AN AMERICAN CHEMICAL SOCIETY JOURNAL VOLUME 9, NUMBER 4
JULYIAUGUST 1995
0Copyright 1995 American Chemical Society
Articles A Calorimetric Study of the Mechanism and Thermodynamics of the Lithium Hydride-Water Reaction at Elevated Temperatures Jonathan Phillips and Michael C. Bradford The Department of Chemical Engineering, 133 Fenske Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802-4400
Martin Klanchar” The Applied Research Laboratory, The Pennsylvania State University, P.O. Box 30, State College, Pennsylvania 16804 Received October 24, 1994. Revised Manuscript Received April 17, 1995@
The lithium hydride-water reaction was studied using a novel flow calorimeter t o determine the mechanism and thermodynamics of lithium compound-water interactions a t elevated temperatures. A concomitant consideration of heat and hydrogen evolution data led to a simple physical reaction model for temperatures of 505 and 588 K. Initially, water interacts directly with lithium hydride to form lithium oxide and hydrogen. However, the lithium oxide surface layer which forms via this reaction subsequently reacts with additional water to form lithium hydroxide. The net effect of this mechanism is that both heat and hydrogen evolution decline per amount of water injected in a batch-type reaction system.
Introduction Current investigations of advanced hydrogen-powered energy systems have also spurred research into more efficient and safer methods of hydrogen st0rage.l For example, hydrogen gas is chemically “generated” and significant heat evolved upon the reaction of solid or molten lithium hydride with water. The storage density of hydrogen in lithium hydride and other chemical forms is greater than high-pressure gas or even cryogenic liquid storage ~ y s t e m s . l - ~However, storage density information is not the only important design aspect of @Abstractpublished in Advance ACS Abstracts, June 1, 1995. (1)Hydrogen as a n Energy Carrier; Winter, C. J., Nitsch, J., Eds.; Springer-Verlag: Berlin, Germany, 1988. (2) CRC Handbook of Chemistry and Physics, 63rd ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL; 1982.
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a hydrogen-generating process. A controlled release of hydrogen is also essential. Therefore, a detailed understanding of the sequence of reaction events in chemical hydrogen-generating systems is necessary for the design of processes requiring a controlled delivery of hydrogen (for example, engines and fuel cells). In this investigation, the lithium hydride-water reaction was studied a t 505 and 588 K by combing in situ heat of reaction and hydrogen production measurements. From these data, a simple model of the overall reaction mechanism at elevated temperatures was developed. (3) Weber, L. A. Thermodynamic and Related Properties of Parahydrogen from the Triple Point to 300 K a t Pressures to 1000 Bar; NASA SP-3088 and NBSIR 74-374; U.S. Department of Commerce, National Bureau of Standards: Washington, DC, 1975.
0 1995 American Chemical Society
Phillips et al.
670 Energy & Fuels, Vol. 9, No. 4, 1995 Flowmeter
Water Supply
Figure 1. Apparatus for conducting lithium hydride-water experiments.
A number of workers previously investigated the thermodynamics and the kinetics of the lithium hydride-water reaction at temperatures ranging from 298 to 393 K.4-7 Some disagreement is evident regarding the exact sequence of reactions. Machin and T ~ m p k i n s , ~ using a combination of manometric and gravimetric methods to study reaction kinetics, indicate that lithium oxide and hydrogen form initially. Upon the addition of more water, the lithium oxide product of the first stage reacts with water to form lithium hydroxide, but without hydrogen release. In contrast, Montgomery7 suggests that water reacts t o form hydrogen gas and lithium hydroxide, which subsequently reacts with additional lithium hydride t o form lithium oxide and more hydrogen gas. Other studies suggest the results of Montgomery are not reasonable. MyersBused ion backscattering to study the kinetics of the lithium hydride-lithium hydroxide reaction from 473 to 553 K. The results of that study and othersg imply that this reaction is extremely slow. Experimental Section Materials. Crystalline lithium hydride (-16 to $40 mesh) was obtained from FMC-Lithium Division. The purity of the sample was reported t o be 98% by the vendor. Virtually all the impurities (oxides) result from reactions with oxygen or water during processing. Ultra-high-purity argon was used as an inert blanket during experiments and also as a carrier gas for water vapor. Distilled and deionized water served as the limiting reagent. Apparatus. The experimental apparatus, shown in Figure 1, has been described in greater detail in a previous report.1° (4)Grunn, S.R.;Green, L. G. J. Am. Chem. Soc. 1968,80, 4782. 1966,62, (5)Machin, W. D.; Tompkins, F. C. Trans. Faraday SOC. 2205. (6) DeVries, G. Pyrodynamics 1966,6, 147. (7)Montgomery, C. D. Nucl. Eng. Des. 1973,25,309. ( 8 )Myers, S. M. J . Appl. Phys. 1974,45, 4320. (9)Holcombe, C. E.; Powell, G. L. J.Nucl. Muter. 1973,47, 121. (10)Bradford, M. C.; Phillips, J.; Klanchar, M. Reu. Sci. Znstrum. 1995,66 (l),171.
The calorimeter consisted of a small (18 cm3) cylindrical stainless steel test reactor that was initially filled with lithium hydride ( ~ 1 5g) and then enclosed inside a commercially supplied high-temperature Calvet-type calorimeter (International Thermal Instrument Co., custom version of Model CA100-1). This assembled unit was housed inside a standard convection oven to allow the reaction to be studied above room temperature. The reactor test cell was constantly purged with a steady flow of argon (1.55 & 0.05 mUs), and water doses (149 4 pmol) were introduced into this carrier stream using a four-port fixed volume injector. The resulting argodwater mixture then passed through a heat exchanger inside the oven prior to introduction into the reactor. Standard heat transfer calculations showed that the size of the exchanger was adequate to vaporize the water and raise the temperature of the water and the argon stream to the oven temperature. Additionally, empty cell (no lithium hydride) tests confirmed no heat flux arising from water injection alone. This and other instrument tests are described in further detail elsewhere.1° Hydrogen production was quantified with a thermalconductivity-type gas analyzer attached t o the reaction cell exhaust port (Figure 1). The advantages of combining reaction calorimetry with gas evolution measurement are emphasized in the literature1' and were especially important in analyzing the data from this study. The gas analyzer was also used t o detect unreacted water in the emuent stream; however, none was detected during the investigation. The heat flow calibration constant was determined t o be 0.103 & 0.006 J/(V-s) at 588 K by a standard electrical calibration procedure.1° This parameter was also shown to be independent of heat input, rate of heat input, and argon flow over the range of experimental conditions. Additionally, the gas analyzer used to quantify hydrogen gas in the effluent was found to have a calibration constant of 0.970 f 0.007 pmol Hd(V.mL) Ar. This constant was independent of the quantity of hydrogen and carrier gas flow rate at the conditions studied. Procedure. Lithium hydride was sealed within the test reactor under an argon atmosphere and then plumbed into the calorimeter flow network while maintaining an argon purge. The reactor and calorimeter were then heated t o the (11)Lambert, P. G.;Amery, G.; Watts, D. J. Chem. Eng. Prog. 1992, 88,53.
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Lithium Hydride- Water Reaction
injected into the test reactor. In fact, as will be demonstrated below, a fit of the reaction equation coefficients to the data leads to a quantitative interpretation of the reaction process consistent with hydrogen evolution and heat of reaction data. The general reaction equation can be written as: yLiH(s)
x
=
1%
I
(1)
0
0
0 O 0
O .
2
where x , y , z, and w represent the stoichiometric coefficients of each compound normalized to the molar amount of water dose (149 f 4 pmol). The reaction assumes lithium hydroxide monohydrate does not form, which is consistent with other investigations reporting its decomposition temperatures well below the temperatures maintained in this study.12J3 The stoichiometric coefficient z in reaction model (1) can be directly calculated from hydrogen evolution measurements by
O
10
Water Dose
f
z = nH2
z
s. s 5 8 K
+ H,O(g) - wLiOH(s) + xLi,O(s) + zH,(g)
”
(2)
where T Z His ~ the molar amount of hydrogen generated and T Z H ~ Ois the molar amount of water injected. An atom balance on hydrogen, lithium, and oxygen reduces (after some algebra) to the following simple expressions for the remaining stoichiometric coefficients in terms of 2.
t
“I e4
y=z
(3)
w=2-z
(4)
40
,IE] 0
2
4
6
8
10
12
Results and Discussion The lithium hydride-water reaction was studied at temperatures of 505 and 588 K. At each temperature, up t o 10 sequential measurements of heat of reaction and hydrogen production were made. Although the results for only one experimental sequence at each temperature are presented, the experiments were repeated at each temperature with nearly identical results. The experimental data clearly indicate that the amount of hydrogen generated declines with each subsequent water dose (Figure 2A). The heat of reaction data for the same experiments, shown in Figure 2B, also appear to decline slightly with dose, although the scatter in the data makes it more difficult to discern trends. Note that error bars associated with the heat data are calculated from the uncertainty in the calibration constant. These results lead to the general conclusion that reaction chemistry changes as more and more water is
x=z-1 (5) Thus, measurement of z served to satisfy the only degree of freedom in the material balance. Using the hydrogen generation data in Figure 2A, the stoichiometric coefficients for hydrogen (z), lithium hydride b), lithium hydroxide (w),and lithium oxide ( x ) were calculated from eqs 2, 3, 4,and 5, respectively, as a function of water dose. These coefficients, plotted in Figure 3, demonstrate a trend where the production of lithium oxide and hydrogen generally decrease with each water injection, while the amount of lithium hydroxide produced increases. The accuracy of the coefficients can be further verified by comparing the experimental heat of reaction data t o heats calculated from tabulated thermodynamic data. The overall energy balance for reaction 1is
+
AHm = xAH&Li,O) wAHdLiOH) - yAHdLiH) AHdHZO) ( 6 ) where AHH,, is the total heat of reaction and the AH@ are heats of formation for reactant and product compounds. Using the stoichiometric coefficients determined from hydrogen evolution and heat of formation data from the JANAF’ database14 allows calculation of the expected heat of reaction as defined in eq 6. As demonstrated in Figure 4,the agreement between the (12) Bach, R. 0.; Boardman W. W. Jr.; Forsyth, M. W. Chimia 1964, 18, 110.
(13) Popescu, C.; Jianu, V.; Alexandrescu, R.; Minailescu, I. N.; Morjan, I.; Pascu, M. L. Thermochim. Acta 1998,129, 269. (14) Chase, M. W., et. al. J m A F Thermochemical Tables, 3rd ed.; American Chemical Society and American Institute of Physics: New York, 1986.
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572 Energy & Fuels, Vol. 9, No. 4, 1995
1.w
-
‘O 20
n-.--nn
I
0
2
0
1
8
8
1
.
10
’
”
2
E 8 0
1.25
240
Lilhlum H y d r h (y) LHhlum Hydmsida (w)
-
’
’
10
Water Dose 280
1.50
“
S
Water Dose
.-I
-
”
f
P
pJ
4-
-
1BO
0 120
loo 0.50
40
20 6
10
Water DOM
“0
2
4
6
8
10
Water Dose
Figure 3. Stoichiometric coefficients for the lithium hydridewater reaction at (A) 505 K and (B) 588 K as determined from hydrogen evolution measurements and a material balance.
Figure 4. Experimental and calculated (from eq 6 energy balance)heat of reaction values for the lithium hydride-water reaction at (A) 505 K and (B) 588 K.
values measured calorimetrically, and those calculated with tabulated data and eq 6 coefficients, is excellent at 505 K. At 588 K, however, the agreement between measured and calculated values is only fair, primarily because of an increase in baseline drift of the calorimeter signal. Generally, there is a gradual increase in drift with increasing temperature, leading to greater data scatter at 588 K than at 505 K. Nonetheless, linear regression of the experimental data shows a gradual decrease in the heat of reaction, which supports the trends calculated from the hydrogen evolution data. This agreement, quantitative at 505 K and qualitative at 588 K, supports the supposition that the coefficients computed from the hydrogen evolution data are reasonable. The change of stoichiometric coefficients with water injection indicates that there is a gradual decrease in the amount of lithium oxide formed, but a gradual increase in the amount of lithium hydroxide formed. This can be interpreted with the following simple physical model. Initially, when the test reactor contains a fresh charge of lithium hydride, water vapor reacts primarily with lithium hydride to produce lithium oxide and hydrogen gas according t o
As indicated from a free energy minimization calculation,15 this reaction is thermodynamically favored. Reaction 7, however, results in the formation of a surface layer of lithium oxide. Subsequently, as experiments continue, water will begin to both diffuse and react within the lithium oxide layer to form lithium hydroxide according t o
2LiH(s)
+ H,O(g) - Li,O(s) + 2H2(g)
(7)
The partial reaction of water with lithium oxide in the surface layer, through reaction 8, results in decreased hydrogen production. This mechanism is supported by the experiments of DeVries6and Dunn et al.,16 whose room temperature data also showed that the formation of a surface layer on lithium hydride gradually inhibits hydrogen production. The reaction model assumed here is also identical t o that proposed earlier by Machin and T ~ m p k i n s .Note ~ also that reactions 7 and 8 can be added together to yield reaction 1 after molar coefficients are normalized and rearranged. (15) Peters, J. A. “Development of a Database of Thermochemical Parameters for Use With the SOLGASMIX Computer Program”; ARL Technical Report 88-008; The Pennsylvania State University, Applied Research Laboratory: University Park, PA, 1988. (16) Dunn, P. M.; Egan, C. J.;Harbison W. L.; Pitcher, G. K. Proc. Intersoc. Energy Conuers. Eng. Conf. 1991,26(31,527.
Lithium Hydride- Water Reaction
It is interesting to note that apparently even the first dose of water creates both lithium oxide and lithium hydroxide. For example, even for the first water dose, hydrogen generation is less than expected for the amount of water injected via reaction 7 . It is suggested that this is due to the interaction of some of the water with a surface impurity of lithium oxide (on the material as received) t o form lithium hydroxide. In the reaction model represented by reactions 7 and 8, the production of hydrogen occurs a t the lithium hydride-lithium oxide interface. Thus, it is expected that the diffusion of hydrogen out of the test reactor should get progressively slower as the layer of lithium oxidehydroxide becomes thicker, especially since the diffusivity of hydrogen in lithium oxide has been reported as extremely s10w.l~ Figure 5 displays the hydrogen evolution signal from the gas analyzer for the first, third, and seventh consecutive water doses. Note that the signal broadens with each additional dose, verifying slower diffusion of hydrogen as the oxidel hydroxide layer develops.
Conclusions In summary, the lithium hydride-water reaction proceeds with the initial formation of a lithium oxide surface layer and hydrogen gas. As the reaction continues, diffusion of water through this surface layer (17)Krikorian, 0. H.High Temp.-High Press. 1988,20, 183.
Energy & Fuels, Vol. 9, No. 4, 1995 573 1.2
8
B
'1 0.81
!+-
Ij
II
x)
Figure 5. Hydrogen analyzer response for lithium hydridewater reactions at 588 K during the first, third, and seventh water doses.
produces some lithium hydroxide. There is clearly a balance of kinetic and thermodynamic considerations which determine the relative amount of water that reacts via the competing reaction paths. It is suggested the strongest influences are the fraction of oxidized surface, the oxide layer thickness, and the relative rates of diffusion and reaction.
Acknowledgment. This work was performed under the sponsorship of the Office of Naval Research. EF940202E
