Mass transfer in fluidized beds of inert particles. Part I: The role of collision currents in mass transfer to the electrode

The effect of particle-wall collision on the mass transfer rate mechanism in liquid fluidized bed electrochemical cells was studied. Collision frequencies and currents were measured at microelectrodes set in the bed wall. It is postulated that, at each particle–electrode collision, a specific microvolume of bulk concentration electrolyte is introduced into the near-electrode diffusion layer during particle movement towards the electrode causing an enhancement of the limiting diffusion current. Based on measurements made at microelectrodes calculations of the contribution of the particle collision mechanism to total mass transfer to a planar electrode are attempted and are in good agreement with experimental values.     

Particle surface temperature measurements with multicolor band pyrometry

A noncontact, color‐band pyrometer, based on widely available, inexpensive digital imaging devices, such as commercial color cameras, and capable of pixel‐by‐pixel resolution of particle‐surface temperature and emissivity is demonstrated and described. This diagnostic instrument is ideally suited to many combustion environments. The devices used in this method include color charge‐coupled device (CCD), or complementary metal oxide semiconductor (CMOS) digital camera, or any other color‐rendering camera. The color camera provides spectrally resolved light intensity data of the image, most commonly for three color bands (Red, Green, and Blue,), but in some cases for four or more bands or for a different set of colors. The CCD or CMOS sensor‐mask combination has a specific spectral response curve for each of these color bands that spans the visible and often near infrared spectral range. A theory is developed, based on radiative heat transfer and camera responsivity that allows quantitative surface temperature distribution calculation, based on a photograph of an object in emitted light. Particle surface temperature calculation is corrected by heat transfer analysis with reflection between the particle and reactor wall for particles located in furnace environments, but such corrections lead to useful results only when the particle temperature is near or below the wall temperatures. Wood particle‐surface temperatures were measured with this color‐band pyrometry during pyrolysis and combustion processes, which agree well with thermocouple measured data. Particle‐surface temperature data simultaneously measured from three orthogonal directions were also mapped onto the surface of a computer generated 3‐D (three‐dimensional) particle model. © 2008 American Institute of Chemical Engineers AIChE J, 2009     

Diffusion-limited chronoamperometry at conical-tip microelectrodes

In this paper, we present simulated diffusion-limited time-variant currents at conical-tip microelectrodes fabricated by depositing a carbon film in and on pulled quartz capillaries. These mechanically strong microelectrodes are suitable probes for detecting neurotransmitters in vivo. The simulated results show that the currents obtained at conical-tip microelectrodes are larger than those at finite conical microelectrodes (e.g. etched carbon fibres protruding from an insulating plane) of comparable dimensions. The currents at conical-tip microelectrodes and finite conical microelectrodes both converge to that of a microdisk electrode at small cone heights and large cone angles, and to that of a cylindrical electrode portion of equal length and half the radius at large cone heights and small cone angles. At short times (scaled by the electrode dimensions), Cottrellian current is achieved at conical-tip microelectrodes and the current densities collapse to the expected chronoamperometric response at a microdisk electrode, subject to some simulation errors. Comparison between a simulated chronoamperogram and an experimental chronoamperogram then allows an estimate of parameters (such as electrode surface area and dimensions) that define the electrode geometry. Steady-state currents based on empirical functions have also been computed for conical-tip microelectrodes and finite conical microelectrodes.     

Thermophoresis of axisymmetric aerosol particles along their axes of revolution

The axisymmetric thermophoretic motion of an aerosol particle of revolution in a uniformly prescribed temperature gradient is studied theoretically. The Knudsen number is assumed to be small so that the fluid flow is described by a continuum model. A method of distribution of a set of spherical singularities along the axis of revolution within a prolate particle or on the fundamental plane within an oblate particle is used to find the general solutions for the temperature distribution and fluid velocity field. The jump/slip conditions on the particle surface are satisfied by applying a boundary‐collocation technique to these general solutions. Numerical results for the thermophoretic velocity of the particle are obtained with good convergence behavior for various cases. For the axisymmetric thermophoresis of an aerosol spheroid with no temperature jump and frictional slip at its surface, the agreement between our results and the available analytical solutions is very good. The thermophoretic velocity of a spheroid along its axis of revolution in general increases with an increase in its axial‐to‐radial aspect ratio, but there are exceptions. For most practical cases of a spheroid with a specified aspect ratio, its thermophoretic mobility is not a monotonic function of its relative jump/slip coefficients and thermal conductivity. © 2008 American Institute of Chemical Engineers AIChE J, 2009     

Physical and mathematical models of an inert macroelectrode modified with active hemispherical microelectrodes

The physical model of an inert electrode partially activated with hemispherical microelectrodes was formed by the deposition of silver grains on a graphite substrate. It is shown that the process on the microelectrodes can be under activation control despite the fact that the overall rate is determined by the diffusion layer of the macroelectrode. On the basis of this conclusion a mathematical model of mass transfer on an inert electrode partially covered with active hemispherical particles (microelectrodes) is given and verified qualitatively by appropriate experiments. It was found that the degree of activation does not depend on the size of the particles, but on the ratio of the radius of the particles to the interparticle distance. This means that the quantity of catalyst required for the transformation of an inert electrode into an active one decreases strongly with decreasing active particle size. It was also shown that the maximum current density to the activated inert electrode was equal to the limiting diffusion one to the massive active electrode, as well as that the activity of the modified inert electrode at the same coverage of catalyst strongly depends on the exchange current density of the electrochemical process taking place on it. The larger is the exchange current density, the lower is the quantity of catalyst required for the same effect on the activity of the modified electrode.     

Shielding effect of double microelectrode tips in a scanning electrochemical microscope

Electrochemical interaction of coaxial double microelectrodes, in which a ring microelectrode was surrounded by another ring microelectrode, was investigated. Mass-transfer reactions that occurred on both inner and outer microelectrodes interfered with each other and showed a “shielding” effect depending on potentials and geometries of microelectrodes. Application of the inner microelectrode of the double microelectrodes for a probing tip of a scanning electrochemical microscope (SECM) revealed that the shielding effect by the outer microelectrode affected the electrochemistry on the inner microelectrode in the vicinity of the substrate surface. The effect was intensified above the insulator but attenuated above the conductor as the microelectrodes approached in feedback mode of the SECM. Approach to a critical interelectrode distance also intensified the shielding effect in the substrate generation/tip collection mode. An SECM line-scan using a platinum/epoxy resin-model substrate was carried out to investigate the shielding effect on current sensitivity and lateral resolution of an SECM image.     

Study of pulsing flow in a trickle bed using the electrodiffusion technique

Absract An electrochemical technique is employed for measuring local, instantaneous liquid-solid mass transfer coefficients in downward cocurrent gas-liquid flow through a packed bed under pulsing flow conditions. The technique involves specially designed electrodes of the same dimensions as the packing material. Also microelectrodes on the surface of a particle are tested for flow diagnostics in the microscale. The feasibility of the method is examined. Interpretation of measurements from various electrodes provides information on the pattern of mass transfer and liquid distribution in the packing. This paper was presented at the International Workshop on Electrodiffusion Diagnostics of Flows held in Dourdan, France, May 1993.     

Deliqueszenz und Effloreszenz hygroskopischer Salzpartikeln in Partikel‐Wand‐ und Partikel‐Partikel‐Kontakten

In case of dust separation with surface filters, adhesive bonds at particle‐wall and particle‐particle contacts are of great importance. The deliquescence and efflorescence of hygroscopic salt particles can significantly influence the adhesive bonds at particle contacts with such salt particles. Therefore, to investigate these phenomena, various types of particle contacts were produced by the separation of aerosols on filtering collectors and subsequently observed in the chamber of an environmental SEM during the moisture‐induced deliquescence and efflorescence.     

Role of fluorocarbon surfactant in the preparation of polytetrafluoroethylene‐modified polyacrylate emulsion

Role of fluorocarbon surfactant in the preparation of polytetrafluoroethylene‐modified polyacrylate emulsion is investigated. The fluorocarbon surfactant has an efficient preemulsification to polytetrafluoroethylene (PTFE) powder. It enables PTFE powder to be introduced into the copolymer of n‐butyl acrylate, n‐methyl methacrylate, n‐styrene, and α‐methacrylic acid. Thereby, stable PTFE‐modified polyacrylate emulsion can be formed. The effects of fluorocarbon surfactant on the surface tension, particle size and particle size distribution of the emulsion, as well as the relation between fluorocarbon surfactant and the amount of PTFE powder are fully investigated. The particle size and the surface tension of emulsion strongly depend on the fluorocarbon surfactant concentration in the reaction system. The particle size distribution becomes narrower and the stability of the emulsion is improved with the increasing of the fluorocarbon surfactant concentration. According to the experiments, a possible mechanism of fluorocarbon surfactant in polymerization is proposed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Monte Carlo modeling of fluidized bed coating and layering processes

A stochastic modeling approach based on a Monte Carlo method for fluidized bed layering and coating is presented. In this method, the process is described by droplet deposition on the particle surface, droplet drying and the formation of a solid layer due to drying. The model is able to provide information about the coating coverage (fraction of the particle surface covered with coating), the particle‐size distribution, and the layer thickness distribution of single particles. Analytical solutions for simplified test cases are used to validate the model theoretically. The simulation results are compared with experimental data on particle‐size distributions and layer thickness distributions of single particles coated in a lab‐scale fluidized bed. Good agreement between the simulation results and the measured data is observed. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2670–2680, 2016     

Effects of the content of silane coupling agent KH‐560 on the properties of LLDPE/magnesium hydroxide composites

Based on traditional idea of molecular coupling effect, surface modifiers were helpful to improve the performances of inorganic/organic composites. However, it was also widely accepted that the content of surface modifier should be controlled in a suitable range, and more or less modifiers could not reach to optimal properties. The intrinsic reason for this phenomenon was not clear until now. In this article, the influences of the content of surface modifier: silane coupling agent KH‐560, on the final performances of linear low‐density polyethylene (LLDPE)/magnesium hydroxide (MH) composites, have been studied. The performance tests of LLDPE/MH composites, including mechanical properties measurements, thermal oxidative stability analysis, and surface morphology observation, all confirmed that there was an optimal content range of surface modifier. However, further morphology investigation of the modified MH by particle size and particle size distribution analyzer showed that, the particle size and particle size distribution of MH both increased as the content of KH‐560 increased. ATR‐FTIR analysis proved that silane grafting was achieved on the MH particle surface, which not only improved the compatibility between MH and LLDPE for molecular coupling effect, but also caused bigger particle size and wider particle size distribution, which were disadvantageous to improve the performances of the composites. The two opposite effects of the surface modifiers mentioned above affected the final performances of the composites. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Grafting of titanium dioxide microspheres with a temperature‐responsive polymer via surface‐initiated atom transfer radical polymerization without the use of silane coupling agents

Titania microspheres with narrow size distribution and diameters of about 1 µm were prepared and subsequently functionalized using surface‐initiated atom transfer radical polymerization (ATRP) of N‐isopropylacrylamide. The ATRP initiator was immobilized on the particle surface via acylation of surface hydroxyl groups with α‐bromoisobutyryl bromide. Subsequently, an established ATRP reaction system was used for the preparation of titania surface‐grafted poly(N‐isopropylacrylamide) (PNiPAAm). Characterization was performed with electron microscopies, X‐ray diffraction, infrared spectroscopy and dynamic light scattering. It was found that the particle size in aqueous dispersions changed reversibly with temperature as expected for a shell of PNiPAAm, a polymer with a lower critical solution temperature at 32 °C. This confirmed the successful preparation of functional, stimuli‐responsive TiO₂ microparticles via a straightforward controlled surface‐initiated polymerization method.     

Chemical reactors of special geometry for the precipitation of mineral particles

Precipitation of pseudo‐boehmite particles is experimentally investigated. The experiments have been carried out in three chemical reactors specially designed to achieve an effective control of the properties of mineral particles produced by precipitation. The chemical reactors are the Sliding‐Surface Mixing Device, the Special Rushton Turbine Reactor and the Vortex Effect Reactor. The mixing efficiency of these new mixing devices is experimentally evaluated and the influence of reactor geometries, operating parameters and initial supersaturation on pseudo‐boehmite particle properties is studied. The particle properties evaluated are particle size distribution, crystallite size, BET surface area, porous volume, mean pore radius and particle shape. A comparison of the particle properties obtained with the three reactors is made.     

Optimizing Microwave‐Assisted Extraction Conditions to Obtain Phenolic‐Rich Extract from Chromolaena odorata Leaves

Phenolic compounds are secondary metabolites that are mainly responsible for different pharmacological activities of plant extracts. In this study, microwave‐assisted extraction (MAE) variables were optimized for the extraction of phenolic compounds from Chromolaena odorata leaves by using a Box‐Behnken design of response surface methodology. The concentration of polyphenols and functional characteristics of the extracts were identified by using liquid chromatography quadrupolar time‐of‐flight mass spectrometry and FTIR analyses, respectively. The obtained results indicated the MAE conditions for the optimal recoveries of the total phenolic content and total flavonoid content from C. odorata leaves. The results reveal that the C. odorata leaf is enriched with phenolic compounds.     

2‐Hydroxyethylmethacrylate carrying uniform porous particles: preparation and electron microscopy

Uniform macroporous particles carrying hydroxyl groups have been obtained in the size range 3–11.5 µm by seeded polymerization. For this purpose, uniform polystyrene particles in the size range 1.9–6.2 µm were used as seeds. The seed particles were successively swollen by dibutyl phthalate (DBP) and a monomer mixture comprising styrene, 2‐hydroxyethylmethacrylate (HEMA) and a crosslinker. Two different crosslinkers, divinylbenzene (DVB) and ethylene glycol dimethacrylate (EGDMA), were tested. Size distribution properties together with bulk and surface structures of the particles have been characterized by both scanning and transmission electron microscopy. While EGDMA provides uniform particles with a non‐porous surface, DVB produces uniform particles having a highly porous surface and interior. The comparison of FTIR and FTIR‐DRS spectra shows that the HEMA concentration is higher on the particle surface than within the particle interior. Seed latex size and monomer/seed latex ratios are identified as the most important variables affecting the final particles. Different seed latexes have been tried; the result is that highly macroporous particles with a sponge‐like pore structure both on the surface and in the particle interior have been obtained by use of the seed latex with the largest particles and the lowest molecular weight. An increase in the HEMA feed concentration leads to final particles with a non‐porous surface and a crater‐like porosity in the particle interior. The average pore size significantly decreases with increasing DBP/seed latex and monomer/seed latex ratios. © 2001 Society of Chemical Industry     

Full‐physics simulations of spray‐particle interaction in a bubbling fluidized bed

Numerical simulations of a gas‐particle‐droplet system were performed using an Euler‐Lagrange approach. Models accounting for (1) the interaction between droplets and particles, (2) evaporation from the droplet spray, as well as (3) evaporation of liquid from the surface of non‐porous particles were considered. The implemented models were verified for a packed bed, as well as other standard flow configurations. The developed models were then applied for the simulation of flow, as well as heat and mass transfer in a fluidized bed with droplet injection. The relative importance of droplet evaporation vs. evaporation from the particle surface was quantified. It was proved that spray evaporation competes with droplet deposition and evaporation from the particle surface. Moreover, we show that adopting a suitable surface coverage model is vital when attempting to make accurate predictions of the particle's liquid content. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2569–2587, 2017     

Arbitrary shaped ice particle melting under the influence of natural convection

This work is devoted to numerical simulations of an arbitrary shaped ice particle melting inside water under the influence of natural convection. Specifically, four different shapes of the ice particle have been studied: sphere, cylinder, cross shaped cylinder, and irregular sphere with radial bumps on its surface. A 2D axisymmetric particle‐resolved numerical model has been employed on a fixed grid to study the detailed melting dynamics of an ice particle. The solid‐liquid interface is treated as a porous medium characterized by the permeability coefficient which is used to damp the velocity values inside the interface. The model results have been compared with an existing experimental results produced by A. Shukla et al. (Metal Mater Trans B. 2011; 42(1):224–235). Very good agreement between our predictions and experimental data have been achieved. Based on the analysis of numerical simulation results, melting process is found to advance through two distinct regimes, namely, establishment of the natural convection and active melting of ice particle exhibiting substantial amount of fluid‐particle interactions. A set of dimensionless parameters have been identified to distinguish between regimes. Finally, we developed a semi‐empirical to predict the melting of any arbitrary shaped ice particle and validated it against the particle‐resolved numerical simulation and experimental results. The comparison showed good agreement. Finally, the presented semi‐empirical model can be used as sub‐grid model in Euler‐Lagrange based numerical models to study the phase change phenomena in particulate flow systems. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3158–3176, 2017     

Creating a smart textile via the self‐assembly of responsive polymer particles on poly(ethylene terephthalate) fibers

A novel method is presented for the construction of a hierarchical microstructure/nanostructure via the chemical self‐assembly of poly(N‐isopropyl acrylamide‐co‐acrylic acid) particles on polyester fibers. The textile of the particle‐bound fibers possessed smart wettability and an oil‐absorbing capacity. The wettability was dependent on the initial contact angle and spreading time of water droplets on the particle‐bound textile, which could be controlled with the content of acrylic acid inserted into the polymer chains. The wettability of the particle‐bound textile was responsive to the pH value. In addition, the textile was superoleophilic on an air–solid surface and could absorb oil quickly from water. The oil‐absorbing capacity could be controlled by the alteration of the pH value. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46834.     

Modeling Growth,Surface Kinetics,and Morphology Evolution in PETN

Pentaerythritol tetranitrate (PETN) is a commonly used energetic material with both military and civilian applications. Good ignition properties mandate a powdered material with a high surface area. However, existing experimental data on PETN powder suggest an active surface that leads to particle coarsening and gradual reduction of the specific surface area over time. In this work we review some of the atomic‐level and coarse‐grained potential models developed for PETN and discuss their applications for studying particle morphology, growth, and surface kinetics, including molecular diffusion and evaporation/condensation rates. Simulation methods include classical molecular dynamics, kinetic Monte Carlo, and transition state calculations.     

Electric field controlled electrospray deposition for precise particle pattern and cell pattern formation

Photolithography, soft lithography, and ink jetting have been used for automated micropattern fabrication. However, most of the methods for microfabrication of surface pattern are limited to the investigation of material properties of substrates with high‐cost and complex procedures. In the present study, we show a simple (single‐step) yet versatile and robust approach to generate biodegradable polymeric particle patterns on a substrate using electrospray deposition through a mask. Various particle patterns including patterned dots, circles, squares, and bands can be easily formed and the features of particle patterns could also be tailored using different masks and electrostatic focusing effects. Furthermore, cell patterns can be achieved on the surface of particle patterns by blocking the areas without particle deposition on the substrate and culturing cells on the substrate. Polymeric particle patterns and cell patterns developed in this study could be used in the high throughput screening of sustained release formulations, cell‐based sensing, and drug discovery. In addition to experimental results, an analysis of the associated electric field is used to investigate quantitatively the nature of focusing effect. Scaling analysis is also applied to obtain the dominate terms in electrospray deposition process. © 2010 American Institute of Chemical Engineers AIChE J, 2010