Effect of Water Vapor on the LiOH-C02 Reaction Dynamic Isothermal System Dale D. Williams and Roman R. Miller Inorganic Chemistry Branch, Chemistry Division, U.S. Naval Research Laboratory, Washington, D.C. 20390
The role of water vapor in the reaction of carbon dioxide with lithium hydroxide in a dynamic system was evaluated. This parameter was isolated by utilizing low concentrations of COS,thus maintaining an essentially isothermal system. LiOH is shown to b e hygroscopic, and the rate of water pickup is directly related to the relative humidity of the feed system. The critical partial pressure of water a t 2 5 ° C is approximately 4 torr. It is demonstrated that hydration to the monohydrate (LiOH.HzO) is a necessary precursor to COSreaction but that the rate of hydration must not exceed the rate of carbonation if high efficiencies are to be realized. The controlled rate of hydration is necessary to maintain the high surface area and porosity of pelletized COS + LizCOa HzO, is the summation of the two-step LiOH. The previously accepted equation, 2LiOH 2H20 2LiOH HzO. 2LiOH H10 reaction, 2LiOH COZ -+ Li2CO3 3Hz0.
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THE
desire of man to move deeper into the sea has imposed many new parameters on atmosphere-control equipment. Machines and materials that function efficiently and reliably in ‘‘normal” situations often refuse to adapt to the novel, even hostile, environments. The control of metabolic carbon dioxide (COS) is a case in point. Meager data from SEALAB I1 (Pauli and Clapper, 1967) indicated that the COz absorbent used, lithium hydroxide (LiOH), had performed a t only 50 to 60% of its theoretical efficiency, whereas predictions based on prior surface experience had indicated that 75 to 80y0 efficiencies should have been attained. While a portion of the less-thanpredicted performance could be laid to premature changing of the canisters, the experience emphasized the need for a critical evaluation of the COz-removal problem. The physiological action of COZ in the respiration process is a function of its partial pressure, and the desired upper limit for prolonged exposure is 3.8 torr, which is 0.5 volume yo atmospheric or surface equivalent. Maintaining this partial pressure as one increases total pressure-Le., depth-means the removal of lower and lower “per cent” concentrations. Since most prior experience with COZ removal involved emergency conditions of 2 to 4y0 COS and high humidities, little or no knowledge existed as to the role of individual parameter effects upon the reaction. Background
The historically accepted expression for COz removal by LiOH has been Equation 1: 2LiOH
+ COS
+ HzO
Li2C03
(1) It has been known for some time, however, that this reaction was affected by the presence of water vapor. Empirical data accumulated over the years could be interpreted to show that a gas stream could be both too “dry” and too “wet” for effective C02-LiOH reaction in nonisothermal C02-air systems. The study reported here was aimed a t defining the mechanism by which water limits or enhances the reaction, in order to explain past anomalies and to establish more orderly ground rules for performance prediction. 454
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Ind. Eng. Chem. Fundam., Vol.
9, No. 3, 1970
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Since the application of these data is to dynamic absorption systems, a brief discussion of the physical form of the LiOH used is in order. With the exception of one vapor pressure run made on virgin crystals of lithium hydroxide monohydrate (LiOH.HzO), the base material for this study was standard S a v y LiOH prepared according to Mil Spec MILL-20213D. This material is pelletized to minimize dusting and to ensure low pressure drop in packed beds. The pellets are made by pressing, cracking, sizing, and dehydrating crystalline LiOH HzO. The anhydrous pellets resulting from this process have high porosity and a surface area of about 10 m 2 per gram. Table I lists porosity values (Musick, 1963) for various forms of this material that are pertinent to this report. The L i O H . H 2 0 data in Table I were obtained on pellets from a cake pressed to 75y0of crystal density. S o significant increase in density was observed for a pressing range of 25,000 through 135,000 psi. From this it is assumed that the indicated porosity represents the “base” condition for the pressed cakes from which anhydrous pellets are made, and to n-hich those pellets would return if completely rehydrated. Data
Since this study was not concerned with redemonstrating the effectiveness of LiOH as a COz-removal agent, a singlesystem geometry, not necessarily ideal, but approximating recommended parameters as to residence time and lengthdiameter ratio, was employed for both the water absorption and COZ reaction runs. The principal variable was the water vapor content, or relative humidity ( R H ) of the feed gas. By the use of a low COz level, heat of reaction temperature effects on the system were virtually eliminated. Similarly, the amount of water produced by the C02-LiOH reaction did not significantly alter the relative humidity of the gas stream. Vapor Pressure of Water over LiOH .HzO. T h e vapor pressure of water over virgin LiOH.HzO crystals and completely and partially rehydrated pellets of X a v y LiOH was determined in order to substantiate t h e literature values (Bach et al., 1964; Thakker et al., 1968). Excellent agreement with those d a t a was found for all three materials, indicating t h a t the rehydration process had produced
LiOH H20 a n d confirming t h e reversible hygroscopicity of LiOH. T h e vapor pressure pertinent t o this report was 4 torr at 25OC. The rate a t which water exchange would occur in the dynamic system used for the COZabsorption studies was demonstrated in a number of runs involving various relative humidities in Con-free helium over LiOH and LiOH.HzO. T h e tube geometry and loadings were identical to those for the COZ runs. The Ivater exchange rates were determined gravimetrically. For this single geometry, the dynamic rates of water gain or loss by mixtures of LiOH-LiOH.HzO us. humidity in helium are shown in Figure 1. These rates are essentially constant until the affected component of the system nears extinction, a t which time the rates taper off asymptotically. The data in Figure 1 demonstrate t h a t the LiOH.HZO system will reversibly equilibrate in response to the vapor pressure of water over LiOH.HzO, and that the point of no loss or gain occurs when the partial pressure of water vapor in the gas stream equals the vapor pressure of water over LiOH.HzO. -kt 25OC, this point is about 17y0 RH, or 4 torr. COz Absorption. T h e feed gas source for all runs was laboratory-compressed air (100 psia). Periodic analysis for COz content over t h e several months of experimentation gave values of 300 =k 25 p p m COZ. T h e relative humidity of this air was less t h a n 1% a t room conditions. T h e room temperature was not strictly controlled a n d varied between 20' a n d 26OC during t h e various experiments. T h e humidi t y of t h e feed gas for a given experiment was controlled originally b y shunting a n appropriate portion of t h e feed stream through a water-filled Xilligan bottle, and later b y feeding t h e entire stream through various constant-RH salt solutions in either 1 he Milligan bottle or a spinning-disc saturator based on a n NRL-developed gas stripper (Williams and Miller, 1962). R H between 35 and 95% was measured with a Labline Alodel2200 electric hygrometer. Calibration of the hygrometer and RH determination below 35% were made with Draeger 0.1 water detector tubes. T h e LiOH used for all tests was from a single lot of Navy stock material. This material analyzed 98.2% LiOH and 1.8% Li2C03 at the beginning, middle, and end of the time involved for the completion of :all runs. All absorption and reaction runs were made in cylindrical polyethylene tubes. The geometry of the reaction bed was 1.6 cm in diameter by 3.5 cm long. The charge tared into this volume represented 3.2 0.28 grams calculated as anhydrous LiOH. The bed was held
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Table 1. Porosity of Pelleted lithium Compounds" Pore Size Distribution, % of Total Volume Compound
>lo0
100-0.012
p
II
