Water Partial Pressure Research Articles

Application of water-tolerant Co/β-SiC catalysts in slurry phase Fischer–Tropsch synthesis

High surface area, porous β-SiC has significant potential for use as a catalyst support in Fischer–Tropsch synthesis (FTS) due to its high hydrothermal stability. In recent times there has been a large amount of work done utilizing β-SiC as catalyst support for FTS but these have been applied to fixed bed reactor (FBR) systems. In this study we first review the existing literature for cobalt supported on β-SiC in FTS and secondly outline selected aspects of our in-house study of water tolerant catalysts for application in slurry phase reactors, which present a more hydrothermally demanding environment than FBR’s. Co/β-SiC catalysts have been prepared by slurry impregnation of cobalt nitrate and tested for the first time in slurry phase CSTR reactors at high water partial pressures (ca 10bar). These catalysts appear to have a high water tolerance compared to standard refractory oxide catalysts and upon optimal calcination afford excellent activities and low methane selectivity’s. Further modification of the β-SiC surface by acid washing and extended oxidation or surface modification with TiO2 can lead to significant improvements in catalytic performance over the unmodified supports.

Two-fold stoichiometry relaxation — Simulated relaxation kinetics of ionic and electronic defect concentrations

The relaxation kinetics of defect concentrations in an oxide containing protons, oxygen vacancies, and holes after an increase of water partial pressure are simulated using materials parameters representative for perovskite proton conductors. Different regimes are identified (e.g. single-fold chemical diffusion of water at low pO2, two-fold relaxation at high pO2). It is shown that the analysis of ionic defect concentrations (which could be measured e.g. by thermogravimetry) and electronic defect concentrations (detectable by conductivity relaxation) yields different effective diffusion coefficients. Owing to the transition between different defect chemical and diffusion regimes, extracted effective activation energies are related to the fundamental transport parameters in a complex way.

Protonic Conduction of Nanostructured Y-Doped BaZrO3

Nanostructured ionic conductors have recently attracted our attention due to the expectation that they may lead to new functionalities absent in microcrystalline conductors. In this study, nanostructured barium zirconate with perovskite crystal structure was prepared and its grain and grain boundary protonic conduction was investigated using ac impedance measurements as a function of temperature (RT ~ 400°C) and water partial pressure. The grain was highly conductive of protons, which is governed by the concentration of protonic defects at all temperatures. On the other hand, the grain boundary was not the preferred route for protonic conduction due to high resistance. However, enhanced protonic transport was observed at certain temperatures (<100°C). The protonic resistivity below that temperature decreased with decreasing temperature, showing positive activation energy in relation to temperature. The conduction route for the enhanced transport was the serial grain boundary. In addition, the route was compared with that of nanostructured zirconia with fluorite crystal-structure.

Water distribution in high temperature polymer electrolyte fuel cells

In this work a high temperature polymer electrolyte fuel cell based on a phosphoric acid doped polybenzimidazole membrane is operated at 160 °C on dry air and dry hydrogen. The anodic stoichiometry was varied to resemble typical operation with pure hydrogen or reformate gas. At the outlet of the fuel cell liquid water was collected by condensers. The resulting amounts of liquid water from cathode and anode were computationally analyzed. The results yielded an effective diffusion coefficient of water vapor of D≈2.7⋅10−7 m2 s−1, which is the average value for the cell based on the water partial pressures at the outlets. From the calculated water partial pressures at the anode and cathode it can be concluded that the concentration of phosphoric acid differs significantly within catalyst layer of the anode and cathode. Finally, the crossover of hydrogen and oxygen was shown to depend on the swelling of the membrane.

Potential Stable Low-Permeation Rate Standard Based on Micro-machined Silicon.

Silicon wafers with micro-machined holes were evaluated for use as low-permeation-rate standard artifacts. Accuracy, stability, and reliability were assessed. Two independent experimental techniques for evaluating permeation were used: chilled mirror hygrometer and mass loss. The wafers exhibited a well-defined linear relationship between hole area and resultant water partial pressure for both techniques, although the mass loss curve exhibited a constant vertical offset from the hygrometer curve, attributed to water loss through the O-ring seal. In contrast to polymer permeation standards, Si wafers provided long-term reproducible permeation rates. However, they were also highly fragile, with most of them cracking during the course of the investigation.

Protonic Conduction Properties of Nanostructured Gd-doped CeO<sub>2</sub> at Low Temperatures

The electrical properties of nanostructured Gd-doped CeO₂ (n-GDC) as a function of temperature and water partial-pressure were investigated using ac and dc measurements. For n-GDC, protonic conductivity prevails under wet condition and at low temperatures ( 200℃) under both dry and wet conditions. The grain boundaries in n-GDC were highly selective, being conductive for protonic transport but resistive for oxygen ionic transport. The protonic conductivity reaches about 4 × 10 −7 S/cm at room temperature (RT).

Air–water fluxes and sources of carbon dioxide in the Delaware Estuary: spatial and seasonal variability

Abstract. Distributions of surface water partial pressure of carbon dioxide (pCO2) were measured on nine cruises in the Delaware Estuary (USA). The Delaware River was highly supersaturated in pCO2 with respect to the atmosphere during all seasons, while the Delaware Bay was undersaturated in pCO2 during spring and late summer and moderately supersaturated during mid-summer, fall, and winter. While the smaller upper tidal river was a strong CO2 source (27.1 ± 6.4 mol-C m−2 yr−1), the much larger bay was a weak source (1.2 ± 1.4 mol-C m−2 yr−1), the latter of which had a much greater area than the former. In turn, the Delaware Estuary acted as a relatively weak CO2 source (2.4 ± 4.8 mol-C m−2 yr−1), which is in great contrast to many other estuarine systems. Seasonally, pCO2 changes were greatest at low salinities (0 ≤ S < 5), with pCO2 values in the summer nearly 3-fold greater than those observed in the spring and fall. Undersaturated pCO2 was observed over the widest salinity range (7.5 ≤ S < 30) during spring. Near to supersaturated pCO2 was generally observed in mid- to high-salinity waters (20 ≤ S < 30) except during spring and late summer. Strong seasonal trends in internal estuarine production and consumption of CO2 were observed throughout both the upper tidal river and lower bay. Positive correlations between river-borne and air–water CO2 fluxes in the upper estuary emphasize the significance of river-borne CO2 degassing to overall CO2 fluxes. While river-borne CO2 degassing heavily influenced CO2 dynamics in the upper tidal river, these forces were largely compensated for by internal biological processes within the extensive bay system of the lower estuary.

Estimation of Partial Pressure of Carbon Dioxide and Air-Sea Fluxes in Hooghly Estuary Based on In Situ and Satellite Observations

An empirical model is developed and used with remotely sensed predictors: sea surface temperature (SST) and chlorophyll-a concentration (Chl-a), to compute surface water partial pressure of carbon dioxide (pCO2w) and air-sea fluxes of CO2 in the Hooghly estuary and its adjacent coastal oceans. In situ observations used here were based on measurements carried out in this region during winter and summer periods in 2008. The estimated pCO2w compares well with the in situ observations at root mean square error ±18 μatm. In winter, estimated pCO2w ranges between 320 and 500 μatm with large values (>400 μatm) on the south-western and south-eastern flanks of the coastal domain and lower values (340–375 μatm) on the main-channel. In summer, it remained spatially uniform at 450 μatm. Extrapolation of the results over the study region based on the Moderate Imaging Specroradiometer (MODIS) measured SST and Chl-a suggests that the region is a strong source of atmospheric CO2 during the summer with net release of 0.095 Tg C year−1 (equivalent to mean flux of 90 molC m−2 year−1) and is a weak source during the winter with net release of 0.006 Tg C yr−1 (0.5 molC m−2 year−1) from the geographical extent of 6000 Km2 area.

Electrochemical Analysis for the Identification of Electrode Reaction in Ni-YSZ Anode as for Solid Oxide Fuel Cell

The impedance of Ni-YSZ cermet electrodes was investigated as a function of different partial pressure of hydrogen and water. The impedance spectra for the hydrogen/water reaction show two reactions by DRT analysis. Reaction at high-frequency region was assigned to transport of electrode – oxidation reaction within the electrode structure and the electrode / electrolyte interface. Whereas low frequency response is the adsorption of a gaseous H2O molecule from the gas phase onto the active electrode site. The data are analyzed using an equivalent circuit representing two parallel RC. The values of activation overpotential are separated under the fixed current load, which are well-fitted to the Butler-Volmer equation. Under the current load, the variation of exchange current density ( ) with pO2 indicated that the total reaction-rate is determined by charge transfer reaction at the high temperature.

Geometry of α-Cr2O3(0001) as a Function of H2O Partial Pressure.

Surface X-ray diffraction has been employed to elucidate the surface structure of α-Cr2O3(0001) as a function of water partial pressure at room temperature. In ultra high vacuum, following exposure to ∼2000 Langmuir of H2O, the surface is found to be terminated by a partially occupied double layer of chromium atoms. No evidence of adsorbed OH/H2O is found, which is likely due to either adsorption at minority sites, or X-ray induced desorption. At a water partial pressure of ∼30 mbar, a single OH/H2O species is found to be bound atop each surface Cr atom. This adsorption geometry does not agree with that predicted by ab initio calculations, which may be a result of some differences between the experimental conditions and those modeled.

Characteristic Studies of Micron Zinc Particle Hydrolysis in a Fixed Bed Reactor

Abstract Zinc fuel is considered as a kind of promising energy sources for marine propeller. As one of the key steps for zinc marine energy power system, zinc hydrolysis process had been studied experimentally in a fixed bed reactor. In this study, we focus on the characteristics of micron zinc particle hydrolysis. The experimental results suggested that the steam inner diffusion is the controlling step of accumulative zinc particles hydrolysis reaction at a relative lower temperature and a relative higher water partial pressure. In other conditions, the chemical reaction kinetics was the controlling step. And two kinds of chemical reaction kinetics appeared in experiments: the surface reaction and the gas-gas reaction. The latter one occurs usually for larger zinc particles and high reaction temperature. Temperature seems to be one of the most important parameters for the dividing of different reaction mechanisms. Several parameters of the hydrolysis process including heating rate, water partial pressure, the particle size and temperature were also studied in this paper. Results show that the initial reaction temperature of zinc hydrolysis in fixed bed is about 410°C. And the initial reaction temperature increases as the heating rate increases and as the water partial pressure decreases. The total hydrogen yield increases as the heating rate decreases, as the water partial pressure increases, as the zinc particle size decreases, and as the reaction temperature increases. A hydrogen yield of more than 81.5% was obtained in the fixed bed experiments.

The response of inorganic carbon distributions and dynamics to upwelling-favorable winds on the northern Gulf of Mexico during summer

Upwelling-favorable winds and an offshore-distributed Mississippi and Atchafalaya River plume trajectory were observed in summer 2009 in contrast to the mean conditions from 2002 to 2010 (upwelling-unfavorable winds and an alongshore river plume trajectory), a set of conditions which was also observed in summer 2007. The responses of dissolved inorganic carbon (DIC) distributions and dynamics to upwelling-favorable winds are studied by comparing the contrasting conditions between summer 2009 and summer 2007 on the northern Gulf of Mexico. Patterns of surface water partial pressure of CO2 (pCO2), DIC, δ13C in DIC, and total alkalinity (TA) determined in July 2009 and August 2007 were strongly related to river plume trajectories, and differed between the two summers.The slope of the relationship between dissolved oxygen (DO) and DIC in summer 2007 was comparable to the Redfield O/C ratio of 1.3, which was attributed to respiration of organic matter in the bottom water. The slope of the DO and DIC relationship and δ13CDIC values in bottom waters during July 2009 were clearly affected by mixing since their salinities were <35. A three end-member mixing model was used to remove mixing effects in (1) δ13CDIC, to estimate the organic source of respiration, and (2) in DIC concentrations, to calculate DIC removal and release. δ13CDIC results in both summers were consistent with an apparent release of DIC in hypoxic waters (DO less than 2mgL−1) associated with respiration of surface organic matter. The area-weighted surface DIC removal (i.e., biological production) was lower in 2009 than in 2007 on the shelf, as the plume was distributed offshore. The release of DIC in bottom waters was higher over the shelf in 2009 and was surmised to be related to stronger mixing, which was favorable for the DO supply for respiration. Overall, surface waters on the continental shelf in the region of study in July 2009 acted as a weak CO2 source to the atmosphere, but a weak CO2 sink in August 2007. We contend that the inorganic carbon distribution and concentrations on the shelf were related to regional wind forcing, through its influence on the distribution of coastal currents and plume trajectories and their subsequent impact on biogeochemical processes.

Analysis of parageneses of metapelite gneisses of the Okhotsk granulite complex by minimization of Gibbs thermodynamic potential

The problem of modeling of real parageneses has been solved by minimization of Gibbs thermodynamic potential for metapelites of the Okhotsk granulite complex. Model mineral assemblages completely reproduce the composition of minerals and their modal contents in the studied rocks. This fundamental fact directly verifies the solution of the problem, proving the validity of the principle of local equilibrium in the studied assemblages and the agreement of all thermodynamic data accepted on the modeling. The pressure and temperature during the metamorphism of granulites of the Okhotsk complex, estimated by modeling, are 5.2–7.0kbar and 620–770°C, which corresponds to the near-boundary conditions of the amphibolitic and granulitic facies. Model mineral assemblages similar to real parageneses in the composition of minerals and their modes can be successfully obtained with the Selektor software under conditions of both inert and moving water. The composition of the external metamorphic fluid and the approximate weight ratio of fluid to rock have been determined. The oxidation potential of this fluid is similar to the potential of oxygen at the buffer C–CO–CO2 if the fluid/rock ratio is 0.03–0.30 and the low partial pressure of water varies from 1.80 to 0.35kbar. The Okhotsk metamorphic complex is not an analog of the granulites of the southern Aldan Shield, because considerably higher pressure and temperature are typical of the latter.

Direct In-Situ Investigation of Selective Surface Oxidation During Recrystallization Annealing of a Binary Model Alloy

The study of high temperature oxidation processes depends mainly on ex situ investigations after the high temperature process is completed. In this study Ambient Pressure Photoelectron Spectroscopy was used to investigate its potential for an in situ analysis of the underlying mechanisms of selective oxidation occurring during high temperature annealing. As an example, the evolution of the surface oxide states of an iron-manganese model alloy (with 2 wt% Mn) during an annealing step simulating recrystallization annealing was studied. It was shown that an investigation of the surface after the annealing step does not provide reliable information about the oxidation state during the annealing step. At low partial pressures of water in a forming gas atmosphere the kinetics of water dissociation on the surface are not high enough to provide enough oxygen for reaching the thermodynamically expected activities. This unexpected low oxygen activity on the surface is also shown to affect the uptake of hydrogen into the Fe–2Mn sample.

Reaction path analysis for 1-butanol dehydration in H-ZSM-5 zeolite: Ab initio and microkinetic modeling

Dispersion corrected periodic density functional theory (DFT) is used to construct a microkinetic model for 1-butanol dehydration in H-ZSM-5. The latter is applied to determine the effect of reaction conditions on dehydration rates, product selectivity and dominant reaction pathway. The consecutive reaction scheme of 1-butanol dehydration to ether followed by ether decomposition offers lower energy barriers as compared to the direct conversion of 1-butanol to 1-butene. The direct dehydration of 1-butanol to 1-butene occurs via an E2 (anti) elimination at low 1-butanol partial pressure and shifts to a 1-butanol assisted 1,2-syn-elimination with increasing 1-butanol partial pressure. The ether formation reaction proceeds via an SN2-type nucleophilic substitution mechanism, involving substitution of the –OH2 group of the protonated alcohol by 1-butanol, while ether decomposition predominantly occurs via a 1,2-syn-elimination mechanism. The effect of reaction conditions viz. reaction temperature, site time, 1-butanol and water partial pressure is studied. The reaction conditions govern the coverage of key surface species which in turn has a significant role in determining the dominant reaction mechanism and product selectivity. Under industrially relevant conditions, the presence of water has no significant effect on 1-butanol conversion and product selectivity. A higher reaction temperature, higher site time and lower 1-butanol partial pressure favor a higher 1-butene selectivity.

Multiscale Modeling of a Proton Exchange Membrane Fuel Cell: Macroscopic Fuel Cell Model and Experimental Validation

In this work is presented a multiscale model for a Proton Exchange Membrane Fuel Cell (PEMFC) composed by a kinetic model for the oxygen reduction reaction (ORR) studied with Density Functional Theory (DFT), and a macroscopic transport model for the diffusion layers and the membrane. Values of water partial pressure at the boundaries of the electrodes and global electro-osmotic drag coefficients are obtained for different humidity of reactive gases and current densities. These quantities are compared with: i) the model itself but assuming the kinetics described by Faraday Laws and ii) experimental data. It is found good consistency between the calculated values and the experimental data. Also, it was obtained better degree of prediction with respect to the kinetics described by only the Faraday laws, especially for low current densities and similar inlet humidity of reactive gases. Because of the results obtained, the multi-scale strategy applied was considered appropriated and a notorious improvement is obtained for the description of the kinetic and transport phenomenology involved in the operation of PEMFC’s.

Assessing the performance of an industrial SBCR for Fischer–Tropsch synthesis: Experimental and modeling

The main objective of this study is to predict the performance of an industrial‐scale (ID = 5.8 m) slurry bubble column reactor (SBCR) operating with iron‐based catalyst for Fischer–Tropsch (FT) synthesis, with emphasis on catalyst deactivation. To achieve this objective, a comprehensive reactor model, incorporating the hydrodynamic and mass‐transfer parameters (gas holdup, εG, Sauter‐mean diameter of gas bubbles, d32, and volumetric liquid‐side mass‐transfer coefficients, kLa), and FT as well as water gas shift reaction kinetics, was developed. The hydrodynamic and mass‐transfer parameters for He/N2 gaseous mixtures, as surrogates for H2/CO, were obtained in an actual molten FT reactor wax produced from the same reactor. The data were measured in a pilot‐scale (0.29 m) SBCR under different pressures (4–31 bar), temperatures (380–500 K), superficial gas velocities (0.1–0.3 m/s), and iron‐based catalyst concentrations (0–45 wt %). The data were modeled and predictive correlations were incorporated into the reactor model. The reactor model was then used to study the effects of catalyst concentration and reactor length‐to‐diameter ratio (L/D) on the water partial pressure, which is mainly responsible for iron catalyst deactivation, the H2 and CO conversions and the C5+ product yields. The modeling results of the industrial SBCR investigated in this study showed that (1) the water partial pressure should be maintained under 3 bars to minimize deactivation of the iron‐based catalyst used; (2) the catalyst concentration has much more impact on the gas holdup and reactor performance than the reactor height; and (3) the reactor should be operated in the kinetically controlled regime with an L/D of 4.48 and a catalyst concentration of 22 wt % to maximize C5+ products yield, while minimizing the iron catalyst deactivation. Under such conditions, the H2 and CO conversions were 49.4% and 69.3%, respectively, and the C5+ products yield was 435.6 ton/day. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3838–3857, 2015

The effect of preparation method on the proton conductivity of indium doped tin pyrophosphates

Indium doped tin pyrophosphates were prepared by three synthetic routes. A heterogeneous synthesis from metal oxides with excess phosphoric acid produces crystalline phosphate particles with a phosphorus rich amorphous phase along the grain boundaries. The amorphous phase prevents the agglomeration of particles, hydrolyzes in moist atmosphere as revealed by FT-IR and solid state NMR, and facilitates a high proton conductivity (above 2.5×10-2Scm−1) with high stability at above 120°C under a water partial pressure of 0.15atm. This phase can be removed by washing with water, resulting in a dramatic decrease in conductivity as well as significant agglomeration of the particles, as evident in TEM and from particle size distribution measurements. Homogeneous synthesis with soluble metal acetates or chlorides as precursors results in a single crystalline phase with a small particle size, but strongly agglomerated, and a low conductivity at 10−7–10−6Scm−1 level. Further impregnation of the agglomerates with phosphoric acid does not lead to formation of the phosphorus rich amorphous layers on the surface of the crystals. An intermediate conductivity of 10−3Scm−1 was observed for the acid treated phosphates from the chloride synthesis but no improvement for the acid treated phosphates from the acetate synthesis was observed.

Oxidation behavior of 2D C/SiC composites coated with multi-layer SiC/Si–B–C/SiC coatings under wet oxygen atmosphere

Oxidation behavior of 2D C/SiC composites coated with multi-layer SiC/Si–B–C/SiC coatings had been investigated under wet oxygen atmosphere for 50h. The multi-layer SiC/Si–B–C/SiC coatings showed excellent oxidation protection at all temperatures due to the effective crack sealing of borosilicate glass. Especially, the coated samples also had a high residual strength retention ratio (106.7%) and a low weight loss (0.058%) after oxidation at 1000°C for 50h. However, the increasing water partial pressure could improve the volatilization of borosilicate glass and weaken the sealing function of the glass at some extent when the oxidation temperature was above 1200°C.

Sensitivity of the air–sea CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; exchange in the Baltic Sea and Danish inner waters to atmospheric short-term variability

Abstract. Minimising the uncertainties in estimates of air–sea CO2 exchange is an important step toward increasing the confidence in assessments of the CO2 cycle. Using an atmospheric transport model makes it possible to investigate the direct impact of atmospheric parameters on the air–sea CO2 flux along with its sensitivity to, for example, short-term temporal variability in wind speed, atmospheric mixing height and atmospheric CO2 concentration. With this study, the importance of high spatiotemporal resolution of atmospheric parameters for the air–sea CO2 flux is assessed for six sub-basins within the Baltic Sea and Danish inner waters. A new climatology of surface water partial pressure of CO2 (pCO2w) has been developed for this coastal area based on available data from monitoring stations and on-board pCO2w measuring systems. Parameterisations depending on wind speed were applied for the transfer velocity to calculate the air–sea CO2 flux. Two model simulations were conducted – one including short-term variability in atmospheric CO2 (VAT), and one where it was not included (CAT). A seasonal cycle in the air–sea CO2 flux was found for both simulations for all sub-basins with uptake of CO2 in summer and release of CO2 to the atmosphere in winter. During the simulated period 2005–2010, the average annual net uptake of atmospheric CO2 for the Baltic Sea, Danish straits and Kattegat was 287 and 471 Gg C yr−1 for the VAT and CAT simulations, respectively. The obtained difference of 184 Gg C yr−1 was found to be significant, and thus ignoring short-term variability in atmospheric CO2 does have a sizeable effect on the air–sea CO2 exchange. The combination of the atmospheric model and the new pCO2w fields has also made it possible to make an estimate of the marine part of the Danish CO2 budget for the first time. A net annual uptake of 2613 Gg C yr−1 was found for the Danish waters. A large uncertainty is connected to the air–sea CO2 flux in particular caused by the transfer velocity parameterisation and the applied pCO2w climatology. However, as a significant difference of 184 Gg C yr−1 is obtained between the VAT and CAT simulations, the present study underlines the importance of including short-term variability in atmospheric CO2 concentration in future model studies of the air–sea exchange in order to minimise the uncertainty.