Mass Transport Steps Research Articles

Kinetic study of Ga electrodeposition on different configuration of Cu cathodes

• The steady-state polarization of Ga electrodeposition at Cu cathode is researched accompanied by HER. • Cu foam narrows the Tafel region but hardly change the kinetic parameters. • Hydrogen bubbles enhance the mass transfer process and cause the increase of limiting current density. • The transformation of electrode surface at early stage affects Ga electrodeposition. Different configuration of Cu cathodes (Cu plate and Cu foam) has been applied for Ga electrodeposition due to the low hydrogen evolution activity of copper. It is found that Ga electrodeposition is controlled by electrochemical reaction step at low current densities and by mass transport step at high current densities. The steady-state polarization process of Ga electrodeposition accompanied by hydrogen evolution is researched, and the results show that Cu foam can hardly change the kinetic parameters of Ga electrodeposition, but narrows the Tafel region due to its large surface area. The hydrogen evolution reaction promotes the mass transfer to the electrode, and the promotion effect of Cu foam is obviously better than that of Cu plate. The absolute value of mass transfer enhancement degree (exponent b ) of Cu plate and Cu foam is 0.39 and 1.12 respectively. The transformation of electrode surface structure at early stage can also affect Ga electrodeposition.

Chitin-biocalcium as a novel superior composite for ciprofloxacin removal: Synergism of adsorption and flocculation

The ubiquitous present antibiotics in aquatic environment is attracting increasing concern due to the dual problems of bioaccumulation toxicity and antibiotic resistance. In this study, a low-cost chitin-biocalcium (CC) composite was developed by a facile alkali activation process from shell waste for typical antibiotics ciprofloxacin (CIP) removal. Response surface methodology (RSM) was utilized to optimize synthesis methodology. The optimized CC products featured superior CIP removal capacity of 2432 mg/g at 25 °C (adsorption combined with flocculation), rapid adsorption kinetics, high removal efficiency (95.58%) and wide pH adaptability (under pH range 4.0–10.0). The functional groups in chitin and high content of biocalcium (Ca2+) endowed CC abundant active sites. The kinetic experimental data was fitted well by pseudo-second-order and intraparticle diffusion model at different concentrations, revealing the removal was controlled by chemisorption and mass transport step. From the macroscopic aspect, flocs were produced with the increase of CIP concentration during the reaction, adsorption combined with flocculation were related to the CIP removal. From the microcosmic aspect, the superior removal performance was attributed to cation bridging, cation complexation among biocalcium-CIP and hydrogen bond between functional groups of chitin and CIP.

Effects of the magnetic field on the anodic dissolution of Ni│H3PO4 + KSCN system

Dynamic processes at the Ni│H3PO4+KSCN interface during the anodic dissolution were observed in situ without and with the magnetic field (MF) by the digital holography. Potentiostatic current oscillations appeared in the pre-passive region, because the newly-formed NiOOH passive layer could be dissolved by SCN− ions. MF enhanced the mass-transport step and accelerated the dissolution of the surface film, which changed the current oscillations into the active dissolution in the pre-passive region and induced the current oscillations at the potential near the passive region. Moreover, MF promoted the SCN−- induced intergranular corrosion.

Microstructure and deposition kinetics of Nb prepared by chemical vapor deposition

The deposition kinetics and microstructure of chemical vapor deposition (CVD) of Nb on the Mo substrate at different deposition variables is investigated. The morphology of CVD Nb is columnar, it exhibits a strong preferred orientation and its growth direction is perpendicular to the substrate surface, the deposition rate and grain size increased with the increase of deposition temperature. The deposition rate conforms to the Arrhenius formula, the activation energy [Formula: see text] at high temperature and low temperature is 0.85 kJ/mol and 7.2 kJ/mol, respectively. The rate-limiting step for CVD Nb at high temperature is chemical reaction step, whereas that is the mass transport step at low temperature. Chlorination temperature has a weak influence on deposition rate and grain structure, the deposition rate and grain size of CVD Nb increased with the increase of the chlorine flow and hydrogen flow, the maximum deposition rate is [Formula: see text], thus, the optimum deposition temperature is 1200[Formula: see text]C, chlorination temperature is 350[Formula: see text]C, hydrogen flow is 400 ml, chlorine flow is 200 ml.

Kinetics of Na2O evaporation from CaO–Al2O3–SiO2–MgO–TiO2–Na2O slags

The present work is carried out to study the evaporation of Na2O from CaO–Al2O3–SiO2–TiO2–MgO–Na2O slags with high basicity and high alumina in the temperature range of 1500–1560°C. The ratio of evaporation was determined by monitoring the Na2O content change of the slag melt under isothermal reduction conditions. The results show that the evaporation ratio increases with increasing the temperature. Higher basicity and increasing concentrations of Na2O, Al2O3 are also found to increase the evaporation ratio of Na2O, while MgO addition only slightly enhances the evaporation ratio. With TiO2 content increasing, the evaporation ratio first increases and then decreases. The evaporation rate of Na2O appears to be controlled by chemical reaction at the slag/gas interface in the beginning, followed by a mixed reaction-mass transfer regime, and finally a liquid-phase mass transport step. The apparent activation energy is 134.74 kJ mol−1 for the chemical reaction regime and 268.53 kJ mol−1 for the liquid-phase mass diffusion step.

The inherent coupling of charge transfer and mass transport processes: the curious electrochemical reversibility

As a complement to a previous contribution from us, the mass transport mechanisms of the electroactive species to and from the electrode in an uncomplicated electrode mechanism are considered. The electrode process as a whole is discussed, with emphasis to its reversibility degree, as results from the relevant responses in controlled potential techniques, such as chronoamperometry and current sampling voltammetry, linear sweep and cyclic voltammetry, and in rotating disk voltammetry. The electrode process as a whole, composed by charge transfer and mass transport steps that concur to condition the current flowing, is analysed on the basis of the relative rates of the two steps, as well of the time window within which the process is observed. The so-called ‘boundary value problem’ for uncomplicated charge transfers with different reversibility degrees is outlined. Supplementary Material is available, in which the simulated concentration profiles for reduced and oxidised species reacting at an electrode, at which a triangular potential waveform is applied, are linked to the corresponding current densities.

Modeling of Progesterone Release from Poly(3-Hydroxybutyrate) (PHB) Membranes.

Poly(3-hydroxybutyrate) (PHB) biodegradable polymeric membranes were evaluated as platform for progesterone (Prg)-controlled release. In the design of new drug delivery systems, it is important to understand the mass transport mechanism involved, as well as predict the processkinetics. Drug release experiments were conducted and the experimental results were evaluated using engineering approaches that were extrapolated to the pharmaceutical field by our research group. Membranes were loaded with different Prg concentrations and characterized by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR). SEM images showed that membranes have a dense structure before and after the progesterone addition. DSC and FTIR allowed determining the influence of the therapeutic agent in the membrane properties. The in vitro experiments were performed using two different techniques: (A) returning the sample to the receptor solution (constant volume of the delivery medium) and (B) extracting total volume of the receptor solution. In this work, we present a simple and accurate "lumped" second-order kinetic model. This lumped model considers the different mass transport steps involved in drug release systems. The model fits very well the experimental data using any of the two experimental procedures, in the range 0 ≤ t ≤ ∞ or 0 ≤ M t ≤ M ∞. The drug release analysis using our proposed approaches is relevant for establishing in vitro-in vivo correlations in future tests in animals.

Fine tuning of magnetite nanoparticle size distribution using dissymmetric potential pulses in the presence of biocompatible surfactants and the electrochemical characterization of the nanoparticles.

The effects of varying the surfactant concentration and the anodic pulse potential on the properties and electrochemical behaviors of magnetite nanoparticles were investigated. The nanoparticles were synthesized with an electrochemical method based on applying dissymmetric potential pulses, which offers the advantage that can be used to tune the particle size distribution very precisely in the range of 10 to 50 nm. Under the conditions studied, the surfactant concentration directly affects the size distribution, with higher concentrations producing narrower distributions. Linear voltammetry was used to characterize the electrochemical behavior of the synthesized nanoparticles in both the anodic and cathodic regions, which are attributed to the oxidation of Fe(2+) and the reduction of Fe(3+); these species are part of the spinel structure of magnetite. Electrochemical impedance spectroscopy data indicated that the reduction and oxidation reactions of the nanoparticles are not controlled by the mass transport step, but by the charge transfer step. The sample with the highest saturation magnetization was that synthesized in the presence of polyethylene glycol.

Triphasic Segmented Flow Millireactors for Rapid Nanoparticle—Catalyzed Gas-Liquid Reactions — Hydrodynamic Studies and Reactor Modeling

In this paper, we present detailed experimental and modeling studies of a recently developed triphasic segmented flow millireactors for rapid nanoparticle-catalyzed gas–liquid reactions. We first present detailed observations of the hydrodynamics and flow regimes in a pseudo-biphasic mode of operation, which enable the design and selection of optimal operating conditions for the triphasic millireactor. We particularly focus on and analyze the presence of wetting films of the organic phase on the reactor walls at high flow speeds, a consequence of the phenomenon of forced wetting, which is a key ingredient for optimal reactor performance. Next, we describe the development of a simple phenomenological model, incorporating the key mass transport steps that accurately captures the observed experimental trends for the rhodium nanoparticle (RhNP) catalyzed hydrogenation of a model substrate (1-hexene). We further discuss and analyze the consequences of this model.

Kinetics of Na2O and B2O3 Loss From CaO-SiO2-Al2O3 Slags

The present work was carried out to study the loss of Na2O and B2O3 from CaO-SiO2-Al2O3 slags containing high content of Al2O3 and Na2O in the temperature range of 1573 K to 1773 K (1300 °C to 1500 °C). The rate of loss was determined by monitoring the weight change of the slag melt by thermogravimetric analysis under isothermal conditions. As expected, the evaporation rate was found to increase with increasing the temperature. Higher basicity and larger B2O3 and Na2O concentrations were also found to increase the evaporation rate. The evaporation rate appears to be controlled by chemical reaction at the surface in the beginning, followed by a mixed reaction-mass transfer regime, and finally a liquid-phase mass transport step. The apparent activation energies for the evaporation reaction were calculated for different slag compositions.

Rate‐limiting step analysis of the microbial desulfurization of dibenzothiophene in a model oil system

A mechanistic analysis of the various mass transport and kinetic steps in the microbial desulfurization of dibenzothiophene (DBT) by Rhodococcus erythropolis IGTS8 in a model biphasic (oil-water), small-scale system was performed. The biocatalyst was distributed into three populations, free cells in the aqueous phase, cell aggregates and oil-adhered cells, and the fraction of cells in each population was measured. The power input per volume (P/V) and the impeller tip speed (vtip ) were identified as key operating parameters in determining whether the system is mass transport controlled or kinetically controlled. Oil-water DBT mass transport was found to not be limiting under the conditions tested. Experimental results at both the 100 mL and 4 L (bioreactor) scales suggest that agitation leading to P/V greater than 10,000 W/ m(3) and/or vtip greater than 0.67 m/s is sufficient to overcome the major mass transport limitation in the system, which was the diffusion of DBT within the biocatalyst aggregates.

Mathematical modeling of drug dissolution

The dissolution of a drug administered in the solid state is a pre-requisite for efficient subsequent transport within the human body. This is because only dissolved drug molecules/ions/atoms are able to diffuse, e.g. through living tissue. Thus, generally major barriers, including the mucosa of the gastro intestinal tract, can only be crossed after dissolution. Consequently, the process of dissolution is of fundamental importance for the bioavailability and, hence, therapeutic efficacy of various pharmaco-treatments. Poor aqueous solubility and/or very low dissolution rates potentially lead to insufficient availability at the site of action and, hence, failure of the treatment in vivo, despite a potentially ideal chemical structure of the drug to interact with its target site. Different physical phenomena are involved in the process of drug dissolution in an aqueous body fluid, namely the wetting of the particle's surface, breakdown of solid state bonds, solvation, diffusion through the liquid unstirred boundary layer surrounding the particle as well as convection in the surrounding bulk fluid. Appropriate mathematical equations can be used to quantify these mass transport steps, and more or less complex theories can be developed to describe the resulting drug dissolution kinetics. This article gives an overview on the current state of the art of modeling drug dissolution and points out the assumptions the different theories are based on. Various practical examples are given in order to illustrate the benefits of such models. This review is not restricted to mathematical theories considering drugs exhibiting poor aqueous solubility and/or low dissolution rates, but also addresses models quantifying drug release from controlled release dosage forms, in which the process of drug dissolution plays a major role.

Facilitated transport enhances spray layer-by-layer assembly of oppositely charged nanoparticles

We investigate the fundamental mechanism of spray-assisted layer-by-layer (LbL) assembly of oppositely charged silica nanoparticles. Alteration of the major operating parameters such as suspension concentration, spray flow rate, and spray duration allows us to develop a kinetic model for spray-assisted LbL deposition. The spray-deposition mechanism is broken down into two sequential mass transport steps: bulk spray flux to the substrate surface followed by random motion of nanoparticles through a thin liquid film that adheres to the substrate throughout the spray procedure. We demonstrate that the second transport step limits the overall deposition rate, and we examine the importance of a convective driving force in accelerating this random motion close to the substrate by controlling the depth of the liquid film atop the substrate. Finally, we generalize the idea of convective acceleration and find that sufficient agitation can be used to enhance the rate of dip layer-by-layer assembly as well.

Development and validation of a dynamic model for regeneration of passivating baths using membrane contactors

This work aims at the development of a dynamic model for the mathematical description of facilitated transport separation processes carried out in membrane contactors where mass transport phenomena are coupled with chemical reactions. A general model that takes into account the description of all possible mass transport steps and interfacial chemical reactions is initially presented, allowing its application to a wide range of separation processes and operation conditions. The analysis of the specific system under study, regeneration of trivalent chromium spent passivating baths by removal of zinc using the emulsion pertraction technology, allowed to define several assumptions obtaining simplified models with minimum number of uncertain parameters and mathematical complexity. The final equations and parameters were validated with experimental data reported in a previous work ( Urtiaga, Bringas, Mediavilla, & Ortiz, 2010).

Reduction of Iron-Oxide-Carbon Composites: Part I. Estimation of the Rate Constants

A new ironmaking concept using iron-oxide-carbon composite pellets has been proposed, which involves the combination of a rotary hearth furnace (RHF) and an iron bath smelter. This part of the research focuses on studying the two primary chemical kinetic steps. Efforts have been made to experimentally measure the kinetics of the carbon gasification by CO2 and wustite reduction by CO by isolating them from the influence of heat- and mass-transport steps. A combined reaction model was used to interpret the experimental data and determine the rate constants. Results showed that the reduction is likely to be influenced by the chemical kinetics of both carbon oxidation and wustite reduction at the temperatures of interest. Devolatilized wood-charcoal was observed to be a far more reactive form of carbon in comparison to coal-char. Sintering of the iron-oxide at the high temperatures of interest was found to exert a considerable influence on the reactivity of wustite by virtue of altering the internal pore surface area available for the reaction. Sintering was found to be predominant for highly porous oxides and less of an influence on the denser ores. It was found using an indirect measurement technique that the rate constants for wustite reduction were higher for the porous iron-oxide than dense hematite ore at higher temperatures (>1423 K). Such an indirect mode of measurement was used to minimize the influence of sintering of the porous oxide at these temperatures.

Effect of Cl/H input ratio on the growth rate of MoSi 2 coatings formed by chemical vapor deposition of Si on Mo substrates from SiCl 4–H 2 precursor gases

Under chemical vapor deposition (CVD) conditions of Si on Mo substrate limited by mass transport of reactant gas species through a gas boundary layer, the effect of the Cl/H input ratio on the growth rate of MoSi 2 coating at 1200 °C was investigated using a horizontal hot-wall reactor and SiCl 4–H 2 gas mixtures. The growth kinetic of the MoSi 2 coating obeyed a parabolic rate law irrespective of Cl/H input ratios. The growth rate of MoSi 2 coating initially increased with increasing Cl/H input ratio until the maximum growth rate was reached, and then decreased inversely proportional to the Cl/H input ratio at higher Cl/H input ratio. This suggests that there is a limit to the increase in growth rate for MoSi 2 coating due to the etching effect of Si with respect to the input ratio of Cl/H. The etching effect of the Cl/H input ratio on the growth rate of the MoSi 2 coating was explained by thermodynamic calculations based on the variation of silicon activity on the surface of the MoSi 2 coating. The mass balance between the Si flux supplied by the mass transport step and that consumed by solid-state diffusion to form MoSi 2 coating was considered to be responsible for the activity of silicon.

Kinetics of chemical vapor deposition of Si on Ni substrates from a SiCl 4–H 2 gas precursor mixture

The kinetics of chemical vapor deposition (CVD) of Si on the Ni substrate at deposition temperatures between 850 and 1050 °C using hot-wall reactor and SiCl 4–H 2 gas mixture was investigated. The deposition rate of Si on the Ni substrate obeyed a linear rate law. The activation energy for Si deposition was approximately 117 kJ/mol at low temperatures, indicating that the rate-limiting step for deposition of Si was the chemical reaction step to deposit Si on the surface of substrate. At high temperatures above 975 °C, activation energy was approximately 29 kJ/mol, indicating that that was the mass transport step of reactant gas species through a gas boundary layer from the main gas stream to the surface of Ni substrate. Under CVD conditions of Si limited by the mass transport step, there was a limit in the increase of the Cl/H input ratio to increase the deposition rate of Si on Ni substrate due to etching effect of Si.

MODELLING OF PERVAPORATION: MODELS TO ANALYZE AND PREDICT THE MASS TRANSPORT IN PERVAPORATION

The modelling of the ma ss transfer in pervaporation is one of the fundamental aspects to understand and therefore improve the process performance. This paper reviews the different models to analyse and predict the mass transport through the selective layer in pervaporation. It therefore provides an overview combined with some guidance about the different models proposed in the literature and the applicability range of these models. The different models reviewed cover the two key mass transport steps in pervaporation: (1) sorption into the membrane, and (2) diffusion through the membrane. For the two different steps individual models will be proposed as well as models covering the mass transport across the membrane as a whole. The different models will be grouped with respect to the nature of the models: theoretical, semi-empirical or empirical. Further the applicability range of the different models regarding the different polymer classes such as glassy, semi-crystalline, or rubbery will be shown. Finally, it will be commented on the applicability of the models with regard to two main research fields in pervaporation: development of membranes and design of processes and modules. Inorganic and composite-material membranes involve additional models to analyse and predict the mass transport and have been excluded from this review.

Interfacial electric properties of beta″-alumina and electrode by AC impedance

Abstract In order to enhance cell power density and to study the interfacial electric property between beta″-alumina and an electrode, test cells of Na(l)/beta″-alumina/M, where M=TiN or TiB 2 or Na–Sn or Na–Pb molten alloys as electrode materials, were set up and run within the temperature range of 400°–800°C. The performance of the test cells and the interfacial electric properties were investigated by measuring current–voltage characteristics and AC impedance. The maximum power density of 0.18 W cm −2 for TiN and 0.24 W cm −2 for TiB 2 could be achieved with a large electrode-area of 30 cm 2 at 800°C. A simplified model and equivalent circuit were given, based on the impedance data. The effect of microstructure of the porous electrode and roughness of the beta″-tube on the cell electric performance and impedance has been studied and discussed. The electron-transport through the porous electrode to the interface of the electrode and the beta″-tube surface is the control step for the electrode reaction, Na + +e→Na, rather than the mass-transport step, for a cell of Na(l)/beta″-alumina/porous thin film electrode. The AC impedance data demonstrated that wetting of the beta″-alumina electrolyte plays an important roll in reducing the cell resistance for the molten Na–Sn or Na–Pb electrode, and the molten alloy electrodes have a smaller cell-resistance, 0.3–0.35 Ω cm 2 , at 700°C after 10–20 h. The comparison with sputtered thin, porous film electrodes, showed that the microstructure and thickness of electrode, and the interfacial resistance between electrode and the surface of the beta″-alumina is crucial to enhance cell power density.

Surface structural transformations during ammonia oxidation on Rh(110)

A metastable surface structure, which mediates the conversion from an O-induced (1\ifmmode\times\else\texttimes\fi{}4) to N-induced (2\ifmmode\times\else\texttimes\fi{}1) substrate reconstruction, is observed by scanning tunneling microscopy during the oxidation of ammonia on Rh(110). The units of the metastable phase are short three-atomic segments of [001] Rh rows forming a hexagonal superlattice of a c(2\ifmmode\times\else\texttimes\fi{}6) symmetry. The issue of this study is the impact of the initial structural anisotropy of the surface on the mass transport and elementary steps of the structural transformations resulting in an intermediate that cannot be formed by either of the reaction participants alone.