Reduced Time Plots Research Articles

Reactive Molecular Dynamics Simulations of Polystyrene Pyrolysis.

Polymers' controlled pyrolysis is an economical and environmentally friendly solution to prepare activated carbon. However, due to the experimental difficulty in measuring the dependence between microstructure and pyrolysis parameters at high temperatures, the unknown pyrolysis mechanism hinders access to the target products with desirable morphologies and performances. In this study, we investigate the pyrolysis process of polystyrene (PS) under different heating rates and temperatures employing reactive molecular dynamics (ReaxFF-MD) simulations. A clear profile of the generation of pyrolysis products determined by the temperature and heating rate is constructed. It is found that the heating rate affects the type and amount of pyrolysis intermediates and their timing, and that low-rate heating helps yield more diverse pyrolysis intermediates. While the temperature affects the pyrolytic structure of the final equilibrium products, either too low or too high a target temperature is detrimental to generating large areas of the graphitized structure. The reduced time plots (RTPs) with simulation results predict a PS pyrolytic activation energy of 159.74 kJ/mol. The established theoretical evolution process matches experiments well, thus, contributing to preparing target activated carbons by referring to the regulatory mechanism of pyrolytic microstructure.

Elucidating of the influence of tin and cadmium in the Bi–Pb alloy on its thermal and nuclear properties as a coolant in fast neutron reactors

The purpose of this research is to investigate the physical properties and shielding characteristics of prepared (Bi–Pb) and (Bi–Pb–Sn–Cd) alloys, to be used as a coolant in a liquid metal nuclear fast reactor. The physical properties density, kinetics and mechanism of melting applying the reverse Johnson–Mehl–-Avrami rule and reduced time plot were applied, showing that melting processes take place in two dimensions, enthalpy and heat capacity were measured on the other hand for the prepared alloys within the solid and liquid states, respectively. Moreover, the results indicated that the bulk density and melting temperature of (Bi–Pb–Sn–Cd) alloys are lower than (Bi–Pb) alloys. Likewise, the thermal properties of (Bi–Pb–Sn–Cd) alloys seemed more suitable applicants than that of (Bi–Pb) alloys. Additionally, eight different gamma-ray energy lines in the range 0.12178–1.40792 MeV were used to determine the mass attenuation for the alloy samples. Also, gamma radiation attenuation characteristics, the half-value layer and effective atomic number were determined using Phy-x/PSD software. Furthermore, three types of neutron energies (slow neutrons, total slow neutrons, as well as neutrons of energies greater than 0.4 eV) were used to determine the macroscopic neutron cross-sections for the prepared alloys. The calculated values of mass attenuation coefficients exhibited a very good agreement with the experimental values of those parameters. The values of neutron macroscopic cross sections at fast neutrons for (Bi–Pb) and (Bi–Pb–Sn–Cd) alloys are small, this means that there is no consequence on fast neutrons used in a nuclear breeder reactor. Finally, we may reach the conclusion that Bi–Pb–Sn–Cd alloys are a possible candidate to be used as a coolant in fast nuclear reactors, as well as it is most suitable as an outer cooling and shielding blanket vessel as well.

Thermal decomposition of un-irradiated and γ-ray irradiated holmium acetate tetrahydrate. Part 1: kinetics of nonisothermal dehydration of un-irradiated and γ-ray irradiated Ho(CH3COO)3⋅4H2O

Abstract Nonisothermal dehydration of un-irradiated and γ-ray irradiated holmium acetate tetrahydrate with 103 kGy total γ-ray dose absorbed was studied in air atmosphere. The thermal decomposition experiments were conducted at heating rates of (5, 7.5 and 10°C/min). The results showed that for un-irradiated material, the dehydration process proceeds in two decomposition steps with the elimination of 3.0 and 1.0 moles of H2O, respectively. The apparent activation energy, E a , as given by both linear and nonlinear isoconversional methods showed dependence upon the conversion degree, α, in the range of 0.2–0.75 for the two dehydration steps. In the first dehydration step, the Ea decreases from 228.0 kJ/mol at the beginning of the decomposition to ≈64.0 kJ/mol at the end of the process. In the second dehydration step, the Ea increases from 42.0 to 72.0 kJ/mol by progressively increasing in α. Compared with solid state reaction models, the two reactions are best described by diffusion (D 4) and nucleation (A 3) models for the first and second dehydration steps, respectively. The results derived from nonisothermal data present a reliable prediction of isothermal kinetics. Straight lines and reduced time plots methods were applied for the determination of the kinetic triplet [Ea , ln A, and reaction model f(α)] from predicted isothermal data. For γ-ray irradiated samples of Ho(CH3COO)3⋅4H2O with 103 kGy total absorbed dose, the dehydration proceeds in two overlapped steps controlled by D 3 model. X-ray data showed phase transformation from monoclinic (SG P2/m) to tetragonal phase (SG P4/mmm) by the elimination of water content from the entire structure of Ho(CH3COO)3⋅4H2O. γ-Ray irradiation effects on the thermal decomposition of Ho(CH3COO)3⋅4H2O were evaluated and discussed based on the formation of trapped electrons, point defects, cation and anion vacancies and cluster imperfections in the host lattice of Ho(CH3COO)3⋅4H2O.

Isothermal kinetics approach to investigating the oxidation process of red phosphorus in air

The isothermal oxidation process of red phosphorus (RP) was investigated by thermogravimetric analysis in the temperature range of 320–480 °C. The model-free, model-fitting and reduced-time plot methods were used to research the oxidation kinetics of RP. According to the dα/dt − α and model-free curves in different temperatures, the oxidation temperatures are divided into two ranges: lower temperatures (320–380 °C) and higher temperatures (390–410 °C). The apparent activation energy and oxidation controlling mechanisms in different temperature ranges are obtained. Besides, the trend of the oxidation rate with mass loss or temperature ranges is discussed.

Decomposition properties of PVA/graphene composites during melting-crystallization

The thermal decomposition of PVA and PVA composites during the melting-crystallization process is still unclear due to indistinct changes in chemical compositions. Using graphene as a model, the decomposition properties of PVA and PVA–graphene composites were systematically analyzed under multiple melting-crystallization cycles. And a series of isothermal decomposition experiments around the melting-crystallization temperature were carried out to simulate the corresponding decomposition kinetics. Based on multiple cycle melting-crystallization, the weight loss of PVA and PVA/graphene composites was successfully quantified. Further morphology investigation and chemical structure analysis indicated that the decomposition was non-uniformly distributed, rendering the possibility of crystallization for PVA and PVA/graphene composites after multiple heating–cooling cycles. In addition, isothermal decomposition analysis based on reduced time plot approach and model-free iso-conversional method indicated that Avrami-Eroffev model could well match the decomposition process of the neat PVA and PG-0.3 composite, while the Avrami-Eroffev and first order models could precisely forecast the decomposition of PG-0.9 composite. Both analyses during multiple cycle melting-crystallization and isothermal decomposition demonstrated that graphene served as decomposition accelerator in the whole thermal decomposition process, and particularly the decomposition of neat PVA and PVA/graphene composites was highly related to the band area ratios of C–H and O–H vibrations in Fourier transform infrared (FTIR) spectrum.

Leaching behaviour of high surface area synthetic boehmite in NaOH solution

This study pertains to alkali leaching of a high surface area synthetic boehmite prepared by thermal decomposition of gibbsite. Synthesised boehmite particles are found to be agglomerates of platelets. These particles are characterised by a monomodal size distribution with a median size of 110μm and high specific surface area (~264m2/g). Leaching studies, in the temperature range 70 to 90°C, have been carried out in NaOH solutions of Na2O concentration, CNa2O=100 and 180g/l. Significant dissolution of this boehmite is seen to occur in NaOH solutions, unlike in many other studies, owing to the high surface area. Both alkali concentration and temperature have influenced the dissolution. Reduced time plot analysis indicates that the shrinking core with surface reaction model (R3) describes the initial stages of the dissolution process reasonably well. Particle size analysis data complemented the results of the reduced time analysis. The value of apparent activation energy for the initial stages of dissolution is evaluated to be around 65kJ/mol.Deviation from shrinking core model R3 is observed in the later stages. Besides the shrinking of size due to dissolution, particle size reduction is also identified due to the detachment of platelets from the agglomerates of particles. Detailed characterisation of the leach residues involving morphological and particle size analyses has established such detachment of platelets and ensuing particle size reduction. Deviation from shrinking core model in the later stages of leaching has been attributed to particle fragmentation arising from platelet separation.

Characteristics of hydrogen-rich gas production of biomass gasification with porous ceramic reforming

In the present study, an updraft biomass gasifier combined with a porous ceramic reformer was used to carry out the gasification reforming experiments for hydrogen-rich gas production. The effects of reactor temperature, equivalence ratio (ER) and gasifying agents on the gas yields were investigated. The results indicated that the ratio of CO/CO2 presented a clear increasing trend, and hydrogen yield increased from 33.17 to 44.26gH2/kg biomass with the reactor temperature increase, The H2 concentration of production gas in oxygen gasification (oxygen as gasifying agent) was much higher than that in air gasification (air as gasifying agent). The ER values at maximum gas yield were found at ER=0.22 in air gasification and at 0.05 in oxygen gasification, respectively. The hydrogen yields in air and oxygen gasification varied in the range of 25.05–29.58 and 25.68–51.29gH2/kg biomass, respectively. Isothermal standard reduced time plots (RTPs) were employed to determine the best-fit kinetic model of large weight biomass air gasification isothermal thermogravimetric, and the relevant kinetic parameters corresponding to the air gasification were evaluated by isothermal kinetic analysis.

Solid-state kinetics and gas-phase prediction of 1,4-bis(trimethylsilyl)benzene

The solid-state kinetics and gas-phase predictions of the 1,4-bis(trimethylsilyl)benzene (TMSB) are visualized by utilizing thermogravimetric and mass spectral data. The statistical analyses and reduced time plots of zero order (F0) and Avrami-Erofeev (A2) nucleation and growth models provides the best fit to experimental data for isothermal evaporation process for TMSB. The activation energy for non-isothermal evaporation processes of TMSB is calculated using isoconversional methods. The molecular structure and energetics of the predicted gas phase molecules and species in chemical vapor deposition process are investigated using semi-empirical quantum chemical calculations.

Kinetics of Fe(II)-Catalyzed Transformation of 6-line Ferrihydrite under Anaerobic Flow Conditions

The readsorption of ferrous ions produced by the abiotic and microbially mediated reductive dissolution of iron oxy-hydroxides drives a series of transformations of the host minerals. To further understand the mechanisms by which these transformations occur and their kinetics within a microporous flow environment, flow-through experiments were conducted in which capillary tubes packed with ferrihydrite-coated glass spheres were injected with inorganic Fe(II) solutions under circumneutral pH conditions at 25 degrees C. Synchrotron X-ray diffraction was used to identify the secondary phase(s) formed and to provide data for quantitative kinetic analysis. At concentrations at and above 1.8 mM Fe(II) in the injection solution, magnetite was the only secondary phase formed (no intermediates were detected), with complete transformation following a nonlinear rate law requiring 28 and 150 h of reaction at 18 and 1.8 mM Fe(II), respectively. However, when the injection solution consisted of 0.36 mM Fe(II), goethite was the predominant reaction product and formed much more slowly according to a linear rate law, while only minor magnetite was formed. When the rates are normalized based on the time to react half of the ferrihydrite on a reduced time plot, it is apparent that the 1.8 mM and 18 mM input Fe(II) experiments can be described by the same reaction mechanism, while the 0.36 input Fe(II) experiment is distinct. The analysis of the transformation kinetics suggests that the transformations involved an electron transfer reaction between the aqueous as well as sorbed Fe(II) and ferrihydrite acting as a semiconductor, rather than a simple dissolution and recrystallization mechanism. A transformation mechanism involving sorbed inner sphere Fe(II) alone is not supported, since the essentially equal coverage of sorption sites in the 18 mM and 1.8 mM Fe(II) injections cannot explain the difference in the transformation rates observed.

Isothermal degradation of poly(3‐hydroxybutyrate)/organically modified montmorillonite nanocomposites

AbstractPolymer nanocomposites consisted of biodegradable poly(3‐hydroxybutyrate) (PHB) and organically modified montmorillonite Cloisite25A (OMMT), prepared by the solution‐casting method, were isothermally degraded for 120 min at 230, 235, 240, and 245°C in the nitrogen atmosphere. The addition of OMMT increases the thermal stability of PHB, and the most pronounced effect has the addition of 7 wt% of OMMT. Kinetic analysis was performed using reduced time plots and model‐free isoconversional methods. The empirical kinetic triplets (E, A, and g(α)) for the isothermal degradation of pure PHB and PHB/OMMT nanocomposites were determined. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers

Poly(3-hydroxybutyrate) nanocomposites: Isothermal degradation and kinetic analysis

Polymer nanocomposites consisted of biodegradable poly(3-hydroxybutyrate) (PHB) and organically modified montmorillonite Cloisite30B (30B) have been prepared by the solution casting method and isothermally degraded at 230, 235, 240 and 245 °C. The addition of 30B increases the thermal stability of PHB and the most pronounced effect has the addition of 1 wt.% of 30B. Kinetic analysis was performed using model-free isoconversional and reduced time plots (RTP) methods. The isothermal degradation of pure PHB and PHB/30B nanocomposites can be separated in two distinct regions: the first where small mass loss appears and the second, assigned as the main degradation mechanism, where the main mass loss takes place. The empirical kinetic triplets ( E, A, and g( α)) for the main degradation mechanism were determined.

Using isothermal kinetic results to estimate kinetic triplet of pyrolysis reaction of polypropylene

This paper describes how to estimate the Arrhenius parameters as well as the reaction model of the pyrolysis of polypropylene (PP) from isothermal kinetic results. We introduced a custom-made thermobalance that is able to record weight decrease with time under pure static condition. A best fit of experimental reduced-time-plot (RTP) to theoretical ones led to a conclusion that the pyrolysis reaction model and Arrhenius parameters of PP vary with reaction temperature. The pyrolysis reaction model of PP is accounted for by “Contracting Cylinder” model at lower-temperature reactions (683–693 K), whereas that of PP by “Avrami-Erofeev” model at higher-temperature reactions (728–738 K). Medium-temperature reactions (693–728 K) manifest the shift pattern of reaction model from “Contracting Cylinder” model to “Avrami-Erofeev” model. There is a critical temperature (705–720 K) that triggers the explosive generation of volatiles, leading to bubble nucleation and growth, simulating the “Avrami-Erofeev” model. The nth order model adopted by most previous studies appears to be inappropriate to represent the pyrolysis reaction model of PP. Consequently, the Arrhenius parameters derived from the assumption of nth order model would be improper as well.

Kinetic Analysis of Thermal Decomposition of Unirradiated and γ-Irradiated Tris(ACEtylacetonato)-Ruthenium(III) [Ru(acac)3

The thermal decompositions of unirradiated and γ-irradiated Ru(acac)3 were studied in air. The results show that decomposition proceeds in one major step in the temperature range of 150-250°C with the formation of RuO2 as a final solid residue for unirradiated Ru(acac)3. For γ-irradiated Ru(acac)3, with a total γ-ray dose of 102kGy, the decomposition goes eventually to completion with almost 100% decomposition and proceeds in one major step which contains four overlapping decomposition stages in the temperature range of 200 −320°C. The kinetics were followed isothermally and non-isothermally using both model-fitting and model-free approaches. IR spectroscopy and X-ray powder diffraction techniques were employed to follow the chemical composition of the solid residues obtained at different temperatures. In a model-fitting approach, the kinetic triplet, i.e. the apparent activation energy Ea, the pre-exponential factor, ln A, and the reaction model f (α), were analyzed and calculated using straight line plots, and reduced time plots by isothermal measurements. For non-isothermal measurements we have applied the Coats and Redfern, and Kennedy and Clark methods. In the model-free approach, the non-thermal data for both unirradiated and γ-irradiated Ru(acac)3 samples were analyzed using the Flynn-Wall-Ozawa (FWO), Tang (T), and Vyazovkin (Vyz) isoconversional methods. The results of application of model-free and model-fitting approaches to the thermal decompositions of both unirradiated and γ-irradiated Ru(acac)3 were compared and evaluated to deduce the most probable kinetic triplet that describes correctly the thermal behavior of Ru(acac)3. Discussion of the results is preceded by a short review of the models relevant to this study.

Modeling of TG curves of isothermal oxidation of graphite

By thermogravimetric analysis technique, TG curves of isothermal oxidation of graphite were obtained and the curves of weight loss versus reduced time were also obtained. The trend of weight loss versus reduced time curves of oxidation of graphite for different temperatures is unified in the temperature range with identical oxidation mechanism. Based on the reduced time plot method, modeling of the TG curves is presented by the method of polynomial regression of weight loss versus reduced time curves without a need of kinetic analysis. By comparing the experimental and simulated TG curves, the method of polynomial regression of weight loss versus reduced time curves can better predict weight loss of the isothermal oxidation of graphite.

Oxidation kinetics and mechanisms of a 2D-C/C composite

The isothermal oxidation of a 2D-C/C composite was investigated by thermogravimetric analysis in the temperature range of 745–900 °C and SEM observation. The model-free, model-fitting and reduced-time plot methods were applied to oxidation kinetic analysis. SEM investigation shows that oxidation starts from the fiber/matrix interfaces, and matrix carbon is oxidized much more rapidly than the carbon fibers. According to the model-free curve, the oxidation temperatures are divided into two ranges: lower temperatures (745–800 °C) and higher temperatures (850–900 °C). The apparent activation energy and oxidation controlling mechanisms in different temperature ranges are obtained. Furthermore, the trend of the oxidation rate with weight loss or temperature ranges is discussed by the combination of microstructure and mechanisms of oxidation of 2D-C/C composite.

Estimation of kinetic triplet of cellulose pyrolysis reaction from isothermal kinetic results

This paper presents a new approach to estimate the Arrhenius parameters as well as the reaction model function of cellulose pyrolysis reaction. Reduced time plot (RTP) was employed to choose a proper form of reaction model function for cellulose pyrolysis reaction. A state-of-the-art thermobalance (TB) that is able to form real isothermal reaction conditions was introduced to construct RTPs from isothermal decomposition kinetic data. The reaction model function of cellulose pyrolysis reaction would be accounted for by Avrami-Erofeev function, n(1-α){-ln(1-a)}1-1/n where n is determined to be 3.69.

Identification of Pyrolysis Reaction Model of Linear Low Density Polyethylene (LLDPE)

Abstract This paper described how to estimate the Arrhenius parameters as well as the pyrolysis reaction model of LLDPE from isothermal kinetic data. We used a custom-made thermobalance that is capable of monitoring a weight decrease with time under pure static condition and performed six isothermal kinetic experiments at 713, 718, 723, 728, 733, and 738 K that were chosen within a temperature range (710–740 K) where main decompositions were observed from non-isothermal kinetic results. Comparing experimental reduced-time-plots (RTPs) with theoretical ones, the pyrolysis reaction model of LLDPE is accounted for by “Avrami–Erofeev” model in the investigated temperature region, allowing its functional form to be 2(1 − α)[−ln(1 − α)]1/2. It is, hence, expected that the reaction order model adopted by the previous studies without verification is inappropriate to represent the pyrolysis reaction model of LLDPE. Bubble nucleation may be a major pyrolysis reaction mechanism of LLDPE. As heat is applied, the LLDPE is melted. Volatiles may be accumulated inside the melt until reaching a critical concentration where bubble nucleation sets forth. The rates of heat, mass, or momentum transfer in the LLDPE melt and its physico-chemical properties (especially viscosity) play an important role in characterizing the thermal decomposition kinetics of LLDPE.

Identification of Pyrolysis Reaction Model of Polypropylene

Abstract This paper attempted to identify a pyrolysis reaction model of polypropylene (PP) and to elucidate its macroscopic decomposition mechanism correspondingly. Among various reaction models, we chose a reaction model that allowed the best fit to experimental reduced-time-plots (RTPs). Surprisingly, the pyrolysis reaction model of PP varies with reaction temperature within a temperature range between 683 and 738 K. At lower temperatures (683–693 K), the “contracting-cylinder” model accounts for the pyrolysis reaction of PP. Bubble nucleation may be a major reaction mechanism of PP pyrolysis at higher temperatures (728–738 K). High generation rates of volatiles lead to the accumulation of volatiles in the melt until reaching a critical concentration where bubble nucleation sets forth, thus mimicking “Avrami–Erofeev” model. Hence, the classical expression of the reaction model, f(α) = (1 − α)n, assumed by most previous studies is inappropriate for pyrolysis reaction of PP. At lower temperatures, chemical reactions may limit the pyrolysis reaction of PP, whereas at higher temperatures the rate of bubble nucleation closely related to heat, mass, and momentum transfer along with thermodynamic properties of melt may limit its pyrolysis reaction.

Using isothermal kinetic results to estimate the kinetic triplet of the pyrolysis of high density polyethylene

The Arrhenius parameters and the reaction model of the pyrolysis of high density polyethylene (HDPE) have been estimated from isothermal kinetic results. We introduced a custom-made thermobalance that is able to record mass decrease with time under pure static condition. A best fit of the theoretical reduced-time plot (RTP) to an experimental one led to the conclusion that “contracting cylinder” model would represent the HDPE pyrolysis. The Arrhenius parameters obtained from the model-fitting method, using the contracting cylinder model, agreed excellently with the values from the model-free methods using both isothermal and non-isothermal measurements.

Identification of Reaction Model of Pyrolysis Reaction of HDPE

Abstract This paper demonstrates how to derive Arrhenius parameters as well as reaction model of pyrolysis reaction of high density polyethylene (HDPE) from isothermal kinetic data. We introduced a state-of-the-art thermobalance that enables to record a weight decrease with time under pure static condition. As a result of comparing experimental reduced time plots (RTPs) with theoretical ones, the pyrolysis reaction of HDPE accommodates “contracting cylinder” model, allowing its model to be 2(1−α)1/2. The reaction model derive here was used to evaluate each Arrhenius parameters from isothermal and non-isothermal data. A couple of evidences strongly support the properness of the reaction model derived from RTP and the corresponding Arrhenius parameters.