High Mass Transfer Rates Research Articles

Influence of static pressure on toluene oxidation efficiency in groundwater by micro-nano bubble ozonation

Micro-nano bubble ozonation has been widely applied in the purification of drinking water due to its superior characteristics such as high mass transfer rate and long resistance time. However, its application in groundwater remediation is limited, partially due to the unclear effect of static water pressure on the oxidation efficiency. This study constructed a batch reactor to investigate the influence of static pressure on toluene oxidation by ozone micro-nano bubble water. To achieve constant pressure, weight was added above the mobile reactor roof, and the initial concentrations of toluene and dissolved ozone were 1.00 mg L−1 and 0.68 mg L−1 respectively. Experimental results demonstrated that as the static water pressure increased from 0.0 to 2.5 m, the average microbubble diameter decreased significantly from 62.3 to 36.0 μm. Simultaneously, the oxidation percentage of toluene increased from 40.3% to 58.7%, and the reaction rate between toluene and hydroxyl radical (OH·) increased from 9.3 × 109 to 1.39 × 1010 M−1 s−1, indicating that the shrinkage of micro-nano bubbles generated an abundance of OH· that quickly oxidized toluene adsorbed at the bubble interface. A greater enhancement of oxidation efficiency for nitrobenzene, as compared to p-xylene, was observed after the addition of 2.5 m water pressure, which verified the larger contribution of OH· under static pressure. Although the improvement of oxidation efficiency was reduced under acid and alkaline environments, as well as in practical groundwater matrices, the overall results still demonstrated the promising application of micro-nano bubble ozonation in groundwater remediation.

Anomalous orbital expansion of the low-mass X-ray binary 2A 1822-371: the existence of a circumbinary disk?

Source 2A 1822-371 is an eclipsing low-mass X-ray binary (LMXB) consisting of a neutron star (NS) and a ∼0.5 M⊙donor star in an orbit of 5.57 h. Based on the timing of the eclipse arrival times, this source was found to experience a rapid orbital expansion with an orbital period derivative asṖorb= (1.51 ± 0.05) × 10−10s s−1, implying that the mass-transfer rate is probably higher than at least three times the Eddington accretion rate. The standard magnetic braking (MB) model cannot produce a mass-transfer rate this high. The modified MB model can produce a high mass-transfer rate, resulting in a highṖorb. We propose an alternative model to account for the anomalously high mass-transfer rate andṖorbof 2A 1822-371. During the mass transfer, a tiny fraction of the transferred material is thought to form a circumbinary (CB) disk around the LMXB, which can efficiently extract orbital angular momentum from the system by the interaction between the CB disk and the binary. We used the MESA code to model the formation and evolution of 2A 1822-371 for different CB-disk masses. When the CB-disk mass is 2.3 × 10−8 M⊙, the simulation can reproduce the observed donor-star mass, orbital period, and orbital period derivative. This CB disk can accelerate the evolution of the binary and produce a high mass-transfer rate of 1.9 × 10−7M⊙yr−1, driving the binary to evolve toward a wide-orbit system. Therefore, we propose that CB disks may cause the rapid orbital changes observed in some LMXBs.

Efficient multi-stage electrochemical flow-through system for refractory organic pollutant treatment: Kinetics, mass transfer, and thermodynamic analysis

Improving the kinetics rate and mass transfer is essential for expanding the potential of electrochemical technologies in wastewater treatment. The electrochemical flow-through configuration promises a high oxidation efficiency and low energy consumption. We aimed to provide a thorough understanding of the enhanced kinetics, mass transfer, and thermodynamic parameters during the degradation of amoxicillin (AMX) in a multi-stage flow-through (MSFT) system using porous Ti-ENTA/SnO2–Sb anodes. All operating conditions strongly influenced the kinetics of AMX degradation and followed pseudo-first-order rate kinetic model (R2 > 0.85), with the highest kobs of 0.228 min−1 at high temperature (318 K).In comparison to the flow-by mode, the AMX removal rate in the three-stage flow-through mode was greatly enhanced by 70%, exhibiting the superior capacity of a porous anode. This system exhibited outstanding performance regarding the high kinetics rate and mass transfer rate (km), which increased by factors of 3.46 and 10.74, respectively, obtained in the flow-by mode. It also revealed that •OH generation was 5.64 times higher, and the EE/O was 19.89-fold lower than those in flow-by mode. Temperature plays a vital role in the reaction process, and thermodynamic features found the positive enthalpy (ΔHo) of +27.06 kJ mol−1, signifying the process was endothermic. A Hatta number (Ha) of >0.02 at all temperatures proved this finding, confirming an undeniable role in mass transfer. Finally, these findings reveal the system's performance and offer the possibility of establishing a multi-stage flow-through for wastewater treatment.

Micro-structured growth of 1,2,4-triazole by electro-polymerization for highly selective detection of gallic acid and uric acid

The detection of gallic acid (GA) and uric acid (UA) is desirable due to their important role in various biological functions. However, their simultaneous detection is still challenging due to the mutual interference with similar oxidation potentials. Electro-grafted polymers on carbon electrode surfaces with desirable composition and structure can induce excellent selectivity towards their electrochemical detection. Herein, the present study aims at developing the electro-grafting of poly-3,5-diamino-1,2,4-triazole (DAT) on glassy carbon (GC) electrodes by controlled electro-polymerization. The presence of nitrogen-rich hydrazo bonds (–NH) in triazole can establish strong intramolecular interactions (N-H) with free- OH (gallic acid) and quinone (uric acid) groups, which facilitate good selectivity at high mass transfer rates. This phenomenon was experimentally examined using differential pulse voltammetry thus revealing a significant separation of potentials of about 110 mV, with peak potentials of E = 0.16 and 0.27 V for gallic and uric acid, respectively. Using electropolymerized triazole (p-DAT/GC) as a modified electrode, the sensing response for gallic and uric acid was found to be linear within a concentration range from 10 to 128 µM with a limit of detection (LOD) of about 2.8 and 1.5 µM. The p-DAT/GC electrode exhibited a good recovery range when used for realistic detection in human urine samples. Therefore, this work is a valuable toolkit for the detection of gallic and uric acid thus paving the way for a wide range of opportunities regarding real-time detection in electrochemical sensing applications.

Experimental study of interface behavior and sound pressure oscillation of direct contact condensation of a steam jet in flowing water

Direct contact condensation of a vapor jet in liquid is frequently encountered in lots of industrial applications because of its high heat and mass transfer rates. However, severe interface and sound pressure oscillation associated with condensation might cause vibration and even failure of pipeline systems. In this paper, the interface behavior and the sound pressure oscillation of a steam jet in flowing water are captured respectively by a high-speed camera and a high-frequency hydrophone. According to the gray profile of the image, an identification method is proposed to determine the interface characteristics, including the plume shape, width, and penetration length of the jet. The statistical intensity and power spectrum density of the interface position and sound pressure oscillation of the steam jet are calculated. Two typical condensation regimes of Chugging and Oscillation-I under low steam mass flux (Gs < 346 kg/m2s) are observed. The intensities of the interface oscillation and sound pressure oscillation decline with increasing steam mass flux and a significant correlation coefficient of about 0.94 between them is revealed. With the incorporation of sound pressure parameters, two correlations of the dimensionless jet length and its oscillation intensity are established, with relative deviations of 20% between the experimental and predicted values. For Oscillation-I, the dominant frequencies of the interface oscillation and sound pressure oscillation are quantitatively consistent. From the time and frequency domain perspectives, it is quantitatively confirmed that the sound pressure oscillation of the steam jet under low steam mass flux is caused by its interface oscillation.

New scaling law for turbulent boundary layers with high surface mass transfer

A fundamental understanding of the phenomena occurring in the turbulent boundary layer in the presence of surface blowing is limited, and considerable disagreements persist even in describing primary quantities, such as the boundary layer profile. The theories based on the linear boundary layer equations show that the thickness of the sublayer increases in the presence of surface blowing; therefore, the viscous sublayer and law of the wall modify. In this study, direct numerical simulations (DNS) of turbulent boundary layers with uniform surface mass transfers are carried out in order to scale the velocity profile. Emphasis is placed on moderate to high mass transfer rates, which are relevant to the most common hybrid rockets configuration. DNS data are used to establish a functional law of the wall and a law of wake by means of the relation between the wall shear stress and surface mass transfer. Analysis of the mean kinetic energy budget shows that the magnitude of turbulent kinetic energy increases by surface mass transfer, and the production rate extends significantly in the inner layer as the injection rate increases. DNS data of various surface blowing are used to complete the closure of turbulence kinetic energy equation and develop an eddy viscosity model. The predicted turbulent kinetic energy and eddy viscosity agree with DNS data for moderate to high blowing rates.

High-Temperature “Ex-Situ” Cyclic Voltammetry to Study the Oxygen Reduction Reaction on Electrocatalysts in Conditions Mimicking Fuel Cell Operation

The massive efforts dedicated over the last decades to the development of improved electrocatalysts (ECs) and membrane-electrode assemblies (MEAs) have already yielded a significant reduction of the loading of platinum needed to devise low-temperature fuel cells (LT-FCs) achieving the performance and durability level that is required for applications. However, more research is still needed to further curtail the amount of Pt in a LT-FC; this is particular relevant to prevent supply bottlenecks in the perspective of a large-scale rollout of this technology.In a conventional LT-FC running on direct hydrogen most of the Pt loading is concentrated on the cathode electrode, where it is needed to promote the sluggish oxygen reduction reaction (ORR). It is often very time-consuming to optimize the features of a MEA to maximize its performance in a LT-FC running in operating conditions. This is especially the case if the MEA mounts innovative ORR ECs exhibiting features that are very different from benchmark ECs in terms of chemical composition of the active sites and morphology. Thus, the general practice is to screen developmental ORR ECs through “ex-situ” techniques before focusing MEA optimization only on the most promising systems.The most widespread “ex-situ” approach to screen developmental ORR ECs involves the use of a rotating (ring) disk electrode (R(R)DE) setup. Though very practical and accurate, conventional R(R)DE experiments are able to study the ORR features of an EC only in quite a narrow set of operating conditions (T = 30-60°C; P = 1 bar). These latter are different from those that are most commonly adopted at the cathode of a LT-FC (T = 80-100°C; P = 2 – 3 bar). As a result, the information obtained on the ORR features of an EC by means of conventional R(R)DE studies may not be fully representative of the behavior of the EC in an operating LT-FC.To address this shortcoming herein we present an innovative setup able to elucidate the ORR features of both benchmark and innovative ECs in experimental conditions that are very close to those found in operating low-temperature FCs. Specifically, the setup consists of a homemade channel flow electrode (CFE) cell operated in a rather simple closed system with a controlled oxygen concentration and maintaining a high level of cleanliness.The modeling, materials selection, design, and testing of a versatile CFE cell developed from scratch is presented. The cell is compatible with commercially-available RDE components and special care is devoted to ease the overall assembly of the experimental setup. The cell is characterized by a well-defined hydrodynamics, a low dead volume, a high mass-transfer rate and a high signal/noise ratio. The cell also allows to measure accurately the pressure and the temperature in close proximity of the working electrode. It is demonstrated the possibility to study the ORR features of ECs up to a temperature of 80°C and a pressure of 3 bar. Both benchmark and innovative ECs are taken into consideration. The latter exhibit features that are very different in comparison with the conventional Pt/C ECs, especially in terms of: (i) chemical composition of the active sites, that typically includes more than one element; and (ii) morphology of the support, that is based on different carbon nanostructures. The proposed setup is implemented to determine crucial ORR features of the ECs, including the kinetically-controlled current, the activation energy and the accessibility of O2 to the active sites. Acknowledgements This research has received funding from: (a) the European Union’s Horizon 2020 research and innovation program under grant agreement 881603; (b) the project ‘Advanced Low-Platinum hierarchical Electrocatalysts for low-T fuel cells’ funded by EIT Raw Materials; (c) Alkaline membranes and (platinum group metals)-free catalysts enabling innovative, open electrochemical devices for energy storage and conversion – AMPERE, FISR 2019 project funded by the Italian Ministry of University and Research; and (d) the German Federal Ministry of Education and Research (BMBF) under grant agreement number 03SF0585.

Tailored monolithic catalyst and reactor by direct photo-curing 3D printing for oxidative desulfurization of fuels

Structured monolithic catalysts have attracted growing attention in the petrochemical industry due to significant advantages of reusability, easy scale-up, and high heat and mass transfer rate. 3D printing offers enormous flexibility in accurately regulating the configuration of monolithic catalysts to realize the strengthening of the reaction process. However, this remains challenging in terms of reliable manufacturing, high resolution, and the distribution of active sites. Here, we report a direct photo-curing 3D printing based method for manufacturing precise monolithic catalysts for oxidative desulfurization of fuels. Numerous molybdenum oxide nanoparticles with a high density of oxygen vacancies are spatially distributed in three-dimensional Al2O3 by 3D printing technology and post-treatment, thus favoring the catalytic reaction. This 3D-MoOx/Al2O3 exhibits excellent catalytic performance with a 99.2 % conversion of dibenzothiophene. Furthermore, this tailored catalyst is 3D-printed to manufacture a monolithic agitating impeller as a stirring reactor, which plays a dual role in mixing and catalysis. The study provides an effective strategy for precisely manufacturing structured catalysts with excellent catalytic activity and can be readily applied to a range of chemical reactions.

Surface-molecularly imprinted ratiometric fluorescence sensor for fast, sensitive and selective determination of rhodamine 6G

Rhodamine 6G (Rh6G) is a classic dye that has been banned as a food additive due to its harm to human bodies. The illegal addition of Rh6G in foods should be strictly monitored. In this study, a surface-molecularly imprinted fluorescence sensor (TPP-SMIPs) for Rh6G detection was prepared on SiO2 using an aggregation-induced emission molecule as a fluorescence signal part. TPP-SMIPs exhibited ratiometric response to Rh6G. Favorable linearity between the fluorescence intensity ratios and Rh6G was obtained in the range of 0–10.0 μmol/L. Better site accessibility and higher mass transfer rate were achieved in TPP-SMIPs than in non-surface imprinted polymers. High sensitivity with a low detection limit (2.82 nmol/L) and excellent selectivity were also realized in TPP-SMIPs. Its applications in real sample detection showed satisfactory recoveries and relative standard deviations. This study demonstrated that this sensor can provide fast, sensitive and selective determination to Rh6G in real samples. This work not only reported a new detection method for rhodamine dyes, but also provided a universal strategy for the design of high-performance molecularly imprinted fluorescence sensors.

Computational investigation on the flow and mass transfer properties inside ejector of loop reactor based on CFD-PBM simulation

Conventional stirred kettle reactors suffer from inadequate gas-liquid mixing in many production processes, and their reaction rates are greatly limited. The loop reactor can achieve high mass transfer rates with low energy consumption. It is well suited for multiphase reactions with limited mass transfer and has the advantage of being excellent sealing and low cost. In this paper, ejector as the core of the loop reactor was investigated. Combining the Computational Fluid Dynamics (CFD) and Population Balance Model (PBM), the industrial-scale ejector model was constructed. The detailed motion behavior of bubbles and bubble size distribution were predicted. Due to the entrainment of the liquid phase, the gas enters the suction chamber and throat of the ejector, where the gas is mainly sheared and broken, providing a large specific surface area of the gas-liquid interface. Moreover, the flow and transfer properties were closely related to parameters such as nozzle-throat diameter ratio and liquid inlet velocity. Based on the results of the simulation, the gas-liquid mass ratio and the volumetric mass transfer coefficient were correlated with the structure and process parameters, and the correlation model was obtained, which provides guidance for the design of the loop reactor.

Stacked nano FAU zeolite as hierarchical Murray material for enhancing CO2 diffusion kinetics

Zeolite materials are widely used in the field of carbon capture and storage (CCS) due to their high specific surface area, good stability, and low cost. However, the microporous structure of zeolite is not conducive to the mass transfer and diffusion processes, which limits the efficiency and practical application of CO2 capture. In this study, we designed a hierarchical Murray material consisting of ultra-small particle size (<20 nm) zeolite stacks based on Murray's law to enhance the diffusion kinetics of CO2. The results demonstrate that the material exhibits a CO2 adsorption capacity of 107.07 cm3·g−1 and a CO2/N2 selectivity of 120.58, meeting the requirements for high-performance CO2 capture materials. Kinetic CO2 adsorption and breakthrough experiments confirm that the nano-FAU zeolite enables fast and efficient CO2 capture compared to micron-sized zeolite. The stacked nano-zeolites with hierarchical pore structure not only shorten the diffusion path but also facilitate the surface permeation process, which contributes to improved diffusion kinetics. This work provides a valuable strategy for the development of CO2 adsorption materials with high mass transfer rates.

Effect of orange waste pretreatment on the fluid dynamics of spouted beds with mechanical agitation

The spouted bed has a vigorous cyclic movement of the solids and high rates of heat and mass transfer, allowing for effective contact between the different phases. However, the spouted bed may exhibit instabilities for inherently cohesive solids due to unwanted agglomeration, preferential channels, particle segregation and high energy consumption. In this context, the present work analyzed the effect of a preliminary treatment of the orange waste, through the addition of lime and mechanical pressing, on the fluid dynamics in two spouted bed configurations of same dimensions, one conventional (CSB) and the other one modified with mechanical agitation (modified mechanically spouted bed, MMSB). The preliminary treatment of the biomass allowed reducing the moisture content by 31.7%, thereby improving the movement of solids in the CSB as when the in natura (raw) biomass was loaded, as the latter underwent agglomeration until it no longer circulated. Persistent instability issues in the CBS were overcome through mechanical stirrers, which made the bed uniform by improving the movement of solids in the annular region. Furthermore, mechanical agitation allowed increasing the maximum static bed height from 21.1 to 27.0 cm, as well decreasing the air flow by up to 91.74%, since the air stream is only that required for bed expansion.

Efficient C(sp3)-H Carbonylation of Light and Heavy Hydrocarbons with Carbon Monoxide via Hydrogen Atom Transfer Photocatalysis in Flow.

Despite their abundance in organic molecules, considerable limitations still exist in synthetic methods that target the direct C-H functionalization at sp3-hybridized carbon atoms. This is even more the case for light alkanes, which bear some of the strongest C-H bonds known in Nature, requiring extreme activation conditions that are not tolerant to most organic molecules. To bypass these issues, synthetic chemists rely on prefunctionalized alkyl halides or organometallic coupling partners. However, new synthetic methods that target regioselectively C-H bonds in a variety of different organic scaffolds would be of great added value, not only for the late-stage functionalization of biologically active molecules but also for the catalytic upgrading of cheap and abundant hydrocarbon feedstocks. Here, we describe a general, mild and scalable protocol which enables the direct C(sp3)-H carbonylation of saturated hydrocarbons, including natural products and light alkanes, using photocatalytic hydrogen atom transfer (HAT) and gaseous carbon monoxide (CO). Flow technology was deemed crucial to enable high gas-liquid mass transfer rates and fast reaction kinetics, needed to outpace deleterious reaction pathways, but also to leverage a scalable and safe process.

Experimental study of fracture conductivity in dolomite reservoirs treated with different acid fracturing technologies

The acid fracturing stimulation mechanism of dolomite reservoirs is entirely different from that of limestone. Most existing research on dolomite acid fracturing still uses the research method of limestone. There is no unified understanding of dolomite reservoirs' acid rock reaction mechanism and stimulation mechanism. The rock outcrops of the Dengying Formation of the Sinian system in the Sichuan Basin, China, with the size of about 17.8 cm × 3.8 cm × 2.5 cm, were used to test the acid-etching morphology, corrosion volume, reaction kinetic parameters, and conductivity under different acid types, injection modes, displacements, and alternating stages with the acid corrosion experiment. The materials used in the experiment are gelling acid, crosslinking acid, and diverting acid, the dolomite content of the rock sample is about 93.79%. The experimental results show that the diverting acid had a lower viscosity and higher mass transfer rate than gelling acid, so it could form a more effective etching, the dissolution volumes in the two acids were 22.15 and 23.38 cm3, respectively. Compared with the injection mode of a single acid fluid, the alternative injection of two fluids with different viscosities can improve the nonuniform etching effect and conductivity, under the “fracturing fluid + acid fluid” alternate injection mode, the dissolution volume of rock was approximately 17.16 cm3, under the “crosslinking acid + diverting acid” alternate injection mode, the dissolution volume of rock was approximately 20.83 cm3. The reaction rate of dolomite is low, and it may be dissolved along the joint and natural fractures. The research results can provide a reference for optimizing the dolomite reservoir's stimulation technology and injection parameter design.

Experimental investigation on continuous reaction crystallizer for CO2 capture by phase-change method

Reducing the energy consumption of the regenerator has been the most intractable problem for CO2 capture processes. An energy-efficient phase change-based carbon dioxide absorption process by potassium carbonate/bicarbonate solution is proposed to reduce the capture energy consumption to less than 1.5 GJ/(t·CO2), and an energy-saving continuous reaction crystallizer integrating reaction and separation is developed here to collect CO2-rich solid products. This capture process is expected to significantly reduce the energy penalty due to high absorbent concentration, high mass transfer rate from the microbubbles, and no repeated solvent heating and vaporization procedure during regeneration. The effects of superficial gas velocity and fluid jet velocity on liquid circulating velocity, gas holdup, and volumetric mass transfer coefficient in the reaction crystallizer are thoroughly investigated. The experimental results demonstrate that the superficial gas velocity is crucial in improving the solid–liquid separating efficiency, particle size classification, and reaction rate. Empirical models which can accurately describe the hydrodynamics are proposed to select proper operating conditions of CO2 absorption in such a kind of multiphase continuous crystallizer. The reaction system shows an effective behavior in absorbing CO2 and producing KHCO3 crystals, and the mean diameters are verified to be controllable by manipulating the superficial gas velocity and fluid jet velocity.

DESIGN AND DEVELOPMENT OF 3D ROTATIONAL BIOREACTOR FOR 3D CELL CULTURE

Currently, cell and tissue culture are a key to success for regenerative medicine and the pharmaceutical industry for implementing new therapeutic concepts. Cell culture is a complex process in which cells are grown under controlled conditions. Three-dimensional (3D) scaffolds provide space for cell adhesion and growth in 3D structures. However, in 3D cell culture, the ability to supply nutrients and oxygen, and the simultaneous removal of waste products and metabolites are the limiting factors for cells survival, due to nutrient starvation and metabolite toxicity. Therefore, an in-house 3D rotational bioreactor, for 3D cell culture, was designed, fabricated, and developed based on a chitosan porous scaffold. The 3D rotational bioreactor provides a continuously moving culture medium led to high rates of mass transfer throughout the scaffolds. The rotation speed and duration are programable and provide a continuous free flow during the rotating period. The results demonstrated the potential of an in-house 3D rotational bioreactor for better nutrient supply and waste transfer systems outweighs the conventional cell culture technique.

Experimental mass and heat transfer at a serpentine tube heat exchanger located at the wall of a square stirred tank reactor

• Rates of mass and heat transfer of a serpentine tube heat exchanger were studied. • The serpentine is located at the wall of a square stirred tank reactor. • Variables studied were impeller speed and geometry, tube diameter, and tube pitch. • Dimensionless correlations were developed to design and operate the heat exchanger. • The possibility of using the serpentine tube as a catalyst support was highlighted. Although the study of the heat transfer behavior of stirred tank heat exchangers such as helical coils, vertical tube baffles, and cooling jackets has received much attention, no study has been reported on the heat or mass transfer behavior of serpentine tube heat exchangers, so the present work aims to study rates of mass and heat transfer (by analogy) at the outer surface of a serpentine tube heat exchanger located at the wall of a square stirred tank reactor by the electrochemical technique. Variables studied were impeller rotational speed, impeller geometry, tube diameter, and tube pitch. The mass or heat transfer coefficients were found to increase with increasing impeller speed and decrease with increasing cylinder diameter and pitch. For a given set of conditions, a radial flow turbine produces higher rates of mass transfer than an axial flow turbine. The data were correlated by dimensionless mass transfer correlations which can be used to scale up and operate the heat exchanger. When the mass transfer data at the serpentine tube were compared with those at an active flat wall of equal height, it was found that the mass transfer rate at the serpentine tube is higher than that of the flat wall by an amount ranging from 10% to 210% depending on the operating conditions. The possibility of using the serpentine tube as a catalyst support for conducting diffusion-controlled exothermic immobilized cell biochemical reactions which need rapid cooling to avoid thermal degradation of the biomass was highlighted. The dual-use of the serpentine tubes as a catalyst support and a cooler would reduce the capital and operating costs of immobilized enzyme or cell stirred tank biochemical reactors compared, for instance, with the traditional stirred slurry reactor owing to the elimination of the costly and time-consuming process of separating the final product from the catalyst particles.

Coping with loss

Context. The stability of mass transfer is critical in determining pathways towards various kinds of compact binaries, such as compact main-sequence white-dwarf binaries, and transients, such as double white-dwarf mergers and luminous red novae. Despite its importance, very few systematic studies of the stability of mass transfer exist. Aims. We study the behaviour of mass-losing donor stars in binary systems in a systematic way. We focus on identifying and understanding the parameter space for stable mass transfer in low- and intermediate-mass binaries with post-main-sequence donor stars as well as the properties of ultimately unstable binary systems at the onset of the instabilities. Methods. We employed the 1D stellar evolution code MESA to simulate the mass-transfer evolution of 1404 binary systems with donor-star masses between 1 M⊙ and 8 M⊙. We studied the behaviour of the binaries during mass transfer, without assuming that the donor star responds adiabatically to mass loss. We treated the accretor as a point mass, which we do not evolve, and assumed the mass transfer is conservative. Results. We considered several criteria to define when unstable mass transfer occurs. We find that the criterion that best predicts the onset of runaway mass transfer is based on the transition to an effectively adiabatic donor response to mass loss. Using this quasi-adiabatic criterion, we determine the location of the stability boundary to within a relative uncertainty of five per cent in the mass ratio at the onset of mass transfer. Defining this critical mass ratio (qqad) in terms of accretor mass over donor mass, we find that qqad ∼ 0.25 for stars with radiative envelopes that cross the Hertzsprung gap, while for convective giants qqad decreases from ∼1 at the base of the red giant branch to ∼0.1 at the onset of thermal pulses on the asymptotic giant branch. Compared with recent similar studies, we find increased stability of mass transfer from convective giants. This is because an effectively adiabatic response of the donor star only occurs at a very high critical mass-transfer rate due to the short local thermal timescale in the outermost layers of a red giant. Furthermore, we find that for q &gt; qqad mass transfer is self-regulated, but that for evolved giants the resulting mass-transfer rates can be so high that the evolution becomes dynamical and/or the donor can overflow its outer lobe. Conclusions. Our results indicate that mass transfer is stable for a wider range of binary parameter space than typically assumed in rapid binary population synthesis. Moreover, we find a systematic dependence of the critical mass ratio on the donor star mass and radius, which may have significant consequences for predictions of post-mass-transfer populations.

Oxygen reduction reaction monitored during corrosion by means of pumped-micropipette delivery/substrate collection (Pumped-MD/SC) mode of scanning electrochemical microscopy

The key improvement of the corrosion resistance of metals requires understanding and resolving the electrochemical reactions involved at the local scale. Here, we present the use of the pumped-micropipette delivery/substrate collection (Pumped-MD/SC) mode of scanning electrochemical microscopy (SECM) to monitor the oxygen reduction reaction (ORR) during corrosion of an Aluminum (Al) / Copper (Cu) galvanic coupled electrode, a model system of copper-rich aluminum alloys. In this case, the micropipette scans the substrate electrode pumping out a controlled flow of air-saturated electrolyte solution, which allows, through convective transport, to achieve high mass-transfer rates under steady-state conditions at the substrate electrode. The most relevant parameters in pumped-MD/SC mode were studied, it was demonstrated that the ORR current is linearly dependent on the injection flow rate. In contrast, the maximum ORR current was reached regardless of the tip-substrate distance (d) within the range of L (d/ micropipette radius (a)) < 15, which represents a clear advantage in comparison with other modes of SECM. It was also proved that the ORR might take place on both Cu and Al electrodes depending on the applied potential. In addition, the pumped-MD/SC mode of SECM was successfully used for imaging large dimension surfaces with both high spatial and temporal resolution, as well as high imaging contrast thanks to the high mass-transfer rate of electroactive species achieved by forced convection. Moreover, relevant information to evaluate the effect of different corrosion inhibitors in solution such as Ce2(SO4)3 and Li2CO3 on ORR was provided by individually addressing the substrate electrode in static pumped-MD/SC mode. It was demonstrated that only Ce2(SO4)3 played the role of cathodic inhibitor through the formation of a Ce3+ oxide/hydroxide precipitate on the substrate electrode during ORR, which was confirmed by SEM and EDX analysis.

Task-specific polymeric membranes to achieve high gas-liquid mass transfer

In the current study, Polyimide (P84)-based polymeric membranes were fabricated and used as spargers in the bubble column reactor (BCR) to get a high gas-liquid mass transfer (GL-MT) rate of oxygen in water. Different polymeric membranes were fabricated by incorporating polyvinyl pyrrolidone (PVP) as a porogen and a Zeolitic Imidazolate Framework (ZIF-8) to induce high porosity and hydrophobicity in the membranes. The GL-MT efficiency of membranes was evaluated by measuring the overall volumetric mass transfer coefficient (kLa) of oxygen in air. The kLa of O2 (in air) was measured by supplying the gas through a fixed membrane surface area of 11.94 cm2 at a fixed gas flow rate of 3L/min under atmospheric pressure. The results revealed that adding porogen and ZIF-8 increased the porosity of the membranes compared to the pure polymeric membranes. In comparison, the ZIF-8 (3 wt%) based membrane showed the highest porosity (80%), hydrophobicity (95° contact angle) and kLa of oxygen in air (241.2 h−1) with 78% saturation in only 60 s. ZIF-8 based membranes showed the potential to increase the amount of dissolved oxygen in BCR by reducing the bubble size, increasing the number of bubbles, and improving the hydrophobicity. The study showed that ZIF-8 based membrane diffusers are expected to produce high GL-MT in microbial syngas fermentation. To the best of our knowledge, this is the first study on the fabrication and application of polymeric membranes for GL-MT applications. Further research should be conducted under real fermentation conditions to assess the practicality of the system to support substrate utilization, microbial growth, and product formation.