Interphase Mass Transport Research Articles

Mullite Synthesis Using Porous 3D Structures Consisting of Nanofibrils of Aluminum Oxyhydroxide Chemically Modified with Ethoxysilanes

Nanocrystalline mullite was synthetized by annealing a highly porous 3D structure consisting of nanofibrous aluminum oxyhydroxides treated with ethoxysilanes. The chemical, structural, and phase transformations in the aluminosilicate nanosystem were studied in the temperature range between 100 and 1600 °C. The features of the solid-phase synthesis of mullite at the interface of crystalline alumina with a liquid silica layer are discussed. It was established that chemical modification of the alumina surface with ethoxysilanes significantly limits the interphase mass transport and delays the phase transformation of the amorphous oxide into γ-Al2O3, which begins at temperatures above 1000 °C, while the basic structural nanofibrils are already crystallized at ~850 °C. The formation of mullite was completed at temperatures ≥ 1200 °C, where the fraction of γ-Al2O3 sharply decreased.

Numerical Two-Phase Simulations of Alkaline Water Electrolyzers

Alkaline water electrolyzers (AWE) have several advantages over other types of electrolyzers, including their high efficiency and especially their relatively low cost due to the usage of non-precious metal catalysts, such as nickel and iron, for the electrodes. Information about local quantities and physical phenomena such as the formation of gas bubbles, current densities, temperatures or local species concentrations within a running cell are important for their improvement. Multiphysical computational fluid dynamics (CFD) simulations of electrochemical components using detailed three-dimensional models can provide valuable insight on local behaviors and characteristics that are difficult or impossible to measure experimentally.This work extends the CFD library openFuelCell2 1, which has been implemented using the open-source platform OpenFOAM®, to simulate AWE cells. The model considers the major transport phenomena, including two-phase fluid flow, heat and mass transfer, electrochemical reactions, species transfer and charge transfer in the various functional regions of the cell. It employs an Eulerian-Eulerian approach to characterize the behavior of each phase comprising interphase mass transport, momentum exchange and heat transfer. Appropriate mapping functions are used to couple the physically distinct regions together. A Butler-Volmer equation characterizes the electrochemical reactions that are assumed to occur in electrodes of finite thickness.This model is used to simulate a single zero-gap AWE cell, depicted in Fig. 1, for different operating conditions such as varying temperatures and volumetric flow rates. The conducted studies provide insight into the local formation of the created gas phase (bubbles), the distribution of species within the gas and electrolyte and their impact towards the performance of the running cell. These numerically obtained results are compared to in-house available and gathered experimental data. Figure 1 demonstrates that the polarization curves obtained at various temperatures are in good agreement with the experimental data. Figure 1

Single-Flow Batteries Leveraging Multiphase Electrolytes

The share of electricity generated from renewable sources is growing rapidly, and thus grid-scale battery storage is becoming more prevalent. Aqueous redox flow batteries have the potential to provide safe and scalable energy storage, but the high cost of storage, particularly the membrane and balance of plant costs, has inhibited commercialization. The recently developed single-flow battery leveraging a multiphase electrolyte promises a low-cost system [1], as it is membraneless and uses only one tank and flow loop, but suffers from low Coulombic efficiency [1]. To unlock the potential of such a system, the interplay between interphase mass transport, multiphase flow phenomena, and battery performance must be unraveled. Here, we will compare our previously developed theoretical battery model derived from a boundary layer analysis [2,3] to results from a dedicated experimental program [4]. This led to several key findings, including that our battery operates in a regime characterized by a high Stanton number, and that our analytical solutions led to excellent predictions in various operational regimes when using the interphase mass transport coefficient as a single-valued fitting parameter [4]. In other regimes, such as at low electrolyte velocity, results indicate that gravity acting on the denser polybromide phase played a significant role, and progress towards incorporating gravitational effects into models will be discussed [4]. Figure 1: Schematic of a discharging single-flow battery leveraging a multiphase flow electrolyte. The flow consists of a continuous, bromine-poor aqueous phase and dispersed, bromine-rich polybromide phase. Bromine in the polybromide phase is largely electrochemically inactive, thus such an electrolyte enables membraneless operation by limiting the crossover of bromine to the zinc anode.

Single-flow multiphase flow batteries: Experiments

The share of electricity generated from renewable sources is growing rapidly, and thus grid-scale battery storage is becoming more prevalent. Aqueous redox flow batteries have the potential to provide safe and scalable energy storage, but the high cost of storage, particularly the membrane and balance of plant costs, has inhibited commercialization. The recently developed single-flow battery leveraging a multiphase electrolyte promises a low-cost system, as it is membraneless and uses only one tank and flow loop, but suffers from low Coulombic efficiency. To unlock the potential of such a system, the interplay between interphase mass transport, multiphase flow phenomena, and battery performance must be unraveled. Here, we compare our previously developed theoretical battery model derived from a boundary layer analysis to results from a dedicated experimental program. This led to several key findings, including that our battery operates in a regime characterized by a high Stanton number, and that our analytical solutions led to excellent predictions in various operational regimes when using the interphase mass transport coefficient as a single-valued fitting parameter. In other regimes, such as at low electrolyte velocity, results indicate that gravity acting on the denser polybromide phase played a significant role, and thus gravitational effects should be incorporated in future models.

Correlation Coefficients in Lee’s Model of Multiphase Flows

Multiple technical and biological systems exhibit multiphase flow phenomena. The demands for accurate calculations of the physical phenomena that occur in engineering technologies have increased along with their rapid advancement. However, it is very hard to identify two- and multi-phase flow through experimental measurements, as a result, in addition to experimental measurements, numerical simulations are also performed, which can help improve our physical understanding of the complex phenomenon of phase transformations. The goal of numerical simulations of phase changes is to precisely simulate the real progress and experiment. These simulations operate on the basis of physical principles and correlation coefficients of phase changes. These correlation coefficients have a different range of values. The examination of the Lee model's correlation coefficients from the ANSYS Fluent environment, which is now the most popular for multiphase flow simulations, is the subject of the article that is being given. In this article is also described and tested the numerical simulation of interphase mass transport in a closed space. Research background: The article is focused on the problematics of multiphase flows. In ANSYS Fluent, there are many types of models, which are used for the numerical simulations of this phenomena. In models are included correlation parameters, which are specific for every single situation and are within the given ranges. This paper is about the Lee model, which correlation coefficients are in the range from 10-3 to 102. Purpose of the article: A detailed description of Lee's model with its testing on a heat pipe in the ANSYS Fluent program with determined correlation coefficients. Methods: The use of CFD simulation of multiphase flow to determine correlation coefficients. Findings & Value added: Testing the correlation coefficients of Lee's model and finding their appropriate values for the given situation.

Microcavity-Enabled Local Oscillation of Taylor Bubbles in a Microchannel

Interphase mass transport is important for many industrial applications ranging from multiphase reaction and separation but remains a great challenge in maximizing the interface area and mass-trans...

A quasi 2D model for the interpretation of impedance and polarization of a planar solid oxide fuel cell with interconnects

In this work, a one dimensional plus one dimensional (1D+1D) physical model of a high temperature solid oxide fuel cell (HT-SOFC) is presented. The model is distributed-charge and dynamic, and allows to predict polarization curves and impedance spectra of applicative sized planar cells (up to 10 cm x 10 cm) mounted between interconnects with rectangular ducts. The model solves rigorous conservation equations of mass, charge, momentum and energy, along the interconnect channels and within the cell's layers. The kinetic parameters of the hydrogen oxidation reaction and of the oxygen reduction reaction are fitted to a literature dataset measured between 700 °C and 800 °C, with a 20% H2, 5% H2O and 75% N2 mixture, on a standard anode-supported HT-SOFC (Ni-YSZ/YSZ/GDC/LSCF). Once calibrated, the model is used to study the evolution of local impedance spectra along the channel, as well as the occurrence of gradients of temperature, concentration, and current density within and along the cell structure. Remarkable differences emerge between global impedance spectra, based on the average current density extracted from the whole cell's surface, and local impedance spectra, based on the local current density value at each position along the channel. Local spectra reveal very specific features (negative-resistance arcs), which are absent in the average spectra, and which question the opportunity of collecting spatially resolved impedance measurements on a fine scale. The analysis of the steady state behavior highlights the severity of temperature gradients along the channel (150 K at 0.75 V and 50% H2 utilization factor), the onset of current density peaks, and the crucial role of interphase mass transport at the gas/electrode interface. The consequences of external diffusion on the polarization performance are analyzed, and the impact of different channel configurations on the local evolution of the spectra is explored.

Simulation of the heterogeneous semi-dry flue gas desulfurization in a pilot CFB riser using the two-fluid model

Simi-dry flue gas desulfurization (FGD) in a pilot circulating fluidized bed (CFB) reactor was studied by the CFD method. Heterogeneous hydrodynamics and desulfurization reaction were simulated in a three-dimensional domain of the CFB riser. Euler–Euler approach was adopted and the O–S drag model was utilized for the unsteady gas–solid two-phase flow, which achieved the typical core-annulus structure of both the particle concentration and the particle velocity. The collision coefficient (e) was selected by comparing with the experimental data of the solid volume fraction. The desulfurization was described by the continuous conversion model and the liquid film thickness equation for interphase mass transport was modified to avoid “divided by zero”. Detailed analysis figured out that the water mass fraction in the particle phase played a significant role in the desulfurization. The sorbent dissolving might be a controlling step when the water mass fraction was very small, and should be taken into consideration rather than neglected. Operating parameters were discussed for their impact on the desulfurization efficiency. Computational results showed that the jet water flow rate was the most significant parameter as the circulating solid mass flow rate followed closely behind, suggesting that uniform water jetting will be the most improving method.

Deep desulfurization from fuel oil via selective oxidation using an amphiphilic peroxotungsten catalyst assembled in emulsion droplets

A recyclable amphiphilic catalyst, which composed of peroxotungsten anion and quaternary ammonium cation assembled in the interface of the emulsion droplets, shows high selectivity and activity in the oxidation of sulfur-containing molecules to sulfones with hydrogen peroxide in diesel under mild reaction conditions. All the sulfur-containing molecules present in diesel can be completely oxidized into sulfones with ∼100% selectivity of sulfones in W/O (H 2O 2-in-diesel) emulsion system. The sulfones can be readily separated from the diesel, and the sulfur level of the desulfurized diesel can be lowered from about 500 ppm to below 1 ppm with about 98% yield of oil. In addition, the S-containing molecules in straight-run gas oil are also readily oxidized to their corresponding products in the O/W emulsion reaction medium. The S-level of oil layer of the oxidized oil was reduced to below 1 ppm from 312 ppm in the original straight-run gas oil with above 99% yield of oil. The amphiphilic catalysts serve as not only a catalyst but also an emulsifying agent to stabilize the emulsion droplets. The emulsion droplets are not only in the highly dispersed form but also behave like homogenous catalyst with high activity. The limitations due to interphase mass transport are greatly reduced in emulsion reaction medium. Moreover, the emulsion catalysts can be easily separated and recycled by demulsifying.

Heavy Metals Uptake by Sardinian Natural Zeolites: Experiment and Modeling

Sardinian natural clinoptilolites are examined to evaluate their performance for heavy metals, i.e., lead, cadmium, copper, and zinc, removal. The natural material is either used as received or once converted into the sodium homoionic form. Equilibrium data for heavy metals in aqueous solutions and the natural material are obtained. The corresponding behavior is quantitatively correlated using classical adsorption isotherms as well as the ion-exchange equilibria model developed by Melis et al. (Ind. Eng. Chem. Res. 1995, 34, 3916), whose parameters are estimated by fitting the equilibrium data. To evaluate removal performance under dynamic conditions, breakthrough experiments are also conducted. The latter ones are compared with different ion-exchange rate laws (namely, equilibrium model, linear driving force (LDF), and film model rate approximations, along with Nernst−Planck model) with the aim of pointing out the importance of the hindrance effect of intra- and interphase mass transport when using the n...

Detailed modelling of MMA dispersion polymerization in supercritical carbon dioxide

A comprehensive model of dispersion polymerization in supercritical media is presented. With respect to previous ones reported in the literature, this model accounts in detail for the interphase mass transport of the active polymer chains and specifically for its dependence on the chain length. All the model parameters have been evaluated from independent literature sources in the case of poly(methyl methacrylate) dispersion polymerization in supercritical carbon dioxide. The corresponding a priori predictions of the model have been validated with various sets of experimental data, covering a rather wide range of operating conditions, in terms of conversion, pressure and molecular weight distribution as a function of time. It has been found that the interphase mass transport of active chains plays a significant role in determining the number of reaction loci and consequently the final polymer properties and, in particular, the molecular weight distribution.

How adaptation and mass transfer control the biodegradation of linear alkylbenzene sulfonate by activated sludge.

We use a nonsteady-state model to evaluate the effects of community adaptation and sorption kinetics on the fate of linear alkylbenzene sulfonate (LAS) in batch experiments conducted with activated sludge that was continuously fed different concentrations of LAS. We observed a sharp decrease in the biodegradation rate between 30 and 60 minutes and the presence of an LAS residual at the end of the batch experiments. The modeling analysis indicates that these phenomena were caused by relatively slow inter-phase mass transport of LAS. The modeling analyses also showed that the amount of LAS-degrading biomass increased when the continuous activated sludge was fed a higher LAS concentration. Although community adaptation to LAS involved accumulation of more LAS degraders, the increase was not proportional to the feed concentration of LAS, which supports the concept that LAS degraders also utilized portions of the general biochemical oxygen demand (BOD) fed to the continuous activated sludge systems.

Consistent structure and thermodynamic characteristics of transport potential model proposed for liquid–liquid solvent extraction

The essential features of the macroscopic transport potential model has been outlined. This model proposed for two-phase mass transport adopts a new practical state variable as the reaction coordinates, which corresponds approximately to the change in Gibbs free energy or chemical affinity of reaction. It was shown that the model integrates the two conventional models, i.e. the diffusional and pseudo-first-order reaction models. All the new fundamental variables including the transport potentials and driving forces are approximately redefined using chemical potential to study the thermodynamic aspects of the transport potential model. It was revealed that the transport potentials correspond to the driving force in nonequilibrium thermodynamics, and they contain a similar quantity, i.e. chemical potential divided by temperature. Besides, the expression of net mass flux derived from the present model agreed exactly with that resulted from nonequilibrium thermodynamics. Thus the transport potential model, which proposes a new theoretical structure of formulations for interphase mass transport, has been located in thermodynamics.

Cobalt-Catalyzed Amination of 1,3-Propanediol: Effects of Catalyst Promotion and Use of Supercritical Ammonia as Solvent and Reactant

The catalytic synthesis of 1,3-diaminopropane from 1,3-propanediol and ammonia was studied in a continuous fixed-bed reactor in the pressure range 50 to 150 bar. The unsupported Co-based catalysts applied were characterized by N2physisorption, XRD, XPS, TPR, and ammonia adsorption using pulse thermal analysis and DRIFT spectroscopy. The latter investigations revealed that the best catalyst, 95 wt% Co–5 wt% Fe, contained only very weak acidic sites, unable to chemisorb ammonia. The absence of strong acidic and basic sites was crucial to suppress the various acid/base-catalyzed side reactions (retro-aldol reaction, hydrogenolysis, alkylation, disproportionation, dimerization, oligomerization). Other important requirements for improved diaminopropane formation were the use of excess ammonia (molar ratio NH3/diol>20) and the presence of the metastable β-Co phase. A small amount of Fe additive could efficiently hinder the transformation of this phase into the thermodynamically stable α-Co phase and thus prevent catalyst deactivation up to 10 days on stream. Application of supercritical ammonia almost doubled the selectivity to amino alcohol and diamine. The selectivity enhancement in the near-critical region is attributed to elimination of the interphase mass transport limitations and to the resulting higher surface ammonia concentration.

Production of ethene oxide in a sieve plate electrochemical reactor Part II: Mathematical reactor model and factors affecting the relative concentrations of ethene and bromine

The sieve plate electrochemical reactor is a unit for enabling electrochemical reaction of gaseous species in the liquid phase. The reaction can occur either directly at the surface of the electrodes or indirectly with electrochemical mediators generated in situ. A sieve plate absorber is used for the gas-to-liquid mass transfer and, on appropriate stages, stacks of electrodes are supported on the sieve plates and are electrically connected in a bipolar configuration. Data are reported for the indirect oxidation of ethene using bromine formed by the anodic oxidation of bromide solutions on graphite electrodes. The effect of ethene partial pressure and current density on the current efficiency, selectivity and energy consumption of ethene oxide is reported. The effect of using a co-solvent ( t-butyl alcohol), to increase ethene solubility, on the performance of the reactor is also reported. The reactor is capable of achieving conversions of ethene in the gas phase greater than 80%, although current efficiencies are relatively low, approximately 50%. A mathematical model for the unit is developed which accommodates interphase mass transport of reactants, and product species, between gas and liquid.

Membrane reactors for electrochemical synthesis processes

An application of membrane reactors in electrochemical synthesis is presented. The membrane separations utilise an appropriate solvent extraction process either as a pertraction, an emulsion membrane filtration or a supported liquid membrane. The electrosynthesis considered is an indirect process involving the oxidation of an organic species using an appropriate active metal ion species generated anodically. A model of this membrane reactor system is presented involving interphase mass transport across the membrane, in the form of a hollow fibre, from the organic reagent to the aqueous electrolyte. The model provides the basis for simulation of the process operation and thus a cost estimation of the process. Preliminary costing of the use of membrane reactors shows that significant benefits can result with certain systems.

Process simulation of the anodic oxidation of benzene to p-benzoquinone in a two-phase electrolyte

Experimental data for the anodic oxidation of benzene to p-benzoquinone are analysed to construct a reaction rate model for its synthesis. This model is based on a second-order oxidation of benzene to p-benzoquinone to maleic acid, which can be further oxidized to carbon dioxide. The reaction is carried out in an emulsion electrolyte and the model incorporates both interphase mass transport, between the organic benzene phase and the aqueous sulphuric acid electrolyte, and interphase mass transport at the electrode. The behaviour of a batch reactor during galvanostatic operation is simulated and the importance of the specific mass transfer coefficient k L a demonstrated. The synthesis block of a proposed industrial process for p-benzoquinone production, incorporating a two-phase electrochemical reactor, a liquid-liquid separator and an organic feed addition stage in a recycle loop, is simulated.

Electro‐Organic Synthesis in Supercritical Organic Mixtures

Dimethyl carbonate (DMC), formed electrochemically from carbon monoxide and methanol, has been synthesized at 90°C in the mixture‐critical region of carbon dioxide‐methanol, near a methanol mole fraction of 0.35. By operating in the single‐phase region, we have avoided interphase mass transport. In our experiments, the rate of carbonate formation depended on the concentration of dissolved . For a mole fraction of carbon monoxide of 0.05, current efficiencies near 100% were obtained. For these studies tetrabutylammonium bromide (TBAB) served as the electrolyte and as a source of bromide ions for a reactive intermediate. The selectivity for the carbonate relative to methyl formate and dimethoxymethane was sensitive to the composition of the cosolvent as well as the concentration of TBAB.

Effect of thermodynamic non-idealities on fluid-fluid interphase mass transfer

The two film resistance model for n -component mass transfer across fluid-fluid interfaces is developed. Arguments are presented in favour of the use of the Generalized Maxwell-Stefan (GMS) diffusion coefficients to describe the transport process. The GMS diffusion coefficients are amenable to molecular interpretation and prediction and multicomponent mass transfer coefficients defined in terms of them are shown to possess simple addivity property. The effect of thermodynamic non-idealities on intra- and inter-phase mass transport processes are brought out in the analysis.

On the relative importance of intraparticle and interphase transport effects in gas-solid catalysis

Criteria expressed in terms of several new dimensionless groups are derived to assess the relative importance of intraparticle and interphase heat and mass transport effects in gas-solid catalysis. An overall criterion incorporating all the effects is also presented. Evaluation of the criteria for typical cases shows that in experimental reactors with low flow rates the relative importance of the effects frequently fall in the order: interphase heat transport > intraparticle mass transport > interphase mass transport > intraparticle heat transport.