, the oxygen gas does not adsorb onto the surface of the adsorption medium and does not condense during the cryogenic adsorption step. The cryogenic adsorption step is carried out using a cryogenic adsorption apparatus including a process chamber (also known as an "adsorption chamber") containing an internal volume with an adsorbent and a cryogenic cooling system. The cooling system controls the temperature inside the chamber, along with the temperature of the oxygen gas flowing through the interior of the chamber and the adsorbent contained within the chamber.

[0025] The apparatus is adapted to carry out the cryogenic adsorption step to remove impurities from the oxygen gas without allowing condensation of oxygen within the adsorption chamber and without condensing the oxygen. The cryogenic adsorption apparatus includes a control system by which the cryogenic cooling system prevents reaching a temperature at which oxygen condenses. The control system is programmed to include a "setpoint" temperature that is approximately equal to or slightly higher than the temperature at which oxygen condenses inside the chamber. During the cryogenic adsorption step, when the control system senses the temperature inside the cryogenic adsorption apparatus (e.g., oxygen gas or adsorbent) equal to the temperature setpoint, the cryogenic cooling system provides no additional cooling to the cryogenic adsorption apparatus (e.g., oxygen gas or adsorbent) and gradually raises the temperature.

[0026] During the purification process, oxygen gas containing impurities can flow through the adsorption chamber and come into contact with the adsorption medium. When the oxygen gas contacts the adsorption medium, a certain amount of impurities in the oxygen gas are adsorbed onto the adsorption medium, and the impurities are removed from the oxygen gas. The impurities can be adsorbed onto the surface of the adsorbent at positions outside the adsorbent or onto the surface within the pores of the adsorbent by physical adsorption or "physisorption" mechanism.

[0027] A variety of useful adsorbent media materials are known. Examples of useful adsorbents that are known to be useful for adsorbing impurities from a gas stream include activated carbon (including carbon molecular sieves), zeolite materials, metal-organic framework adsorbents ("MOF"), zeolitic imidazolate framework adsorbents ("ZIF"), and zeolite-templated carbon adsorbents (ZTC).

[0028] Useful types of adsorbent media include those having pore diameters on the angstrom scale, which can be referred to as "molecular sieves." Exemplary adsorbents of useful types as "molecular sieves" can be characterized based on pore diameter (diameter). Molecular sieve adsorbent media can have pore diameters in the angstrom range, for example, less than 20 angstroms. Molecular sieves having pore diameters of 10 angstroms, 8 angstroms, or 5, 4, or 3 angstroms are commercially available.

[0029] Adsorbent media (or simply "adsorbents") can also be described in terms of surface area. Useful adsorbents can have any useful surface area, and in some cases, higher surface area adsorbents may be preferred. Very generally, useful adsorbents can have surface areas in the range of 100 square centimeters / gram to several thousand square centimeters / gram. Zeolite adsorbents can have surface areas at the lower end of this range, for example, 100 - 1500 square centimeters / gram. Other types of adsorbents can have significantly higher surface areas. For example, some MOF and ZIF can have surface areas up to 5,000, 6,000, or 7,000 square centimeters / gram.

[0030] The adsorbent can be in the form of small microparticles, granules, pellets, shells, cubes, etc., of various sizes and shapes that can form a powder.

[0031] Exemplary adsorbents that have been identified as useful in the cryogenic adsorption step described for adsorbing impurities from oxygen gas at cryogenic temperatures include activated carbon having pore diameters associated with molecular sieves and molecular sieve adsorbents comprising zeolite materials.

[0032] One type of useful adsorbent is a type of crystalline aluminosilicate known as "zeolite". Zeolites include synthetic hydrated aluminosilicates and natural zeolites having the function of screening molecules. The zeolite structure has many orderly arranged pores with relatively uniform pore diameters.

[0033] Zeolite adsorbents are made of a three-dimensional interconnected network of alumina and silica tetrahedra that form a porous solid. This structure makes it possible to remove natural crystal water and leave a porous crystal structure. Zeolites are crystalline microporous materials mainly composed of SiO4 and AlO4 vertex-sharing tetrahedral building units. These are grown to form a three-dimensional (3D) crystal structure with well-defined channels and cavities of molecular dimensions. The pores or "cages" have a high affinity for re-adsorbing water or other molecules of a specific size, which is assisted by strong ionic forces due to the presence of cations such as sodium, calcium, and potassium, as well as a high internal surface area. The presence of highly positive cations such as sodium, potassium, and calcium in the structure can increase the polarity of these sieves.

[0034] Examples of synthetic zeolites include "type A" zeolites and "type X" zeolites having various pore diameters and various chemical diversities. In conventional forms, zeolite molecular sieve products can be in the form of beads, pellets, or powders, or porous particles, and can be characterized based on their pore diameters (diameters). Zeolites characterized by having pore diameters in the angstrom range, for example less than 20 angstroms, for example 10 angstroms, 8 angstroms, or 5, 4, or 3 angstroms, are commercially available.

[0035] Examples of commercially available molecular sieve products include the following types based on pore size and chemistry. "4A" type sieves (Na2O·Al2O3·2SiO2·9 / 2H2O) have a continuous three-dimensional network of channels approximately 4 Å in diameter, in addition to larger "cages" approximately 7 Å in diameter. "3A" molecular sieves (2 / 3K2O·1 / 3Na 22 O·AI2O3·2SiO2·9 / 2H2O) are made by substituting the sodium ions inherent in the 4A basic structure with potassium cations and have a nominal pore size of 3 angstroms (3 Å).

[0036] "5A" type sieves (3 / 4CaO·1 / 4Na2O·Al2O3·2SiO2·9 / 2H2O) are described as a three-dimensional network of intersecting channels using calcium substituted in place of potassium cations. Entry into the channels is restricted by the eight oxygen atoms (with a diameter of approximately 3 - 5 Å) by which they are formed. When these channels intersect, larger pores or cages with a diameter of 11.4 Å are formed.

[0037] "13X" type sieves have an effective pore size of 10 angstroms (10 Å) and are formed by Na2O·Al2O3·(2.8 ± 0.2)SiO2·(6 - 7)H2O.

[0038] Also useful as an adsorbent in the cryogenic adsorption step is a "carbon adsorbent", which refers to a series of carbon-based materials synthetically derived from carbon-containing polymer materials or carbon-based materials of natural origin. Examples include carbon formed by the pyrolysis of synthetic hydrocarbon resins such as polyacrylonitrile, sulfonated polystyrene-divinylbenzene, and polyvinyl chloride; cellulose carbide; charcoal; and activated carbon formed from natural source materials such as coconut shells, pitch, wood, petroleum, and coal; nanoporous carbon, etc.

[0039] Carbon adsorbents (as well as other types of adsorbents) can have any suitable form, such as in the form of granules (also called "particles"). Granules are individual pieces of the carbon adsorbent, and each piece has a relatively small size, such as less than 2 centimeters, or less than 1 or 0.5 centimeters. The particles may have any useful particle size, shape, and particle size range. Exemplary shapes include beads, granules, pellets, platelets, shells, saddles, powders, irregularly shaped particulate matter, extrudates of any shape and size, materials in the form of cloth or web, as well as composites (of the adsorbent and other components), and pulverized or crushed forms of the aforementioned types of adsorbent materials.

[0040] The adsorbent may also be of a type called carbon molecular sieve ("CMS"). Carbon molecular sieves are commercially available and are generally prepared by the pyrolysis or carbonization of polymer precursors under a controlled vacuum or inert atmosphere.

[0041] Zeolite materials, including zeolite-type imidazolate framework adsorbents, may also be useful as adsorbents in the cryogenic adsorption processes described. Zeolite-type imidazolate framework adsorbents are metal-organic frameworks containing tetrahedrally coordinated transition metals such as iron (Fe), cobalt (Co), copper (Cu), or zinc (Zn) connected by imidazolate linkers, which may be the same or different for a particular ZIF composition or for a single transition metal atom of the ZIF structure. The ZIF structure contains 4-coordinate transition metals linked via imidazolate units to form an extended structure based on a tetrahedral topology. ZIFs are said to form a structural topology equivalent to that found in zeolites and other inorganic microporous oxide materials.

[0042] Zeolitic imidazolate frameworks can be characterized by features including, among numerous other physical and chemical properties, specific transition metals of the framework (e.g., iron, cobalt, copper, or zinc), the chemical nature of the linker (e.g., chemical substituents of the imidazolate unit), the pore diameter of the ZIF, the surface area of the ZIF, and the pore volume of the ZIF. Dozens (at least 105) of unique ZIF species or structures are known, each having a different chemical structure based on the type of transition metal and the type of linker (or linkers) that make up the structure. Each topology is identified using a unique ZIF name, such as ZIF-1 to ZIF-105. For an account of ZIFs including the specific chemical compositions and associated properties of a number of known ZIF species, see Phan et al., “Synthesis, Structure, and Carbon Dioxide Capture Properties of Zeolitic Imidazolate Frameworks”, Accounts of Chemical Research, 2010, 43(1), pp58 - 67 (received April 6, 2009).

[0043] The pore diameter of the ZIF can affect the performance of the ZIF as an adsorbent. Exemplary ZIFs can have a pore diameter in the range of about 0.2 to 13 angstroms, such as 2 to 12 angstroms or 3 to 10 angstroms.

[0044] As another example, a useful adsorbent can be of a type called zeolite-templated carbon adsorbent (“ZTC”). Zeolite-templated carbon (ZTC) contains regular microporous carbon synthesized by using zeolite as a sacrificial template. Zeolite-templated carbon is thought to be made of a curved monolayer graphene structure exhibiting significantly uniform micropore diameters and a relatively high surface area.

[0045] Optionally, the described method can include a step of converting hydrogen present in oxygen gas into water. The step of converting hydrogen into water can be carried out catalytically, for example, by contacting the oxygen gas with a catalyst based on a transition metal such as palladium, nickel, or platinum, including a metal-containing catalyst, for example, a transition metal oxide catalyst. One example is a palladium-containing catalyst such as palladium oxide (PdO).

[0046] Optionally, the described method can include a step of reducing the temperature of the oxygen gas to cryogenic temperatures before treating the oxygen gas by a cryogenic adsorption step. For example, the oxygen gas stream can be cooled (i.e., "pre-cooled") in a "pre-cooling" temperature reduction step before treating the oxygen gas in a cryogenic adsorption step to remove impurities from the oxygen gas. The pre-cooling step can use a heat exchanger to reduce the temperature of the oxygen gas to cryogenic temperatures such as temperatures below -100 °C, or below -120 °C, below -130 °C, or below -140 °C.

[0047] The cryogenic adsorption process can be carried out using an apparatus designed for the described cryogenic adsorption process. A useful cryogenic adsorption apparatus can include a process chamber (or "adsorption chamber") containing an internal volume with an adsorbent, a cryogenic cooling system for reducing the temperature of the interior, the oxygen gas within the interior, and the adsorbent within the interior, a control system for controlling the temperature of the process within the interior and maintaining a minimum temperature higher than the temperature at which oxygen condenses, and ancillary flow control structures such as pipes, valves, sensors, flow control meters, pressure meters, etc. for guiding the oxygen gas stream into, through, and out of the adsorption chamber.

[0048] Useful or preferred cryogenic adsorption devices can be of the type designed to process very high purity oxygen gas without the device becoming a source of contamination. The oxygen gas can flow through the device for processing, and minimal impurities or contaminants are introduced into the oxygen gas from the device. Such devices may also be referred to as ultra-high purity flow control devices or components.

[0049] To reduce the possibility of introducing new impurities into the oxygen gas stream from the process device during processing, the surface of the process device can be treated to remove impurities and reduce surface morphologies that allow material to migrate from the surface into the oxygen gas in contact with the surface. The process device includes process chambers; heat transfer surfaces; surfaces supporting solid catalyst particles or solid adsorbent particles; flow control devices such as valves, baffles, conduits (e.g., pipes); or any other surfaces in contact with the oxygen gas stream during processing, such as processing devices and flow control devices.

[0050] In useful or preferred systems, the inner surfaces of one or more process chambers, valves, flow control conduits, metering devices, sensors, etc. can be treated to a very smooth finish to reduce the surface area at the microscopic level and thereby reduce the surface reactivity. A useful finishing process is one that treats and cleans the surface and makes it less reactive, i.e., reduces the reactivity, to ensure and maintain the high purity of the oxygen gas when the gas is processed by the system. The process of reducing the surface reactivity can be referred to as a process of "passivating" the surface, and the treated surface can be referred to as a "passivated" surface. Examples of passivation techniques include spraying, polishing, grinding, sanding, electropolishing, electroplating, electroless plating, coating, zinc plating, anodizing, etc.

[0051] The components of the system of the present specification that can be prepared for a process to avoid introduction of impurities into oxygen gas include any surface that contacts oxygen gas during the process. This includes any surface of a cryogenic adsorption device, any surface of a heat exchanger when used in the system, any surface of a device for catalytically converting hydrogen to water when used in the system, and any conduit (pipe), valve, side wall of the process chamber of a catalytic converter, or of a heat exchanger, or of a cryogenic medium layer, or other device used to monitor or control the oxygen gas flow during the described process.

[0052] Specific examples of useful surfaces include surfaces made of electropolished stainless steel and surfaces made of chemically passivated stainless steel. Electropolished stainless steel is a known type of stainless steel material having a surface treated by electropolishing. The electropolishing process, also called the "reverse plating" process, uses an electrochemical solution to remove a very small amount of the outer surface of a stainless steel part. By stripping a small, uniform layer from the stainless steel surface, the electropolishing process also removes embedded inclusions, heat tint and oxide scale, burrs, microcracks, pits, and features that can cause the transfer of materials that can become inclusions in the oxygen gas from the stainless steel surface to the oxygen gas that contacts that stainless steel surface. A useful electropolished surface can meet the ASTM B912 requirements for electropolished stainless steel.

[0053] Chemically passivated stainless steel is also a known type of stainless steel material having a surface treated by a chemical passivation step. The chemical passivation step is a process of contacting the stainless steel surface with a liquid acid such as citric acid or nitric acid. The acid can remove free iron and other contaminants from the surface and leave a passivating metal oxide film on the surface. The passivation and the formed metal oxide film can be effective in suppressing the movement of inclusions from the stainless steel surface to the oxygen gas contacting the stainless steel surface. A useful chemically passivated stainless steel surface can meet the ASTM B912 requirements for chemically passivated stainless steel.

[0054] Also, in an exemplary system, the surface of the device that contacts oxygen gas during processing may not be lubricated. The system may include lubrication, but the exemplary system does not contain any lubricant and does not contain a fluorinated lubricant. Certain types of flow control systems may be designated as "cleaned for oxygen supply," meaning that the system may contain a fluorinated lubricant. Systems containing fluorinated lubricants may be less preferred and may be excluded from the described process.

[0055] Referring to FIG. 1, system 100 is an example of a system of devices and flow control devices that may be useful for implementing the described method. In FIG. 1, system 100 includes an oxygen gas source 102, a catalytic process chamber 104, a heat exchanger 106, and a cryogenic adsorption layer 108. The step of catalytically removing hydrogen using process chamber 104 is optional and may be excluded. Similarly, oxygen source 102 and heat exchanger 106 may be provided in any useful form including, but not limited to, the arrangement shown in FIG. 1.

[0056] Referring again to FIG. 1, oxygen gas 110 is contained in oxygen gas source 102. Oxygen gas 110 can be at any temperature and pressure, for example, a temperature in the range of 0 to 40 degrees Celsius and a pressure of approximately atmospheric pressure.

[0057] Oxygen gas 110 flows into the catalytic process chamber 104 through a conduit with a valve, and the catalytic process chamber 104 contains a catalyst 112 effective to catalytically convert hydrogen in the oxygen gas 110 to water. The oxygen gas 110 flowing through the device 104 can be at any temperature and pressure, for example, a temperature in the range of 0 to 40 degrees Celsius, and a pressure in the range of 17 to 140 pounds per square inch (gauge).

[0058] After the catalytic process chamber 104, the oxygen gas 110 flows into a heat exchanger (“precooler”) 106 through a conduit with a valve, and reduces the temperature of the oxygen gas 110 to an extremely low temperature, for example, less than -100 degrees Celsius. The heat exchanger 106 includes a processing chamber and a cooling conduit passing through the processing chamber. The cooling conduit contains an extremely low temperature nitrogen stream. By contacting the conduit containing the extremely low temperature nitrogen stream, the temperature of the oxygen gas 110 is reduced. The oxygen gas 110 flowing through the heat exchanger 106 can be at any useful pressure, for example, a pressure in the range of 17 to 140 pounds per square inch (gauge).

[0059] From the heat exchanger 106, the oxygen gas 110 flows into an extremely low temperature adsorption layer 108 containing an adsorbent 114 described herein, for example, a molecular sieve adsorbent. The temperature inside the extremely low temperature adsorption layer 108 is measured by a temperature sensor (e.g., a thermocouple) 122 and controlled to an extremely low temperature by a control system 120. The temperature in the extremely low temperature adsorption layer 108 is maintained at an extremely low temperature effective to adsorb impurities such as nitrogen (among others) present in the oxygen gas 110 onto the surface of the adsorbent 114. The temperature is controlled so as not to drop to the temperature at which oxygen condenses within the extremely low temperature adsorption layer 108. In an exemplary method, the temperature in the extremely low temperature adsorption layer 108 is maintained at a temperature higher than -190 degrees Celsius, for example, in the range of -190 degrees Celsius to -140 degrees Celsius, for example, -185 degrees Celsius or -180 degrees Celsius to -150 degrees Celsius. The purified oxygen 118 is delivered from the extremely low temperature adsorption layer 108.

[0060] According to this system and other systems of this specification, purified oxygen (e.g., 118) may contain very small amounts of impurities such as hydrogen (H2), nitrogen (N2), nitrogen oxides, water (H2O), carbon dioxide (CO2), carbon monoxide (CO), and hydrocarbons (C n H m ). The amounts of these impurities can vary based on the content of each type of impurity initially contained in the oxygen gas before treatment.

[0061] By way of an exemplary method, in an exemplary system, the amount of one or a combination of these impurities (hydrogen (H2), nitrogen (N2), nitrogen oxides, water (H2O), carbon dioxide (CO2), carbon monoxide (CO), and hydrocarbons (C n H m )) can be less than 1 ppb, for example less than 0.5 ppb or less than 0.1 ppb.

[0062] In a first aspect, a method for purifying oxygen gas to form purified oxygen gas, the method comprising contacting the oxygen gas with an adsorption medium at a temperature below -100 degrees Celsius to adsorb nitrogen-containing impurities contained in the oxygen gas onto the surface of the adsorption medium without condensing the oxygen.

[0063] A second aspect according to the first aspect further includes controlling the temperature of the oxygen gas to prevent condensation of the oxygen gas.

[0064] A third aspect according to the first aspect further includes controlling to maintain the temperature of the oxygen gas within the range of -140 degrees Celsius to -190 degrees Celsius.

[0065] In a fourth aspect according to any of the above aspects, the method is implemented in a cryogenic adsorption layer containing an adsorption medium, and the cryogenic adsorption layer includes a passivated surface.

[0066] A fifth aspect according to the fourth aspect, the passivated surface includes electropolished stainless steel or chemically passivated stainless steel.

[0067] A sixth aspect according to any of the above aspects further includes catalytically converting hydrogen contained in oxygen gas into water.

[0068] In a seventh aspect according to the sixth aspect, catalytically converting hydrogen includes contacting oxygen gas with solid catalyst particles contained in a process chamber, and the process chamber includes a passivated surface.

[0069] In an eighth aspect according to the seventh aspect, the passivated surface is an electropolished stainless steel surface or a chemically passivated stainless steel surface.

[0070] In a ninth aspect according to any of the above aspects, contacting oxygen gas with an adsorption medium includes passing oxygen gas through a cryogenic adsorption layer including an adsorption chamber, a solid adsorption medium contained in the adsorption chamber, and a passivated surface.

[0071] In a tenth aspect according to the ninth aspect, the passivated surface is an electropolished stainless steel surface or a chemically passivated stainless steel surface.

[0072] In an eleventh aspect according to any of the above aspects, the adsorption medium includes zeolite particles or carbonaceous particles.

[0073] In a twelfth aspect according to any of the above aspects, the adsorption medium includes molecular sieve particles.

[0074] In a thirteenth aspect according to any of the above aspects, the adsorption medium has pores with a pore diameter of less than 20 angstroms.

[0075] A fourteenth aspect according to any of the above aspects further includes pre-cooling oxygen gas to a temperature below -100 °C using a heat exchanger, and contacting the pre-cooled oxygen gas with an adsorbent medium at a temperature in the range of -140 °C to -190 °C in order to adsorb impurities contained in the oxygen gas onto the surface of the adsorbent medium, wherein the oxygen gas contains one or more impurities selected from nitrogen-containing impurities (e.g., nitrogen (N2), nitrogen oxides (e.g., N2O, NO, NO2), ammonia (NH3), and amines), water (H2O), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), non-methane hydrocarbons (NMHC), argon (Ar), and hydrogen (H2), and the adsorbent medium contains molecular sieve particles.

[0076] In a fifteenth aspect according to any of the above aspects, the nitrogen-containing impurities are nitrogen (N2) or nitrogen oxides (e.g., N2O, NO, NO2).

[0077] In a sixteenth aspect according to the fifteenth aspect, pre-cooling the oxygen gas includes passing gaseous oxygen through a heat exchanger including a process chamber and a cooling surface within the process chamber, wherein the cooling surface, the surface of the process chamber, or both include a passivated surface.

[0078] In a seventeenth aspect according to the sixteenth aspect, the passivated surface is an electropolished stainless steel surface or a chemically passivated stainless steel surface.

[0079] In an eighteenth aspect according to any of the above aspects, the oxygen gas contains a maximum of 0.1 percent impurities.

[0080] In a nineteenth aspect according to any of the above aspects, prior to contacting the oxygen gas with the adsorbent medium, the oxygen gas contains at least 99.9 volume percent oxygen.

[0081] In a twentieth aspect according to any of the above aspects, prior to contacting the oxygen gas with the adsorbent medium, the oxygen gas contains less than 100 ppm total N2 and Ar.

[0082] In a 21st aspect according to any of the above aspects, before the oxygen gas is brought into contact with the adsorption medium, the oxygen gas contains H2O of less than 30 ppm.

[0083] In a 22nd aspect according to any of the above aspects, after the oxygen gas is brought into contact with the adsorption medium, the purified oxygen gas contains N2 of less than 1 ppb.

[0084] In a 23rd aspect according to any of the above aspects, after the oxygen gas is brought into contact with the adsorption medium, the purified oxygen gas contains N2 of less than 0.5 ppb.

[0085] In a 24th aspect, a system for removing impurities from oxygen gas, comprising an oxygen gas source and a cryogenic adsorption layer connected to the oxygen gas source, the cryogenic adsorption layer being maintained at a cryogenic temperature and containing adsorbent particles adapted to adsorb nitrogen contained in the oxygen gas without condensing oxygen, and a temperature control system for monitoring the temperature inside the cryogenic adsorption layer and controlling the temperature to a set value higher than the condensation temperature of gaseous oxygen, is disclosed.

[0086] In a 25th aspect according to the 24th aspect, the temperature control system is adapted to maintain the temperature above -190 degrees Celsius.

[0087] In a 26th aspect according to the 24th or 25th aspect, the cryogenic adsorption layer includes a passivated surface.

[0088] In a 27th aspect according to the 26th aspect, the passivated surface is an electropolished stainless steel surface or a chemically passivated stainless steel surface.

[0089] A 28th aspect according to any of the 24th to 27th aspects further includes a catalytic process chamber containing catalytic particles capable of converting gaseous hydrogen present in the oxygen gas into water, and the catalytic process chamber includes a passivated surface.

[0090] In a 29th aspect according to the 28th aspect, the passivated surface is an electropolished stainless steel surface or a chemically passivated stainless steel surface.

[0091] A 30th aspect according to any one of the 24th to 29th aspects further includes a heat exchanger connected to the cryo-adsorption layer, the heat exchanger being capable of reducing the temperature of the oxygen gas to less than -100 degrees Celsius, and the heat exchanger including a passivated surface.

[0092] In a 31st aspect according to the 30th aspect, the passivated surface is an electropolished stainless steel surface or a chemically passivated stainless steel surface.

[0093] In a 32nd aspect according to any one of the 24th to 31st aspects, the adsorption medium includes zeolite particles or carbonaceous particles.

[0094] In a 33rd aspect according to any one of the 24th to 32nd aspects, the adsorption medium includes molecular sieve particles.

[0095] In a 33rd aspect according to any one of the 24th to 33rd aspects, the adsorption medium has pores having a pore diameter of less than 20 angstroms.

Claims

1. A method for purifying oxygen gas to form purified oxygen gas, comprising contacting the oxygen gas with an adsorption medium at a temperature below -100°C in order to adsorb nitrogen-containing impurities contained in the oxygen gas onto the surface of the adsorption medium without condensing the oxygen.

2. The method according to claim 1, further comprising controlling the temperature of the oxygen gas to prevent condensation of the oxygen gas.

3. The method according to claim 1, further comprising controlling to maintain the temperature of the oxygen gas within the range of -140°C to -190°C.

4. The method according to any one of claims 1 to 3, which is carried out in a cryogenic adsorption layer containing an adsorption medium, and the cryogenic adsorption layer includes a passivated surface.

5. The method according to claim 4, wherein the passivated surface includes electropolished stainless steel or chemically passivated stainless steel.

6. The method according to any one of claims 1 to 5, further comprising catalytically converting hydrogen contained in the oxygen gas to water.

7. The method according to claim 6, wherein catalytically converting hydrogen includes contacting the oxygen gas with solid catalyst particles contained in a process chamber, and the process chamber includes a passivated surface.

8. The method according to claim 7, wherein the passivated surface is an electropolished stainless steel surface or a chemically passivated stainless steel surface.

9. The method according to any one of claims 1 to 8, wherein contacting the oxygen gas with the adsorption medium includes passing the oxygen gas through a cryogenic adsorption layer including an adsorption chamber, a solid adsorption medium contained in the adsorption chamber, and a passivated surface.

10. The method according to claim 9, wherein the passivated surface is an electropolished stainless steel surface or a chemically passivated stainless steel surface.

11. The method according to any one of claims 1 to 10, wherein the adsorption medium includes zeolite particles or carbonaceous particles.

12. The method according to any one of claims 1 to 11, wherein the adsorption medium includes molecular sieve particles.

13. The method according to any one of claims 1 to 12, wherein the adsorption medium has pores with a pore diameter of less than 20 angstroms.

14. Pre-cooling the oxygen gas to a temperature below -100°C using a heat exchanger, To adsorb impurities contained in oxygen gas onto the surface of an adsorption medium, pre-cooled oxygen gas is brought into contact with the adsorption medium at a temperature in the range of -140 °C to -190 °C further comprising Oxygen gas contains one or more impurities selected from nitrogen-containing impurities (e.g., nitrogen (N ２ ), nitrogen oxides (e.g., N ２ O, NO, NO ２ ), ammonia (NH ３ ), and amines), water (H ２ O), carbon monoxide (CO), carbon dioxide (CO ２ ), methane (CH ４ ), non-methane hydrocarbons (NMHC), argon (Ar), and hydrogen (H ２ ). the adsorption medium containing molecular sieve particles The method according to any one of claims 1 to 13

15. The nitrogen-containing impurity is nitrogen (N ２ ), or a nitrogen oxide, and the method according to any one of claims 1 to 14.

16. Pre-cooling the oxygen gas comprises passing gaseous oxygen through a heat exchanger comprising a process chamber and a cooling surface within the process chamber the cooling surface, the surface of the process chamber, or both comprising a passivated surface The method according to claim 15

17. The method according to claim 16, wherein the passivated surface is an electropolished stainless steel surface or a chemically passivated stainless steel surface

18. The method according to any one of claims 1 to 17, wherein the oxygen gas contains up to 0.1 percent impurities

19. The method according to any one of claims 1 to 18, wherein the oxygen gas contains at least 99.9 volume percent oxygen before being brought into contact with the adsorption medium

20. Before bringing the oxygen gas into contact with the adsorption medium, the oxygen gas contains a total of N of less than 100 ppm ２ and Ar, and the method according to any one of claims 1 to 19.

21. Before bringing the oxygen gas into contact with the adsorption medium, the oxygen gas contains H ２ ２ O of less than 30 ppm, and the method according to any one of claims 1 to 20.

22. After contacting the oxygen gas with the adsorption medium, the purified oxygen gas contains N of less than 1 ppb ２ The method according to any one of claims 1 to 21

23. After bringing the oxygen gas into contact with the adsorption medium, the purified oxygen gas contains N of less than 0.5 ppb ２ The method according to any one of claims 1 to 22

24. A system for removing impurities from oxygen gas, comprising an oxygen gas source a cryogenic adsorption layer connected to the oxygen gas source, the cryogenic adsorption layer containing adsorbent particles maintained at a cryogenic temperature and adapted to adsorb nitrogen contained in the oxygen gas without condensing the oxygen a temperature control system that monitors the temperature within the cryogenic adsorption layer and controls the temperature to a setpoint higher than the condensation temperature of gaseous oxygen A system comprising

25. The system according to claim 24, wherein the temperature control system is adapted to maintain the temperature above -190 °C

26. The system according to claim 24 or 25, wherein the cryogenic adsorption layer comprises a passivated surface

27. The system according to claim 26, wherein the passivated surface is an electropolished stainless steel surface or a chemically passivated stainless steel surface

28. The system according to any one of claims 24 to 27, comprising a catalytic process chamber containing catalytic particles capable of converting gaseous hydrogen present in the oxygen gas to water, the catalytic process chamber comprising a passivated surface

29. The system according to claim 28, wherein the passivated surface is an electropolished stainless steel surface or a chemically passivated stainless steel surface. **Claim 30** The system according to any one of claims 24 to 29, further comprising a heat exchanger connected to the cryogenic adsorption layer, the heat exchanger being capable of reducing the temperature of the oxygen gas to less than -100 degrees Celsius, the heat exchanger including a passivated surface. **Claim 31** The system according to claim 30, wherein the passivated surface is an electropolished stainless steel surface or a chemically passivated stainless steel surface. **Claim 32** The system according to any one of claims 24 to 31, wherein the adsorption medium includes zeolite particles or carbonaceous particles. **Claim 33** The system according to any one of claims 24 to 32, wherein the adsorption medium includes molecular sieve particles. **Claim 34** The system according to any one of claims 24 to 33, wherein the adsorption medium has pores with a pore diameter of less than 20 angstroms.

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