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Electrochemical gas evolution reactions are common but essential in many electrochemical processes including water electrolysis. During these processes, gas bubbles are constantly nucleating on reaction interfaces in electrolyte a...
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Electrochemical gas evolution reactions are common but essential in many electrochemical processes including water electrolysis. During these processes, gas bubbles are constantly nucleating on reaction interfaces in electrolyte and consequently exert an impact on catalysts and the performance. In the past few decades, extensive studies have been conducted to characterize bubbles with emerging advanced technologies, manage behaviors of bubbles, and apply bubbles to various domains. In this review, we summarize representative discoveries as well as recent advancements in electrochemical gas evolution reactions from the perspective of gas bubbles. Finally, we end up this review with a profound outlook on future research topics from the combination of experiments and theoretical techniques, non-negligible bubble effects, gravity-free situation, and reactions under practical industrial conditions.
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Aquatic ecosystems with organic-rich sediments are a globally significant source of methane to the atmosphere. In shallow waters, ebullition is often a dominant emission pathway of methane. Current knowledge on the processes contr...
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Aquatic ecosystems with organic-rich sediments are a globally significant source of methane to the atmosphere. In shallow waters, ebullition is often a dominant emission pathway of methane. Current knowledge on the processes controlling gas bubble formation and persistence in aquatic sediments is limited. An important prerequisite for accurate quantification of the structure and methane bubbles in sediment samples is to preserve the ambient in situ conditions during the withdrawal process and further analysis. A novel freeze corer has been developed that facilitates sampling of gas-bearing soft sediments for X-ray computer tomography. The sampler allows freezing sediment inside a double-walled corer with a mixture of dry ice and ethanol. This corer has moderate costs and offers important advantages for gassy sediment sampling. Its simplicity and robustness allow to perform sampling from a small boat and the ability to characterize in situ sediment features. The applicability of this freeze coring technique for gas bubble quantification was validated during laboratory experiments aimed to investigate the effects of freezing on sediment gas content, bubble size distribution, and their geometry by comparing computer tomography scans of unfrozen vs. frozen cores. The performance of the corer was further evaluated during field conditions in Lake Kinneret (the Sea of Galilee, Israel). The results demonstrate the suitability of the freeze-coring method for in situ preservation of gas-bearing sediments. The sediment structure, however, showed some displacements of sediments layers and bubble abundance in some core regions. Future investigations are needed to address the nature of disturbances of the frozen sediment.
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Chinalco's Toromocho mine, located in the Morococha district of Peru, treated 117,200 mtpd ores during last 10 years. It is currently undergoing expansion, treating approximately 52,740 mtpd of chalcopyrite ore since 2021. As with...
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Chinalco's Toromocho mine, located in the Morococha district of Peru, treated 117,200 mtpd ores during last 10 years. It is currently undergoing expansion, treating approximately 52,740 mtpd of chalcopyrite ore since 2021. As with the commissioning of most large plants, the metallurgical performance levels produced after stability was achieved were below the design criteria; more specifically, the Cu overall recoveries were 80-82% compared with the design value of 85%, and the final concentrate Cu grades were 20-23% compared with the design value of 24% over the past 2 years. It is clear that the copper losses in the fine size fraction (<10 μm) were due to unoptimized hydrodynamics. To overcome this obstacle and improve overall performance, an innovative approach for flotation circuit evaluation was designed and set up, to improve understanding of the nature of the mineral losses, benchmark the cell hydrodynamics as a platform to improve flotation cell operation, and link the circuit setup and operation to gas dispersion characteristics. These needs were fulfilled by the developments in gas dispersion sensor technology and process measurement methodology of CRIST. Effectively utilizing the sensor technology, it was observed that the deficient recovery of the Toromocho expansion plant was due to poor recovery of <10 μm Cu, as a result of a lack of small bubbles (<1 mm) compared to the pilot circuit. The resulting data highlight the potential for recovery improvement (increased kinetics) via bubble size reduction and increased air rate.
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A recently developed measurement method has been applied for the first time to the characterization of a bubble column operated in the heterogeneous regime and in presence of non-Newtonian fluids. The resulting measurements of bub...
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A recently developed measurement method has been applied for the first time to the characterization of a bubble column operated in the heterogeneous regime and in presence of non-Newtonian fluids. The resulting measurements of bubble size are completed with local gas holdup and global mass transfer characterizations. A strong impact of rheology on both bubble size and mass transfer is observed, at any tested superficial gas velocity. A dissociation of the liquid side mass transfer k(L) and the specific interfacial area (a) is proposed. Results show that the k(L) model previously validated in stirred tanks filled with complex fluids is satisfyingly extended to bubble columns. The incoming databank will be of great help to develop and validate multiphase hydrodynamic models, especially in the scope of biotechnology and associated systems of complex rheology.
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Acoustic Emission Analysis (AEA) for textural characterization of granular activated carbons (GACs) is an interesting alternative for assessing the GAC volume of pores in a rapid, sensitive and reliable way. Correlations between A...
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Acoustic Emission Analysis (AEA) for textural characterization of granular activated carbons (GACs) is an interesting alternative for assessing the GAC volume of pores in a rapid, sensitive and reliable way. Correlations between AEA and conventional methods have demonstrated its potentialities for textural characterization of high-porosity materials. In this study, commercial GACs with different porous characteristics were compared and correlated by applying volumetric gas sorption (N-2 at 77 K, CO2 at 273 K), Hg porosimetry and AEA. The energy, amplitude and power of the acoustic signal produced during GAC immersion in water are used as parameters to correlate with textural characteristics of the carbons. The found relationship between conventional porosity assessment techniques and acoustic parameters demonstrated the potentialities of the acoustic method as complementary technique of exploring textural properties of high-porosity materials.
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Over the last 15 years, the satellite constellation of the global positioning system (GPS) has been modernized for more precise applications, with the introduction of the L2C and L5 signals. However, among other effects, they are ...
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Over the last 15 years, the satellite constellation of the global positioning system (GPS) has been modernized for more precise applications, with the introduction of the L2C and L5 signals. However, among other effects, they are susceptible to severe ionospheric effects, particularly in the equatorial and low-latitude regions. Equatorial plasma bubbles, resulting from the combination of the ionospheric electrodynamics with plasma instability mechanisms and thermospheric coupling, may generate irregularity structures with scale sizes ranging from hundreds of kilometers to a few meters (or less). Ionospheric irregularities may cause deep amplitude fades and phase shifts to transionospheric signals. That is, they are responsible for amplitude and phase scintillation, which degrade receiver operations and may cause failures and unavailability to positioning and navigation services under extreme conditions. The objective of the present work is to analyze ionospheric scintillation effects on the L2C and L5 GPS signals, to compare their vulnerabilities with those of the L1 signal. The data used in this analysis were collected between November 2014 and March 2015, during the maximum solar activity of cycle 24 (a period of great scintillation incidence), by scintillation monitors deployed at four different sites in the Brazilian territory: Fortaleza, Presidente Prudente, Sao Jose dos Campos, and Porto Alegre. Intensity fades will be analyzed, considering different thresholds, to reveal their empirical probability distributions of scintillation occurrence, average fading occurrences and durations. The results will show that greater probabilities of strong scintillation occurrences are present in the modernized signals, reaching up to five times more events in the L5 signal in comparison with those in the legacy L1 signal. It will be shown that the L5 average fade duration is distinctly longer than the corresponding ones for the other frequencies, considering the same site, threshold, and L1 amplitude scintillation level. The results will also show that the average fade duration decreases according to the average ratio 0.6 s/3 dB within the threshold range from - 6 to - 15 dB, considering the same amplitude scintillation level and location.
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Acoustic emission analysis (AEA) results for textural characterization of granular activated carbons (GAC) can be related with its porosity. GACs were evaluated using this AEA technique by band-pass filtering from 3.5 to 50 kHz of...
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Acoustic emission analysis (AEA) results for textural characterization of granular activated carbons (GAC) can be related with its porosity. GACs were evaluated using this AEA technique by band-pass filtering from 3.5 to 50 kHz of the sound produced by flooding GAC with water. Used acoustic parameters for evaluation were: area under the signal envelope curve, acoustic energy and power. An exhausted GAC used in a water filter system of engine-based power plant was subjected to thermal and chemical regeneration under different conditions. Acoustic measurement results have been correlated with TGA, XRF, gas adsorption (N-2 at 77 K), GAC stability and elemental analysis for assessing the AEA. Based on all these techniques, it was found that mainly metal ions were adsorbed on the GAC. It was found that chemical regeneration using a 2.4 N HCl solution was the most successful approach in removing the adsorbed metal ions: Ca, Mn and Ba. Using this HCl solution did have the additional benefit in keeping the amount of elemental and fixed carbon almost intact compared to the more diluted HCl solution of 0.6 N and 1.2 N.
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Bubble behaviors in fluidized beds were investigated by a self-designed double-core screened waveguide. Experiments presented that, the bubble dome and vortex would generate AE signals with different shapes when passing through th...
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Bubble behaviors in fluidized beds were investigated by a self-designed double-core screened waveguide. Experiments presented that, the bubble dome and vortex would generate AE signals with different shapes when passing through the waveguide, which could be further used to precisely detect the bubble sizes and velocities both in pseudo-two-dimensional and three-dimensional fluidized beds. Compared with results from pressure fluctuations, bubble sizes measured by AE methods in three-dimensional fluidized beds were more consistent with empirical formulas results and displayed similar robustness. Furthermore, the measured bubble distributions demonstrated that bubbles tended to rise along an annular zone in the cross-section of the fluidized bed, and the annular zone shrank with the increasing height due to the net inward movement of bubbles. Under high gas velocities, the annular zone would shrink until reaching the centerline, while under low gas velocities, a steady annular zone existed at a certain radial position.
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Unexpected gas bubbles in microfluidic devices always bring the problems of clogging, performance deterioration, and even device functional failure. For this reason, the aim of this paper is to study the characterization variation...
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Unexpected gas bubbles in microfluidic devices always bring the problems of clogging, performance deterioration, and even device functional failure. For this reason, the aim of this paper is to study the characterization variation of a valveless micropump under different existence conditions of gas bubbles based on a theoretical modeling, numerical simulation, and experiment. In the theoretical model, we couple the vibration of piezoelectric diaphragm, the pressure drop of the nozzle/diffuser and the compressibility of working liquid when gas bubbles are entrapped. To validate the theoretical model, numerical simulation and experimental studies are carried out to investigate the variation of the pump chamber pressure influenced by the gas bubbles. Based on the numerical simulation and the experimental data, the outlet flow rates of the micropump with different size of trapped gas bubbles are calculated and compared, which suggests the influence of the gas bubbles on the dynamic characterization of the valveless micropump.
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Reliable quantification of natural and anthropogenic gas release (e.g. CO2, methane) from the seafloor into the water column, and potentially to the atmosphere, is a challenging task. While ship-based echo sounders such as single ...
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Reliable quantification of natural and anthropogenic gas release (e.g. CO2, methane) from the seafloor into the water column, and potentially to the atmosphere, is a challenging task. While ship-based echo sounders such as single beam and multibeam systems allow detection of free gas, bubbles, in the water even from a great distance, exact quantification utilizing the hydroacoustic data requires additional parameters such as rise speed and bubble size distribution. Optical methods are complementary in the sense that they can provide high temporal and spatial resolution of single bubbles or bubble streams from close distance. In this contribution we introduce a complete instrument and evaluation method for optical bubble stream characterization targeted at flows of up to 100 ml/min and bubbles with a few millimeters radius. The dedicated instrument employs a high-speed deep sea capable stereo camera system that can record terabytes of bubble imagery when deployed at a seep site for later automated analysis. Bubble characteristics can be obtained for short sequences, then relocating the instrument to other locations, or in autonomous mode of definable intervals up to several days, in order to capture bubble flow variations due to e.g. tide dependent pressure changes or reservoir depletion. Beside reporting the steps to make bubble characterization robust and autonomous, we carefully evaluate the reachable accuracy to be in the range of 1-2% of the bubble radius and propose a novel auto-calibration procedure that, due to the lack of point correspondences, uses only the silhouettes of bubbles. The system has been operated successfully in 1000 m water depth at the Cascadia margin offshore Oregon to assess methane fluxes from various seep locations. Besides sample results we also report failure cases and lessons learnt during deployment and method development.
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