Global Kinetic Parameters Research Articles

Critical Review of the Global Chemical Kinetics of Cellulose Thermal Decomposition

Historical models for cellulose pyrolysis and mathematical approaches to kinetic analysis are reviewed with the objective of identifying the correct global chemical kinetic models and parameters for cellulose pyrolysis. In most recent experiments, cellulose pyrolysis clearly has sigmoidal reaction character, which is consistent with one of a sequential, nucleation–growth, or random-scission global model. The apparent activation energy of ∼47 kcal/mol is consistent with mechanistic modeling if one allows catalytic acceleration of the concerted initiation reaction. There is a possibility that part of the sigmoidal character is due to adsorption effects at low pyrolysis temperatures, but further work is required to resolve this issue. Reasons are given for why fitting data at a single heating rate to a first-order reaction model gives incorrect results.

Anodic Oxidation of COads Derived from Methanol on Pt Electrocatalysts Linked to the Bonding Type and Adsorption Site

A newly formulated four-component modified Butler-Volmer model has been developed to evaluate global oxidation kinetic parameters for the various types of carbon monoxide adsorbates (COads) on a nanoparticle Pt surface determined by the type of bonding as well as the local structure of the adsorption site. Partial coverages of COads were prepared by potentiostatic adsorption of methanol followed by potentiostatic partial oxidation at various elevated potentials and for various durations. Anodic linear sweep voltammetry was then performed, and the COads oxidation peaks were fitted with the model to analyze the kinetics. According to the model, preferential oxidation with respect to COads bonding and Pt substrate structure can be achieved dependent upon the potential and extent of oxidation. Partial oxidation at 450mV vs. RHE for 60min. resulted in a majority population of linearly bonded COads on cubic-packed Pt sites; whereas partial oxidation at 650mV vs. RHE for 220sec. resulted in a majority population of bridged-bonded COads on close-packed Pt sites.

A Comparative Analysis of the Kinetic Experiments in Polyethylene Pyrolysis

A review of literature has been conducted to survey the kinetic data of low-density polyethylene (LDPE) pyrolysis. The review reveals large variations in the reported global kinetic parameters. The cause of variation has been identified to be the difference in the experimental techniques, including thermogravimetric analysis (TGA) and non-TGA methods. Even within the nonisothermal TGA data, large variations have been observed at heating rate of 20 K/min, while the variations are insignificant at lower heating rate regimes (2–10 K/min), indicating the influence of heat/mass transfer resistance controlling the kinetics. Detailed analysis revealed that most of the current techniques are unable to capture all the relevant data necessary for estimating the kinetic parameters of the aforementioned process. The outcome of this review work thrusts the need for a better experimental technique to estimate the kinetic parameters of complex reactions, such as polymer pyrolysis.

Thermal stability of some cryogels based on poly(vinyl alcohol) and cellulose

The aim of this study consisted in the elucidation of the thermal decomposition process of some cellulose and poly(vinyl alcohol) based cryogels as films. The thermal behavior was studied by dynamic thermogravimetry and differential scanning calorimetry in nitrogen atmosphere, up to 600°C. Evolved gases analysis were performed using a coupling to a quadrupole mass spectrometer and a Fourier transform infrared spectrophotometer equipped with external modulus for gas analyses. Global kinetic parameters of the thermal decomposition process were determined by applying the isoconversional differential method of Friedman and the integral method of Flynn–Wall–Ozawa. The final form of the kinetic model and the parameters for each individual stage of thermal decomposition were determined by multivariate non-linear regression method.

Model-based analysis of bench-scale fixed-bed units for chemical-looping combustion

The objective of this work is to compare reaction schemes and kinetic mechanisms, and simulate the dynamic behavior of chemical-looping Reducers operating with methane as the feed and nickel oxide as the oxygen carrier. A dynamic model for the reduction step of chemical-looping combustion in fixed-bed reactors is developed, on the basis of the same kinetic network to predict published experimental data of different fixed-bed reactors and data of a chemical-looping fixed-bed reactor operating at the University of Connecticut. Steam reforming, water gas shift, dry reforming, methane decomposition, and carbon gasification by carbon dioxide and steam are considered as reactions catalyzed by the reduced oxygen carrier. Heterogeneous reactions of the oxygen carrier encompass reactions with methane, carbon monoxide and hydrogen. Kinetic expressions reported in the literature are compared and their parameters are estimated on the basis of experimental data of nickel-based oxygen carriers for chemical-looping combustion of methane. Particle shrinking, molar expansion and surface area changes are accounted for in the transient plug flow reactor model. The applicability of the shrinking core and modified volumetric models, which are typically used for the description of gas–solid reactions is verified and compared against various experimental data, showing the superiority of the latter. A global mechanism and kinetic parameters that can be used to simulate CLC Reducers operating with NiO as the oxygen carrier and CH4 as the fuel are proposed. Finally, discrepancies between published data and the model accuracy for different experimental setups are discussed.

Oxygen Reactivity of Devolatilized Diesel Engine Particulates from Conventional and Biodiesel Fuels

This study reports measurements of the oxygen reactivity of diesel engine particulates generated with a range of conventional and biodiesel fuel blends. Particulate samples were collected from a modern light-duty diesel engine and then devolatilized at 650 °C under argon. Reaction rates of the remaining fixed carbon component with oxygen were measured in a differential fixed-bed reactor. Global Arrhenius kinetic parameters were determined from the measured reaction rates. Surface area measurements were also made in situ during particle burnout and revealed fuel-dependent differences in burning mode. An empirical function for correlating the variation of surface area with burnout is proposed based on these observations. When the observed reaction rates are normalized to the local active carbon surface area (which varied with fuel type and degree of burnout), it is possible to identify a single global Arrhenius activation energy of 113 ± 6 kJ/mol.

Synthesis and Characterization of a New Thermoreversible Polyurethane Network

A new polyurethane network was synthesized by the Diels–Alder cross-linking reaction of a polyurethane to bisfuryl monomer. Attenuated total reflectance in conjunction with Fourier transform infrared spectroscopy (ATR-FTIR) spectra of the network showed the disappearance of the absorption bands of maleimide and the appearance of new bands attributed to furan-maleimide cycloadduct. Chemical shifts characteristic to the cycloadduct appeared in the proton nuclear magnetic resonance spectra. ATR-FTIR and differential scanning calorimetry (DSC) demonstrated the thermal reversibility of the material by the reproduction of the retro-Diels–Alder and Diels–Alder processes upon heating and cooling. Global kinetic nonisothermal decomposition parameters in nitrogen were determined by the Flynn-Wall-Ozawa method. A three successive stage thermal decomposition mechanism depicted by n order reaction model for each stage was proposed. The validity of the chosen kinetic model and the values of the kinetic parameters of the individual decomposition stages were determined by the multivariate nonlinear regression method.

Vapor-Phase Cracking of Eugenol: Distribution of Tar Products as Functions of Temperature and Residence Time

Knowledge of the product distribution and kinetics of the secondary vapor-phase cracking of biomass tar is crucial for the design and optimization of biomass thermochemical conversion processes, such as fast pyrolysis, gasification, and combustion. In this study, we have examined the distribution of condensable tar products from the vapor-phase cracking of eugenol, a model-fuel compound representative of the lignin structural entities found in biomass. Pyrolysis experiments were investigated at different temperatures (300–900 °C) and residence times (τ = 1 and 3 s) in a non-isothermal laminar-flow reactor operated at atmospheric pressure. Tar products were analyzed using gas chromatography with mass spectrometric detection. Eugenol conversion commenced rapidly above 350 °C at τ = 3 s and above 400 °C at τ = 1 s. Complete conversion was attained at 550 and 600 °C at τ = 3 and 1 s, respectively. A numerical model incorporating laminar-flow, non-isothermal temperature profile and pseudo-unimolecular kinetics was developed to model the eugenol conversion data. The model agreed very well with the experimental data, and the derived global kinetic parameters for overall eugenol decomposition were A = 1014 s–1 and Ea = 50.7 kcal mol–1. Tar products were comprised of oxygenates and aromatic hydrocarbons. Identified oxygenates include phenols, furans, ethers, and acids. The aromatics consisted of single-ring aromatic compounds and polycyclic aromatic hydrocarbons. Oxygenates were observed as tar products in the temperature range of 350–850 °C at both τ = 1 and 3 s. Aromatic hydrocarbons were observed as tar products at temperatures above 650 °C. Most of the identified aromatic hydrocarbons showed an increase in the yield above 650 °C at both τ = 1 and 3 s. At all temperatures, yields of oxygenates and aromatics were generally higher at τ = 1 s than at τ = 3 s. Mass balance closure was not achieved above 400 and 500 °C at τ = 3 and 1 s, respectively, with discrepancies in the mass balances increasing with an increase in the temperature. The discrepancies in the mass balances can be attributed to the formation of noncondensable gas products.

Experimental Investigation on the Oxidation Characteristics of Diesel Particulates Relevant to DPF Regeneration

Diesel engines generally employ diesel particulate filter (DPF) systems to meet increasingly stringent emissions regulations. The development of optimum methodologies for DPF regeneration requires detailed information on the oxidation characteristics of diesel particulate matter (PM) that accumulates on the DPF under realistic engine conditions. An experimental investigation on the oxidation behavior of diesel PM collected from a DPF test system connected to the exhaust stream of a 1.9 L, 4-cylinder, light-duty diesel engine is reported. A thermogravimetric analyzer (TGA) was used to measure the instantaneous sample mass and the rate of mass loss during its oxidation for a wide range of conditions, which include initial sample mass, amount of volatile components of soluble organic fraction (SOF) in the sample, oxygen concentration, and various heat treatment schemes in both the inert and oxidizing environments. The global kinetic parameters, i.e., the reaction orders of soot and oxygen, activation energy, and pre-exponential factor, were determined for the diesel PM and surrogate soot samples. Significant differences are observed in the oxidation behavior of surrogate soot and diesel PM. The oxidation rate of surrogate soot decreases continuously as the soot is oxidized, while that of diesel soot is nearly constant until about 80% of the sample mass is oxidized, and then decreases as the sample is completely oxidized. In addition, the oxidation behavior of surrogate soot is found to be essentially independent of the heat treatment schemes used, while that of diesel soot is strongly influenced by them. These differences may be attributable to changes in soot morphology during heating/oxidation and the presence of surface functional groups and heavier SOF components in the diesel PM. The effects of SOF and thermal aging on diesel PM oxidation have also been characterized. Results indicate that the PM oxidation is only weakly influenced by the presence of volatile components of SOF, whereas it is noticeably affected by thermal aging.

Estimating and Optimizing the Amount of Generated Hydrocarbon by Determining Kinetic Parameters (A, E) in Pabdeh, Gurpi, and Kazhdumi Source Rocks With an Open Pyrolysis System

One of the standard methods of estimating the amount of hydrocarbon generated is determining the kinetic parameters of source rock samples. Therefore, three immature samples from well-known Iranian source rocks (Pabdeh, Gurpi, and Kazhdumi) artificially were matured by an open pyrolysis system (Rock-Eval 6) at temperature rates of 5 and 15°C/min in bulk. Optimization was done by Optkin software. Kinetic parameters (A, E) and amount of generated hydrocarbon was determined. The results show different kinetic parameters for each sample and significant differences between them and the global kinetic parameters. Consequently, to get an accurate kinetic model in a petroleum system, a strategy for determining kinetic parameters on any new source rock samples is proposed.

Combining in situ HT-ESEM observations and dilatometry: An original and fast way to the sintering map of ThO2

The sintering of ThO2 pellets prepared from the initial precipitation of thorium oxalate was investigated by the means of HT-ESEM observations and dilatometric measurements. On the one hand, the use of environmental microscope allowed the in situ observation of the pellet behaviour during heat treatments between 1250 °C and 1400 °C. Subsequent image analysis led to the determination of local (i.e. at the grain scale) and global (i.e. at the pellet scale) kinetic parameters. Particularly, the average grain size was plotted versus the holding time for the three considered temperatures. On the other hand, analogous experiments were performed by dilatometry and led to monitor both linear shrinkage and density of the samples. On this basis, the combination of the two sets of data allowed us to establish for the first time a sintering map for ThO2. This latter clearly evidenced two different zones driven by densification (d ≤ 92%) then by grain growth (d ≥ 92%) which can be used efficiently to monitor the final microstructure of sintered thorium oxide.

Analytical solutions of sharp interface models with nth order kinetics. Application to char conversion

When a reaction occurs in a narrow layer of a porous particle, it is practical to simplify the calculation of the reaction rate by assuming global kinetic parameters based on the surface of the reaction front. The assumption of such simplified situation allows formulation of a sharp interface model (SIM). Though, there are three difficulties for the application of a SIM in reactor simulations: (1) No analytical solution is available for general SIM with nth order kinetics with respect to gas reactant. For reactor model simulation, with a variety of particles and operating conditions, a numerical model is still necessary to be applied, leading to numerical difficulties and time consumption; (2) The global surface kinetic coefficient is not a priori known. The reason is that this coefficient depends not only on the intrinsic reactivity, but also on physical factors such as the size and density of the solid, as well as operation conditions of the reactor like gas reactant concentration and temperature, varying during conversion of the particle; (3) The SIM is applicable when chemical reaction is rapid compared to intraparticle diffusion because in this situation the reaction occurs within a narrow region compared to the size of the particle. However, no quantitative criteria has been developed to delimit the conditions for application of SIM. In the present work these questions are answered. Char conversion (combustion and gasification) is taken as reference, but most conclusions are applicable to isothermal non-catalytic gas–solid irreversible reactions with a single gaseous reactant.

Data analysis of the inactivation of foodborne microorganisms under high hydrostatic pressure to establish global kinetic parameters and influencing factors.

The inactivation rate of foodborne microorganisms under high hydrostatic pressure (HHP) is influenced by factors such as substrate, species, strain, temperature, pH, and stage of growth of the cell. In this study, 445 D(P)-values from previously published data were analyzed, including those from bacterial spores, vegetative cells, and yeasts. Three secondary linear inactivation models with pressure and/or temperature as process parameters were tested to estimate global log D(P)-, z(P)-, and z(T)-values, and the influence of these parameters and additional factors was assessed. The results show that significant differences in microbial resistance are mainly the result of temperature, highlighting the need for its inclusion as a process parameter. Perhaps due to the large number of data and very distinct factors, the remaining factors showed no significant differences in microbial resistance, except in the case of Clostridium spp. in soy milk, which showed decreased resistance in this substrate compared with its behavior in other products. These results serve to establish priorities among factors influencing HHP inactivation and to estimate global kinetic parameters as a basis for setting target levels of inactivation. Moreover, they can be used as a benchmark for comparison of microbial HHP inactivation data gathered in future studies.

Experimental Method for Kinetic Studies of Gas–Solid Reactions: Oxidation of Carbonaceous Matter

A methodology comprising careful consideration of sample preparation, reactor design, experimental procedures, and data evaluation routines for precise analysis of the kinetics of gas–solid reactions, specifically the oxidation of carbonaceous materials, has been developed and validated. The well-controlled solvent-free deposition of the carbonaceous material onto cordierite monolith substrates ensures experimental studies in the absence of diffusion limitations, temperature gradients, and hot zones. These critical aspects are supported also by theoretical considerations. Temperature-programmed oxidation and isothermal oxygen step-response experiments in a continuous gas-flow reactor using a homogeneous synthetic carbon-black material demonstrate excellent reproducibility, and the conversion profiles agree well with previously reported data. An independent set of global kinetic parameters was estimated for each 5% subconversion interval using linear regression such that the conversion dependence of each parameter could be analyzed separately and compared to previously published data. The results show that the evolution of reactive carbons cannot be described with a single global reaction order. This is supported by intermittent ex situ measurements of the specific surface area of the carbon-black material during the course of isothermal oxidation, which reveals a developing microporous structure at high conversions. Physically the changes in carbon reaction order are interpreted as changes in fraction of accessible reactive carbon atoms during progressing oxidation. Moreover, at high conversions, the carbon reaction order approaches 0.7 implying not only that the evolution of the concentration of reactive carbon atoms is proportional to the external surface area of shrinking spheres but also that these spheres have approximately the same size.

Study of sawdust pyrolysis and its devolatilisation kinetics

AbstractPyrolysis of sawdust was studied using a thermogravimetric analyser (TGA) to understand the devolatilisation process and to obtain its global kinetic parameters. The influences of particle size, initial weight of the sample and heating rate on the devolatilisation of sawdust particles have been studied. Results from proximate analysis show that smaller particle size has more ash content compared to larger particle size. The TG and derivative TG curve for variation in particle size and initial weight of the sample showed significant difference in the third stage of the pyrolysis. In addition, the pyrolysis of sawdust differed significantly for variation in heating rate. As the heating rates increased, the char yield also increased. The devolatilisation kinetics was studied considering different stages of pyrolysis. The kinetic parameters for thermal devolatilisation of the sawdust were determined through a nonlinear optimisation method of two independent parallel nth‐order reaction models. The kinetic parameters such as activation energy, frequency factor and order of the reaction for the two stages considered in the model were: E2 = 79.53 (kJ/mol), E3 = 60.71 (kJ/mol); k02 = 1.90 × 106 (1/min), k03 = 1.01 × 103 (1/min); n2 = 0.91, n3 = 1.78, respectively. The results show good agreement between the proposed model and the experimental data of the sawdust pyrolysis.

Measurement and modelling of adsorption equilibrium, adsorption kinetics and breakthrough curve of toluene at very low concentrations on to activated carbon

Indoor air pollution, characterized by many pollutants at very low concentrations, is nowadays known as a worrying problem for human health. Among physical treatments, adsorption is a widely used process, since porous materials offer high capacity for volatile organic chemicals. However, there are few studies in the literature that deal with adsorption as an indoor air pollution treatment. The aim of this study was to investigate the adsorption of toluene on to activated carbon at characteristic indoor air concentrations. Firstly, global kinetic parameters were determined by fitting Thomas's model to experimental data obtained with batch experiments. Then, these kinetic parameters led to the determination of Henry's coefficient, which was checked with experimental data of the adsorption isotherm. Secondly, we simulated a breakthrough curve made at an inlet concentration 10 times higher than the indoor air level. Even if the kinetic parameters in this experiment are different from those in batch experiments, it can be emphasized that the Henry coefficient stays the same.

Thermogravimetric Analysis of Photodegraded Acrylic Coated Lime Wood

A series of experiments were carried out to investigate the photodegradation of lime wood (Tilia cordata Mill.) coated with acrylic copolymer during artificial UV/Vis light irradiation for 600 h. Photodegradation of the Paraloid B72 films and Paraloid B72 treated lime wood samples was evaluated by thermogravimetry throughout the irradiation period of 100 h. The results obtained indicate a shifting of the DTG maxima to lower temperatures, which may be related to a decrease in the stability of the copolymer and wood during photodegradation. The decrease of weight loss with increasing time of exposure was observed, while the global kinetic parameters for the main peak increases when increasing exposure time of wood to the UV light. Even when the surface of the wood was covered with a thin layer of acrylic resin, some photodegradation reactions of the wood surface occurred. The modifications in the wood structure may be influenced by the newly formed structures from acrylic resin photodegradation.

Effects of a Pt/Ce0.68Zr0.32O2 catalyst and NO2 on the kinetics of diesel soot oxidation from thermogravimetric analyses

In this study, the reactivity of well-characterized diesel soot samples is investigated by thermogravimetry under different kinds of oxidizing atmospheres (20% O2 or 10% O2+700ppm NO2) either under catalyzed or non-catalyzed conditions. Whatever the atmosphere used, the catalyst Pt/ceria–zirconia was able to lower significantly the ignition temperature of soot, but the catalytic effect was found to be more pronounced when the oxidation process was assisted by NOx. This is due mainly to the efficiency of both catalyst components (the noble metal and the OSC material) in recycling the NO released after attack of the soot by NO2. By contrast, the NO2 is of very limited use in the absence of catalyst under our experimental conditions. The global kinetic parameters representative of the carbonaceous matrix oxidation are determined using a methodological approach combining thermogravimetric experiments and non-linear multivariate regression. The kinetic parameters obtained are consistent both with the literature results and the postulated mechanistic pathways for soot oxidation assisted or not by NOx.

Thermal behavior of biodegraded lime wood

The effects of the soft-rot fungus Trichoderma viride Pers., on the thermal behavior of lime wood ( Tillia cordata Mill .) were investigated. The lime wood pieces were inoculated with the fungus over a 12-week period. At pre-established time intervals two samples were withdrawn from the medium and analyzed by thermogravimetry and differential calorimetry, and the results were correlated with mass loss. Fungal activity was indicated by continuous decrease of sample mass. Modification of the wood because of the presence of the fungus was evidenced by structural changes that affected its thermal properties, both in respect to the hydrophilicity of the wood (evidenced mainly in desorption process) and in its decomposition behavior. The shape of DTG curves depends on the exposure time of wood to the action of microorganisms. The peak temperature assigned to the decomposition of wood components increases, while the global kinetic parameters for the main peak decrease with increasing exposure time of the wood to the attack by microorganisms. The increased characteristic temperatures of water desorption and cellulose decomposition processes and lower thermal stability could be explained by newly formed structures, mainly the oxidized ones.

Pyrolysis of wheat straw in a thermogravimetric analyzer: Effect of particle size and heating rate on devolatilization and estimation of global kinetics

The influence of different parameters such as particle size, initial weight of the sample, and heating rate on the devolatilization of wheat straw particles have been studied using thermogravimetric analysis. In addition, the variations in proximate analysis with different particle sizes of wheat straw have also been investigated. Results show that the curves corresponding to the third stage of pyrolysis differ for variations in particle size, initial weight, and heating rate of the pyrolysis process. A devolatilization model with three parallel nth-order reactions has been considered to determine the global kinetic parameters using thermogravimetric data. The kinetic parameters such as activation energy (kJ/mol), frequency factor (1/min), and order of the reaction for the three stages considered in devolatilization model were E 1 = 69, E 2 = 78, E 3 = 80; k 01 = 2.57 × 10 12, k 02 = 3.97 × 10 7, k 03 = 3.17 × 10 6; and n 1 = 2.3, n 2 = 0.65, n 3 = 2.7, respectively. It was noted from the order of the reaction that the second stage of the pyrolysis curve corresponds to the degradation of cellulose and hemicellulose, and the third stage to the lignin degradation.