NZ248804A

NZ248804A - Argon purification by cryogenic adsorption

Info

Publication number: NZ248804A
Application number: NZ248804A
Authority: NZ
Inventors: Ravi Jain; Mark J Andrecovich
Original assignee: Boc Group Inc
Priority date: 1992-10-01
Filing date: 1993-09-29
Publication date: 1995-10-26
Also published as: AU4753793A; ZA937162B; KR940009063A; CN1088471A; AU659759B2

Description

<div class="application article clearfix" id="description"> Patents Form 5 N.Z. No. NEW ZEALAND Patents Act 1953 COMPLETE SPECIFICATION PURIFICATION OF ARGON BY CRYOGENIC ADSORPTION We, THE BOC GROUP, INC., a Corporation organized under the laws of the State of Delaware, United States of America, of 575 Mountain Avenue, Murray Hill, New Providence, New Jersey 07974, United States of America do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - -1 - (Followed by 1A) -1A.- PURIFICATION OF ARGON BY CRYOGENIC ADSORPTION BACKGROUND OF THE INVENTION This invention relates to the purification of argon, and more particularly to the removal of nitrogen and oxygen from an argon stream by adsorption at cryogenic temperatures. Crude argon produced by the cryogenic distillation of air by conventional techniques generally contains 3-5% by volume oxygen and up to about 1% by volume nitrogen. If 1t is desired to produce higher purity argon, the oxygen and nitrogen can be removed from the argon stream by various chemical or physical techniques. Oxygen is commonly removed from an argon stream by reacting the oxygen with excess hydrogen over a suitable catalyst, such as a noble metal, thereby forming water. The removal of oxygen from an argon stream by catalytic treatment is carried out at relatively high temperatures; accordingly a significant quantity of energy is expended in raising the temperature of the crude argon product to the reaction temperature and then in cooling it down for further purification. Furthermore, if the argon stream contains significant amounts of oxygen, considerable quantities of hydrogen must be added to effect complete removal of the oxygen from the gas stream. After oxygen removal by catalysis, the argon stream is dried by adsorption and nitrogen and excess hydrogen are often removed from the stream by one or more cryogenic distillation steps. It is also known to remove oxygen from an argon stream by chemisorption using getters such as reduced Cu and Ni. This technique is effective primarily when the oxygen concentration in the argon stream is below 0.8%. Argon product gas with such low oxygen content can be obtained by using structured packing in the crude argon side column. The oxidized getter beds are regenerated by passing hydrogen therethrough. Any nitrogen in the oxygen-free argon stream is generally removed by cryogenic distillation. Catalytic and chemisorption reaction steps and cryogenic distillation steps are complex and significantly increase the cost of high purity argon production. Furthermore, the high purity hydrogen required for these processes is not always available at locations where it is desired to operate such argon purification plants. Argon has been separated from nitrogen and oxygen by pressure swing adsorption (PSA) at ambient temperatures. U.S. Patents 4,144,038 and 4,477,265 disclose adsorption of oxygen and nitrogen from an oxygen-rich feedstock withdrawn from the rectification column of a cryogenic air separation plant. These processes suffer from low yield and low purity of the argon product. Removal of both oxygen and nitrogen from argon at below ambient temperatures (-100 to 0° C.) by PSA and a combination of PSA and temperature swing adsorption (TSA) is described in: German Patent 2,826,913 (which discloses the use of a mixture of 4A and 5A zeolites as adsorbents); Japanese Patent Kokai 59/064,510 (which uses a mixture of mordenite and faujasite as adsorbent); and Japanese Patent Kokai 58/187,775 (which uses type A zeolite as adsorbent). In the TSA embodiments adsorption capacities are fairly low resulting in very large bed requirements, and in the PSA embodiments high purity argon product yields are low. The removal of oxygen alone or the removal of both oxygen and nitrogen from argon at cryogenic temperatures (90 to 173° K.) by adsorption using 4A type sieve is described: in Japanese Patent Kokai TiwrtiAfli n? pui.hli. iiu niiqfin^ -3- 2 4 8804 62/065.913; by Fedorov et al. in Khira. Neft. Mashinostr. (Vol 6, page 14, 1990); and by Kovalev et al. in Energomashinostroenie (Vol 10, page 21, 1987). The disadvantage of this technique is that when both nitrogen and oxygen are present in the gas stream being treated, 4A sieve has a very low capacity for oxygen; consequently very large beds are required for complete oxygen removal. If nitrogen is removed by cryogenic distillation prior to adsorption, 4A zeolite sieve is effective for oxygen removal; however, this increases the cost of argon purification. Since crude argon generally contains both oxygen and nitrogen, single step high efficiency and high yield processes for removing both of these impurities from an argon stream are constantly sought. The present invention provides such a process. In a broad embodiment of the invention, high purity argon, i.e. argon containing no more than about 5 ppm by volume each of nitrogen and oxygen, is produced by subjecting a crude argon stream containing nitrogen and oxygen as impurities to cryogenic TSA. The adsorption step is generally carried out at temperatures between the dew point of the feed gas entering the adsorption unit and about 150°K and at absolute pressures in the range of about 1.0 to 20 atmospheres in a two layer adsorbent bed, the first layer of which comprises one or more adsorbents which preferentially adsorb nitrogen from a gas stream comprising nitrogen, argon and oxygen, and the second layer of which comprises one or more adsorbents which preferentially adsorb oxygen from a substantially nitrogen-free gas stream comprising argon and oxygen. Preferred adsorbents for use in the first layer include calcium-exchanged type X zeolite, calcium-exchanged type A zeolite, 13X zeolite, and carbon molecular sieve (CMS). Preferred adsorbents for use in the second layer include CMS or 4A type zeolite. The invention also provides a process for producing a substantially nitrogen-free argon-oxygen gas mixture by cryogenics!ly adsorbing nitrogen from nitrogen-argon-oxygen gas mixtures. SUMMARY OF THE INVENTION WRIUUI y2 ■» "ftULkyL IIU. sYlUoiw -4- 24 8 8 0 4 The adsorption is preferably carried out in a battery of two or more adsorption beds arranged in parallel and operated out of phase, so that at least one bed is undergoing adsorption while another is undergoing regeneration. The process is most effective for the removal of up to about 5% by volume each of oxygen and nitrogen. The adsorption is preferably carried out at temperatures in the range of about 1.0 to 20 atmospheres. Upon completion of the adsorption step, flow of the feed gas through the adsorption bed is terminated and the bed is regenerated by passing a warm nitrogen-free purge gas therethrough, when only nitrogen is to be removed from the gas stream, and a warm nitrogen-free and oxygen-free purge gas therethrough, when both nitrogen and oxygen are to be removed from the gas stream. The purge gas preferably is at a temperature of about -20° to 250° C. The preferred purge gas is the high purity argon or argon-oxygen gas mixture being produced during the adsorption step. In another embodiment of the process of the invention an argon stream from a cryogenic fractional distillation air separation unit is distilled, preferably at a temperature of about 90 to 110° K., to produce an oxygen-enriched bottoms product stream and a gaseous argon-enriched overhead product stream. Part of the argon-enriched product stream is then subjected to a TSA process at cryogenic temperatures to remove residual nitrogen and oxygen from this stream, thereby producing a high purity argon product stream. The high purity argon stream, now containing not more than about 5 ppm each of nitrogen and oxygen, may be passed to product as a gas or condensed and passed to product as high purity liquid argon. In this embodiment the portion of the argon-enriched overhead product stream not subjected to adsorption is condensed and returned to the crude argon distillation column as reflux. mm c\r^^s^ hatcmt ^t'jza ■ rnhrt tin m?"^ 24 88 04 In an alternate embodiment, all of the argon-enriched overhead product is subjected to a cryogenic TSA process and all or a portion of the high purity argon nonadsorbed product stream is condensed, and all or part of the condensed argon stream is returned to the crude argon distillation unit as reflux. In a preferred embodiment of the combined disti1lation-TSA process, the adsorption is conducted in a two-layer adsorbent bed of the type described above, i.e. a first layer containing one or more adsorbents which preferentially adsorb nitrogen and a second layer containing one or more adsorbents which preferentially adsorb oxygen. As was the case in the broad embodiment, the preferred adsorbents for use in the first layer include calcium-exchanged type X zeolite, calcium-exchanged type A zeolite, 13X zeolite and CMS, and preferred adsorbents for use in the second layer include CMS or type 4A zeolite. In another aspect, the invention comprises apparatus for producing high purity argon from a crude argon stream. The first element of the apparatus is a distillation column with a top outlet for removing a gaseous argon-enriched stream and a bottom outlet for removing an oxygen-enriched bottoms stream. An inlet is provided near the bottom of the distillation column for introducing an argon-containing stream into the column. In a preferred embodiment of the apparatus aspect of the invention, some or all sections of the distillation column contain structured packings. The second element of the apparatus is a TSA system comprising an adsorbent bed containing one or more adsorbents. The adsorbents remove nitrogen or both nitrogen and oxygen from the argon stream. The inlet to the adsorption system is connected to the top outlet of the distillation column. The adsorption system also has a nonadsorbed product outlet and a desorbed product outlet. In one arrangement of the apparatus embodiment of the invention a condenser inlet is also connected to the top outlet of the crude argon distillation column. The outlet of the condenser is connected to the top of the distillation column. This provides a means for condensing part of -Wtt rrsms^- — n. .i. . i. ... -6- 2 4 the argon-enriched stream exiting the distillation column and returning" the condensed argon-enriched stream to the column as reflux. The remainder of the argon-enriched stream is passed to the TSA system. In another arrangement the nonadsorbed product outlet of the TSA system is connected to a condenser and the outlet of the condenser is connected to the top of the crude argon distillation column. This permits part or all of the condensed high purity argon product to be returned to the distillation column as reflux. In a preferred arrangement of the system of the invention the adsorbents in the TSA system comprise a first layer of calcium exchanged X zeolite, calcium exchanged A zeolite, CMS, 13X zeolite or mixtures of two or more of these, and a second layer comprising CMS or 4A zeolite, or mixtures of these. In another preferred arrangement, the crude argon distillation column is partly or completely filled with low pressure drop structured packing. In still another preferred arrangement, the TSA system comprises a single adsorbent bed packed with an adsorbent which adsorbs nitrogen in preference to both oxygen and argon. The invention is illustrated in the drawings, in which: Fig. 1 depicts a system for recovering substantially pure argon from a crude argon feed in accordance with the principle of the invention; and Fig. 2 illustrates a variation of the system illustrated in Fig. 1. BRIEF DESCRIPTION OF THE DRAWINGS Like characters designate like or corresponding parts throughout the several views. Auxiliary valves, lines and equipment not necessary for an understanding of the invention have been omitted from the drawings. -7- 24 0 8 0 4 DETAILED DESCRIPTION OF THE INVENTION In the broad aspect of the invention, an argon feed stream containing nitrogen and oxygen as impurities is passed through a single-layer adsorption bed at cryogenic temperatures, thereby removing nitrogen but not substantial quantities of oxygen from the feed stream. In another broad aspect the argon feed stream is passed through a two-layer adsorption bed at cryogenic temperature, thereby removing nitrogen and oxygen from the feed stream. The adsorption process is a TSA cycle. In a specific aspect, a feed stream consisting predominantly of oxygen and argon but also containing a small amount of nitrogen is distilled in a cryogenic distillation column to produce an argon-enriched stream by removing a significant amount of oxygen therefrom, and the argon-enriched stream is subjected to the above-described TSA process. In this specific aspect a portion of the high purity nonadsorbed product stream from the adsorption system may be condensed and returned to the argon distillation column as reflux. Both of these aspects are illustrated in Fig. 1. Turning now to Fig. 1, the system illustrated therein includes a crude argon distillation column, D, a pair of parallel disposed adsorption beds, A and B, and a nonadsorbed product gas condenser, C. Argon-containing gas enters the system through feed line 2, which is preferably located in the lower part of column D. The feed generally enters the system at a temperature in the range of about 90 to 150° K. and an absolute pressure of about 1 to 20 atmospheres as it enters column D and is preferably at a temperature of about 90 to 110° K. and at an absolute pressure of about 1 to 1.5 atmospheres. Column D may contain trays, packings or both. Packed columns are preferred, however, since they offer the advantage of a smaller pressure drop. When packed columns are used, the column may be partially or completely filled with the packing. In the most preferred embodiment of the invention column D contains structured packings. The use of structured packings in column D can reduce the amount of oxygen in the crude argon effluent from column D to 0.5% or less. This can substantially reduce the load on adsorber I * LKHIUUUl!^ ffWm* flf;Zt Tnr'"t Nn "-'H 24 8 8 i vessels A and B, thereby reducing their size requirements substantially. The use of structured packings in crude argon columns is described in U.S. Patents 4,994,098; 5,019,144 and 5,019,145, the specifications of which are incorporated herein by reference. 4 The adsorption system illustrated in Fig. 1 is depicted as comprising two parallel arranged beds; however the invention is not limited to a two parallel arranged bed system. A single bed adsorption system can be used, but in such a case a vessel would have to be provided in line 6 to store argon enriched gas exiting column D during regeneration of the single bed. Similarly, the adsorption system can comprise more than two parallel arranged adsorption beds. The number of adsorption beds in the system is not critical to the operation of the invention. In the two bed system illustrated in the drawings one bed is in adsorption service while the other bed is being regenerated. Beds A and B are identical and each contains a first layer of adsorbent, 12A and 12B and a second layer of adsorbent, 14A and 14B. The adsorbent in layers 12A and 12B preferentially adsorbs nitrogen and the adsorbent in layers 14A and 14B preferentially adsorbs oxygen from the argon feed gas. Layers 12A and 12B are generally packed with one or more adsorbents selected from X type zeolites, mordenites, CMS and A type zeolites other than type 4A zeolite, and layers 14A and 14B are generally packed with at least one adsorbent selected from CMS and 4A zeolite. Preferred adsorbents for layer 12A and 12B include calcium-exchanged type X zeolite, type 5A zeolite and 13X zeolite, and the preferred adsorbent for layers 14A and 14B is 4A. In the most efficient embodiment of the system of the invention the nitrogen adsorbent layer precedes the oxygen adsorbent layer. In the adsorption system illustrated in Fig. 1, valves 16A and 16B control the flow of feed gas to beds A and B, respectively; valves ISA and 18B control the flow of vent gas and desorbed gas from adsorbers A and B, respectively; valves 20A and 20B control the flow of purge gas to adsorbers A and B, respectively; and valves 22A and 22B control the flow of nonadsorbed product gas from adsorbers A and B, respectively. r>**rrn"r *mrr ■dulKli mu jinn , 24 88 0 During operation of column D oxygen-enriched liquid is withdrawn from the column through line 4, located at or near the bottom of column D, and argon-enriched gas is withdrawn from column D through line 6, located at or near the top of the column. Argon-enriched gas leaving column D passes through valve 8 and enters the adsorption system through line 10. Before the initial start-up, adsorbent beds A and B are preferably heated to temperatures up to 300°C to remove any residual moisture contained therein. This step is not repeated during the regular operation. The operation of the adsorption system will first be described with bed A in the adsorption mode and bed B in the regeneration mode. In this half of the cycle, valves 16A, 18B, 20B and 22A are open and valves 16B, 18A, 20A and 22B are closed. The feed gas entering the system through line 10 can contain up to 5% nitrogen and about 3 to 5% oxygen. The feed gas passes through valve 16A and line 24A and enters layer 12A of bed A. As the gas passes through layer 12A, nitrogen is preferentially adsorbed therefrom. The nitrogen-depleted gas stream next passes through layer 14A wherein oxygen is preferentially adsorbed from the stream. The gas stream leaving bed A, now containing no more than about 5 ppm each of nitrogen and oxygen, passes through line 26A and valve 22A and leaves the adsorption system through line 28. A portion of the high purity argon product stream leaving the adsorption units may be removed from the system as a gas via line 30 by opening valve 32 and the remainder Introduced into condenser C, or alternatively, all of the product stream may enter condenser C. The gaseous argon product 1s condensed 1n condenser C by means of a coolant which enters condenser C through line 34 and leaves the condenser through line 36. High purity liquid argon leaves condenser C via line 38 and is returned to the top of column D through valve 40 and line 42, where it serves as a reflux to remove oxygen from the vapor rising in column D. If desired, a portion of the high purity liquid argon can be passed to 24 88 04 product storage by opening valve 44 in line 46. Thus, when operating the system of Fig. 1, a high purity gaseous argon product can be produced or a high purity liquid argon product can be produced or high purity gaseous and liquid products can be simultaneously produced. While high purity argon is being produced in bed A, bed B is being regenerated. During regeneration, a warm purge gas is introduced into the adsorption section through line 48 and open valve 20B. The purge gas temperature is typically between -20 and 250° C. The flow of purge gas through line 4B is typically between 5 and 15X of the flow of feed gas to the adsorption system. The warm purge gas passes through bed B, thereby desorbing and sweeping oxygen and nitrogen from the bed. If the adsorption bed is contacted directly with the purge gas, it is preferred to use high purity argon as the purge gas to avoid contaminating the adsorption bed. On the other hand, if the bed is indirectly contacted with the purge gas, as by passing it through heat transfer tubes embedded in the adsorbent, it is not necessary to use high purity argon as the purge gas since the purge gas will not cause contamination of the bed. In any event a final flush with pure argon and/or evacuation is usually desirable. The desorbed oxygen and nitrogen are removed from the adsorption section of the system through open valve 18B and line 50. This gas may be vented to the atmosphere or reintroduced into the system to recover the argon used as purge gas. This can be accomplished, for example, by introducing the desorbed gas stream into the plant feed air compressor located upstream of column D. During the course of the adsorption step, the adsorbed gas front in each layer of the adsorbent progresses toward the outlet end of the bed. Hhen the front in the nitrogen adsorbing bed or the oxygen adsorbing bed, whichever is used to determine the extent of the adsorption cycle, reaches a predetermined point 1n the bed, the first half of the cycle is terminated and the second half is begun. 24 8 8 0 4 During the second half of the adsorption cycle, bed B is put into adsorption service and bed A is regenerated. During this half of the cycle valves 16B, 18A, 20A and 22B are open and valves 16A, 18B, 20B and 22A are closed. Feed gas now enters the adsorption system through line 10, and passes through bed B through valve 16B, line 24B, line 26B, valve 22B and line 28. Meanwhile bed A is now being regenerated. During regeneration of bed A, the warm purge gas passes through bed A via line 48, valve 20A, valve 18A and line 50. When the adsorption front in bed B reaches the predetermined point in this bed the second half of the cycle is terminated and the cycle is repeated. The adsorption system of Fig. 1 can be operated independently of distillation column D by closing valves 8 and 40 and introducing the argon stream to be further purified in the adsorption system through line 52 by opening va'ive 54. By virtue of this feature, other argon streams such as the ones from a liquid storage station can be treated.in the adsorption system of Fig. 1. Fig. 2 illustrates a variation of the system illustrated in Fig. 1. With the exception of the modification of the column D reflux section, the system illustrated in Fig. 2 is identical to the system of Fig. 1. In the system of Fig. 2, a portion of the gaseous enriched-argon stream leaving distillation column D 1s diverted to condenser E through line 56. The gas is condensed as 1t passes through the condenser by means of a coolant, which enters condenser E through line 58 and leaves the condenser through line 60. Condensate leaving condenser E is returned to column D via line 62. In this embodiment, all of the high purity argon leaving the adsorption section of the system is sent to product storage as gas, liquid or both. It may sometimes be desirable to produce a product gas containing a mixture of argon and oxygen. Such gas mixtures are useful as shielding gases in welding operations. If this is desired, the adsorption system of the invention can be operated 1n a manner such that only nitrogen is adsorbed from the gas feed to the adsorption system. This can be accomplished by eliminating beds 14A and 14B and operating the adsorption I ■"uw i uu i yu ■ -12- 2'1 8 8 0 4 system with only beds 12A and 12B. Alternatively, if the adsorbent in layers 14A and 14B adsorb both oxygen and nitrogen, but preferentially adsorbs nitrogen, the system can be operated with all four layers intact, but the duration of the adsorption step will be increased to the extent that the nitrogen adsorption front enters layers 14A and 14B and reaches a desired point near the nonadsorbed gas exit end of these layers, at which time the adsorption step in the relevant beds will be terminated and the regeneration step initiated. A typical cycle for the adsorption process of the invention is given in Table I. TABLE I Typical Cycle Sequence for the Cryogenic TSA Process Step Time, hrs Pressurize Bed A, purify using Bed B 0.5 Purify using Bed A, vent Bed B to atmosphere 0.5 Purify using Bed A, regenerate Bed B with warm purge gas 4.0 Purify using Bed A, cool Bed B with cold purge gas 3.0 Pressurize Bed B, purify using Bed A 0.5 Purify using Bed B, vent Bed A to atmosphere 0.5 Purify using Bed B, regenerate Bed A with warm purge gas 4.0 Purify using Bed B, cool Bed A with cold purge gas 3.0 Total 16.0 hr *4 88 0 The invention is further exemplified by the following examples, in which parts, percentages and ratios are on a volume basis, unless otherwise indicated. EXAMPLE I Commercially available 5A zeolite, 4A zeolite, 13X zeolite, CaX zeolite and CMS were chromatographically evaluated for oxygen, nitrogen and argon separation. In these experiments, a 3.0 ft column of 1/8" diameter was packed with 60-80 mesh size adsorbent particles. The columns were regenerated at a temperature of 250° C. in a gas chromatograph (GC) equipped with a Thermal Conductivity Detector (TCD). The columns were then cooled first to ambient temperature and then to a temperature of 87° K. by placing them in a dewar containing liquid argon at one atmosphere pressure. Hydrogen at a flow rate of 50 cc/min was used as the carrier gas. A one ml sample containing M nitrogen, 1% oxygen and the balance argon was injected into the columns and the effluent from the column was analyzed using the TCD. Argon was the first to elute in all cases. For 5A, 13X, CaX and CMS, very good argon/nitrogen and good argon/oxygen separations were obtained. In the presence of nitrogen, argon/oxygen separation on 4A zeolite was not as good. The experiment was repeated using 4A zeolite and a gas stream which contained only argon and oxygen. Very good argon/oxygen separation was obtained for 4A sieve in this case. EXAMPLE II In order to determine the dynamic nitrogen adsorption capacity, a 2" vessel containing 930 gms of a calcium-exchanged zeolite (CaX) manufactured by UOP in the U.S.A. was regenerated with high purity argon at a temperature of 250° C. to remove residual moisture. The vessel was then Immersed 1n liquid argon at superatmospheric pressure to cool it to a temperature of -170° C. Argon containing 0.5% nitrogen was passed through the bed at a pressure of 10 pounds per square inch gauge (psig) and an average flow rate of 10.5 standard liters per min (SLPM - standard conditions refer to 70°F and 1 atmosphere). The nitrogen concentration Ahli.lL iiu. Qiiiiiaa» 2 4 8 Q in the adsorber vessel effluent was measured continuously using a Gow-Mac GC with a dissociation Ionization Detector (DID). The nitrogen detection limit for this instrument was 10 ppb. Nitrogen breakthrough (defined as the point when the nitrogen content in the bed effluent reached a concentration of 1 ppm) time was about 24 hours. Nitrogen adsorption capacity for the CaX sieve was about 16 weight %. This is considered to be a high nitrogen removal capacity. EXAMPLE III In this example the same conditions and sieve (CaX) as in Example II were used. Argon containing 0.5% nitrogen and 0.5% oxygen at a pressure of 10 psig was passed through the bed at a pressure of 10 psig and an average flow rate of 10.5 SLPM. The nitrogen concentration in the adsorber vessel effluent was measured continuously using a Gow-Mac GC with DID and the oxygen concentration in the effluent was measured using a Teledyne Liquid Cell Oxygen Analyzer. The oxygen detection limit for the Teledyne Analyzer was about 0.1 ppm. Nitrogen breakthrough time remained the same (about 24 hours). Nitrogen adsorption capacity was again about 16 weight %. However, the oxygen breakthrough (defined as the point when the oxygen content in the bed effluent reached a concentration of 1 ppm) time was about 20 minutes. After about 4 hours, the oxygen concentrations in the bed outlet and the feed were the same. This experiment indicates that for the CaX sieve, when both nitrogen and oxygen are present 1n the feed, nitrogen adsorption is virtually unaffected. However, oxygen removal capacity for the CaX sieve is very poor, indicating that a second sieve Is necessary for efficient oxygen removal. This experiment also Indicates that when removal only of nitrogen 1s desired from a gas stream containing argon, nitrogen and oxygen, 1t can be accomplished using CaX type sieve. jyinlui> Hi QliMPfU 24 88 0 In this example the same conditions as in Examples II and III were used. However, instead of CaX, a 5A sieve was used. Argon containing 0.5% nitrogen and 0.5% oxygen at a pressure of 10 psig was passed through the bed at an average flow rate of 10.5 SLPM. Very high nitrogen adsorption capacity (over 10 wt %) and long nitrogen breakthrough time were obtained. Oxygen breakthrough time was again very rapid (less than 25 minutes) and oxygen adsorption capacity was very poor (complete breakthrough in about 4.0 hours). The 5A sieve works very similar to the CaX sieve in this application and is a good candidate for nitrogen removal from argon but not very good for oxygen removal. EXAMPLE V The vessel used in Examples II to IV was filled with 930i gms of a commercially available 4A sodium A (NaA) zeolite and the adsorbent was regenerated with purified argon at a temperature of 250° C., thereby removing residual moisture contained in the adsorbent. Experiments were carried out using an argon feed containing 0.5% oxygen at a flow rate of 10.5 SLPM, a pressure of 10 psig and a temperature of -170° C. Oxygen breakthrough time was about six hours and the oxygen adsorption capacity was about 11 wt %. The sieve in this Example, 4A sieve, is a good candidate for oxygen removal from argon (when nitrogen is absent from the gas stream). EXAMPLE VI In this example the vessel, sieve material and operating conditions were the same as those used 1n Example V. The argon feed contained 0.5% nitrogen and 0.5% oxygen. Oxygen and nitrogen breakthroughs were monitored using Gow-Mac GC and the Teledyne Oxygen Analyzer. Oxygen and nitrogen breakthrough times were each less than 30 minutes. Also oxygen adsorption capacity was reduced to less than 1.5 wt compared to about 11 wt % for Example V. Nitrogen adsorption capacity was less than 1 wt %. This experiment indicates that 4A sieve 1s not very good for oxygen I -15- EXAMPLE IV 24 8 8 0 4 removal when nitrogen 1s present in the gas stream being treated. This experiment confirms the results observed in Japan Kokai 62/065,913 wherein a 4A type sieve was used at cryogenic conditions for oxygen and nitrogen removal from argon. In the presence of nitrogen, very low oxygen adsorption capacity (about 0.03%) was obtained. Examples II to VI show that nitrogen can be efficiently removed from argon by cryogenic adsorption using 5A or CaX type sieve and that the presence of oxygen in the gas stream has a very small effect on nitrogen removal. The examples further show that oxygen can also be efficiently removed from argon in the absence of nitrogen using a 4A type sieve. However, oxygen adsorption on 4A from an argon gas stream is very adversely effected by the presence of nitrogen in the feed gas being treated. The examples also illustrate that both nitrogen and oxygen can be removed from an argon stream containing nitrogen and oxygen as impurities by first removing nitrogen using CaX or 5A type sieve and then removing oxygen from the nitrogen-depleted stream using a 4A type sieve. Although the invention is described with reference to specific examples, the scope of the invention is not limited thereto. For example, impurities other than nitrogen and oxygen, such as hydrocarbons, can be removed from an argon gas stream by the process of the invention. Furthermore, nitrogen alone can be removed from a gas stream comprising argon, nitrogen and oxygen. The scope of the invention is limited only by the breadth of the appended claims. </div>

Claims

<div class="application article clearfix printTableText" id="claims"> 248804 ■UUUUUItfi. PATEIfro <rnu JterIrnt Nn_ niiwi77 ; -17- what we claim is: 1- A temperature swing adsorption process for purifying a gaseous argon stream which contains nitrogen and oxygen as impurities, comprising the step of passing said argon stream, at a temperature below substantially 150* K., first through a nitrogen-selective adsorbent and then through an oxygen-selective adsorbent.
2. The process of Claim 1 wherein said nitrogen-selective adsorbent is selected from calcium-exchanged type A zeolite, calcium-exchanged type X zeolite, 13X zeolite, a carbon molecular sieve and mixtures of these.
3. The process of Claim 1, wherein said oxygen-selective adsorbent is selected from a carbon molecular sieve, 4A type zeolite and mixtures of these.
4. The process of Claim 1 further comprising regenerating said adsorbent(s) at a temperature in the range of substantially-20 to 250*C.
5. The process of Claim 1, wherein said argon stream contains up to substantially 1% nitrogen.
6. The process of Claim 1, wherein said argon stream contains up to substantially 5% oxygen.
7. The process of Claim 1, wherein said adsorption process 1s carried out at an absolute pressure in the range of substantially 1.0 to 20 atmospheres. «*\ i jl T7rrnrr Jlniiilipfr 2TW54 -18-
8. The process of Claim 1 carried out in a plurality of adsorption beds operated out of phase with one another such that at least one bed is undergoing adsorption while at least one other bed is undergoing regeneration.
9. A temperature swing adsorption process for producing an oxygen-containing argon product stream containing not more than substantially 5 ppm nitrogen from air comprising the steps of: (a) cryogenically distilling said air, thereby producing a gaseous argon-rich stream containing less than substantially 5% each of nitrogen and oxygen; and (b) adsorbing nitrogen from said gaseous argon-rich stream by passing said argon-enriched gaseous stream through a nitrogen-selective adsorbent at a temperature between the dew point of said gaseous argon-ri ch stream and substantially 150* K. and at an absolute pressure of 1 to 20 atmospheres.
10. A temperature swing adsorption process for producing an argon product stream containing not more than substantially 5 ppm each of nitrogen and oxygen from air comprising the steps of: (a) cryogenically distilling said air, thereby producing a gaseous argon-rich stream containing less than substantially 5% each of nitrogen and oxygen; and (b) adsorbing nitrogen and oxygen from said gaseous argon-rich stream by passing said stream, at a temperature between the dew point of said gaseous argon-rich stream and substantially 150* K and an absolute pressure of 1 to 20 atmospheres first through a nitrogen-selective adsorbent and then through an oxygen-selective adsorbent.
11. The process of Claim 9 or Claim 10, wherein the cryogenic distillation step is carried out at a temperature in the range of substantially 90 to 125° K.
12. The process of Claim 9 or Claim 10, wherein said nitrogen-selective adsorbent is selected from calcium-exchanged type A zeolite, calcium-exchanged type X zeolite, 13X zeolite, acarbon mixtures of these. fprrrrrt -19- 248 8 0
13. The process of Claim 10, wherein said oxygen-selective adsorbent 1s selected from a carbon molecular sieve, 4A type zeolite and mixtures of these.
14. A process according to claim 1, 9 or 10, substantially as herein described or exemplified. THE BOC GROUP, INC By Their Attorneys </div>