GB1605041A

GB1605041A - Trapping and reuse of radioactive xenon

Info

Publication number: GB1605041A
Application number: GB2845677A
Authority: GB
Original assignee: Novo Industri AS
Current assignee: Novo Nordisk AS
Priority date: 1978-05-26
Filing date: 1978-05-26
Publication date: 1981-12-16

Description

(54) IMPROVEMENTS IN OR RELATING TO TRAPPING AND REUSE OF RADIOACTIVE XENON (71) We, NOVO INDUSTRI A/S, a Danish Company, of Novo Allé, DK-2880 Bagsvaerd, Denmark, 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: This invention relates to a method suitable for recovering, from a mixture of gases containing radioactive xenon, a mixture of gases containing an increased concentration of the radioactive xenon.

The usefulness of radioactive xenon, for example 133we or 127we, for medical diagnostic purposes is well known. Examples of such applications of radioactive xenon are functional pulmonary studies and blood flow measurements.

In most applications of ratioactive xenon, a gas containing radioactive xenon is inhaled by a patient from a spirometer so as to allow the radioactive xenon to be absorbed in the respiratory system. However, almost all the activity absorbed in the body of the patient will be exhaled within five to ten minutes after the inhalation of xenon has ended.

Because of the radiation problems which the exhaled radioative xenon may present and the relatively high costs of radioactive xenon, it is desirable to recover xenon from the exhaled air so as to enable the radioactive xenon to be re-used.

Various trapping techniques have been suggested for recovering radioative xenon from gases exhaled during medical diagnostic studies. One of these techniques (Mantel, J. et al: Radioactive Krypton and Xenon, Radiology 90, March 1968, 590-591) is based on the use of liquid nitrogen in a cryogenic trapping system. In such a system, the radioactive xenon is frozen out of the exhaled air coming from a patient. In low temperature operations, condensable gases, such as water vapour and carbon dioxide, must be removed before refrigeration of the exhaled air, otherwise ice and other solids may give rise to acute problems. There is also an explosion hazard with refrigeration systems due to the possible accumulation of explosive materials, such as liquid ozone, which is formed by radiolysis of oxygen.

Furthermore, there is a danger of pressure build-up if refrigeration is lost.

Another suggested xenon trapping technique comprises the steps of passing the exhaled gas through a tube containing activated charcoal to collect the radioactive xenon therein and subsequently heating the charcoal to a temperature of about 100"C to remove the radioactive xenon from the charcoal (Liuzzi, A. et al, J. Nucl. Med. 13, 673-676 (1972)). Such a trapping technique allows the xenon to be trapped almost quantitatively and about 80% of the xenon contained in the trap can be recovered.

A further suggested trapping technique comprises passing exhaled xenon-contaimng air through a tube containing activated charcoal immersed in a dry ice-ethanol bath (-78"C). Following the collection of xenon in the charcoal, the trap is heated to about 20 C and steam is passed through the trap to desorb xenon from the charcoal.

It is an object of the present invention to provide an improved recovery method wherein the radioactive xenon is trapped almost quantitatively on a bed of activated charcoal and the xenon rapidly desorbed from the charcoal so as to enable the formation of a gas having a xenon concentration sufficiently high to allow the gas to be re-used.

According to the present invention there is provided a method suitable for recovering, from a mixture of gases containing radioactive xenon, a mixture of gases containing an increased concentration of radioactie xenon, which method comprises passing a xenon-containing gas through a bed of activated charcoal to adsorb the xenon therein, thereafter heating the charcoal bed containing the adsorbed xenon to a temperature within the range of from 200 to 400"C, passing a mixture free sweep gas through the bed when heated to said temperature to desorb the xenon from the charcoal bed and then collecting the xenoncontaining gas thus formed.

By effecting the release of adsorbed radioactive xenon at such high temperatures while passing a sweep gas through the bed of activated charcoal, the desorption is effected rapidly and almost quantitatively and consequently the released radioactive xenon can be contained in a sufficiently small volume of gas to allow it to be recycled and re-used.

An additional advantage obtained as a result of the heating of the activated charcoal is that the xenon-containing gas is sterilized and the charcoal regenerated.

Regeneration of the charcoal is important since water vapours, carbon dioxide etc.

absorbed therein will reduce its trapping efficiency.

Various types of activated charcoal can be used in the method of the present invention.

However, it has been found that optimum adsorption results are achieved by using activated charcoal which is commercially available under the trade names Picatif 210 and GX 180 (Pica, France). Examples of other commercially available types of activated charcoal are two products supplied by KEBO (Stockholm, Sweden), namely grade 0.5-1 mm and grade 2-3 mm, respectively. The charcoal is preferably packed in pipes (tubes), for example metal and glass pipes having internal diameters in the range of from 10 to 200 mm, preferably from 50 to 100 mm, most preferred 78 mm, and lengths ranging from 200 to 2000 mm, preferably 1000 mm. For the same trapping efficiency, a wider pipe needs more charcoal than a corresponding narrower one. The word "Picatif' is a Trade Mark.

In order to obtain a high trapping efficiency and consequently a high purification capacity of the charcoal used, it is important to have maximum charcoal density in the trap. Furthermore, it is preferred that the charcoal trap being mounted vertically.

The trapping efficiency depends on the temperature of the charcoal bed and the efficiency increases with decreasing temperatures. It is preferred to use a trap temperature in the range of from 0 to 30"C, most preferably at a out 5 to 10 C.

The xenon-containing gas is preferably passed through the charcoal bed at flow rates of about 10 to 15 1/min. for about 5 to 10 min. The decontaminated gas stream from the charcoal bed can be released directly into the room but, in case of leakage, it is safer to release the gas via a ventilation system.

After a predetermined time, the flow of gas to be decontaminated is stopped and the charcoal bed is gradually heated to a temperature in the range of from 200 to 400"C.

During the first part of the heating, for example within the temperature range of from 30 to 1000C or even a higher temperature such as 200 or 300"C, air which has been adsorbed during the adsorption step, is degassed. These gases, which will occupy only a few litres and contain a negligible amount of xenon (less than 1%), are allowed to escape into the room or the ventilation system in order to increase the concentration of xenon in the gas liberated from the trap. According to a preferred embodiment of the invention, it is not necessary to carry out any degassing phase.

In order to reduce the formation of carbon monoxide during the initial heating of the charcoal bed, nitrogen is preferably blown through the charcoal to remove gaseous oxygen prior to the heating.

When the temperature has been increased to at least 200 C, a sweep gas is blown through the charcoal to rapidly and efficiently remove xenon therefrom, the xenon being collected or recycled for re-use. A preferred sweep gas is nitrogen but also but also other inactive gases, such as carbon dioxide, may be used. The carbon dioxide can be removed from the xenon liberated from the trap by refrigeration. If carbon dioxide is used as a sweep gas, one must use a catalytic system in order to oxidize any carbon monoxide, if the xenon is to be reused for medical purposes.

There is a danger of carbon monoxide formation if oxygen is present. Therefore, it is preferable to use high purity sweep gases, for example high purity nitrogen, and a catalytic system, for example CuO, for oxidizing any carbon monoxide.

The flow rate of the sweep gas is preferably of the order of 10 1/min. The sweeping time depends on the temperature and mass of the charcoal bed. Thus, for a charcoal mass of 450 g, at a temperature of about 200"C, it is preferably about 1 minute, at a temperature of about 250"C, it is preferably about 30 seconds and, at a temperature of 300"C, it is preferably about 20 seconds.

The air exhaled by a patient is preferably fed to a reservoir balloon for a period of about 10 min. From the balloon, the air is sucked through a water absorption system, such as a silica gel, and subsequently through a carbon dioxide absorber, such as a soda lime absorber. After having passed through the water absorption system and the carbon dioxide absorber, the air is preferably pumped through the bed of activated charcoal. Thus, it is preferable to pass the air through the charcoal bed under a superatmospheric pressure instead of generating a subatmospheric pressure at the downstream side of the charcoal bed. When sucking the gas through the charcoal bed, flow channels tend to be formed therein.

Also, the decontaminated xenoncontaining gas is preferably passed through a water absorption system and eventually a carbon dioxide absorber to remove water vapour and carbon dioxide therefrom. The xenon-containing gas may be introduced into a steel tank which serves as a reservoir.

Oxygen is subsequently added in an amount sufficient to provide a concentration corresponding to that of normal air. It is thus possible to obtain a gas mixture suitable for medical diagnostic purposes, which gas mixtue may be directly passed to a spirometer.

For a better understanding of the invention and to show how the same may be put into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 schematically shows a clinical recovering and recycling system for carrying out a method in accordance with the present invention, Figure 2 shows a curve illustrating the release of radioactivity as a function of time from an injection of a xenon pulse in the system as shown in Figure 1, and Figure 3 shows a curve illustrating effluent activity as a function of elapsed time from injection of a xenon pulse (see Example 2 hereinafter).

The system shown in Figure 1 comprises a conduit 1 connecting a spirometer (not shown) with a mask 2 for a patient 3. The mask 2 is also connected with a buffer bag 4 via a conduit 5. The buffer bag 4 is connected with a charcoal trap 6 via a conduit 7 containing a silica gel bed 8, a soda lime absorber a pump 10 and a valve 11.

A nitrogen inlet conduit 12 containing a valve 13 is connected to the conduit 7. The charcoal trap 6, which comprises a pipe packed with activated charcoal, is provided with an electric heating unit in the form of a heating coil 14. The outlet end of the charcoal trap 6 is connected to a valve 15.

The valve 15 has two outlets, one being connected to a discharge conduit 16 containing a GM-Counter 17, the latter being connected with another conduit 18. A silica gel bed 19 and a soda lime absorber 20 are inserted in the conduit pipe 18 which is connectable to the above mentioned spirometer.

The operation of the system shown is as follows: When the patient 3 has inhaled xenoncontaining air from the spirometer for a few minutes, the inhalation is switched to normal air and the exhaled air is passed into the buffer bag 4. Most of the xenon introduced into the body of the patient during the inhalation of xenon-containing air will be lost within the first ten minutes of the inhalation of normal air and passes with the exhaled air into the buffer bag 4. From the buffer bag 4, the exhaled air containing the xenon thus liberated is sucked through the silica gel bed 8 which absorbs water vapour, and subsequently through the soda lime absorber which absorbs carbon dioxide. The exhaled air thus purified is subsequently pumped into the charcoal trap 6 in which the xenon is adsorbed so as to decontaminate the exhaled air (the valve 13 is closed).

The decontaminated air passes through the valve 15 and into the conduit 16 which may be connected to the atmosphere or to a ventilation system. The GM-Counter 17 serves to measure the xenon concentration and is preferably connected with a ratemeter and a strip-chart recorder.

When the charcoal has been saturated with xenon, the valve 11 is closed. In order to minimize the amount of carbon monoxide formed during the warm-up of the trap 6, nitrogen may be blown through the charcoal bed to remove gaseous oxygen prior to heating of the trap 6. The heating unit 14 is then switched on. During the warm-up of the trap 6, it is connected with the conduit 16 in order to remove the gas which is released during the warm-up phase. Since the released gas will only be a few litres and only contains a very negligible amount of xenon (less than 1%), the gas may be allowed to escape through the ventilation system.

When the temperature has been raised to a value in the range of from 200 to 4000C, preferably about 325"C, the valve 15 is switched so as to connect the trap 6 with the conduit 18. The valve 13 is then opened and nitrogen is blown through the charcoal bed 6. Only a relatively small amount of nitrogen is required to remove almost all (90-95So) of the absorbed xenon from the charcoal bed. The xenon thus removed is passed through the silica gel bed 19 to remove any water vapour and subsequently through the soda lime absorber 20 to remove any carbon dioxide. The xenoncontaining gas thus formed may be recycled to the spirometer and may be re-used therein.

The release of radioactivity from a charcoal bed consisting of a brass pipe having an inner diameter of 37 mm and a length of 1m and packed with charcoal (Picatif G 210, Pica, France) was investigated in the following manner: The buffer bag 4 of the system illustrated in Figure 1 was removed so as to allow atmospheric air to flow through the system.

The flow rate was 15.5 litres per minute. A pulse of 133we was introduced into the system through the nitrogen inlet valve 13.

The trap 6 was connected with the conduit 16 during the entire experiment. The amount of 133we released from the trap 6 was measured with the GM-Counter 17 which was connected with a ratemeter and a strip-chart recorder.

The effluent activity is shown in Figure 2 as a function of elapsed time from the injection of the xenon pulse.

The experiment was carried out at 20"C.

As will appear from Figure 2, only a very negligible amount of l33Xe is released within the first 10 minutes of operation (corresponding to 155 litres of air passing through the trap).

The Picatif charcoals referred to herein are produced from coconut shells.

The following Examples further illustrate the invention.

Example I A prototype unit corresponding to the system illustrated in Figure 1 was used.

A patient breathed about 10 litres of an air-xenon mixture containing approximately 5 mCi xenon per litre from a spirometer (mCi represents millicurie). When the spirometer had been emptied, the patient was allowed to breathe atmospheric air for a total time of 10 minutes. The air exhaled during this period was passed into the buffer bag 4 and subsequently pumped through the charcoal bed 6. The total amount of exhaled air was about 100 litres. During the trapping procedure, the valve 13 was crossed and the trap 6 was connected with the conduit 16.

The valve 11 was then closed and the heating unit 14 was turned on. When the temperature had been raised to 3250C, the valve 15 was switched on so as to connect the trap 6 with the conduit 18. About 5 litres of nitrogen were then blown through the charcoal bed 6. The desorbed gas was passed through the silica gel bed 19 and the soda lime absorber 20 and recycled to the spirometer. The amount of 13 Xe recycled was about 90% of the original amount.

It should be noted that the recycling efficiency depends on various factors but that in most cases it is between 80 and 95% or even higher.

Example 2 The charcoal trap shown in Figure 1 was replaced by another charcoal trap having a diameter of 78 mm, a length of 570 mm and a charcoal mass of 1.25 kg.

The buffer bag 4 of the system illustrated in Figure 1 was removed so as to allow atmospheric air to flow through the system.

The flow rate was 10 litres per min. A pulse of 133we was introduced into the system through the nitrogen inlet valve 13. The trap 6 was connected with the conduit 16 during the entire experiment. The amount of 133Xe released from the trap 6 was measured with the GM-Counter 17 which was connected to a rate meter and a strip-chart recorder.

The effluent activity is shown in Figure 3 as a function of elapsed time from the injection of the xenon pulse, CPS standing for counts per second.

The experiment was carried out at 120C.

As will appear from Figure 3, only a very negligible amount of the xenon is released within the first 45 minutes of operation, corresponding to 450 litres of air passing through the trap.

Example 3 The experimental set-up described in Example 2 was used for trapping 133we in the same way as described above. However, the air flow was interruped after a 45 minute run.

The valve 11 was then closed and the valve 13 opened, thus allowing nitrogen to flow through the charcoal trap in order to remove all gaseous oxygen prior to the heating of the unit. After about 20 litres of nitrogen flow, the valve 13 was closed. The valve 15 was then switched to the spirometer, that is to connect the trap with the conduit 18, and the heating unit was turned on. Thus, during warm up, the degassed gases were directed to the spirometer balloon. When the temperature inside the charcoal bed had reached 325"C, the heating was turned off. It could be observed that the degassed gases occupied a volume of approximately 10-15 lites. The valve 13 was then opened, thus allowing nitrogen to sweep through the charcoal and carry the xenon out to the spirometer. The total volume of nitrogen used was approximately 10 litres. It was then observed that the amount of xenon in the spirometer balloon corresponded to 90-98% of the amount of xenon injected in the trap.

Reference is directed to the article by Bolmsjo and Persson in "Physics in Medicine & Biology", Vol 23, 1978, pp 72 to 89.

WHAT WE CLAIM IS: 1. A method suitable for recovering, from a mixture of gases containing radioactive xenon, a mixture of gases containing an increased concentration of radioactive xenon, which method comprises the steps of passing xenon-containing gas through a bed of activated charcoal to adsorb the xenon therein, thereafter heating the charcoal bed to a temperature within the range of from 200 to 400"C, passing a moisture-free sweep gas through the bed when heated to said temperature to desorb xenon therefrom and then collecting the xenon-containing gas thus formed.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The flow rate was 15.5 litres per minute. A pulse of 133we was introduced into the system through the nitrogen inlet valve 13. The trap 6 was connected with the conduit 16 during the entire experiment. The amount of 133we released from the trap 6 was measured with the GM-Counter 17 which was connected with a ratemeter and a strip-chart recorder. The effluent activity is shown in Figure 2 as a function of elapsed time from the injection of the xenon pulse. The experiment was carried out at 20"C. As will appear from Figure 2, only a very negligible amount of l33Xe is released within the first 10 minutes of operation (corresponding to 155 litres of air passing through the trap). The Picatif charcoals referred to herein are produced from coconut shells. The following Examples further illustrate the invention. Example I A prototype unit corresponding to the system illustrated in Figure 1 was used. A patient breathed about 10 litres of an air-xenon mixture containing approximately 5 mCi xenon per litre from a spirometer (mCi represents millicurie). When the spirometer had been emptied, the patient was allowed to breathe atmospheric air for a total time of 10 minutes. The air exhaled during this period was passed into the buffer bag 4 and subsequently pumped through the charcoal bed 6. The total amount of exhaled air was about 100 litres. During the trapping procedure, the valve 13 was crossed and the trap 6 was connected with the conduit 16. The valve 11 was then closed and the heating unit 14 was turned on. When the temperature had been raised to 3250C, the valve 15 was switched on so as to connect the trap 6 with the conduit 18. About 5 litres of nitrogen were then blown through the charcoal bed 6. The desorbed gas was passed through the silica gel bed 19 and the soda lime absorber 20 and recycled to the spirometer. The amount of 13 Xe recycled was about 90% of the original amount. It should be noted that the recycling efficiency depends on various factors but that in most cases it is between 80 and 95% or even higher. Example 2 The charcoal trap shown in Figure 1 was replaced by another charcoal trap having a diameter of 78 mm, a length of 570 mm and a charcoal mass of 1.25 kg. The buffer bag 4 of the system illustrated in Figure 1 was removed so as to allow atmospheric air to flow through the system. The flow rate was 10 litres per min. A pulse of 133we was introduced into the system through the nitrogen inlet valve 13. The trap 6 was connected with the conduit 16 during the entire experiment. The amount of 133Xe released from the trap 6 was measured with the GM-Counter 17 which was connected to a rate meter and a strip-chart recorder. The effluent activity is shown in Figure 3 as a function of elapsed time from the injection of the xenon pulse, CPS standing for counts per second. The experiment was carried out at 120C. As will appear from Figure 3, only a very negligible amount of the xenon is released within the first 45 minutes of operation, corresponding to 450 litres of air passing through the trap. Example 3 The experimental set-up described in Example 2 was used for trapping 133we in the same way as described above. However, the air flow was interruped after a 45 minute run. The valve 11 was then closed and the valve 13 opened, thus allowing nitrogen to flow through the charcoal trap in order to remove all gaseous oxygen prior to the heating of the unit. After about 20 litres of nitrogen flow, the valve 13 was closed. The valve 15 was then switched to the spirometer, that is to connect the trap with the conduit 18, and the heating unit was turned on. Thus, during warm up, the degassed gases were directed to the spirometer balloon. When the temperature inside the charcoal bed had reached 325"C, the heating was turned off. It could be observed that the degassed gases occupied a volume of approximately 10-15 lites. The valve 13 was then opened, thus allowing nitrogen to sweep through the charcoal and carry the xenon out to the spirometer. The total volume of nitrogen used was approximately 10 litres. It was then observed that the amount of xenon in the spirometer balloon corresponded to 90-98% of the amount of xenon injected in the trap. Reference is directed to the article by Bolmsjo and Persson in "Physics in Medicine & Biology", Vol 23, 1978, pp 72 to 89. WHAT WE CLAIM IS:

1. A method suitable for recovering, from a mixture of gases containing radioactive xenon, a mixture of gases containing an increased concentration of radioactive xenon, which method comprises the steps of passing xenon-containing gas through a bed of activated charcoal to adsorb the xenon therein, thereafter heating the charcoal bed to a temperature within the range of from 200 to 400"C, passing a moisture-free sweep gas through the bed when heated to said temperature to desorb xenon therefrom and then collecting the xenon-containing gas thus formed.

2. A method according to Claim 1,

wherein the temperature of the charcoal bed is within the range of from 0 to 300C during the adsorption phase.

3. A method according to Claim 1 or 2, wherein the xenon-containing gas is passed through a silica gel bed to absorb water vapours and through a soda lime absorber to absorb carbon dioxide before it is passed through the charcoal bed.

4. A method according to any one of Claims 1 to 3, wherein the xenon-containing gas is introduced into the charcoal bed under super-atmospheric pressure.

5. A method according to any one of Claims 1 to 4, wherein the sweep gas is nitrogen.

6. A method according to any one of Claims 1 to 5, wherein nitrogen is passed through the charcoal bed prior to the heating of the bed.

7. A method according to any one of Claims 1 to 6, wherein the gas leaving the charcoal bed during warm-up of the charcoal bed is released into the surrounding atmosphere.

8. A method according to any one of Claims 1 to 7, wherein the charcoal bed is a pipe densely packed with charcoal.

9. A method according to any one of Claims 1 to 8, wherein the desorbed xenoncontaining gas is passed through a silica gel bed to absorb water vapour and a soda lime absorber to absorb carbon dioxide.

10. A method according to any one of Claims 1 to 8, wherein the activated charcoal is charcoal produced from coconut shells.

11. A method according to any one of Claims 1 to 10, wherein the charcoal bed is heated to a temperature within the range of from 300 to 350"C during the desorption phase.

12. A method according to Claim 11, wherein the temperature is 325"C.

13. A method suitable for recovering, from a mixture of gases containing radioactive xenon, a mixture of gases containing an increased concentration of radioactive xenon, substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

14. A method suitable for recovering, from a mixture of gases containing radioactive xenon, a mixture of gases containing an increased concentration of radioactive xenon, substantially as described in foregoing Example 1.

15. A method suitable for recovering, from a mixture of gases containing radioactive xenon, a mixture of gases containing an increased concentration of radioactive xenon, substantially as described in foregoing Example 3.

16. An apparatus suitable for recovering from a mixture of gases containing radioactive xenon, a mixture of gases containing an increased concentration of radioactive xenon, substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

17. An apparatus suitable for recovering, from a mixture of gases containing radioactive xenon, a mixture of gases containing an increased concentration of radioactive xenon, substantially as hereinbefore described with reference to Figure 1 of the accompanying drawing and foregoing Example 2.