Kinetics of hydrogen absorption in a stirred metal hydride slurry

Abstract

Isothermal, isobaric measurements were made of the rate of hydrogen absorption by a slurry of LaNi, powder suspended in n-undecane in a baffled stirred reactor. The data were interpreted using a model embodying three resistances in series: a gas-liquid mass transfer resistance, a liquid-solid mass transfer resistance, and an "overall reaction resistance."At the highest agitator speeds employed, 2,000 rpm, conversions of LaNi, to hydride of 90% or better were obtained within one minute or less, thereby confirming the potential of the metal hydride slurry concept for commercial applications. SCOPEMetal hydrides and their formers have been under study for some years as hydrogen-absorbing media.[Strictly speaking, hydrogen is absorbed by a metal hydride former, such as lanthanum nickel-five (LaNi,), also called the a-solid phase, to form a metal hydride, such as lanthanum nickel-five hexahydride (LaNi,H,), called the P-solid phase. Upon heating and/or lowering the pressure of the latter, it is transformed back to the metal hydride former, with the release (desorption) of hydrogen. However, in common discussions of these phenomena, distinction between metal hydrides and their formers is omitted and the simple term, metal hydride, is used for both, as in the present paper.]The large capacity of metal hydrides for hydrogen storage coupled with their ready release of hydrogen at moderate temperatures and pressures and their ability to undergo many cycles of absorption and desorption with little decrease in capacity, have prompted examination of their usefulness in a variety of industrial applications. These include hydrogen storage in energy conversion cycles, chemical heat pumps and compressors, hydrogen purification, and hydrogen isotope separation.

Kinetics of hydrogen absorption and desorption in LaNi5‐xAlx slurries

“…6 gives the absorption rate as a function of F/F,,,,,, this is represented by the solid line in Figure 8. The dotted line represents a hypothetical change of the reaction rate according to a three-dimensional nucleation and growth process (Rudman, 1983 andTung et al, 1986):…”

“…According to Ptasinski et al (1986) and Tung et al (1986) the value of the k p is not influenced appreciably by small metal particles at low volume concentrations. In the present investigations the slurries contain only 1-2 vol Yo metal alloy.…”

“…This value is in good agreement with values reported by others. Holstvoogd et al (1989) have found 32 kJ/mol for a LaNiS/Pd5 silicon oil slurry and Tung et al (1986) about 47 kJ/mol for a LaNiS/undecane slurry. In both cases surface adsorption was found to be the rate limiting step.…”

“…k,a,JE,= 37.6 l / s (present work) n-undecane: Pd5 silicon oil (Bayer): k,as/E,7=21.3 l/s k,a,/t,= 14.6 l/s (Tung et al, 1986, from data at t,=0.12) (Holstvoogd et al, 1989) The absorption rate is initially limited by a bulk process. As soon as the nuclei growth rate has become large enough, either of the two surface processes, chemisorption or dissociation of hydrogen, will be the rate determining step.…”

“…When a metal hydride is suspended in an inert solvent, two additional mass-transfer steps have to be included, as was done earlier by Ptasinski et al (1986) and Tung et al (1986). The following steps can be distinguished:…”