EP2307836A1

EP2307836A1 - Apparatus and method for purifying a gas

Info

Publication number: EP2307836A1
Application number: EP08766755A
Authority: EP
Inventors: Johann Carl Rudolf Van Der Hart
Original assignee: Jcr Van Der Hart Holding Bv
Current assignee: Jcr Van Der Hart Holding Bv
Priority date: 2008-05-30
Filing date: 2008-05-30
Publication date: 2011-04-13
Also published as: WO2009145611A1

Abstract

The invention relates to a method and an apparatus for purifying a xenon-comprising gas, comprising a cryostat (1) having a cryostat inlet (4) and a cryostat outlet (5), said cryostat further having an internal vessel (2) for holding said gas, a cooling jacket (6), at least partially surrounding said internal vessel and having a cooling medium inlet (7) an a cooling medium outlet (7') for allowing a cooling medium to be introduced into said cooling jacket for cooling said internal vessel for cooling said gas, and heating elements reaching into said internal vessel of said cryostat, an overflow vessel which is coupled via a valve to the upper part of said internal vessel of said cryostat, for allowing said overflow vessel to be in fluid communication with the upper part of the cryostat volume, and a vacuum pump which is operationally coupled to said overflow vessel for reducing the pressure in said overflow vessel.

Description

Apparatus and method for purifying a gas

Background

The present invention relates to a method and apparatus for purifying a gas.

In the past, many ways of purifying a gas have been proposed. In particular, many ways of purifying a gas which is relatively rich in xenon or krypton have been proposed. Xenon, krypton and neon are noble gasses which are used in many products and processes. For instance, these gasses are for instance used in laser applications, i.e. excimer lasers, rocket thrusters, as a filler gas in light bulbs, and in semiconductor fabrication processes. Recently, it is proposed to administer xenon gas to patients during surgical procedures. The recovery and purification of gas mixtures comprising these noble gasses are in particular desirable, as the costs of these gasses are high.

Known methods relate to extracting xenon from ambient air. Many of the known purifying processes start from a relatively pure xenon of at least about 4.7 quality (99.997% xenon). None of the known purifying methods seems robust enough to be able to deal with a wide range of impure xenon.

WO-87/06684 discloses a method for krypton separation. In that method, two streams of liquid oxygen are produced, one of which being richer in krypton and xenon than the other one. The relatively enriched stream is subjected to further steps for enriching.

US-7,368,000 discloses a method for recovering xenon or krypton from a gas mixture. In this method, the gas mixture is conveyed through a gas chromatography column.

Other known methods use cryogenic distillation. In that process, a gas is cooled and at selected temperatures condensation products are recovered. FR-2 848 650, WO- 2005/031168 and WO-2005/045530 seem to refer to such a process. US-2007/0033968 also discloses a method for obtaining krypton or xenon, in which cryogenic distillation is used. Additional halogen compounds are removed by using TiO2 and ZrO2 catalyst beds.

Summary of the Invention

The invention aims to provide a robust method for purifying a gas, in particular a method suited for obtaining purified xenon gas.

According to an aspect of the invention this is realized with a method for purifying a xenon-comprising gas for obtaining purified xenon, in which the xenon-comprising gas is introduced into a cryostat, and said xenon-comprising gas is subjected to cycles comprising the steps of cooling the xenon-comprising gas below a temperature required to make the xenon gas liquid in order to obtain a fluid having a liquid phase and a gas phase, removing the gas phase from the cryostat in a time which is short in comparison to the evaporation or sublimation time of the liquid phase, and heating the remaining liquid phase until the largest part of the xenon-comprising gas is in its gas phase.

The invention further provides an apparatus for purifying a gas, comprising a cryostat having a cryostat inlet and a cryostat outlet, said cryostat further having an internal vessel for holding said gas, a cooling jacket, at least partially surrounding said internal vessel and having a cooling medium inlet an a cooling medium outlet for allowing a cooling medium to be introduced into said cooling jacket for cooling said internal vessel for cooling said gas, and heating elements reaching into said internal vessel of said cryostat, an overflow vessel which is coupled via a valve to the upper part of said internal vessel of said cryostat, for allowing said overflow vessel to be in fluid communication with the upper part of the cryostat volume, and a vacuum pump which is operationally coupled to said overflow vessel for reducing the pressure in said overflow vessel.

The use of the particular steps provide a method which is robust. It allows a flow of highly contaminated xenon gas to be purified. The gas can even be purified up to a level of 4.0, and even levels of up to 6.0 and higher can be obtained. The method allows a continuous process for the purification of effluent gas from production processes. To that end, the method may require a buffer vessel for the storing xenon- comprising gas before applying the method of the invention. Economically, it is preferred if all the xenon-comprising gas is heated until it is in its gas phase. The buffer vessel can also be used for recycling gas which originates from the overflow vessel. This enhances the performance of the apparatus and method. It enables repeating the cycles of the method almost without loss of xenon gas.

In this patent, the word 'cryostat' is used for a vessel which has provisional for keeping its contents at a cryogenic temperature. In this context, a cryogenic temperature is a temperature below about -70 degrees Celsius (203K).

In an embodiment, the cryostat is filled with such an amount of xenon-comprising gas that said liquid phase fills less than about 90 % of the volume of the cryostat. Filling the cryostat further with liquid phase will reduce the efficiency of the method. For an efficient process, a filling of about 50-70 % liquid phase is efficient.

In an embodiment, said xenon-comprising gas is cooled to a temperature below about - 80 degrees Celsius (193K).

In an embodiment, said xenon-comprising gas is cooled to a temperature below about - 100 degrees Celsius (173K). In an embodiment, the xenon-comprising gas is cooled to a temperature below the temperature a which the xenon becomes liquid. In practise, this will result in a temperature below 165 K. In a further embodiment, it is preferred that solidification of the xenon-comprising gas is prevented. In fact, cooling too low below the boiling temperature of xenon, when purifying xenon, will make it necessary to use more energy to heat in the next step of a cycle. In fact, removing the gas phase will also lower the temperature of the remaining liquid phase. Thus, the temperature is above the solidification temperature, i.e. for xenon just above 161 K, just before the step of removing the gas phase.

In an embodiment, said xenon-comprising gas comprises at least about 50% xenon. In is possible to purify a gas which gas less than about 50 % xenon. In most cases, contaminated xenon will have a maximum of about 80% xenon. In an embodiment, it will be possible to purify gas which has at least about 20% xenon. It will be clear that it will also be possible to purify cleaner xenon. In fact, it may be possible to clean for instance 4.7 xenon (99.997% xenon). Further purification may require more cycles.

In an embodiment, in said heating step said liquid phase is heated to a temperature of at least about -90 degrees Celsius (183K), in an embodiment to a temperature of at least about -80 degrees Celsius (193K). In fact, it may be heated even further, but it has to be kept in mind that after heating, the gas may need to be cooled again (liquefied). The temperature should be high enough to turn largest part of the contents of the cryostat into a gas. This will make the process give the best energy effectiveness. Part of the contents, however, may in practise remain solid. Thus, heating to slightly above 165K (the boiling temperature of xenon) may be enough to run the process. However, this may require more time before cooling again, i.e. a slower process cycle.

In an embodiment, said gas phase is removed in about 1-10 seconds. In fact, as mentioned, it should be fast enough to prevent a substantial part of the liquid phase to evaporate. Fast removal may require low pressure of an overflow vessel. In may also require a large volume of the overflow vessel. It may also require a large cross section of a connection between cryostat and overflow vessel.

In an embodiment, the pressure of the xenon-comprising gas in the first cycle is introduced in the cryostat at a pressure of less than about 50 bar. Using a higher pressure may result in solid xenon. A high pressure reduces the filling time and may reduce required cooling

In an embodiment, the pressure of the gas phase of the xenon-comprising gas in the subsequent cycles in the cryostat is less than about 1 bar. This is the pressure before removing the gas atmosphere. In fact, when the amount of contamination is low, it may be much lower.

In an embodiment, said gas phase is removed by bringing the part of said cryostat holding said gas phase into contact with a volume which is at a lower pressure than said gas phase. In fact, using a volume which is at least 2 times the volume of the gas phase showed to be efficient. Furthermore, a pressure of below 10^Λ-3 mbar also provides fast removal of the gas atmosphere. Using a lower pressure may increase the removal speed. Furthermore, it may increase the amount of contamination which can be removed. In fact, the pressure may be increased at each cycle.

In an embodiment, said gas phase is removed by bringing the part of said cryostat holding said gas phase into contact with a volume which is below about 10^Λ-3 mbar. This will give an efficient removal. It may be considered at the first cycle to use a higher pressure, if the amount of contamination is very high.

In an embodiment, said gas phase is removed by bringing the part of said cryostat holding said gas phase into contact with a volume which is at least about 2 times the volume of said gas phase. As stated before, a volume of up to 10 times the volume of the gas phase may be used. The removed volume of gas may be returned into the process when starting with a next batch of gas in a first cycle. Usually, the gas removed during the first cycle may be thrown away as this will usually have a very large amount of contaminant.

In an embodiment, said xenon-comprising gas is introduced at a temperature of more than about -50 degrees Celsius. This prevents forming of solid xenon in the system.

In an embodiment, said method is substantially a batch process. However, in an embodiment, a quantity of gas is stored and batch- wise, portions of the gas are purified, thus making the method a continuous batch process. In the method, further intermediate storage buffers may be used.

In an embodiment, before said xenon-comprising gas is introduced into said cryostat, moisture and carbon dioxide are largely removed from said xenon-comprising gas. In fact, freezing out moisture and carbon dioxide are very efficient processes. These may be used for removing as much moisture and carbon dioxide as economically efficient before entering the cycles of the method. In an embodiment, said step of removing said gas phase is done in such a way that the temperature of said liquid phase is lowered not more than 40 degrees. As stated above, solid material in the cryostat may slow the process down.

In an embodiment, said xenon-comprising gas which is introduced into said cryostat has a feed purity level (FPL) and said purified xenon has a set purity level (SPL), and the difference between the FPL and SPL is used for setting the number of cycles. In fact, when measuring the FPL before entering gas in the method, and measuring again for instance during each cycle may optimize energy requirements. A controller may determine the number of cycles before the cycles start. It may also adjust the number of cycles during the process of purification.

In an embodiment, said xenon-comprising gas is cooled by contacting it indirectly with a cooling medium, in an embodiment liquid nitrogen. This has proven to be efficient.

In an embodiment, said steps are repeated at least four times. The process can in fact run very many cycles, depending on the required purity. Usually, about ten cycles will be sufficient.

In an embodiment, said removed gas phase is returned to a buffer of xenon-comprising gas if the contents of xenon in said gas phase is above a set level. This can be automatically done after the first cycle. In another embodiment, this is done after several cycles.

In an embodiment, the apparatus further comprises temperature sensors in said cryostat. This allows good process control.

In an embodiment, said temperature sensors are provided at various levels in said internal vessel in said cryostat. Thus, the process can be monitored sufficiently.

In an embodiment, the apparatus further comprises a flow sensor for measuring a mass flow of a gas into said cryostat. Thus, it can be monitored if the cryostat is filled sufficiently. Furthermore, it may prevent filling the cryostat too far. In an embodiment, the apparatus further comprises a controller, operationally coupled to said temperature sensors and to valves, for steering a cooling and heating process of the cryostat. During cooling, in an embodiment the temperature of the cooling fluid entering the cryostat and the temperature inside the cryostat are compared. Especially, as xenon is a good isolator, there will be a difference which decreases during cooling. For preventing solidification of xenon, it may be possible to increase the temperature of the cooling liquid if the difference between the temperature of the cooling fluid entering the cryostat and the temperature inside the cryostat is blow a set value.

In an embodiment, the apparatus further comprises a buffer vessel for storing a gas which needs to be purified, which buffer vessel is operationally coupled to said cryostat. This allows the batch process to be a continuous batch process.

In an embodiment, the apparatus further comprises a device for freezing out moisture and carbon dioxide from a gas, which device is operationally coupled to the inlet of said cryostat.

In an embodiment, the apparatus further comprises at least one molecular sieve, operationally coupled to the outlet of the cryostat. Using this device, it is possible to remove remaining traces of contaminants.

In an embodiment said molecular sieve comprises an A12O3 skeleton which has been calcinated to have an pore cross section adapted to block molecules of the gas which needs to be purified, in an embodiment in which said gas is xenon, said pore cross section is smaller than about 0.3 nanometre. For other gasses, other pores sizes may be selected. In an embodiment, a molecular sieve us used which has an Al-Si basis. In an embodiment, a 6 ring zeolite or an 8 ring zeolite may be used.

The invention further pertains to a cryostat, specifically designed for use in the method or the apparatus of the preceding claims. The invention further relates to a method for purifying a krypton-comprising gas for obtaining purified krypton. In such a method, it should be clear that the krypton- comprising gas should be cooled to a temperature at or below which krypton is mainly liquid.

The invention further relates to a method for purifying a neon-comprising gas for obtaining purified neon. In such a method, it should be clear that the neon-comprising gas should be cooled to a temperature at or below which neon is mainly liquid.

If the price for instance for argon increases, it may even be possible to apply this method to the purification of argon. In that case, the temperatures used should b adapted to the temperature at which argon liquefies.

The various aspects discussed in this patent can be combined in order to provide additional advantages. Furthermore, various aspects of this application may form the basis for one or more divisional applications.

Description of the Drawings

The invention will be further elucidated referring to an embodiment of a method and apparatus for purifying a gas shown in the attached drawings, showing in:

Fig. 1 a cross sectional view of an embodiment of a cryostat which can be used;

Fig. 2 a cross sectional view through the lid of the cryostat of figure 1; Fig. 3 a top view of the lid of figure 2;

Fig. 4 a view on the bottom of the lid of fig. 2;

Fig. 5 a process cycle of the method of the invention for purifying an effluent gas which is rich in xenon;

Figs. 6a-6c a process sheet showing the process cycles, cut in three parts; Figs. 7a and 7b a simplified system chart cut into two parts fitting together, showing the most important apparatus parts of the apparatus of the invention.

Detailed Description of an Embodiment In figure 1 , an example of a cryostat which can be used in the method and apparatus of the current invention is shown. The cryostat 1 has an internal vessel 2 for holding a gas (liquefied). The cryostat 1 is closed by a lid 3 which has an inlet 4 to the internal vessel 2 and an outlet 5 for extracting the purified gas from the internal vessel 2.

The internal vessel 2 is surrounded by a cooling jacket 6. Through this cooling jacket, a cooling medium can circulate around the internal vessel 2 in order to cool its contents. In an embodiment for purifying xenon, liquid nitrogen is used for cooling the contents of the internal vessel 2. The cooling jacket 6 has inlets 7 for the cooling medium, and outlets 7' for the cooling medium. In the drawings, these inlets 7 and outlets 7' are depicted at one side of the cryostat 1. In practice, however, they will be located at opposite positions of the cryostat to allow a proper flow of cooling medium. In this embodiment, the cooling jacket has been divided into three sections, indicated with numbers 8, 9 and 10. Using three sections 8, 9, 10, it is possible to better control the temperature in the cryostat 1.

The cryostat 1 further has an insulating jacket 11 which in this embodiment consists of an isolating jacket 11 surrounding the cooling jacket. The space between the wall of isolating jacket 11 and the cooling jacket 6 is usually put at a reduced pressure

('vacumated'). In the drawing, a connection for a pump can be seen at the bottom of the isolating jacket 11.

Fig. 2 shows a cross section through lid 3 of the cryostat 1. Lid 3 has a through hole 4 as inlet 4 for introducing gas into the cryostat 1. Lid 3 further has an outlet 5 formed by a through hole. Lid 3 further has means for cooling 13 and means for heating 12 the gas in the cryostat 1. In this embodiment, the lid 3 has a set of channels 13 in the plane of the lid through which a cooling medium/fluid can flow. The lid 3 also has channels 12 through which hot fingers can be inserted for heating.

Figure 3 shows a top view of lid 3. In this embodiment, the lid 3 has a central outlet 5 and an inlet 4. The cooling channels 13 are indicated with dotted lines. The cooling channels 13 are in this embodiment three sets of circular channels, each having its own inlet and outlet for cooling fluid.

The channels 12 for heating are three channels which form three sides of a triangle, also indicated with dotted lines.

In figure 4 the bottom of lid 3 is shown.

In figure 5, a temperature cycle of an embodiment of the method of the invention used for purifying xenon is shown. The temperature starts at an ambient temperature of 20 degrees Celsius (293 K). The cryostat in this embodiment is filled at a rate of 58 nl/min, during which the gas is cooled to a temperature of -112 degrees Celsius (161 K). Usually, the cryostat is filled up to 2/3 of its total volume with liquid gas. Thus, 1/3 of the volume is filled with a gas phase. After reaching this temperature, at which most of the xenon in the gas is in a liquid state, the gas atmosphere is rapidly removed, in this case in 5 seconds. At this speed, the xenon does not have the time to substantially evaporate.

Next, the liquid in the cryostat which now has a top layer or gas atmosphere which is at a low pressure, usually of between l*10^Λ-3 and 5 * 10^Λ-3 mbar, is heated to a temperature at which the liquid becomes gaseous again. For xenon, a temperature of about -80 degrees Celsius (193 K) proved sufficient. As soon as the contents of the cryostat becomes gaseous, all the contaminating compounds will be distributed evenly again. Heating will be done as quickly as possible. In this embodiment, it is done within one hour. In fact, in order to speed up the process as much as possible, heating is done as fast as possible. The construction of the cryostat usually is the limiting factor in this.

As soon as the contents is in gas phase again, it will be cooled back to liquid phase. To that end, liquid nitrogen will be circulated through the cooling jacket of the cryostat of fig. 1. In this case, if xenon is purified, this cooling process can be optimized using some special control. Xenon is a highly insolating gas. Thus, in this optimized embodiment, using temperature sensors measuring the inflowing cooling fluid and temperature sensors measuring the temperature inside the cryostat are compared. If the temperature difference is too large, the temperature of the cooling fluid is allowed to raise a little to allow a better distribution of temperature.

The cycle of cooling, removal of the remaining gas phase, and heating is repeated several times. The gas can then be sampled in order to check is a desired degree of purity is already reached. If this is the case, the gas can be processed further using other purification techniques, or used as such.

Figures 6a-6c shows a process cycle which is used in this embodiment for the purification of xenon. The schedule is cut into three parts in order to be readable, and shows the different steps the controller will take in the process. Figs. 7a and 7b shows the system chart, cut into two parts for clarity reasons.

The apparatus in this embodiment uses three identical cryostats 1, 1 ' and 1 " in order to be failsafe and in order to provide sufficient production capacity. Some of the features of the cryostats 1, 1 ' and 1 " are already discussed above. The apparatus has an inlet for contaminated gas 15, in this case contaminated xenon gas. The contaminated feed gas is then passed though a device for removing a large fraction of the moisture and carbondioxide from the gas. In this embodiment, the gas is passed through a cooled body in order to freeze out these components. In this embodiment, this is done because these compounds can be removed easily, in particular if they are present in relatively large amounts. It is not required to use this pre-phase.

Next, the gas is stored in a storage vessel 17. This storage vessel 17 is used for transforming the batch process of the cryostat 1 into a continuous batch process. The volume of this storage vessel 17 is matched to the flow of feed gas and to the processing capacity of the cryostats 1, 1 ' and 1". From the storage vessel 17, batches of feed gas are fed to the cryostats 1, 1 ' and 1", via inlets 4, 4' and 4". During and after being introduced into the cryostat, the gas is cooled using a cooling fluid. In this embodiment, liquid nitrogen in introduced into the apparatus via cooling fluid inlet 23. The cooling fluid cools the gas via the cooling jacket of the cryostat and via the lid of the cryostat. In this embodiment, the cryostat further comprises a hollow spiral 25 running through the inner vessel. Through this hollow spiral 25, cooling fluid can be passed. In fact, the spiral 25 is designed in such a way that it can pass cooling fluid through its core down and through its wall up, and vise versa. In this way, it is possible to further control the temperature in the cryostat at different locations.

In this embodiment, each cryostat has its own overflow vessel 20, 20' and 20", which is coupled via valves 22, 22' and 22", respectively, to the upper part of the inner vessel of a cryostat. Each overflow vessel is coupled via a line to a vacuum pump 22. The exact nature of this type of pumps is known to a person skilled in the art. Using this vacuum system, the pressure in the overflow vessels is reduced. In an embodiment, the overflow vessel 17 is brought at a pressure below the pressure of the gas atmosphere which exists above the liquid gas in the cryostat. The pressure should be such that the gas atmosphere is removed before the liquid is evaporated. On the other hand, the volume of an overflow vessel 17 should be such that the gas atmosphere is removed quickly. In an embodiment, both volume and pressure will be chosen for fast removal. Thus, the pressure will be between about 10*10^Λ-3 and l*10^Λ-3 mbar. Furthermore, the volume will be selected to be at least 2 times the gas atmosphere volume. It is also conceivable to use one overflow vessel for all the cryostats.

The purified gas which is removed from the cryostats is then stored intermediately into an intermediate storage vessel 26. This can be done for further improving the continuous nature of the apparatus.

From the intermediate storage vessel 26, the gas can be further purified up to very high levels using techniques which are well known as such. For instance commercially available getters, molecular sieves, and other techniques. In this embodiment, the gas is fed to specially designed and constructed molecular sieves 40, 40' 40" and 40'". These molecular sieves are based upon an aluminium oxide (A12O3) skeleton. This skeleton was calcinated to obtain a pore size cross section of less than 0.3 nanometres, which is effective for adsorbing molecules below the kinetic diameter of xenon. Thus allowing other, smaller molecules to adsorbed. Next, the further purified xenon is stored in a storage vessel 50. For monitoring and controlling the process, the cryostat has various temperature sensors. In this embodiment, PTlOO temperature sensors are used. These sensors are operationally coupled to a PLC process controller. In fact, the cryostat has various temperature sensors, indicated with PT(number) and T(number). The temperature is measured at several levels in the cryostat. Furthermore, the temperature of the cooling fluid entering and leaving the cryostat can be measured. A hot finger is indicated with H7, for instance.

The process and apparatus described in this text are specifically designed for purifying xenon. The parameters can, however, be modified in order to make the process and method suited for purifying, for instance, krypton. To that end, krypton has a boiling point at 120 Kelvin and a melting point at 116 K, whereas xenon has its boiling point at 165 K and its melting point at 161 K. Thus, all the temperature mentioned for xenon should be lowered by 45 K in order to use the process for krypton. For Argon the boiling temperature of Argon should b kept in mind. Thus, temperatures should be lowered with about 78 K with respect to xenon. For Neon, the temperatures should be lowered with about 138 K, i.e. the boiling temperature of neon should be kept in mind.

It will be clear that the above description and drawings are included to illustrate some embodiments of the invention, and not to limit the scope of protection. Starting from this disclosure, many more embodiments will be evident to a skilled person which are within the scope of protection and the essence of this invention and which are obvious combinations of prior art techniques and the disclosure of this patent.

Claims

1. A method for purifying a xenon-comprising gas for obtaining purified xenon, in which the xenon-comprising gas is introduced into a cryostat, and said xenon- comprising gas is subjected to cycles comprising the steps of:

- cooling the xenon-comprising gas below a temperature required to make the xenon gas liquid in order to obtain a fluid having a liquid phase and a gas phase;

- removing the gas phase from the cryostat in a time which is short in comparison to the evaporation or sublimation time of the liquid phase, and

- heating the remaining liquid phase until the largest part of the xenon-comprising gas is in its gas phase.

2. The method according to claim 1, wherein the cryostat is filled with such an amount of xenon-comprising gas that said liquid phase fills less than about 90 % of the volume of the cryostat.

3. The method according to claims 1 or 2, wherein said xenon-comprising gas is cooled to a temperature below about -80 degrees Celsius (193K).

4. The method according to claim 3, wherein said xenon-comprising gas is cooled to a temperature below about -100 degrees Celsius (173K).

5. The method according to any one of the preceding claims, wherein said xenon- comprising gas comprises at least about 50% xenon, in an embodiment at least about 20% xenon.

6. The method according to any one of the preceding claims, wherein in said heating step said liquid phase is heated to a temperature of at least about -90 degrees Celsius (183K), in an embodiment to a temperature of at least about -80 degrees

Celsius (193K).

7. The method according to any one of the preceding claims, wherein said gas phase is removed in about 1-10 seconds.

8. The method according to any one of the preceding claims, wherein the pressure of the xenon-comprising gas in the first cycle is introduced in the cryostat at a pressure of less than about 50 bar.

9. The method according to claim 8, wherein the pressure of the gas phase of the xenon-comprising gas in the subsequent cycles in the cryostat at a pressure of less than about 1 bar.

10. The method according to any one of the preceding claims, wherein said gas phase is removed by bringing the part of said cryostat holding said gas phase into contact with a volume which is at a lower pressure than said gas phase.

11. The method according to any one of the preceding claims, wherein said gas phase is removed by bringing the part of said cryostat holding said gas phase into contact with a volume which is below about 10^Λ-3 mbar.

12. The method according to any one of the preceding claims, wherein said gas phase is removed by bringing the part of said cryostat holding said gas phase into contact with a volume which is at least about 2 times the volume of said gas phase.

13. The method according to any one of the preceding claims, wherein said xenon- comprising gas is introduced at a temperature of more than about -50 degrees

Celsius.

14. The method according to any one of the preceding claims, wherein said method is substantially a batch process.

15. The method according to any one of the preceding claims, wherein before said xenon-comprising gas is introduced into said cryostat, moisture and carbon dioxide are largely removed from said xenon-comprising gas.

16. The method according to any one of the preceding claims, wherein said step of removing said gas phase is done in such a way that the temperature of said liquid phase is lowered not more than 40 degrees.

17. The method according to any one of the preceding claims, wherein said xenon- comprising gas which is introduced into said cryostat has a feed purity level (FPL) and said purified xenon has a set purity level (SPL), and the difference between the FPL and SPL is used for setting the number of cycles.

18. The method according to any one of the preceding claims, wherein said xenon- comprising gas is cooled by contacting it indirectly with a cooling medium, in an embodiment liquid nitrogen.

19. The method according to any one of the preceding claims, wherein said steps are repeated at least four times.

20. The method according to any one of the preceding claims, wherein said removed gas phase is returned to a buffer of xenon-comprising gas if the contents of xenon in said gas phase is above a set level.

21. An apparatus for purifying a gas, comprising:

- a cryostat having a cryostat inlet and a cryostat outlet, said cryostat further having an internal vessel for holding said gas, a cooling jacket, at least partially surrounding said internal vessel and having a cooling medium inlet an a cooling medium outlet for allowing a cooling medium to be introduced into said cooling jacket for cooling said internal vessel for cooling said gas, and heating elements reaching into said internal vessel of said cryostat; - an overflow vessel which is coupled via a valve to the upper part of said internal vessel of said cryostat, for allowing said overflow vessel to be in fluid communication with the upper part of the cryostat volume;

- a vacuum pump which is operationally coupled to said overflow vessel for reducing the pressure in said overflow vessel.

22. The apparatus of claim 21 of 22, further comprising temperature sensors in said cryostat.

23. The apparatus of claim 23, wherein said temperature sensors are provided at various levels in said internal vessel in said cryostat.

24. The apparatus of any one of the preceding claims 21-24, further comprising a flow sensor for measuring a mass flow of a gas into said cryostat.

25. The apparatus of claims 22-24, further comprising a controller, operationally coupled to said temperature sensors and to valves, for steering a cooling and heating process of the cryostat.

26. The apparatus of any one of the preceding claims 21-25, further comprising a buffer vessel for storing a gas which needs to be purified, which buffer vessel is operationally coupled to said cryostat.

27. The apparatus of any one of the preceding claims 21-26, further comprising a device for freezing out moisture and carbon dioxide from a gas, which device is operationally coupled to the inlet of said cryostat.

28. The apparatus of any of the preceding claims 21-27, further comprising at least one molecular sieve, operationally coupled to the outlet of the cryostat.

29. The apparatus of claim 28, wherein said molecular sieve comprises an A12O3 skeleton which has been calcinated to have an pore cross section adapted to block molecules of the gas which needs to be purified, in an embodiment in which said gas is xenon, said pore cross section is smaller than about 0.3 nanometre.

30. A cryostat, specifically designed for use in the method or the apparatus of the preceding claims.

31. Apparatus comprising one or more of the characterising features described in the description and/or shown in the attached drawings.

32. Method comprising one or more of the characterising features described in the description and/or shown in the attached drawings.

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