Practical Sorption Kinetics of TiCl 3 Catalyzed NaAlH 4
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Practical Sorption Kinetics of TiCl3 Catalyzed NaAlH4 Xia Tang, Daniel A Mosher and Donald L Anton United Technologies Research Center 411 Silver Lane East Hartford, CT 06108 Abstract Sodium alanate has been studied as a promising candidate material for reversible hydrogen storage due to its intermediate temperature range and relatively high storage capacity. Its rates of desorption and absorption of hydrogen have been shown to be enhanced by the addition of Ti in various compounds. To date, the sorption kinetics, especially absorption kinetics, is not well understood. In this study, a practical sorption kinetics model for TiCl3 catalyzed NaAlH4 has been developed to assist in the engineering design and evaluation of a prototype hydrogen storage system. Introduction The design of a hydrogen storage system using any exothermic hydriding compound, such as NaAlH4, requires detailed consideration of local heat management. This is especially important in the critical hydrogen absorption stage, where high kinetics are required and heat flow is at its maximum. Thermal transport architectures such as cooling tubes and metal foam structures need to be designed to meet the optimum operational characteristics of the hydrogen storage media. In order to design and model these architectures and obtain a gravimetrically and volumetrically optimized storage system, absorption and desorption kinetic models need to be identified and validated. Many current models, such as the well-known Arrhenius model, are insufficient to characterize materials behavior under transient or partially discharged conditions. Previous kinetics studies of NaAlH4 mainly focused on the desorption reaction [1-4]. Aborption and desorption kinetics models were developed by Luo and Cross [5] to simulate NaH+Al ↔NaAlH4 reactions using NaH and Al as starting materials. No kinetics model was reported to simulate transient hydriding rate and hydrogen absorption capacity of NaH+Al derived from NaAlH4. In this study, a solid/gas chemical kinetics model originally developed by El-Osery [6-9] to design conventional metal hydride systems was utilized. This model was adapted for use in the multi-step hydrogen absorption mechanisms of NaH+Al→NaAlH4. Basic Kinetics Model The dehydrogenation and hydrogenation of sodium alanate involve the following well-known reactions: NaAlH4 ↔ 1/3 Na3AlH6 + 2/3 Al + H2 ↔ NaH + Al + 3/2 H2 For compactness, the compositional state can be tracked by a single variable for each product/reactant, C1, C2 and C3 as: C 1: NaH + Al + 3/2 H2 1/3 Na3AlH6 + 2/3 Al + H2 C 2: NaAlH4 C 3: The nomenclatures for all reactions are listed in Table 1.
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Table 1 Label r1 r2 r3 r4
Nomenclature for All Reactions
Action Dehydriding of Na3AlH6 Hydriding of NaH Dehydriding of NaAlH4 Hydriding of Na3AlH6
Reactant C2 C1 C3 C2
Product C1 C2 C2 C3
Reaction rates can be represented by equation (1) based on the metal hydride model developed by El-Osery [6-7]: ⎛ dC j ⎞ ⎜ ⎟ (1) ⎜ dt ⎟ = f T (T ) * f P ( P ) * f C (C k ) ⎝ ⎠ ri i for reaction ri j for
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