FRANK J.
HENDEL
WATER RECOVERY FROM FUEL CELLS AND LUNAR MINERALS Water supplies f o r space missions-a challenging problem f o r applied chemical research n selecting suitable life support systems for space of long duration, waste disposal, water recovery, and atmospheric control are of major concern. Graphs which consider the launch weight of different water recovery systems, with and without environmental control, for missions up to 1000 days are available (4, 7). Present day technology indicates that a man can be supported in space up to three months with a launchweight expenditure of 400 pounds per man. This includes carbon dioxide removal, oxygen supply, and, if sufficient power is available, water and oxygen reclamation. Food supply is not included. For missions of one year, the launch-weight expenditure must be doubled, and for three year missions, tripled. A moderately active man requires on the average at least 7 pounds of water per day, including a minimum amount for washing. Several methods of water recovery from human metabolic wastes are possible, and if hydrogen-oxygen fuel cells are used for secondary power generation, water in substantial quantities will be the by-product. Also, for lunar expeditions, moon raw materials heated in solar or nuclear furnaces may provide additional water. Many processes for water recovery have been reviewed (2-4, 9). This article reviews in detail the recovery of water from fuel cells and moon minerals.
I missions
Fuel Cells
Many space missions will depend on fuel cells as a source for the secondary (nonpropulsive) power necessary to operate the air conditioning, telecommunication, and other equipment aboard the spacecraft. The gaseous hydrogen and oxygen used as fuel and oxidizer form water that must be removed continuously. The rate of water recovery is approximately 1 pound per hour per kw. Thus, a I-kw. fuel cell can supply up to 24 pounds of water per day. The fuel cells anticipated for space missions use either an electrolyte held between two diaphragms, one diaphragm being an anode and the other a cathode, or an Frank J . Hendel is Staff Sczentzst with the North American Aviation, Inc., in Downey, Cal;f. He was Consultant to the Space #ScienceBoard, of the National Academy of Sciences, and teaches courses zn space vehicles and missiles at University of California, Los Angeles. Suggesttons of D r . G. C. Clementson are gratefully acknowledged. AUTHOR
ion membrane held between two electrodes. The cells with electrolytes use potassium hydroxide, sodium hydroxide, or sulfuric acid as water solutions, either diluted or concentrated. The Bacon fuel cell uses highly concentrated potassium hydroxide and must be operated at high temperatures (above the freezing point of the potassium hydroxide solution). The advantage of this is that heat rejection of the hydrogen-oxygen reaction (which is approximately half of the total energy produced) through the space radiators occurs a t high temperatures, thus requiring less radiating surface on the spacecraft. The ion-membrane fuel cell (8),developed by General Electric for use in the Gemini spacecraft, uses, instead of an electrolyte, a reinforced solid membrane placed between two electrodes. Membranes of acidic (sulfonated) polymers have been most successful. A thin, catalytic, porous electrode is applied to each surface of the membrane to provide a large number of three-phase (gas, conductor, and electrolyte) reaction sites per unit of projected area. The complete membrane-electrode assembly, usually about 0.03 inch thick, is relatively tough and pliable. These cells operate at approximately 150' F. Their primary safety feature is that the catalytic electrodes have a n immediate scavenging effect if a gas leak develops anywhere in a battery system. Each cell is, in effect, a catalytic oxidizer that immediately converts the smallest hydrogen-oxygen mixture into water. If gases should leak, they cannot accumulate to the degree that damage could occur. I n Figure 1 which illustrates the operation of the previously described fuel cells, the cell is represented by one element only, but actually there are many of these elements. Gaseous hydrogen and oxygen are supplied a t pressures ranging from 15 to 60 p.s.i.a. on opposite sides of each element. The gases penetrate the porous electrodes to contact either the electrolyte or the ion membrane. Hydrogen ionizes on the anode, giving up its electrons : Anode 2Hz + 4H (chemisorbed) 4H -+4 H + 4e-
+
Cathode 2 0 (chemisorbed) 4H+ 2 0 --+ 2H20
O2 4e-
---f
+
+
Over-all reaction 2Hz 0 2 + 2H20
+
The electrons are conducted through the load (doing work) to the cathode. At the cathode, the electrons, hydrogen ions, and gaseous oxygen react, forming water. If pressure of the gaseous hydrogen is slightly greater VOL. 5 6
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Figure 1. Ful cell c l m n t md typical water recouny. Only one clcmm: is shown h e , but a operating cell uctaully contoins m n y
Figure 2. Watm rccomy from 4n ion-membrone f u l cell. To make th .water fully potable, pncolatim throargh utioatcd chmcool m ion exchange rcsinr and also we of rnmbranc p m a t i o n is recommended
than that of gaseous oxygen, then the water formed is expelled on the oxygen side. If the pressure of the gaseous oxygen is greater than the pressure of gaseous hydrogen, the water formed will be expelled on the hydrogen side. The liquid electrolyte must be kept a t exactly the same temperature and pressure to avoid accumulation or loss of water from the electrolyte. The gas on that side of the fuel cell from which the water is expelled is used for recirculation. Figure 1 shows that the recirculation is done on the oxygen side. The humid oxygen is cooled in a condenser to room temperature, which causes the water vapors to liquefy and form droplets. The mixture goes to a centrifugal separator, where the water is centrifuged (at approximately 10,000 r.p.m.) and separated from the oxygen; then it enters a zero-gravity container. The oxygen, free from water, is blown back to the fuel cell by a blower. A recirculating fluid, Freon or a water solution of glycol, cools the gas in the condenser. The coolant, cooled in a space radiator which is turned away from the sun, removes not only the latent heat of condensation and the sensible heat of water but cools the gas as well, thus removing the waste heat formed during the fuel cell reaction. Cells operating at high temperatures must have an additional heat exchanger between the gas leaving the fuel cell with water vapors and the gas leaving the blower before it re-enters the fuel cell. In the ion-membrane fuel cell, the heat of the reaction is removed a t the electrode (Figure 2). The cathode is in contact with a flat plate, called a bipolar collector, through which the coolant flows. Water is condensed on the cold collector and absorbed by a layer of wicks (sponges). Capillary forces drive the water through a system of intermediate wicks to a stuffing box, which is a tube filled with transporting wicks, a t the end of the battery and to a zero-gravity water container (or to a container filled with sponges). Only water is transported through the wicks; gaseous oxygen is excluded from the water container. The tubes used for the stuffing box may be either rigid or flexible. When several batteries are present, these stuffing boxes are combined to form a common feeder connected to the water
container. A 50-watt battery requires a ‘/,-inch diameter stuffing box. I n General Electric fuel cells, water is transported upward via the wick connectors to water containers filled with a fibrous cellulose absorbent. A good absorbent draws six times its own weight, with a volume efficiency of 90%; volume efficiency equals the weight of absorbed water divided by the weight that could be held in the container without absorbents. Water from the fuel cells is usually slightly contaminated, either by potassium hydroxide or sulfuric acid. These contaminants come from liquid electrolytes through microscopic (or even macroscopic) cracks in electrode diaphragms and, in case of ion-membrane fuel cells, from acidic (sulfonated) polymers. In these solid polymers, the hydrogen ions or protons are exchanged from one SO8group to the next. The protons are hydrated and carry water molecules with them which pass through the porous electrode and usually contain minute amounts of acidic contaminants. T o make the water potable, alkalinity or acidity must be neutralized. Percolation through activated charcoal or ion exchange resins or application of membrane permeation is also recommended. In case of macroscopic cracks in the electrodes of fuel cells with liquid electrolytes, the recovered water may be very strongly alkaline or acidic, which requires shutting down the defective fuel cell. Such cracks must be detected as mon as possible, and hence, the conduits carrying water from each fuel cell require instruments to measure either pH or conductivity. Neutralization of highly contaminated water is not practical, not only because the weight of the required chemicals would be prohibitive, but also because the water from fuel cells is usually applied in the environmental control system where it is used for evaporative cooling of air in air coolers, called “boilers.” Water containing dissolved solids would leave a scale on the boiler and block the exit to the vacuum of space.
ao
INDUSTRIAL
A N D ENGINEERING C H E M I S T R Y
Moon Minerals
The rocks in the earth’s crust contain from 1 to 12% of chemically bound water and/or water of crystallization.
Similar percentages may be expected on the moon, a t least in shallow, subsurface layers. Green (7) thinks that the order of probability for occurrence of water sources on the moon is as follows: chemically bound water present in the volcanic extrusives; ice in permanently shadowed zones in volcanic fractures and fissures; water of crystallization in volcanic sublimates ; permafrost in dust basins; and water in carbonaceous chondrite and other rock types. Although the lunar atmosphere has a pressure perhaps as low as mm. of mercury which would vaporize ice even at -150’ C., it may be possible that in the fractures and fissures, the pressure is higher (possibly from radon gas leakage). T. Gold and Hibbs (5) suggest that half of the moon under the lunar surface may consist of dirty ice, since hydrogen and oxygen are very common elements in the solar system. This may account for the low mean density of the moon, which is 3.3 (as compared to 5.5 for earth). Kopal (6) thinks that small, clustered domes have underground connections filled with ice. The top of the lunar surface is very likely covered with meteoritic material similar in nature to the meteorites that fall to earth. The lack of atmosphere on the moon permits not only the macrometeorites (density 2.7 to 8.0) but also the micrometeorites (density about 3.6) to fall from dusty space onto the lunary surface. The accretion rate of these bodies per day may be as high as 1000 tons per day for the entire lunar surface. The meteorites falling on the moon are assumed to be of the same composition as those falling on earth: chondrites and achondrites, which account for 90% of the meteorites, and irons and stony irons, which account for the remainder. Chondrites contain approximately 3391, of silicon dioxide, while achondrites are composed of approximately 50%. Their water content is very low, about O.lyc. This includes the water that can be obtained by applying a high temperature (up to 1000° C.). Some of the chondrites, however, are carbonaceous and contain a remarkable amount of water (up to 16%). All chondrites and achondrites in space could be carbonaceous and contain a high water content. This would agree with the present theory of Whipple that the nucleus of each comet is composed of frozen water, methane, and ammonia, which hold together other elements found in meteoroids, whether they are stones, ANALYSIS O F CARBONACEOUS CHONDRITES [Stone meteorites ( l o ) ] Composztzon Si02 MgO A1203 FeO FeS Fe (free) NiO CaO
HzO C Organics MnO, TiOz, NazO, KzO, P205, Cr203, COO
I
Percentage 22-33
16-24 1-3 10-24
6-1 6 0-2
1 1-2
1-16 0.4-4 0-5
In trace.
irons, or stony irons. It is also theorized that, when a comet approaches the sun, it is heated, vaporizing a small part of the methane, which causes a partial disintegration, thus forming the comet’s “tail.” T h e intense electromagnetic and corpuscular radiation in the vicinity of the sun catalyzes chemical reactions between methane, ammonia, and water, forming such free radicals as CZ, CH, OH, “2, NH, and CN, which have been found in the spectrum of comets’ tails. T h e energetic free radicals probably form carbon, organic materials, water, and silicates such as olivine, pyroxene, orthopyroxene, enstatite, hypersthene, and ferroaugite. The comets’ tails leave particles in their wakes, which account for most of the meteoroids in the solar system. When the meteoroids reach the surface of the moon or the earth (they are then called meteorites), they may preserve part of their original composition, depending on the heat of re-entry through the earth’s atmosphere or on the velocity a t impact with the moon (usually between 10 and 30 kilometers per second). I n November 1963, Kopal (6) noticed a temporary luminescence around the crater Kepler, which was probably caused by bombardment of the local achondritic enstatites with energetic protons from a solar flare. Thus, the lunar surface is probably covered with micrometeorites and macrometeorites, which, by heating to a high temperature, may produce substantial amounts of water, carbon dioxide, and nitrogen. The meteorites, especially the larger ones, at impact with the moon are mixed with ultrabasic rocks. These rocks may contain serpentine 3Mg0.2Si02.2H-20,which has 12.7yc water. During its existence of 4l/2 billion years, the moon has probably had some volcanic activities, and hence, volcanic extrusives will be found on the lunar surface. Extrusive materials may vary their silicon dioxide content from 17.3 (olivine mellilite) to 37.5y0 (ash tuff). Their water content varies between 0.3 and 7.05!&,. Polarization curves of lunar light, obtained by checking the polarization during different phases of the moon, show that maria are covered with a surface material similar to ocherous, limonite, which contain ferric hydroxide and clay, and that the highlands are covered with ash tuff. These materials exhibit similar polarization curves when illuminated a t different angles. REFERENCES (1) Green J., “Geoscience Applied to Lunar Exploration ” p. 214 of “The Moon ” by K o p h , Z., and Mikhailov, 2. K., Academic Press,’London and New York, 1962. (2) Hendel, F. J. “Water Recovery During Space Flights,” Proc. Div. of Water and Waste Chehstry, Am. Cbem. SOC.,pp. 3-9,1962. (3) Hendel, F. J., “Water Recovery During Space Missions bv Catalytic and Other Processes,” Proc. Inst. of Environmental Sciences, pp. 225-40, 1962. (4) Hendel, F J., “Recovery of Water During Space Missions,” A m . Rocket SOL.J. 12, 1847-59 (December 1962). (5) Hibby: A. R. I n Discussion on “The Implications of Water as a Lunar Resource p. 53 Proc. Lunar and Planetary Explor. Collo North Amer. Av., Inc., S‘pace anb Infor. Systems Div., 9(3), November 1963:) ( 6 ) Kopal 2. “Structure of the Lunar Surface” (F. J. Hendel, Ed.), North Amer. Av., inc., Space and Infor. Systems Div., SID64-152, Downey, Calif,, January 1964. (7) Popma, D. C “Life Support for Long-Duration Missions,” Artronaufics and Aerospace Eng. 1YNo. 7, 53-56 (August, 1963). (8) Schanz J. L. Bullock E. K., “Gemini Fuel Cell Power Source,” Am. Rocket SOC.Spade Poder Conf.,’Santa Monica, Calif. (September 1962). (9) Slonim A. R. Hallam A P. Jensen D. H. Kamrnermeyer K . “Water Recover; from bh siolo&ai Sources fo; Space k p lications,” Airosiace Med. Div., Terh. Doc. J e p o r t MRL-TDR-62-75, July If62, (10) Urey, H. C., Craig, H., “Composition of the Stone Meteorites,” Geochemica et Coamochimica Acta, Vol. 4, pp. 38-62, 1953,
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