| The objective of this work is to determine the self-hydrolysis kinetics of sodium borohydride (NaBH4) in a solution with liquid water (H 2O) for hydrogen (H2) generation and storage purposes. In order to be able to derive two semi-global rate expressions, an extensive series of batch process experiments was performed in a glass flask where the following parameters were systematically varied: solution temperature varied from 298 K (25°C) to 348 K (75°C), NaBH4 concentration ranged from 0.5 wt% to 25.0 wt%, and the concentration of the hydrolysis stabilizer sodium hydroxide (NaOH) varied from 0.0 wt% to 4.0 wt%, which corresponds to initial solution pH values ranging from 9.5 to 12.95. Transient hydrogen generation rates and transient solution pH were measured during hydrolysis. Based on the data collected, the decomposition rate for NaBH4 when the effect of the transition period diminishes is found to be: -dNaBH4 dt=A1˙e 49860J/molRT NaBH4 H+0.788 Where A1=1.72x109 1s Lmol0.788 [NaBH4] represents the NaBH4 concentration in units of moles per kilogram of solution, [H+] represents the concentration of the hydrogen ion, H+, in units of moles per liter of water, R in the Arrhenius term is the universal gas constant and T is temperature in Kelvin. The expression for the rate of change of [H +] (or equivalently pH level) is: -dH+ dt=A2f NaBH4O ˙e-7930J/m olRT˙e -4.5291-NaB H4NaBH 4O H+b 2f&parl0;&sqbl0;NaBH4&sqbr0;O&parr0; Where A2fNaBH4 O= -2.236˙NaBH4 O+36.6081 sL molb2 b2f&parl0;&sqbl0;NaBH4&sqbr0; O&parr0;=0.0973˙&sqbl0;NaBH4&sqbr0; O-2.763 The quantity [NaBH4]O represents the initial concentration of sodium borohydride and f is symbolic for a function. In principle, given initial conditions (temperature, NaBH4 concentration and H+ concentration), the two quasi-global coupled equations can be integrated to obtain the transient time history of H2 (or NaBH4) and solution pH (or H+). The model transient hydrogen generation rate and transient pH trends were compared with experimental data. Good agreement between analytical and experimental data was reached for many conditions, especially for elevated solution pH values, levels at which NaBH4 solutions are used practically. |
