Variation Of Activation Energy Research Articles

Thermogravimetric study on the pyrolysis kinetic mechanism of waste biomass from fruit processing industry

The detailed kinetic analysis of slow pyrolysis process of apricot (Prunus armeniaca L.) kernal shells has been estimated, under non-isothermal conditions, through thermogravimetric analysis and derivative thermogravimetry. Thermal decomposition was implemented using four different heating rates (5, 10, 15, and 20?C per minute), with consideration of how this parameter effects on the process kinetics. The higher heating rates provoke the shift of thermoanalytical curves towards more elevated temperatures. Using isoconversional differential method, the variation of activation energy, Ea, with conversion fraction, ?, was detected, and pyrolysis reaction profile was discussed. After resolving the pyrolysis rate curves of individual biomass constituents, the temperature and conversion ranges of their thermal transformations were clearly identified. In the latter stage of analysis, every identified reaction step was considered through mechanistic description, which involves selection of the appropriate kinetic model function. The comparison of the results as well as discrepancies between them has been discussed. The corresponding rate-law equations related to thermal decomposition reactions of all biomass constituents present in the tested agricultural waste material have been identified.

Analytical fast pyrolysis of nitrogen-rich mosquito species via pyrolysis-FTIR and pyrolysis-GC/MS

This study presents a thorough thermal analysis of the dead remains of mosquitoes, in order to unravel the composition of vapors evolved from the bug zappers decimating them. Analytical pyrolysis-gas chromatograph/mass spectrometry (Py-GC/MS) and pyrolysis-Fourier transform infrared (Py-FTIR) spectrometric techniques were used to understand the pyrolysate composition and their time evolution from mosquitoes. Thermogravimetric mass loss data obtained at different heating rates (5–30 °C min−1) were used to determine the variation of apparent activation energy with conversion using isoconversional methods of Friedman, Flynn-Wall-Ozawa and Kissinger–Akahira–Sunose. The apparent activation energies varied in the range of 90–200 kJ mol−1. Increase in pyrolysis temperature from 400 to 700 °C resulted in the formation of alcohols, indoles, nitriles, monoaromatics and alkenes in higher selectivities, while the selectivity to long chain carboxylic acids decreased with increasing temperature. The formation of various organics was corroborated by the evolution of functional groups at short timescales. The significant nitrogen content in the char (∼9.7 wt.%) was evidenced in the form of NH and NO groups in the FTIR spectrum. The deconstruction of proteins and fatty acids present in mosquito was similar to that of microalgae in terms of the pyrolysis products.

Low-temperature oxidation reactions of crude oils using TGA–DSC techniques

In this research, combustion behaviour of three crude oils with different °API gravities was analysed. The thermogravimetric analyser (TGA) and differential scanning calorimeter (DSC) experiments were performed using three different heating rates (2, 5, and 10 °C min−1) under air atmosphere. The reaction regions, burn-out temperatures, and peak temperatures were determined accordingly. Two main reaction regions, particularly the low-temperature oxidation (LTO) and high-temperature oxidation (HTO) reactions, were figured out in TGA and DSC curves, which suggested exothermic behaviour. Kinetic parameters of the crude oils were determined using model free methods (Ozawa–Flynn–Wall, OFW, and Kissinger–Akahira–Sunose, KAS), which allows the calculation of kinetic properties at progressive conversion degrees without the requirement of any reaction models and f(α) estimations. The variation in activation energy with respect to the conversion degree pointed out that the LTO reaction stage can be subdivided into three subzones. The results indicated that the activation energies were changing depending on the conversion degree, which is an indication of complex reaction mechanisms. The change in activation energy with respect to conversion degree showed slight variation for heavy crude oil compared with that in light crude oil.

Characterization of the residue of a commingled post-consumer plastic waste treatment plant: a thermal, spectroscopic and pyrolysis kinetic study

A plastic packaging residue provided by a central Italy recycling facility was the subject of study of the present paper. The aim is to propose the valorization of plastic film residue (PFR) through a pyrolysis process. The PFR was thermo-chemically characterized through heating value, proximate and ultimate analysis. Fourier transformed infrared measurements have shown that PFR is constituted by 92–95 mass% of polyethylene (PE) film, around 5 mass% of PP, polystyrene (PS) < 1 mass%, PET < 1 mass% and traces of foreign materials. The extremely high percentage (98.7 mass%) of volatile matter and the low content of ash (2.1 mass%), humidity (0.6 mass%) and chlorine (0.1 mass%) make PFR an optimum candidate as load of a pyrolytic reactor. Thermogravimetry (TG) experiments were carried out at five different heating rates (2, 5, 7, 10, 12 K min−1) to determine the kinetic parameters of pyrolysis (activation energy E, pre-exponential factor A and the reaction model). No significant variation of activation energy, calculated by an integral isoconversional method proposed by Vyazovkin, is observed with increasing the degree of conversion. An average value of 264 ± 5 kJ mol−1 was estimated. Then, the Coats–Redfern method and the compensation effect were used to determine the lnA versus α data (being 41.9 the average value within the 0.25 < α < 0.85 range) and the reaction mechanism (R2 contracting cylinder model). Furthermore, the minimum energy required to pyrolyze 1 kg of PFR, about 2.27 MJ, was estimated by differential scanning calorimetry. It corresponds to about 5.5% of the exploitable energy of the input material.

Kinetics of water freezing from inorganic salt solution confined in mesopores

In the presented study the kinetics of pore-confined water freezing is analysed using differential scanning calorimetry. The process is investigated in mesopores of two silica gels with pore diameter equal to 6.5 nm and 11.3 nm. Based on isoconversional study of ice crystallization caused by cooling of pure water and 1%, 3% NaCl aqueous solutions, the negative value of activation energy is obtained. The observed variation of effective activation energy might be explained by the nucleation theory, namely, the dominant mechanism changes from homogeneous through heterogeneous nucleation to diffusion in the final stage of phase transition. At the low concentrations the bulk solution surrounding the silica gel particles does not exhibit the eutectic solidification.

Effect of A-site disubstituted of lanthanide perovskite on catalytic activity and reaction kinetics analysis of coal combustion

Perovskite catalysts are increasingly devoted to the field of coal combustion due to their special crystal structure and excellent catalytic effect. In this study, the lanthanide perovskite type catalyst La(0.8−x)Sr0.2KxMnO3 (x is the doping amount of K+) was prepared by sol-gel method, and the effect of K+ amount on the catalytic activity for coal combustion was investigated in detail. The catalyst was well characterized and the thermal dynamic parameters of coal catalytic combustion were calculated by the single reaction (SR) model and the distributed activation energy (DAE) model. The results show that the doping amount of K+ affects the specific surface area and particle size of the catalyst, due to the relatively large ionic radius of K+ and the effect of doping on the valence of other cations. The highest combustion reaction rate was achieved over La0.66Sr0.2K0.14MnO3, and it will change the specific surface area and particle size of the catalyst extremely. The dynamic calculation shows that there is a compensation relationship between the activation energy and the frequency factor. The DAE model can get the variation of activation energy in different reaction stages, and the final calculation results are in good agreement with the experimental results.

Роль термической активации при получении глинозема из низкокачественных бокситов

The purpose of this article is giving the rationale for preliminary thermal activation under alumina production from low-quality bauxites via a comparative analysis of liquid and solid phases of desiliconization of initial and calcined bauxite. The following methods are used: thermal activation of the product through firing of initial bauxite at the temperature of 1000 ºC, its desiliconization by alkaline solutions of 150 g/dm 3 of Na 2 O к where the liquid-to solid ratio is 10:1, production of the concentrate suitable for refining by the Bayer method. The chemical analysis of solid phase is determined by the x-ray fluorescence method using XRF-1800 spectrometer ("Shimadzu", Japan). The content of SiO 2 in the liquid phase is determined using PE-5400UF spectrophotometer. Phase separation is intensified by means of a centrifuge. The solid phase in the sample cell (after liquid phase sampling) is washed by decantation, dried and analyzed. Graphs are built that describe the dependence of silica extraction degree on desiliconization time for different temperatures. Graphic dependences also take into consideration the use of raw material thermoactivation, show the variations of the process rate depending on the desiliconization temperature, reflect the changes in the apparent activation energy of desiliconization depending on the degree of silica extraction in solution. Conclusions are drawn on the expediency of firing for increasing the silica extraction degree and obtaining a higher value of the silicon modulus of bauxite concentrate. The feasibility of thermal activation in processing of high-silicon bauxites is proved as it allows to increase the degree of silica extraction from calcined bauxite into an alkaline solution as well as to intensify the desiliconization process. Based on the variations of the apparent activation energy for different temperature intervals and SiO 2 extraction degree for thermoactivated and initial bauxite an attempt is made to reveal the mechanism and chemical nature of the desiliconization process.

Microstructure, mechanical properties and sintering mechanism of pressureless-sintered porous Si3N4 ceramics with YbF3-MgF2 composite sintering aids

Porous Si3N4 ceramics were fabricated by pressureless sintering at 1550 °C for 2 h with YbF3-MgF2 composite sintering aids, systematically investigated for the effect of sintering aids addition on the microstructure and mechanical properties, further explored the formation process and sintering mechanism. The α-β phase transition rate of all samples approached 100% and MgF2 showed a positive effect on the elongating of β-Si3N4 crystal grains in morphology. The apparent porosity of porous Si3N4 ceramics ranged from 34% to 39%. In addition, when 2.5 wt% YbF3 was replaced by the same amount of MgF2, the flexural strength sharply increased to 298 MPa, an increase of ~49%, while the porosity increased to 37%, an increase of ~15%. With MgF2 content increasing from 0 to 7.5 wt%, the structure factor reached a minimum of 3.23 when MgF2 addition was 2.5 wt%, which demonstrated unparalleled superiority in mechanical properties compared with previous studies due to a more preferred form of morphology of connected rod-like β-Si3N4 crystals network. Besides, our research indicated that MgF2 addition could develop higher aspect ratio of β-Si3N4 grains and had obviously positive effect on the elongating characteristic of β-Si3N4 grains, which was related to the activation energy variation for nucleation on the different facets.

The transition state conformational effect on the activation energy of ethyl acetate neutral hydrolysis

We report a first-principles study on ethyl acetate neutral hydrolysis in which we focus on the activation energy variation resulting from the conformational effect in the transition state. We use the conformers of ethyl formate, ethyl acetate, ethyl fluoroacetate, and ethyl chloroacetate as the ester models and one water molecule with a one-step reaction mechanism. We also consider the long-range interaction and the surrounding water in the form of PCM. Our results show that the various conformers yield a significant range of activation energy. Moreover, the gauche conformer has lower activation energy than the trans conformer. The activation energy in its own right is lowered by the halogen atoms. Finally, we remark that the long-range correction and PCM stabilize the transition state geometry but raise the activation energy.

Thermal degradation kinetics study of polyvinyl chloride (PVC) sheath for new and aged cables

The thermal degradation dynamics of new and aged PVC sheaths was studied in detail. The results illustrated that compared to new PVC sheath, the onset decomposition of aged PVC sheath mainly happened at higher temperature with larger peak value of mass loss rate. Three model-free methods most commonly used were employed to estimate the activation energy values at different conversions. It was noted that the aged PVC sheath exhibited greater entire activation energy than new PVC sheath. Two thermal degradation regions were observed based on the activation energy variation with conversion. The threshold of conversion for two regions was 0.6 for new PVC sheath and 0.5 for aged PVC sheath. The possible reaction mechanism was predicted by generalized master-plots method. The reaction model corresponding to each region showed observed difference between new and aged PVC sheaths. The compensation effect was also used to calculate the related pre-exponential factor. The variation of thermal degradation behavior could be ascribed to the changes of chemical composition, molecular structure, composition proportion and various additives after thermal aging. Besides, the thermal degradation process was reconstructed by an ANN model and it indicated that the predicted data fitted well with the experimental data.

Structural Relaxation in Polyanionic Sodium Fluorophosphate Glasses

Structural heterogeneity is a common feature of all glasses, however, little is known about the underlying contributions of chemical fluctuations and modulations in free volume in concrete glass forming systems. In this investigation, we relate the dynamics of structural relaxation of (100-x)NaPO3–xAlF3 glasses to their heterogeneous structure as determined from multinuclear magnetic resonance spectroscopic analysis. For this, we evaluate differential scanning calorimetry (DSC) data using the integral isoconversional method to determine the variation in activation energy, Ea, of the glass transition as a function of temperature and conversion progress. Specific heat measurements from DSC allow for the determination of the effective size of the cooperatively rearranging region (CRR). From 31P, 19F and 27Al NMR, we observe that the introduction of AlF3 into the NaPO3 network increases the average connectivity (i.e., the number of heteronuclear Al—O—P bonds), rationalizing the higher Ea determined from the DSC measurements. We find highly constrained regions of Al(OP)4F2 with Al—F—Al cross-linking (high Ea) and, simultaneously, more flexible regions of phosphate chains containing P—F··(Na+)n bonds (low Ea); this results in a topologically and dynamically heterogeneous structure as evidenced by the increased variability in Ea with higher AlF3 content. The decreasing size of the CRR reflects the increased heterogeneity: at low AlF3 the CRR is large, while at high AlF3 (high heterogeneity), the CRR is significantly smaller (by a factor of 103). Finally, we relate the heterogeneity to other macroscopic properties, such as Tg and mechanical properties.

Coarsening of nanoporous Au during catalytic CO oxidation

This study focuses on the effects of catalytic activity on nanoporous Au catalysts fabricated by dealloying. Nanoporous Au structures are shown to coarsen with similar kinetics over a broad temperature range. A variation in the apparent activation energy suggests that coarsening is controlled by different rate-limiting steps at high and low temperatures.

Glass transition of a polystyrene surface as detected via two-dimensional diffusion of Au atoms during physical vapor deposition

We investigated two-dimensional diffusion of Au atoms across the surface of a polystyrene (PS) film during physical vapor deposition to determine the surface glass transition temperature (Tg). Deposited Au atoms underwent two-dimensional diffusion on the PS surface until they coalesce with other atoms or aggregated particles. The diffusivity of Au on the PS surface was evaluated from the size and the number density of yielded Au nanoparticles. The diffusion coefficient D of Au was found to exhibit a drastic change near 356 K, which was assigned to the surface Tg of the PS substrate. A stronger temperature dependence of D observed near Tg suggests that Au diffusion is coupled with the segmental dynamics of PS. The change in the diffusion coefficient suggests an essential difference in the diffusion mechanism below and above the Tg of the PS surface. We assume that diffusion on a glassy PS surface reflects a more heterogeneous feature of the PS surface than that above Tg. This was qualitatively supported by the results from Monte Carlo simulations with varying activation energy distributions.

Evaluation on activation energy of deposited clay based on thermo-gravimetric analysis (TGA) and four kinetics models

In order to evaluate and compare the accurate activation energy variations of two kinds of deposited clays (i.e. non-organic clay and organic clay), thermo-gravimetric analysis (TGA) was firstly conducted to study the reaction characteristics of the two clay samples. Two periods of weight losses were observed according to each TGA curve for both clay samples. The first period happened due to water evaporation, while the second period was mainly caused by the composition reactions of clay minerals, especially influenced by the de-hydroxylation of kaolinite. An obvious phenomenon is that the organic clay with a high soil organic matter (SOM) has a higher activity energy. Then, four common kinetics models [i.e. the Coats-Redfern method, the Doyle integral method, the maximum rate method, and the distributed activation energy model (DAEM)] were used to calculate and compare the activation energy values. And the Doyle integral method and the DAEM method are considered as two recommended models to analyze the activation energy performance during thermal treatment. Some interesting conclusions can be summarized as: (1) The multi-period reactions of deposited clays during the heating process are common, especially for organic clay. It would be beneficial knowing its biomass resource feasibility concerning the second weight loss; (2) The high SOM has a significant influence on the potential energy unitization of deposited clays, while the variations of activity energy for deposited clays are influenced by the mineral compositions and microstructure features; (3) It is possible to evaluate the accurate activity energy of deposited clays combined with TGA tests and two recommended models (i.e. the Doyle integral method and DAEM method).

Thermal Behavior and Kinetic Triplets of Heavy Crude Oil and Its SARA Fractions during Combustion by High-Pressure Differential Scanning Calorimetry

In situ combustion (ISC) has been regarded as an efficient technique for the exploitation of heavy oil reserves. In this work, the thermal behavior of one heavy crude oil and the fractions of its saturates, aromatics, resins, and asphaltenes (SARA) during combustion was thoroughly investigated using high-pressure differential scanning calorimetry. Two typical isoconversional methods were adopted to determine the variation of activation energy (E) and frequency factor (Ar) versus conversion degree in the course of the reaction, followed by the evaluation of the reaction model, f(α), via the master plot method. The results indicated that the heavy oil encountered larger thermal release caused by low-temperature oxidation (LTO) reactions rather than high-temperature oxidation (HTO) reactions, suggesting that appreciable heat could be available within the low-temperature range. Saturates showed a notably apparent heat release in the LTO reactions. For aromatics, the exothermic effect at the LTO stage was appa...

Low-temperature oxidation and reactivity of coal in O2/N2 and O2/CO2 atmospheres, a case of carboniferous–permian coal in Shaanxi, China

Liquid carbon dioxide has an excellent ability of endothermic cooling and inhibition on coal fire, which was an effective coal spontaneous combustion prevention technology. To analyze the oxidation characteristics and variation of apparent activation energies, a carboniferous–permian coal sample was investigated in O2/N2 and O2/CO2 atmospheres by the coal spontaneous combustion oxidation and the Fourier transform infrared spectroscopy experiments. The results indicated that with temperature, carbon monoxide (CO) concentration and oxygen (O2) consumption rate increased. While O2 concentration decreased, CO concentration and oxygen consumption rate reduced. At the same O2 concentration, the oxygen consumption rate and CO concentration on the O2/CO2 atmospheres were less than on the O2/N2 atmospheres. Therefore, O2 concentration reduced, or added CO2 significantly inhibited coal oxidation. As the temperature elevated, the apparent activation energy gradually increased. Furthermore, the apparent activation energy increased when the oxygen concentration reduced in the physical–chemical adsorption stage and the slow oxidation stage. In the rapid oxidation stage, the apparent activation energy lessened with increase in oxygen concentration. Through correlation analysis, the key functional groups in the physical–chemical adsorption stage were hydroxyl, C–O, –COO–, and aliphatic hydrocarbons. During the slow oxidation, the key functional groups were –COO– and aliphatic hydrocarbons. The key functional groups in the rapid oxidation stage were hydroxyl and C–O.

Dielectric behavior and AC electrical conductivity analysis of PMMA films

PMMA films of different thickness (0.006, 0.0105, 0.0206, 0.0385 and 0.056cm) were synthesized by casting process. The temperature and frequency dependence of dielectric constant and AC electrical conductivity measurements at various frequencies (10kHz-10MHz) and temperatures (293-373K) were carried out. Few anomalies in dielectric studies were observed near 313 and 373 K respectively. These points were related to glass transitions temperature. The variation of activation energy and conduction behavior was studied .From the AC conduction studies, it is confirmed that the mechanism responsible for the conduction process is hopping of carriers. The variations of the dielectric constant and loss as function of frequency at different temperature was observed and the results were discussed. The calculated activation energy varied with the thickness, temperature, and applied frequency. Conductivity plots against frequency suggested that the response obeying the universal power law concerning the AC conductivity and dielectric behavior of polymer. The polarizability a increases with temperature but decreases with thickness indicating weekends and rising of intermolecular forces respectively.

Thermal and thermo-oxidative degradation kinetics and characteristics of poly (lactic acid) and its composites

Thermal degradation behavior and kinetics of starch/PLA composites in an inert and oxygen atmosphere were investigated by TG and TG-FTIR techniques. It is shown that the thermal degradation process can be divided into three stages, and a significant difference between thermal and thermo-oxidative degradation is the third stage in which the residual chars are further cracked for the thermo-oxidative degradation. The activation energy was calculated using the iso-conversional Flynn-Wall-Ozawa (FWO) method. The lower activation energy for thermo-oxidative degradation indicated that oxygen had an accelerated effect on thermal degradation. The variation of activation energy indicated that the decomposition of all the samples was a complex process that included at least two different mechanisms. Evolution of the evolved gaseous products with temperature was studied to understand the thermal and thermo-oxidative decomposition comprehensively. According to the TG-FTIR analysis, the gaseous products mainly contained lactide, cyclic oligomers, aldehydes, CO2, CO, and H2O.

Optoelectronic properties of transparent conducting silver beta alumina and indium doped silver beta alumina thin films prepared by multi source vacuum evaporation method

The present study reports the optoelectronic properties of crystalline transparent conducting silver beta alumina (AgAl11O17) and indium doped silver beta alumina (AgAl11O17:In) thin films. In contrast to the p-type conductivity of the delafossite AgAlO2, both the hexagonal crystalline AgAl11O17 and AgAl11O17:In are n-type as confirmed by hot probe, hall effect and thermoelectric measurements. The variation in electrical conductivity (10−5-10−2 S/cm) and activation energy are correlated with carrier concentration and mobility. The films exhibit a wide direct band gap of ∼3.93 ± 0.02eV and transparency uptõ ~62%.

Pressure-induced phase transitions for goethite investigated by Raman spectroscopy and electrical conductivity

ABSTRACTWe reported two pressure-induced phase transitions of goethite up to ∼35 GPa using a diamond anvil cell in conjunction with ac impendence spectroscopy, Raman spectra at room temperature. The first pressure-induced phase transition at ∼7.0 GPa is manifested in noticeable changes in six Raman-active modes, two obvious splitting phenomena for the modes and the variations in the slope of conductivity. The second phase transition at ∼20 GPa was characterized by an obviously drop in electrical conductivity and the noticeable changes in the Raman-active modes. The variations in activation energy with increasing pressure were also discussed to reveal the electrical properties of goethite at high pressure.