Gas Phase Saturation Research Articles

CO2 foam to enhance geological storage capacity in hydrocarbon reservoirs

Geological CO2 storage is an emerging topic in energy and environmental community, which is, as a commonly-accepted sense, considered a powerful approach to mitigate the global carbon emissions during the transition to net-zero. The geological media initially considered cover the saline aquifers, oil and gas reservoirs, coal beds, and potentially basalts. Up to now only the first two choices have been proven to be the most capable storage sites and successfully implemented at pilot/commercial scales. Here, novel strategies were proposed for the first time, by synthesizing and utilizing new high-dryness CO2 foam, to enhance geological CO2 storage capacity in oil and gas reservoirs. As a major storage site, the oil and gas reservoirs ranked only second to the saline aquifer, due to their huge underground spaces, and more importantly, spontaneous economic benefits by injecting CO2 for geological storage. However, at any development stage, particularly late stage with high water cut, CO2 injection usually tends to cause gas channeling and therefore result in low-efficient storage. In this paper, a series of dynamic evaluation experiments are carried out to investigate the effect of newly-synthesized high-dryness CO2 foam in oil and gas reservoirs. A comprehensive set of parameters, including the fluid production volume, rate, efficiency, pressure, porous media weight, water cut, mobility reduction factor, oil-gas-water ternary phase saturation, etc., are specifically analyzed. The lab results show that when the foam quality is 85 %, the oil recovery performance and gas phase saturation reach their highest values, of 82.68 % and 75.36 %, respectively; meanwhile, the water consumption is at the lowest level, 43.88 g/mol, for the CO2 storage in oil and gas reservoirs. This indicates the synthesized high-dryness CO2 foam, attributed to its superb stability, modifies the mobility ratio, suppresses the viscous fingering, and finally results in higher oil recovery and more CO2 storage. Similar work has been done for the reservoirs with permeabilities from high to low levels. The study provides experimental evidences that suggest benefits from applying CO2 flooding in oil and gas reservoirs.

Migration behavior of fugitive methane in porous media: Multi-phase numerical modelling of bench-scale gas injection experiments

Two-dimensional multi-phase numerical simulations based on detailed laboratory experiments are used to provide insight into the key processes of methane migration in porous media and to analyze the suitability of a continuum approach for modelling gas migration at the intermediate bench-scale. The simulations were conducted using the multi-phase numerical model DuMux, including groundwater flow, transport of free-phase methane subject to capillary and buoyancy forces, kinetic-controlled dissolution of methane into the flowing groundwater, and advective-diffusive transport of dissolved-phase methane. Mass transfer kinetics are controlled by the water-gas interfacial area, avoiding the assumption of thermodynamic equilibrium which was shown not applicable under these conditions of high aqueous velocity in sandy materials.The model is applied to a series of 2D laboratory experiments in which gas-phase methane was injected near the bottom of a 1.5 m square vertical flow cell under a background flow gradient and with homogeneous and heterogeneous configurations. Simulations are compared to the observed behavior with respect to gas-phase mass distribution over time, gas-phase saturations, and to breakthrough of the dissolved-phase methane at selected monitoring points. In this first comparative study (simulations vs real data), insights are provided into the role of spatial property distributions on methane migration, in particular gas-phase pooling below low-permeability layers. Similar to the laboratory results, the simulations confirmed that the concentrations of dissolved-phase methane are strongly dependent on the structure of the gas-phase plume which in turn is influenced by the geology. We show that at the local metre-scale, kinetic-controlled mass transfer is needed to reproduce the dissolved-phase methane concentrations. Nevertheless, an assessment of capillary parameter values typically encountered in sandy aquifer materials suggests that an equilibrium approach could still be suitable for field-scale simulations.

Bubble dynamic behaviors in the anode porous transport layer of proton exchange membrane electrolyzers using a microfluidic reactor

Gas bubbles in proton exchange membrane (PEM) electrolyzers are the main source of mass transport loss in the porous transport layer at high current densities. However, the complexity and opacity of the porous transport layer make it challenging. In this study, a microfluidic reactor with an electrochemical reaction boundary is designed to observe the bubble behaviors in the porous transport layer. The detailed bubble growth and detachment mechanisms are explored based on the visualization results. Results show that the gas phase evolution undergoes the unsteady growth stage and the steady stage. Bubble behaviors including free movement, coalescence, breakthrough and detachment are observed for a single bubble in the porous transport layer. The gas phase saturation in the porous transport layer as well as the applied voltage increases with increasing the current density. Moreover, as the flow rate increases, the time for the first bubble detachment and the number of bubble detachment gradually decrease, and the average detachment radius of the bubble gradually increases. A lower applied voltage is needed under a higher flow rate due to low mass transfer resistance. The proposed method can help us understand the dynamic behavior of bubbles in the porous transport layer of PEM electrolyzers.

Computational study on the three phase displacement characteristics of foam fluids in porous media

Foam technology has shown great potentials not only on enhancing oil recovery (EOR) but also on the Carbon Capture Storage Utilization (CCUS) applications. In this paper, the foam three phase displacement characteristics in the porous media is investigated based on the mechanistic Stochastic Bubble Population Balance (SBPB) model. With taking the foam generation rate Kg and maximum bubble density nmax as the key influential parameters, the dynamic distributions of the water, oil and gas phase saturations as well as the bubble densities along the foam flow direction are successfully obtained. Firstly, the overall computation frame is validated based on good agreements between the numerical and the reported experimental results on the foam displacement efficiency on water and oil phases. Then, the parameter studies are thoroughly performed concerning the two key factors of Kg and nmax in the SBPB model. Numerical results show the parameter of Kg dominates the foam flow behavoir in the inlet region of the porous media therefore doesn't show significant effect on the final oil and gas phase recovery rates. On the other hand, it is found larger nmax could lead to higher oil and water recovery ratio, which indicates to increase the nmax values of the foam fluid could act as the favorable means in oil recovery enhancement practices.

Prototype commercial evapoporometer instrument

Determining the pore size is important for assessing the performance of porous materials such as membranes. Evapoporometry is a relatively new characterization method based on the Kelvin equation that predicts the vapor-pressure reduction of a volatile wetting liquid in the pores. Evapoporometry employs a test cell that maintains a saturated gas phase above the porous sample so that evaporation progresses from the largest to the smallest pores. Continuously weighing the evaporated mass permits determining the pore-size distribution. Implementing evapoporometry requires considerable expertise; hence, it has been used as a research tool but not for routine characterization. Dedicated instruments are available for competitive pore-size-characterization methods such as liquid-displacement and gas-adsorption/desorption porometry. In this paper the development of a dedicated evapoporometer instrument is described that requires no user intervention once the porous sample is placed in the test cell. Moreover, the software also gives the flow-average and molecular-weight-cutoff metrics not reported in prior studies and corrects for the adsorbed t-layer mass on the pore walls that has been ignored or accounted for incorrectly in prior studies. The t-layer mass ranged from 8% to 72% for the three commercial PVDF membranes in this study; hence, ignoring it will seriously distort the pore-size distribution.

CFD Simulation of a bubble column evaporator

The combined principles of computational fluid dynamics (CFD) and heat and mass transfer are applied to design process equipment capable of performing both batch and continuous solvent swap operations at low temperature in the pharmaceutical industry. A bubble column evaporator was chosen a low temperature evaporator as it is a primitive, scalable and robust unit operation employed in a broad range of industries. A CFD analysis using OpenFOAM was subsequently performed on batch and continuous bubble column evaporators (BCE) for the methanol air system to provide data which experimental analysis cannot collect (with the same degree of detail) such as temperature distributions, concentration gradients and velocities across the column, which can then be used to predict methods of enhancing efficiency and evaporation rate. The effects of the gas flowrate, gas temperature and liquid flowrate on mass transfer in a BCE were investigated and the resulting system was analysed. Detailed analysis revealed that the gas was saturated almost instantaneously upon entering the column. It was concluded that both the gas flowrate and gas temperature are important factors affecting the rate of evaporation. An increased outlet gas temperature enhanced the rate of mass transfer in the system by up to 100% by increasing the gas phase saturation concentration of the solvent. This method proved particularly successful for the methanol-air system as the liquid had a significantly higher specific heat capacity than the gas and therefore the increased inlet gas temperatures had negligible effects on the liquid temperature.

Thermodynamic Modeling the Solubility of CO2 in Aqua System of Methyldiethanolamine and 2-(2-Aminoethylamino)ethanol Using the Nonelectrolyte Wilson Nonrandom Factor

Alkanolamines are used to remove acidic gases such as CO2 and H2S from natural gas. In this study, thermodynamic modeling of the binary component CO2+MDEA, three component MDEA+H2O+CO2 and the quaternary MDEA+AEEA+H2O+CO2 systems were developed using an additional Gibbs argillic model for the first time in the modeling of CO2 solubility in different solutions. The appropriate model was considered using the assumption of an entirely molecular system without any occurrence of chemical reactions and saturated gas phase from the CO2 gas. The nonelectrolyte Wilson nonrandom factor (N-Wilson-NRF) model and the activity coefficient method (γ_φ Aproach) were used to calculate solubility of CO2. The two-component water- CO2 model was modeled and the results were obtained by the accuracy of 1.38 of experimental results. In a three-component, water-CO2-MDEA system with the amount of 6.913, the optimization was developed. The quaternary water-CO2-MDEA-AEEA system was optimized with an overall approximation of 19.537 for all experiment data.

Hydrogen Accumulation and Distribution in Titanium Coatings at Gas-Phase Hydrogenation

This work is devoted to studying the accumulation of hydrogen in titanium coatings to use a completely new concept of hydrogen accumulators based on a system of easily replaceable cartridges. Modern hydrogen accumulators based on magnesium powder have several problems associated with uneven heating during hydrogen desorption. Increasing the efficiency of hydrogen accumulators and the possibility of their reuse and/or repair remains a topical problem. For the analysis of the microstructure of the received titanium coatings, scanning electron microscopy (SEM) was used, the structural-phase state was studied using x-ray diffraction (XRD) analysis. The coatings were hydrogenation by gas-phase saturation at 450–550 °C. Increased film thickness reduced the storage capacity of coatings. Besides hydrogenation at 450 °C, 20 µm of titanium coatings accumulated 3.96 wt.%, while 80 µm of coatings accumulated 3.98 wt.%. The chemical composition of the coatings before and after the hydrogenation was controlled using glow-discharge optical emission spectroscopy.

Investigating biases in the measurement of apparent alveolar septal wall thickness with hyperpolarized 129Xe MRI.

To investigate biases in the measurement of apparent alveolar septal wall thickness (SWT) with hyperpolarized xenon-129 (HXe) as a function of acquisition parameters. The HXe MRI scans with simultaneous gas-phase and dissolved-phase excitation were performed using 1-dimensional projection scans in mechanically ventilated rabbits. The dissolved-phase magnetization was periodically saturated, and the dissolved-phase xenon uptake dynamics were measured at end inspiration and end expiration with temporal resolutions up to 10 ms using a Look-Locker-type acquisition. The apparent alveolar septal wall thickness was extracted by fitting the signal to a theoretical model, and the findings were compared with those from the more commonly use chemical shift saturation recovery MRI spectroscopy technique with several different delay time arrangements. It was found that repeated application of RF saturation pulses in chemical shift saturation recovery acquisitions caused exchange-dependent gas-phase saturation that heavily biased the derived SWT value. When this bias was reduced by our proposed method, the SWT dependence on lung inflation disappeared due to an inherent insensitivity of HXe dissolved-phase MRI to thin alveolar structures with very short . Furthermore, perfusion-based macroscopic gas transport processes were demonstrated to cause increasing apparent SWTs with TE (2.5 μm/ms at end expiration) and a lung periphery-to-center SWT gradient. The apparent SWT measured with HXe MRI was found to be heavily dependent on the acquisition parameters. A method is proposed that can minimize this measurement bias, add limited spatial resolution, and reduce measurement time to a degree that free-breathing studies are feasible.

Noninvasive Monitoring of the Response of Human Lungs to Low-Dose Lipopolysaccharide Inhalation Challenge Using MRI: A Feasibility Study.

Development of antiinflammatory drugs for lung diseases demands novel methods for noninvasive assessment of inflammatory processes in the lung. To investigate the feasibility of hyperpolarized 129 Xe MRI, 1 H T1 time mapping, and dynamic contrast-enhanced (DCE) perfusion MRI for monitoring the response of human lungs to low-dose inhaled lipopolysaccharide (LPS) challenge compared to inflammatory cell counts from induced-sputum analysis. Prospective feasibility study. Ten healthy volunteers underwent MRI before and 6 hours after inhaled LPS challenge with subsequent induced-sputum collection. 1.5T/hyperpolarized 129 Xe MRI: Interleaved multiecho imaging of dissolved and gas phase, ventilation imaging, dissolved-phase spectroscopy, and chemical shift saturation recovery spectroscopy. 1 H MRI: Inversion recovery fast low-angle shot imaging for T1 mapping, time-resolved angiography with stochastic trajectories for DCE MRI. Dissolved-phase ratios of 129 Xe in red blood cells (RBC), tissue/plasma (TP) and gas phase (GP), ventilation defect percentage, septal wall thickness, surface-to-volume ratio, capillary transit time, lineshape parameters in dissolved-phase spectroscopy, 1 H T1 time, blood volume, flow, and mean transit time were determined and compared to cell counts. Wilcoxon signed-rank test, Pearson correlation. The percentage of neutrophils in sputum was markedly increased after LPS inhalation compared to baseline, P = 0.002. The group median RBC-TP ratio was significantly reduced from 0.40 to 0.31, P = 0.004, and 1 H T1 was significantly elevated from 1157.6 msec to 1187.8 msec after LPS challenge, P = 0.027. DCE MRI exhibited no significant changes in blood volume, P = 0.64, flow, P = 0.17, and mean transit time, P = 0.11. Hyperpolarized 129 Xe dissolved-phase MRI and 1 H T1 mapping may provide biomarkers for noninvasive assessment of the response of human lungs to LPS inhalation. By its specificity to the alveolar region, hyperpolarized 129 Xe MRI together with 1 H T1 mapping adds value to sputum analysis. 1 Technical Efficacy Stage: 2 J. Magn. Reson. Imaging 2020;51:1669-1676.

Impact of fluid property shift and capillarity on the recovery mechanisms of CO2injection in tight oil reservoirs

AbstractThe phase equilibria with the confinement effect could shift in nano‐pores, which could have a great impact on the recovery mechanisms of CO2injection in tight oil reservoirs; this has not been systematically studied. In this paper, the confinement effect with property shift and capillarity effect is introduced into the flash calculation of confined fluids. The Soave modification of the Redlich–Kwong equation of state is extended by the molecular‐wall collision parameter to describe the shifted pressure–volume–temperature properties of confined fluid, and the Young–Laplace equation is applied to evaluate the capillary pressure. This developed model could effectively be applied for phase equilibrium calculation in tight porous media because of the verification of experimental results. A binary mixture is investigated to study the different effect of capillary pressure and property shift on phase equilibria. Subsequently, a typical hydrocarbon fluid from Middle Bakken tight oil reservoirs is studied with CO2injection. Results illustrate that the confinement effect could play an increasingly important part in the phase equilibrium state. The CO2solubility and mass transfer driving force in tiny pores would be greater than those in large pores under the same conditions. The gas phase saturation would be smaller with the same compositions, which could extend the single‐phase region of fluid flow in porous media. Furthermore, bubble‐point pressure, the minimum miscible pressure of CO2/hydrocarbon, and the viscosity of tight oil dissolved with CO2both decrease with the pore size, which has a good influence on tight oil recovery. In general, the confinement effect could effectively reinforce the recovery mechanisms of CO2injection, which is conducive to the enhancement of tight oil recovery. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd.

Geophysical response to simulated methane migration in groundwater based on a controlled injection experiment in a sandy unconfined aquifer

Geophysical methods have the capacity to detect and characterize gas-phase dynamics in groundwater. Suitable methods can be deployed at surface or within boreholes depending on the required depth of investigation, spatial/temporal resolution, and geologic conditions. While the application of geophysical methods to monitor immiscible phase contaminants in the subsurface has been extensively documented, the effects of hydraulic properties and flow system conditions on the nature of the geophysical responses used to elucidate multi-phase fluid flow remains underdeveloped. A series of numerical 2-dimensional multi-phase flow and geophysical model simulations based on a controlled methane release experiment in the Borden unconfined sand aquifer was carried out to assess the influence of porous media hydraulic properties and flow system conditions on geophysical signatures associated with transient gas-phase saturation and gas migration behaviour. Specifically, the utility of electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) to monitor gas-phase plume dynamics in shallow groundwater flow systems is examined. ERT and GPR responses to gas-phase distribution and migration during a 72-day methane gas injection and subsequent recovery period was calculated using a numerical multi-phase flow model (CFbio) simulating four distinct parameterizations of the sandy aquifer system. Geophysical models showed that ERT was effective at imaging the central position of the plume but was less effective at detecting thinner lateral migration pathways extending beyond the primary high gas saturation bulb. Conversely, GPR was able to detect thin gas pools emanating from the primary gas bulb and small-scale vertical preferential pathways arising from capillary boundaries with contrasting saturations; however, gradational boundaries proved to be more difficult to resolve using GPR. This study demonstrates that ERT and GPR can be very useful tools in combination for longer-term monitoring of stray gas leakage from decommissioned hydrocarbon wells in shallow granular media freshwater aquifers, especially given the likelihood of strong lateral migration.

Impacts of salinity on CO 2 spatial distribution and storage amount in the formation with different dip angles.

Formation dip angle and the distortion of salinity affect the spatial distribution and storage capacity of carbon dioxide (CO2). In this numerical study, based on an actual CO2 injection demonstration project (Shiqianfeng group in the Ordos Basin) in China, CO2 was injected for a period of 20years at four different formation dip angles (0°, 5°, 10°, 15°). In conjunction, some salinity values were chosen, ranging from saturation salinity to no salinity. A three-dimensional (3D) model was established to systematically explore the influence of different formation dip angles and salinities on the CO2 spatial distribution and storage amount. The simulation results showed that larger salinity and higher pressure near the injection well will lead the CO2 gas-phase saturation and mass fraction to be smaller for a given formation dip angle. When salinity is held constant at the saturation value, a larger dip angle will cause a smaller CO2 gas saturation in the upper right units of the injection well, and a larger gas saturation in the lower left units at the 20th year of CO2 injection. For large salinity values (full, half, and quarter saturation salinity), the larger the formation dip angle is, the greater the CO2 total storage amount. For smaller salinity values (0.00 and 0.03), a transition point existed (at 8 and 18.2years) during the 20-year injection period. Before the transition point, the CO2 total storage amount also increases with the dip angle. After the transition point, however, the larger the formation dip angle is, the smaller the CO2 total storage amount becomes. In addition, a lower salinity may lead to the earlier appearance of the transition point.

Impact of temperature and pressure on the characteristics of two-phase flow in coal

Temperature and pressure are key factors affecting gas–liquid two-phase flows in porous media. Although evidence supports the effect of temperature on coal-bed methane (CBM) exploration, the fluid production is, at present, not sufficiently understood. By conducting nitrogen and water two-phase displacement experiments under different temperatures and pressures and monitoring the production of fluids at different times, this study obtained quantitative experimental measures for investigating the multi-phase seepage properties. The results indicated that (1) in the temperature range of 30–180 °C, the liquid produced was mainly the free water in the coal, and heating a coal reservoir facilitated the desorption of the water; (2) experimental testing provided a novel characteristic curve for the entire two-phase displacement process, which could be separated into the liquid seepage stage, gas–liquid multi-phase seepage stage, and gas flow stage; (3) increasing the temperature affected the gas and liquid phase velocities, production, and saturations during displacement; (4) with an increase in pressure, the irreducible water increased, whereas the two-phase percolation area decreased. The experimental methodology and obtained results provided evidence that during heat injection enhanced CBM recovery, the gas production rate might increase after breakthrough, with a simultaneous increase in the liquid production. Heating coal seams can thus improve the gas production efficiency, but has disadvantages compared to the initial drainage.

Effects of formation dip angle and salinity on the safety of CO2 geological storage - A case study of Shiqianfeng strata with low porosity and low permeability in the Ordos Basin, China

The safety of CO2 geological storage is a key problem that hinders its implementation. In particular, the formation dip angle and salinity can directly affect the spatial migration distribution, storage form and storage amount of CO2. A three-dimensional simulation model was established to evaluate the effects of the formation dip angle and salinity on the safety of CO2 geological storage. The simulation results showed that a larger dip angle resulted in a greater CO2 migration distance. The greatest CO2 migration distances with formation dip angles of 0°, 5° and 10° were 60%, 73.3%, and 86.7%, respectively, compared with a formation dip angle of 15° in the 200th year of CO2 migration. A larger formation dip angle was not conducive to CO2 geological storage. When the salinity was greater, smaller values were obtained for the CO2 liquid phase mass fraction, CO2 gas phase, liquid phase and total storage amount. With salinity values of S,3/4S,1/2S,1/4S, and 0.03, the proportions of liquid-phase storage amount in the salt-free strata were determined as 26.2%, 38.0%, 54.3%, 74.5%, and 88.1%, respectively, after CO2 migration for 200 years. Higher salinity resulted in less dissolved CO2 and decreased CO2 geological storage safety. After considering the combined influence of the dip angle and salinity, we found that the effects of the dip angle were obvious on the unit pressure and gas phase saturation, and the effect of salinity on the CO2 liquid phase mass fraction was significant. A higher dip angle and salinity greatly decreased the CO2 geological storage safety. Thus, a reservoir with a smaller dip angle and lower salinity should be selected as a site for CO2 geological storage in future studies.

Cryovolcanism on the Earth: Origin of a Spectacular Crater in the Yamal Peninsula (Russia)

Geological activity on icy planets and planetoids includes cryovolcanism. Until recently, most research on terrestrial permafrost has been engineering-oriented, and many related phenomena have received too little attention. Although fast processes in the Earth’s cryosphere were known before, they have never been attributed to cryovolcanism. The discovery of a couple of tens of meters wide crater in the Yamal Peninsula aroused numerous hypotheses of its origin, including a meteorite impact or migration of deep gas as a result of global warming. However, the origin of the Yamal crater can be explained in terms of cryospheric processes. Thus, the Yamal crater appears to result from collapse of a large pingo, which formed within a thaw lake when it shoaled and dried out allowing a large talik (that is layer or body of unfrozen ground in a permafrost area) below it to freeze back. The pingo collapsed under cryogenic hydrostatic pressure built up in the closed system of the freezing talik. This happened before the freezing completed, when a core of wet ground remained unfrozen and stored a huge amount of carbon dioxide dissolved in pore water. This eventually reached gas-phase saturation, and the resulting overpressure came to exceed the lithospheric confining stress and the strength of the overlying ice. As the pingo exploded, the demarcation of the crater followed the cylindrical shape of the remnant talik core.

Anaerobic biodegradation of dissolved ethanol in a pilot-scale sand aquifer: Gas phase dynamics

Groundwater contamination from ethanol (e.g., alternative fuels) can support vigorous biodegradation, with many possible reactions producing dissolved gases. The objective of this study was to improve the understanding of the development and evolution of trapped gas phase changes occurring within an ethanol plume undergoing biodegradation. The experiment performed involved highly detailed spatial and temporal monitoring of gas phase saturations using Time Domain Reflectometry probes embedded in a 2-dimensional (175 cm high × 525 cm long) synthetic aquifer (homogeneous sand tank with horizontal groundwater flow). Ethanol injection immediately promoted gas-producing reactions, including: fermentation, denitrification, sulphate-reduction and iron(III)-reduction, with methanogenesis developing between 69 and 109 days. Substantial in situ increases in trapped gas were observed over ~330 days, with maximum gas saturations reaching 27% of the pore volume. Despite sustained gas production, this maximum was never exceeded, likely due to the onset of gas phase mobilization (i.e., ebullition) upon reaching a buoyancy-capillarity threshold. Reductions in the quasi-saturated hydraulic conductivity, resulting from the gas phase accumulation, were restricted by ebullition to a factor of ≤2; but still appeared to alter the groundwater flow field. Overall, trapped gas saturations exhibited high spatial and temporal variability, including declines within the plume and increases outside of the plume. Influential factors included vertically-shifting ethanol inputs and resultant secondary redox reactions, microbial controls on redox zonation, ebullition, and altered groundwater flows. These observations have implications for the transport of gases and volatile compounds within plumes and above the water table at sites with groundwater contamination from ethanol or other highly degradable organics.

The mass transfer at Taylor cones

Electrospraying is a highly attractive method for generation of small particles and also for measurement applications of particles in suspensions. In the cone jet mode, the particle generation rate increases with decreasing flow rate. For applications in the field of aerosol technology flow rates of less than 10µl/h are attractive as particle sizes of less than 200nm can be obtained in this flow rate range. At the same time generation frequency of drops reaches the values required for size measurements by Scanning Mobility Particle Sizers (SMPS). A special challenge in operation of electrosprays at small flow rates is the change of liquid composition due to mass transfer processes between the Taylor cone and the surrounding gas phase. Typically used organic solvents such as alcohols, acetone or acetonitrile evaporate fast (or even evaporate completely) and mixtures containing such volatile species will change their composition significantly due to evaporation from the Taylor cone. Based on key experiments we showa)that the evaporating solvent can be reduced by as much as a factor of 20 by controlling the gas phase saturation. This allows spraying volatile solvents such as e.g. methanol at flow rates down to less than 1µl/h and makes the attractive sub-10µl/h flow rate range accessible to quite volatile solvents as well.b)that changes in the liquid composition by not considering the mass transfer (evaporation) problem lead to significant differences in liquid properties between the feed liquid and the Taylor cone. For example, an increase of surface tension by a factor of 2 is observed for water and t-butanol feed liquid.c)that employing a saturated sheath gas flow allows for very effective manipulation of the solvent composition in the Taylor cone. Based on water as feed liquid we show that a sufficient amount of organic solvent is easily transferred to the Taylor cone liquid from the gas phase to allow spraying the water without electron scavenging gases.Measurement results of the Taylor cone liquid composition are in good agreement with predictions by the presented new model describing the mass transfer between the Taylor cone and the surrounding gas phase. The efficiency of the mass transfer, the predictability of the process and the opportunity of comparably quick changes of the Taylor cone liquid by changing the gas phase saturation state result in a powerful tool for the entire field of electrospraying at low rates.

Strontium isotope fractionation during strontianite (SrCO3) dissolution, precipitation and at equilibrium

In this study we examine the behavior of stable Sr isotopes between strontianite [SrCO3] and reactive fluid during mineral dissolution, precipitation, and at chemical equilibrium. Experiments were performed in batch reactors at 25°C in 0.01M NaCl solutions wherein the pH was adjusted by bubbling of a water saturated gas phase of pure CO2 or atmospheric air. The equilibrium Sr isotope fractionation between strontianite and fluid after dissolution of the solid under 1atm CO2 atmosphere was estimated as Δ88/86SrSrCO3-fluid = δ88/86Sr SrCO3−δ88/86Srfluid=−0.05±0.01‰. On the other hand, during strontianite precipitation, an enrichment of the fluid phase in 88Sr, the heavy isotopomer, was observed. The evolution of the δ88/86Srfluid during strontianite precipitation can be modeled using a Rayleigh distillation approach and the estimated, kinetically driven, fractionation factor αSrCO3-fluid between solid and fluid is calculated to be 0.99985±0.00003 corresponding to Δ88/86SrSrCO3-fluid = −0.15‰. The obtained results further support that under chemical equilibrium conditions between solid and fluid a continuous exchange of isotopes occurs until the system approaches isotopic equilibrium. This isotopic exchange is not limited to the outer surface layer of the strontianite crystal, but extends to ∼7–8 unit cells below the crystal surface. The behavior of Sr isotopes in this study is in excellent agreement with the concept of dynamic equilibrium and it suggests that the time needed for achievement of chemical equilibrium is generally shorter compared to that for isotopic equilibrium. Thus it is suggested that in natural Sr-bearing carbonates an isotopic change may still occur close to thermodynamic equilibrium, despite no observable change in aqueous elemental concentrations. As such, a secondary and ongoing change of Sr isotope signals in carbonate minerals caused by isotopic re-equilibration with fluids has to be considered in order to use Sr isotopes as environmental proxies in aquatic environments.

An efficient approach for treating composition-dependent diffusion within organic particles

Abstract. Mounting evidence demonstrates that under certain conditions the rate of component partitioning between the gas and particle phase in atmospheric organic aerosol is limited by particle-phase diffusion. To date, however, particle-phase diffusion has not been incorporated into regional atmospheric models. An analytical rather than numerical solution to diffusion through organic particulate matter is desirable because of its comparatively small computational expense in regional models. Current analytical models assume diffusion to be independent of composition and therefore use a constant diffusion coefficient. To realistically model diffusion, however, it should be composition-dependent (e.g. due to the partitioning of components that plasticise, vitrify or solidify). This study assesses the modelling capability of an analytical solution to diffusion corrected to account for composition dependence against a numerical solution. Results show reasonable agreement when the gas-phase saturation ratio of a partitioning component is constant and particle-phase diffusion limits partitioning rate ( < 10 % discrepancy in estimated radius change). However, when the saturation ratio of the partitioning component varies, a generally applicable correction cannot be found, indicating that existing methodologies are incapable of deriving a general solution. Until such time as a general solution is found, caution should be given to sensitivity studies that assume constant diffusivity. The correction was implemented in the polydisperse, multi-process Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) and is used to illustrate how the evolution of number size distribution may be accelerated by condensation of a plasticising component onto viscous organic particles.