Bojan Janković

The comparative kinetic analysis of Acetocell and Lignoboost® lignin pyrolysis: The estimation of the distributed reactivity models

The non-isothermal pyrolysis kinetics of Acetocell (the organosolv) and Lignoboost® (kraft) lignins, in an inert atmosphere, have been studied by thermogravimetric analysis. Using isoconversional analysis, it was concluded that the apparent activation energy for all lignins strongly depends on conversion, showing that the pyrolysis of lignins is not a single chemical process. It was identified that the pyrolysis process of Acetocell and Lignoboost® lignin takes place over three reaction steps, which was confirmed by appearance of the corresponding isokinetic relationships (IKR). It was found that major pyrolysis stage of both lignins is characterized by stilbene pyrolysis reactions, which were subsequently followed by decomposition reactions of products derived from the stilbene pyrolytic process. It was concluded that non-isothermal pyrolysis of Acetocell and Lignoboost® lignins can be best described by n-th (n>1) reaction order kinetics, using the Weibull mixture model (as distributed reactivity model) with alternating shape parameters.

The pyrolysis process of wood biomass samples under isothermal experimental conditions—energy density considerations: application of the distributed apparent activation energy model with a mixture of distribution functions

This work deals with the isothermal pyrolysis of Pine and Beech wood samples and kinetic studies, using the thermo-analytical technique, at five different operating temperatures. Pyrolysis processes were investigated by using the distributed apparent activation energy model, which involves the complex mixture of different continuous distribution functions. It was found that decomposition processes of wood pseudo-components take place in different conversion areas during entire pyrolyses, whereby these areas, as well as the changes in apparent activation energy (Ea) values, are not the same for softwood and hardwood samples. Bulk density (Bden) and energy density (ED) considerations have shown that both biomass samples suffer from low Bden and ED values. It was concluded that pyrolysis can be used as a means of decreasing transportation costs of wood biomass materials, thus increasing energy density. The “pseudo” kinetic compensation effect was identified, which arises from kinetic model variation and wood species variation. In the current extensive study, it was concluded that primary pyrolysis refers to decomposition reactions of any of three major constituents of the considered wood samples. Also, it was established that primary reactions may proceed in parallel with simultaneous decomposition of lignin, hemicelluloses and cellulose in the different regions of wood samples, depending on the operating temperature. It was established that endothermic effects dominate, which are characterized with devolatilization and formation of volatile products. It has been suggested that the endothermic behavior that arises from pyrolyses of considered samples may indicate the endothermic depolymerization sequence of cellulose structures.

Kinetic Analysis of Isothermal Decomposition Process of Zinc Leach Residue in an Inert Atmosphere. The Estimation of the Apparent Activation Energy Distribution

Thermal decomposition of zinc leach residue has been studied in a tubular furnace under a constant nitrogen gas flowing, at four different operating temperatures (600°C, 750°C, 950°C, and 1150°C). Using a detailed kinetic analysis, it was shown that the investigated process can be described by a two-parameter autocatalytic Šesták–Berggren reaction model. It was noted that the apparent activation energy values Ea increase progressively with a degree of conversion, accompanied by the appearance of a convex Arrhenius dependence. This behavior is a characteristic of a system of parallel competing reactions. It was concluded that the investigated isothermal decomposition process is characterized by unusually very low preexponential factor and low values of the apparent activation energy. Based on the derived density distribution function of Ea values, it was concluded that the isothermal decomposition process probably occurs through four reaction steps, where each step is characterized by one parallel reaction.

On-line pyrolysis kinetics of swine manure solid samples collected from rearing farm

The on-line dynamic pyrolysis kinetics of swine manure solid samples was investigated in this paper. It has been found that the model which includes combined Friedman’s isoconversional and multi-distributed reactivity approaches can best describe the pyrolysis process. Pyrolysis kinetics of swine manure samples proceeds through two major pyrolysis zones, where in first zone exists the kinetically complex reactions related to decompositions of hemicelluloses and fat/proteins, as well as decomposition reactions related to degradation of cellulose and lignin structures. Second zone includes the reactions attached to follow-up devolatilization and carbonization processes. It was found that identified high reaction orders represent the measure of complexity and multiplicity of the mechanism. It has been found that the change in reaction order (n) values is the result of reactions that are taking place due to the instability of products, or the secondary reactions, change in apparent activation energy, and effect of varying of the pre-exponential factor values.

TG-DTA-FTIR analysis and isoconversional reaction profiles for thermal and thermo-oxidative degradation processes in black chokeberry (Aroniamelanocarpa)

The thermal and thermo-oxidative processes in Aroniamelanocarpa (black chokeberry) were investigated using combined thermo-analytical (TG-DTA) and spectroscopic (FTIR) experimental techniques. Isoconversional analysis revealed that the process in an inert (argon) atmosphere was probably governed by chlorogenic acid degradation, where autocatalysis (described by the empirical Šesták-Berggren model) might occur due to water already present in the early stages of the process through hydrolysis. Thermal degradation is described by the intrinsic kinetic parameters, where the degradation rate increases proportionally with an increase in the heating rate. Under oxidative conditions, the process was found to be primarily driven by neochlorogenic acid degradation. The thermo-oxidative degradation of Aroniamelanocarpa fresh samples can be described by two competitive reactions, where it was established that a cyanidin-3-glucosylrutinoside degradation made a significant contribution to a comprehensive kinetics. This study showed the targeting of the neochlorogenic acid in Aroniamelanocarpa fresh samples to have a strong hydrogen-donating activity, thereby rendering it capable of very efficiently entrapping the peroxy radicals. Current research has demonstrated that the relative contribution of the two competitive reactions to the overall process is highly dependent on the heating rate of the system under consideration.

Thermal stability and nonisothermal kinetics of Folnak degradation process.

Aim: The purpose of this article is to investigate the thermal stability and nonisothermal kinetics of Folnak® drug degradation process using different thermoanalytical techniques. Methods: The nonisothermal degradation of Folnak® powder samples was investigated by simultaneous thermogravimetry–differential thermal analysis, in the temperature range from ambient to 810°C. Results: It was found that the degradation proceeds through five reaction stages, which include the dehydration, the melting process of excipients, the decomposition of folic acid, corn starch, and saccharose. The presence of compounds such as excipients increases the thermal stability of the drug and some kind of solid–solid and/or solid–gas interaction occurs. Conclusion: It was concluded that the main degradation stage of Folnak® sample represents the decomposition of folic acid. It was established that the folic acid decomposition cannot be explained by simple reaction order model (n = 1) but with the complex reaction mechanism that includes higher reaction orders (n > 1). The isothermal predictions of the folic acid decomposition at four different temperatures (Tiso = 180°C, 200°C, 220°C, and 260°C) were established. It was concluded that the shapes of conversion curves at lower temperatures (180–200°C) were similar, whereas they became more complex with further temperature increase because of the complexity of the decomposition reaction.

Kinetic Analysis of Nonisothermal Reduction of Silica-Supported Nickel Catalyst Precursors in a Hydrogen Atmosphere

A series of silica-supported nickel catalyst precursors was synthesized with different SiO2/Ni molar ratios. Reduction of Ni catalyst precursors with different SiO2/Ni molar ratios under a hydrogen atmosphere was investigated at different heating rates. Kinetic parameters were determined using Kissinger–Akahira–Sunose isoconversional and invariant kinetic parameter methods. It was found that for all molar ratios, the apparent activation energy (Ea) is practically constant in the conversion range of 0.20 ≤ α ≤ 0.80. In the considered conversion range, following values of Ea were found: 134.5 kJ mol−1 (SiO2/Ni = 0.20), 139.6 kJ mol−1 (SiO2/Ni = 0.80), and 128.3 kJ mol−1 (SiO2/Ni = 1.15). It was established that the reduction of Ni catalyst precursors with different SiO2/Ni molar ratios is a complex process and can be described by the Šesták–Berggren autocatalytic model. It was found that the reaction is more Langmuir–Hinshelwood type, as hydrogen dissociates rapidly on surface nuclei and the dissociated hydrogen reacts with the Ni–O active system. It was concluded that the reduction process proceeds through bulk nucleation, which is a dominant mechanism, where three-dimensional growth of crystals with polyhedron-like morphology exists. It was found that the Ni/Si ratio decreases after the reduction process. This has been explained by low Ni and higher Si surface concentrations. It has been disclosed that Ni dispersion decreases.

Tritium concentration analysis in atmospheric precipitation in Serbia.

Tritium activity concentration were monitored in monthly precipitation at five locations in Serbia (Meteorological Station of Belgrade at Zeleno Brdo, Vinča Institute of Nuclear Sciences, Smederevska Palanka, Kraljevo and Niš) over 2005, using electrolytic enrichment and liquid scintillation counting. The obtained concentrations ranged from 3.36 to 127.02 TU. The activity values obtained in samples collected at Zeleno Brdo were lower or close to the minimum detectable activity (MDA), which has a value of 3.36 TU. Significantly higher tritium levels were obtained in samples collected in Vinča Institute of Nuclear Sciences compared with samples from the other investigated locations. Amount of precipitation were also recorded. A good linear correlation (r = 0.75) for Zeleno Brdo and VINS between their tritium activity was obtained. It was found that the value of the symmetrical index n (which indicates the magnitude of tritium content changes with time (months) through its second derivative) is the highest for Vinča Institute of Nuclear Sciences compared to other locations, which is in accordance with the fact that the highest concentrations of tritium were obtained in the samples from the cited place.

TGA-DSC-MS Analysis of Pyrolysis Process of Various Biomasses with Isoconversional (Model-Free) Kinetics

Slow pyrolysis characteristics of different biomasses (hazelnut shell (HS), sawdust (Beech), and sawdust chemically treated (SDCT)) were investigated by simultaneous thermal analysis (STA), coupled with mass spectrometry (MS). Thermal decomposition of these samples was divided into three stages corresponding to removal of water, devolatilization, and formation of bio-char. It was found that differences in thermal behavior of the samples are due to differences in their composition. Mass spectrometry results showed that H2, CH4, H2O, CO2 (C3H8), CO, and C2H6 were the main gaseous products released during pyrolysis. It was shown that HS could be a good fuel, since during its pyrolysis at high temperature, more gaseous products are released compared to other systems. Isoconversional (model-free) method was used in order to determine variation magnitudes of effective activation energy (Ea) values on conversion fraction (α) during pyrolysis. It was found that identified variations of Ea with α arise from the different chemical structures among cellulose, hemicelluloses and lignin in tested samples that may affect on their effective activation energies.

Reliable method for determining the complete kinetic and thermodynamic information for thermal degradation of polymers in a multi-step process

Proposed approach enables a reliable determination of kinetic and thermodynamic behaviors for thermal degradation of polymers. As a model polymer, poly(ethylene oxide) with average molecular weight Mv ~ 100.000 (PEO100) was used. Two-component continuous distribution kinetics best describes PEO100 thermal degradation, involving two main reactions with different values of reaction orders. Procedure enables the check of existence of true or real enthalpy–entropy compensation (EEC). It was established that in the case of random scission reaction step, the artificial EEC emerges. For second reaction step which involves β-scission reaction step (“slow” step), true (real) EEC emerges. It was found that this reaction step is responsible for occurrence of successive micro-phase transitions. These transitions are associated with significant reduction of PEO100 crystallinity through a hinder effect on polymer chains. It was concluded that these transitions are cyclic and reversible, related to the change of mechanical properties of the polymer.