Method and apparatus for recovering xenon or a mixture of krypton and xenon from air

Abstract: The concentration of xenon or a mixture of xenon and krypton in a liquid oxygen stream (103) containing, in addition to xenon and or krypton and xenon, other trace impurities is increased by removal of carbon dioxide and nitrous oxide (104) prior to separation (113) of the stream into an oxygen enriched vapour (115) and a liquid (117) enriched in xenon or krypton and xenon mixture. Usually, said liquid (1170 enriched xenon or krypton and xenon mixture will be vaporized (119) for transportation (123) to a purification plant. The invention has particular application to small and medium sized air separation plants (101) for producing oxygen.

Description: The present invention pertains to economical recovery of xenon or mixtures of xenon and krypton from air processed in a cryogenic air separation plant. The average concentration of rare gases in atmospheric air is extremely small. For example, xenon is present in amounts of about 0.09 part per million (ppm) and krypton is present in amounts of about 1.1 ppm. In order to recover xenon and/or krypton from air it is necessary to process large volumes of air. To build a facility to produce only rare gases from air would not be economical utilizing current technology. In practice a small stream more concentrated in xenon and/or mixtures of krypton and xenon is usually withdrawn from an oxygen plant for further treatment. Due to the fact that the volatility of krypton and xenon is lower than the volatility of oxygen, the stream is usually in a form of a liquid oxygen purge. This purge stream is then further concentrated by stripping some of the oxygen in the distillation column to produce a raw xenon or krypton and xenon stream. Because the raw stream contains other non-volatile components, there are several factors limiting the maximum degree of concentration of xenon in the raw stream. These include, among others, solubility of carbon dioxide (CO 2 ), solubility of nitrous oxide (N 2 O) and the Lower Explosion Limit (LEL) of hydrocarbons present in the raw stream. The raw stream is then subjected to a series of operations in order to purify the xenon or a krypton-xenon mixture completely by vaporizing the stream, treating the stream to remove hydrocarbons (usually by chemical reaction), removing carbon dioxide, N 2 O and water (usually by adsorption) and cooling the stream to cryogenic temperature, e.g. -290°F (-180°C), for final distillation. Due to the cost of the facility to accomplish the large number of process steps that are necessary to purify xenon or a krypton-xenon mixture, xenon recovery from small and medium oxygen plants, (e.g. up to 1000 tons (900 tonnes) per day) is not economically attractive. On the other hand, the number of small and medium oxygen plants that are either existing or are in the process of being, or are recently, built is relatively high, with potentially large amounts of xenon and/or krypton and xenon that are not presently being recovered. Therefore, it is the primary objective of the present invention to provide an economically attractive way to recover xenon and/or krypton and xenon from existing oxygen plants. There is no disclosure in the prior art concerning the issues of economics of producing xenon and/or krypton-xenon mixtures as a function of the size of an air separation plant. In all of the prior art related to xenon or krypton-xenon mixture recovery, it is assumed that a recovery and purification system has to be built. The prior art describe only technical details and possible advantages of various recovery systems. US-A-3,191,393 describes a krypton/xenon separation and process consisting of an initial (raw) distillation column, a catalytic reactor, carbon dioxide separator and dryer, a batch distillation device and the necessary heat exchangers. A similar process, with an additional distillation column for rejection of methane, is disclosed in US-A-4,421,536. The characterizing feature of US-A-4,421,536 is that a liquid oxygen feed is rectified and concentrated in a first concentrating column to produce concentrated liquid having a relatively low concentration of methane. The concentrated liquid is stripped of methane by contact with oxygen gas in a methane purging column to reduce the methane concentration, the resultant methane-purged liquid is then vaporized and subjected to catalytic combustion to generate water and carbon dioxide. The water and carbon dioxide is adsorbed from the catalytic combustion product and thereafter a mixture of krypton and xenon is separated from the purified gas in a second concentrating column. US-A-3,596,471 discloses a process for recovering a mixture of krypton and xenon from air with an argon stripper. Other parts of the process include hydrocarbon reactor, a CO 2 separator and dryer, and a continuous distillation column for final purification. US-A-3,609,983 discloses a krypton-xenon recovery system using a two-stage distillation process, hydrocarbon contaminant removal by adsorption and catalytic combustion with the resultant water and carbon dioxide being frozen out in heat exchangers. US-A-4,384,867 describes a more complex process for recovery of krypton and xenon, where, in addition to krypton and xenon, a liquid oxygen stream is produced and an argon recycle stream is used to provide the necessary heat for rectification. US-A-4,401,448 and US-A-5,067,976 disclose air separation processes for the production of krypton and xenon where the raw mixture from the first distillation column is further concentrated using a mixing column with a feed that also contains nitrogen. Therefore, the rare gases (together with hydrocarbons) are concentrated safely in a nitrogen environment, instead of oxygen. US-A-3,751,934; US-A-3,768,270; US-A-3,779,028; US-A-4,586,528; US-A-4,647,229; US-A-5,122,173; US-A-5,309,719; and US-A-5,313,802 disclose various methods for removing hydrocarbons so they will not concentrate in to great of quantity with krypton and xenon in the bottom of the raw column. Concentration control is realized by reducing the reflux ratio in the raw distillation column by replacing the single feed to the column with various combinations of multiple feeds and/or bypasses. This permits most of the methane to be stripped and leave the raw column with the top vapour while krypton and xenon are retained in the bottom product. Also hydrocarbon adsorbers are discussed for removal of heavier hydrocarbons. For example, US-A-3,768,270 discloses the production of a krypton/xenon concentrate by passing oxygen vapour from an air separation unit into a rectification column having an upper section and a lower section with the ratio of the number of trays in the upper section to the number of trays in the lower section being in the range 6 to 12.. A minor proportion of the oxygen vapour is provided by vaporization of a small liquid oxygen fraction, containing most of the impurities, withdrawn from the air separation unit. As further exemplification, US-A-5,122,173 discloses a process in which the krypton and xenon content of, and methane rejection from, a liquid oxygen stream are simultaneously maximized by operating a krypton/xenon cryogenic distillation column such that the ratio of liquid to vapour in an oxygen enriching section of the column is in the range 0.05 to 0.2. The liquid oxygen feed to the column has been withdrawn from the main distillation column system of an air separation unit and passed through a hydrocarbon adsorber to remove C 2+ hydrocarbons and nitrous oxide. None of the prior art describes an economical process for recovery xenon and/or mixtures of krypton and xenon from small and medium size oxygen plants. The present invention pertains to a method and apparatus for recovering xenon or a mixture of krypton and xenon from air by removing at least one oxygen-enriched stream from an air separation plant, the oxygen stream containing, in addition to krypton and xenon, carbon dioxide, nitrous oxide, and hydrocarbons, removing the carbon dioxide and nitrous oxide from the stream and thereafter concentrating the xenon or a mixture of krypton and xenon to produce an oxygen-enriched vapour stream and a xenon or krypton-xenon enriched liquid stream, vaporizing the liquid to produce a vapour enriched in xenon or a krypton-xenon mixture, collecting the enriched vapour and transporting the enriched vapour to a central purification facility for final treatment. In its method aspect, the present invention provides a method for recovering one of xenon or a mixture of krypton and xenon in a feed stream of liquid oxygen from a cryogenic air separation plant producing at most 1000 tons (900 tonnes) oxygen per day and said feed containing, in addition to one of xenon or a mixture of krypton and xenon, trace amounts of carbon dioxide, nitrous oxide, and hydrocarbons comprising the steps of: treating said liquid oxygen feed stream to remove completely carbon dioxide and nitrous oxide therefrom; subjecting said liquid oxygen stream, after carbon dioxide and nitrous oxide removal, to a further processing step to produce an oxygen enriched vapour stream and a liquid stream enriched in one of xenon or a mixture of krypton and xenon, and containing methane and C 2+ hydrocarbons; vaporizing said liquid stream enriched in one of xenon or a mixture of krypton and xenon; and transporting the vaporized stream to a central purification facility for processing into a commercial product. In its apparatus aspect, the invention provides a system for recovering, by a method of the invention, one of xenon or a mixture of krypton and xenon from air, comprising in combination: a cryogenic air separation plant (101) providing a liquid oxygen feed stream producing at most 900 tonnes 1000 tons (900 tonnes) oxygen per day; carbon dioxide and nitrous oxide removal means for treating said liquid oxygen feed stream to remove completely carbon dioxide and nitrous oxide therefrom; separation means to separate a liquid stream from said carbon dioxide and nitrous oxide removal means into an oxygen-enriched vapour stream and a liquid stream enriched in xenon, or a mixture of krypton and xenon, and containing hydrocarbons; conduit means for feeding said feed stream from said carbon dioxide and nitrous oxide removal means to said separation means; conduit means to withdraw said liquid stream from said separation means for further processing; vaporization means for vaporizing said liquid stream enriched in one of xenon or a mixture of krypton and xenon and transport means to collect said vaporized liquid for transport to a processing facility. The following is a description by way of illustration and with reference to the drawing of a presently preferred embodiment of the invention. The single figure of the drawing is a schematic representation of the method and apparatus according to the preferred embodiment. Referring to the drawing, a preferred embodiment of the present invention is shown generally at 100. According to the present invention a liquid oxygen stream containing xenon or mixtures of krypton and xenon and other components, including but not limited to argon, nitrogen, carbon dioxide, nitrous oxide and hydrocarbons is withdrawn from that portion of a single or dual distillation column where there is greater than 95%, preferably greater than 99%, oxygen in the liquid, e.g. distillation column 101 of a conventional cryogenic air separation plant. Such plants are well known in the art and are disclosed, for example, in a classic double column built by Linde in 1910 and described extensively in cryogenic literature, for example in the book "The Separation of Gases" by M. Ruhemann, Oxford University Press, Second Edition, London 1949, page 158 or in the Encyclopaedia of Separation Technology, Douglas M. Ruthven-Editor, John Wiley & Sons, 1997, Vol. 1, under "Cryogenic Distillation", both references incorporated herein by reference. The liquid oxygen stream is conducted via line 103 to a carbon dioxide and nitrous oxide removal system 104. In a preferred embodiment the carbon dioxide and nitrous oxide removal system includes a pair of cryogenic adsorption devices 105 and 106. Cryogenic adsorption systems are available from Air Products and Chemicals Inc. of Allentown, Pennsylvania. The stream exiting the carbon dioxide and nitrous oxide removal section 104 is conducted via line 107 to a distillation column 113. The stream identified in line 107 can be divided into sub-streams shown as 108 and 111 which can be fed into different locations in the column 113. The division of stream 107 into 108 and 111 is done to adjust Liquid to Vapour (L/V) ratio in column 113. This allows for operation of column 113 in such a way that volatile hydrocarbons (methane) leave column 113 with the top vapour 115 (Liquid to Vapour ratio must be low enough). On the other hand if krypton is recovered, the L/V is high enough to prevent krypton from escaping with vapour 115. Column 113 contains mass transfer devices (such as trays or packing) corresponding to 5-10 theoretical stages. Column 113 results in an oxygen enriched vapour being withdrawn from the top of the column in line 115. A xenon or krypton and xenon enriched liquid is withdrawn from the bottom of column 113 via line 117 and passed through a heat exchanger 119 where it is vaporized to form a gas enriched in xenon or a krypton-xenon mixture and withdrawn in line 121. The vapour in line 121 can be then collected in gas cylinders or a tube trailer such as shown as 123 for transport to a central location to further process the vapour to concentrate and/or purify xenon or a mixture of krypton and xenon for commercial uses. Set forth in Table 1 is an example of a scheme according to the present invention utilized to recover xenon and krypton from a liquid oxygen stream in an oxygen plant used to produce 700 tons (630 tonnes) per day of oxygen product. From Table 1 it is apparent that the final stream identified as 121 is enriched in both krypton and xenon which can be collected for further processing to yield a commercial product. In the event that only Xenon is to be recovered the process and apparatus of the invention can be modified by replacing distillation column 113 with a partial vaporization device. Such devices are well known in the art. It is also within the scope of the present invention to use partial condensation as a means for recovering the rare gas fraction from the liquid oxygen stream 107, vaporized prior to the partial condensation. The most important benefit of the present invention is that it enables a user to recover xenon or a mixture of krypton and xenon from small and medium size oxygen plants in an economical manner. Because the carbon dioxide and nitrous oxide are removed upstream of the raw distillation column 113, krypton and xenon can be concentrated to a much higher degree than in conventional plants with the hydrocarbon contents still substantially below the Lower Explosion Limit (LEL). This enables transportation of the concentrate to be less expensive and the use of a central purification system to be economically attractive. On the other hand additional concentration of the xenon or a krypton-xenon mixture is not an important economic advantage when the mixture does not have to be transported, i.e. when the final purification plant is connected to the raw purification unit.

Claims: A method for recovering one of xenon or a mixture of krypton and xenon in a feed stream (103) of liquid oxygen from a cryogenic air separation plant (101) producing at most 900 tonnes (1000 tons) oxygen per day and containing, in addition to one of xenon or a mixture of krypton and xenon, trace amounts of carbon dioxide, nitrous oxide, and hydrocarbons comprising the steps of: treating (104) said liquid oxygen feed stream (103) to remove completely carbon dioxide and nitrous oxide therefrom; subjecting said liquid oxygen stream (107), after said carbon dioxide and nitrous oxide removal, to a further processing step (113) to produce an oxygen enriched vapour stream (115) and a liquid stream (117) enriched in one of xenon or a mixture of krypton and xenon, wherein said enriched liquid stream (117) contains methane and C 2+ hydrocarbons and is vaporized (119); and the vaporized stream is transported (123) to a central purification facility for processing into a commercial product. A method according to Claim 1, wherein said further processing step of the feed stream after carbon dioxide and nitrous oxide removal is by distillation (113). A method according to Claim 2, wherein xenon concentration is increased by maintaining the liquid-to-vapour ratio in the column (113) low enough that methane in removed in the oxygen-enriched vapour stream (115). A method according to Claim 1, wherein said further processing step of the feed stream after carbon dioxide and nitrous oxide removal is by partial evaporation. A method according to Claim 1, wherein said further processing step of the feed stream after carbon dioxide and nitrous oxide removal is vaporization and partial condensation. A method according to any one of the preceding claims, wherein the step of removing carbon dioxide and nitrous oxides is by cryogenic adsorption. A method according to any one of the preceding claims, wherein said liquid oxygen feed stream (103) contains greater than 95% oxygen. A method according to Claim 7, wherein said liquid oxygen feed (103) stream contains greater than 99% oxygen. A system for recovering, by a method as defined in Claim 1, one of xenon or a mixture of krypton and xenon from air, comprising in combination: a cryogenic air separation plant (101) producing at most 900 tonnes (1000 tons) oxygen per day and providing a liquid oxygen feed stream (103); impurity removal means (104) for treating said liquid oxygen feed stream to remove completely carbon dioxide and nitrous oxide therefrom; separation means (113) to separate a liquid stream from said impurity removal means (104) into an oxygen-enriched vapour stream and a liquid stream enriched in xenon or a mixture of krypton and xenon; conduit means (107, 108, 111) for feeding said feed stream from said impurity removal means (104) to said separation means (113); and conduit means (117) to withdraw said liquid stream from said separation means for further processing; vaporization means (119) to vaporize said withdrawn liquid enriched in xenon or a mixture of krypton and xenon and transport means (121, 123) to collect said vaporized liquid for transport to a processing facility. A system according to Claim 9, wherein said carbon dioxide and nitrous oxide removal means (104) comprises a cryogenic adsorption system (105, 106). A system according to Claim 9 or Claim 10, wherein said separation means (113) is a distillation column. A system according to Claim 9 or Claim 10, wherein said separation means (113) is partial evaporation means. A system according to Claim 9 or Claim 10, wherein said separation means (113) is vaporization and partial condensation means.

Inventors: FIDKOWSKI ZBIGNIEW TADEUSZ, SWEENY WILLIAM PAUL, FIDKOWSKI, ZBIGNIEW TADEUSZ, SWEENY, WILLIAM PAUL
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 2000
Main classification: F25J3
IPC codes: F25J3