ARTICLE pubs.acs.org/JPCC
Diffusion of Atomic Hydrogen through VNi Alloy Membranes under Nondilute Conditions M. D. Dolan,*,† K. G. McLennan,† and J. D. Way†,‡ † ‡
CSIRO Energy Technology, Pullenvale, Queensland 4069, Australia Department of Chemical and Biological Engineering, Colorado School of Mines, Golden, Colorado 80401, United States ABSTRACT:
Vanadium and other group V metals and alloys exhibit much higher hydrogen permeabilities than palladium alloys, and the basis of this lies primarily in their high hydrogen solubility. Because absorbed hydrogen profoundly influences the physical and chemical properties of the host metal, classical absorption and diffusion theories, which assume low concentrations of solute, may not apply to these alloys. To elucidate hydrogen transport through nondilute alloy membranes, a comprehensive series of absorption and flux measurements have been made for three VNi alloys over a range of pressures and temperatures. Alloy disks of three compositions (V95Ni5, V90Ni10, and V85Ni15 atom %) were sectioned from arc-melted ingots and coated on each side with a Pd dissociation catalyst. Hydrogen absorption and desorption isotherms were calculated using the Sieverts’ method, and hydrogen flux was measured using the constant-pressure permeation method. The pressureconcentration relationships of these alloys were nonideal, particularly at high hydrogen concentrations. As a result, the diffusion coefficients for each alloy exhibited a significant hydrogen concentration dependence, which illustrates the nonapplicability of Fick’s first law of diffusion to these alloys. A strong dependence on Ni content was also observed. During permeation the hydrogen concentration gradient increases with increasing distance from the feed surface.
1. INTRODUCTION Alloy membranes are an emerging hydrogen separation technology for industrial applications which operate on the basis of solutiondiffusion, whereby1 (i) molecular hydrogen adsorbs to the high-pressure surface and dissociates into atomic hydrogen; (ii) atomic hydrogen dissolves into the alloy and diffuses across the membrane under a concentration gradient; (iii) atomic hydrogen recombines to molecular hydrogen at the low-pressure surface, where it is desorbed. Palladium alloy membranes are the benchmark alloy membrane technology, but the high cost of Pd is a severe limitation, particularly in light of the 2015 DOE cost target of ∼$1000 m2.2 This is driving the development of less-expensive alloy alternatives, most notably group V metals such as vanadium and niobium. These alloys exhibit hydrogen permeability values 12 orders of magnitude greater than Pd, largely due to very high hydrogen absorption.3 r 2011 American Chemical Society
Under most operating conditions, and for all but the thinnest membranes, the rate of hydrogen transport across alloy membranes (both Pd-based and otherwise) is diffusion-limited, i.e., diffusion of atomic hydrogen is the slowest process. For example, palladium membranes are diffusion-limited when the thickness exceeds 1 μm and the temperature is 573 K or greater.4 Maximizing the rate of mass transfer through the bulk of the membrane is therefore the major focus of current alloy membrane research and development. 1.1. Descriptions of Hydrogen Diffusion. Classical descriptions of hydrogen mass transfer across alloy membranes assume that hydrogen diffusion and gasmetal partition follow several simple, well-understood relationships. Membranes which behave according to these relationships can be considered “ideal”. Received: September 8, 2011 Revised: October 20, 2011 Published: December 02, 2011 1512
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The most commonly applied model of hydrogen transport across alloy membranes is that proposed by Fick in 1855:5 ∂cH ∂x
JH ¼ DH
ð1Þ
where JH is flux of the solute (atomic H), cH is solute concentration, x is distance, and DH the diffusion coefficient of the solute. Fick’s law of diffusion assumes the solution is dilute, i.e., there is no solutesolute interaction, and the properties of the solvent are unchanged. The quantity of hydrogen which dissolves in an alloy is dependent on the hydrogen partial pressure in the adjacent atmosphere, according to Sieverts’ law: cH ¼ spn
ð2Þ
where p is the hydrogen partial pressure in equilibrium with the alloy, s is a constant of proportionality, and n is a constant. In ideal hydrogenmetal systems, n = 0.5 because of the dissociative chemisorption of hydrogen and the partition between the gas and alloy phases of molecular and atomic hydrogen, respectively (i.e., H2(g) T 2H(s)). Sieverts’ law would predict that membrane performance can be characterized by the H2 permeability (ΦH2): ϕ H2 ¼
ðp1
JH2 d p2 0:5 Þ
0:5
ð3Þ
where p1 and p2 are the hydrogen partial pressures in the feed and permeate streams, respectively, JH2 is the measured flux of molecular hydrogen, d is the membrane thickness, and permeability is a constant for a given alloy at a given temperature, independent of driving force and membrane thickness (d). 1.2. Diffusion under Nonideal Conditions. During permeation at conditions relevant to the industrial hydrogen production (i.e., ∼400 °C, p1 < 10 bar), Pd alloy membranes are reasonably dilute (H/M < 0.05) and would be expected to follow Sieverts’ law, but group V metals and alloys are nondilute (H/M up to 0.8, depending on alloy and conditions).1 The likelihood that a given hydrogen jump will be blocked because the interstitial destination is already occupied will be significant. Moreover, absorption of hydrogen increases the interatomic spacing of the metal lattice,6 a phenomenon which has been measured for body-centered cubics (bcc’s) like vanadium, niobium, and tantalum,7 and for face-centered cubic Pd.8 In addition, uptake of hydrogen changes the band structure of the host alloy. This means the properties of the solvent phase (the alloy) change considerably with solute concentration, and with these resulting variations in jump length and activation barriers, it can therefore be expected that the rate of diffusion of hydrogen will have a significant concentration dependence. A more representative description of diffusion than that offered by Fick’s law is therefore required to describe diffusion through alloy membranes, especially bcc alloys under nondilute conditions. Several groups have adopted varying mathematical treatments to account for the nonideality of hydrogen permeation through palladium alloy membranes. The simplest approach is to determine the exponent directly from experimental flux data by plotting dpn against flux for various n values to determine the best fit. This approach was taken by Morreale et al.,9 who determined the optimum n value for thick Pd membranes to be 0.62, and by Hurlbert and Konecny (0.68).10 This approach is simplistic, however, in that the varying Sieverts’ exponent is used to account
for behavior which may be influenced by several factors, including concentration-dependent diffusivity, nonideal hydrogen absorption, or interfacial resistances. In their study of hydrogen permeation through palladium, Hara et al.11 proposed a pressure-dependent permeability which varied with p0.5 according to a second-order polynomial. Permeation flux for a given upstream and downstream pressure could then be calculated from the integral of the polynomial between the specified pressures. Vanadium alloys show significant deviations from n = 0.5 behavior,12 but as there is no theoretical basis for n 6¼ 0.5, the absorption behavior is a reflection of changes to the substrate with increasing hydrogen concentration. Correspondingly the term s in eq 2 should not be a constant, but rather a function of the dissolved hydrogen concentration. 1.3. Scope of This Research. VNi bcc alloys are among the leading alternatives to palladium alloys for hydrogen separation membranes, and a detailed understanding of atomic hydrogen transport will accelerate further development. This paper will describe a comprehensive and systematic investigation of the hydrogen diffusion and absorption properties of VNi alloys at various nonideal conditions. These data will be used to form a description of hydrogen concentration profiles and diffusion coefficients during permeation under conditions representative of an industrial membrane plant. It is hoped this work will guide the design of suitable membrane alloys and lead to a greater understanding of failure modes in bcc alloy membranes which arise from hydrogen absorption.
2. EXPERIMENTAL SECTION 2.1. Alloy Preparation. Three alloys were investigated (V95Ni5, V90Ni10, V85Ni15 (atom %). The alloys were formed from highpurity vanadium (99.99%) and nickel (99.99%), purchased from ESPI (U.S.A.). Appropriate quantities of each metal were arcmelted together on a water-cooled copper hearth under argon. Each ingot was turned and remelted around 10 times to ensure homogeneity. The presence of a single bcc solid solution was confirmed with X-ray diffraction (XRD), and this was consistent with previous investigations of these alloys.13,14 Ingots were sectioned into disks with the approximate dimensions 1 mm thick 25 mm diameter using electrical discharge wire machining. The surfaces were polished mechanically using 1 μm diamond paste to remove oxides and other contamination from each surface of the disks. Disks were then chemically cleaned, rinsed in deionized water, and dried in a desiccator. Each surface was then sputter-coated with a 500 nm Pd layer to promote hydrogen dissociation and recombination at the surface. 2.2. Permeation Measurements. Hydrogen permeation tests were undertaken using the constant pressure method. The disks were compression-sealed into a custom inconel module between copper gaskets. Disks were unsupported during testing to minimize mass transfer limitations at each surface. The total exposed surface area was 2.01 cm2. The module was inserted into a tube furnace and connected via compression fittings to a custom permeation testing apparatus. The disks were annealed at 400 °C for 12 h under flowing Ar prior to the introduction of hydrogen to ensure good adhesion between the Pd and underlying VNi alloy. After the module was stabilized at the appropriate temperature, a feed mixture of 90% H2 + 10% CO2 was introduced to one side of the membrane at a total flow rate of 800 mL min1 (N). The purpose of the CO2 was to detect the presence of defects in the membrane: the detection of CO2 in the permeate stream was 1513
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indicative of a membrane failure, and the experiment was terminated accordingly. A 100% Ar sweep was passed over the opposing surface, and the H2 content in the product stream was measured at 2 min intervals using a gas chromatograph. Prior to testing, the chromatograph was calibrated using certified standard gas mixtures. The Ar flow rate was varied throughout the experiment to maintain the H2 content to
