[0001] The present invention relates to a system for the purification of inert gases. More specifically, the present disclosure relates to a system for the onsite removal of trace impurities, such as nitrogen and methane, from an argon stream using cryogenic distillation separation.

Background of the Invention

[0002] The use of argon as an inert gas is important to a number of industries including the production of monocrystalline and polycrystalline silicon, steel making (e.g., using the hot isotatic pressing (HIP) process to make steel, aluminum and nonmetallic castings from powders), heat treating, and semiconductor, wafer, and electronics manufacturing. Given the rising costs of argon gas, onsite purification of waste argon streams to recycle back to the manufacturing process is increasingly common.

[0003] One manufacturing process suitable for argon purification and recycle is the production of monocrystalline silicon using the Czochralski Process. High purity argon, i.e., having a purity of 99.999% or greater, is used in the process to control the atmosphere and to purge contaminants and volatilized materials from the process as a waste stream. Purity of the argon stream is critical because it affects the both the purity and the quality of the silicon ingot grown in the Czochralski Process.

[0004] The waste stream from the process, which may include between about 95 to about 99% argon (Ar), may contain any number of contaminants including: oxygen ((¾), hydrogen (H ₂ ), volatilized silicon oxide (SiO), carbon dioxide (CO ₂ ), carbon monoxide (CO), nitrogen (N ₂ ), water (H ₂ 0), methane (CH ₄ ), other hydrocarbons, and other impurities. Although commercial technologies currently address the removal of contaminants such as (¾, H ₂ , CO ₂ , CO, H ₂ 0, and hydrocarbons, cryogenic distillation is still the preferred route for the removal of N ₂ and CH ₄ .

[0005] Current methods for the purification of argon typically utilize a cryogenic distillation process used in conjunction with a pre-treatment process. Current pre-treatment process systems begin by compressing the waste stream. The compressed gas is fed, along with excess O ₂ and ¾ if necessary, to a series of commercially available catalyst beds that oxidize CO and CH ₄ to CO ₂ and H ₂ 0. The CO ₂ and H ₂ 0 can be removed by adsorption, either by using molecular-sieve or a combination of silica gel and molecular-sieve. The waste stream now contains primarily argon, N ₂ , and ¾, and may also contain minute quantities of CH ₄ (methane slippage), which must be removed prior to recycle to the manufacturing process. With the CO ₂ and ¾0 removed, the argon waste stream may be fed to a cryogenic distillation system capable of removing the N ₂ , any remaining ¾.

[0006] Argon purification systems, as described above, are available in packaged skids typically sold to companies for housing on their property for use in their manufacturing process. However, current systems do not address methane slippage and are incapable of removing impurities below part per million (ppm) levels.

[0007] Therefore, it would be beneficial to have a simpler cryogenic distillation system with fewer mechanical parts. Additional benefits could be gained if a simpler system was capable of removing more impurities and addressing methane slippage from the pre-treatment process. Summary of the Invention

[0008] The present invention relates to a system for the purification of inert gases. More specifically, the present disclosure relates to a system for the onsite removal of trace impurities, such as nitrogen and methane, from an argon stream using cryogenic distillation separation.

[0009] In one embodiment, an argon purification system is provided. The argon purification system includes a cryogenic heat exchanger configured to remove heat from a pre-treated argon waste stream to create a cold feed stream. A cryogenic distillation column includes a reboiler and an overhead condenser, said column having an upper portion and a lower portion and is configured to receive a liquid feed stream and to produce a bottoms argon product stream and a gas waste stream. The reboiler is positioned in the lower portion of the cryogenic distillation column and is configured to remove heat from the cold feed stream such that the cold feed stream condenses to the liquid feed stream. The condenser is positioned in the upper portion of the cryogenic distillation column and is configured to heat the bottoms argon product stream such that the bottoms argon product stream evaporates to a purified vapor phase argon stream.

[0010] In certain embodiments, the cryogenic heat exchanger heats the purified vapor phase argon stream with the heat removed from the pre-treated argon waste stream. In certain embodiments, the cryogenic heat exchanger heats the gas waste stream with the heat removed from the pre-treated argon waste stream. In certain embodiments, the reboiler uses the heat removed from a cryogenic distillation column overheads to heat the bottoms argon product stream. In certain embodiments, the overhead condenser uses heat removed from the cryogenic distillation column overheads to heat the bottoms argon product stream. In certain embodiments, the bottoms argon product stream undergoes expansion in an expansion device before being fed to the overhead condenser, such that the temperature of the bottoms argon product stream is reduced in the expansion device. In certain embodiments, a pump provides the driving force to get the bottoms argon product stream to the overhead condenser.

[0011] In certain embodiments, the cryogenic distillation column is configured with additional stages above the overhead condenser. In certain embodiments, the argon purification system includes a sorbent bed system, such that the sorbent bed system accepts the bottoms argon product stream, removes contaminants from the bottoms argon product stream, and then feeds a sorbent bed outlet stream to the overhead condenser. In certain embodiments, the sorbent bed system is filled with regenerable adsorbents. In certain embodiments, the sorbent bed system is filled with getter materials.

Brief Description of the Drawings

[0012] These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

[0013] FIG. 1 shows one preferred embodiment of the invention.

[0014] FIG. 2 shows another preferred embodiment of the invention.

[0015] FIG. 3 shows an alternative preferred embodiment of the invention.

[0016] FIG. 4 shows another preferred embodiment of the invention. Detailed Description

[0017] While the invention will be described with several embodiments, it is understood that one of ordinary skill in the relevant art will appreciate that many examples variations, and alterations to the following details are within the scope and spirit of the invention. Accordingly, the exemplary embodiments of the invention described herein are set forth without any loss of generality, and without imposing limitations, relating to the claimed invention.

[0018] In one embodiment, the present invention describes a cryogenic distillation system capable of removing trace impurities from an argon stream.

[0019] FIG. 1 provides an illustration of an embodiment of the present invention. Pre-treated argon waste stream 2 is the outlet of the pre-treatment process. The waste gas stream from the manufacturing process will include argon, H ₂ , (¾, CO _2, CO, CH ₄ , and H ₂ 0, along with other trace impurities. The pre-treatment process begins by compressing the waste gas stream. The compressed gas is then fed, along with excess O ₂ and H ₂ , or air, if necessary, to a series of commercially available catalyst beds that oxidize CO, CH ₄ , and O2 to CO2 and H2O. The CO2 and H ₂ O are removed by adsorption, either by using molecular-sieve or a combination of silica gel and molecular-sieve, resulting in pre-treated argon waste gas stream 2.

[0020] Pre-treated argon waste stream 2 contains greater than about 95% argon, less than about 4% N ₂ , less than about 1% H ₂ , less than about 1 ppm H ₂ , 0 ₂ , CO, and CH ₄ , and no CO ₂ or H2O. The exact temperature, pressure and composition of pre-treated argon waste stream 2 will depend on the specific manufacturing and pre-treatment processes that are employed. In certain embodiments, the pre-treatment process removes as much O ₂ , CO, and CH ₄ as possible, to achieve concentrations from about 1 ppm to 10 ppm, alternatively to below 1 ppm. [0021] Pre-treated argon waste stream 2 is fed into cryogenic heat exchanger 10, where it is cooled to a temperature between about -150°C and -160°C, alternatively between about - 152°C and -157°C, alternatively between about -153°C and -155°C, alternatively about - 153.6°C. Cryogenic heat exchanger 10 may be any heat exchanger of the type and materials commonly employed in a cryogenic air separation process. In certain embodiments, the heat exchanger is constructed of brazed aluminum.

[0022] Cold feed stream 12 exits cryogenic heat exchanger 10 and enters reboiler 30. Additional energy is removed from cold feed stream 12 in reboiler 30, and the heat from cold feed stream 12 is used to provide reboil liquid at the bottom of column 20. This results in condensing of cold feed stream 12. Liquid feed stream 32 exits reboiler 30 at a temperature between about -150°C to about -160°C, preferably about -156.9°C. Liquid feed stream 32 is then fed to cryogenic distillation column 20 at a point above the bottom section and below the upper section, preferably in the packing section.

[0023] Argon stream 4 is fed to cryogenic distillation column 20 to make up for argon lost as waste in gaseous waste stream 26 and the liquid purge 28. Any refrigeration requirements for the process such as heat leakage, temperature differences in heat exchanger 10, liquid purge or other losses can be compensated by the addition of argon stream 4. In one embodiment, Argon stream 4 is a high purity source of liquid argon containing at least about 99.999% argon, less than about 0.001% (¾, and less than about 0.001% N ₂ . Advantageously, argon feed stream 4 can also provide additional cooling for cryogenic distillation column 20. The temperature of argon feed stream 4 is preferably close to its dew point.

[0024] Bottoms argon product stream 22 exits cryogenic distillation column 20. Bottoms argon product stream 22 has a purity of at least about 99.999% argon. After exiting cryogenic distillation column 20, bottoms argon product stream 22 is cooled by expansion in expansion device 50. Expansion device 50 may be any device capable of reducing the pressure and temperature of bottoms argon product stream 50. In one embodiment, bottoms argon product stream 22 is cooled to at or about its bubble point.

[0025] Bottoms argon product stream 22 can be supplied to overhead condenser 40. In one embodiment, argon lift stream 6 (a small stream of argon gas, less than about 1% by mass of bottoms argon product stream 22) can be added to bottoms argon product stream 22 after expansion valve 50 as a lift gas to drive the stream to overhead condenser 40. This feature can be useful to ease the liquid transfer when distillation column 20 is tall, particularly due to the relatively high density of liquid argon and the low differential pressure between the bottom of distillation column 20 and overhead condenser 40. A partial vapor stream that includes argon lift stream 6 and bottoms argon product stream 22 (i.e., bottoms lift stream 52), operates by virtue of the buoyancy of a gas-liquid slug.

[0026] Bottoms lift stream 52 provides cooling to overhead condenser 40, which vaporizes bottoms lift stream 52 to create purified vapor phase argon stream 24. Vapor from the top of cryogenic distillation column 20 partially condenses in overhead condenser 40 due to the cooling provided by bottoms lift stream 52. A small liquid purge stream 28 containing some heavy impurities such as hydrocarbons, carbon dioxide, etc... can be removed from overhead condenser 40.

[0027] Purified vapor phase argon stream 24 can be fed to cryogenic heat exchanger 10 to provide cooling for pre-treated argon waste stream 2. Product stream 14, having a temperature of about ambient temperature, may be returned to the manufacturing process requiring pure argon, may be stored, may be diverted for other processes, or may be distributed off-site. The purity of product stream 14 is greater than about 99.999% argon. [0028] Nitrogen and ¾, being more volatile than argon, collect near the top of cryogenic distillation column 20 and are removed from the distillation column as gaseous waste stream 26. Gaseous waste stream 26 can be supplied to cryogenic heat exchanger 10 to provide cooling of pre-treated argon waste stream 2. Warm gas waste stream 16 may then be recycled to the manufacturing process, disposed of, or used to regenerate the mole-sieve beds of the pre-treatment process.

[0029] In a second embodiment of the system in FIG. 1, a pump (not shown) can be used to supply the driving force to drive bottoms argon product stream 22 to overhead condenser 40. The pump may be of any type and materials commonly used in a cryogenic distillation system. In one embodiment, argon lift stream 6 is not used when the pump is used.

[0030] FIG. 2 demonstrates an additional embodiment of the invention. In this embodiment, in addition to the features shown in FIG. 1 and described above, additional separation stages can been included in cryogenic distillation column 20 above overhead condenser 40. The additional separation stages allow for further purification of the argon stream by removing CH ₄ not removed during the pre-treatment process, thus addressing methane slippage. Methane, which is less volatile than argon, will be carried out the bottom of cryogenic distillation column 20 in bottoms argon product stream 22. The additional stages in cryogenic distillation column 20 allow for the separation and removal of this CH ₄ prior to purified vapor phase argon stream 24 exiting cryogenic distillation column 20. Methane waste stream 28 may be bled from the column below the additional stages and disposed of or redirected as necessary. The flowrate of argon stream 4 can be adjusted to account for additional argon losses in methane waste stream 28. It is understood that the additional separation stages shown in Figure 2 can also be added to the additional embodiments described herein, for example FIG. 3 and FIG. 4. [0031] FIG. 3 illustrates an alternative embodiment of the invention. In this embodiment, in addition to the features shown in FIG. 1 and described above, a portion of product stream 14 can be diverted to compressor 60 through recycle stream 15. Compressor 60 compresses recycle stream 15 to the pressure of cryogenic distillation column 20, creating compressed argon recycle stream 68. Compressed stream 68 can be fed to cryogenic heat exchanger 10 to be cooled. The cooled and compressed argon recycle stream 18 is cooled to a temperature at or about its dew point, and is fed to cryogenic distillation column 20. Between about 5 and 25% of product stream 14 by volume can be recycled to cryogenic distillation column 20 to improve the overall recovery of argon. In alternate embodiments, between about 5 and 10% by volume is recycled, alternatively between about 10 and 15% by volume, alternatively between about 15 and 20% by volume, alternatively between about 20 and 25%. In certain embodiments, by recycling approximately 10% of product stream 14 to cryogenic distillation column 20 can increase overall argon recovery from about 83% to greater than 90%, alternatively greater than 95%, alternatively to about 99%.

[0032] FIG. 4 illustrates yet another embodiment of the invention. In this embodiment, in addition to the features shown in FIG. 1 and FIG. 3 and described above, bottoms argon product stream 22 can be supplied to sorbent bed system 80 to reduce contaminants in bottoms argon product stream 22. Pump 70 can provide the drive necessary so sorbent bed feed stream 72 can pass through sorbent bed system 80 and can enter overhead condenser 40.

[0033] Sorbent bed system 80 can include two or more beds in parallel, or alternatively can include two or more beds such that at least one is a back-up for use when another bed is being serviced. Sorbent bed system 80 may be filled with regenerable adsorbents or nonregenerable getter materials. Getter materials can include finely divided metals (such as, for example, zirconium, tantalum, copper, nickel, etc.) on a substrate, such as alumina, ceria or kieselguhr. In certain embodiments, sorbent bed system 80 will remove contaminants such that the concentration of CO, CH ₄ , H ₂ , O2, N ₂ , and H ₂ 0 is reduced to between about 1 ppm to about 10 ppm, alternatively to between about 1 ppb to 10 ppb. Outlet stream 82 from sorbent bed system 80 may be returned to overhead condenser 40, or alternatively may be diverted for other processes.

Example 1:

[0034] The present invention is further demonstrated by the following illustrative embodiment, which does not limit the claims of the present invention.

[0035] Table 1 provides data for a representative process, utilizing the embodiment illustrated in FIG. 1. The manufacturing process pre-treated argon gas waste stream is provided from a monocrystalline silicon production process. A molar gas flowrate of 500 m ³ /h was chosen for simulation purposes. The process conditions of pre-treated argon waste stream 2 are based on the outlet of a pre-treatment process, which typically takes place at an ambient temperature, but with a compressed gas stream, thus for simulation purposes an inlet temperature of 20°C and 147 psig were chosen. Finally, the composition of pre-treated argon waste stream 2 assumes some CH ₄ leakage and impurities of CO and O2 on the part per million scale.

Table 1: Representative Data

[0036] As shown in Table 1, the present invention is capable of removing trace impurities to produce an argon stream (product stream 14), having an argon purity of greater than 99.999%. Cold feed stream 12 leaving cryogenic heat exchanger 10 was cooled to a temperature of -153.6°C by using the cold from purified vapor phase argon stream 24 and gaseous waste stream 26.

[0037] While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

[0038] The singular forms "a", "an" and "the" include plural referents, unless the context clearly dictates otherwise. [0039] "Comprising" in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of "comprising"). "Comprising" as used herein may be replaced by the more limited transitional terms "consisting essentially of and "consisting of unless otherwise indicated herein.

[0040] "Providing" in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary a range is expressed, it is to be understood that another embodiment is from the one.

[0041] Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

[0042] Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such particular value and/or to the other particular value, along with all combinations within said range.

[0043] All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.