WO1995005885A1

WO1995005885A1 - Crude argon purification (system one)

Info

Publication number: WO1995005885A1
Application number: PCT/US1994/009606
Authority: WO
Inventors: Ashok V. Joshi
Original assignee: Ceramatec, Inc.
Priority date: 1993-08-26
Filing date: 1994-08-26
Publication date: 1995-03-02

Abstract

Method and apparatus for effectively removing trace amounts of oxygen and/or nitrogen from impure argon are disclosed. Crude argon (10), such as that obtained from a cryogenic distillation unit for air is directed into a system wherein oxygen is first removed (11), e.g. electrolytically, and nitrogen is removed by contrast with a nitride forming metal (12). The metal nitride formed in the second stage of the purification process may be regenerated electrolytically by first forming an oxide of the metal to displace the nitrogen, then electrolytically removing the oxide to recover the base metal.

Description

CRUDE ARGON PURIFICATION (SYSTEM ONE) BACKGROUND OF THE INVENTION Field of the Invention: This invention relates to a method and apparatus for removing trace amounts of oxygen and nitrogen from crude argon and to particular apparatus useful in said removal wherein the crude argon is generally obtained by cryogenic distillation of air.

State of the Art: Argon is a useful inert gas which has many application such as in light bulbs, in the welding of metals, as inert atmosphere for steel production as well as in various electronic industries, and the like. A major source of argon is atmospheric air, about 1 % of which is argon.

Commercially, argon is produced as a valuable by-product in cryogenic air separation plants which produce oxygen and nitrogen. Commercial grade argon produced cryogenically usually contains trace amounts of nitrogen (100 ppm to 1 %) and appreciable quantities of oxygen (100 ppm to 7%). This crude argon stream must be purified to reduce nitrogen and oxygen before it is suitable for use as an inert gas for certain purposes. Because of the proximity of the boiling point of argon (87.28°K) and oxygen (90.19°K), distillative separation of argon and oxygen (especially to a very low oxygen content), in particular, is very difficult and energy intensive. Heretofore, oxygen has been removed from crude argon streams by catalytic reduction to water with excess hydrogen over platinum catalyst beds, referred to herein as the deoxo process, followed by drying to remove the water and then by dual pressure distillation to remove nitrogen and excess hydrogen. See, for example, R.E. Latimer, "Distillation of Air," Chemical Engineering Process. pp. 35-59, February 1967, which illustrates a typical scheme.

Although argon streams purified by this method usually contain only ppm levels of nitrogen, oxygen and hydrogen, the process does have significant drawbacks. First, the hydrogen used in conventional cryo/deoxo processes is expensive. Secondly, hydrogen, because of its explosive nature, must be very carefully handled and processed. Further, hydrogen is not always conveniently available in many parts of the world. Another shortcoming of the cryo/deoxo process for purifying argon is that the water produced from the deoxo reaction must be removed completely before the argon is fed to the final cryogenic distillation column. This requires feeding the argon stream through a dryer preliminary to the cryogenic distillation. Capital and operating costs associated with this additional step add significantly to overall cost. Further, the excess hydrogen introduced to remove the oxygen in the first place must itself be removed and recovered before a pure argon stream can be produced. This adds further to the complexity and cost of the overall design and operation of the process. Other techniques for purifying argon gas streams have also been suggested.

For example, U.S. Patent Nos. 4,144,038 and 4,477,265 suggest separating argon from oxygen using aluminosilicate zeolites and molecular sieves. Such processes trade argon recovery for purity.

U.S. Patent No. 4,230,463 suggests using polymeric membranes such as polysulfones, polysiloxanes, polyaryleneoxides, polystyrenes, polycarbonate, cellulose acetate and the like for separating pairs of gases such as hydrogen and argon and polymeric membranes such as polysulfones have been suggested for the removal of oxygen from argon. Studies of hybrid processes involving cryogenic distillation and membrane separation have been reported as, see. for example, Jennings, et al., "Conceptual Processes for Recovery of Argon with Membranes in an Air Separation Process," American Institute of Chemical Engineers. 1987 Summer National Meeting, and Agrawal, et al., "Membrane/-Cryogenic Hybrid Scheme for Argon Production from Air. " American Institute of Chemical Engineers. 1988 Summer Meeting in Denver, Colo. Selectivity and recovery in such hybrid schemes has been rather poor. Much of the argon permeates with oxygen through membranes and must be recycled to crude argon distillation columns.

A more recent proposal for purifying crude argon is disclosed in U.S. Patent No. 5,035,726, assigned to Air Products. The patent suggests deoxygenating crude argon by use of an oxygen ion transporting membrane, i.e.. a solid electrolyte oxygen concentrator (SEOC) unit, and then removing nitrogen by returning the oxygen-depleted argon stream to an argon/nitrogen cryogenic distillation unit. Such an argon purification system must necessarily be located close to an argon/nitrogen/oxygen production facility - which is usually a large plant.

Also, the Air Products system involves heating cold crude argon to the operating temperature of the SEOC unit, which is typically about 700°C and above, then cooling the oxygen-depleted argon and refrigerating it to a cryogenic distillation temperature (approximately 87.28 °K or approximately -185°C). Heating and cooling of the argon gas to such temperature extremes tends to be energy inefficient. Further, the Air Products system contemplates Bi₂O₃ as an electrolyte, which is generally unstable under conditions of very low oxygen concentration, tending then to reduce to bismuth.

Therefore, there is a need in the industry for an improved process for purifying crude argon produced by cryogenic air separation and, preferably, a system which need not be necessarily located adjacent a cryogenic distillation facility.

SUMMARY OF THE INVENTION The instant invention involves a method of removing trace amounts of oxygen and nitrogen from a crude argon stream and to apparatus which are especially adapted and suitable for such purpose. The crude argon stream generally emanates from a cryogenic distillation unit which produces oxygen, nitrogen and argon from air. Crude argon typically contains less than about 5 % oxygen and frequently less than about 1 % nitrogen. However, for many purposes, for example, as an inert gas in the electronics industry, it is desired that the oxygen and nitrogen content of argon used as inert gas approach zero. The term "crude argon" is used herein to describe argon which contains minor quantities of oxygen and/or nitrogen. Argon produced as a by-product or principal product from the cryogenic distillation air is crude argon as described herein.

The method of the invention comprises contacting the crude argon with a solid, oxygen-ion conducting electrolyte under oxygen-ion conducting conditions to remove oxygen from the crude argon stream. The oxygen-depleted argon is then contacted with a nitride-forming metal such as titanium under nitride forming conditions to remove nitrogen from the crude argon stream and to form a metal nitride. Purified argon is recovered from the nitrogen removing stage. The metal nitride is then separated from non-nitrided metal and sent to a system for commercial utilization of the metal nitride or, alternatively, the metal nitride is reconverted to metal and returned to the nitrogen removal process.

The oxygen removing stage preferably, in a first stage, comprises a solid state oxygen-ion conducting electrolyte stable under conditions of low oxygen concentration such as zirconia, ceria, hafnia, thoria or the like operated under either a high pressure differential with a mixed ion conductor, which is an electrolyte which conducts both oxygen ions and electrons to cause oxygen ions to flow through the electrolyte leaving the oxygen depleted argon on one side of the electrolyte or, as preferred, under the influence of a significant imposed voltage to cause the oxygen ion migration to occur. Under conditions of very low oxygen concentration and high current densities, zirconia is especially preferred as an electrolyte.

Alternative systems for separating oxygen from other gases by a solid state mixed conductor, i.e. one which conducts electrons as well as oxygen ions, may be utilized. Such mixed conductors are known in the art and are generally operated under a significant pressure differential so that the oxygen partial pressure in the crude gas, e.g. argon, is much higher than the oxygen partial pressure on the pure oxygen side of the conductor. This system may be advantageously used in conjunction with a commercial argon plant wherein the argon is available from a cryogenic unit at a high pressure.

Stage two of the crude argon purification process preferably involves a device which includes finely divided nitride forming metals such as titanium, zirconium, tantalum, silicon and the like operated at elevated temperatures whereby the metal reacts with the nitrogen in the oxygen-depleted argon stream to form a metal nitride.

Stage One and Stage Two of the invention can be conveniently combined into a single stage as disclosed in more detail hereinafter.

A significant advantage of the instant invention is that the invention may be used as a large scale system adjacent to a cryogenic distillation plant to purify crude argon at the point of manufacturing or, alternatively, in miniaturized form, for example, portable or mobile units. The system may thus be utilized at a point-of- use whereby the crude argon is delivered to a user in cylinders, tank cars or the like and the crude argon is then purified by the instant invention at its point-of-use. There are many advantages for purifying argon at the point-of-use inasmuch as the cylinders or other containers may become contaminated to some small degree with oxygen or nitrogen, especially if the cylinders are interchanged for transport of compressed gases whereby the same cylinder is one which may have been previously used to contain compressed oxygen or nitrogen. Thus, filling such cylinders with pure argon at its production site may still result in contaminated argon when delivered to its point-of-use. Also, in many point-of-use systems there is always the possibility of some small leakage of air into the inert gas system so that if inert gas is being recycled through the argon purification system then any trace amounts of oxygen or nitrogen picked up may be readily removed.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic illustration of the instant invention; FIG. 2 is a schematic illustration of an embodiment of the invention having parallel nitrogen removal devices which are alternatingly employed; and FIG. 3 is a schematic illustration of an embodiment of the instant invention in which a single cell is used to remove oxygen and nitrogen from crude argon.

DETAILED DESCRIPTION OF INVENTION The process and systems of this invention remove trace amounts of oxygen and nitrogen from crude argon. Generally, the invention includes a first stage, preferably an oxygen removal stage, in which oxygen is removed from the crude argon stream by causing dissociation of the oxygen ions and passage of the oxygen ions through a solid electrolyte such as zirconia, ceria, hafnia, thoria or La,._x Sr_x Y_].z Ca_z O₃ and the like, preferably by means of imposing an electrical current across the electrolyte to induce such migration. Alternatively, the electrolyte may be a mixed electrolyte wherein both electrons and oxygen ions are transported across the electrolyte and the driving force is provided by a significant pressure differential of oxygen across the electrolyte. Zirconia and La,._x Sr_x Y,__z Ca_z O₃ (where x varies from 0.1 to 0.9 and z varies from 0.1 to 0.9) are especially preferred because of their stability under extremely low O₂ concentrations, although thoria and hafnia are also especially useful electrolytes under conditions of large voltage potentials and low oxygen concentrations.

Although bismuth oxide is a known oxygen ion conductor and has been suggested as an electrolyte for trace oxygen removal, it is generally not useful for such purpose because it tends to disassociate, i.e. convert to bismuth and release oxygen, under conditions of very low oxygen partial pressures, especially when the large voltage potentials, e.g. greater than one volt, are employed to get efficient trace oxygen removal.

In a typical first stage oxygen removal unit, the crude argon is fed into that unit after preferably being preheated to a temperature of at least 300 °C and preferably to a temperature of about 500 °C and above. The operating temperature of the first stage oxygen removal unit is generally in the range of about 500 °C to about 900°C and preferably in the range of about 700°C to 800°C. In the preferred process, the argon is introduced into the oxygen removal unit at a temperature close to the preferred operating temperature of the unit.

The oxygen removal unit may be of the type described in detail in issued U.S. Patents 5,021 ,137; 4,879,016 and 4,725,346.

The nitrogen removal unit is generally downstream from the oxygen removal units so that an oxygen depleted stream of argon containing trace amounts of nitrogen is fed to the nitrogen removal unit. Alternatively, the nitrogen removal unit may be constructed as part of the oxygen removal unit so that oxygen and nitrogen are removed substantially simultaneously in a single stage process. A down-stream nitrogen removal unit preferably acts also as a heat exchanger wherein the cool cnide argon is passed through the nitrogen removal unit to preheat the crude argon before it has been fed to the oxygen removal unit. The oxygen-depleted argon stream is at an elevated temperature, usually close to the operating temperature of the oxygen removal unit.

A down-stream nitrogen removal unit may be constructed in the form of a finned heat exchanger. The operating temperature of the nitrogen removal unit is preferably above about 150°C and preferably about 250" C. The nitrogen removal unit utilizes a nitrogen forming metal such as titanium in a foπn having a large surface area per unit of volume so that the nitrogen within the crude argon may react with the metal and thereby remove the nitrogen from the argon. Generally, it is desirable to operate the nitrogen removal units in a batch wise operation. A nitride-forming metal, such as titanium, zirconium, hafnium, tantalum or silicon, reacts with nitrogen in the argon. Generally, titanium is preferred although zirconium is also especially useful. The nitrogen removal unit is maintained in operation until small traces of nitrogen are detected in the purified argon outlet of the unit. The oxygen depleted argon is then directed from the oxygen removal device to a second nitrogen removal device while the first nitrogen removal device is taken out of operation and the titanium or other metal denitrided either in situ or by removal of the, metal nitride material -and replacement with pure metal. The process of the instant invention is illustrated schematically in FIGS. 1 and 2.

It is, of course, technically achievable to have a continuous introduction of fresh nitride-forming metal fed to the nitrogen removal unit while continuously removing nitrided metal from the unit. The incoming and outgoing streams of metal could preferably pass through a gas-lock and/or exit under a blanket of pure argon at a positive pressure to the removal unit and to ambient pressure.

The systems of this invention in preferred embodiments are illustrated in FIGS. 1 and 2.

FIG. 1 is a schematic fundamental representation of the invention wherein a crude argon stream 10 from a source of crude argon, e.g., bulk argon in cylinders or storage units or directly from a crude argon manufacturing facility, such as a cryogenic air distillation facility, is directed to an oxygen removal cell 11 of the type described herein. The oxygen-depleted crude argon, which contains trace N₂, is directed to a nitrogen removal device 12 of the type described herein.

The oxygen removal cell 11 is preferably one which employs an oxygen ion transport membrane, esp. a metal oxide electrolyte. These are generally operated at an elevated temperature, generally above 500°C and preferably at about 700°C to 800°C. Thus, it is generally advantageous to preheat the cnide argon entering the oxygen removal cell 11.

The oxygen-depleted, nitrogen contaminated argon stream 13 is hot (500°C to 800 °C) when it leaves cell 1 1 and enters device 12. The device 12 typically contains a nitrogen reactive material, esp. a metal such as Ti, Zr, Hf, Th, Ta, Si and the like which are reactive at temperatures above about 250°C. Preferred operating temperatures are from about 300°C to about 800°C. Because of the heat content of argon stream 13, it may be unnecessary to add any heat in device 12. Purified argon stream 14 exits device 12. The purified argon stream is essentially devoid of the presence of either oxygen or nitrogen. If any oxygen or nitrogen is present, it is present in quantities less than 0.0001 %, i.e., less than about 1 part per million (ppm).

The purification process illustrated in FIG. 2 is a thermally efficient one which exchanges the heat content of the oxygen-depleted argon stream 13 with the cool crude argon stream 10. The process uses a pair of nitrogen removal devices 12a and 12b which are dual units operated alternatingly, i.e. , one unit is "on-stream" while the other unit is being recharged or regenerated. As indicated elsewhere herein, the nitrogen removal stage is a batch wise operation.

The units 12a and 12b are structured as heat exchangers so that the cool crude argon is not in direct contact with purified argon. The units may employ finned tubes wherein the cool argon stream 13 passes in contact with the external surface of banks of finned tubes while the nitrogen reactive metal is contained within the tubes and through which the hot, oxygen-depleted argon flows.

The system of FIG. 2 can be operated with valves 15, 16 and 17 in an open condition and valves 18, 19 and 20 in a closed condition. Any make-up heat necessary in the system due to radiation losses and the like can be supplied by a heating element within the oxygen removal cell 11.

Electrolytic oxygen removal cells employing a zirconia electrolyte may provide sufficient heat without other additions of heat because of the internal electrical resistance of the cell.

Generally, it is very desirable to maintain a relatively uniform temperature within the oxygen removal cell 11 , esp. when the cell employs a metal oxide, e.g., ZrO₂, electrolyte. Thermal shock or thermal cycling is generally to be avoided or at least minimized as much as possible. These devices often contain glass seals and ceramic components wherein an exact match of coefficients of thermal expansion is not possible, so that temperature cycling can result in undesirable thermal stresses. The system of FIG. 2 is advantageous inasmuch as the oxygen removal unit

11 can be operated continuously at a relatively uniform temperature, while devices 12a and 12b can be operated batch wise.

Another very useful embodiment of the invention involves a single purification unit which removes both oxygen and nitrogen in a single stage. Such a unit is illustrated in FIG. 3.

The embodiment of FIG. 3 is structured and operated to remove oxygen and nitrogen substantially simultaneously in an electrochemical cell which is operated to remove oxygen wherein titanium or other nitride-formi metal is disposed in the cathode chamber of the electrochemical cell. Crude argon is introduced into the cathode chamber, which is maintained at a temperature above 600°C and preferably above 700°C. The crude argon is preferably near or at the temperature of the cathode chamber when introduced into it. The unit illustrated in FIG. 3 may be employed in tandem so that one unit may be operated while the other unit is being recharged with fresh titanium or the titanium is being regenerated in situ. A preferred electrochemical cell is one employing a zirconia electrolyte and a porous platinum electrode. While ceramic electrodes, e.g. LSM or LSCo are especially useful for oxygen generating devices, i.e. ones designed to produce significant quantities of oxygen from air. the removal of trace amounts of oxygen from an inert gas such as argon requires a large voltage potential, i.e. generally above about 1.0 volt, typically above about 1.5 volts and preferably above about 2.0 volts. Such large voltages at the temperatures involved seem to work best with platinum electrodes. A silver layer, pref erably porous, to distribute current over the entire electrode may be advantageously utilized. Also because of the high voltages used it has been found that bismuth oxide, while useful in oxygen generating devices typically operated at about 1.0 volt or less, tends to be unstable at the higher voltages used to remove trace amounts of oxygen from inert gases such as argon. The flow rate of crude argon containing typically about 5 % O₂ or less and about 1 % nitrogen into the cathode chamber is controlled such that the chamber, especially near its discharge end (where the purified argon exits) contains essentially no oxygen so that any oxide of titanium which is formed immediately is reduced, i.e. , the oxygen disassociated from the titanium because of the voltage potential in the cathode chamber and the oxygen starved condition of at least a portion of the electrode/electrolyte surface. Thus, while titanium at the temperatures of the cathode chamber preferentially reacts with oxygen rather than nitrogen, any titanium oxide formed is reduced so that it may then be nitrided, thereby removing nitrogen from the argon.

Regeneration of the titanium, once nitrided, can be accomplished by introducing a measured (controlled) quantity of air into the cathode chamber at a temperature sufficiently high to cause the titanium nitride to convert to titanium sub-oxides, i.e. TiOx wherein X is < 2.0. Conversion of the titanium nitride to TiO₂ makes subsequent reduction more difficult.

Once the titanium nitride is converted to TiOx then the electrochemical cell can be used to cause dissociation, i.e. TiOx + 2xe → Ti + XO⁼ e.g. if X = 1 then TiO + 2e → Ti + O⁼

Wherein the electrons have an electroconductive path into the Ti. The Ti is preferably in the circuit, e.g.. the titanium may be part of the current collector. Both titanium and titanium nitride are electroconductive.

The titanium nitride formed during purification of crude argon could also be regenerated, i.e. converted to Ti, by exposing the titanium nitride to hydrogen under conditions appropriate for reduction of TiN (or TiO₂, if it is formed from TiN) to TiH. A regeneration unit containing a proton conducting electrolyte can then be used to "pump" the hydrogen from the TiH to convert it to Ti. The proton conductor can be structured in the purification unit so that such regeneration may take place in situ.

An alternative technique for removing oxygen and nitrogen from cnide argon is to utilize a device containing an oxide-forming metal and a nitride-forming metal whereby oxygen and nitrogen are substantially simultaneously removed when crude argon is placed in contact with said metals under oxide and nitride forming conditions. The process, of course, could be practiced sequentially wherein the crude argon was first placed in contact with an oxide-forming metal and then a nitride-forming metal, or vice versa, so that oxygen was removed in one step and nitrogen removed in a second step. Generally, those metals which form nitrides at elevated temperatures, as disclosed hereinabove, also form oxides under similar conditions. Generally, when oxygen and nitrogen are both present, these metals tend to form oxides more readily or form oxides under conditions which are less favorable for nitride formation, e.g., at lower temperatures.

Some metals such as copper, lithium, sodium, etc. form oxides at temperatures as low as 200 °C to effectively remove substantially all the minor quantities of oxygen present in cnide argon. The rate of oxygen removal, however, is enhanced by operation at higher temperatures, e.g. temperatures from about 100°C to about 500°C or higher may be used.

The oxides and nitrides of such metals may be regenerated in the manner described herein for an electrolytic removal/regeneration process.

EXAMPLE I Inlet gas — Cnide argon 10 liters/min of 100 ppm O₂ and 50 ppm of N₂

System has zirconia based O₂ removal unit followed by titanium sponge based N₂ removal unit.

First Stage — 10 to 15 tubes of zirconia (¹ " dia x 18" leng) will remove 100 ppm of oxygen to less than one ppm of oxygen under 1-5 Vol 3 (Temp = 750 °C) Second Stage - 10 liters of argon/min. contains 0.5 cc of N₂/min. i.e. 30 cc of N₂ per hr

3 Ti + 2 N, -> Ti₃ N₄ (Temp 750°C)

144 gm + 56 gm

144 gm (3 moles) will treat 44.8 liters (2 moles) of N₂ 48 gm (one gm-mole) will treat approx 15 liters of N₂

1 kg of Ti (approx 20 gm-moles) will treat approx 300 liters of N₂ 1 kg of titanium is enough for 300 liters x 1000 cc/liter ÷ 33 cc/hr =

- 100,000 hrs i.e. 1 kg of titanium is enough to last more than 1 year for complete removal of nitrogen. From a flowrate of 10 liters/min. of argon containing 50 ppm of nitrogen.

In the above example, the flow is suggested for an end use application.

Thus, a small unit could readily purify the argon requirements of an industrial process utilizing flow rates up to about 50 liters/min of pure argon derived from impure argon containing significant quantities of oxygen and nitrogen. The oxygen and nitrogen may exist in the argon source as it may be picked up during circulation of argon through a system blanketed by a stream of argon inert gas.

Claims

CLAIMS What is claimed is:

1. A method of removing O₂ and N₂ from crude argon comprising contacting said crude argon with a solid oxygen-ion conducting electrolyte under oxygen-ion conducting conditions to remove oxygen from said crude argon; contacting said oxygen-depleted crude argon with a nitride-forming metal under nitride-forming conditions to remove nitrogen and to foπn a metal nitride; and recovering purified argon substantially free of oxygen and nitrogen.

2. The method of Claim 1 wherein metal nitride is regenerated to a nitride-forming metal.

3. The method of Claim 1 wherein said crude argon is contacted with said oxygen-ion conducting electrolyte at a temperature of about 500 "C to about

900 °C.

4. The method of Claim 1 wherein said electrolyte is an ionically conductive zirconia ceramic.

5. The method of Claim ϊ wherein said electrolyte is ionically conductive La,._x Sr_x Y^ Ca_z O₃ ceramic, where x varies betv/een 0.1 and 0.9 and z varies between 0.1 and 0.9.

6. The method of Claim 1 wherein said oxygen-depleted crude argon is contacted with a nitride-forming metal at an elevated temperature.

7. The method of Claim 1 wherein the current collector in the electrochemical cell is a nitride-forming metal.

8. The method of Claim 1 wherein said nitride-forming metal is one selected from a group consisting of titanium, tantalum, zirconium, lithium, sodium, calcium, magnesium and their alloys, and composites thereof with other metals and ceramics.

9. The method of Claim 8 wherein said metal is titanium.

10. A method of removing substantially simultaneously O₂ and 4N₂ from crude argon comprising contacting said crude argon with an oxide-forming metal and a nitride-forming metal under oxide and nitride forming conditions to remove nitrogen and oxygen and recovering purified argon substantially free of oxygen and nitrogen and treating said metal oxide and metal nitride to reform them to said oxide-forming metal and nitride-forming metal.

11. An apparatus for removing oxygen and nitrogen from crude argon comprising: an oxygen-removing cell having a solid oxygen-ion conducting electrolyte; means for introducing cnide argon to said cell; means for discharging said oxygen-depleted argon from said cell; a nitrogen-removing device containing a nitride-forming metal; means for introducing said oxygen-depleted argon to said nitrogen-removing device; and means for discharging said purified argon from said nitrogen removing device.