Molar Flux Research Articles

Emission of volatile halogenated organic compounds over various Dead Sea landscapes

Abstract. Volatile halogenated organic compounds (VHOCs), such as methyl halides (CH3X; X is Br, Cl and I) and very short-lived halogenated substances (VSLSs; bromoform – CHBr3, dibromomethane – CH2Br2, bromodichloromethane – CHBrCl2, trichloroethylene – C2HCl3, chloroform – CHCl3 – and dibromochloromethane – CHBr2Cl) are well known for their significant influence on ozone concentrations and oxidation capacity of the troposphere and stratosphere and for their key role in aerosol formation. Insufficient characterization of the sources and the emission rate of VHOCs limits our ability to understand and assess their impact in both the troposphere and stratosphere. Over the last two decades, several natural terrestrial sources for VHOCs, including soil and vegetation, have been identified, but our knowledge of emission rates from these sources and their responses to changes in ambient conditions remains limited. Here we report measurements of the mixing ratios and fluxes of several chlorinated and brominated VHOCs from different landscapes and natural and agricultural vegetated sites at the Dead Sea during different seasons. Fluxes were generally positive (emission into the atmosphere), corresponding to elevated mixing ratios, but were highly variable. Fluxes (and mixing ratios) for the investigated VHOCs ranged as follows: CHBr3 from −79 to 187 nmol m−2 d−1 (1.9 to 22.6 pptv), CH2Br2 from −55 to 71 nmol m−2 d−1 (0.7 to 19 pptv), CHBr2Cl from −408 to 768 nmol m−2 d−1 (0.4 to 11 pptv), CHBrCl2 from −29 to 45 nmol m−2 d−1 (0.5 to 9.6 pptv), CHCl3 from −577 to 883 nmol m−2 d−1 (15 to 57 pptv), C2HCl3 from −74 to 884 nmol m−2 d−1 (0.4 to 11 pptv), methyl chloride (CH3Cl) from -5300 to 10,800 nmol m−2 d−1 (530 to 730 pptv), methyl bromide (CH3Br) from −111 to 118 nmol m−2 d−1 (7.5 to 14 pptv) and methyl iodide (CH3I) from −25 to 17 nmol m−2 d−1 (0.4 to 2.8 pptv). Taking into account statistical uncertainties, the coastal sites (particularly those where soil is mixed with salt deposits) were identified as sources of all VHOCs, but this was not statistically significant for CHCl3. Further away from the coastal area, the bare soil sites were sources for CHBrCl2, CHBr2Cl, CHCl3, and probably also for CH2Br2 and CH3I, and the agricultural sites were sources for CHBr3, CHBr2Cl and CHBrCl2. In contrast to previous reports, we also observed emissions of brominated trihalomethanes, with net molar fluxes ordered as follows: CHBr2Cl > CHCl3 > CHBr3 > CHBrCl2 and lowest positive flux incidence for CHCl3 among all trihalomethanes; this finding can be explained by the soil's enrichment with Br. Correlation analysis, in agreement with recent studies, indicated common controls for the emission of CHBr2Cl and CHBrCl2 and likely also for CHBr3. There were no indications for correlation of the brominated trihalomethanes with CHCl3. Also in line with previous reports, we observed elevated emissions of CHCl3 and C2HCl3 from mixtures of soil and different salt-deposited structures; the flux correlations between these compounds and methyl halides (particularly CH3I) suggested that at least CH3I is also emitted via similar mechanisms or is subjected to similar controls. Overall, our results indicate elevated emission of VHOCs from bare soil under semiarid conditions. Along with other recent studies, our findings point to the strong emission potential of a suite of VHOCs from saline soils and salt lakes and call for additional studies of emission rates and mechanisms of VHOCs from saline soils and salt lakes.

Application of the microchannel reactor to carbon dioxide absorption

In this study, the absorption of carbon dioxide (CO2) in a microchannel reactor by use of the three solvents monoethanolamine (MEA), diethanolamine (DEA) and activated methyldiethanolamine (a-MDEA) was investigated under different operating conditions, including temperature, solvent and gas flow rates, and amine concentration. The results show that increasing solvent flow rate or amine concentration has effect of increasing the mass transfer flux. Raising the gas flow rate increased the mass transfer flux for MEA and DEA, while for a-MDEA it initially increased and then reduced the flux. The temperature was found to be a less effective parameter. Additionally, the performance of the microreactor in the enhancement of the absorption rate was demonstrated. The optimum molar flux of volumetric mass transfer was determined to be 12,826, 11,374 and 8241 kmol per cubic meter-hour for MEA, DEA and a-MDEA respectively.

Mechanism of Selective Ion Removal in Membrane Capacitive Deionization for Water Softening.

Capacitive deionization (CDI) is an emerging technology capable of selective removal of ions from water. While many studies have reported chemically tailored electrodes for selective ion removal, the selective removal of divalent cations (i.e., hardness) over monovalent cations can simply be achieved using membrane CDI (MCDI) equipped with ion exchange membranes (IEMs). In this study, we use both experimental and modeling approaches to systematically investigate the selective removal of Ca2+ over Na+. Specifically, the impacts of current density, hydraulic retention time, and feed composition on the selectivity of Ca2+ over Na+ were investigated. The results from our study suggest a universal correlation between the ratio of molar fluxes and the ratio of spacer channel ion concentrations, regardless of operating conditions and feed composition. Our analysis also reveals inherent and universal trade-off relationships between selectivity and the Ca2+ removal rate and between selectivity and the degree of Ca2+ removal. This fundamental understanding of the mechanism of selective ion removal in MCDI can also be applied to flow-electrode CDI processes that employ IEMs.

Utilization of CO2 in Flue Gas for Sodium Bicarbonate Production in a Bubble Column

Utilization of CO2 in flue gases for the production of sodium bicarbonate is an environmentally friendly process. A mathematical model was constructed for the design and simulation of utilizing a low concentration CO2 (2-18%) in flue gas to produce sodium bicarbonate in a bubble column reactor. The model is based on the mass balance equations for three phases (gas, liquid, and solid). Danckwerts theory for mass transfer from the gas phase to the liquid phase coupling with chemical reaction, and crystallization mechanism was used. The effect of process variables; gas molar velocity or flux (G=2.5-10 mole/m2.s), liquid mass flow rate (mL=2800-3400 kg/h), sodium bicarbonate concentration (x1=0.04-0.1), CO2 gas mole fraction (y=0.02-0.18), column height (h=11-33 m), and column diameter (dR=1-3 m) on the objective variables; solid molar velocity (S), CO2 conversion, precipitation zone height (Zi), and crystal size distribution (CSD) were studied. The conversion of CO2 varied from 34% to 71% whereas the particle size range varied from 0 to 400 μm. The particle size range and the CO2 absorption efficiency of about 50% for 20 m column height are in agreement with the literature.

Simulation study on the microscopic characteristics of electrochemical Bunsen reaction in the sulfur–iodine cycle for renewable hydrogen production

The sulfur–iodine (SI) thermochemical water-splitting cycle is a promising hydrogen production process with high thermal efficiency and low pollution. The Bunsen reaction in the cycle using an electrochemical cell (EC) has been proposed and R&D on the cell has been carried out. To fill the knowledge gap in the microscopic characteristics of the electrochemical Bunsen reaction, a numerical study was conducted in this work. A two-dimensional, steady state, laminar and isothermal model of EC with detailed processes of flow, species transfer and electrode reactions was developed, and the simulated results showed good agreement with experimental data. The reaction rates, indicated in the form of species molar flux variation, increase with rising current density due to more electron transfer at a certain time, along with the increase of electric energy consumption from 9.1 W/m2 to 227.9 W/m2. Increasing the temperature inhibits the conversion and generation of species. Although higher flow rate reduces the flow residence time as well as the concentration variation of species, faster flow and higher bulk reactants concentration around the electrode surface lead to the increase of overall reaction rates. The consumption of pumping power also increases from 0.264 W/m2 to 6.661 W/m2 with rising flow rate. The optimal operating conditions of 4–5 A/dm2, 303–313 K and 0.01–0.02 m/s are obtained. The simulation favors a better understanding of the phenomena occurring in the EC and its further optimization.

Membrane Absorption Coupling Process for CO2 Capture: Application of Water-Based ZnO, TiO2, and Multi-Walled Carbon Nanotube Nanofluids

In this work, stable water-based zinc oxide (ZnO), titania (TiO2), and multi-walled carbon nanotube (MWCNT) nanofluids are prepared and examined as CO2 absorbents in a polypropylene hollow fiber membrane contactor. Different operating variables, such as liquid flow rate, gas flow rate, and concentration of nanoparticles, and their effects on CO2 molar flux are investigated. The long-term stability of nanofluids is monitored using ultraviolet–visible spectroscopy. Also, ζ-potential measurements and sediment photography are applied to confirm the results of nanofluid stability. Dynamic light scattering is used to determine the size distribution of dispersed nanoparticles. The results show that the increase in the nanoparticle concentration to 0.15 wt % has a favorable effect on CO2 absorption efficiency as a result of the increase in Brownian motion and other related mechanisms. However, it adversely affects the CO2 absorption by lowering the nanofluid stability at higher concentrations. The obtained result...

A New Unified Gas-Transport Model for Gas Flow in Nanoscale Porous Media

SummaryA new unified gas-transport model has been developed to characterize single-component real-gas flow in nanoscale organic and inorganic porous media by modifying the Bravo (2007) model. More specifically, a straight capillary tube is characterized by a conceptual layered model consisting of a viscous-flow zone, a Knudsen-diffusion zone, and a surface-diffusion zone. To specify the contributions of the viscous flow and the Knudsen diffusion to the gas transport, the virtual boundary between the viscous-flow and Knudsen-diffusion zones is first determined using an analytical molecular-kinetics approach. As such, the new unified gas-transport model is derived by integrating the weighted viscous flow and Knudsen diffusion, and coupling surface diffusion. The model is also comprehensively scaled up to the bundles-of-tubes model considering the roughness, rarefaction, and real-gas effect. Nonlinear programming methods have been used to optimize the empirical parameters in the newly proposed gas-transport model. Consequently, the newly proposed gas-transport model yields the most accurate molar fluxes compared with the Bravo (2007) model and four other analytical models. One of the advantages of the new unified gas-transport model is its great flexibility, because the Knudsen number is included as an independent variable, which also endows the newly proposed model with the capability to cover the full-flow regimes. In addition, the apparent permeability has been mathematically derived from the new unified gas-transport model. A series of simulations has been implemented using methane gas. It is found through sensitivity analysis that apparent permeability is strongly dependent on pore size, porosity, and tortuosity, and weakly dependent on the surface-diffusivity coefficient and pore-surface roughness. The increased viscosity can reduce the total molar flux in the inorganic pores up to 66.0% under the typical shale-gas-reservoir conditions. The viscous-flow mechanism cannot be neglected at any pore sizes under reservoir conditions, whereas the Knudsen diffusion is found to be important when pore size is smaller than 2 nm and pressure is less than 35.0 MPa. The contribution of surface diffusion cannot be ignored when the pore size is smaller than 10 nm and the pressure is less than 15.0 MPa.

Modeling of convective drying of cornstarch-alginate gel slabs

This work aimed at developing numerical models to predict the moisture and size of slices of gels containing gelatinized or native cornstarch and calcium alginate, during convective drying. The coupling of mass transfer and shrinkage simulated the drying kinetics with evaluation of effective mass diffusivity. Drying of gels containing 34% wb cornstarch (GC90 and RC90 samples) were well fitted by the analytical solution of Fick's second law, with R2 of 0.963–0.998. The numerical model developed in finite element method via COMSOL Multiphysics better described the drying of gels formulated with 5.4% wb of cornstarch (GC50 and RC50 samples), yielding R2 of 0.958–0.992. Shrinkage was estimated by the molar flux of water and simulated by the Arbitrary Lagrangian-Eulerian (ALE) method. The shrinkage term modified the drying rate profiles and a pseudo-constant rate period was observed. The developed model can be applied in drying studies of gels or foodstuffs with high shrinkage ratio.

Intensification of zirconium and hafnium separation through the hollow fiber renewal liquid membrane technique using synergistic mixture of TBP and Cyanex-272 as extractant

Abstract The novel synergistic mixture of TBP and Cyanex-272 is used as the extractant in the hollow fiber renewal liquid membrane (HFRLM) technique for Zr/Hf separation. The effects of the chemical and operational parameters such as HNO3 concentration in the donor phase, NH4F concentration in the acceptor phase, Cyanex-272 and TBP concentration in the liquid membrane phase, the lumen and shell side flow rates, and aqueous/organic volume ratio on the mass transfer and separation performance of HFRLM method were investigated. The obtained results reveal the intensification potential of proposed HFRLM technique for selective extraction of Zr over Hf with separation factor higher than 100. The HFRLM method provides two times higher mass transfer flux in comparison with hollow fiber supported liquid membrane (HFSLM). Also, the HFRLM method shows satisfactory stability for 700 min of continuous operation. The Zr ion transport through the LM phase follows the coupled co-transport mechanism and the diffusion in the renewal layer is recognized as the rate-controlling step in the HFRLM process. Moreover, the Zr mass transfer coefficient and molar flux in the HFRLM method are calculated in the range of 1 × 10−8 to 8.4 × 10−7 m·s−1 and 4.9 × 10−6 to 20.1 × 10−6 mol·m−2·s−1, respectively.

An Adaptive Observer Design for Real-Time Parameter Estimation in Lithium-Ion Batteries

This paper addresses the problem of parameter identification in an electrochemical model of a Lithium-ion battery. The starting point is the development of an extended model based on the absolute nodal coordinate formulation (ANCF-e), which provides high accuracy, yet low computational complexity. Using such a model, this paper proposes an adaptive observer to carry out real-time parameter estimation. The complex spatiotemporal relation between inputs, states, and outputs, and the combined presence of multiple nonlinearities related to the open circuit, electrolyte, solid, and overpotentials prevents the application of standard tools of parameter estimation and adaptive observers. These challenges are overcome by breaking the ANCF-e model into four subsystems which consist of key dynamic relationships between the molar flux and either measurable quantities or states of the solid and electrolyte dynamics, as well as a nonlinear subsystem that relates inputs, outputs, and states on one hand, and potentials on the other. Adaptive observers are proposed to identify parameters of each subsystem and are validated using simulation studies. Significant extensions of currently available adaptive observers are proposed for this purpose.

Analysis and Interpretation of Pressure-Decay Tests for Gas/Bitumen and Oil/Bitumen Systems: Methodology Development and Application of New Linearized and Robust Parameter-Estimation Technique Using Laboratory Data

Summary Diffusion mixing is a dominant process in the absence of convective mixing in various reservoir processes, such as carbon dioxide (CO2) flooding of fractured reservoirs, heavy-oil and bitumen recovery, solution-gas-drive processes, and the gas-redissolution process in a depleted reservoir. In these processes, the diffusivity governs the rate and extent of mixing of light hydrocarbons/nonhydrocarbons with the oil that enhances the oil recovery through in-situ viscosity reduction. It is one of the key parameters for the design and understanding of displacement processes. Because of its significance in various aspects of oil-recovery processes, several experimental and theoretical studies were recently performed on the measurement of gas diffusivity in oils. Experimental work most commonly uses the pressure-decay (PD) concept because of its simplicity and the potential extraction of other necessary parameters, such as Henry's constant. However, the parameter estimation from these tests is dependent on nonlinear regression, which might have several issues such as nonconvergence, nonuniqueness or multiplicity in solution, and high sensitivity toward noise and the time span of the data. Therefore, in this paper, Ratnakar and Dindoruk (2015) is extended and New experimental data are provided from a PD test for CO2 diffusion into bitumen at 80°C and approximately 700 psi. A robust inversion technique for parameter estimation is presented for exponentially decaying late-transient data, which can be used with any PD model used in the literature. The validity and applicability of the inversion technique is demonstrated against numerical data that are generated for a PD system by solving a diffusion model with continuity in the state variable (using Henry's constant) and molar flux at the gas/oil interface. Most importantly, the issues with the nonlinear-regression technique are resolved using the linearized technique. The inversion technique presented in the work is dependent on a combination of linear regression and numerical integration using a modified, more-convenient form of the fundamental equations rather than a nonlinear regression on the fundamental equations as derived. This integral-based linear representation avoids the multiple solutions and can be used with limited data sets and/or when noise in the experimental data is significant, especially in industrial-grade experiments.

Investigation of gas sweetening by nanofluid in isothermal tower with consideration of thermodynamic equilibrium; experimentally and theoretically

The hydrogen sulfide is a basic contaminant which is exited to atmosphere from chemical factories. The study focuses on initialization of Stefan-Maxwell equations to calculate the hydrogen sulfide removal molar flux by nanofluid in an absorption process. The sweetening process is done in a packed bed tower, isothermally. The aluminum oxide nanofluid as solvent in the role of the liquid phase is considered as a non-ideal solution. The activity coefficient for the vapor-liquid equilibrium calculation is based on molar transfer equations is proposed. No ideality within vapor phase is considered based on the modified state equation of Virial. The hydrogen sulfide removal molar flux is based on two cases of film theorem and KGa calculation are investigated in this paper. Results show increasing percentage of mass transfer coefficient for increasing of solvent flow rate from 2 m3/h to 3 m3/h is 71.2%. The theoretical values of hydrogen sulfide molar flux which is based on film theorem using Fick’s diffusion factor shows much deviation with experimental hydrogen sulfide molar flux. Also, H2S removal molar flux based on KGa is calculated in this research. Finally, the performance of absorber is modelled by ANN. The value of RMSE and R in this theoretical model is 0.2979 and 0.9537, respectively.

Quantitative elucidation of the elusive role of defects in polycrystalline MFI zeolite membranes on xylene separation performance

The defect structure in a c-out-of-plane oriented MFI membrane was quantitatively analyzed by processing images obtained by fluorescence confocal optical microscopy (FCOM). The MFI membranes were placed in contact with a dye solution at a fixed concentration (1 mM) for 2, 4, and 8 d and at different concentrations (0.01, 0.1, and 1 mM) for 8 d. This approach led to the identification and understanding of two types of defects (cracks and grain boundary defects). The representative quantitative properties (porosity and tortuosity) relevant to the defects were obtained via the image processing. Furthermore, the estimation of the defect sizes was complemented by the use of a one-dimensional permeation model for the molar flux of the p-/o-xylene components across the c-oriented MFI membrane. Using this combination, we found that although the amount of defects in the whole zeolite membrane was close to ~ 1%, they provided non-selective, facile pathways that accounted for ~ 58% of the total molar flux of faster permeating p-xylene. Surprisingly, despite the lower density (pixel-based area fraction) of cracks, wider cracks (~ 7.8–8.2 nm) accounted for the much higher molar flux of permeation components compared to major, but narrower grain boundary defects (~ 1–2 nm). Indeed, the cracks mainly deteriorated a p-/o-xylene separation performance of the c-oriented MFI membrane. Finally, we found that the crack size increased in the presence of p-xylene so that the molar flux of o-xylene in the binary mixture was significantly increased, thus reducing the p-/o-xylene separation performance. This behavior was ascribed to the flexible MFI zeolite structure after the adsorption of p-xylene.

A Single Tube Contactor for Testing Membrane Ozonation

A membrane ozonation contactor was built to investigate ozonation using tubular membranes and inform computational fluid dynamics (CFD) studies. Non-porous tubular polydimethylsiloxane (PDMS) membranes of 1.0–3.2 mm inner diameter were tested at ozone gas concentrations of 110–200 g/m3 and liquid side velocities of 0.002–0.226 m/s. The dissolved ozone concentration could be adjusted to up to 14 mg O3/L and increased with decreasing membrane diameter and liquid side velocity. Experimental mass transfer coefficients and molar fluxes of ozone were 2.4 × 10−6 m/s and 1.1 × 10−5 mol/(m2 s), respectively, for the smallest membrane. CFD modelling could predict the final ozone concentrations but slightly overestimated mass transfer coefficients and molar fluxes of ozone. Model contaminant degradation experiments and UV light absorption measurements of ozonated water samples in both ozone (O3) and peroxone (H2O2/O3) reaction systems in pure water, river water, wastewater effluent, and solutions containing humic acid show that the contactor system can be used to generate information on the reactivity of ozone with different water matrices. Combining simple membrane contactors with CFD allows for prediction of ozonation performance under a variety of conditions, leading to improved bubble-less ozone systems for water treatment.

Flux-dependent anisotropic pellet diffusivity in particle-resolved CFD simulations of fixed beds

The implementation of multicomponent diffusion in porous catalyst particles into resolved-particle fixed bed computational fluid dynamics (CFD) presents challenges. Usually an isotropic effective diffusivity is employed for each species, which includes molecular diffusion, Knudsen diffusion and viscous flow. To obtain this quantity, constant and isotropic molar flux ratios are assumed. In fixed bed models, pellets composed of isotropic materials are situated in non-isotropic fluid-phase gradients in the bed, which call the assumptions into question. In this paper we present an extended model using a non-isotropic effective diffusivity tensor, which allows evaluation of the flux assumptions. The method is illustrated for resolved particle steady-state CFD simulations of ethylene oxidation in small test beds of spheres and four-hole cylinders. Comparisons to the original constant molar flux ratio model are made, which show that the flux changes affect reaction, species and temperature profiles in the particles, and are stronger in particles with longer diffusion paths. Larger differences between results from the two diffusion models are also seen when the effective diffusivity is decreased by reducing porosity and increasing tortuosity, typical of a reaction system where deactivation increases by carbon deposition.

Deformation of giant unilamellar vesicles under osmotic stress.

Biological membrane plays an important role in maintaining an osmotic equilibrium between the cytoplasm and the extracellular solution of cells. Here, the giant unilamellar vesicles (GUVs) as cell models were used to investigate the effect of osmotic stress on phospholipid membranes. The deformation of GUVs, including inward budding and outward budding, was systematically investigated by the osmotic press from glucose, sucrose, LiCl, and KCl solutions. The permeability (P) of DMPC, DMPC/10 mol% Chol GUVs, DMPC/25 mol% Chol GUVs, and DMPC/40 mol% Chol GUVs in glucose, sucrose, LiCl, and KCl solutions were all obtained. The P value decreases with the addition of more cholesterol in the bilayer. The monovalent cations caused higher permeability of lipid bilayer membranes due to their combination with phospholipids. The molar flux of water (J) value was found to be the key factor for determining the deformation state from mainly inward budding to mainly outward budding. The findings in this paper may help us to understand cell transformation triggered with osmotic stress.

Mass transfer investigation and operational sensitivity analysis of amine-based industrial CO2 capture plant

Abstract In this article, the industrial process of CO2 capture using monoethanolamine as an aqueous solvent was probed carefully from the mass transfer viewpoint. The simulation of this process was done using Rate-Base model, based on two-film theory. The results were validated against real plant data. Compared to the operational unit, the error of calculating absorption percentage and CO2 loading was estimated around 2%. The liquid temperature profiles calculated by the model agree well with the real temperature along the absorption tower, emphasizing the accuracy of this model. Operational sensitivity analysis of absorption tower was also done with the aim of determining sensitive parameters for the optimized design of absorption tower and optimized operational conditions. Hence, the sensitivity analysis was done for the flow rate of gas, the flow rate of solvent, flue gas temperature, inlet solvent temperature, CO2 concentration in the flue gas, loading of inlet solvent, and MEA concentration in the solvent. CO2 absorption percentage, the profile of loading, liquid temperature profile and finally profile of CO2 mole fraction in gas phase along the absorption tower were studied. To elaborate mass transfer phenomena, enhancement factor, interfacial area, molar flux and liquid hold up were probed. The results show that regarding the CO2 absorption, the most important parameter was the gas flow rate. Comparing liquid temperature profiles showed that the most important parameter affecting the temperature of the rich solvent was MEA concentration.

(Invited) Material Advances Toward Vertically-Conducting AlGaN Power Transistors

Due to their large bandgaps (≥ 3.4 eV) compared to Si and SiC, AlGaN alloys have superior material properties that are appealing for the next generation of high-voltage power devices. Most notable is that the critical electrical field scales as the bandgap to the 2.0-2.5 power. Thus, diodes employing wide bandgap semiconductors are expected to operate at higher breakdown voltages with thinner and more heavily-doped drift regions, leading to lower resistive losses compared to Si-based devices. Additionally, the formation of AlGaN-based heterojunctions and the utilization of polarization fields offer design options not possible for devices based on materials such as SiC and diamond. Noteworthy demonstrations of high voltage (> 3 kV) PN and Schottky barrier diodes in both GaN and AlGaN alloys confirm the potential of III-Nitride materials for power devices. However, full exploitation of the efficiency gains promised by III-Nitrides for power management systems also requires vertically-conducting transistors for current switching, in addition to diodes. Analogous to power diodes, vertically-conducting power transistors such as D-MOSFETs employ a thick n-type drift layer to stand-off high reverse voltage, but unlike simple PN diodes, have the p-type material limited to spatially defined regions over the surface of the device. “Selective area p-doping” is typically accomplished in SiC and Si devices by implantation and activation of p-type dopants. However, implantation processing in III-Nitride structures has to date only yielded PN junctions that exhibit significantly higher reverse current leakage than continuously-grown PN junctions. Selective-area epitaxial regrowth of p-type material in regions etched into the drift layer offers an alternative to implantation. However, regrown pn junctions on plasma-etched III-Nitrides with low reverse leakage current have not been demonstrated. In this presentation we will discuss the advances in epitaxial p-regrowth for forming pn junctions on inductively-coupled plasma (ICP) etched n-type GaN and Al0.3Ga0.7N drift layers. The role and sources of unintentionally-introduced Si at the regrown interface will be presented. Additionally, approaches to ameliorate damage and defect states introduced by ICP etching through wet-chemical and in-situ reactor thermal treatments prior to p-regrowth will be related to current-voltage characteristics of regrown diodes. While high voltage GaN-based diodes have been fabricated on vertically-conducting GaN substrates, AlGaN-based high voltage diodes with thick drift layers (≥ 5 μm) have been fabricated on AlN and sapphire substrates. The insulating nature of these substrates necessitates planar device geometries that result in high ohmic losses, heating, and degraded device performance. However, the thick drift layer and the smaller lattice constant of AlGaN alloys present significant challenges for realizing vertically-conducting AlGaN power devices on GaN substrates. Here we report a method for fabricating vertically-conducting AlxGa1-xN PN junctions grown on conducting GaN substrates. An n-type GaN substrate is patterned with parallel, sub-micron-wide mesas separated by trenches etched > 0.5 μm deep prior to AlxGa1-xN overgrowth using metal-organic vapor phase epitaxy. We find that growth temperature, V/III ratio, and group-III molar flux can be used to manipulate the crystalline facets formed along the etched pattern prior to AlxGa1-xN coalescence. Using the described process, coalesced planar AlxGa1-xN epitaxial layers were achieved with less than 6 μm of overgrowth for Al mole fractions up to 0.45, which is the highest Al mole fraction reported to date. Cathodoluminescence measurements of an overgrown Al0.3Ga0.7N epilayer indicate that the density of threading dislocations is mid x 108 cm-2 which is similar to that measured for Al0.3Ga0.7N epilayers overgrown on patterned Al0.3Ga0.7N on sapphire. The current-voltage characteristics of vertically-conducting AlGaN PN diodes grown on GaN substrates will be presented. The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency – Energy (ARPA-E), U.S. Department of Energy under the PNDIODES program managed by Dr. Isik Kizilyalli. This work was also supported, in part, by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525

Analysis of molar flux and current density in the electrodialytic separation of sulfuric acid from spent liquor using an anion exchange membrane

Separation of sulfuric acid from a dilute solution involved a plate and frame type electrodialysis unit using a commercial anion exchange membrane. Experiments were conducted in batch with catholyte concentrations ranging from 1 to 5 wt%. Effect of applied current density, initial catholyte concentration and initial concentration difference of catholyte and anolyte on the molar flux was studied extensively. The maximum molar flux was estimated to be 10.52×10-8 mol cm-2s-1 at 4.45 wt% catholyte concentration and applied current density of 30 mA cm-2. Current efficiencies were observed to be 75 to 85% at lower current density, which rose to more than 100% at 20 and 30mA cm-2, at equal initial concentration of catholyte and anolyte. Diffusive flux and flux due to membrane potential contributed very less compared to total flux in presence of applied electric current. An equation was developed to predict the practical molar fluxes, which fitted satisfactorily with minor standard deviation. Pristine and used membrane specimens were characterized using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM).

Ionic liquids as bulk liquid membranes on levulinic acid removal: A design study

Ionic liquids are considered as green solvents because of their remarkable properties. In view of this, imidazolium-based ionic liquids were used as a bulk liquid membrane (BILM) for levulinic acid removal. Four different hydrophobic ionic liquids, 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][Tf2N], 1-Butyl-3-methylimidazolium hexafluoro phosphate [BMIM][PF6], 1-Hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [HMIM][Tf2N], 1-Hexyl-3-methylimidazolium hexafluorophosphate [HMIM][PF6], were used in the membrane phase. Tributyl phosphate was used as a carrier (0–2 mol/L) in the membrane, and NaOH solutions (0–4 N) were used as stripping phase respectively. Various parameters such as initial levulinic acid concentration in the feed, carrier concentration and ionic liquid type in the membrane, and NaOH concentration in the stripping phase were examined by using D-optimal design. D-optimal design based on the Response Surface Methodology (RSM) is a design method that provides the relationship between independent and dependent factors with a minimum number of experiments. Levulinic acid was successfully transported through BILM using imidazolium-based ionic liquids. Extraction and stripping removal efficiencies were selected as dependent variables and calculated from the experimental results. The experimental results were analyzed by analysis of variance (ANOVA). According to ANOVA, the second order model equations were obtained for dependent variables and have been the good fit with the experimental results. The optimal conditions for BILM process were performed to obtain the higher removal efficiencies. The highest extraction and stripping efficiencies have obtained in case of higher concentration values of TBP and NaOH. Additionally, the molar fluxes at extraction and stripping interfaces were calculated from experimental data. As a result, this design study showed that ionic liquids as bulk liquid membranes can be used on levulinic acid removal.