Single Porous Particle Research Articles

DRUG RELEASE FROM POROUS SPHERICAL PARTICLE: DIFFUSION MODEL WITH AN INTERMEDIATE COMPLEX FORMATION

This publication proposes a new model of drug release from a spherical particle that takes into account the transfer of the drug and the porous particle into the solution by forming an intermediate complex with the bio carrier, one of the components of the solution. The mathematical model is reduced to a dimension-less form that is convenient for qualitative analysis of the process. The problem is realized numerically. The initial stage of filling a porous particle with a bio carrier and stage of drug release into environment were analyzed. Two main types of kinetic curves corresponding to diffusive and convective modes were revealed. The kinetic curves with the initial stage of drug release delay were revealed. The model was in effects (unobvious at first glance) related to the interaction of opposite factors affecting the drug release. The different types of kinetic curves obtained on its basis correspond to different observational conditions, types of particles, and properties of bio fluids. This model can be improved to describe and predict drug release kinetics not only from single porous particles, but also from containers of more complex shape.

Study and comparison of different drying processes of porous particle at high temperature

This work presents a comparative study of four drying processes of a porous media: superheated steam drying at atmospheric pressure (APSSD), humid air (HA), low pressure superheated steam drying (LPSSD) and vacuum drying (VD). A single porous particle model has been developed to simulate the four drying processes. The model is based on the method of averaging volume. Spherical porous particles of coal are used as the model material in this paper. The evaporation rates are equal for these processes in point called the inversion temperature. This temperature was calculated during the constant rate period (CRP) and the falling rate period (FRP). A variation in the values of inversion temperature was observed (363-503 K). The effect of drying parameters such as: particle radius, gas mass flux, permeability, porosity and operating pressure were investigated. Several researchers have reported the noticeable variation of the inversion temperature values according to the drying period used to calculate this key parameter. Our results are compared with those obtained from a front model reported in the literature. A good agreement is found.

Drag and heat transfer characteristics around and through two interactive porous particles

When the concentration of porous particles reaches a certain level, the interactions between the particles can't be ignored. Therefore, this paper numerically investigated the interactions between two porous particles using the lattice Boltzmann method. In this paper, two-dimensional steady flow and heat transfer around and through two porous particles with the same diameters (D) was studied numerically. The effects of Reynolds number (Re), Darcy number (Da), center-to-center distance (expressed as dimensionless form L/D), angle between two porous particles (β) and particles with different permeability on the flow and heat transfer characteristics were investigated in detail. The investigated ranges of the parameters were 10 ≤ Re ≤ 40, 10−6 ≤ Da ≤ 10−2, D ≤ L ≤ 4D and 0° ≤ β ≤ 90°. It is observed that these parameters have significant effects on the flow and temperature fields, drag coefficient and average Nusselt number. The drag coefficient and average Nusselt number of the leading particle (P1) is much larger than those of the trailing particle (P2) in most instances. And the two porous particles show different change tendency to the same changes of these parameters. The effects of Da on P1 are more prominent compared with P2 while the effects of L/D on P2 are more obvious compared with P1. Besides, the drag coefficient of P2 increases with β increasing. In addition, we define drag force ratio and heat transfer enhancement ratio to compare the sensitivity of particles to the changes of Re and/or Da when two particles coexist and when one particle exists alone. The results indicate the heat transfer efficiency of P1 is more sensitive to changes in Da and Re compared with a single porous particle.

Confocal Raman Microscopy Enables Label-Free, Quantitative, and Structurally Informative Detection of DNA Hybridization at Porous Silica Surfaces.

Characterization of DNA at solid/liquid interfaces remains a challenge because most surface-sensitive techniques are unable to provide quantitative insight into the base content, length, or structure. Surface-enhanced Raman scattering measurements of DNA hybridization on plasmonic-metal substrates have been used to overcome small Raman-scattering cross-sections; however, surface-enhanced Raman spectroscopy measurements are not generally quantitative due to the fall-off in the scattering signal with the decay of the electric field enhancement from the surface, which also limits the length of oligonucleotides that can be investigated. In this work, we introduce an experimental methodology in which confocal Raman microscopy is used to characterize hybridization reactions of ssDNA immobilized at the solid/liquid interface of porous silica particles. By focusing the femtoliter confocal probe volume within a single porous particle, signal enhancement arises from the ∼1500-times greater surface area detected compared to a planar substrate. Because the porous support is a purely dielectric material, the scattering signal is independent of the proximity of the oligonucleotide to the silica surface. With this technique, we characterize a 19-mer capture strand and determine its hybridization efficiency with 9-mer and 16-mer target sequences from the scattering of a structurally insensitive phosphate-stretching mode. Changes in polarizability and frequency of scattering from DNA bases were observed, which are consistent with Watson-Crick base pairing. Quantification of base content from their duplex scattering intensities allows us to discriminate between hybridization of two target strands of equivalent length but with different recognition sequences. A duplex having a single-nucleotide polymorphism could be distinguished from hybridization of a fully complementary strand based on differences in base content and duplex conformation.

Numerical investigation of lithium-sulfur batteries by cyclic voltammetry

Abstract The lithium-sulfur batteries have the potential to become a leading energy storage system as they are showing promising characteristics such as high theoretical capacity and energy density. However, their performance is hindered by poor cyclability, self-discharge and capacity loss. Numerical simulations of the processes occurring in lithium-sulfur batteries might bring a better understanding of the governing mechanisms, which will allow the necessary development of the applicable cells. This paper presents a custom electrochemical model of a lithium-sulfur cell implemented into Ansys Fluent. The model describes a multi-component transport of species through Nernst-Plank equations in a dilute solution. The redox mechanism was described through a multi-step mechanism, with kinetics being defined through the Butler-Volmer equation. The model was used to model cyclic voltammetry at a planar electrode and at single porous particles. The results indicate that the cyclic voltammetry response of the whole porous sulfur cathode could be solved as a response of a single particle, which might lead to a simplification of numerical simulations.

Light-driven motion of self-propelled porous Janus particles

We introduce a versatile mechanism of light-driven self-propelled motion applied to porous Janus-type particles. The mechanism is based on the generation of local light-driven diffusio-osmotic (l-LDDO) flow around each single porous particle subjected to suitable irradiation conditions. The photosensitivity is introduced by a cationic azobenzene containing surfactant, which undergoes a photoisomerization reaction from a more hydrophobic trans-state to a rather hydrophilic cis-state under illumination with light. The negatively charged porous silica particles are dispersed in a corresponding aqueous solution and absorb molecules in their trans-state but expel them in their cis-state. During illumination with blue light triggering both trans-cis and cis-trans isomerization at the same time, the colloids start to move due to the generation of a steady-state diffusive flow of cis-isomers out of and trans-isomers into the particle. This is because a hemi-spherical metal cap partially sealing the colloid breaks the symmetry of the otherwise radially directed local flow around the particle, leading to self-propelled motion. Janus particles exhibit superdiffusive motion with a velocity of ∼0.5 μm/s and a persistence length of ca. 50 μm, confined to microchannels the direction can be maintained up to 300 μm before rotational diffusion reverts it. Particles forming dimers of different shapes can be made to travel along circular trajectories. The unique feature of this mechanism is that the strength of self-propulsion can be tuned by convenient external optical stimuli (intensity and irradiation wavelength) such that a broad variety of experimental situations can be realized in a spatiotemporal way and in situ.

Behavior of a porous particle in a radiofrequency plasma under pulsed argon ion beam bombardment

The behavior of a single porous particle with a diameter of 250 μm levitating in a radiofrequency (RF) plasma under pulsed argon ion beam bombardment was investigated. The motion of the particle under the action of the ion beam was observed to be an oscillatory motion. The Fourier-analyzed motion is dominated by the excitation frequency of the pulsed ion beam and odd higher harmonics, which peak near the resonance frequency. The appearance of even harmonics is explained by a variation of the particles's charge depending on its position in the plasma sheath. The Fourier analysis also allows a discussion of neutral and ion forces. The particle's charge was derived and compared with theoretical estimates based on the orbital motion-limited (OML) model using also a numerical simulation of the RF discharge. The derived particle's charge is about 7–15 times larger than predicted by the theoretical models. This difference is attributed to the porous structure of the particle.

Drying Kinetics of a Porous Spherical Particle and the Inversion Temperature

A model is presented for drying of a single porous particle with superheated steam and humid air. Experimental data for spherical porous ceramic particle reported in the literature were used for the validation of the model. An inversion temperature at which the evaporation rates within superheated steam and humid air are equal was predicted. The effect of thermophysical properties of the particle (permeability 10−14 − 10−17 m2, diameter 3 × 10−3 − 10 × 10−3 m) and operating variables (gas mass flux 0.26 − 0.78 kg m−2 s−1, drying agent temperature 120–200°C) is tested. The inversion temperature is shown to be affected by the thermophysical properties of the porous particle and of the drying agent.

Modern Modelling Methods in Drying

Several modern modelling techniques are presented as tools for drying science and technology, namely pore networks, discrete element method and population balances. After first presenting results from their own research, the authors indicate what future contributions to a better understanding of the drying process at different levels—single porous particles, agitated and fluidised beds—may be expected.

Computational Modeling of CO/CO2 Ratio Inside Single Char Particles during Pulverized Coal Combustion

A recently developed model was used to study the CO/CO2 ratio inside a burning pulverized coal particle, to better understand the effect of bulk gas composition on the equilibrium partial pressure of reduced metal species at the surface of ash inclusions. The motivation was to improve the ability to model submicrometer particle formation by ash vaporization, as a function of furnace conditions. Assumptions for the CO/CO2 ratio that have been made in previous studies are compared to predictions from a psuedo-steady-state model for a single porous particle that considers homogeneous and heterogeneous reaction kinetics and mass transfer both in particle pores and in the boundary layer. This is the first publication of model predictions for the CO/CO2 ratio as a function of radius for a coal char particle in a furnace with a bulk gas CO2 concentration in the range of 0%−79%. A method is proposed for summarizing the effects on the CO/CO2 ratio that are due to changes in the bulk furnace gas O2 and CO2 concentration, furnace temperature, and particle size, using an empirical equation that is suitable for incorporation as a submodel into comprehensive computational fluid dynamics-based codes for combustion simulation. Trends from the model simulations show general agreement with experimental data; however, the accuracy of the predictions is limited by the lack of fuel-specific input data.

Drying kinetics of single porous particles in superheated steam under pressure

The kinetics of single porous particles of ceramic and lignite in pressurised superheated steam was investigated experimentally. Particles of diameters in the range 10–14 mm were used. The experiments were performed over a range of pressure, from 1.7 to 8.4 bar, and a range of superheated steam temperature, from 155 to 197 °C. The steam velocity was also varied, from 1.1 to 4.3 m/s. The results from these experiments show that the rate of drying is limited by the rate of external convective heat transfer. The operating pressure appears to have no influence on the drying rate. The experimental results, however, show that the equilibrium moisture content is higher in a higher pressure steam and that this equilibrium is attained earlier when drying is carried at higher pressure. A mathematical model for the drying of single particles in pressurised superheated steam was also developed and tested against experimental results of ceramic drying. This model is based on the receding core assumption and is solved using the Crank–Nicholson method. Prediction of drying rate, drying time and moisture content in the case of porous ceramic particles is encouraging. In the case of lignite particles the model always under-predicts the drying rate and this is thought to be due to cracking of the lignite particles which increases the area for heat transfer.

Measuring and modelling gas adsorption kinetics in single porous particles

Mass transfer in single porous particles is studied by measuring uptake rates in porous single particles and by modelling these rates with a non-isothermal, non-isobaric Maxwell–Stefan model. Uptake curves of carbon dioxide, methane and ethane in specially prepared activated carbon extrudates were measured in a gravimetric analyser in a broad range of temperatures, concentrations and pressures. The model describes mass transfer due to pore diffusion (bulk and Knudsen diffusion), viscous flow and surface diffusion. To identify the different types of mass transport in the porous particles two types of kinetic uptake measurements were performed: responses to pure component pressure steps and responses to concentration steps at constant feed pressure. All transport mechanisms contribute significantly to the total molar flux. Surface diffusion is an important transport mechanism at low temperatures and at small step changes in both concentration and pressure. Viscous flow is not only important in experiments with a total pressure change but also in concentration step experiments at constant total pressure.

Steam drying of coal. Part 2. Modeling the operation of a fluidized bed drying unit

Drying of coal in steam offers, besides reduction of fire and explosion hazards, improved thermal efficiency. In addition, steam-dried coals appear to be less prone to spontaneous combustion. In a previous paper, a mathematical model was developed for the drying of a single porous particle in steam. In this paper, the single particle model has been integrated with a two-phase hydrodynamic model to simulate the continuous drying of coal in a bed fluidized with superheated steam. The model incorporates: (i) variation of particle temperature and moisture content with residence time within the bed; (ii) the effect of the initial condensation during the heating period of coal fed to the fluidized bed; and (iii) variation in the superficial gas flow—arising from the vaporization of moisture from the wet coal or the condensation of inlet steam—for various operating conditions. Model studies show that, for a specified reactor size, system performance is most sensitive to the steam-tube duty and the initial moisture content of the wet coal. The conceptual problems in defining an inversion temperature, above which vaporization rate in steam is greater than that in air, for the continuous drying of porous particles are discussed. It is shown that the inversion temperature is a consequence of departure from thermal equilibrium between the gas and solid phases. Model studies show that the inversion temperature depends on design and other operating parameters.

99/02848 High pressure steam drying of single porous particles

99/02848 High pressure steam drying of single porous particles