Xenon recycling in an anaesthetic closed-system using carbon molecular sieve membranes

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1 Journal of Membrane Science 301 (2007) Xenon recycling in an anaesthetic closed-system using carbon molecular sieve membranes S. Lagorsse, F.D. Magalhães, A. Mendes LEPAE, Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, Porto, Portugal Received 29 November 2006; received in revised form 16 April 2007; accepted 30 May 2007 Available online 2 June 2007 Abstract This work presents a new envisioned application of carbon molecular sieve membranes technology. It proposed the on-line recycling of xenon in anaesthetic closed re-breathing circuits. The cost and rarity of the so-called ideal anaesthetic gas xenon have impaired its widespread use in clinical practice. Reducing its waste through recycling will help xenon to find its place among anaesthetic substances. In collaboration with the only two companies worldwide that ever produced carbon molecular sieve membranes, several samples were selected and tested for xenon recovery from a gas mixture containing carbon dioxide and nitrogen. Mono- and multicomponent permeation experiments were performed with the most promising membranes. Very high ideal selectivities CO 2 /Xe and N 2 /Xe were obtained in combination with high permeabilities. Multicomponent permeation results suffered from pore blocking effects of xenon. Multicomponent permeabilities were used to simulate a membrane module separation unit for recycling anaesthetic xenon. Simulation results indicate that xenon recovery above 97% can be achieved Elsevier B.V. All rights reserved. Keywords: Carbon molecular sieve membrane; Xenon recycling; Multicomponent permeation 1. Introduction Xenon s anaesthetic properties in humans were described for the first time nearly 60 years ago [1]. Xenon is described as having many of the characteristics of an ideal anaesthetic agent, offering many medical and environmental advantages over the nowadays-used nitrous oxide [2]. The most important medical positive effects seem to be cardiovascular stability, neuroprotection and favourable pharmacokinetics [3]. Moreover, contrary to xenon, most of the conventional anaesthetic agents are involved in the destruction of the ozone layer and contribute to the greenhouse effect. At present, the only concern regarding the introduction of xenon into anaesthetic practice is the increase of the costs of anaesthesia. Xenon is a far more expensive gas (about 11 euro per normal liter) than nitrous oxide. Xenon is present in the atmosphere at very low concentrations ( %) and is produced mainly by cryogenic distillation. Xenon recycling is the only economically acceptable technique for anaesthetic purposes using conventional breathing circuits [4]. Corresponding author. Tel.: ; fax: address: mendes@fe.up.pt (A. Mendes). Table 1 shows typical gas mixtures composition in an inhalation anaesthetic circuit [5,6]. The breathing gas mixture fed to the patient is composed by gaseous anaesthetic (nitrous oxide), oxygen, one volatile anaesthetic (sevoflurane, desflurane, isoflurane, etc.) and water vapour. If xenon was used instead of nitrous oxide, a volatile anaesthetic does not necessarily to be present. The gas mixture exhaled from the patient contains additionally: (a) carbon dioxide, which is continuously generated by the patient; (b) nitrogen that is slowly desorbed from the patient s internal organs, muscles and bones, or that permeates from the outside atmosphere through the patient s skin or through the anaesthetic machine pipes and fittings [7]. The exhaled gas mixture also contains a very small amount of at least 102 various organic compounds like methane, isoprene and acetonitrile [8]. In average, a patient produces about 15 dm 3 h 1 of carbon dioxide and releases about 1.2 dm 3 h 1 of nitrogen during the first anaesthesia period (about 15 min), decreasing afterwards [6]. To re-use the exhaled mixture, nitrogen and carbon dioxide concentrations cannot increase above 3 and 0.5%, in volume, respectively [5]. Currently, some re-breathing, i.e., re-use of the exhaled gas mixture, is possible using available anaesthetic closedcircuits. These closed-circuits are in reality semi-closed /$ see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.memsci

2 30 S. Lagorsse et al. / Journal of Membrane Science 301 (2007) Table 1 Typical anaesthetic mixture flow rate and composition in a closed breathing circuit [6] Composition (%) Flow rate (dm 3 h 1 ) Inhaled mixture Exhaled mixture Anaesthetic (nitrous oxide) Volatile anaesthetic (VA) Isoflurane Sevoflurane <1.2 Desfluorane <2.5% 360 Oxygen Nitrogen Must be lower than Carbon dioxide Must be lower than circuits. Carbon dioxide is removed by passing the anaesthetic flow stream through a canister containing caustic materials, generally soda lime, that have a high affinity towards carbon dioxide. Nitrogen is removed by periodically venting all the gas volume in the circuit. The use of soda lime presents several problems. Soda lime in canisters can react with the volatile anaesthetic sevoflurane originating toxic side products in the gas-circuit, i.e., compounds A and B. Toxic concentrations of compound A are easily achievable in normal clinical practice thus it is recommended to make re-breathing together with some fresh anaesthetic gas flow. Moreover, if allowed to dry up (e.g. after a weekend), a solid lime canister can produce hydrogen and heat and give rise to an explosion hazard. In contact with volatile anaesthetics (mainly desflurane and enflurane), carbon monoxide is also produced, principally when the absorbents is dry. Finally, the exhausted soda lime canisters must be changed regularly and are dangerous and expensive hospital solid wastes [9]. An alternative solution have been proposed by University of Porto (Faculty of Engineering, FEUP) for the continuous removal of carbon dioxide from conventional closed-circuits [6,9]. To replace soda lime canister, the use of membrane contactor technology is proposed. The anaesthetic gas is fed to one side of a dense polymeric hollow fiber membrane module. On the membrane permeate side an aqueous solution of a carbon dioxide absorbent removes it selectively from the anaesthetic stream. The aqueous solution is continuously regenerated in a second membrane contactor by rising the temperature and lowering the carbon dioxide partial pressure. The replacement of xenon by nitrous oxide using conventional anaesthetic closed-circuits, where the circuit is periodically vented, would imply valuable losses. Since 1996, several technological solutions have been proposed for xenon external recycling, i.e., off-line xenon recycling with the closed breathing circuit. The main concern in xenon external recycling is to recover xenon from gas mixtures containing oxygen and nitrogen. A group at the University of Ulm (Germany) [10] developed a device where the anaesthetic gas passes through a cooling trap, activated carbon and a molecular sieve filter to remove all volatile substances. The remaining O 2 /N 2 /Xe mixture is compressed to 60.8 bar and cooled to below 289 K such that only the xenon liquefies. The liquid xenon is transferred to another container for re-use. This system allowed 67% of the xenon to be recovered, at a purity of 89%. A group from the Botkin hospital (Russia) [11] developed a device composed by several containers of adsorbent such as charcoal, cooled in liquid N 2 and evacuated by a pump. Waste gas is slowly passed through them and all the components solidify. The containers are then allowed to warm up very slowly so that xenon boils off first. This passes to an evacuated gas cylinder, also cooled in liquid N 2. When this cylinder is then allowed to warm up to room temperature, the xenon within pressurizes the cylinder rendering it ready for reattachment to the anaesthetic machine. This system produces xenon of purity higher than 99% but the percentage of xenon recovered is not known. More recently, a group of Porto University (Portugal) [12] developed a recycling unit where the anaesthetic gas mixture is directed through a filter system which reduces all the condensable present in the anaesthetic gas mixture (water vapour, VA anaesthetics) up to a molar concentration below 0.5%. The filtered anaesthetic gas mixture (70% Xe, 24% O 2 and 6% N 2 ) is directed to a packed column, which is filled with a selective adsorbent, e.g. zeolite 5A, in which xenon is selectively adsorbed. Then, xenon is purified in an adsorption system using Vacuum Swing Adsorption (VSA), in which the adsorbent (Takeda CMS) selectively retains all components but xenon. For the mentioned fed composition the VSA unit can achieve a xenon purity of 97% or above with a xenon recuperation above 70%. The recycled xenon is stored in a stainless steel reservoir either by compressing the gas, or by using a column containing a specific adsorbent (e.g. zeolite 5A). None of these technologies are currently used. The present work provides, for the first time, a solution for on-line removal of carbon dioxide and nitrogen from anaesthetic closed-circuits. It suggests the use of carbon molecular sieve membrane (CMSM) technology. At present, most of the research on CMSM reports on novel approaches to control membrane microporous morphology and to fabricate CMSM with superior separation performance, by modifying synthesis protocols [13 21]. Through the rigorous control of the pyrolysis conditions, i.e., temperature, heating rate, atmosphere and thermal soaking time, gas flow rate, pressure and concentration, and pre/post-treatment conditions, the pore aperture can be nearly continuously tuned, so that a membrane module can be specifically designed for gas separation and gas separation and reaction. Although commercial applications are still very limited, CMSM are expected to undergo

3 S. Lagorsse et al. / Journal of Membrane Science 301 (2007) Fig. 1. Anaesthetic closed-circuit using xenon. steady commercial production in the near future since they have been identified as having a very large potential for some important industrial application. These include: (a) the production of low cost and high purity nitrogen from air [22]; (b) the separation of alkane alkene, for example, propene/propane or propane ethene mixtures [23]; (c) the removal of carbon dioxide from poor quality landfill gas, in order to make it suitable for use as an engine fuel or to upgrade it to pipeline quality natural gas [23,24]; (d) hydrogen purification from reformed gas, in order to remove the carbon monoxide that is a serious poison of the fuel cell catalyst and others [25]; (g) dehydrogenation reactions [26]. These inorganic membranes have critical pores of comparable sizes to the diameters of diffusing species (ranging from 0.3 to 0.5 Å [27,28]) therefore the separation of the components of a mixture occurs on the basis of molecular size. Since xenon is a bulky species, CMSM can take advantage of this fact. CMSM can be accurately tuned to show high CO 2 /Xe and N 2 /Xe selectivities and high carbon dioxide permeabilities. A simplified sketch of the closed anaesthetic circuit using CMSM module separation system is shown in Fig. 1 If the exhaled anaesthetic gas stream is fed to a CMSM module especially designed to suit this particular application, carbon dioxide and nitrogen should permeate through the membrane while xenon stays in the loop, together with other large molecules such as volatile anaesthetics (when used). This work presents results from mono and multicomponent permeation experiments using carbon molecular sieve membranes supplied by Blue Membranes GmbH (Germany) and by Carbon Membranes Ltd (Israel). Multicomponent experiments were conducted with only membranes supplied by Blue Membranes GmbH, because the Israeli company closed in meanwhile. The simulation of the membrane module separation is also provided. 2. Experimental 2.1. Materials The production processes of the CMSM supplied from Blue Membranes GmbH and Carbon Membranes are described elsewhere [27,28]. Blue Membranes GmbH supplied flat membranes (designated as samples BM) constituted by a thin microporous layer with an average thickness of 10 m on a highly porous support made of silica that gives mechanical support. Carbon Membranes Ltd supplied hollow fiber membranes (designated as samples CM) with 10 m thickness and an external diameter of 170 m. Figs. 2 and 3 show SEM photographs of the crosssection of the flat and hollow fibers CMSM used in this study. In close collaboration with these two companies, several CMSM samples were tuned towards this specific gas separation, so as to achieve good membrane performance. Table 2 shows the desired separation characteristics of CMSM for xenon recycling. Tailoring of correct pore size to obtain an optimal balance between selectivity and permeability is a cumbersome task, demanding for a good understanding of the optimum production conditions with respect to the pyrolysis and pos-treatment (chemical vapour deposition and oxidation) conditions Permeation and adsorption experiments The experimental set-up assembled for performing mono and multicomponent permeation experiments is shown in Fig. 4. It was assembled inside a thermostatic cabinet and consists of a permeation module in which the pressure gradient can be generated both by pressurizing the feed and by evacuating the permeate side of the membrane. Permeation modules for samples CM consisted on tubing housing containing a bundle of about one hundred fibers with an effective area of 180 cm 2.For

4 32 S. Lagorsse et al. / Journal of Membrane Science 301 (2007) Fig. 2. SEM pictures of a cross section of the supported CMSM: (a) two layers are visible: ultramicroporous layer supported on a meso/macroporous structure; (b) ultramicroporous layer (selective layer). Table 2 Separation characteristic of CMSM for xenon recycling Size exclusion Loading capacity (adsorption) Diffusion Monocomponent permeability Xenon High Negligible Extremely low Extremely low Carbon dioxide No High High High Nitrogen No Low Moderate Low Oxygen No Low High Moderate samples BM, a flat sheet cell with an effective membrane area of 33.7 cm 2 was used. In monocomponent experiments, the feed was high-purity gas supplied directly from compressed gas cylinders. In multicomponent experiments a 2.5 dm 3 mixture vessel was used. The feed pressure was controlled with a pressure regulator (Joucomatic, 0 4 bar). The feed and permeate pressure were read using pressure transducers (Druck, model PMP 4010, accuracy ± 0.04%, 0 7 bar and 0 2 bar, respectively). The permeation flux through the membrane was determined by measuring the mass flow rate Fig. 4. Sketch of mono and multicomponent permeation experimental set-up. of the permeating species (Bronkhorst HI-TEC flow meters, F101, accuracy ±1% FS, flowrate capacity chosen in accordance with the flowrate range). The pressure transducers and the flowmeters were logged directly into a computer (using Labview). In multicomponent experiments, the composition of the permeate stream was measured using a Dani GC 1000 gas chromatograph equipped with a thermal conductivity detector (TCD) and two types of capillary columns for separation (VP-Hayesep D and VP-Molesieve columns phase). Pure component adsorption equilibrium isotherms were determined using a volumetric type apparatus, described elsewhere [27]. 3. Results and discussion 3.1. Monocomponent permeabilities Fig. 3. SEM pictures of a cross section of the hollow fiber CMSM. Five CMSM samples were selected to be the most suitable for xenon recycling. These were tailored through a precise control of

5 S. Lagorsse et al. / Journal of Membrane Science 301 (2007) Table 3 Single gas CMSM performance at 303 K at a feed pressure of 2 bar and at a permeate pressure of 0.01 bar and of 1 bar for samples BM and CM, respectively Monocomponent membrane performance CMSM samples BM #1 BM #2 CM #1 CM #2 CM #3 Permeability ( 10 8 m 3 STP m 2 kpa 1 s 1 ) Xe <0.014 <0.014 CO N O Ideal selectivity CO 2 /Xe >1000 >6020 N 2 /Xe >40.0 >180 O 2 /N the selective layer activation step after chemical vapour deposition, in terms of the resulting pore size, differing from each other on the duration and temperature of the treatment. Single gas permeabilities were measured at 303 K at a feed pressure of 2 bar and at a permeate pressure of 0.01 and of 1 bar for samples BM and CM, respectively. Permeabilities and ideal selectivities (ratio between monocomponent permeabilities) obtained are listed in Table 3. The monocomponent permeabilities obtained for all samples are high in combination with high ideal selectivities towards xenon separation. The mean ultramicropore size seems to be very close to the molecular dimension of xenon (0.394 nm, Xe molecular dimension based on liquid molar volume [29]), leading to the exclusion of this species from the pore system. Sample CM #2 and CM #3 from Carbon Membranes Ltd show exceptional molecular sieve capabilities. For example, the more permeable membrane, sample CM #3, presents selectivities higher than 6020 for CO 2 /Xe and 180 for N 2 /Xe. Even though some ideal perm-selectivities seem very promising, these values may be misleading when dealing with multicomponent separation of absorbable gases in CMSM. Multicomponent measurements are therefore essential to evaluate the membrane separation performance toward a specific application Multicomponent experiments Multicomponent experiments were performed using the membranes supplied from Blue Membranes GmbH (samples BM #1 and BM #2). The experimental conditions used in the multicomponent gas permeation runs are listed in Table 4. The synthetic gas mixture, fed to the permeation cell, has a composition close to a typical anaesthetic stream. The feed and permeate pressures were selected to be the same as the one used in monocomponent permeation experiments. Figs. 5 and 6 show the permeate stream composition and flow-rate, for samples BM #1 and BM #2, respectively. Despite the high feed concentration (66%), the permeate streams are very poor in xenon. The CMSM tested evidenced good stability during the experimental runs period. This is an important observation, since membrane stability is one of the key factors for commercialization. Sample BM #1 proved to perform better, in accordance with the monocomponent permeation data. Table 4 Multicomponent experimental conditions Experimental conditions CMSM samples BM #1 BM #2 Temperature (K) 300 Feed composition (%) Xe CO N O Feed pressure (bar) Permeate pressure (bar) Experiment time (min) Gas permeabilities and selectivities, i.e., real selectivity (ratio between multicomponent permeabilities) based on multicomponent permeation experiments are given in Table 5. In general, permeabilities are quite lower than the ones obtained in monocomponent experiments. Roughly, real selectivities are nine and five times lower than ideal selectivities for CO 2 /Xe and N 2 /Xe, respectively. It was observed that the tested CMSM show a multicomponent permeability towards xenon higher than the one obtained Fig. 5. Molar fraction and flow rate of the permeate stream of sample BM #1 at 300 K. Solid lines are the mole fraction and the dotted line is the total permeate flowrate.

6 34 S. Lagorsse et al. / Journal of Membrane Science 301 (2007) Fig. 6. Molar fraction and flow rate of the permeate stream of sample BM #2 at 300 K. Solid lines are the mole fraction and the dotted line is the total permeate flowrate. for monocomponent permeability, as shown in Tables 5 and 3 respectively. The determined permabilities are very small and large errors could be involved. Moreover, the multicomponent determinations are more complex and involve even larger errors. However, this observation might indicate that faster species act as transport facilitators of xenon. The experimental multicomponent permeabilities/selectivities provide realistic or, in the worst scenario, conservative parameters for computing the CMSM separation performance. Although ideal selectivities are very high, multicomponent results revealed that these values are misleading. The complex multicomponent behaviour may be difficult to interpret with detail but is clearly related to the interaction of transport and sorption within the nanopore space and their concentration dependence. From adsorption equilibrium isotherms it is possible to qualitatively predict competitive adsorption effects in multicomponent permeation. However, for samples BM #1 and BM #2, it was not possible to determine adsorption equilibrium data of xenon, even when allowing it to contact the CMSM samples for a reasonably long time, because of the narrow micropores in these samples. Thus, a third sample, designated BM #3, with a critical pore size slightly higher than for samples BM #1 and BM #2, was used to obtain the adsorption equilibrium isotherms of xenon and the other gases and therefore allow for Table 5 CMSM multicomponent permeabilities and selectivities at 300 K Fig. 7. Adsorption equilibrium data of sample BM #3 at 300 K. comparison of adsorption behaviours. Fig. 7 shows the adsorption isotherms of xenon, oxygen, nitrogen, carbon dioxide and sulphur hexafluoride at 300 K obtained on sample BM #3. As shown, when xenon is able to penetrate into the CMSM pore system, it attains high adsorption concentrations that correspond to an also high adsorption affinity. The CMSM pore network is usually depicted as being formed by relatively wide pores diameter intermediated by constrictions, these being responsible for the molecular sieving [28,30]. Xenon atoms, which probably adsorb at the entrance of these constrictions, reduce the free pore space and limit the diffusion of other species, especially for the less adsorbable gases that are unable to displace the adsorbed xenon. This behaviour is the so-called blockage effect, which has a negative influence on the multicomponent permeabilities. Even when xenon is effectively excluded from entering into the CMSM micropore system, the blockage effect can occur just right at the pore mouth, at the membrane surface. The transport behavior of the individual species in a mixture is also a strong function of the manner how these species accommodate within the pore structure. The size distribution and shape of the overall pore system should play an important role in multicomponent transport. Steel and Koros [31] idealized the pore structure of CMSM based on three key dimensions: (a) d c, the constriction dimension; (b) d λ, the jump length dimension which is believed to be correlated to diffusion coefficients and (c) the cross-section dimension, d tv (see Fig. 8). These authors Multicomponent membrane performance CMSM samples BM #1 BM #2 Permeability ( 10 8 m 3 STP m 2 kpa 1 s 1 ) Xe CO N O Real selectivity CO 2 /Xe N 2 /Xe O 2 /N Fig. 8. Different blockage behaviors on an idealized pore structure of a CMSM based on three key dimensions d c, d λ and d tv : (a) total blockage; (b) partial blockage; (c) no blockage.

7 S. Lagorsse et al. / Journal of Membrane Science 301 (2007) Fig. 9. Schematic of the honeycomb-like membrane module flow configuration. suggested that to achieve high sorption affinities with high diffusion in a carbon molecular sieve material, one might expect d c d λ d tv (in the micropore range). They also argued that d tv is not critical to understand the molecular sieving nature of the carbon membranes. One believes, however, that this parameter is a key to understand the multicomponent behavior in CMSM. Fig. 8 represents schematically three pore dimensions illustrating the blocking effects of large molecules adsorbed in the cavities at the entrance of constriction and affecting the passage of small molecules. Depending on the pore characteristic dimension, d tv, the molecule might block, total or partially the passage of the other species. The characteristic dimensions of a CMSM depend upon the preparation protocol and the precursor type. Modifying these parameters, it should be possible to produce membranes with the same molecular sieving characteristics but with different blockage effects. The science and technology of carbon molecular sieve membranes are still in its infancy. Since so many variables are involved in this kind of experimental work, systematic synthesis approaches are needed, in an effort to establish cause effect relationships between preparation protocols and final membrane characteristics. Improvements in this field could be obtained by combining the use of a design of experiments (DoE), and combining molecular modeling and experimental characterization work. These studies are now being undertaken in our lab. It might also be questioned if the volatile anaesthetic gases interfere with the CMSM by blocking the pores. A set of experiments was performed by Mendes [6] to evaluate the pore blockage effects on carbon molecular sieve adsorbents to the presence of sevoflurane, one of the most common volatile anaesthetics. Two breakthrough experiments were performed with a Takeda carbon molecular sieve adsorbent using a gas mixture made of helium and carbon dioxide (about 6%) on the first experiment and using helium, sevoflurane (about 0.6%) and carbon dioxide (about 6%) on the second experiment. It was concluded that the column adsorption capacity stayed nearly constant. This indicates that sevoflurane will probably not interfere significantly with the CMSM performance when used for xenon recycling Simulation of the membrane separation The CMSM module idealized to perform the xenon separation is being developed by Blue Membranes GmbH [32]. It has a honeycomb-like geometry and was already described elsewhere [28]. This new design is obtained starting from a pleated flat sheet precursor and the pyrolysis is done with the membrane module already assembled. The honeycomb-like geometry membrane module has a high packing density, up to 2500 m 2 m 3. The flow configuration of the honeycomb module is represented schematically in Fig. 9. As shown, the permeate bulk flow, the feed/retentate flows and the transmembrane flow are in perpendicular directions. Taking into consideration the membrane module arrangement, cross-flow operation is assumed. This considers that the local permeate composition depends only on the retentate-side composition, which means on the local driving force [33]. Fig. 10 schematically shows the idealized flow pattern. The module is considered to be divided into N small modules where complete mixing occurs on both sides of the membrane. The main assumptions of the present model are: (a) cross-flow operation; (b) retentate flow-pattern is described by the axially dispersed plug flow model; (c) negligible pressure drop in both chambers, retentate and permeate; (d) constant permeability over the whole operating pressure range. In Fig. 10, F k and V k are the total feed and permeate flow rates leaving stage k. The mole fractions of component i leaving the retentate and permeate sides of the membrane at stage k are w i,k and y i,k, respectively. The retentate gas stream from stage k is assumed to become the feed of stage k +1.V is the permeate Fig. 10. Idealized membrane module divided into N stages with complete mixing conditions.

8 36 S. Lagorsse et al. / Journal of Membrane Science 301 (2007) flow rate and y i is the average mole fraction of component i in the permeate. In each stage k, the mass balance can be written as y i,k = N comp (F k 1 /F k )p hk w i,k 1 (V k /P i A) + p hk (V k /F k ) + p lk, i = 1,..., N s (1) y i,k = 1 (2) i=1 F k 1 = F k + V k (3) w i,k = 1 F k (w i,k 1 F k 1 y i,k V k ) i = 1,..., N s (4) where P is the permeance, A the membrane area of one stage, p h and p l are the feed and permeate pressures, respectively, N s the number of species in the mixture and N is the number of perfectly mixed stage required to perform the separation. The above 2 N s + 2 set of equations allow us to calculate the 2 N s +2 unknown variables (F k, w i,k, V k, y i,k ). The number of stages is related to the dispersion of the retentate flow, where the Peclet number is numerically equal to 2 N [34]. APe = 2000 was assumed. The molar fraction of carbon dioxide and nitrogen at the inhaled stream (after replacing the lost amounts of oxygen and xenon) should be smaller or equal to 0.5 and 3.0%, respectively (see Table 1). The model was then solved in order to meet this constrain and the membrane area needed for the separation obtained. The input values used in the simulations are listed in Table 6. The feed composition and flow rate are similar to a typical exhaled anaesthetic gas mixture. Also listed in Table 6 are the simulation results and the characteristics of the recycled stream for two different feed pressures: (i) 2 bar, the same pressure employed to determine the multicomponent permeabilities; (ii) 1 bar, to simulate a real closed anaesthetic loop where only a ventilator is available. In both cases the permeate side was considered to be at 10 mbar pressure. The results obtained in terms of membrane area needed and xenon losses are very good for both membranes and operating conditions. Xenon losses through the permeate are lower than 3.6 and 7.2 dm 3 h 1 using data from CMSM BM #1 and BM #2, respectively. Xenon recovery is higher than 97%. If a standard honeycomb module is considered, an effective 10 m 2 can be packed in 4 dm 3 volume. Considering that the tested membranes are now being improved and even higher selectivities and Table 6 Membrane simulation results for xenon recycling application BM #1 BM #2 #1 #2 #1 #2 Input Feed flowrate (dm 3 h 1 ) 360 Feed composition (%) N O CO Xe 65.0 Feed pressure (bar) Permeate pressure (bar) Output Retentate flowrate (dm 3 h 1 ) Retentate composition (%) N O CO Xe Area (m 2 ) Xe recuperation (%) Recycled stream O 2 reposition (dm 3 h 1 ) Xe reposition (dm 3 h 1 ) Recycled flowrate (dm 3 h 1 ) 360 Recycled composition (%) N O CO Xe 66.5 Xe reposition (euros h 1 ) a a Considering xenon cost equal to 11 euros L 1.

9 S. Lagorsse et al. / Journal of Membrane Science 301 (2007) is able to block the pores close to the membrane surface, hindering the passage of other species. It is suggested that pore blockage effects can be diminished by modifying the preparation protocol and/or the precursor type in order to obtain a membrane with wider pore cavities, but maintaining, however, the same molecular sieving ability (related to the number and size of constrictions). Even using multicomponent experimental data from the least performing BM membranes on the simulation of the separation process, good results were obtained. Using a feed pressure of 1 bar and using vacuum (0.01 bar) in the permeate side, a 13 m 2 membrane module (which corresponds to an approximated volume of 5.2 dm 3 ), allows for 98.4% xenon recovery. References Fig. 11. A schematic of xenon recycling system using BM #1. permeabilities are expected, the CMSM technology is foreseen viable to accomplish on-line xenon recycling. Shown in Fig. 11 is a schematic of the technology proposed, which uses permeabilities data obtained with BM #1. The CMSM separation system considers two membrane modules in parallel for safety reasons and to allow periodic regeneration of the membranes. The objective of regeneration is to clean the membrane by removing contaminants and restore the membrane performance. Based on data from sample BM #1 a 13 m 2 membrane module will be needed, which should have an approximated volume of 5.2 dm 3. Presently our lab is studying the pore blocking minimization and the water vapour management. These are, together with CMSM aging, some of the most important issues to be addressed in this area. 4. Conclusion Carbon molecular sieve membranes revealed to be a promising technology for on-line recycling xenon in closed anaesthetic breathing circuits, proving to be an alternative to the soda lime canister currently used in these systems. A set of carbon molecular sieve membranes were successfully designed for removing carbon dioxide and nitrogen from a gas mixture containing the anaesthetic xenon. Ideal selectivities CO 2 /Xe and N 2 /Xe above 1000 and 30, respectively, in combination with high permeabilities towards carbon dioxide, were found using samples supplied both by Blue Membranes GmbH and Carbon Membranes Ltd. This was indicative of the molecular sieving capability of these materials, which proved to be finely tunable in order to exclude the xenon atom. Multicomponent performance of CMSM was adversely affected by the presence of xenon which made multicomponent permeabilities and real selectivities smaller. Xenon has a high adsorption affinity and when is able to penetrate into the pore system its loading capacity is close to that of carbon dioxide. Even when it is mostly excluded from the micropores, xenon [1] S. Cullen, E. 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