Abstract:
A device for the concurrent oxygen generation and control of carbon dioxide for life support system involves two stages, where a first stage removes CO 2  from an exhalent side of a ventilation loop and a second stage employs Ceramic Oxygen Generators (COGs) to convert CO 2  into carbon and O 2 . The first stage includes a plurality of chambers and means to switch the ventilation loop through at least one of the chambers, where CO 2  removal is carried out before discharge of the CO 2  depleted gas to an inhalant side of the ventilation loop, and to exclude the ventilation loop from the remaining chambers of the first stage, where these chambers are placed in communication with the second stage. The second stage has two portions separated by the COGs such that CO 2  and the formed carbon remain on an intake portion from the O 2  rich atmosphere on the exhaust side, which is plumbed via a metering valve to introduce the O 2  rich atmosphere to the inhalant side of the ventilation loop.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is the U.S. national stage application of International Patent Application No. PCT/US2008/062047, filed Apr. 30, 2008, which claims the benefit of U.S. Provisional Patent Application No. 60/926,909, filed Apr. 30, 2007, the disclosures of which are hereby incorporated by reference in their entireties, including all figures, tables and drawings. 
    
    
     GOVERNMENT SUPPORT 
     The subject matter of this application has been supported by a research grant from the National Aeronautic and Space Administration under Grant No. NNJO6HB01C. Accordingly, the government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a device to generate oxygen concurrently with the reduction of CO 2  for life support systems. 
     BACKGROUND OF THE INVENTION 
     Weight is one of the most crucial factors in determining the viability of manned space travel, such as a mission to mars. Ostensibly, much of the technology needed for manned space travel is already in place but its weight is prohibitive. Particularly, life support related technologies are currently too heavy for an astronaut to carry for a reasonable duration without exhaustion. The deficit caused by weight for a life support system is also a deficit for other endeavors where a relatively long use of the system, including the use in a mining or other environment during emergency or other situations where the atmosphere is not conducive to breathing. 
     There are two crucial parts of such a life support system, those being carbon dioxide (CO 2 ) removal and oxygen (O 2 ) supply. Current CO 2  removal technologies are not suitable for use on a long space mission, such as a mars mission, because of their weight and may not be appropriate for interaction with the Martian environment. State of the art advanced CO 2  removal systems employ lithium hydroxide canisters for CO 2  removal. Such a system requires approximately sixty pounds of consumable lithium hydroxide for a proposed five day Mars surface mission. To decrease such weight penalties on a mission to mars, it is imperative to employ an alternative system. Cycling bed and permeable membranes systems are also unsuitable because the high CO 2  concentration in the Martian atmosphere, along with other environmental variables, precludes simply venting the CO 2  after its removal from a spacesuit. Additionally a source of O 2  is also a requirement for the life support system, and again the weight of an O 2  source, such as an oxygen tank, must be considered. Vital O 2  is bound up in the CO 2  molecule, yet, none of the CO 2  removal technologies mentioned above capitalize on the use of the CO 2  as an O 2  source. 
     Hence, new methods must be developed or older/existing alternatives must be perfected to remove CO 2  from the space suit, without the need for venting, and to recover the oxygen from the CO 2  molecule. Indeed, if a system is able to capture and recycle all the oxygen from the exhaled CO 2 , it may be possible to considerably reduce the need for additional O 2  sources (such as oxygen tanks) and hence gain significant weight savings. 
     SUMMARY OF THE INVENTION 
     The invention is a device for continuous conversion of CO 2  to O 2  thereby providing a breathable atmosphere for at least one individual in an environment where the individual must be isolated from the environment by a suit or other containment where a ventilation loop delivers exhaled air from the containment to the device and delivers inhalable air from the device to the containment. The device has at least one first stage that includes at least one inlet, at least one outlet, a plurality of sorption bed chambers where each chamber has at least one CO 2  sorption bed, a means to direct a gas flow to and from the chambers, and a means to actuate the means to direct. The device also has at least one second stage where each second stage includes at least one ceramic oxygen generator (COG) unit, at least one CO 2  intake, at least one O 2  exhaust, and a means, such as a metering valve, to control the O 2  exhaust from the second stage into the outlet of the first stage. 
     The means to direct the gas flow can be a first valve between the inlet and the chamber and a second valve between the chamber and the outlet. When the first valve is in an open position, the gas flow is directed to the chamber such that CO 2  is absorbed on the sorption bed and the second valve is positioned to direct the CO 2  depleted gas flow to the outlet. When the first valve is in a closed position, the gas flow is restricted from the chamber and the second valve is positioned to provide fluid communication between the chamber and the intake of the second stage such that CO 2  can be desorbed from the sorption bed into the second stage for conversion of the CO 2  to C and O 2  within the second stage. 
     The device can also include at least one CO 2  sensor in the outlet or at least one CO 2  sensor in each of the chambers. The sensors provide a signal to a means to actuate the valves or other means to direct the gas flow. When a level of CO 2  in the gas flow at the sensor exceeds a predetermined level of CO 2 , the first and second valves of all chambers positioned to be in the gas flow of the ventilation loop are positioned to restrict the gas flow to the chambers that have exceeded the predetermined CO 2  level and place these chambers in communication with the second stage. The valves or other means of directing the gas flow of other chambers where the CO 2  has been desorbed from the sorption beds are simultaneously or subsequently placed into the position such that the exhalent gas at the intake is directed into these chambers to absorb CO 2  and released as a CO 2  depleted gas to the outlet and ultimately into the inhalant side of a ventilation loop when mixed with O 2  rich gas from the second stage and delivered to the containment. The sorption bed can be polyethyleneimine deposited on mesoporous molecular sieves. The chambers can include heat exchangers such that heat of absorption by sorption beds of chambers positioned between the inlet and outlet can be transferred to the sorption beds of chambers in communication with the second stage, thereby reducing any quantity of heat from an additional heating source that may be required to cause CO 2  desorption from sorption beds to the COGs of the second stage during use of the device. 
     The second stage of the device preferably has a catalytic carbon deposition layer (CCDL) on a cathode of the COG. This CCDL can be metal containing particles on a support, where the metal containing particles can be, for example, Fe, Ni, Co, alloys thereof, or oxides thereof and the support can be a metal wire mesh or a SiO 2  fiber cloth. The metal containing particles can be nanoparticle of about 10 to about 50 nm in size. The COG unit can employ a graded bilayer electrolyte comprising a layer of erbia-stabilized bismuth oxide (ESB) and a layer of samaria- or gadolinia doped ceria (SDC or GDC) between a cathode and an anode. The cathode can be a Pt-ceria cermet or a Cu-ceria cermet and the anode can be a silver-ESB cermet. The second stage can include CO sensors to provide a signal to an alarm that a predetermined level of CO has be exceeded in the second stage. The high level of CO can indicate saturated CCDLs or other factors that reduce the efficiency of the COGs. The CCDLs can be replaced when saturated. 
     The invention is also directed to a method of providing a breathable gas for life support, with the steps of: absorbing CO 2  from an exhaled gas flow in at least one chamber of the first stage of the device described above; directing the CO 2  depleted gas to an inhalable gas flow, converting the CO 2  desorbed from chambers restricted from the exhaled gas flow and in communication with a second stage of the device described above, to C and O 2 ; and directing the O 2  rich gas from the second stage to the inhalable gas flow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       There are shown in the drawings, embodiments, which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
         FIG. 1  shows a conceptual representation of a Ceramic Oxygen Generator (COG). 
         FIG. 2  is a schematic of the CO 2  removal system of the invention where the features of the CO 2  absorbance, of a first stage, are shown in (a) and the features of the COG, of a second stage, are shown in (b) 
         FIG. 3  is plot of the conductivities of different solid oxide electrolytes vs. temperature. 
         FIG. 4  shows a SDC-ESB bilayered electrolyte illustrating how the relative thickness can be varied to control the interfacial oxygen partial pressure(P O2 ) to avoid ESB decomposition. 
         FIG. 5  shows tubular (a) and planar (b) ceramic oxygen generators with removable catalytic carbon deposition layers (CCDL). 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is a device for CO 2  removal from and O 2  addition to a life support system, such as a spacesuit. CO 2  will be removed using a catalytic layer in series with a ceramic oxygen generator (COG) so that it is reduced to carbon and oxygen as in the chemical equation below:
 
CO 2 →C+O 2  
 
     Carbon dioxide is electro-catalytically reduced at the cathode  2  of a COG  17 , illustrated in  FIG. 1 , which transports the oxygen as O 2   −  ions from the cathode  2  leaving behind solid carbon. The driving force is an electric potential applied across a dense oxide ion-conducting electrolyte  1 , as shown in  FIG. 1 . Under the influence of this potential, CO 2 , adsorbed on the cathode  2 , releases oxygen species, which are then incorporated into the lattice of the electrolyte  1  as oxide ions and transported to the anode  3 , where they recombine to form O 2  molecules. The oxygen flux is given by Faraday&#39;s equation:
 
 J   O2   =i   app    n   −1   F   −1   =W   app    V   app   −1   n   −1   F   −1  
 
where J O2  is the molar flux of oxygen (mol/s), i app  is the applied current (A), W app  is the applied power, V app  is the applied voltage, F is Faraday&#39;s constant and n is the number of electrons transferred in the ionization of an oxygen molecule.
 
     Complete reduction of CO 2  to C and O 2 , requires about 1.1 V/mole at 500° C. to 700° C. Given that CO 2  is produced metabolically at the rate of 25 mg/s (0.00057 mol/s), complete reduction of CO 2  (or equivalently removing all the oxygen released in the reaction) requires a power consumption of about 240 W if no operational losses occur. Such operational losses (due to Ohmic resistances and electrode overpotentials) must be included and, for accuracy, these values can be obtained from experiments. Furthermore, careful system design can allow for a reduction in the required oxygen removal rate and, hence, the applied power required. 
     The CO 2  removal system of the present invention comprises two stages, as shown schematically in  FIGS. 2   a  and  2   b , respectively. Each stage can be, and, generally, the first stage is, comprised of a plurality of redundant combinations of functional components. The first stage has common features with a system for NO x  removal, as disclosed in U.S. Pat. No. 5,456,807 to Wachsman et al., which is incorporated herein by reference. In this first stage CO 2  is concentrated before it is released to at least one COG  17  within a second stage  4 . By concentrating the CO 2 , the amount of oxygen pumped through the COG  17  is limited, primarily to that liberated by the reduction of the CO 2 . In this manner, power consumption by the COG  17  is reduced by a considerable extent.  FIG. 2   a  shows the removal and concentration of CO 2  in the first stage. The exhalant gas from a life support system is piped from the system&#39;s ventilation loop through an inlet  5  to one of a plurality of chambers, illustrated in  FIG. 2   a  as two chambers  6  and  7 , each containing a sorption bed  8  and  9  to remove CO 2 . By closing valves  10  and  11  while opening valves  12  and  13 , chamber  7  is excluded from the ventilation loop such that CO 2  is removed from the exhalant gas stream by the absorbent  8  in chamber  6  and the CO 2  depleted gas is reintroduced to the ventilation loop to the life support system through an outlet  14 . The saturation level of the sorption bed in chamber  6  can be monitored by using at least one CO 2  sensor  15  to measure the CO 2  concentration in the effluent from the sorption bed. Once  15  senses a predetermined CO 2  concentration, the sorption bed  8  has achieved a desired degree of saturation and chamber  7  is automatically switched into communication with the ventilation loop by switching valves  10 ,  11 ,  12  and  13  such that CO 2  removal is handled by chamber  7 , while simultaneously sealing chamber  6  from the ventilation loop and opening a connection from chamber  6  to second stage  4 . The CO 2  is desorbed from the sorption bed  8  of chamber  6  into the second stage  4 , where CO 2  is reduced to carbon and O 2 . Once sensor  16  indicates that the desired level of CO 2  saturation has been achieved in sorption bed  9  of chamber  7 , valves  10 ,  11 ,  12  and  13  are simultaneously switched such that the chamber  7  can vent to the second stage  4  while chamber  6  can absorb the CO 2  passing through the ventilation loop. 
     The O 2  recovered by at least one COG  17  is reintroduced to the ventilation loop as needed. The second stage  4 , as shown in  FIG. 2   b  comprises a chamber housing  18 , at least one planar or tubular COG  17 , a removable catalytic carbon deposition layer (CCDL)  19 , and its support  20 . Each COG  17  is oriented so that the cathode  2 , as shown in  FIG. 1 , is directed to the intake  21  of stage  4  and the anode  3 , as shown in  FIG. 1 , is directed to the exhaust  22  of the second stage  4 . At least one gas-tight seal  23  isolates the intake  21  and the cathode  2  from the exhaust  22  and the anode  3  of the COG  17 . A metering valve at the exhaust  22  allows O 2  produced at the anode  3  to be readily collected and reintroduced with the CO 2  depleted gas of the outlet  14 , such that a breathable inhalant is provided to the ventilation loop. The amount of O 2  gas released from exhaust  22  into the ventilation loop is controlled via a metering valve  24 . When the carbon buildup on the CCDL  19  surpasses an experimentally predetermined level, the CCDL  19  can be replaced with a carbon free CCDL. The carbon deposits may then be removed from the CCDL after which the CCDL can be reused if desired. As desired, the metering valve  24  can be controlled based on an input of an O 2  sensor (not shown) between the metering valve  24  and the outlet to the ventilation loop. 
     The CCDL  19  is designed such that its replacement is required only once in a predetermined period, for example replacement is required only once during an eight hour period. The second stage  4  preferably contains at least one carbon monoxide (CO) sensor  25  within the CO 2  rich side of the chamber housing  18 . This CO sensor  25  should be sensitive to only a few parts per billion of CO in the atmosphere. Formation of CO can result from excessive carbon accumulation on the CCDL  19  and can be used to indicate a need for its replacement. When CO is sensed, the quantity of oxygen being recovered by the COG is necessarily less than optimal for the life support system. 
     An efficient CO 2  sorbent  8  and  9  in the first stage helps reduce weight and decreases the energy demand of the system, as heating is generally required to desorb the CO 2  to the second stage  4 . The sorbent  8  and  9  is chosen to maximize the mass of CO 2  absorbed per unit mass of sorbent and permit regeneration with a low thermal energy requirement. Preferred sorbents are solids. Most preferably they are solids with a high selectivity for absorption of CO 2  selectively from the ventilation loop. The mode of CO 2  absorption can be physisorption or chemisorption. Physisorption can be carried out using zeolites, activated carbons and molecular sieves. Chemisorption systems that can be used include metal oxides, for example, CaO and/or MgO, and mixed metal oxides, for example, hydrotalcite compounds and/or perovskites. Chemisorption systems are generally more selective than physisorption systems. A polymeric adsorbent such as polyethyleneimine (PEI)-deposited on mesoporous molecular sieve MCM-41 can be used. The PEI/MCM-41 absorbent is a high-capacity and highly selective CO 2  sorbent that has been used to remove CO 2  from exhalants in the space shuttle. The adsorption/desorption temperatures are relatively low where desorption can occur at temperatures as low as about 75° C. Carbon nanotubes can be used as the sorbent. Furthermore, chemisorbents, for example, CaO, can be dispersed onto a high surface area support, for example, Al 2 O 3 , zeolites, activated carbon or carbon nanotubes. The absorption process is generally exothermic. For example, the enthalpy of reaction to absorb CO 2  on CaO is about −178 kJ/mol. In one embodiment of the invention, the first stage can be designed, such that the sorption beds and chambers behave as a heat exchanger, where the heat generated in one sorption chamber, for example, referring to  FIG. 2   a , absorbing chamber  6  can transfer heat to desorbing chamber  7 . 
     Full CO 2  reduction results in carbon build up on the surface of cathode  2  of the COGs. If the amount of carbon deposited exceeds a critical level, carbon deposits can potentially: (i) block gas diffusion paths through the cathode to the triple-phase boundaries (TPBs)—i.e., lines where cathode  2 , electrolyte  1  and gas phase coincide; and (ii) reduce the TPB length. As the gas diffusion paths become blocked, concentration overpotential increases until COG operation ceases due to starvation. As TPB length decreases, activation overpotential increases until COG operation ceases due to the absence of oxygen incorporation sites on the electrolyte surface. As illustrated in  FIG. 2   b , the CCDL  19  is placed between the intake  21  and the cathode  2  of the COG  17  to collect carbon before it reaches the cathode  2 . The CCDL can include metals which are prone to “coking”, i.e. carbon deposition, and include Ni, Fe and Co. However, coke formation is not a sufficient characteristic of an efficient CCDL. A CCDL must have the ability to adsorb and decompose CO 2 . Most catalysts for CO 2  decomposition contain iron, and comprise oxygen-deficient Fe or Ni—Fe oxides. Although for the purpose of the invention the CCDL is referred to as catalysts, they are not catalysts in the truest sense, since the carbon is deposited onto the surface and the oxygen is incorporated into the lattice of the catalytic compounds. Consequently, the amount of CO 2  adsorbed and decomposed on the surface is proportional to the number of oxygen vacancies in the catalytic compounds, which, once fully oxidized, oxygen deficiency has to be reestablished before more CO 2  can react. For the present invention to generate O 2  efficiently oxygen must be continuously removed from the catalyst and the catalyst must have the ability to incorporate a large amount of carbon on the surface without catalyst deactivation. The COG of the invention reacts with the oxygen present in the system at the CCDL and effectively serves as an oxygen pump resulting in the continuous removal of the oxygen from the CCDL. The amount of carbon deposited on the catalysts can range from a few grams per gram of catalyst up to 380 g/g catalyst. An important catalyst property for forming carbon is the particle size of the active metal. Particle sizes of Ni, Fe and Co in the range of about 10 to 60 nm are desirable. The CCDL can be removed from the system when its catalytic activity decreases below a critical value due to excessive carbon deposition on the cathode. After the CCDL is removed from the second stage, the CCDL can be regenerated. 
     In one embodiment, Ni wire meshes and SiO 2  fiber cloths is used as structural supports for the catalyst of the CCDL. Ni wire mesh is commercially available in a range of different wire sizes and various mesh counts and the high temperature SiO 2  material is also available with different yarn sizes and thicknesses. Perhaps the main advantage with the SiO 2  material is the weight, which ranges from 3.6 to 40 oz/yd 2  dependent on the thickness and the weave construction. The particular dimensions of the Ni wire mesh depend on whether it is used as the actual support for the active metal, or if it is used to contain the catalyst. Small diameter wires permit a higher surface area of the nickel per mass of metal than do larger diameter wires. The openings in the wire mesh form an open structure that can allow carbon to grow. The CCDL catalysts can be prepared by dipping a Ni wire mesh or a SiO 2  fiber cloth in a dispersion of Ni or Fe nanoparticles. Although Fe and Ni nanoparticles are commercially available in nanoparticle sizes of 10 to 50 nm, in one embodiment of the invention oxides of these metals are used to avoid the flammability of the metal nanoparticles. Fe 2 O 3 , NiO, NiFe 2 O 4  oxides or iron-based carbon nanotube catalyst can be used, and are commercially available. The nanoparticles can be dispersed in a solvent comprising either deionized water or alcohol and placed in an ultrasonic bath to break up some of the agglomerates before the Ni wire mesh or SiO 2  fiber cloth is dipped into the solution. Alternatively, the solvent can be sprayed onto the Ni wire mesh or SiO 2  fiber cloth. The Ni mesh and SiO 2  cloth can be drip dried before placed in a 110° C. oven. Before use in the second stage of the system, the CCDL catalysts are heated in air or nitrogen at the reaction temperature, or slightly above, to obtain a stable catalyst. 
     Current COG technology uses yttria-stabilized zirconia (YSZ) electrolytes, and is operated in the range of 700 to 1000° C. to avoid unacceptably high Ohmic losses. These high temperatures demand heavy materials for insulation and require significant energy input to achieve the operating temperature. For the present invention, it is critical to reduce the thermal gradient between the COG O 2  output and the ventilation loop gas temperature to achieve the goal of oxygen recovery and recycling and to control the temperature to which an individual using the system is exposed. Therefore, it is preferable to use a COG that has a reasonable power at low temperatures. Having a lower operating temperature provides the benefits of: (i) reduced thermal stresses (e.g., from thermal expansion mismatches); (ii) lowered radiative losses; and (iii) negligible electrode sintering. At lower temperatures, the oxide ion conductivity of YSZ is too low for viable operation in a COG, as can be seen in  FIG. 3 . As shown in  FIG. 3 , erbia-stabilized bismuth oxide (ESB) and an acceptor-doped ceria, gadolinia-doped ceria (GDC), have the highest conductivity for the targeted temperature range of about 500° C. However, in reducing conditions, such as high CO 2  reduction potential or low oxygen partial pressure, acceptor-doped ceria becomes a mixed conductor with a reduced efficiency, and bismuth oxide decomposes. 
     In one embodiment of the invention, a functionally graded bilayer electrolyte, comprised of a layer of cubic-stabilized bismuth oxide, for example erbia-stabilized bismuth oxide (ESB), on the oxidizing side and a layer of acceptor-doped ceria, for example samaria- or gadolinia doped ceria (SDC or GDC), on the reducing side, is used. As illustrated in  FIG. 4 , this bilayer can employ constituent layers that have a thickness ratio where the ceria layer protects the bismuth oxide layer from decomposing by shielding it from very low oxygen partial pressures. Concomitantly, the ESB layer blocks electronic flux through the electrolyte and thereby neutralizes the efficiency decreasing electronic leakage of the ceria layer. Furthermore, small dopant concentrations of SDC in ESB or ESB in SDC have conductivities comparable to the host lattice and these solid solutions that can form at the SDC-ESB interface are not detrimental to the performance of the bilayer. 
     Both tubular and planar designs for the COG, as shown in  FIGS. 5   a  and  5   b , can be used. In general, tubular designs permit simplified sealing, which has generally been more reliable than seals in planar configurations. However, planar designs are generally simpler to fabricate. 
     The thickness of the bilayer electrolyte  1  is critical for minimizing Ohmic losses. An electrolyte thickness less than about 8 μm on a porous Ni-ceria substrate generally compromises its density, resulting in pinhole formation through the electrolyte  1 . Consequently, a bilayer electrolyte prepared via this technique has a minimum thickness of about 16 μm, or greater for thickness ratios other than 1:1. Increased electrolyte thickness leads to greater power consumption. 
     Because ceria sinters at temperatures in excess of 1500° C. while bismuth oxide sinters at about 890° C., the ceria layer is applied and sintered before applying the bismuth oxide film. After sintering of the ceria layer, application of the bismuth oxide layer by dip-coating (or spray-coating) becomes more difficult. To get a dense film, the thickness of the bismuth oxide layer is applied at thicknesses greater than 8 μm. For planar designs, as shown in.  FIG. 5   b , other thin film techniques can be used to form one or both constituent layers. Pulsed Laser Deposition (PLD) is a thin film technique that can deposit dense, stoichiometric and homogenous oxide films. 
     Prior art bilayer COGs use platinum (Pt) cathodes and Ag-ESB anodes. To increase the performance of bilayer COGs, in one embodiment of the invention, porous Pt-ceria or Cu-ceria cermets are used as the cathode  2 . Cermets increase the concentration of triple phase boundaries (TPBs)—where gas, ionic/ceramic and electronic/metallic phases meet. Moreover, since the TPBs are no longer confined to the electrolyte surface, the average distance traveled by the gas molecules to the reaction sites (i.e., the mean free path) is reduced significantly. This results in a reduction of the cathode  2  overpotential so that less power is used for CO 2  reduction/oxygen generation. In one embodiment of the invention, porous silver-ESB (erbia-stabilized bismuth oxide) cermets can be used as the anode  3 . In another embodiment of the invention, bismuth ruthenate-ESB composites can be used as the anode  3 . 
     All patents, patent applications, provisional applications, and publications referred to or cited herein, supra or infra, are incorporated by reference in their entirety, including all figures and tables, to the extent they are not inconsistent with the explicit teachings of this specification. 
     It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application.