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Separation of oxygen by using zirconia solid electrolyte membranes
Separation of oxygen by using zirconia solid electrolyte membranes* D.J. Clark, R.W. Losey and J.W. Suitor Jet Propulsion Laboratory, California Institute of Technology, 4800 M/S 125-l 77, Pasadena, California 9 1109, USA
Oak Grove Drive,
Received 7 April 1992
Oxygen separation from air and other oxygen-containing gases and gas mixtures by using zirconia solid electrolyte membranes has the potential to meet the oxygen needs of many applications. The high purity of the product oxygen and the electrochemical nature of the process are particularly attractive. The technology has been proposed for use in large-scale industrial applications, medical applications and in processing of off-planet resources. These applications are made possible by the ability of stabilized zirconia to conduct oxygen ions at high temperature. The performance of oxygen-separation systems built at JPL, capable of up to 1 I min-’ output, is discussed. Factors influencing the efficiency of the oxygen separation process and systems analysis of conceptual oxygen production plants are also addressed. Keywords:
zirconie;
solid electrolyte;
oxygen separation
Introduction zirconia (YSZ) is one of a number of ceramic materials that can conduct electrical current via oxygen ion transport. For the past five years. the Jet Propulsion Laboratory (JPL) of the California Institute of Technology has been developing zirconia-based oxygen separation systems for use in coal gasification. Other applications, including production of medical oxygen and off-planet processing oflunar and Martian resources, have also been proposed. In contrast to the concentration gradient-driven separation employed by traditional membrane separators. separation of oxygen in an electrolytic cell is an electrochemical process, driven by electrical energy supplied by a direct current power source. This fact gives solid electrolyte membrane separators several advantages over traditional membrane-based oxygen separation schemes. including infinite selectivity and the ability to deliver oxygen at elevated pressures (pumping). Yttria-stabilized
Theory The fluorite crystal lattice of yttria-stabilized zirconia (ZIO~),,~~ (Y,O,),,,, contains oxygen ion vacancies. When an electric field is applied to such a crystal. oxygen ions migrate between vacancies. creating an appreciable oxygen ion conductivity. An oxygen separation cell is formed when a membrane of yttria-stabilized zirconia (YSZ) is sandwiched between two gas-permeable electrically conductive electrodes (Figure 1). *Presented at GAS Separation International,Austin, TX, USA. The US Government paper.
retains certain rights in data presented
0950-4214/92/040201-05 0 1992 Butterworth-Heinemann
Ltd
in this
In operation, an oxygen-containing feedstock, such as air, is supplied to the cathode side of the cell. Oxygen gas diffuses through the porous cathode and arrives at the cathode-electrolyte interface. There, oxygen molecules are dissociated, ionized, and enter oxygen ion vacancies in the YSZ lattice. The overall reaction (for air as the feedstock) is: O2 (gas 0.2 atm) + 4-e- (cathode)
* 20”- (electrolyte)
(1)
Once in the crystal lattice, oxygen ions move under the influence of the electric field provided by the power supply. The transport of ions between vacancies is moderated by the presence of an activation energy barrier. resulting in an effective ionic resistivity of the form:
wherep is the ionic resistivity, E, is the activation energy of migration (18 kcal mol-’ is typical for 8 mol% YSZ). kB is the Boltzmann constant (= 1.4 X IO-” J K-‘), T is the absolute temperature and C is the proportionality constant, a function ofvacancy concentration. The typical operating temperature of 1273 K yields ionic resistivity of about 10 n cm.’ After being conducted through the YSZ electrolyte. oxygen ions are recombined at the electrolyte-anode interface: 20’-
(electrolyte)
* O2 (gas 1.0 atm) + 4e- (anode)
(3)
The product oxygen gas is available diffusing through the porous anode.
for collection
after
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Zirconia oxygen separation: D. J. Clark et al.
starvation at the cathode-electrolyte enrichment at the electrolyte-anode
interface and oxygen interface.
Zirconia solid electrolyte
Advantages
Figure 1
Oxygen separation cell
Examination of reactions (1) and (3) indicates a direct correlation between current flow (transport of electrons in the circuit) and oxygen mass flow rate; transport of four electrons results in the conduction of exactly one oxygen molecule. Use of the Faraday constant and the molecular weight of oxygen allows calculation of the mass flow rate of product oxygen for 1 A of current: l/j=
lCe-X S
kmol
0,
32 kg
9.6 X 10’ C ’ &? ’ kmol
= 8.3 x IO-* kg O2 (4)
S
Because power is the product of voltage and current, for a given mass flow rate of product oxygen the separation power is dictated by the voltage appearing across the cell. Thus, to maximize efficiency, the voltage appearing across the cell should be minimized. The magnitude of applied cell voltage is determined by several phenomena. The ratio of partial pressure of oxygen on the cathode (input) side of the cell, P,, to that on the anode (output) side, P2, results in a Nemst potential:
(5) where R is the gas constant (8.31 X lo3 J kmol-’ K-‘), T is the absolute temperature (1273 K typical), n is the number of electrons transferred (four) and F is Faraday’s constant (9.6 X 10’ C kmol-‘). This voltage is a result ofthe fact that in a typical oxygen separation cell, the oxygen chemical potential increases as oxygen is ‘pumped’ from a condition of low partial pressure (0.2 atm for atmospheric air) to a condition of high partial pressure (1 .O atm for pure oxygen at atmospheric pressure) (see, for example, reference 2). When oxygen transport occurs against such a chemical potential, the Nemst potential adds to the power consumntion. Electrical resistance in the electrodes and ionic resistance in the electrolvte are additional contributors to the overall cell voltage.’ A cell’s ionic resistance is the product of the ionic resistivity of the YSZ and the cell thickness, divided by its area. Voltage drop due to ionic resistance can be minimized by making the electrolyte membrane large in area and as thin as possible. In addition to the effects noted above, additional voltage requirements occur at each of the two electrodeelectrolyte interfaces. These effects, known as electrode polarization, are due to physical limitations to the speed at which reactions (1) and (3) can occur. An example of electrode polarization is the resistance to gaseous diffusion in the electrodes that results in regions of oxygen
202
Gas Separation
Because the vacancies responsible for oxygen ion conduction in materials like YSZ accept only oxygen ions, membrane selectivity is infinite, regardless of the other gases present in the feedstock. This results in 100% pure product oxygen in a single separation stage. Oxygen can also be separated from oxygen-containing gases such as carbon dioxide or steam at the zirconia operating temperature of 1000°C. Electrochemical interactions between the gas, the electrode and the electrolyte result in dissociation fractions substantially larger than those predicted by thermal decomposition reactions. Because the process is electrically driven, the YSZ oxygen separation cell can be operated with negligible pressure differential across the membrane. This reduces the cost and complexity of a separation plant by eliminating the need for compressors. In addition. the oxygen production rate is easily controlled by varying the applied electrical current.
8 Purification 1992
Vol 6 No 4
Experimental
work
JPL’s experimental work has concentrated on development, fabrication and testing of viable YSZ-based oxygen separators. Towards this end, the cell geometry used was designed with the aid of computer models to reduce the power consumption. Fabrication techniques and materials suitable for economical mass production have been extensively employed. For example, use of strontiumdoped lanthanum manganite (LSM), an electrically conductive ceramic, has eliminated the need for expensive platinum electrodes. In addition, manifold schemes have been developed to allow multiple cells to be operated together to increase oxygen output.
Cell geometry Computer models developed early in the project indicated that the power consumption of a circular cell, with air supplied at the perimeter of the cell and removed from its centre, was lower than that of a tubular ce113.4, so the development has concentrated on this circular geometry. In this scheme, feedstock flows in a radial direction towards the centre of the cell (Figure 2). Note that electrical current is supplied over the entire surface of the cell.
Membrane
fabrication
A fabrication technique was developed for production of thin, circular separation membranes. Based on the method of tape casting, the technique begins with preparation of YSZ slip, a mixture of powdered YSZ and oil-based binders. A slip of LSM is also prepared. The YSZ slip is cast with a flat blade on a glass table over a layer of LSM slip. The two-layer tape is folded upon itself to form a three-layer sandwich, which is then hydraulically pressed and fired to 1300°C. The resultant three-layer membrane contains a solid YSZ electrolyte layer 125 pm thick surrounded by two electronically conductive porous LSM electrodes.
Zirconia oxygen separation: D. J. Clark et al.
Oxygen-depleted air out
Air in
Electrolyte
Air in
Oxygen
out
Figure 2
Gas flow in circular-disk
Multicell
separation stack design
oxygen
separation
cell
To obtain appreciable amounts ofoxygen, up to 20 cells of the type shown in Figure 2 are stacked and operated simultaneously. To ensure that all cells produce equal amounts of oxygen, the cells are connected in series electrically. Gas connections, however, are made in parallel so that all cells have fresh feedstock. The product oxygen and spent feedstock are collected from each cell by integral manifolds. Non-porous, electrically conductive distribution plates prevent mixing between a cell’s product oxygen and the feedstock of the adjacent cell. Figure 3 is a diagram of the manifold. For development work, the distribution plates, also known as interconnects. were fabricated from A-Lava, a machinable natural stone (alumina silicate) that required a platinum coating to impart electrical conductivity. While convenient for development work use of this material was labour-intensive and not suitable for mass production. Distribution plates were later formed by slip casting with LSM. Multicell
contact resistan&. The second factor resulting in higher than expected power consumption is electrode polarization. Single-cell tests indicated that application of ceria (which can conduct both electrons and oxygen ions) to the electrode-electrolyte interface during membrane fabrication could reduce overall cell voltage by a factor of two. Stack sealing remains a major difficulty. All stacks were operated at 1 atm total pressure on input and output. Seals were made by precision grinding mating surfaces of stack components before assembly and application of mechanical pressure along the length of the stack during operation. Oxygen outputs as a function of applied current of the best-sealed stack is shown in Figure 5. The deviation of actual oxygen production from that predicted by the applied current shown in the figure indicates leakage of product oxygen from the manifold. Development of more robust seals is a high priority for future development work. Applications Air separation
A conceptual air separation plant based on zirconia solid electrolyte technology is, shown in Fi’re6. Heat exchangers are used to cool the product and oxygendepleted air streams while preheating the feedstock. The electrochemical oxygen separation process is modular, thus small and large plants produce oxygen at
stack performance
Several multicell air separation stacks were constructed. The largest of these, built with 20 6.35 cm diameter cells and A-Lava distribution plates, was able to produce one standard litre per minute (SLPM) of oxygen. Six-cell stacks were built by using LSM distribution plates. Figure 4 shows the specific energy of both the A-Lava and LSM stacks (furnace power not included). Note that for a given current density, the power consumption of the stacks constructed with LSM distribution plates was lower than that of the stacks built with A-Lava plates. This is due to the lower electrical resistance of the LSM distribution plates, as well as improved electrical contact between the LSM plates and the threelayer separation membranes. Modelling studies4 indicate that membrane power consumption should be about 100 kWh t-’ when operated at 0.3 A cm-‘. The disparity between measured power consumption (Figure4) and the modelled value can be attributed to two factors. First. four-w-ire resistance measurements made across a single cell during stack operation indicated that there was a significant contact resistance between the distribution plates and the separation membranes. This was confirmed by several cell failures that showed evidence of localized heating caused by
Figure 3
Schematic
diagram
of manifold
4000 X
3600 0
3000
X
X
0.0
0.1
0.2
II
A-Lava, 20 cell
x
LSM, 6cell
0.4
0.3
0.5
0.6
Current density (A/a+ 1
Figure 4
Specific
energy
of oxygen
separation
Gas Separation 8 Purification 1992
stacks
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203
Zirconia oxygen separation: D. J. Clark et al.
q
-
0
1
production with the anticipated maturity of zirconia technology. Oxygen produced in a 1000 t d-’ plant of this type was estimated to cost $39.00 per ton (1988 dollars) including 600 psi oxygen compression(‘. Wright and Copeland estimated oxygen cost for a 10 t d-’ zirconia plant to be $36.00 per ton (1985 dollars)‘. These investigators stated that oxygen could be produced with a pressuredriven system (in which an applied pressure differential overcomes the Nernst potential and external electrical power is not used) by using natural-gas turbine-driven compressors for about $24 per ton. This cost assumes that there is a substantial improvement in actual performance of the system.
Oxygen output Theoretical
2
4
3
Current (A)
Figure 5 Oxygen output (standard cubic centimetres per minute) of 20-cell stack
Product oxygen output
Oxygen
Coal gasification Under US Department of Energy funding. JPL has for the last five years been developing zirconia-based oxygen separation for use in coal gasification. The intent was to use the zirconia unit in an integrated gasification combined cycle (IGCC) plant, as shown in Figure 7. This scheme is similar to an Air Products patentx. Economic analysis indicates that the use of the zirconia air separation plant would be economically competitive with the use of a cryogenic plan?. Aerospace applications
Oxygen-depleted air vent
I suPPrYI Figure 6
Zirconia-based oxygen production plant
roughly the same cost. Thus, a plant of the type shown in Figure 6 could be used in applications ranging from home medical oxygen units producing 3 1 min-’ to industrial units capable of 1000 t d-’ production. A cost estimate, made by using EPFU Technology Assessment Guidelines (TAG), was performed by JPL’. The assumptions of the cost estimate are given in Table 1. It was assumed that the performance of the zirconia system would be at predicted power consumptions rather than the higher values currently measured in the laboratory. This assumption was made in order to compare mature cryogenic oxygen
Planners of future missions to the moon are considering using zirconia solid electrolyte technology to produce oxygen for propellant and life support from lunar mineral resources. The plentiful lunar mineral ilmenite (FeTiO,) would be reacted with hydrogen (imported from earth) at high temperature to produce steam. Zirconia separation cells would electrolyse the steam, producing oxygen for life support and propellant. The hydrogen remaining would be recycled to the reactor’. Plans for manned missions to Mars call for oxygen to be produced by decomposition of the carbon dioxide present in the Martian atmosphere’“. A zirconia oxygen separation unit could be used in this application”.
Conclusions Oxygen separation by using the zirconia solid electrolyte membrane has the potential to meet the needs of many segments of the oxygen market. Operation of separation systems based on commercially viable materials and
Coal in
Table 1
Gasifier
Economic assumptions
Debt cost (%) Debt ratio Preferred stock cost (%) Preferred ratio Common stock cost (%) Common stock ratio Total equity ratio Before-tax discount rate Effective income tax rate investment tax credit Property taxes and insuranc:e Plant tax life Plant book life Depreciation tax method Book depreciation method
4.600 0.500 5.200 0.150 8.700 0.350 0.500 0.061 0.38 0.00 0.02 20 30 ACRS Straight line
air separation
Electricity
Figure 7
204
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Electricity
Steam turbine
Integrated gasification combined cycle plant
Zirconia oxygen separation: D.J. Clark et al.
fabrication processes has been demonstrated. of power consumption to near theoretical development of suitable sealing techniques requiring further investigation.
Reduction levels and are areas
Acknowledgements The authors wish to acknowledge J.F. Ferrall, L.M. Galica and N.K. Rohatgi for their contributions to this work. The research described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Technology, and was sponsored by the Morgantown Energy Technology Center of the United States Department of Energy through an agreement with the National Aeronautics and Space Administration.
References I 2 3
Etsell, T.H. and Flengas, S.N. The electrical properties of solid oxide electrolytes Chem Rev (1970) 70(3) 339-376 Boikess, R.S. and Edelson, E. Chemical Principles Harper & Row. New York (1981) ch I6 Suitor, J.W., Berdahl, C.M. and Mamer, W.J. Apparatus in the form of a disk for the separation of oxygen from other gases and/or for the pumping of oxygen and the method of removing the oxygen US Patent 4.885.142 (5 December 1989)
4
Suitor, J.W., Berdahl, C.M., Ferrall, J.F., Mamer, W.J., Schroeder, J.E. and Shlichta, P.J. Development of an alternate oxygen production source using a zirconia solid electrolytic membrane: Technical progress report for fiscal years 1986 and 1987Jet Prop&ion Laboratory Internal Document D-4320 (May 1987) 5 Electrical Power Research Institute (EPRI). TAG-Technical Assessment Guide EPRIRep. P-4463-SR Vol I (December 1986) and Vat 3 (May 1987) 6 Suitor, J.W., Clark, D.J. and Losey, R.W. Development of alternative oxygen production source using a zirconia solid electrolytic membrane: Technical progress report for fiscal vears 1987. 1988 and 1990 Jet Prom&on Laboratorv Internal document D-7790 (August 1990) ’ 7 Wright, J.D. and Copeland, R.J. Advanced oxygen separation membranes: Topical report Gas Research Institute Repbe TDAGM-W/O303 (Seutember 1990) 8 Hegarty, W.P. Process for producing by-product oxygen from turbine power generation US Patent 4.54.5.787 (5 October 1985) 9 Mamer, W.J., Suitor, J.W., Schooley, L.S. and Cellier,F.E. Automation and control of off-planet oxygen production processes Engineering, Construction and Operations in Space II, Proceedings of Space90 Vol I. ASCE. New York (April 1990) 226-235 IO Zubrin, R. and Baker, D. Humans to Mars in 1999Aerospace America (August 1990) 30-32.41 I I Sullivan, T.A. and McKay, D.S. Using Space Resources NASA Johnson Space Center. Houston (1991)
Gas Separation
8 Purification 1992
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205
Oak Grove Drive,
Received 7 April 1992
Oxygen separation from air and other oxygen-containing gases and gas mixtures by using zirconia solid electrolyte membranes has the potential to meet the oxygen needs of many applications. The high purity of the product oxygen and the electrochemical nature of the process are particularly attractive. The technology has been proposed for use in large-scale industrial applications, medical applications and in processing of off-planet resources. These applications are made possible by the ability of stabilized zirconia to conduct oxygen ions at high temperature. The performance of oxygen-separation systems built at JPL, capable of up to 1 I min-’ output, is discussed. Factors influencing the efficiency of the oxygen separation process and systems analysis of conceptual oxygen production plants are also addressed. Keywords:
zirconie;
solid electrolyte;
oxygen separation
Introduction zirconia (YSZ) is one of a number of ceramic materials that can conduct electrical current via oxygen ion transport. For the past five years. the Jet Propulsion Laboratory (JPL) of the California Institute of Technology has been developing zirconia-based oxygen separation systems for use in coal gasification. Other applications, including production of medical oxygen and off-planet processing oflunar and Martian resources, have also been proposed. In contrast to the concentration gradient-driven separation employed by traditional membrane separators. separation of oxygen in an electrolytic cell is an electrochemical process, driven by electrical energy supplied by a direct current power source. This fact gives solid electrolyte membrane separators several advantages over traditional membrane-based oxygen separation schemes. including infinite selectivity and the ability to deliver oxygen at elevated pressures (pumping). Yttria-stabilized
Theory The fluorite crystal lattice of yttria-stabilized zirconia (ZIO~),,~~ (Y,O,),,,, contains oxygen ion vacancies. When an electric field is applied to such a crystal. oxygen ions migrate between vacancies. creating an appreciable oxygen ion conductivity. An oxygen separation cell is formed when a membrane of yttria-stabilized zirconia (YSZ) is sandwiched between two gas-permeable electrically conductive electrodes (Figure 1). *Presented at GAS Separation International,Austin, TX, USA. The US Government paper.
retains certain rights in data presented
0950-4214/92/040201-05 0 1992 Butterworth-Heinemann
Ltd
in this
In operation, an oxygen-containing feedstock, such as air, is supplied to the cathode side of the cell. Oxygen gas diffuses through the porous cathode and arrives at the cathode-electrolyte interface. There, oxygen molecules are dissociated, ionized, and enter oxygen ion vacancies in the YSZ lattice. The overall reaction (for air as the feedstock) is: O2 (gas 0.2 atm) + 4-e- (cathode)
* 20”- (electrolyte)
(1)
Once in the crystal lattice, oxygen ions move under the influence of the electric field provided by the power supply. The transport of ions between vacancies is moderated by the presence of an activation energy barrier. resulting in an effective ionic resistivity of the form:
wherep is the ionic resistivity, E, is the activation energy of migration (18 kcal mol-’ is typical for 8 mol% YSZ). kB is the Boltzmann constant (= 1.4 X IO-” J K-‘), T is the absolute temperature and C is the proportionality constant, a function ofvacancy concentration. The typical operating temperature of 1273 K yields ionic resistivity of about 10 n cm.’ After being conducted through the YSZ electrolyte. oxygen ions are recombined at the electrolyte-anode interface: 20’-
(electrolyte)
* O2 (gas 1.0 atm) + 4e- (anode)
(3)
The product oxygen gas is available diffusing through the porous anode.
for collection
after
Gas Separation 8 Purification 1992
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Zirconia oxygen separation: D. J. Clark et al.
starvation at the cathode-electrolyte enrichment at the electrolyte-anode
interface and oxygen interface.
Zirconia solid electrolyte
Advantages
Figure 1
Oxygen separation cell
Examination of reactions (1) and (3) indicates a direct correlation between current flow (transport of electrons in the circuit) and oxygen mass flow rate; transport of four electrons results in the conduction of exactly one oxygen molecule. Use of the Faraday constant and the molecular weight of oxygen allows calculation of the mass flow rate of product oxygen for 1 A of current: l/j=
lCe-X S
kmol
0,
32 kg
9.6 X 10’ C ’ &? ’ kmol
= 8.3 x IO-* kg O2 (4)
S
Because power is the product of voltage and current, for a given mass flow rate of product oxygen the separation power is dictated by the voltage appearing across the cell. Thus, to maximize efficiency, the voltage appearing across the cell should be minimized. The magnitude of applied cell voltage is determined by several phenomena. The ratio of partial pressure of oxygen on the cathode (input) side of the cell, P,, to that on the anode (output) side, P2, results in a Nemst potential:
(5) where R is the gas constant (8.31 X lo3 J kmol-’ K-‘), T is the absolute temperature (1273 K typical), n is the number of electrons transferred (four) and F is Faraday’s constant (9.6 X 10’ C kmol-‘). This voltage is a result ofthe fact that in a typical oxygen separation cell, the oxygen chemical potential increases as oxygen is ‘pumped’ from a condition of low partial pressure (0.2 atm for atmospheric air) to a condition of high partial pressure (1 .O atm for pure oxygen at atmospheric pressure) (see, for example, reference 2). When oxygen transport occurs against such a chemical potential, the Nemst potential adds to the power consumntion. Electrical resistance in the electrodes and ionic resistance in the electrolvte are additional contributors to the overall cell voltage.’ A cell’s ionic resistance is the product of the ionic resistivity of the YSZ and the cell thickness, divided by its area. Voltage drop due to ionic resistance can be minimized by making the electrolyte membrane large in area and as thin as possible. In addition to the effects noted above, additional voltage requirements occur at each of the two electrodeelectrolyte interfaces. These effects, known as electrode polarization, are due to physical limitations to the speed at which reactions (1) and (3) can occur. An example of electrode polarization is the resistance to gaseous diffusion in the electrodes that results in regions of oxygen
202
Gas Separation
Because the vacancies responsible for oxygen ion conduction in materials like YSZ accept only oxygen ions, membrane selectivity is infinite, regardless of the other gases present in the feedstock. This results in 100% pure product oxygen in a single separation stage. Oxygen can also be separated from oxygen-containing gases such as carbon dioxide or steam at the zirconia operating temperature of 1000°C. Electrochemical interactions between the gas, the electrode and the electrolyte result in dissociation fractions substantially larger than those predicted by thermal decomposition reactions. Because the process is electrically driven, the YSZ oxygen separation cell can be operated with negligible pressure differential across the membrane. This reduces the cost and complexity of a separation plant by eliminating the need for compressors. In addition. the oxygen production rate is easily controlled by varying the applied electrical current.
8 Purification 1992
Vol 6 No 4
Experimental
work
JPL’s experimental work has concentrated on development, fabrication and testing of viable YSZ-based oxygen separators. Towards this end, the cell geometry used was designed with the aid of computer models to reduce the power consumption. Fabrication techniques and materials suitable for economical mass production have been extensively employed. For example, use of strontiumdoped lanthanum manganite (LSM), an electrically conductive ceramic, has eliminated the need for expensive platinum electrodes. In addition, manifold schemes have been developed to allow multiple cells to be operated together to increase oxygen output.
Cell geometry Computer models developed early in the project indicated that the power consumption of a circular cell, with air supplied at the perimeter of the cell and removed from its centre, was lower than that of a tubular ce113.4, so the development has concentrated on this circular geometry. In this scheme, feedstock flows in a radial direction towards the centre of the cell (Figure 2). Note that electrical current is supplied over the entire surface of the cell.
Membrane
fabrication
A fabrication technique was developed for production of thin, circular separation membranes. Based on the method of tape casting, the technique begins with preparation of YSZ slip, a mixture of powdered YSZ and oil-based binders. A slip of LSM is also prepared. The YSZ slip is cast with a flat blade on a glass table over a layer of LSM slip. The two-layer tape is folded upon itself to form a three-layer sandwich, which is then hydraulically pressed and fired to 1300°C. The resultant three-layer membrane contains a solid YSZ electrolyte layer 125 pm thick surrounded by two electronically conductive porous LSM electrodes.
Zirconia oxygen separation: D. J. Clark et al.
Oxygen-depleted air out
Air in
Electrolyte
Air in
Oxygen
out
Figure 2
Gas flow in circular-disk
Multicell
separation stack design
oxygen
separation
cell
To obtain appreciable amounts ofoxygen, up to 20 cells of the type shown in Figure 2 are stacked and operated simultaneously. To ensure that all cells produce equal amounts of oxygen, the cells are connected in series electrically. Gas connections, however, are made in parallel so that all cells have fresh feedstock. The product oxygen and spent feedstock are collected from each cell by integral manifolds. Non-porous, electrically conductive distribution plates prevent mixing between a cell’s product oxygen and the feedstock of the adjacent cell. Figure 3 is a diagram of the manifold. For development work, the distribution plates, also known as interconnects. were fabricated from A-Lava, a machinable natural stone (alumina silicate) that required a platinum coating to impart electrical conductivity. While convenient for development work use of this material was labour-intensive and not suitable for mass production. Distribution plates were later formed by slip casting with LSM. Multicell
contact resistan&. The second factor resulting in higher than expected power consumption is electrode polarization. Single-cell tests indicated that application of ceria (which can conduct both electrons and oxygen ions) to the electrode-electrolyte interface during membrane fabrication could reduce overall cell voltage by a factor of two. Stack sealing remains a major difficulty. All stacks were operated at 1 atm total pressure on input and output. Seals were made by precision grinding mating surfaces of stack components before assembly and application of mechanical pressure along the length of the stack during operation. Oxygen outputs as a function of applied current of the best-sealed stack is shown in Figure 5. The deviation of actual oxygen production from that predicted by the applied current shown in the figure indicates leakage of product oxygen from the manifold. Development of more robust seals is a high priority for future development work. Applications Air separation
A conceptual air separation plant based on zirconia solid electrolyte technology is, shown in Fi’re6. Heat exchangers are used to cool the product and oxygendepleted air streams while preheating the feedstock. The electrochemical oxygen separation process is modular, thus small and large plants produce oxygen at
stack performance
Several multicell air separation stacks were constructed. The largest of these, built with 20 6.35 cm diameter cells and A-Lava distribution plates, was able to produce one standard litre per minute (SLPM) of oxygen. Six-cell stacks were built by using LSM distribution plates. Figure 4 shows the specific energy of both the A-Lava and LSM stacks (furnace power not included). Note that for a given current density, the power consumption of the stacks constructed with LSM distribution plates was lower than that of the stacks built with A-Lava plates. This is due to the lower electrical resistance of the LSM distribution plates, as well as improved electrical contact between the LSM plates and the threelayer separation membranes. Modelling studies4 indicate that membrane power consumption should be about 100 kWh t-’ when operated at 0.3 A cm-‘. The disparity between measured power consumption (Figure4) and the modelled value can be attributed to two factors. First. four-w-ire resistance measurements made across a single cell during stack operation indicated that there was a significant contact resistance between the distribution plates and the separation membranes. This was confirmed by several cell failures that showed evidence of localized heating caused by
Figure 3
Schematic
diagram
of manifold
4000 X
3600 0
3000
X
X
0.0
0.1
0.2
II
A-Lava, 20 cell
x
LSM, 6cell
0.4
0.3
0.5
0.6
Current density (A/a+ 1
Figure 4
Specific
energy
of oxygen
separation
Gas Separation 8 Purification 1992
stacks
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203
Zirconia oxygen separation: D. J. Clark et al.
q
-
0
1
production with the anticipated maturity of zirconia technology. Oxygen produced in a 1000 t d-’ plant of this type was estimated to cost $39.00 per ton (1988 dollars) including 600 psi oxygen compression(‘. Wright and Copeland estimated oxygen cost for a 10 t d-’ zirconia plant to be $36.00 per ton (1985 dollars)‘. These investigators stated that oxygen could be produced with a pressuredriven system (in which an applied pressure differential overcomes the Nernst potential and external electrical power is not used) by using natural-gas turbine-driven compressors for about $24 per ton. This cost assumes that there is a substantial improvement in actual performance of the system.
Oxygen output Theoretical
2
4
3
Current (A)
Figure 5 Oxygen output (standard cubic centimetres per minute) of 20-cell stack
Product oxygen output
Oxygen
Coal gasification Under US Department of Energy funding. JPL has for the last five years been developing zirconia-based oxygen separation for use in coal gasification. The intent was to use the zirconia unit in an integrated gasification combined cycle (IGCC) plant, as shown in Figure 7. This scheme is similar to an Air Products patentx. Economic analysis indicates that the use of the zirconia air separation plant would be economically competitive with the use of a cryogenic plan?. Aerospace applications
Oxygen-depleted air vent
I suPPrYI Figure 6
Zirconia-based oxygen production plant
roughly the same cost. Thus, a plant of the type shown in Figure 6 could be used in applications ranging from home medical oxygen units producing 3 1 min-’ to industrial units capable of 1000 t d-’ production. A cost estimate, made by using EPFU Technology Assessment Guidelines (TAG), was performed by JPL’. The assumptions of the cost estimate are given in Table 1. It was assumed that the performance of the zirconia system would be at predicted power consumptions rather than the higher values currently measured in the laboratory. This assumption was made in order to compare mature cryogenic oxygen
Planners of future missions to the moon are considering using zirconia solid electrolyte technology to produce oxygen for propellant and life support from lunar mineral resources. The plentiful lunar mineral ilmenite (FeTiO,) would be reacted with hydrogen (imported from earth) at high temperature to produce steam. Zirconia separation cells would electrolyse the steam, producing oxygen for life support and propellant. The hydrogen remaining would be recycled to the reactor’. Plans for manned missions to Mars call for oxygen to be produced by decomposition of the carbon dioxide present in the Martian atmosphere’“. A zirconia oxygen separation unit could be used in this application”.
Conclusions Oxygen separation by using the zirconia solid electrolyte membrane has the potential to meet the needs of many segments of the oxygen market. Operation of separation systems based on commercially viable materials and
Coal in
Table 1
Gasifier
Economic assumptions
Debt cost (%) Debt ratio Preferred stock cost (%) Preferred ratio Common stock cost (%) Common stock ratio Total equity ratio Before-tax discount rate Effective income tax rate investment tax credit Property taxes and insuranc:e Plant tax life Plant book life Depreciation tax method Book depreciation method
4.600 0.500 5.200 0.150 8.700 0.350 0.500 0.061 0.38 0.00 0.02 20 30 ACRS Straight line
air separation
Electricity
Figure 7
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Steam turbine
Integrated gasification combined cycle plant
Zirconia oxygen separation: D.J. Clark et al.
fabrication processes has been demonstrated. of power consumption to near theoretical development of suitable sealing techniques requiring further investigation.
Reduction levels and are areas
Acknowledgements The authors wish to acknowledge J.F. Ferrall, L.M. Galica and N.K. Rohatgi for their contributions to this work. The research described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Technology, and was sponsored by the Morgantown Energy Technology Center of the United States Department of Energy through an agreement with the National Aeronautics and Space Administration.
References I 2 3
Etsell, T.H. and Flengas, S.N. The electrical properties of solid oxide electrolytes Chem Rev (1970) 70(3) 339-376 Boikess, R.S. and Edelson, E. Chemical Principles Harper & Row. New York (1981) ch I6 Suitor, J.W., Berdahl, C.M. and Mamer, W.J. Apparatus in the form of a disk for the separation of oxygen from other gases and/or for the pumping of oxygen and the method of removing the oxygen US Patent 4.885.142 (5 December 1989)
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Suitor, J.W., Berdahl, C.M., Ferrall, J.F., Mamer, W.J., Schroeder, J.E. and Shlichta, P.J. Development of an alternate oxygen production source using a zirconia solid electrolytic membrane: Technical progress report for fiscal years 1986 and 1987Jet Prop&ion Laboratory Internal Document D-4320 (May 1987) 5 Electrical Power Research Institute (EPRI). TAG-Technical Assessment Guide EPRIRep. P-4463-SR Vol I (December 1986) and Vat 3 (May 1987) 6 Suitor, J.W., Clark, D.J. and Losey, R.W. Development of alternative oxygen production source using a zirconia solid electrolytic membrane: Technical progress report for fiscal vears 1987. 1988 and 1990 Jet Prom&on Laboratorv Internal document D-7790 (August 1990) ’ 7 Wright, J.D. and Copeland, R.J. Advanced oxygen separation membranes: Topical report Gas Research Institute Repbe TDAGM-W/O303 (Seutember 1990) 8 Hegarty, W.P. Process for producing by-product oxygen from turbine power generation US Patent 4.54.5.787 (5 October 1985) 9 Mamer, W.J., Suitor, J.W., Schooley, L.S. and Cellier,F.E. Automation and control of off-planet oxygen production processes Engineering, Construction and Operations in Space II, Proceedings of Space90 Vol I. ASCE. New York (April 1990) 226-235 IO Zubrin, R. and Baker, D. Humans to Mars in 1999Aerospace America (August 1990) 30-32.41 I I Sullivan, T.A. and McKay, D.S. Using Space Resources NASA Johnson Space Center. Houston (1991)
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