DE19526226A1 - Ultra-pure xenon prodn. - Google Patents

Abstract

The prodn. of ultra-pure xenon from a contaminated xenon stream comprises passing it into a distn. column (1) whose head is cooled by an indirect heat exchanger (50) in which a liquefied coolant is evaporated. Also claimed is the appts. required for the process.

Description

The extraction of xenon from air separation plants keeps getting higher Purity requirements are placed on the end product. In contrast, are in the intake air more and more impurities such. B. C₂F₆, N₂O, SF₆, CF₄ and / or R115, which mainly accumulate in raw xenon and are difficult to remove. The invention relates to the purest xenon extraction from a raw xenon stream, the predominantly xenon and, for example, a few tenths of a percent of krypton and traces of impurities, for example of the type mentioned above.

So far, it has been common to use batch distillation (batch Distillation). To cool the head of the distillation column, solid becomes Carbon dioxide used in a carrier liquid. The temperature of the melting Carbon dioxide is no longer sufficient for the condensation of the top fraction of the column, if the volatile components are there at a relatively high concentration have enriched. This is not the known method for high xenon yields suitable. To a satisfactory heat transfer between the condensing To reach head gas and the carrier liquid-carbon dioxide mixture must also keep the liquid moving (e.g. by Stir). There are fluctuations in the heat transfer capacity and fluctuating return quantities and pressures in the column.

The invention has for its object a method for the purest xenon production to specify, where the return fluctuations in the distillation column largely be avoided.

This object is achieved in that the head of the distillation column by indirect Heat exchange is cooled with an evaporating first refrigerant.

By using a boiling refrigerant instead of a melting one good and even heat transfer at the top condenser of the distillation column secured. The chemical composition of the first refrigerant should be chosen in this way be that on the one hand its boiling temperature is low enough to the top gas of the Condense the column even at high concentrations of volatile components, and on the other hand it is ensured that the temperature at the top of the column is not below the melting point of xenon drops.  

These conditions are met when ethylene is used as the first refrigerant. With "ethylene" here is a mixture containing predominantly ethylene technically pure ethylene (with for example about 1 vol% impurities). The boiling temperature of ethylene is 182 K at a pressure of 2.0 bar Temperature difference of 8 K at the top capacitor can thus be pure xenon a pressure of 3.5 bar (corresponding to 190 K). The increases Concentration of impurities at the top of the distillation column can be caused by compensated for a drop in pressure on the evaporation side of the top condenser will.

For example, in raw xenon there are contaminations of 5000 vppm krypton contain, which are to be withdrawn with the residual gas at the top of the column, and if you only want to lose about 0.5% xenon, the head gas must be 50% Combine krypton and 50% xenon. Instead of increasing the Column pressure (16.9 bar would be necessary) in the invention Process the ethylene evaporation pressure to 1.0 bar corresponding to one Evaporation temperature of 169 K, can be reduced. That would make one Temperature difference of 8 K a condensation temperature of 50% krypton Set the xenon mixture to 177 K and thus a column pressure of 11 bar.

The first refrigerant can be routed within a primary circuit it is liquefied by indirect heat exchange with a second refrigerant. The second refrigerant preferably evaporates during this indirect heat exchange. The cold for the operation of the distillation column is therefore a second refrigerant delivered; the first refrigerant circulates as a coolant in the primary circuit. Frequently the purest xenon extraction is carried out in larger cryogenic plants, in which liquids with suitable evaporation temperature for use as second refrigerant are available.

Especially in connection with air separation plants, it is beneficial if a Fluid that contains one or more air gases is used as the second refrigerant. Vaporizing oxygen or nitrogen is preferably used. Here is too note that ethylene solidifies at a temperature of 103.4 K, i.e. the Boiling temperature of the liquid used to liquefy the ethylene is not may be lower than 104 K. This corresponds to liquid oxygen of 3.5 bar or liquid nitrogen of 10.2 bar. When operating with ethylene under 2 bar and liquid oxygen below 3.5 bar or liquid nitrogen below 10.2 bar results  on the second condenser-evaporator to re-liquefy the ethylene Temperature difference of 78 K. This large temperature difference results in a relatively small heating area. Even after considering the Leidenfrost effect, this leads for the construction of a compact, self-regulating device.

For this purpose, it is advantageous if the primary circuit is in flow connection with a Gas storage stands and in particular the indirect heat exchange between the first and the second refrigerant is carried out in a heat exchanger, the Liquefaction side is designed so that when a certain value is exceeded the amount of liquid present in the primary circuit is the heat transfer performance is greatly reduced. This makes it possible to go through a specific one Initial pressure in the ethylene cycle and by regulating the temperature at Vaporizing the second refrigerant to control the entire process.

The device according to the invention has a distillation column which is equipped with a crude Xenon feed line and with a first condenser-evaporator in Flow connection is established, the liquefaction side with the upper area of the Distillation column and its evaporation side with a liquid line for supply a first refrigerant is connected.

It preferably comprises a second condenser-evaporator, the Liquefaction side on the one hand via a gas line and on the other hand via the Liquid line with the evaporation side of the first condenser-evaporator connected is.

Especially when on the one hand in the second condenser-evaporator flammable and on the other hand an oxygen-containing refrigerant can be used (especially ethylene on the liquefaction and oxygen on the Evaporation side) it is beneficial if in the second condenser evaporator Evaporation and liquefaction side by a single workpiece from each other are separated. This can avoid that the two refrigerants through leaky seams (e.g. welds) between different Exchange workpieces into an explosive mixture. This will be best realized in that the evaporation and liquefaction side of one own container to be enclosed, the one shell and the only one Workpiece exists, whereby the shells of the two containers are not one another  are connected; there is therefore no connection to be sealed between Liquefaction and evaporation side of the second condenser evaporator.

The invention and further details of the invention are described below of an embodiment shown in the drawing.

A raw xenon feed line 2 is used to introduce raw xenon, which contains a few tenths of a percent of krypton and traces of contaminants such as e.g. B. C₂F₆, N₂O, SF₆, CF₄ and / or R115 contains, in a distillation column 1 , of which only a section is shown in the drawing. At the bottom of column 1 , liquid ultrapure xenon can be removed continuously or discontinuously via a product line 3 . An electrically operated heater serves to supply heat to the column sump. (Alternatively, other types of heating are also possible, for example by burning gas.) The upper section of the distillation column 1 is connected to the liquefaction chamber 51 of a first condenser-evaporator 50 , which is operated as a top condenser. The liquefaction space 51 has an essentially cylindrical shape and is surrounded by the essentially cylindrical evaporation space 52 of the first condenser-evaporator 50 .

A residual gas line is connected to the upper end of the liquefaction space 51 . The valve 7 in the residual gas line 5 also serves to adjust the pressure in the liquefaction space 51 and thus in the distillation column 1 . For this purpose, it can be controlled via a pressure measuring and regulating device 6 (PIC: pressure indication and control). In addition or as an alternative to a liquid level control in the bottom of the column, the same control device 6 can also be used to adjust the heating power, as is shown by the corresponding dashed connection in the drawing.

The evaporation chamber 52 is connected to a gas line in its upper region and to a liquid line 9 in its lower region and communicates via both with the liquefaction chamber 61 of a second condenser-evaporator 60 . The gas line 8 is connected to the evaporation space 52 of the first condenser-evaporator 50 at a point which is geodetically higher than the point of connection of the gas line 8 to the liquefaction space 61 of the second condenser-evaporator 60 . The latter is preferably arranged at the upper end of the liquefaction space 61 of the second condenser-evaporator.

The evaporation chamber 62 of the second condenser-evaporator 60 each has a line for the supply of liquid ( 63 ) and for the removal of steam ( 64 ). The level in the evaporation chamber 62 is regulated via the liquid supply (liquid level regulator 65 , LIC: liquid indication and control), and the pressure is removed via the gas extraction (pressure regulator 66 ). A safety drain 72 is used for occasional liquid removal at the lower end of the evaporation space 62 . The evaporation space 62 is enclosed by a container which is formed by a casing 67 and a heat transfer plate 68 . The container of the liquefaction space 61 is formed by a further shell 69 and also by the heat transfer plate 68 . The heat transfer plate 68 is made from a single workpiece. The two sleeves 67 , 69 are each connected to the heat transfer plate by an annular weld seam 70 , 71 , but not to one another.

The distillation column 1 and the two condenser evaporators 50 , 60 are arranged within a cold box, which is used for thermal insulation.

The gas line 8 is connected via a compensating line 10 to a gas storage 11 which has a relatively large filling volume, for example approximately 100 times the volume of the primary circuit (evaporation space 52 of the first condenser-evaporator 50 , liquefaction space 61 of the second condenser-evaporator 66 , gas line 8 and liquid line 9 ). The pressure in the gas storage can be adjusted by supplying additional quantities of the first refrigerant circulating in the primary circuit via line 12 .

In the exemplary embodiment, ethylene with a purity of approximately 99 vol% is used as the first refrigerant. In the second condenser-evaporator 60 , oxygen is evaporated as the second refrigerant.

To understand the operation of the process and the system is as follows their operation including starting from the warm state is described.

The amount of liquid ethylene in operation or the amount of gas of ethylene in the warm state are designed in the example so that in the warm state, that is to say at ambient temperature, the ethylene pressure is approximately 6.5 bar. When starting up, liquid oxygen is first filled into the evaporation space 62 of the second condenser-evaporator 60 ; the oxygen evaporates strongly. The pressure in the evaporation chamber 62 is kept constant at 3.5 bar. After the components have cooled accordingly, the liquefaction of the initially gaseous ethylene begins in the liquefaction chamber 61 of the second condenser-evaporator. Since the volume of the liquefied gas is much smaller than the volume in the gaseous state, the pressure in the primary circuit, in the compensating line and in the gas storage device slowly drops. This continues until the liquid level in the liquefaction space 61 has reached the heat transfer plate 68 . An ethylene pressure (pressure of the gas in the primary circuit) of 2 bar is established. The liquid ethylene communicates via the liquid line 9 with the heating surface in the first condenser-evaporator 50 .

After raw xenon has been fed into the distillation column 1 (preferably via branch line 2 a at the top of the column), this is liquefied in the liquefaction space 51 of the first condenser-evaporator 50 , runs down the column and collects in its sump. When enough liquid has accumulated in the sump, it is heated with constant power and the feed is switched from a line 2 to line 2 b, which ends at an intermediate point in the distillation column. 1 A pressure is initially set in the column which is so high that the sum of the amount of crude xenon drawn in and the amount of xenon evaporated in the sump is condensed in the first condenser-evaporator 50 . After some time, when the trace impurities have accumulated at the top of the column, the pressure in the column begins to rise so that the lower-boiling constituents can also be liquefied. At a target pressure previously determined on the basis of the amount of impurities in the xenon and the desired yield of ultrapure xenon, these impurities are regulated as residual gas with a fine control valve 7 .

If the impurities in the raw xenon are large, for example up to 1 vol% krypton, and you want to achieve a high concentration of the impurity components in the residual gas in order to lose as little xenon as possible with the residual gas, the ethylene pressure can be released from the primary circuit or from the Gas storage can be reduced to, for example, 1.0 bar. The temperature difference at the first condenser-evaporator 50 is thus initially increased. Over time, with increasing pressure, more and more pure xenon accumulates in the bottom of the column, which is discharged as a product continuously or from time to time via line 3 .

The device described can be used for small quantities of raw material to be processed. Xenon can also be operated completely discontinuously (batch operation). Doing so the raw xenon quantity to be processed is first condensed and in the sump  collected. The procedure is then as described above, only none continuous supply of raw xenon. Residual gas must also be removed here to draw off a pure product in the sump at the end of batch distillation can. Part of this residual gas is initially discarded; the bulk of the Residual gas can, however, be collected for recycling.

Claims (10)

1. A process for recovering high-purity xenon (3) containing impurities from a crude xenon stream (2), wherein the raw xenon current introduced (2) into a distillation column (1) (2a, 2b), whose head is cooled by indirect heat exchange ( 50 ) with an evaporating first refrigerant ( 9 ). 2. The method according to claim 1, in which ethylene is used as the first refrigerant ( 9 ). 3. The method according to any one of claims 1 or 2, wherein the first refrigerant is conducted in a primary circuit, within which it is liquefied ( 61 ) by indirect heat exchange ( 60 ) with a second refrigerant ( 63 ). 4. The method of claim 3, wherein the second refrigerant ( 63 ) evaporates ( 62 ) in the indirect heat exchange ( 60 ) with the first refrigerant. 5. The method according to claim 3 or 4, wherein oxygen or nitrogen is used as the second refrigerant ( 63 ). 6. The method according to any one of claims 3 to 5, wherein the primary circuit is in flow connection ( 10 ) with a gas reservoir ( 11 ). 7. The method according to any one of claims 3 to 6, wherein the indirect heat exchange between the first and second refrigerant is carried out in a heat exchanger ( 60 ), the liquefaction side ( 61 ) is designed such that when a certain value of the amount in the primary circuit is exceeded existing liquid the heat transfer performance is greatly reduced. 8. Apparatus for obtaining ultrapure xenon from a crude xenon stream containing impurities with a distillation column ( 1 ) which is in flow connection with a crude xenon feed line ( 2 ) and with a first condenser-evaporator ( 50 ). the liquefaction side ( 51 ) of which is connected to the upper region of the distillation column ( 1 ) and the evaporation side ( 52 ) of which is connected to a liquid line ( 9 ) for supplying a first refrigerant. 9. The device according to claim 8 with a second condenser-evaporator ( 60 ), the liquefaction side ( 61 ) on the one hand via a gas line ( 8 ) and on the other hand via the liquid line ( 9 ) with the evaporation side ( 52 ) of the first condenser-evaporator ( 50 ) connected is. 10. The device according to claim 9, wherein the evaporation and liquefaction side ( 62 , 61 ) in the second condenser-evaporator ( 60 ) are separated from one another by a single workpiece ( 68 ).