Rare gases recovery process for triple column oxygen plant

The present invention is a process for recovering rare gases from a multiple column oxygen plant, wherein the multiple column oxygen plant comprises a higher pressure column, a lower pressure column, a middle pressure intermediate column, and a low pressure intermediate column, said middle pressure intermediate column comprising a first bottom reboiler and said low pressure intermediate column comprising a second bottom reboiler. The process includes providing a first oxygen rich liquid stream containing rare gases from the higher pressure column, wherein said first oxygen rich liquid stream is introduced to the first bottom reboiler. The process also includes removing a second oxygen rich liquid stream rich in rare gases from the bottom of the middle pressure intermediate column, wherein said second oxygen rich liquid stream is introduced to the low pressure intermediate column. The process also includes removing a first liquid purge stream concentrated in rare gases is removed from the low pressure intermediate column, wherein said first liquid purge stream is further concentrated downstream. And the process includes removing a third oxygen rich liquid stream lean in rare gases at a location that is at least one tray above the first bottom reboiler, wherein said third oxygen rich liquid stream is introduced to the lower pressure column.

BACKGROUND

In recent years the demand for rare gases, in particular Krypton and Xenon, is becoming very important. New applications and advances in electronics, medical, glass insulation etc. are greatly contributing to this high demand.

Krypton and Xenon are produced as the by-products of a cryogenic air separation plant. The basic recovery scheme is well known in the art. Since Kr and Xe are heavier than oxygen and will accumulate in liquid oxygen, the recovery technique usually calls for the refining of a liquid oxygen purge stream of the low pressure column of a double column cycle. The rare gases contained in the purge stream are further concentrated in a first concentrating column along with other heavy components in liquid oxygen such as hydrocarbons, CO2, nitrogen oxide etc. For safety considerations, the limit of this first concentrating operation corresponds to about 10% of the limit of flammability of hydrocarbons in oxygen. The first concentrated stream is then either treated in an on-site purification plant or transported to a central purification center where it is vaporized, heated and treated in a catalytic reactor at high temperature of about 500° C. to remove the hydrocarbons. This oxidation reaction forms CO2 and moisture. The mixture is then dried, its CO2 content is removed in an adsorber. The dried and CO2-free mixture is then cooled and distilled to yield the product which is usually a mixture of Kr and Xe. The product is then further refined to remove oxygen, argon and some other impurities such as CFC compounds, green house gases, remaining traces of hydrocarbons etc. and to yield pure Krypton and pure Xenon as final products.

Kr and Xe are present in very small concentration in atmospheric air (1.14 ppm Kr and 0.086 ppm Xe by volume). Therefore it is currently only economically viable to produce Kr—Xe in large oxygen plants, preferably above 1000 T/D and even larger.

If the purification portion of the process can be a standardized process to refine different types of first concentrated streams, either from an oxygen plant, nitrogen plant, low purity or high purity oxygen plant etc. then the same remark cannot be applied for the process involved to extract a stream containing Krypton and Xenon from the air separation columns. Indeed, because of the above-mentioned variety of air separation plants/processes, it is not possible to have one type of extraction process applicable for all types of air separation plants. For example, a plant producing gaseous oxygen product from the low pressure column would require a different type of rare gases extraction from a plant producing liquid oxygen product for pumping from the low pressure column.

Heavy industrial demand for oxygen for gasification, IGCC, GTL, oxyfuels have increased significantly the size of trains of oxygen plants. Because of the limitation of the size of distillation columns by transport regulations the technological trend in cryogenic process is shifting toward elevated air pressure plants wherein the feed air and the columns' pressure are at higher pressure than traditional oxygen plants. The triple column process is designed to address this type of application and there is a need to provide a technique for extracting rare gases from this type of process.

This triple column process is described in details in several patent such as U.S. Pat. No. 5,231,837, and U.S. Pat. No. 5,341,646.

The technique of recovering Krypton and Xenon from an oxygen plant have been covered extensively in several patents:

U.S. Pat. No. 6,776,004: this prior art taught the technique of recovering rare gases of a mixing column plant for oxygen production. The liquid purge of the low pressure column is treated in an enrichment column reboiled by the top gas of the mixing column to recover the rare gases.

PCT WO 2004/023054: air feeds to the high pressure column is separated into a nitrogen rich stream and 2 oxygen rich liquid streams: rare gases rich liquid and rare gases lean liquid. The rare gases-rich stream is treated in a column located above the crude argon column to yield a Krypton Xenon concentrate at the bottom.

U.S. Pat. No. 6,662,593: the rare gases in the feed air are confined in a rare gases rich liquid stream of the high pressure column and then its oxygen content is stripped in a side column to yield the rare gases concentrate stream. By extracting the rare gases prior to the final distillation in the low pressure column the oxygen product can be quite lean in rare gases and can then be pumped and vaporized to high pressure as final product without incurring losses of rare gases.

U.S. Pat. No. 6,612,129: Krypton and Xenon containing liquid from the high pressure column is partially evaporated in the top condenser of the side-arm argon column of the double column plant. The liquid purge and the vaporized streams of the condenser are then treated in an enrichment column to yield the Krypton Xenon concentrate at the bottom.

U.S. Pat. No. 6,220,054: a column is used to treat the bottom liquid of the crude argon column to yield final oxygen product which is depleted of Krypton and Xenon since the feed to the crude argon column is also depleted in Krypton and Xenon. A stream concentrated in Krypton and Xenon is extracted at the bottom of the low pressure column.

As can be seen, most of the prior art addressed the rare gases recovery for oxygen plant equipped with argon production for high purity oxygen and in some cases, mixing column. Those processes operate at relatively low pressure at about 1.5 to 2 bar in the low pressure column which would yield an air pressure of about 6 to 7.5 bar. Higher pressure than these values would deteriorate the distillation performance especially for the argon recovery. Elevated pressure plant in contrary produces low purity oxygen and operates at about 10 to 16 bar air pressure with the low pressure column operates at about 4 to 6 bar. In order to maintain a good oxygen recovery rate an intermediate column is used to generate more liquid nitrogen reflux from the top of the intermediate column.

SUMMARY

The present invention is a process for recovering rare gases from a multiple column oxygen plant, wherein the multiple column oxygen plant comprises a higher pressure column, a lower pressure column, a middle pressure intermediate column, and a low pressure intermediate column, said middle pressure intermediate column comprising a first bottom reboiler and said low pressure intermediate column comprising a second bottom reboiler. The process includes providing a first oxygen rich liquid stream containing rare gases from the higher pressure column, wherein said first oxygen rich liquid stream is introduced to the first bottom reboiler. The process also includes removing a second oxygen rich liquid stream rich in rare gases from the bottom of the middle pressure intermediate column, wherein said second oxygen rich liquid stream is introduced to the low pressure intermediate column. The process also includes removing a first liquid purge stream concentrated in rare gases is removed from the low pressure intermediate column, wherein said first liquid purge stream is further concentrated downstream. And the process includes removing a third oxygen rich liquid stream lean in rare gases at a location that is at least one tray above the first bottom reboiler, wherein said third oxygen rich liquid stream is introduced to the lower pressure column.

DESCRIPTION OF PREFERRED EMBODIMENTS

As illustrated inFIG. 1, elevated pressure air7at about 10 to 16 bar is fed to a high pressure column100to form a nitrogen rich gas at the top and an oxygen rich liquid10at the bottom. A liquid air stream8is fed to an intermediate tray location of column100. A liquid stream20with a composition close to liquid air is extracted from the liquid of the tray above the feed tray of liquid air stream8. Nitrogen rich gas is condensed to yield a first reflux40to the low pressure column200. The oxygen rich liquid10is then fed to the bottom reboiler of an intermediate column300wherein it is distilled to form a second nitrogen rich gas at the top and a second oxygen rich liquid31at the bottom. The second nitrogen rich gas is condensed to yield a second reflux44to the low pressure column200. Stream20is fed to column200or to both columns200and300. It can be seen that most of the Kr and Xe contained in the air feeds7and8of the high pressure column is collected in stream10. Column300operates at a pressure lower than column100's pressure but higher than column200's pressure. In order to balance out the system a third oxygen rich liquid32is extracted at a tray location above the bottom reboiler of column300. By adopting an adequate tray location and flow of stream32, it is possible to yield a stream32very lean in Kr and Xe and therefore almost all Kr and Xe of the feed stream10can be captured in stream31. Stream31is then fed to the bottom reboiler72of a column400, which is reboiled by condensing nitrogen from the top of the intermediate column. This column400contains about 5 to 15 theoretical trays and operates at about the same pressure as column200. A portion33of stream32is used as reflux for column400. A liquid purge50rich in Kr and Xe is then extracted at the bottom of column400for further concentrating operation.

In some plants low pressure air expander12expanding air feed into the low pressure column200is used. This expanded stream15also contained rare gases which would be lost if sent to the low pressure column200. In this case it is possible to send the expanded air15to the bottom of column400in order to wash out the contained rare gases and maintaining high recovery of Kr and Xe.

In the process without rare gases production, the bottom stream of the intermediate column300is normally divided into 2 portions: the first one is vaporized in the overhead condenser of the intermediate column, the second one is fed as liquid feed to the low pressure column200. If the same process is applied for rare gases production, the Kr—Xe contained in the second portion of bottom liquid feeding the low pressure column200would have been lost in the liquid oxygen product30. In order to remedy this situation a liquid stream32free of Kr—Xe is extracted at a tray located above the bottom reboiler to substitute this second portion of bottom liquid. By doing so the process efficiency is essentially unchanged, and the bottom stream31containing the rare gases can be isolated and treated, either in a column or a vaporizer, to recover the rare gases prior to sending it to the low pressure column200to produce oxygen. A fraction33of stream32is used to reflux the KrXe column400to further improve the recovery of rare gases.

In reference to the process described inFIG. 1, for a total feed air of 1000 containing 1.14 ppm Kr and 0.086 ppm Xe:

Stream1032313633KrXeFlow454150225254345ppm Kr2.490.264.90.040.26219ppm Xe0.190.000120.3800.0001217.2ppm: parts per million by volume

In this process simulation, stream32is extracted at 2 trays above the bottom reboiler. In another embodiment, stream32may be extracted at least one tray above the bottom reboiler. Range of composition of stream31:about 5.5 ppm to 3 ppm Krabout 0.5 ppm to 0.3 ppm Xe

Stream32has very low content of Kr and Xe, preferably a maximum at about 1.5 ppm of Kr and 0.01 ppm Xe. The rich liquid10is fed to the bottom of the intermediate column.

In another embodiment described inFIG. 2, the vaporized stream36from condenser72is treated in a short column401to recover the Kr and Xe carried over in stream36. Column401operates at about the same pressure as the low pressure column200. Column401is refluxed by a portion33of stream32. The reboil of column401can be supplied by heating the bottom reboiler75with any suitable stream such as air, nitrogen, oxygen rich liquid, liquid air etc. The liquid purge stream50of the top condenser can be optionally sent to the bottom of column401and the combined collected Kr and Xe is recovered is bottom stream53. Again, the expanded air stream (not shown), if existed, can be fed to the bottom of column401to recover its rare gases content.

It is also possible to just vaporize the bottom liquid31in the condenser72without the use of the column400or401as illustrated inFIG. 3. The Krypton recovery will be reduced significantly because of the carry-over of Kr and, at a lesser proportion, of Xenon in the vaporized stream36. This process is slightly simpler and can be used in cases when Krypton recovery does not need to be very high.

A Krypton recovery higher than 96% and a Xenon recovery higher than 99% in the liquid purge bottom are expected for this type of process as illustrated inFIGS. 1 and 2.