Low Equilibrium Pressures Research Articles

AB5-type intermetallic compounds for biohydrogen purification and storage

Intermetallic compounds (IMC) provide a promising solution for a biohydrogen purification problem due to their selective hydrogen absorption from gaseous mixtures produced during dark fermentation. A main IMC characteristic for use in biohydrogen purification is hydrogen equilibrium pressure, which has to be as low as possible during hydrogen sorption from a mixture. Intermetallic compounds LaNi5–xMx (M−Al, Sn, Fe; x = 0.1–0.3) were prepared and their PCT properties were investigated. Compositions LaNi4.6Fe0.2Al0.2 (0.049 MPa at 293 K) and LaNi4.6Sn0.2Fe0.2 (0.055 MPa at 293 K) are found to be the most promising for practical applications due to low equilibrium pressures.

Reversible hydriding in YFe2-xAlx (x = 0.3, 0.5, 0.7) intermetallic compounds

To restrain the irreversible disproportionating reaction of YFe2 phase in the hydrogenation process, the structure of YFe2 was modified by partly substituting Fe with Al. And then, the hydrogen storage properties of YFe2-xAlx (x = 0.3, 0.5, 0.7) alloys were systematically investigated. It was found that the C15 Laves phase structure of those three Y-Fe-Al alloys kept stable at different states of as-annealed, hydrogenated and dehydrogenated, but their lattice constants vary with the Al and hydrogen contents. The YFe1.7Al0.3 alloy shows maximum reversible hydrogen storage capacity of 1.38 wt%, which is lower than its initial hydrogen absorption capacity of 1.56 wt%. All the three Y-Fe-Al alloys show fast hydrogenation rate at relative low temperature of 100 °C, however, their dehydrogenation properties are still limited due to their rather low desorption equilibrium pressure. The present finding provides a possibility of developing new types of rare earth-transition metal alloys for hydrogen storage.

Low-pressure equilibrium binary argon–methane gas mixture adsorption on exfoliated graphite: Experiments and simulations

Adsorption equilibrium measurements of pure methane, pure argon, and binary mixtures over exfoliated graphite were carried for different initial compositions, temperatures, and total pressures in the range of 0.1–1.5Torr using the volumetric static method. Diagrams for gas and adsorbed phase compositions were constructed for the conditions explored, and isosteric heats of adsorption were calculated. Experimental results were compared with predictions obtained with Monte Carlo simulations and using the Ideal Adsorbed Solution Theory (IAST).

Starting transient flows in a chevron ejector-diffuser system

The vacuum ejector-diffuser system has been widely used in many applications such as refrigeration systems, high altitude test facilities and fluid transportation devices. We simulated the starting transient flows of a supersonic vacuum ejector-diffuser system and its performance characteristics and analyzed by the numerical methods. Newly designed chevron lobes were installed at the inlet of the primary stream of the vacuum ejector-diffuser system for the purpose of its performance improvement. A CFD method based on transient scheme was applied to simulate the equilibrium flow behavior inside the secondary chamber. Primary numerical analysis results show that the performance of the vacuum ejector-diffuser system is considerably improved under chevron effects. Compared to the conventional ejector-diffuser system with a convergent nozzle, the updated system with a chevron nozzle leads to a lower equilibrium pressure inside the secondary chamber with less starting time. The flow characteristics obtained from the numerical results highlight the importance of the vortex generation effects of the chevron nozzle. The generation process of the longitudinal vortexes and its effects on the ejector-diffuser performance has been discussed in detail.

Thermodynamic and mechanistic studies on recovering phenol crystals from dilute aqueous solutions using pervaporation–crystallization coupling (PVCC) system

High purity phenol crystal was recovered from dilute aqueous solution via a novel pervaporation–crystallization coupling (PVCC) system, which used a two-stage condenser to collect the permeate. The first stage was used as a crystallizer of phenol and the second stage was to collect remaining permeate that was not trapped in the first stage. Due to the low equilibrium pressure of phenol in the crystallizer operated at low temperatures (e.g., −9°C) and excellent membrane permselectivity for phenol, a large proportion of phenol vapor leaving from membrane surface was condensed as high purity phenol crystals in the first condenser (crystallizer). The crystallization mechanism of phenol under vacuum condition was discussed. Phenol crystals were obtained when the temperature of the crystallizer was maintained in the range of −7 to 19°C. Phenol crystals at a purity of 99.8% were recovered in the crystallizer cooled at −9°C under a permeate pressure of 280Pa, and the quantity of phenol collected as crystals accounted for 75.9–96.1% of the total amount of phenol permeated through the membrane for a broad feed concentration range of 220–10,100ppm at an operating temperature of 80°C. Phase equilibrium analysis revealed that water vapor would also be precipitated when the temperature was below a certain point at a given pressure. Thus, the crystallizer should be operated at an appropriate temperature in order to prevent water condensation in the phenol crystallizer.

H2S interaction with HKUST-1 and ZIF-8 MOFs: A multitechnique study

The interaction of H2S with HKUST-1 and ZIF-8 MOFs has been studied by means of FT-IR, Raman, DRUV-Vis-NIR and PXRD techniques. At very low equilibrium pressure (below 5 mbar) a stepwise structural distortion is observed in both HKUST-1 and ZIF-8. At higher equilibrium pressures (20–60 mbar), PXRD technique, in particular, showed the structural destruction of HKUST-1 with formation of a covallite (CuS) phase and some structural distortion of ZIF-8.

Hydrogen-sorption properties of Nb4M0.9Si1.1 (M = Co,Ni) hydrides

Nb4M0.9Si1.1 (M = Co,Ni) is synthesised using high temperature techniques and its properties characterised using PXRD, PCI and TPD. It is shown that the hydrogen-sorption properties of Nb4M0.9Si1.1 (M = Co,Ni) are dominated by a low equilibrium pressure, low storage capacity and slow kinetics. These properties can in large part be explained by the crystal structure of the materials: kinetics are slow because of the arrangement of available H-sites (which are tetrahedral voids coordinated Nb-atoms) in 1d-“chains” possibly limiting diffusion in the two other dimensions; the high affinity for hydrogen of Nb and the large size of these voids explain the low equilibrium pressure; close proximity of most available H-sites to sites partially occupied by Si explains the low storage capacity.

The effects of porous medium and temperature on exothermic tetrahydrofuran hydrate formation

Exothermic energy affects the sustained and rapid formation of hydrates. Therefore, it is necessary to clarify the thermal behaviour during hydrate formation. In this study, tetrahydrofuran (THF) was chosen as the hydrate former component due to its low hydrate equilibrium pressure. Five types of glass beads were used to form a porous medium that was saturated with 19wt% THF solution. Magnetic resonance imaging (MRI) was used to monitor the distribution of the solution in the vessel. The results showed that the “induction time” of THF hydrate formation is short in the glass beads. The exothermic character of the hydrate formation process was evident in the MRI images. The temperature profile in the porous medium was determined based on the MRI mean intensity and temperature. There were obvious higher temperature regions around the hydrate formation zones. It was found that the hydrate formation rate was slowly at multiple points and aggregated in large glass beads, and that the heat release was homogeneous. The residual MI value is higher than that in the small glass beads. Then, the hydrate began to grow from the vessel wall in the small glass beads. The temperature rose significantly during the hydrate formation. The effect of initial temperature on the experimental results is small. There are two factors that cause the temperature rise that cannot be monitored: if hydrate formation is finished and the exothermic energy has been lost, or if the hydrate does not begin to form abundantly.

Polycyclotrimers of 1,4‐Diethynylbenzene, 2,6‐Diethynylnaphthalene, and 2,6‐Diethynylanthracene: Preparation and Gas Adsorption Properties

Hyperbranched partly cross‐linked polycyclotrimers of 1,4‐diethynylbenzene, 2,6‐diethynyl­naphthalene, and 2,6‐diethynylanthracene, Pc(1,4‐DEB), Pc(2,6‐DEN), and Pc(2,6‐DEA), respectively, are prepared using TaCl5/Ph4Sn catalyst. Brunauer–Emmett–Teller (BET) surface area, microporosity, and maximum sorption capacity for H2 and CO2 decrease in the order of decreasing relative content of branching points in polycyclotrimers Pc(1,4‐DEB) > Pc(2,6‐DEN) > Pc(2,6‐DEA), the highest values for Pc(1,4‐DEB) being SBET = 1299 m2 g−1, aH2 = 1.26 wt% (100 kPa, 77 K), and aCO2 = 10.8 wt% (100 kPa, 273 K). N2 isotherms show that adsorption/desorption hysteresis occurs already at low equilibrium pressures. CO2 isotherms show that the time allotted to the measurement influences both the maximum adsorption capacity and the hysteresis upon desorption. image

The Effect of 3He on Low Pressure Hydride Absorption Measurements with Tritium

Absorption isotherm data exists for a wide variety of hydrogen-metal systems. When working with high purity gases, appropriately sized equipment, and hydrides with equilibrium pressures above several hundred Pa, data collection is relatively straightforward. Special consideration must be given to experiments involving low equilibrium pressure hydrides, as even sub-ppm levels of gas impurities can generate partial pressures many times greater than the equilibrium pressures to be measured. Tritium absorption experiments are further complicated by the continuous generation of helium-3. The time required to transfer and absorb a known quantity of tritium onto a sample ultimately limits the minimum pressure range that can be studied using the standard technique. Equations are presented which show the pressure of helium-3 in a sample cell based on the amount of tritium to be absorbed, the sample cell volume and temperature, and the decay time of tritium. Sample calculations for zirconium show that at 300°C, the estimated helium-3 pressure in the cell will be equal to the hydrogen absorption pressure after only milliseconds of tritium decay. An alternate method is presented that permits the collection of equilibrium data at pressures orders of magnitude lower than possible using a direct approach.

Prediction of metal–silicate partition coefficients for siderophile elements: An update and assessment of PT conditions for metal–silicate equilibrium during accretion of the Earth

Experimental studies of the partitioning of siderophile elements between metallic and silicate liquids have provided fundamental constraints on the early history and differentiation conditions of the Earth. With many new studies even in the last 20 yr, several models have emerged from the results, including low pressure equilibrium, high pressure equilibrium, and combined high and low pressure multi-stage models. The reasons – silicate melt composition, pressure effects on silicate melt structure, different methods for calculating metal activity coefficients, and the role of deep mantle phases – for the multitude of resulting models have not been specifically addressed before, yet are critical in evaluating the more likely and realistic models. The four reasons leading to the divergence of results will be discussed and evaluated. The behavior of the moderately siderophile elements Ni and Co will be compared using several approaches, each of which results in the same conclusion for Ni and Co. This consistency will eliminate the supposition that one or the other approaches gives a more accurate answer for element partitioning. Newly updated expressions for 11 elements are then derived and presented and applied to the early Earth to evaluate the idea of a late stage equilibration between a core forming metal and silicate melt (or magma ocean). It is possible to explain all 11 elements at conditions of 27–33 GPa, 3300–3600 K, ΔIW = − 1, for peridotite and a metallic liquid containing 10% of a light element. The main difference between the current result and several other recent modeling efforts is that Mn, V, and Cr are hosted in deep mantle phases as well as the core. The other elements – Ni, Co, Mo, W, P, Cu, Ga, and Pd – are hosted in core, and detailed modeling here shows the importance of accounting for oxygen fugacity, silicate and metallic liquid compositions, as well as temperature and pressure. The idea of late stage metal–silicate equilibrium at a restricted pressure and temperature range leaving a chemical finger print on the upper mantle remains viable.

The low-pressure phase diagram of sulfur

The experimental values of the ( p, T) phase diagram of sulfur have been assessed through empirical equations commonly used. This treatment shows insufficiencies, namely those leading to significant uncertainties in the coordinates of the low-pressure triple points. An alternative way to try and find reliable equations for the phase equilibrium curves is provided by the exact integration of Clapeyron equation established and developed in a previous work. No fitting to ( p, T) experimental data is then necessary. According to this method equations have been found to all the sulfur low-pressure equilibrium lines, and at the same time providing the enthalpies of transition between the phases as functions of temperature, and reliable values for the coordinates of the low-pressure triple points. The details of all these calculations and of the corresponding results are the object of the present paper. The ( p, T) low-pressure phase diagram of sulfur is reconstructed on the basis of the results of the calculations carried out following these lines.

Studies of Ti 1.5Zr 5.5V 0.5(M xNi 1− x) 9.5 (M = Cr, Mn, Fe, Co, Cu, Al): Part 2. Hydrogen storage and electrochemical properties

In Part 2 of this two part series of papers, the gas phase and electrochemical properties of the quinary alloy Ti 1.5Zr 5.5V 0.5(M x Ni 1− x ) 9.5 with M = Cr, Mn, Fe, Co, Cu, or Al and x = 0.1 or 0.2 are studied and compared to the structural properties reported in Part 1. The electrochemical properties of the alloys are strongly related to the non-Laves phases. For example, the Zr 7Ni 10 phase provides a more catalytic surface and improves the high rate discharge capability of the alloy at the expense of discharge capacity due to the higher Ni to Zr ratio; the Zr 9Ni 11 phase, on the contrary, improves the discharge capacity but hinders the high rate discharge capability. In the Laves phases, C14 provides a stable hydride with lower equilibrium pressure, higher gas phase storage capacity and lower electrochemical rate capability. The C15 phase provides a less stable hydride with higher equilibrium pressure, lower gas phase storage capacity and higher electrochemical storage capacity. The main effect of each modifying element can be summarized as follows: Mn is the most beneficial substitution element found in this study. Mn increases both the gas phase and electrochemical capacities with good reversibility; it also improves the activation, hydrogen diffusion, surface reaction, and high rate discharge capability. Co increases the degree of disorder in the alloy, reduces both gas phase and electrochemical capacities, destabilizes the hydride, improves activation, and reduces surface reactivity. Fe decreases the gas phase capacity but improves the electrochemical discharge capacity. Fe facilities activation, but its surface reaction is the least active among the substituents used in this study. Al, Cr, and Cu are less desirable because they reduce: (1) the electrochemical and gas phase capacities, (2) the reversibility, and (3) the rate capability. They also slow activation.

Evidencing three distinct FeII sites in Fe–FER zeolites by using CO and NO as complementary IR probes

Fe–FER zeolites were characterized by FTIR spectroscopy of adsorbed CO and NO. Two aged samples (Fe–FER-1 and Fe–FER-4 with Fe content of 1.1 and 3.7wt%, respectively) and one freshly prepared 57Fe rich sample (57Fe–FER, designed for Mössbauer studies and containing 1.5wt% Fe) were studied. Both CO and NO are adsorbed onto Fe2+ cations and have different sensitivities to their location and/or coordination state. CO adsorption on Fe–FER-1 reveals two kinds of Fe2+ sites with the respective complexes observed at 2195 and 2189cm−1. The principal carbonyls (2195cm−1) are easily converted into dicarbonyls (2188cm−1) at low temperature and high CO equilibrium pressure. NO is less sensitive than CO to the environment of Fe2+ ions and NO adsorption gives rise to a single band at 1878cm−1. However, careful analysis reveals that this band consists of two closely located components. With the Fe–FER-4 sample a third family of iron sites was detected by CO at 2196cm−1. These carbonyl species are stepwise converted to di- (∼2188cm−1) and tricarbonyls (∼2180cm−1). With these sites NO forms another nitrosyls clearly detected at 1895cm−1. The latter are converted with time into polynitrosyls. These new sites are very sensitive to the preliminary treatment and easily change their oxidation state, forming Fe2+/Fe3+ redox couples. The sample preliminary treated with oxygen at 673K is characterized by Fe3+–OH groups (3674cm−1) and reactive oxygen that produces carbonates when reacting with CO, and NO+ when interacting with NO. Adsorption of NO on a freshly prepared 57Fe–FER sample confirms the presence of the three distinct Fe2+ sites which is consistent with Mössbauer data. Finally, summarizing all the data, location of the different sites inside the FER structure is proposed. The results obtained are discussed in relation with the catalytic performance of Fe–FER.

Hydrate Equilibrium Data for Methane + tert-Butylamine + Water

Hydrate equilibrium pressures of methane + tert-butylamine + water were investigated in the temperature range of (281.2 to 295.2) K and in the pressure range of (2.69 to 8.46) MPa at (0.01, 0.056, and 0.097) mole fraction of tert-butylamine. Measurements were made using an isochoric method. The addition of tert-butylamine causes the hydrate equilibrium pressure to be lowered. In the case of higher mole fraction x = (0.056, 0.097), the hydrate has a lower equilibrium pressure than that of mole fraction x = (0.01).

Hydrogen storage in carbon fibers activated with supercritical CO 2: Models and the importance of porosity

In this work we studied the adsorption of H 2 at 77 K and 0.0–0.12 MPa onto carbon fibers activated with supercritical CO 2 (ACFs) and with different burn-offs (10–53%). The highest amount of H 2 stored was 2.45 wt% in an ACF with a burn-off of 51% at 0.12 MPa. The measured isotherms were analyzed using an equilibrium model derived by analogy with a multiple-site Langmuir-type adsorption model. The different equilibria correspond to adsorption in pores of different sizes. The experimental results fitted a model with two different adsorption sites satisfactorily, allowing such sites to be related to the microporous structure of the ACFs. Thus, a high-energy adsorbent–adsorbate interaction site, associated with very small micropores, accessible only to very small molecules such as H 2, and another lower-energy site associated with larger pores can be proposed. The model also predicts the adsorption behavior under equilibrium conditions at higher pressures, allowing the maximum adsorption capacity of the ACFs to be determined. The results show that the ACFs adsorb most of the H 2 molecules at low equilibrium pressures, and that they become almost saturated at pressures around 1.0 MPa. The maximum H 2 storage capacity in these ACFs lies between 1.50 and 3.15 wt%.

Response of CPO-27-Ni towards CO, N2 and C2H4

Coordinatively unsaturated Ni(2+) atoms in CPO-27-Ni form linear adducts with molecular nitrogen. The framework responds to the adsorption-modifying vibrational properties and local structure around adsorbing sites. The present paper deals with a fundamental infrared (IR) study of the interaction of gases on a microporous adsorbent metallorganic framework CPO-27-Ni containing, after solvent removal, coordinatively unsaturated Ni(2+) atoms [Dietzel et al., Chem. Commun. 2006, 959]. CO, N(2) and C(2)H(4) have been chosen. Notwithstanding the relative medium (CO and C(2)H(4)) and weak (N(2)) adsorption enthalpies and the low equilibrium pressures adopted (100-10(-3) mbar) the CPO-27-Ni framework responds promptly and reversibly to the adsorption process, modifying significantly both vibrational properties and local structure around Ni(2+) adsorbing sites as determined by a parallel EXAFS investigation locating the N(2) molecule 2.27 +/- 0.03 A apart from Ni(2+). For both N(2) and C(2)H(4), IR spectra have been discussed and carefully compared with literature data. Isosteric heat of adsorption of the Ni(2+)...N(2) complex formation has been evaluated from temperature dependent IR study to be -DeltaH(ads) = 17 kJ mol(-1).

Hydrogenation study of suction-cast Ti 40Zr 40Ni 20 quasicrystal

Suction casting was predicted to be an usable method for improving the hydriding kinetics of Ti/Zr-based icosahedral quasicrystals (IQCs) in our previous work. To further determine it, a suction-cast Ti 40Zr 40Ni 20 IQC alloy was used for hydrogenation studies by Pressure Composition Isotherm (PCI) and Temperature Programmed Desorption (TPD) techniques. The results showed that, this alloy absorbed hydrogen rapidly with obvious hydrogen pressure plateau and some reversibility, however, displayed very limited hydrogen capacity (about 0.7 wt.%) and low equilibrium pressure. After several hydrogenation/dehydrogenation cycles, the IQC structure transformed into two hydride phases, ZrH 2− x and one unknown, both of which decomposed at above 600 °C, suggesting high thermo-stability for them. On the whole, indeed the suction-casting method can increase the hydrogen absorption rate of Ti/Zr-based IQCs, however, the hydrogenation properties of the Ti 40Zr 40Ni 20 IQC alloy still need a mighty advancement.

Helium Effects on Tritium Storage Materials

For ten years French Tritium laboratories have been using metal hydride storage beds with LaNi4Mn for process gas (HDT mixture) absorption, desorption and for both short and long term storage. This material has been chosen because of its low equilibrium pressure and of its ability to retain decay helium 3 in its lattice. Aging effects on the thermodynamic behavior of LaNi4Mn have been investigated. Aging, due to formation of helium 3 in the lattice, decreases the desorption isotherm plateau pressure and shifts the α phase to the higher stoichiometries. Life time of the two kinds of tritium (and isotopes) storage vessels managed in the laboratory depends on these aging changes. The Tritium Long Term Storage (namely STLT) and the hydride storage vessel (namely FSH 400) are based on LaNi4Mn even though they are not used for the same applications. STLT contains LaNi4Mn in an aluminum vessel and is designed for long term pure tritium storage. The FSH 400 is composed of LaNi4Mn included within a stainless steel container. This design is aimed at storing low tritium content mixtures (less than 3% of tritium) and for supplying processes with HDT gas. Life time of the STLT can reach 12 years. Life time of the FSH 400 varies from 1.2 years to more than 25 years depending on the application.

Synthesis of GaN bulk crystals and melt growth of GaN layers under nitrogen plasma

Abstract Gallium nitrides of wurtzite structure were synthesized by reacting gallium metal with atomic nitrogen in a microwave plasma operating at 200–400 Pa with the power of 420 W and the frequency of 2.45 GHz to avoid high equilibrium pressure. Polycrystalline GaN can be synthesized at low equilibrium pressures of 200–400 Pa and low temperature of 610–700 °C. The yield of polycrystalline GaN was increased by increasing the nitrogen plasma exposure time and reached 96% at 3 h. The growth of GaN layers on (0 0 0 1) sapphire substrate was carried out using the same technique. When the sapphire substrate was placed in the bottom of the BN crucible, GaN layer of about 3 μm was grown on the substrate with the growth rate of 0.5 μm/h. The layer was nearly oriented to (0 0 0 1)GaN∥(0 0 0 1) sapphire. These results indicate that the direct conversion of gallium metal into GaN layer under nitrogen plasma is a useful method.