GB2608737A

GB2608737A - Improvements in or relating to capture and remanufacture of anaesthetic gas

Info

Publication number: GB2608737A
Application number: GB2214003.2A
Authority: GB
Inventors: Brown Sebastian
Original assignee: Sagetech Medical Equipment Ltd
Current assignee: Sagetech Medical Equipment Ltd
Priority date: 2018-05-29
Filing date: 2018-05-29
Publication date: 2023-01-11
Anticipated expiration: 2038-05-29
Also published as: GB2608737B; GB202214003D0

Abstract

A method of purifying xenon gas 3 by binding it to a filter material 17a, 17b, such as silica gel, zeolites, or a sliver of lithium doped aerogel, then extracting the xenon gas from the filter, such as by exposing the filter to supercritical CO2, and then purifying the xenon gas, such as with a vortex tube 14. The apparatus may be used to recycle anaesthetic gas

Description

IMPROVEMENTS IN OR RELATING TO CAPTURE AND REMANUFACTURE OF ANAESTHETIC GAS

Technical Field

The present invention relates to methods and systems for capturing and recycling xenon. In particular, the present invention relates to methods and systems for capturing and recycling xenon when used as an anaesthetic or neuroprotective agent in medical environments.

Background

Xenon is a noble gas element with uses in lasers, lighting and in medicine. In anaesthesia, xenon at concentrations of 72% in oxygen can deliver a depth of anaesthesia consistent with surgery. Xenon has been suggested to offer neuroprotective effects via inhibition of NMDA receptors and is used for neonates with birth-induced brain injury and potentially for patients following subarachnoid haemorrhage.

Xenon is a rare element, occurring at approximately I part per 11.5 million in air. The majority is produced as a by-product of the fractional distillation of air to form oxygen and nitrogen. However worldwide production is still very small when compared to the potential needs of anaesthesia. Therefore, significant interest exists for technology that is capable of reprocessing xenon in medical devices for anaesthesia.

Prior art focusses on the removal of xenon from oxygen during the cryogenic processing of air using selective absorbents and/or catalysis to remove hydrocarbon contaminants. Absorbents can be silica gel, zeolites, metal doped (e.g. Silver/Lithium) or more recently metal-organic frameworks. Once absorbed, the xenon can be removed by freezing out at cryogenic temperatures or by heating and evacuation with a gas (helium or nitrogen). These processes form part of the cryogenic separation process. Cryogenic processes require very significant capital infrastructure, only being economically viable in large scale and to produce multiple products (e.g. the separation of air). However, this capital-intensive technology is not suitable for the remanufacture of xenon from medical use.

Gas-chromatography has been proposed as a non-cryogenic purification methodology (CN I 02491 293B) to separate xenon from krypton. By this method, helium or nitrogen driving gas is used to pass xenon gas through a gas chromatography column. Due to the strong interaction of the xenon with the column stationary phase, the passage of the Xenon is retarded compared to krypton and other contaminants. The xenon can then be extracted from the driving gas following elution from the top of the column. Gas purification systems suffer from low production rates due to low density and the batch nature of chromatographic processes. Furthermore, purified products must be separated from the driving gas, which is often just as complicated as the original separation.

Limited recirculation measures are used in anaesthesia to preserve volatile anaesthetics. However, even these rebreathing systems (e.g. circle system) run at low-flow conditions of 0.5L/min fresh gas flow will only lead to 20% of the administered xenon being absorbed by the patient. Therefore, the entire world production of xenon would only be sufficient for 400,000 anaesthetics. 4 million anaesthetics are delivered each year in the UK alone. Therefore, technologies outside or in addition to rebreathing systems are required. Ideally these systems would be incorporated into the anaesthesia device as it is likely that even with very high-efficiency systems, xenon use would be restricted to certain patients/long cases. Currently, xenon is operated in an intensive care setting for long-term use. Therefore, it is likely that local recycling systems, either at the patient or within the hospital itself would be most economical.

Description

According to an aspect of the present invention there is provided a method for the extraction of xenon gas bound to a filter material using supercritical CO2 to form a mixture in which both CO2 and xenon are in a supercritical state.

The present invention also provides a method of recovering xenon anaesthetic agent from a filter, comprising the step of subjecting the filter to a supercritical fluid, thereby forming a supercritical solution.

The present invention also provides a method for extraction of xenon by supercritical CO2 by first capturing xenon from the exhaust of a medical device delivering xenon by binding it to a filter material that may include but is not limited to silica gel, zeolites, metal organic frameworks, or metal doped silica/zeolite.

The present invention also provides a method for capturing xenon from the exhaust of a medical device delivering xenon by binding it to a filter material formed of a silver or lithium doped aerogel.

The present invention also provides a method to capture xenon from exhausted anaesthetic gas, the method comprising processing gas containing xenon with filter material.

The method may further comprise the step of releasing xenon from the filter using a supercritical fluid.

Methods formed in accordance with the present invention may further comprise the steps of: passing gas derived from a patient in a medical environment through a filter so that xenon anaesthetic agent becomes bound thereto; subjecting the filter material to a supercritical fluid, thereby forming a supercritical solution; removing contaminants from the supercritical solution; collecting the xenon anaesthetic agent from the supercritical solution; and reintroducing the xenon anaesthetic agent to a patient.

The present invention also provides apparatus to or suitable to perform a method as described herein, comprising a module housing filter material and into which anaesthetic gas can pass so that xenon anaesthetic agent binds to the filter material, and a supercritical fluid source, the module being resistant to supercritical fluid and able to withstand supercritical pressure and temperature so as to enable captured xenon to be reclaimed by exposure to supercritical fluid.

The present invention also provides for the separation of xenon gas derived from a medical device and CO2 using a vortex tube.

The present invention also provides a method of producing medical grade xenon from contaminated xenon derived from the exhaust of a xenon delivery medical device by using liquid CO2 chromatography followed by separation of xenon from CO2 The present invention also provides a method in which liquid CO2 is used as the mobile phase for chromatographic purification of xenon from gaseous contaminants derived from the patient or breathing systems.

The purpose of some aspects and embodiments of this invention is to provide a method for high volume, high purity xenon recycling for medical devices by using carbon dioxide in liquid and supercritical phases for the extraction and purification and re-delivery of xenon to medical devices used in anaesthesia.

Apparatus formed in accordance with the present invention may comprise a chamber containing an absorbent which may include but is not limited to silica gel, zeolites, metal organic frameworks, or metal doped silica/zeolite, most preferably a metal (silver or lithium) doped aerogel is attached to the exhaust of the anaesthetic machine. The anaesthetic exhaust contains xenon at I-100%, most preferably containing clinically relevant concentrations such as 45% for hypnosis and 72% for anaesthesia usually in oxygen. This is contaminated by hydrocarbons and many other compounds present in exhaled breath (e.g. ethanol, acetone) and from the machine/gases (e.g. hydrocarbons, plasticisers). This absorbent selectively absorbs the xenon gas and some contaminants, but oxygen passes through. Preferably, binding of the xenon to the absorbent can be increased by pressurising and/or cooling the exhaust gases into the capture cylinder.

The present invention provides a method for extraction of xenon by supercritical CO2 by first capturing xenon from the exhaust of a medical device delivering xenon by binding it to a filter material that may include but is not limited to silica gel, zeolites, metal organic frameworks, or metal doped silica/zeolite.

The present invention provides a method for capturing xenon from the exhaust of a medical device delivering xenon by binding it to a filter material formed of a metal (silver or lithium) doped aerogel.

Some aspects and embodiments relate to the remanufacture of xenon gas for medical devices.

In an embodiment of the invention, the absorbent is then regenerated by passing supercritical CO2 through the chamber and absorbent. Carbon dioxide becomes a supercritical fluid above its critical temperature (31 degrees Celsius) and pressure (73 bar). At this critical point, carbon dioxide has the properties of both a gas and a liquid. It expands to fill the container it is in and dissolves non-polar compounds like a liquid. This is due to the rapid increase in density at the critical point. Liquid CO2 can also be used for the extraction of xenon at temperatures below the critical temperature.

Supercritical fluids dissolve each other perfectly. The critical point of xenon is 17 degrees Celsius and 59 bar. Therefore, at the critical point of carbon dioxide, xenon will also be a supercritical fluid.

The present invention provides a method for the extraction of xenon gas bound to a filter material using supercritical CO2 to form a mixture in which both CO2 and xenon are in a supercritical state.

In another aspect of the invention, the mixture of xenon and carbon dioxide exits the chamber through the exit port and through the back-pressure regulator and is subsequently depressurised into a vortex tube. The pressure drop is controlled by a pressure-regulating valve situated prior to the vortex tube.

The depressurised gas enters the vortex tube tangentially, creating a vortex that is partially reflected by a throttle valve at the other end of the vortex tube. The increased kinetic energy of the high-density xenon (density 5.894g/L at standard temperature and pressure) forces the xenon to the outside of the vortex tube, whereas the lower density carbon dioxide (density I.964g/L) is maintained in the centre the vortex tube and is reflected to leave at the injection end of the vortex tube. The CO2 is cooled during the process and is then re-compressed and used again to extract more xenon in a circular process. Vortex tubes can be used in sequence with slightly different dimensions to improve collection purity and yield of xenon.

The present invention also provides for the separation of xenon gas and CO2 by the use of a vortex tube.

The present invention also provides for the separation of xenon gas and CO2 using a plurality of vortex tubes arranged in series to increase the purity of the xenon-rich gas stream.

In a further embodiment of this invention, the vortex tube Is used to separate xenon from oxygen following cryogenic separation of air.

In one embodiment of the invention, the xenon is passed through soda lime to absorb any remaining CO2 and is then administered to the patient via the anaesthesia machine breathing system. The xenon exiting the vortex tube can be maintained at pressure to fill an injection chamber for controlled release into the breathing circuit as dictated by xenon detectors and a closed-loop controller present in the anaesthetic machine. This system could be used for the re-administration of anaesthetic to the same patient in a closed loop.

The present invention also provides a method for the re-administration of xenon gas to the same patient by the capture of xenon onto a filter material, extraction of xenon in supercritical CO2, separation and removal of CO2 from the xenon gas and redelivery of the xenon to the breathing circuit within the medical device for administering xenon to the patient.

The use of supercritical extraction conditions enables a much higher throughput than gas chromatography due to the high density of supercritical solutions. Furthermore, the extraction conditions are at near room temperature (31 degrees Celsius) and at pressures that, although are high (73 bar or more), these are pressures common in anaesthesia using bottled oxygen. Due to the presence of bottled oxygen, and high oxygen fractions delivered to patients, elevated temperatures and flammable conditions are avoided. Due to pressurised conditions, the equipment is small and can fit into the space available for conventional anaesthesia equipment.

The use of the vortex tube allows the use of a small, zero maintenance component to achieve gas separation. The pressure drop used to drive separation is already available from the depressurisation of the supercritical mixture, making this an efficient process.

In a further embodiment of this invention, the depressurisation of CO2 can be used as a source of cooling to reduce the temperature of the exhaust gases from the anaesthetic machine entering the collection chamber. This means that industrial chilling equipment is not required for the medical device.

A system in which the cooling of exhaust gases from the xenon medical device, to improve the efficiency of xenon binding to the filter material, is achieved by the adiabatic expansion of CO2 following depressurisation after supercritical fluid extraction.

The use of CO2 to drive the process enables the use of conventional CO2 absorbers used in anaesthesia re-breathe systems to purify the xenon. Furthermore, medical CO2 is commonly available and CO2 concentration is routinely detected as part of the gas monitoring systems on anaesthesia machines. This leads to a significant factor of safety when compared to nitrogen or helium that are not routinely of medical grade or monitored as part of anaesthesia or ICU care. Finally, pure CO2 is very selective, only dissolving non-polar molecules. Therefore, significant purification of the exhaust is achieved by selective binding and desorption by supercritical CO2.

It may be necessary to purify the xenon during use due to the accumulation of contaminants. Furthermore, when the patient is woken, the xenon must be captured and processed ready for use by another patient. Purification can be driven using liquid CO2 chromatography. Although supercritical CO2 chromatography can be used, the liquid phase has performance benefits partly because both the xenon and CO2 are not in the supercritical phase at the same time.

The present invention also provides a method in which liquid CO2 is used as the mobile phase for chromatographic purification of xenon from gaseous contaminants derived from the patient or breathing systems.

Captured and extracted xenon as described previously is liquified and delivered to a chromatography column with a silica stationary phase of 5-10 microns, although other normal and reverse stationary phases can be used as familiar to those skilled in the art. Liquid CO2 at a pressure of 1 to 200 bar and temperatures of less than 31 degrees Celsius and above -80 degrees Celsius can be used as the mobile phase, to drive the chromatographic separation of xenon from contaminants. Most preferably, a temperature of 10 degrees Celsius and a pressure of 70 bar is used. Xenon interacts with the silica stationary phase and its flow is retarded to a different degree to the various contaminants. Upon elution from the column the xenon is detected by mass spectrometry, microthermal (katharometer), x-ray absorption, ultrasound or refractometry although other detection systems know to those familiar with the art can be used. In the case of integration into the xenon anaesthesia machine, the xenon detector used for the measurement of the concentration of xenon in the anaesthetic circuit can also be used for the detection of chromatography product The present invention also provides for the use of mass spectrometry, microthermal (katharometer), x-ray absorption, ultrasound or refractometry methods to detect purified xenon produced by CO2 liquid chromatography.

The detector signal is used to control a three-way valve that directs the Xenon/CO2 mixture to a vortex tube for separation of Xenon from CO2 as described previously. More than one vortex tube can be used to produce high purity Xenon. CO2 from the chromatography process is scrubbed using a silica and activated carbon absorbent, pressurised and used again. The high purity xenon has any remaining CO2 absorbed by a CO2 absorber such as soda lime to produce medical-grade Xenon.

The present invention also provides for the re-pressurisation and recirculation of CO2 with/without any remaining xenon during supercritical fluid extraction and liquid CO2 chromatography.

Steps to prevent microbiological transmission can be implemented at many stages of the process and are familiar to those skilled in the art. It is possible to use liquid CO2 chromatography to purify xenon derived from a medical device by methods other than supercritical fluid extraction and known to those skilled in the art, including but not limited to cryogenic liquefaction and inert gas extraction at non-supercritical conditions (e.g. nitrogen and helium).

In one embodiment of this invention, the xenon can be returned to the same patient by incorporating the liquid CO2 chromatography system into the medical device that delivers xenon to the patient. In another embodiment of this invention, the medical grade xenon can be used for other patients following the normal pharmaceutical regulatory processes for the manufacture and sale of a medicine. This would involve the purification process being delivered in a GMP (Good Manufacturing Practice) environment remote to the medical device. In one embodiment of this invention, the capture and extraction of xenon is performed by the medical device delivering xenon to the patient and then this extracted xenon is transported to a GMP facility for purification and subsequent release as a medicine.

The present invention also provides for the production of medical grade xenon from contaminated xenon derived from the exhaust of a xenon delivery medical device by using liquid CO2 chromatography followed by separation of xenon from CO2.

Furthermore, it is anticipated that the steps of capture and extraction may be separated. In a further embodiment of the invention, a semi-pressure intolerant sleeve is used to house the filter material as described previously. This sleeve is made of a stainless-steel tube which can tolerate extraction pressures up to 80-100 bar working pressure. The ends of the tube are made of plastic and are allowed limited movement. A seal such as EPDM prevents gas leaking between the plastic caps and the stainless-steel tube. A connector links the canister to the exhaust of the anaesthetic circuit and this exhaust may be cooled and the canister may be cooled to improve binding. The pressure vessel that houses the canister for the purposes of extraction has mouldings that fit the cap at either end. During extraction, when pressurised CO2 enters the canister, the caps move outwards slightly, retained by the ends of the pressure vessel and CO2 can flow through the canister only due to seals in the pressure vessel that operate between the cap and pressure vessel and the seals between the canister tube and plastic ends. This system overcomes the problems inherent in manufacturing canisters for atmospheric pressure gas collection if they are also required for high pressure extraction. These canisters would be too large to use or be economical if they were the principal pressure vessel.

In the system in figure I, the chambers can be small because capture and extraction are happening frequently. However, when capture and extraction are separated, the canister needs to be large to hold enough xenon to make transport economical and therefore wall tensions are higher, and the end pressures can be very high-over 4 tonnes. In this system, the stainless-steel tube maintains pressure well as the hoop stress and is thin walled as it is contained within the housing, it only needs a factor of safety of 1.5. The ends are essentially free-floating and therefore the pressure is held by the ends of the pressure vessel rather than the junction between the tube and caps. By this method, the pressure inside the canister is maintained and gas can flow only through the canister. In other systems with pressure-intolerant canisters, gas is required outside the canister to balance the trans-mural pressure, which can lead to gas passing outside the canister and picking up contaminants that have transferred from the operating environment, which are not controlled.

The use of a pressure tolerant stainless-steel tube with sealed, floating end caps to contain the filter material as a canister so that upon pressurisation above the critical pressure of CO2, the end caps move and are retained by a pressure vessel ends and flow of supercritical CO2 is maintained within the canister.

The limited abundance of xenon means that the use of xenon for general anaesthesia will be limited and drug use restricted to those patients who would benefit from its neuro protective effects, such as neonatal hypoxic encephalopathy, hypoxic encephalopathy following cardiac arrest, cardiac surgery, sub-arachnoid haemorrhage, stroke and traumatic brain injury, although other indications requiring neuroprotection are envisaged. Such situations require long-term use of xenon and are often delivered on intensive care units that do not have access to anaesthesia gas scavenging. Therefore, xenon delivery medical devices need to be capable of capture and re-delivery of xenon to the patient, purification of the patient gas volume to remove exhaled contaminants and contaminants derived from the breathing circuitJsystems, and when the xenon is stopped and 'washed out', the capture and processing of xenon for use by another patient.

Some aspects and embodiments of this invention can serve all three scenarios. It is able to capture and re-deliver xenon to the medical device without dedicated purification. It is able to capture and purify xenon for re-delivery to the same patient as part of the medical device. It is able to purify xenon separately from the medical device as part of a process that is fit for regulations under the medicines act so that the product can be delivered to another patient.

The invention uses pressurised systems that, when combined are thermodynamically efficient-using pressure changes that are required as part of the system to drive separation. All components are small due to pressure and require minimal cooling as this is often provided by depressurisation of the Different aspects and embodiments may be used together or separately.

Detailed Description

The present invention is more particularly shown, by way of example, with in the accompanying drawings.

The example embodiments are described in sufficient detail to enable those of ordinary skill in the art to embody and implement the systems and processes herein described. It is important to understand that embodiments can be provided in many alternate forms and should not be construed as limited to the examples set forth herein.

Accordingly, while embodiments can be modified in various ways and take on various alternative forms, specific embodiments thereof are shown in the drawings and described in detail below as examples.

There is no intent to limit to the particular forms disclosed. On the contrary, all modifications, equivalents, and alternatives falling within the scope of the appended claims should be included. Elements of the example embodiments are consistently denoted by the same reference numerals throughout the drawings and detailed description where appropriate.

Unless otherwise defined, all terms (including technical and scientific terms) used herein are to be interpreted as is customary in the art. It will be further understood that terms in common usage should also be interpreted as is customary in the relevant art and not in an idealised or overly formal sense unless expressly so defined herein.

One of ordinary skill in the art will appreciate the many possible applications and variations of the present invention based on the following examples of possible embodiments of the present invention.

Figure I shows a xenon closed circle breathing system with capture, extraction and re-delivery of xenon into the circuit.

It is anticipated that although this system described in figure I is applied to a circle system, the same system can be used to deliver recycled xenon to the gas stream in other anaesthesia systems such as a reflector system or cardiopulmonary bypass machine oxygenator.

A xenon delivery medical device can be a circle system, reflector or cardiopulmonary bypass machine oxygenator.

Oxygen I is delivered to the anaesthetic circuit through a servo valve under electronic control 2. Xenon gas 3 is delivered to the circuit though a solenoid or piezo injection valve 4 under electronic control. Electronic control (not shown) of a negative feedback loop with a target concentration set by medical personnel is determined by pressure 5 and gas monitoring 6 systems. The oxygen/xenon gas mixture passes down the inspiratory limb of the circuit through the inspiratory one-way valve 7. The gas monitoring system detects the concentration of xenon, CO2 and Oxygen at the patient end of the circuit. This is performed by a negative pressure system removing a constant stream of gas from the patient y-piece. Most of this gas is returned to the patient circuit (not shown). Expiratory gases pass down the expiratory limb to the expiratory one-way valve 8 and pressure transducer 5. This reading is used to set the back pressure of the exhaust valve 9. A pressure relief valve 10 protects the circuit from overpressure. Some of the expiratory gases are vented through the exhaust valve (adjustable pressure limiting valve) 9 and the remainder pass through the CO2 absorber 1 I and to the ventilator/bag assembly where either mechanical (ventilator) or manual (bag) means are used to pressurise the circle during the ventilation cycle to produce inspiration and expiration. These recirculated gases then circulate back to the inspiratory limb via the gas injectors, where further gas can be added to regulate the system volume (and therefore pressure) and gas concentrations.

Exhaust gas from the exhaust valve 9 passes down the exhaust limb to one of two collection chambers I 2a I 2b tolerant of supercritical CO2 pressure above 73 bar. In one preferred embodiment the working pressure of the chamber is 100 bar and the vessel is manufactured from 316 stainless steel. Each collection chamber is controlled by two selection valves 13a 13b and two section valves I 4a 14b. These selection valves ensure that each chamber is either set to receive gas from the exhaust valve 9 and ventilate it to air, the suction or Anaesthetic Gas Scavenging System (AGSS) or to receive supercritical CO2 from the pump 15 and heater 16 and pass it to the back-pressure regulator 17. The chambers 12a 12b can have a single input and output through which both exhaust and supercritical fluid can pass or can have separate inputs for the exhaust and supercritical fluid. In a preferred embodiment, separate inputs and outputs are used for the supercritical fluid and exhaust due to the different pressures and flow-rates required for exhaust and supercritical fluid. The selection valves I 3a I 3b I 4a I 4b ensure that each chamber I 2a I 2b is only open to either the exhaust or the supercritical fluid and that one chamber 12a or 12b is exposed to the exhaust while the other chamber 12b or I 2a is exposed to the supercritical fluid. The control of the valves is under electronic control (not shown). The flow of exhaust gas and supercritical fluid can be in the same direction or in a preferred embodiment, in different directions as shown in figure 1. This improves the rate of desorption of the xenon by the supercritical fluid and increases the absorption capacity.

The use of a chamber for the capture of xenon onto a filter material that is capable of withstanding pressures above the critical pressure of carbon dioxide.

The use of two chambers, such that one is exposed to the exhaust of the xenon delivery medical device and the other is exposed supercritical CO2 for extraction.

The use of a single opening at either end of the chamber for the passage of both the exhaust from the xenon delivery medical device and supercritical CO2 for extraction of xenon from the filter material contained within the chamber.

The use of separate openings at either end of the chamber, one for the passage of the exhaust from the xenon delivery medical device and the other for the passage of supercritical CO2 for extraction of xenon from the filter material contained within the chamber.

The chambers 12a 12b are filled with a filter material 17a 17b that absorbs xenon gas. The chambers may be cooled, and the exhaust gas cooled to temperatures from room temperature down to -50 degrees Celsius (not shown) to improve binding. The filter material may include but is not limited to silica gel, zeolites, metal organic frameworks, or metal doped silica/zeolite, most preferably a metal (silver or lithium) doped aerogel. The filter material binds the xenon gas reversibly from the exhaust gases from the exhaust valve 9 when the chamber is connected to the exhaust and releases the xenon gas when exposed to the flow of supercritical CO2.

Carbon Dioxide is provided by a pressurised cylinder 18 and powered valve 19 and one-way valve 20a to a pump 15 that pressurises the CO2 above 73 bar, although lower pressures can be used for liquid CO2 extraction. The liquid is then heated above the critical temperature by a heater 16 to form a supercritical fluid. The supercritical fluid is exposed to the filter material I 3a or 13b in pressure-tolerant chamber I 2a or I 2b, dissolving the xenon to form a supercritical solution. Any non-polar contaminants from the patient or breathing systems may also be absorbed by the filter material and desorbed by the supercritical solution. The supercritical solution passes to the back-pressure regulator 17 and is depressurised into a volume buffer vessel 21 with pressure monitoring 22. The supercritical solution is depressurised further through a pressure reducing valve 23 to enter the vortex tube gas separator through an inlet throttle restriction in the vortex tube 24. The tangential entry and depressurisation at the throttle restriction combined with the gas reflection at the throttle valve at the xenon outlet end 25 cause separation of the gas streams into a xenon-rich gas stream at one end 25 and the xenon-depleted CO2 stream at the other end 26. The xenon-depleted gas stream passes through the one-way valve 20b to the pump 15 for recirculation. The volume of the system is controlled negative feedback from the pressure of the buffer vessel 21 acting on the CO2 inlet valve 19.

The throttle at the xenon-rich outlet 25 of the vortex gas separator can be closed until there is sufficient xenon in the gas stream to allow separation and opened proportionally to the amount of xenon in the system. This concentration can be detected by ultrasound, katharometer or refractive index at any point from the selection valve 13a or 13b and the vortex tube 24.

The xenon-rich gas stream passes through a CO2 absorber 27 and is stored in a vessel 28 ready for re-delivery to the patient circuit via a solenoid or piezo valve 4 under physician target electronic control and negative feedback from the patient gas detector 6 and a CO2 absorber to remove any remaining CO2 29.

Figure 2 shows the purification of xenon by liquid CO2.

Carbon dioxide contained in a pressurised cylinder with liquid and vapour phase 18 (approx. 55 bar at room temperature) passes through a powered valve 19 and one-way valve 20a to a condenser 101 to cool the CO2 to -10 degrees Celsius, although other temperatures and pressures to ensure liquid CO2 can be used. The cold liquid CO2 passes to a liquid CO2 pump 102 increasing the pressure to bar although other liquid CO2 pressures can be used. The fluid passes through a heater 103 to increase the temperature above the critical temperature of CO2, 31 degrees Celsius. In a preferred embodiment the fluid is heated to 50 degrees Celsius. The supercritical CO2 passes to a rotary 6-port injection valve 104. This injection valve links to a fixed volume loop 105 that is filled with extracted xenon with contaminants from the patient or breathing system 106 contained in a pressurised vessel 107 at 70 bar and a temperature below 17 degrees Celsius such that the xenon is a liquid. Other temperatures and pressures to ensure liquid xenon can be used. The liquid xenon is pumped 108 around the loop during the filling setting of the rotary valve 104 and then during the load setting of the rotary valve 104, the valve turns and connects the loop to the flow of supercritical CO2 from the pump 102. This flow takes the bolus of xenon/contaminants 106 into the chromatography column 108 filled with the stationary phase 109. In a preferred embodiment the stationary phase is plain silica although other normal and reverse-phase stationary phases can be used as knows to those skilled in the art.

The xenon 106 is separated from contaminants during passage through the column 108 by its interaction with the stationary phase 109, driven by the flow of CO2 from the pump 102. The purified xenon, diluted in CO2, is detected by the detector 110 immediately after leaving the column. The detection method can be mass spectrometry, microthermal (katharometer), x-ray absorption, ultrasound or refractometry although other detection systems know to those familiar with the art can be used. When the bolus of xenon is detected, the electronic controller (not shown), often a Programmable Logic Controller, activates a three-way valve 112 to pass the xenon and CO2 into the collection system. The xenon and CO2 first pass through a back-pressure regulator III and then the three-way valve 112 into the collection buffer 113 with pressure sensor 114. When sufficient pressure is in the buffer 113, the xenon/CO2 mixture passes through a powered valve 115 and pressure-reducing valve 116 into the vortex tube gas separator 117. The vortex tube gas separator separates the xenon from the CO2 by the virtue of density, with the xenon exiting via the throttle valve at one end 118 and the CO2 via the other end 119. The high xenon fraction coming from 118 is passed through soda lime 120 to remove any remaining CO2 a powered valve 121 then a condenser 122 and stored in a vessel 123. This process may require increasing the pressure of the xenon (pump not shown). It is possible to use more than one vortex tube gas separator in series to increase the purity of the xenon fraction before soda lime.

The CO2 leaves the vortex tube gas separator 119, passing through a one-way valve 124 and an activated carbon 125 filled capture chamber 126 to scrub out any contaminants and then passes through another one-way valve and back to the condenser 101 for recirculation.

CO2 exiting the column without xenon is passed through the back-pressure regulator III, three-way valve 112 and is directed straight to recirculation via a one-way valve 128 and activated charcoal 125 filled capture chamber 126 to remove contaminants.

Further steps may be taken to remove microbiological contaminants, package and present the Xenon ready for re-supply as a medical gas. These steps are not shown but are familiar to those skilled in the art.

Further aspects and embodiments are provided in the following numbered paragraphs.

I. A method for the extraction of xenon gas bound to a filter material using supercritical CO2 to form a mixture in which both CO2 and xenon are in a supercritical state.

2. A method of recovering xenon anaesthetic agent from a filter, comprising the step of subjecting the filter to a supercritical fluid, thereby forming a supercritical solution.

3. A method for extraction of xenon by supercritical CO2 by first capturing xenon from the exhaust of a medical device delivering xenon by binding it to a filter material that may include but is not limited to silica gel, zeolites, metal organic frameworks, or metal doped silica/zeolite.

4. A method for capturing xenon from the exhaust of a medical device delivering xenon by binding it to a filter material formed of a silver or lithium doped aerogel.

5. A method to capture xenon from exhausted anaesthetic gas, the method comprising processing gas containing xenon with filter material.

6. A method as claimed in paragraph 4, further comprising the step of releasing xenon from the filter using a supercritical fluid.

7. A method as claimed in any preceding paragraph, comprising the steps of: passing gas derived from a patient in a medical environment through a filter so that xenon anaesthetic agent becomes bound thereto; subjecting the filter material to a supercritical fluid, thereby forming a supercritical solution; removing contaminants from the supercritical solution; - collecting the xenon anaesthetic agent from the supercritical solution; and reintroducing the xenon anaesthetic agent to a patient.

8. Apparatus to perform the method of any preceding claim, comprising a module housing filter material and into which anaesthetic gas can pass so that xenon anaesthetic agent binds to the filter material, and a supercritical fluid source, the module being resistant to supercritical fluid and able to withstand supercritical pressure and temperature so as to enable captured xenon to be reclaimed by exposure to supercritical fluid.

9. The separation of xenon gas derived from a medical device and CO2 using a vortex tube.

10. A method of producing medical grade xenon from contaminated xenon derived from the exhaust of a xenon delivery medical device by using liquid CO2 chromatography followed by separation of xenon from CO2.

I I. A method in which liquid CO2 is used as the mobile phase for chromatographic purification of xenon from gaseous contaminants derived from the patient or breathing systems.

Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiments shown and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention.

Claims

CLAIMSI. A system to capture and recycle xenon when used as an anaesthetic or neuroprotective agent in a medical environment, the system comprising a filter for processing gas containing xenon, means for extracting bound xenon from the filter, and means for purifying xenon.
2. A method to capture xenon gas from exhausted anaesthetic gas, the method comprising processing gas containing xenon with filter material. 10
3. A system or method as claimed in claim I or claim 2, in which xenon is delivered by a medical device and in which xenon is purified separately from the medical device as part of a process such that it can be delivered to another patient in a medical environment.
4. A system or method as claimed in any preceding claim, comprising the cooling of exhaust gases from a medical device to improve the efficiency of xenon binding to filter material.
5. A system or method as claimed in any preceding claim, comprising subjecting the filter to a supercritical fluid, thereby forming a supercritical solution.
6. A system or method as claimed in any preceding claim, comprising filter material that may include but is not limited to silica gel, zeolites, metal organic frameworks, or metal doped silica/zeolite.
7. A system or method as claimed in any preceding claim, comprising separation of xenon using a vortex tube.
8. A system or method as claimed in any preceding claim, comprising chromatographic purification of xenon.