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Carbon Dioxide Scrubbing Materials in Life Support Equipment

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Carbon Dioxide Scrubbing Materials in Life Support Equipment FAU Institutional Repository http://purl.fcla.edu/fau/fauir This paper was submitted by the faculty of FAU’s Harbor Branch Oceanographic Institute. Notice: ©1982 ASME. This manuscript is an author version Harbor Branch Oceanographic Institute Contribution #293. It may be cited as: Wang, T. C. (1982). Carbon Dioxide scrubbing materials in life support equipment. In M.L. Nucklos and K.A. Smith (Eds.), The Characterization of carbon dioxide absorbing agents for life support equipment (pp. 1-21 ). Presented at the Winter Annual Meeting of the American Society of Mechanical Engineers. Bioengineering Division, ASME. OED-Ocean Engineering Division, ASME, 10. New York, 1982. /'!, ·~(,\ ' I \ r·+-, ·~ CARBON DIOXIDE SCRUBBING MATERIALS IN LIFE SUPPORT EQUIPMENT T. C. Wang Harbor Branch Foundation Fort Pierce, Florida ABSTRACT This review paper gathers the information of carbon dioxide (C0 2) scrubbing materials in life support systems. Four groups of scrubbing materials are classified: Alkali metal hydroxides, alkali metal superoxides and peroxides, regenerative types of adsorbants, and other methods such as membrane separation, freeze-out, and photosynthetic gas exchange are also described. The advantages and disadvantages of each material are also discussed in the paper. For short duration operations, alkali metal hydroxides are the most attractive material to control carbon dioxide. The economics, simplicity and its well developed state-of-the-art are the reasons for the wide acceptance of this material. However, since alkali metal superoxides or peroxides simultaneously remove C02 and generate 02, they appear to be promising. For moderate to long duration operations, regenerable adsorbants becomes necessary. At the same time the need for oxygen recovery from carbon dioxide begins to enter the picture. INTRODUCTION This paper reviews the author's past research and other re­ searcher's work on carbon dioxide scrubbing materials in life support systems. Some of these materials mentioned in the paper are yet to be developed and they may still be far remote for C0 2 scrubber designer's application. However, it is the author's inten­ tion to review all of the available information and present them to the readers. Carbon dioxide scrubbing materials can be classified with re­ spect to their physical and chemical properties, and their function and scrubbing mechanism. Four groups of scrubbing materials are presented as follows: I. Alkali Metal Hydroxides 1. Lithium hydroxide 2. Sodalime 3. Baralyme II. Alkali Metal Superoxides, Peroxides 1. Sodium or potassium superoxide 2. Alkali metal peroxides III. Regenerative Type of C0 2 Adsorbents 1. Molecular sieves 2. Metallic oxides A. Magnesium oxide B. Silver oxide IV. Miscellaneous Methods 1. Carbon dioxide reduction A. Sabatier process B. Bosch reaction 2. Membrane separation 3. Freeze-out 4. Photosynthetic gas exchange ALKALI METAL HYDROXIDES The oxides, hydroxides, peroxides and higher oxide states of the alkali and alkaline earth metals react with carbon dioxide to form carbonate, bicarbonate or hydrates of carbonates and bicarbon­ ates. The theoretical capacities of the various possible C0 2 sorbents is shown in Table 1. Lithium monoxide has a greater C0 2 capacity than any of the other oxides. The use of lithium oxide as a carbon dioxide absorbent has been investigated for short duration space flights. Its dynamic absorption characteristics are not as good as those of lithium hydroxide. Alkali metal hydroxides other than LiOH have a high capacity for C0 2. However, their high degree of hygroscopicity eliminates them from consideration as solid ab­ sorbents. Alkaline earth metal oxides such as MgO and CaO absorb carbon dioxide very slowly, while the addition of water vapor and conversion of the oxide to hydroxide increases the rate of absorption. Dedman and Owen (1), in studying the reaction of CaO with C0 2, showed an initial rapid uptake due to chemisorption, followed by a slower re­ action rate-controlled by the diffusion of C0 2 into the pores of the solid. Comparison of alkaline earth metal hydroxides (see Table 1) shows Mg (OH) 2 to have a high capacity for C0 2 on a unit weight basis. Little work has apparently been done on the absorption of C02 by magnesium oxide under conditions which are applicable to the removal of C0 2 from expired gases. The absorption capacity of calcium hydroxide is somewhat less than that of Mg(OH)2, but the literature suggests that the C0 2 absorption rate of hydrated lime is very slow. This slow reaction rate has been circumvented through the use of additives of other hydroxides to the basic lime structure. The addition of 4.5 wt% sodium hydroxide to lime, containing 17.5% water, results in a product which absorbs C0 2 significantly more rapidly than Ca(OH)2 alone. This material, commercially known as soda lime, has been used for many years in anesthesiology for the absorption of C0 2. Another modification of lime is Baralyme, which is a mixture containing 20 wt % Ba(OH)2.8H 20 in lime. By far, lithium hydroxide, soda lime and Baralyme are the most effective C0 2 absorbents in enclosed space. 1. Lithium Hydroxide LiOH absorbs C0 2 from a gas mixture and in the presence of water vapor. The reaction is given by the following equations: 2 2LiOH + 2H20 + 2LiOH · H20 ------- (1) 2LiOH · H20 + C02 + Li2C03 + 3H20 ------ (2) The reaction of C0 2 with LiOH requires the presence of water in an amount sufficient to produce LiOH·H 20 prior to or simultaneously with the C0 2 reaction (2,3). The properties of LiOH as a C0 2 absorbent are listed below: Properties of LiOH Chemical Formula LiOH Molecular wt. 23.95 Melting point 461°C Density of crystals 2500 kg/m 3 Bulk density of granular LiOH 400-449 kg/m 3 Theoretical C0 2 absorption capacity 0.919 ~ C02/g LiOH Heat of absorption (H20 gas) 3.2xl0 J/kgC02 Water of reaction 0.409 g/g C02 or 0.375 g/g LiOH The dynamic removal efficiency, which expresses the reduction in C0 2 concentration across the bed, varies with time in the operation of LiOH as the absorbent is used up. The theoretical capacity of LiOH for C0 2 absorption can not be achieved in an atmosphere control system because of the bed dynamics, characteristics, canister dimensions, air superficial velocity and the process air inlet temperature. The LiOH particle size is generally kept between 4 to 8 mesh size for high utilization efficiencies on low bed pressure drops. Normally, no channeling occurs with this particle size at gas velocities up to 30 em/sec (12). The C0 2 absorption capacity of LiOH varies with various tempera­ tures and humidities (4). The chemical reaction associated with water production and the rate of dehydration affect the C0 2 absorp­ tion capacity (see discussion). 2. Sodas orb (Soda Lime) Sodasorb consists essentially of hydrated lime [Ca(OH) 2], sodium hydroxide (NaOH), potassium hydroxide (KOH), 14 to 19 percent moisture content, and some inert. It is in the form of granules and contains an indicator which changes from colorless or white to blue when the NaOH has reacted to form Na 2C0 3 • The chemical reaction of C0 2 with Sodasorb is as follows: C02 + H20 + H2C03 ------ (3) 2 H2C0 3 + 2 NaOH + 2 KOH + Na2C0 3 + K2C03 + 4 H20------ (4) Na 2C0 3 + K2C0 3 + 2 Ca(OH)2 + 2 CaC03 + 2 NaOH + 2 KOH------ (5) The water is added as free moisture in the manufacturing process and is necessary for efficient removal of C0 2. Significant water is lost through evaporation if properly sealed storage is not main­ tained and the C0 2 absorption capacity will be decreased. The theo­ retical C02 absorption capacity and physical properties of Sodasorb are shown in Tables 1 and 3. 3. Baralyme Baralyme is a mixture of barium hydroxide octalhydrate [Ba(OH) 2·8H 20], calcium hydroxide [Ca(OH) 2] and potassium hydroxide (KOH). The eight waters of crystallization present in the Ba(OH)2 serve to fuse the mixture into a homogeneous mass which will hold its shape and form under varying conditions of heat and moisture. 3 The water is used as shown in the chemical equations: Ba(OH) 2 ·8 HzO "'COz + BaC03 + 9 RzO (6) 9 HzO + 9 COz + 9 HzC03 (7) 9 HzC03 + Ca(OH)z + 9 CaC03 + 18 HzO --- (8) 2 KOH + HzC03 + KzC03 + 2 HzO ------ (9) Ca(OH)z + KzC03 + CaCo3 + 2 KOH------- (10) When temperature is above 150°F, water has a tendency to liber­ are from Baralyme granules and COz absorption capacity will be de­ creased. Tables 1 and 3 show the properties, characteristics and COz absorption capacity of Baralyme. Discussion Lithium hydroxide, Sodasorb and Baralyme are the most widely used absorbents used for C0 2 removal. Experience .has shown that the C0 2 removal is related to the porosity, granule size, surface area, moisture content, scrubber design and other environmental conditions such as temperature, humidity, pressure, etc. (4, 5, 6, 7). In­ sufficient water vapor in the air stream allows only partial C0 2 reactions as shown in the chemical equations, while an excessive amount of water vapor forms a water film barrier around the absorbent granules also resulting in an incomplete reaction between C0 2 and absorbents (2, 3). Higher temperature normally favors higher re­ action rate and therefore has a higher water production. Too high a temperature in the chemical reaction produces excess amounts of water that could decrease the available surface area for C0 2 re­ action. When this occurs, the COz absorption capacity of absorbents decreases. For a given C0 2 content in the air stream at any given tempera­ ture, there is a corresponding humidity of the feed stream which results in maximum C0 2 absorption efficiency.
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