Stirred Flow Reactor Research Articles

CH4-NH3-NO oxidation: The interplaying role of NO sensitizing effect and DeNOx chemistry

This work presents a systematic experimental and numerical investigation of the oxidation behavior of fuel-lean CH4-NH3-NO mixtures in a Jet Stirred Flow Reactor. The experimental tests were performed as a function of the mixture preheating temperature (750–1250 K), at fuel-lean conditions (0.8), changing the concentration of ammonia in the fuel blends (from 0 up to 70 %). Results showed a significant sensitizing effect of NO on the oxidation of methane and methane/ammonia mixtures. In particular, CH4 reactivity is drastically enhanced by NO additions, while the onset of oxidation is gradually shifted to higher preheating temperatures as the concentration of NH3 increases.The kinetic analyses suggested that the interaction of NO with C-chemistry explicates through the reaction loop NO+HO2=NO2+OH and CH3+NO2=CH3O+NO, which influences both the reactivity and the thermo-kinetic instabilities, by promoting the formation of OH radicals and activating the CH3O oxidation pathway. Specifically, the NO-NO2 loop is identified as a crucial factor when the branching chemistry involves HO2 radicals.The presence of NH3 counterbalances this sensitizing effect through the activation of DeNOx chemistry, which partially inhibits the NO-NO2 -loop since they both compete for the NO consumption. In addition, NH3 can act as OH scavenger through NH3+OH=NH2+H2O, hindering the CH4 conversion via CH4+OH=CH3+H2O, thus slowing down the overall mixture reactivity.

NH3[sbnd]NO interaction at low-temperatures: An experimental and modeling study

The present work provides new insight into NH3NO interaction under low-temperature conditions. The oxidation process of neat NH3 and NH3 doped with NO (450, 800 ppm) was experimentally investigated in a Jet Stirred Flow Reactor at atmospheric pressure for the temperature range 900–1350 K. Results showed NO concentration is entirely controlled by DeNOx reactions in the temperature range 1100–1250 K, while NH3NO interaction does not develop through a sensitizing NO effect, for these operating conditions.A detailed kinetic model was developed by systematically updating rate constants of controlling reactions and declaring new reactions for N2H2 isomers (cis and trans). The proposed mechanism well captures target species as NO and H2 profiles. For NH3NO mixtures, NO profiles were properly reproduced through updated DeNOx chemistry, while NH2 recombination reactions were found to be essential for predicting the formation of H2. The role of ammonia as a third-body species is implemented in the updated mechanism, with remarkable effects on species predictions. For neat NH3 mixture, the reaction H+O2(+M)=HO2(+M) was crucial to predict NO formation via the reaction NH2+HO2H2NO+OH.

New insight into NH3-H2 mutual inhibiting effects and dynamic regimes at low-intermediate temperatures

The hydrogen effect as a “fuel enhancer” on ammonia oxidation features is a relevant topic for ammonia-based energy conversion systems. For the most, scientific literature is focused on high temperature ammonia-hydrogen oxidation chemistry, whereas few works focus on low-intermediate temperatures (900–1350 K), relevant for non-conventional low-temperature combustion processes. Recently, low-intermediate and the shift to high-temperature ammonia oxidation chemistry has been characterized through experimental tests in a Jet Stirred Flow Reactor (JSFR) by the same authors, with the identification of thermo-kinetic instabilities. In addition, the ammonia effect on hydrogen oxidation chemistry has been addressed through a mutual inhibiting interaction for low-intermediate temperatures.Given this background, this work investigates the hydrogen effects on ammonia oxidation and thermo-kinetic instabilities from low-intermediate to high temperatures in a JSFR, parametrically varying the H2 inlet concentration. Maps of combustion behaviours (Tin- ϕ) are then drawn up, on the basis of experimental evidences, in the range 1200K<Tin<1350 K, and 0.2 <ϕ<1.2. Results show H2 only moderately enhances the reactivity of the system for the investigated conditions. Consequently, dynamic regime areas in Tin-ϕ maps are slightly shifted towards lower Tin and restricted to a narrower ϕ range.Numerical simulations were able to predict the main NH3/H2 oxidation features, albeit low-intermediate temperature oxidation chemistry description is very mechanism-dependent. Nonetheless, the H2-NH3 mutual inhibiting interaction oxidation chemistry is congruently addressed: NH3 acts as OH radical scavenger, thus partially inhibiting the direct H2 oxidation, whereas H2 re-coverts back NH2 radicals to NH3, through the reaction NH2+H2=NH3+H. The same reaction produces the sole H radicals able to feed the high-temperature branching reaction of the H2/O2 sub-system. Same concluding remarks on the NH3/NH3--H2 oxidation chemistry open issues are then reported.

Modeling kinetics of Cu(II) release during Fe oxide transformation under the influence of humic substances

The dynamic behavior of Cu(II) in soil is usually affected by Fe oxides and humic substances, both of which are capable of Cu(II) binding. However, Fe oxides may undergo transformation processes and the presence of humic substances may complicate the Fe oxide transformation and Cu(II) binding. There is still a lack of quantitative understanding on how varying Fe oxide compositions and humic substances affect Cu(II) kinetic behavior. In this study, we developed a unified quantitative model for Cu(II) release from Fe oxides or Fe oxide-humic substance composites during the Fe oxide transformation based on the experimental data from our previous studies. The kinetic data were collected in a stirred-flow reactor system, in which Cu(II) release kinetics was studied during the Fe oxide transformation processes. Our model successfully reproduced the temporal changes of effluent Cu(II) concentrations with only one fitting parameter while other parameters were derived from the thermodynamic constraints or from the literatures. The modeling results indicated that Cu(II) release rates decreased with Fe oxide aging processes, while the presence of humic substances increased Cu(II) mobility. Our model was able to quantitatively assess the roles of different adsorbents in controlling the Cu(II) release rates, which highlighted the importance of accounting for the variations of Fe oxide transformation and the impact of humic substances in kinetic modeling. Our model has provided a quantitative tool for quantifying the kinetic behavior of heavy metals in multi-adsorbent systems, which is helpful for predicting the geochemical cycling of heavy metals in the environment.

Thermokinetic Instabilities for Ammonia-hydrogen Mixtures in a Jet Stirred Flow Reactor

The oxidation of ammonia and the interaction between ammonia and hydrogen chemistry have been extensively studied at high temperatures for traditional fames, while no experimental evidences have been provided for conditions relevant to MILD (Moderate or Intensive Low-oxygen Dilution) combustion. The high dilution levels and the relatively low working temperatures have been proven to promote thermo-kinetic instabilities, with detrimental effects on pollutant emissions and process efficiency.Given this background, first, this work reports on an experimental characterization of NH3-O2-N2 instabilities in a Jet Stirred Flow Reactor. Oxidation regimes were consequently reassumed in Tin-?? (preheating temperature Tin, and equivalence ratio ?) maps.Second, the effect of H2 as a fuel “enhancer” on the identified NH3-O2-N2 oxidation regimes was numerically investigated, parametrically changing the H2 concentration itself. Results suggested that small concentrations of H2 strongly enhance the system reactivity and tighten the Tin-? windows where instabilities occur.Kinetic analyses suggested that H2 strongly interacts with NH2 radicals, enhancing the overall NH3 oxidation chemistry, thus, suppresses the instabilit

Ammonia oxidation regimes and transitional behaviors in a Jet Stirred Flow Reactor

The present work was devoted to the experimental identification of combustion regimes for NH3/O2 mixtures diluted in N2 in a Jet Stirred Flow Reactor (JSFR). Low Temperature (LT) combustion and High Temperature (HT) combustion regimes were identified as a function of reactor temperature (Tr) and mixture equivalence ratio (ϕ). The shift between the identified regimes occurs at a noticeable system working temperature (1300 K), independently of the mixture composition. This shift may occur through a transitional Lower Reactivity (LR) regime for ϕ≤1 in-between 1300 and 1310 K. For fuel-lean conditions, damped temperature oscillations were recognized between 1310 and 1320 K.Different detailed kinetic mechanisms were used to simulate system reactivity. To some extent, kinetic models were able to reproduce the establishment of different kinetic regimes along with the dependence on system temperature and mixture composition. Kinetic analyses suggested that NH2 oxidation routes control the shift between LT and HT regimes along with the establishment of dynamic behaviors.The experimental results obtained in this work represent important constraints for a fine tuning of ammonia chemical kinetic mechanisms, with particular regards to intervals of parameters where the transition from LT to HT regimes occurs. Indeed, improving the models in these critical ranges will help seeking for possible fluid-dynamical and chemical strategies to improve combustion stability while reducing pollutants emissions.

Speciation and Release Kinetics Simulation of Zn and Cd from River Sediment Contaminated by Gold Mining

Heavy metals release from contaminated sediments is one of the most important chemical processes affecting overlying water quality in river, lake, and ocean. The objective of this study was to determine the leaching properties, kinetic rate, and leaching amount of zinc (Zn) and cadmium (Cd) from the river sediment contaminated by gold mining. Speciation of Zn and Cd in the sediments was extracted by a modified BCR extraction procedure. Release kinetics of Zn and Cd were studied by a simulated leaching experiment using a stirred-flow reactor and a two-site equilibrium-kinetic model. The sediments we studied were significantly contaminated by Zn (620–5878 mg kg−1) and Cd (2–67 mg kg−1), and both have high content of weak acid extractable forms. There were much more smaller particles on the surface of sediment JH01 and JH02 than sediment JH03 and JH04. The two-site equilibrium-kinetic model fits the release data of Zn and Cd well, and it was demonstrated as an effective tool to describe the kinetic release of Zn and Cd from river sediments. Kinetic rates obtained from curve fitting showed large variation among sediments indicating different reaction mechanisms. The rapid release stage (before the second stop-flow) of Zn and Cd was controlled both by the equilibrium sites and the kinetic sites, while the slow release stage (after the second stop-flow) was mainly controlled by the kinetic sites. The total leaching amount of Zn and Cd in JH01 (27.1 mg kg−1, 0.4 mg kg−1), JH02 (474 mg kg−1, 7.5 mg kg−1), JH03 (320 mg kg−1, 7.6 mg kg−1), and JH04 (52.4 mg kg−1, 2.0 mg kg−1) demonstrated that large amount of Zn and Cd in sediments can be leached into solution. Thus, effective measures should be taken to prevent leaching of heavy metals from river sediment.

Mutual inhibition effect of hydrogen and ammonia in oxidation processes and the role of ammonia as “strong” collider in third-molecular reactions

Ammonia/hydrogen interaction in oxidation chemistry has been extensively valued for traditional flames, while no-experimental evidences have been provided at lower temperatures, relevant for MILD combustion. Herein, ter-molecular reactions play a fundamental role, boosted by “strong” colliders (CO2, H2O). H2O “strong” collider nature derives from its polar nature. Ammonia exhibits a polar behavior similar to water. Given this background, the aim of this work is to value the “strong” collisional nature of ammonia through experimental tests in a Jet Stirred Flow Reactor for H2–O2/Ar or N2 mixtures in presence of H2O or NH3. Results show NH3 inhibits H2 reactivity more than H2O. Numerical analyses suggest a hydrogen/ammonia mutual inhibiting effect under MILD conditions. Simulations are very sensitive to the declaration of NH3 third-body collisional efficiencies in third-molecular reactions in detailed kinetic mechanisms. These effects may infer the possibility to develop economic hydrogen transport/store systems while minimizing ammonia production costs.

Ammonia oxidation features in a Jet Stirred Flow Reactor. The role of NH2 chemistry.

This work focuses on the characterization of the oxidation process of NH3/O2/N2 mixtures in a quartz Jet Stirred Flow Reactor (JSFR). Experimental tests are run as a function of mixture pre-heating temperatures (900–1350 K) and different equivalence ratios (Φ = 0.8, 1, 1.2), while keeping constant pressure, mixture residence time and mixture dilution level (d = 86%).Three different oxidation characteristic regimes are experimentally recognized as a function of the mixture pre-heating temperature (Tin), namely low (Tin < 1100 K), intermediate (1100 < Tin < 1225 K) and high (Tin > 1225 K) temperatures. Ammonia reactivity exhibits no-dependency on the mixture equivalence ratio for low-intermediate temperatures, while it is strongly dependent on it at higher ones.Some effort is devoted to evaluate the heterogeneous surface effects on the homogeneous ammonia oxidation process comparing the previous results with further tests obtained passivating the inner surface of the JSFR with water and comparing experimental results from two tubular Laminar Flow Reactors (LFR) made of different materials (quartz and alumina). Results suggest surface reactions mainly affect NO profiles, with no remarkable influence of key species concentrations.Numerical simulations show that tested models are not able to carefully reproduce the experimental results, specially within the low-intermediate temperature oxidation regimes. The analysis of the Reaction Rates highlights that ammonia oxidation is strongly dependent on the NH2 reaction routes at different temperatures. The discrepancy between the considered kinetic schemes can be ascribed to the different description of NH2 chemistry

On H2–O2 oxidation in several bath gases

The oxidation process of H2/O2 mixtures has received a lot of attention since it represents the first step for the development of detailed kinetic schemes for complex fuels within the idea of their construction in a hierarchical structure. Although much effort has been dedicated to the definition of reliable and robust kinetic mechanisms for the H2–O2 sub-system, still recently updated kinetic mechanisms for the H2–O2 system have been proposed by several authors. While under traditional conditions, available kinetic schemes seem to perform properly, when moving towards the simulation of new combustion modes (i.e. MILD, Oxyfuel, LTC engine conditions) their performance is still under discussion. In particular, the presence of large amount of non-inert diluent species as H2O and CO2 with high collisional efficiencies poses several problems, such as uncertainties on third body collisional efficiencies, the chemical kinetic description of reaction pressure dependence and “mixing rules”.In this work, new experimental tests have been run for a lean H2–O2 mixture in several bath gases (mainly N2, CO2 and H2O) in a Jet Stirred Flow Reactor. Dynamic behaviors have been experimentally detected, with temperature oscillations within the control volume for N2 diluted mixtures. CO2 and H2O act as inhibitors of such phenomenologies.The performance of updated kinetic schemes in different environments is satisfactory when the diluent is N2, while some relevant discrepancies occur for mixtures diluted in other bath gases.Analyses of Rates of Reactions (RR) have allowed to identify the kinetic mechanism responsible for the onset and termination of instabilities. CO2 and H2O may suppress instabilities because of the enhanced role of termolecular reactions in virtue of their high third body collisional efficiencies.

Oxidation and pyrolysis of ammonia mixtures in model reactors

The present work was devoted to an experimental characterization of NH3 oxidation and pyrolysis processes in model reactors. The oxidation tests were performed in a Jet Stirred Flow Reactor (JSFR) for stoichiometric ammonia/oxygen/argon and ammonia/oxygen/argon/water mixtures, as a function of mixture inlet temperature (Tin), at fixed pressure and residence time. Following, the behavior of ammonia/oxygen/argon mixture was analyzed in two tubular laminar flow reactors (LFRs) made of different materials (quartz and alumina), to evaluate potential surface heterogeneous effects.Pyrolysis tests were performed in both the reference model reactors, by substituting oxygen with argon.Experimental results realized in the JSFR allowed to identify three different kinetics regimes in ammonia oxidation: low (Tin < 1130 K), intermediate (1130 < Tin < 1250 K) and high temperatures (Tin > 1250 K). Surface heterogeneous reactions seemed to be negligible on ammonia reactivity and hydrogen profiles, but some effect could be observed on NOx concentration in the low-intermediate temperature range. Experiments realized in presence of water suggested that such species may passivate the surface, minimizing heterogeneous effects.Experimental tests realized in the LFRs confirmed these results, as hydrogen and oxygen profiles were independent of the reactor material, while NOx concentrations exhibited significant changes. Experimental results suggested that heterogeneous reactions were more relevant under pyrolytic conditions.Numerical simulations were performed with different kinetic mechanisms available in literature to value their capability to describe ammonia chemistry. None of them could accurately reproduce the experimental data for ammonia oxidation process, while they completely failed to predict ammonia pyrolysis features.

Impact of bisphenol A influent concentration and reaction time on MnO2 transformation in a stirred flow reactor.

Bisphenol A (BPA) is an endocrine disrupting compound commonly found in natural waters at concentrations that are considered harmful for aquatic life. Manganese(iii/iv) oxides are strong oxidants capable of oxidizing organic and inorganic contaminants, including BPA. Here we use δ-MnO2 in stirred flow reactors to determine if higher influent BPA concentrations, or introduction rates, lead to increased polymer production. A major BPA oxidation product, 4-hydroxycumyl alcohol (HCA), is formed through radical coupling, and was therefore used as a metric for polymer production in this study. The influent BPA concentration in stirred flow reactors did not affect HCA yield, suggesting that polymeric production is not strongly dependent on influent concentrations. However, changes in influent BPA concentration affected BPA oxidation rates and the rate of δ-MnO2 reduction. Lower aqueous Mn(ii) production was observed in reactors at higher BPA introduction rates, suggesting that single-electron transfer and polymer production are favored under these conditions. However, an examination of Mn(ii) sorption during these reactions indicated that the length of the reaction, rather than BPA introduction rate, caused enhanced aqueous Mn(ii) production in reactors with low introduction rates and longer reaction times due to increased opportunity for disproportionation and comproportionation. This study demonstrates the importance of investigating both the organic and inorganic reactants in the aqueous and solid phases in this complex reaction.

Propane oxidation in a Jet Stirred Flow Reactor. The effect of H2O as diluent species

Advanced combustion technologies, such as MILD, LTC (Low Temperature Combustion) reduce emission of pollutants by controlling system working temperatures to values not critical to promote the formation of several classes of pollutants. To access this temperature range, a significant dilution of reactants is required. At the same time, reactants have to be preheated to sustain the oxidation process. Such conditions are achieved by a strong recirculation of exhaust gases. Such a strategy implies that high contents of CO2 and H2O interact with reactants oxidation chemistry. In order to characterize this aspect of the combustion processes under diluted conditions, experimental tests were carried out for propane/oxygen/nitrogen mixtures in presence of variable amounts of H2O in a quartz Jet Stirred Flow Reactor (JSFR). Experiments were carried out at atmospheric pressure, over the temperature range 720–1100 K, from fuel lean to rich conditions and at a residence time of 0.5 s. Temperature and species concentration measurements suggest that the oxidation of propane is significantly altered by H2O in dependence of mixture inlet temperatures and equivalence ratios.Numerical analyses were performed to explore the interaction of H2O with the oxidation chemistry of propane. Results suggested that such a species alters the main radical branching mechanisms, i.e. in termolecular reactions as a third body species with high collisional efficiency or directly participating in bimolecular reactions.

Calcite dissolution kinetics at the sediment-water interface in natural seawater

Abstract The absorption, or uptake, of anthropogenic CO 2 by the oceans results in a decrease in pH and carbonate ion concentration, [CO 3 2 − ]; as a consequence, the saturation state of seawater with respect to CaCO 3 minerals (calcite, aragonite) falls, leading to a shallowing of their saturation depths and triggering an increase in their dissolution at the seafloor. Nearly one-third of the seabed is composed of CaCO 3 -rich sediments, and their dissolution is the ultimate marine sink of anthropogenic CO 2 . Despite numerous past studies, much confusion and uncertainty still surround our understanding of the rates and kinetics of CaCO 3 dissolution at the deep seafloor. Results from in situ studies disagree with laboratory studies, most of which have been carried out under conditions, e.g., mineral suspensions, that are not representative of processes at the seafloor. Herein, we report measurements of the dissolution rate of calcite, formed into synthetic sediment disks by mixing various amounts of this mineral with montmorillonite. These disks were placed in a stirred-flow reactor and exposed to a range of saturation states and shear stress conditions to simulate conditions at the sediment-water interface. The dissolution rates, normalized to the interfacial area of the sediment disks, were linearly dependent on the undersaturation state of the experimental seawater solution and displayed a square-root dependence on the calcite content, under both quiescent and stirred conditions. The rate of release of reaction products from the sediment increased with stirring rate, i.e., shear stress, until it became invariant at higher stirring rates. This latter result argues that calcite dissolution is transport (water-side) controlled for shear stress levels known to exist at the seafloor, which advises a simpler kinetic description of benthic calcite dissolution.

Siderite dissolution coupled to iron oxyhydroxide precipitation in the presence of arsenic revealed by nanoscale imaging

Siderite, the iron carbonate mineral, occurs in several geological environments and contributes to both the global iron and CO2 cycles. Under crustal conditions, this mineral may dissolve, releasing iron that becomes oxidized and then precipitates in the form of iron oxides and oxyhydroxides that have a high affinity for pollutants, such as arsenic. The process of siderite dissolution, dissolved iron oxidation, and oxyhydroxide precipitation is coupled in time and space. Here, we study the entire process using time-lapse in-situ atomic force microscopy. Natural siderite crystals were dissolved at room temperature in acidic aqueous solutions in the presence or absence of arsenic. The dissolution process, whose rate could be measured at a nanometer scale, occurred by the nucleation and growth of etch pits, the retreat of step edges, and the deepening of cleavage steps. Precipitation of iron oxyhydroxide phases coupled to siderite dissolution was imaged in-situ. Nucleated particles have an initial height of 1–2nm after 1minute reaction and then grow with time into aggregate precipitates 130–220nm wide and up to 80nm high after 24h of reaction. Ex-situ stirred-flow reactor measurements confirm the same sequence of siderite dissolution and iron oxyhydroxide precipitation. The arsenic is adsorbed by iron oxyhydroxides and its presence does not change significantly the rate of dissolution-precipitation of the overall process. Results provide a basis for understanding and quantifying the interactions between reduced-iron minerals and aqueous-phase oxidants, as well as potential sequestration of toxic elements such as arsenic.

Study of hydraulics and mixing in roof tanks used in intermittent water supply

Roof tanks are common in low and middle income countries, due to the intermittent water supply. Their hydraulic and water mixing behaviour has not been studied. This paper presents the results of a study on mixing and water demand in roof tanks, based on physical and numerical models. Tracer tests were carried out on a real scale transparent wall laboratory model of a roof tank, and a three-parameter residence time distribution model was applied, showing that the model that best describes mixing in roof water tanks is the one with a completely stirred flow reactor with a small portion of bypassing. This result was confirmed by computational fluid dynamic simulations and visual observation. The instantaneous water flow derived from activating typical home water-using fixtures was measured at the pipe feeding the tank, the pipe exiting the tank, and without a roof tank. Stochastic water demand patterns were generated with the measured data and used in the numerical model of a small distribution network. Based on this model it was found that water demand and pipe flow behave differently in continuous and intermittent water supply networks. The instantaneous flow rate withdrawn from the water distribution network pipes is lower in systems with roof tanks.

Heterogeneous kinetics of H&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O, HNO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; and HCl on HNO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; hydrates (&amp;lt;i&amp;gt;α&amp;lt;/i&amp;gt;-NAT, &amp;lt;i&amp;gt;β&amp;lt;/i&amp;gt;-NAT, NAD) in the range 175–200 K

Abstract. Experiments on the title compounds have been performed using a multidiagnostic stirred-flow reactor (SFR) in which the gas phase as well as the condensed phase has been simultaneously investigated under stratospheric temperatures in the range 175–200 K. Wall interactions of the title compounds have been taken into account using Langmuir adsorption isotherms in order to close the mass balance between deposited and desorbed (recovered) compounds. Thin solid films at 1 µm typical thickness have been used as a proxy for atmospheric ice particles and have been deposited on a Si window of the cryostat, with the optical element being the only cold point in the deposition chamber. Fourier transform infrared (FTIR) absorption spectroscopy in transmission as well as partial and total pressure measurement using residual gas mass spectrometry (MS) and sensitive pressure gauges have been employed in order to monitor growth and evaporation processes as a function of temperature using both pulsed and continuous gas admission and monitoring under SFR conditions. Thin solid H2O ice films were used as the starting point throughout, with the initial spontaneous formation of α-NAT (nitric acid trihydrate) followed by the gradual transformation of α- to β-NAT at T &gt; 185 K. Nitric acid dihydrate (NAD) was spontaneously formed at somewhat larger partial pressures of HNO3 deposited on pure H2O ice. In contrast to published reports, the formation of α-NAT proceeded without prior formation of an amorphous HNO3 ∕ H2O layer and always resulted in β-NAT. For α- and β-NAT, the temperature-dependent accommodation coefficient α(H2O) and α(HNO3), the evaporation flux Jev(H2O) and Jev(HNO3) and the resulting saturation vapor pressure Peq(H2O) and Peq(HNO3) were measured and compared to binary phase diagrams of HNO3 ∕ H2O in order to afford a thermochemical check of the kinetic parameters. The resulting kinetic and thermodynamic parameters of activation energies for evaporation (Eev) and standard heats of evaporation ΔHev0 of H2O and HNO3 for α- and β-NAT, respectively, led to an estimate for the relative standard enthalpy difference between α- and β-NAT of −6.0 ± 20 kJ mol−1 in favor of β-NAT, as expected, despite a significantly larger value of Eev for HNO3 in α-NAT. This in turn implies a substantial activation energy for HNO3 accommodation in α- compared to β-NAT where Eacc(HNO3) is essentially zero. The kinetic (α(HCl), Jev(HCl)) and thermodynamic (Peq(HCl)) parameters of HCl-doped α- and β-NAT have been determined under the assumption that HCl adsorption did not significantly affect α(H2O) and α(HNO3) as well as the evaporation flux Jev(H2O). Jev(HCl) and Peq(HCl) on both α- and β-NAT are larger than the corresponding values for HNO3 across the investigated temperature range but significantly smaller than the values for pure H2O ice at T &lt; 200 K.

Experimental study of the effect of CO2 on propane oxidation in a Jet Stirred Flow Reactor

The use of advanced combustion technologies (MILD, oxy-fuel combustion) is among the most promising methods to reduce emission of pollutants, as the system working temperatures are enough low to boost the formation of several classes of pollutants. To access this temperature range, a significant dilution of reactants is required. At the same time, reactants have to be preheated to sustain the oxidation process. Such conditions are achieved by a strong recirculation of exhausted gases. Such a strategy implies that high contents of CO2 and/or H2O interact with the reactants oxidation chemistry. In order to characterize this aspect of the combustion processes under diluted conditions, experimental tests were carried out for propane/oxygen/nitrogen mixtures in presence of variable amounts of CO2 in a quartz Jet Stirred Flow Reactor (JSFR). Experiments were realized at atmospheric pressure, over the temperature range 720–1100K, from fuel lean to rich conditions and at a residence time of 0.5s. Temperature and species concentration measurements suggest that the oxidation of propane is significantly altered by CO2 in dependence of mixture inlet temperatures and equivalence ratios.Numerical simulations pointed out that kinetic models are not able to correctly reproduce the experimental results. Further numerical analyses were performed to explore the interaction of CO2 with the oxidation chemistry of propane. Results suggested that such a species alters the main radical branching mechanisms, i.e. in termolecular reactions as a third body species with high collisional efficiency or directly participating in bimolecular reactions.

Desorption Characteristics of Three Mineral Oxides and a Non-crystalline Aluminosilicate for Supplying Phosphate in Soilless Root Media

ABSTRACTThe objective was to evaluate phosphate desorption characteristics of synthetic hematite, goethite, and allophane and commercial alumina after loading at maximum adsorbed phosphate levels to determine their potential to release phosphate at a constant, low level to sustain plant growth in soilless media and reduce phosphate leaching. Desorption isotherms were measured at pH 6.4 ± 0.1 using a continuously stirred-flow reactor. The time period during which dissolved phosphate was maintained within the range of 5–0.2 mg·L−1 phosphate-P decreased in the order: allophane (12.4 d) > alumina (4.6 d) > goethite (3.6 d) > hematite (1.9 d). Allophane released the most phosphate during the desorption process (40% of maximum adsorbed phosphate; 12.7 mg∙g−1) followed by alumina and goethite (19–20%; ≈2.5 mg∙g−1) and lastly hematite (5%; 0.1 mg∙g−1). Allophane demonstrated the greatest potential as a phosphate-charged source for soilless root media, in amount and duration of phosphate release.

The mid‐IR Absorption Cross Sections of α‐ and β‐NAT (HNO3 · 3H2O) in the range 170 to 185 K and of metastable NAD (HNO3 · 2H2O) in the range 172 to 182 K

AbstractGrowth and Fourier transform infrared (FTIR) absorption in transmission of the title nitric acid hydrates have been performed in a stirred flow reactor (SFR) under tight control of the H2O and HNO3 deposition conditions affording a closed mass balance of the binary mixture. The gas and condensed phases have been simultaneously monitored using residual gas mass spectrometry and FTIR absorption spectroscopy, respectively. Barrierless nucleation of the metastable phases of both α‐NAT (nitric acid trihydrate) and NAD (nitric acid dihydrate) has been observed when HNO3 was admitted to the SFR in the presence of a macroscopic thin film of pure H2O ice of typically 1 µm thickness. The stable β‐NAT phase was spontaneously formed from the precursor α‐NAT phase through irreversible thermal rearrangement beginning at 185 K. This facile growth scheme of nitric acid hydrates requires the presence of H2O ice at thicknesses in excess of approximately hundred nanometers. Absolute absorption cross sections in the mid‐IR spectral range (700–4000 cm−1) of all three title compounds have been obtained after spectral subtraction of excess pure ice at temperatures characteristic of the upper troposphere/lower stratosphere. Prominent IR absorption frequencies correspond to the antisymmetric nitrate stretch vibration (ν3(NO3−)) in the range 1300 to 1420 cm−1 and the bands of hydrated protons in the range 1670 to 1850 cm−1 in addition to the antisymmetric O–H stretch vibration of bound H2O in the range 3380 to 3430 cm−1 for NAT.