Intraparticle Reaction Research Articles

The promotion effect of pyrolysis conditions on alkali metal pretreatment during pyrolysis of sub-bituminous Zhundong coal: Carbon engulfment control and subsequent sodium transformation

Alkali metal pretreatment in pyrolysis is proposed to address alkali-induced issues in high-alkali coal utilization, on the basis of staged fuel combustion. However, the newfound carbon engulfment remains an intractable problem. This work investigated the effect of pyrolysis conditions on enhancing alkali metal pretreatment during Zhundong coal pyrolysis over 500–1000 °C in a laboratory fixed-bed apparatus, which focused on heating rate, volatile-char interaction, and CH4 atmosphere. Multiple analytical methods were integrated to relate the facilitation effect of pyrolysis conditions on sodium pretreatment with the suppression of carbon engulfment. The results indicated carbon engulfment primarily blocked mesopores with a size <5 nm to retain large quantities of water-soluble sodium species. It resulted from coal swelling induced by the fusibility of semi-fusinites. Fortunately, volatile-char interaction and CH4 atmosphere eliminated carbon engulfment through the intra-particle reaction between gasification agents/active radicals and char matrix. They fully eliminated carbon engulfment at 800 °C, which mainly reopened mesopores with a diameter of 4–5 nm. They also promoted the formation of sodium silicates/aluminosilicates. A slow heating rate weakened carbon engulfment by limiting the fusibility of semi-fusinites, but to a lower extent. Volatile-char interaction and CH4 atmosphere were proposed as universal enhancements for alkali metal pretreatment.

Solvent-Free Synthesized Monolithic Ultraporous Aluminas for Highly Efficient Removal of Remazol Brilliant Blue R: Equilibrium, Kinetic, and Thermodynamic Studies.

In this study, ultraporous aluminas (UPA) were synthesized as new effective adsorbents for Remazol Brilliant Blue R (RBBR) removal from aqueous solutions. The UPA monoliths were grown via facile oxidation process, followed by isochronous annealing treatment in air at different temperatures, through which γ, θ, and α phase polycrystalline fibrous grains of UPA can be accordingly obtained. The experimental factors that affect the material adsorption performances including initial pH, contact time, and temperature were comprehensively studied by batch experiments. The RBBR adsorption isotherms of UPA(γ) and UPA(θ) powders were found almost identical, while UPA(α) powders showed low effectiveness. To obtain the desirable mechanical stability of the UPA monolith with considerable RBBR adsorption capacity, UPA(θ) powders were further studied. The UPA(θ) powders exhibited maximum RBBR adsorption at pH 2 due to the positively charged surface under acidic conditions. Compared with the Lagergren pseudo-first-order model, the pseudo-second-order model was found to explain the adsorption kinetics better. Despite the film diffusion dominating the adsorption process, the contributions of the intraparticle diffusion and chemical reactions were also found significant. The adsorption equilibrium data at different temperatures were fitted by the Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D–R) isotherm models. The Langmuir model was found the most effective in the description of equilibrium data, and the maximum RBBR adsorption capacity retained by UPA(θ) powders was 122.55 mg·g−1 at 295 K. Thermodynamic parameters (ΔG0, ΔH0, and ΔS0) indicated the adsorption process was spontaneous and exothermic in nature.

Numerical Analyses of Heterogeneous CLC Reaction and Transport Processes in Large Oxygen Carrier Particles

Heterogeneous chemical looping combustion (CLC) reactions and conjugate transports in large oxygen carrier particles were numerically investigated with computational fluid dynamics (CFD) approaches, in which a simplified noncatalytic reaction model was implemented for reducing intraparticle modelling computation. Volumic gas-solid reactions were treated as surface reactions based on the equivalent internal surface in the particle model. In large porous particles such as fixed bed CLC reactors, the heterogeneous reactions are often limited by intraparticle diffusion. Comprehensive analyses were conducted on transports across the particle surface and their influences on reactions inside the single particles. A threshold Reynolds number of external convections was found for the enhancement of intraparticle reactions. The heterogeneous reactions, intraparticle diffusions and interstitial transports in a fixed bed CLC reactor randomly packed with 597 spheres were thoroughly analysed with the same numerical approaches. Comprehensive insights of the temporal evolution and spatial distribution of scalars in the packed bed reactor were presented.

Controlling reaction selectivity for sugar fatty acid ester synthesis by using resins with different basicities

A strongly basic ion-exchange resin catalyst was reported to exhibit a high catalytic activity in transesterification to produce a bio-based surfactant, sugar ester under mild condition. However, the side-reactions to decompose the reactant and the product were found to occur. This study was aimed to improve the selectivity of sugar ester synthesis by newly focusing on the basicity of the resin. A weakly basic resin (Diaion WA20) with a lower mass transfer resistance suppressed the decompositions while maintaining synthesis rate. Controlling molar ratio of the reactants in the intraparticle reaction field also increased the reaction selectivity, 72.1% and product yield, 57.5%. Both values were drastically increased compared to the reported values with the strongly basic resin (selectivity 50.9%, yield 14.3%). This is the first knowledge to show a high catalytic activity of weakly basic resin. These results suggest that a more efficient continuous production process would be possible.

On the fate of oxygenated organic molecules in atmospheric aerosol particles.

Highly oxygenated organic molecules (HOMs) are formed from the oxidation of biogenic and anthropogenic gases and affect Earth's climate and air quality by their key role in particle formation and growth. While the formation of these molecules in the gas phase has been extensively studied, the complexity of organic aerosol (OA) and lack of suitable measurement techniques have hindered the investigation of their fate post-condensation, although further reactions have been proposed. We report here novel real-time measurements of these species in the particle phase, achieved using our recently developed extractive electrospray ionization time-of-flight mass spectrometer (EESI-TOF). Our results reveal that condensed-phase reactions rapidly alter OA composition and the contribution of HOMs to the particle mass. In consequence, the atmospheric fate of HOMs cannot be described solely in terms of volatility, but particle-phase reactions must be considered to describe HOM effects on the overall particle life cycle and global carbon budget.

Micromechanical modelling of mechanochemical processes in heterogeneous materials

There is a range of practical problems where advanced engineering heterogeneous materials undergo chemical transformations. The primary example of such system is energy storage materials, in particular anodes of Li-ion batteries containing active Si particles. The exploitation of such anodes involves extreme volumetric expansion of the active particles during the chemical reaction. The expansion is causing mechanical stress, which, in turn, influences the kinetics of chemical reactions even up to their arrest. A particular reaction between Si and Li is localised, as well as a number of other reactions, such as oxidation or precipitate formation. The model presented in this paper accounts for the kinetics of the reactions in a collection of particles inside a matrix material. The microstructure is modelled using the multiscale mean-field (MF) framework based on the incremental Mori–Tanaka (IMT) method. This is the first application of a multiscale MF technique to modelling reaction front kinetics in particles and linking the intra-particle kinetics with the response of the matrix. A number of physical effects arising from the influence of the deformation mechanisms of the matrix on the kinetics of the intra-particle reactions is investigated. Furthermore, the applicability of the proposed model and the IMT homogenisation scheme is studied by comparison to the full-field simulations in the cases of small and finite strains.

Combined organic acid leaching and torrefaction as pine wood pretreatment before fast pyrolysis

Reducing pine wood particle size is beneficial for acid leaching (shorter leaching time, smaller equipment size) and also for pyrolysis (low pyrolysis time, high oil and sugar yields). Torrefaction helps to improve the energy efficiency of grinding. However, it is not well established whether alkali and alkaline earth metals can still be removed effectively, after torrefaction, by leaching with an acetic acid solution, while keeping high oil and sugar yields. To investigate this, an experimental study was carried out combining torrefaction (290?C, 20 minutes) and subsequently acid leaching of pine wood as pretreatment step before fast pyrolysis of the feedstock at 530?C. The oil, char, gas, water, pyrolytic lignin, light oxygenates and levoglucosan yields were compared with the results obtained from fast pyrolysis of untreated pine wood and acid leached pine wood at 530?C. In addition, pyrolysis vapors were condensed step-wise into two distinguished fractions (condensation T = 80?C) to increase the levoglucosan concentration up to 45 wt.% with or without applying torrefaction as pretreatment. Intra-particle reactions during torrefaction are more profound in the presence of alkali and alkaline earth metals. Cellulose and lignin derived products (e.g. levolucosan (~0.2 kg/kg pine wood) and pyrolytic lignin (~0.11 kg/kg pine wood) were not affected by the torrefaction pre-treatment of acid leached pine wood. In general, torrefaction of acid leached pine wood followed by fast pyrolysis showed similar total organics, char, gas, light oxygenates and produced water yields compared to fast pyrolysis of acid leached pine wood. Contrary, when acid leaching is applied after torrefaction the organics (dry bio-oil) and light oxygenates yields are quite comparable to the results of untreated pine wood fast pyrolysis.

Micro-heterogeneous regimes for gasless combustion of composite materials

ABSTRACTReactive Ni/Al composite particles with different internal microstructures were fabricated by ball milling (BM). The propagation of gasless combustion waves through the compacted composite particle media was investigated using high-speed microscope video recording (HSMVR), with a resolution of 10 μm/pixel and 21.25 μs/frame. The microstructural combustion-wave characteristics, including hesitation time, propagation step size, instantaneous velocity, intraparticle reaction time, and average combustion-wave velocity, were studied as functions of measured internal microstructural parameters. The micro-heterogeneous relay-race combustion mechanism prevails across the investigated conditions. Decreasing the metal layer thicknesses in the composite particles leads to significant decrease in hesitation time, while only weakly affecting the instantaneous velocity. Characteristic times of hesitation and thermal relaxation defined two combustion front propagation regimes limited by interparticle heat transfer and by chemical reaction kinetics. Understanding the existence of these two discrete regimes allows us to effectively control the combustion parameters in this high-energy-density system.

Influence of temperature and particle size on structural characteristics of chars from Beechwood pyrolysis

This work investigates the effect of temperature and particle size on the product yields and structure of chars obtained from the pyrolysis of Beechwood Chips (BWC), a lignocellulosic biomass. BWC of three different size fractions (0.21–0.50 mm, 0.85–1.70 mm and 2.06–3.15 mm) were pyrolyzed at atmospheric pressure and temperatures ranging from 300 to 900 °C in a fixed bed reactor. Tar and gas yields increased with increasing temperature, while char yield decreased, particularly between 300 and 450 °C. The effect of particle size was mostly observed at temperatures lower than 400 °C as a larger char yield for larger particles due to intraparticle reactions. At higher temperatures the larger surface area in the char fixed bed favoured reactions increasing char and gas yields from the smaller particles. Pyrolysis chars were characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and Raman spectroscopy. Loss in oxygenated functional groups and aliphatic side chains with increasing temperature was revealed, along with an increase in the concentration of large aromatic systems, leading to a more ordered char structure but no significant graphitization. The changes in char nature at high temperature led to a loss in their combustion reactivity. Raman spectra indicated that the temperature needed to completely decompose the cellulose structure increased with biomass particle size and the enhanced intraparticle reactions in pyrolysis of large particles was likely to give rise to amorphous carbon structures with small fused ring systems.

Effects of pyrolysis atmosphere on the porous structure and reactivity of chars from middle and high rank coals

The influence of a CO2 or N2-based atmosphere on the porous structure and microstructure of the chars obtained from the slow devolatilization (10°C/min) of two coals of different rank (Semi-Anthracite (SA) and High Volatile Bituminous type C (HVBC)) and different particle size distribution was studied. Physicochemical characterization (ultimate and proximate analysis), structural and morphological characterization by Raman spectroscopy, FE-SEM, BET surface area, and volume and diameter microporous by CO2 adsorption measurements were carried out for all the chars. It was found that the kinetic parameters, the physicochemical properties, and reactivity of the chars are different, depending on the pyrolysis atmosphere. It was also determined that for char from SA coal with particle size greater than 0.7mm, the BET surface area increases when the atmosphere is enriched in CO2. This effect appears to be promoted by the interaction of different processes such as intraparticle side reactions (softening, nucleation and coalescence of bubbles, crosslinking, among others), differences in the thermal diffusivity of N2 and CO2, and the reactive effects of the latter. Additionally, tests of oxidative reactivity of chars showed that the char formed in a CO2 atmosphere is more reactive than that formed in N2. With the results of Raman analysis and kinetic parameters quantified, it was concluded that the reaction atmosphere determined the degree of ordering achieved by the char structure and that the thermo-diffusive properties of the reaction atmosphere promoted structural differences in the char even at low heating rates.

Numerical investigation of the effects of some selected thermo-physical properties on products evolution and yields during biomass pyrolysis

ABSTRACTEffects of some selected biomass thermo-physical properties (biomass density, biomass thermal conductivity and char thermal conductivity) on the evolution and yields of pyrolysis products have been investigated. Wood cylinders were modelled as porous solids. Transport, solid mass conservation, intra-particle pressure generation and energy conservation equations were coupled and simultaneously solved to simulate the pyrolysis process. The first-order implicit Euler method was used to solve solid mass conservation equations. The transport, energy conservation and intra-particle pressure generation equations were discretized by the finite volume method. The generated set of linear equations was solved using the tri-diagonal matrix algorithm. Effects of selected thermo-physical properties were investigated on weight loss history, products evolution, intra-particle temperature gradient and tar secondary reactions. Total yields of tar and gas were then estimated. Results show that the selected thermo-physical properties influence weight loss history, product evolution, tar intra-particle secondary reactions and total product yields. The study also revealed that biomass density has the highest effect on product evolution and yields during pyrolysis.

Gasification Effect of Metallurgical Coke with CO2 and H2O on the Porosity and Macrostrength in the Temperature Range of 1100 to 1500 °C

The gasification of metallurgical cokes with CO2 and H2O on their porosity and macrostrength were investigated in the current study. Cokes were reacted with CO2 or H2O in the temperature range of 1100 to 1500 °C. During the reaction, the compositional change of product gases were measured by quadruple mass spectrometry (QMS) for evaluating the gasification rate. Image analysis was carried out to measure the porosity according to the distance from the coke surface. The porosity at the surface of coke gasified with CO2 indicated low values due to its low reactivity, which resulted in the intraparticle reaction to diffuse into the pores at low temperatures while the coke gasified with H2O showed the tendency to react with the coke at the surface. This difference in the reaction behavior can be explained by Thiele modulus. Furthermore, fine powder formation and tensile strength were measured to evaluate macrostrength of cokes. According to the results, the different reaction mode caused an obvious difference ...

Role of Pretreatments in the Thermal Runaway of Hazelnut Shell Pyrolysis

This study examines the effects of various pretreatments (size reduction, pelletization, hot water washing, torrefaction) on the pyrolysis of packed beds of hazelnut shells, exposed to moderate heating along the lateral surface, aimed at identifying the controlling or predominant mechanisms for the high reaction exothermicity leading to thermal runaway. As long as the particles preserve their chief structural properties (roughly or finely crushed shells), the process dynamics are not altered by size variation. Instead exothermic effects are highly reduced or almost disappear for milled shells, even in the pelletized form, depending on the actual powder size. Hence, secondary intraparticle reactions, owing to a peculiar scarcely porous microstructure, play a paramount role in the process exothermicity and thermal runaway. The displacement of the beginning of the reaction process at higher temperature, following sample washing, is enhanced by deepening the pretreatment. In this way sample conversion takes place in the presence of large spatial gradients, which hinder the occurrence of thermal runaway. Finally torrefaction, by partial or complete conversion of the more thermally labile chemical components, also eliminates the conditions that trigger and partly sustain the thermal runaway.

Enrichment and encapsulation of uranium with iron nanoparticle.

The ability to recover uranium from water is significant because of its potential applications on nuclear fuel capture and mitigation of nuclear wastes. In this work, a unique nanostructure is presented by which trace level (2.32-882.68 μg/L) uranium can be quickly separated from water and encapsulated at the center of zero-valent iron nanoparticles. Over 90% of the uranium is recovered with 1 g/L nanoparticles in less than 2 min. Near atomic-resolution elemental mapping on the U(VI) intraparticle reactions in a single iron nanoparticle is obtained with aberration corrected scanning transmission electron microscopy, which provides direct evidence on U(VI) diffusion, reduction to U(IV), and deposition in the core area.

Acetone hydrogenation in exothermic reactor of an isopropanol–acetone–hydrogen chemical heat pump: effect of intra-particle mass and heat transfer

Intra-particle mass and heat transfer plays an important role in performance of the exothermic fixed-bed reactor for an isopropanol–acetone–hydrogen chemical heat pump. In this work, an exothermic fixed-bed reactor model, taking into account the actual packing structure, is established in the commercial software Fluent. A 120° segment of a tube with tube-to-particle diameter ratio (n) of 4, where realistic particles are packed and set to porous media, is used to simulate the 3D external flow, concentration and temperature fields in the exothermic packed-bed reactor. The influence of catalyst particle diameter (d p) and micropore diameter (d 0) on the intra-particle temperature, species distribution, reaction rate and selectivity is discussed. The appropriate d p and d 0 are obtained. Simulation results showed that intra-particle temperature gradient is not obvious. Large d p and small d 0 lead to remarkable gradient of reaction rate inside the catalyst particle and the decrease in the catalyst efficiency and reduce the acetone conversion and the selectivity in isopropanol. The optimal results reveal that the spherical catalyst with d p of 1 mm and d pore of 10 nm is appropriate for high-temperature acetone hydrogenation.

Sequestration of Arsenate in Zero-Valent Iron Nanoparticles: Visualization of Intraparticle Reactions at Angstrom Resolution

Reactions of arsenate [As(V)] in a single nanoscale zero-valent iron (nZVI) particle are mapped by spherical aberration-corrected scanning transmission electron microscopy integrated with X-ray energy dispersive spectroscopy. Near-atomic-resolution distributions of arsenic, iron, and oxygen demonstrate the diffusion and reactions of arsenic in nZVI. The reactions occur primarily in the surface layer, especially at the oxide–metallic iron [Fe(0)] interface. As(V) is attracted to the surface via electrostatic interactions and embedded in the nanoparticle via chemical reduction to form a thin (1.0–1.2 nm) layer of elemental arsenic inside the shell. Results offer unprecedented details about the intraparticle reaction mechanisms and demonstrate intrinsic advantages of nZVI for arsenic encapsulation, treatment, and remediation.

A multidimensional population balance model to describe the aerosol synthesis of silica nanoparticles

The aim of this work is to present a new detailed multivariate population balance model to describe the aerosol synthesis of silica nanoparticles from tetraethoxysilane (TEOS). The new model includes a chemical representation of the silica particles to facilitate a detailed chemical description of particle processes. Silica nanoparticles are formed by the interaction of silicic acid monomers (Si(OH) 4) in the gas-phase as reported in a previous study. A multidimensional population balance model is developed where each particle is described by its constituent primary particles and the connectivity between these primaries. Each primary, in turn, has internal variables that describe its chemical composition, i.e., the number of Si, free O and OH units. Different particle processes, such as inception, surface reaction, coagulation, sintering, and intra-particle reactions, are formulated from first-principles that alter the particle ensemble and are two-way coupled to the gas-phase. The free parameters in the model are estimated by fitting the model response to experimental values of collision and primary particle diameters using low discrepancy Sobol sequences followed by the simultaneous perturbation stochastic approximation algorithm. The simulation results are finally presented at different process conditions. A strong dependence of particle properties on process temperature and inlet concentration is observed. The desirable operating conditions for different industrial applications are also highlighted. This work illustrates the significance of adopting a multidimensional approach to understand, and hence control, complex nanoparticle synthesis processes.

Kinetics of Hydrolysis of Chloride Salts in Model Crude Oil

Chloride salts are difficult to remove from heavy crude oils and bitumens. In the presence of steam, the chloride salts form hydrochloric acid, causing significant corrosion in the downstream equipment. The rate and the extent of the hydrolysis reactions for magnesium, calcium, and sodium chloride salts were measured by contacting with flowing steam at 100°C to 350°C. The extent of hydrolysis was controlled by salt cations and by the external particle area. The non-volatile products of the reactions were metal hydroxychlorides. Intraparticle reaction of the water limited the rate of hydrolysis, therefore, the water of hydration was most reactive.

Tailoring of Phase Composition and Photoresponsive Properties of Ti-Containing Nanocomposites from Layered Precursor

Titanium-containing nanocomposites have been synthesized by calcination of a Ni−Ti layered double hydroxide as precursor. The nanocomposites were characterized by powder X-ray diffraction (PXRD), high-resolution transmission electron microscopy (HRTEM), the Brunauer−Emmett−Teller (BET) method, and UV−vis diffuse reflection spectroscopy (DRS). The transformation of the metastable Ni2TiO4 spinel phase to the stable NiTiO3 ilmenite phase was observed during the thermal process. The phase transformation at a calcination temperature below and at 800 °C belongs to the intraparticle reaction, while a phase separation occurs at 900 °C. UV−vis DRS indicates that the nanocomposites are not only responsive but also activated in both the UV- and visible-light regions. Their UV−vis-induced photocatalytic activities were evaluated by the degradation of methylene blue. The results show that the Ti-containing nanocomposites exhibit good photocatalytic activity under UV−vis-light irradiation.

Synthesis of Polymeric Nanoparticles by Cross-Linking Copolymerization

Soluble cross-linked nanoparticles were prepared by free radical copolymerization of styrene (ST, M1) and ethylene glycol dimethacrylate (EGDM, M2) monomers in homogeneous solution. Spherical nanoparticles were obtained in the early stage of polymerization, and no gelation occurred up to 66% of yield. The isolated particles are decorated with pendant vinyl groups that are target for further polymerization. The molar feed ratio was ST/EGDM = 5/5, the monomer concentration 0.556 mol/dm3 and the initiator concentration 5 mol %. It was found that as the conversion increased, the ratio of pendant vinyl groups reached a maximum value; however, the size of nanoparticles continued to increase with the reaction time. The molecular weight distributions were monomodal at short reaction times (30 and 60 min), however, at longer reaction times (90, 120, and 180 min) these became multimodal. This was caused by intraparticle reactions (branching and cross-linking) and possibly also by interparticle reactions (coupling). The reactive vinyl groups were detected by nuclear magnetic resonance spectroscopy (NMR). The size of nanoparticles in swollen state was determined by dynamic laser light scattering (DLS) method and in dried form by scanning electron microscopy (SEM). Molecular weight and distributions were measured by gel permeation chromatography (GPC).