Values Of Pre-exponential Factor Research Articles

Thermochemical conversion of livestock wastes: Carbonization of swine solids

Slow pyrolysis or carbonization promotes the conversion of animal manures such as swine manure into charcoal. In this paper, the carbonizing kinetics of swine solids taken from different treatment stages were investigated with a thermogravimetric analyzer. Compared to their biologically stabilized counterpart (lagoon sludge) with an activation energy of 160 kJ mol −1, the activation energies for fresh swine solid samples such as homogenized flushed manure and dewatered solids were much lower between 92 and 95 kJ mol −1. Compared to the kinetics of first order decomposition of cellulose, the pyrolytic decomposition of the swine manures were more complex with the reaction orders varying at 3.7 and 5.0. The two different mathematical methods employed in this paper yielded the similar values of activation energy ( E) and pre-exponential factor ( A), confirming the validity of these methods. The results of this study provide useful information for development of farm-scale swine solid carbonization process.

A kinetic study of the thermal decomposition process of potassium metabisulfite: Estimation of distributed reactivity model

The thermal decomposition kinetics of potassium metabisulfite was studied by thermogravimetric (TG) and differential thermogravimetric (DTG) techniques using non-isothermal experiments. The apparent activation energy ( E a ) is determined using the differential (Friedman) isoconversional method. The results of the Friedman's isoconversional analysis of the TG data suggests that the investigated decomposition process follows a single-step reaction and the observed apparent activation energy was determined as 122.4±2.1 kJ mol −1. A kinetic rate equation was derived for the decomposition process of potassium metabisulfite with contracting area model, f( α)=2(1− α) 1/2, which is established using the Malek's kinetic procedure. The value of pre-exponential factor ( A) is also evaluated and was found to be A=1.37×10 12 min −1. By applying the Miura's procedure the distributed reactivity model (DRM) for investigated decomposition process was established. From the dependence α versus E a , the experimental distribution curve of apparent activation energies, f( E a ), was estimated. By applying the non-linear least-squares analysis, it was found that the Gaussian distribution model (with distribution parameters E 0=121.3 kJ mol −1 and σ=1.5 kJ mol −1) represents the best reactivity model for describing the investigated process. Using the Miura's method, the A values were estimated at five different heating rates and the average A values are plotted against E a . The linear relationship between the A and E a values was established (compensation effect). Also, it was concluded that the E a values calculated by the Friedman's method and estimated distribution curve, f( E a ), are correct even in the case when the investigated decomposition process occurs through the single-step reaction mechanism.

Study of Curing Kinetics and Thermal Degradation of UV Curable Epoxy Acrylate Resin

Blends of epoxy acrylate resins (acid values 3, 6.5 & 10 mg KOH/gm Solid) with monofunctional monomers (ethoxylated phenol monoacrylate) were prepared by physical mixing, having weight ratio 50:50. These blends were cured by using UV radiations in presence of photo initiator (Darocure 1173). The thermal degradation kinetics of these resin blends were studied, using thermo gravimetric analysis in nitrogen atmosphere at a heating rate of 10°C/min. by applying Coats-Red fern equation. According to the analysis, all the coating films degrade in two steps. In the first step of degradation kinetics, R2M follows 1.75 order (n=1.75) and all other coating films follow second order (n=2) kinetics. In second step, R2M & R3M follow half order (n=0.5) kinetics and R1M follow first order (n=1) degradation kinetics. Order of the reaction is obtained on the basis of best fit analysis, and all the parameters were confirmed by regression analysis. From the reaction order, value of activation energy (E) and pre exponential factor (Z) were calculated by the slop and intercept of the plot between X and Y, respectively.

Kinetic analysis of the oxidative decomposition in γ-zirconium and γ-titanium phosphate intercalation compounds: The case of 2,2′-bipyridyl and its copper complex formed in situ

The thermal behaviour of the heterocyclic compound 2,2′-bipyridyl and its copper complex formed in situ, both intercalated between the layers of γ-zirconium and γ-titanium phosphates was studied by simultaneous thermogravimetry and differential scanning calorimetry techniques using non-isothermal constant heating rate conditions up to 873 K under a stream of air. In all the obtained intercalation materials, which are found to be stable up to 600 K, the ligand shows an oxidative decomposition practically in only one step. Model-free isoconversional method of Ozawa–Flynn–Wall, applied to the vaporization of pure 2,2′-bipyridyl and to the oxidative decomposition step occurring in each intercalation material examined, yields practically constant activation energies for values of the fraction decomposed α ≤ 0.7. The activation energies calculated using the best fit between calculated and theoretical g( α) models do not differ significantly from the corresponding mean E values selected using the isoconversional OFW method in the range α ≤ 0.7. Activation energies derived by the Kissinger method show a good agreement with the mean values derived by the former method, and the Arrhenius rate constants determined using also the pre-exponential factor values enable to conclude that the bipyCu intercalation materials show a destabilizing effect with respect to the corresponding bipy intercalation materials (negligible difference in the oxidative decomposition temperatures, but a significant difference in the rate constant values: at least one order of magnitude).

Structural and thermal studies on the solid products in the system MnSeO3-SeO2-H2O

The solubility of MnSeO3-SeO2-H2O system was studied in the temperature region 25–300°C. The compounds of the three-component system were identified by the Schreinemaker’s method. The phase diagram of manganese(II) selenites was drawn and the crystallization fields for the different phases were determined. Depending on the conditions for hydrothermal synthesis, MnSeO3·H2O, MnSeO3·3/4H2O, MnSeO3·l/3H2O and MnSe2O5 were obtained. The different phases were proven and characterized by chemical, powder X-ray diffraction and thermal analyses, as well as IR spectroscopy. The kinetics of dehydration and decomposition of MnSeO3·H2O was studied under non-isothermal heating. Based on 4 calculation procedures and 27 kinetic equations, the values of activation energy and pre-exponential factor in Arrhenius equation were calculated for both processes.

Kinetics and mechanisms of monoammonium phosphate-induced potassium release from selected potassium-bearing minerals

A high concentration of phosphate in the immediate vicinity of the phosphate fertilizer zone of soil could induce K release from K-bearing minerals to soil solution. In this study, the kinetics of K release by monoammonium phosphate (NH4H2PO4) from biotite, muscovite, and microcline were investigated. The results indicate that K release from all of these minerals was promoted by 1 M NH4H2PO4 (pH 4.0). The release of K from these minerals can be described by the zero-order rate equation. The rate coefficients of K released from the minerals (2–5 µm) in 1 M NH4H2PO4 solution ranged from 0.20 to 2.09 mg kg-1 h-1 at 25°C and from 1.22 to 9.00 mg kg-1 h-1 at 45°C, respectively. The amount of K release from the same mineral samples in 1 M NH4Cl solution (pH 4.0) at 25°C was not detectable and was too low to be determined accurately at 45°C. The dfferences in the rate of NH4H2PO4-induced K release from the different minerals were explained by the chemical composition, crystal structure, and the crystallization sequence of the minerals. The activation energies of the phosphate-induced K release from these minerals ranged from 37 to 72 kJ mol-1 and the pre-exponential factor values (Arrhenius equation) ranged from 1.2 × 106 to 7.2 × 1011 mg kg-1 h-1. The examination of the phosphate-reacted minerals shows the formation of crystalline NH4-taranakite in the muscovite and microcline systems and noncrystalline products in the biotite system. The absence of the formation of crystalline NH4-taranakite in the biotite system is attributed to the Fe perturbation. These data indicate that the major mechanism of NH4H2PO4-induced K release from the K-bearing minerals was the combined effect of phosphate and protons on the alteration of minerals. Therefore, the K-supplying rate of soils in the immediate vicinity of fertilizer zones may be enhanced through phosphate fertilization. Key words: Potassium release, kinetics and mechanisms, phosphates, potassium-bearing primary minerals

An explanation of the peculiar behavior of TSDC peaks at Tg: a simple model of entropy relaxation

Thermally stimulated depolarization current (TSDC) signals measured in polymers at the glass transition Tg often exhibit intricate shapes that much depend on the thermal history and on the non-stationary condition used during experiments. This peculiar behavior is frequently explained in terms of a physical singularity of the molecular motions. We show that this singularity, i.e. unexpected values for activation energy and pre-exponential factor, obtained around Tg, result from the misuse of the Arrhenius law for treating the experimental data obtained in non-stationary experimental conditions. A simple model using time dependent configurational entropy for the material evolution and using a single Debye relaxation for the dielectric relaxation process is therefore proposed to explain the experimental behavior. The influence of the heating rate and of a non-Debye relaxation on the TSDC signal is also studied and clearly shows that the peculiar behavior of TSDC peaks can originate from the conjugated effects of entropy relaxation and non-Debye response

An explanation of the peculiar behavior of TSDC peaks at Tg: a simple model of entropy relaxation

Thermally stimulated depolarization current (TSDC) signals measured in polymers at the glass transition T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> often exhibit intricate shapes that much depend on the thermal history and on the non-stationary condition used during experiments. This peculiar behavior is frequently explained in terms of a physical singularity of the molecular motions. We show that this singularity, i.e. unexpected values for activation energy and pre-exponential factor, obtained around T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> , result from the misuse of the Arrhenius law for treating the experimental data obtained in non-stationary experimental conditions. A simple model using time dependent configurational entropy for the material evolution and using a single Debye relaxation for the dielectric relaxation process is therefore proposed to explain the experimental behavior. The influence of the heating rate and of a non-Debye relaxation on the TSDC signal is also studied and clearly shows that the peculiar behavior of TSDC peaks can originate from the conjugated effects of entropy relaxation and non-Debye response

Development of a predictive kinetic model for homogeneous Hg oxidation data

Several researchers have developed kinetic models to predict the effects of various flue gas components on homogeneous mercury (Hg) oxidation. Most of these models make use of over 50 reversible reactions that involve radicals in a combustion or post-combustion system, where temperatures are similar to that of a power plant boiler, in excess of 1700 ∘C. Reaction rate constants are expressed in the form of k = A ∗ T n ∗ exp ( − E / R / T ) where the unknown variables are A (pre-exponential factor), n (the exponent of temperature) and E (activation energy). The focus of this paper is the development of a simple kinetic model to predict experimental data in a laboratory scale apparatus. The results obtained show that only five reactions are required to predict the data obtained in the experimental work. Two of these reactions are reversible and three are irreversible. None of the reactions involve radicals, and only the pre-exponential factor and activation energy values in the rate constant term for the reactions are determined — the value of ‘ n ’ is set at 0. It is found that the predicted values correspond very well with the observed experimental data.

Prediction of Experimental Methanol Decomposition Rates on Platinum from First Principles

A microkinetic model for methanol decomposition on platinum is presented. The model incorporates competitive decomposition pathways, beginning with both O–H and C–H bond scission in methanol, and uses results from density functional theory (DFT) calculations [Greeley and Mavrikakis, J. Am. Chem. Soc. 124 (2002) 7193, Greeley and Mavrikakis, J. Am. Chem. Soc. 126 (2004) 3910]. Results from reaction kinetics experiments show that the rate of H2 production increases with increasing temperature and methanol concentration in the feed and is only nominally affected by the presence of CO or H2 with methanol. The model, based on the values of binding energies, pre-exponential factors and activation energy barriers derived from first principles calculations, accurately predicts experimental reaction rates and orders. The model also gives insight into the most favorable reaction pathway, the rate-limiting step, the apparent activation energy, coverages, and the effects of pressure. It is found that the pathway beginning with the C–H bond scission (CH3OH→H2COH→HCOH→CO) is dominant compared with the path beginning with O–H bond scission. The cleavage of the first C–H bond in methanol is the rate-controlling step. The surface is highly poisoned by CO, whereas COH appears to be a spectator species.

The effect of orientation of various phenols on the degradation kinetics of blends of resole and epoxy

Resoles were prepared separately with different phenols, with formaldehyde having phenol-to-formaldehyde mole ratio of 1:2 in basic medium at 70°C. These resoles were physically blended with different weight percentages of diglycidyl ether of bisphenol-A (DGEBA)–epoxy resin. The blends were cured with 40, 50, and 60 wt % polyamide (based on total amount of the blended resin). Decomposition kinetics of blend sample was studied by dynamic thermogravimetric (TG) analysis. It was found that the degradation of most of the blend samples followed two-step second-order degradation kinetics with 50 wt % polyamide and varied reaction orders with 40 and 60 wt % polyamide and type of phenols used in the resole. The energy of activation (E) was found to be maximum with the blend system containing p-cresolic resole and epoxy. The value of preexponential factor (Z) decreased when phenolic resole was changed to p-cresolic resole in the blend. Also, the plotted values of E and Z were found to be in close resemblance with those obtained from calculated values. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:4171–4176, 2006

'Up-Hill' Interdiffusion and Accompanying Effects in Ternary Cu-Ni-5 at. % Sn(In) Systems

Kinetics of phase growth in multiphase Cu-Sn and Ni-Sn systems and effect of third substitutive component (Ni or Cu respectively) were studied experimentally in temperature range 160-220oC and analyzed theoretically. Kinetic equations for calculation of diffusion coefficients in growing competitive phases were obtained. It was found that the substitutive components accelerated phase growth lowering the values of diffusion activation energy and pre-exponential factor. As a result the growth rates of competitive phases Cu6Sn5 and Cu3Sn in Cu-Sn system changed with substitutive Ni so, that only one (CuNi)6Sn5 phase survived whereas (CuNi)3Sn phase was suppressed. Obtained results are discussed in terms of higher disordering caused by substitutive components in ternary phases as compared with binary ones.

Kinetic models for the pyrolysis and combustion of two types of sewage sludge

Pyrolysis and combustion of two types of sewage sludge have been studied. The two sludges considered, considering their thermal decomposition, have different behaviors when comparing pyrolysis and combustion TG runs of each sludge. In one case, the combustion can be considered as an oxidative pyrolysis, where the decomposition process is accelerated by the presence of oxygen. In the other case, the TG combustion run has two parts, the first one coinciding with the pyrolysis run and the second one with combustion of the char formed previously. For the pyrolysis of each sludge, three thermogravimetric runs at different heating rates (5, 15 and 30 K/min) were carried out to obtain a kinetic model, considering three parallel reactions. Following the simplex method, and with the same set of parameters for the three runs, and for each one of the three fractions considered, the values of pre-exponential factors, the apparent activation energies (103.9, 169.7 and 209.4 kJ/mol), the reaction orders and the maximum amount of volatiles were obtained. A discussion of the different values, in comparison with others obtained from literature is presented. Following a similar methodology, for the combustion of each sludge, two series of three runs at different oxygen concentrations (10 and 20%) and heating rates were carried out. For the sludge with oxidative pyrolysis behaviour, the scheme of three parallel reactions (as in pyrolysis) was appropriate for correlating the data, using the same values of activation energy, reaction order and maximum amount of volatiles evolved, and optimizing only the value of the pre-exponential factor, that is correlated with oxygen pressure. On the other hand, for the other sludge, with the first stage coinciding with pyrolysis and the second stage concerning the char combustion, the kinetic parameters of the second stage are obtained, with an apparent activation energy of 144.1 kJ/mol.

A simple model of entropy relaxation for explaining effective activation energy behavior below the glass transition temperature

Strong changes in relaxation rates observed at the glass transition region are frequently explained in terms of a physical singularity of the molecular motions. We show that the unexpected trends and values for activation energy and preexponential factor of the relaxation time tau, obtained at the glass transition from the analysis of the thermally stimulated current signal, result from the use of the Arrhenius law for treating the experimental data obtained in nonstationary experimental conditions. We then demonstrate that a simple model of structural relaxation based on a time dependent configurational entropy and Adam-Gibbs relaxation time is sufficient to explain the experimental behavior, without invoking a kinetic singularity at the glass transition region. The pronounced variation of the effective activation energy appears as a dynamic signature of entropy relaxation that governs the change of relaxation time in nonstationary conditions. A connection is demonstrated between the peak of apparent activation energy measured in nonequilibrium dielectric techniques, with the overshoot of the dynamic specific heat that is obtained in calorimetry techniques.

Compensation Effects In The Non-isothermal Pyrolysis Of Wood

A considerable variation exists in the kinetics constants that were derived using Arrhenius equation in the study of the reaction kinetics for wood pyrolysis. It is of interest to evaluate whether compensation behaviours that usually associate with a series of related reactions might have contributed to the observed variations in the nonisothermal pyrolysis kinetics. The study of the compensation effects was intended to seek a logical explanation for the observed variations of the apparently large and differed values of activation energy and pre-exponential factors. Statistical regression analyses were performed to correlate the Arrhenius parameters derived from the non-isothermal Thermogravimetry Analysis (TGA) data. To gain an insight on the competing reactions so as to expound the varied kinetic data, these transition kinetics were also analyzed for exothermic transition using Differential Scanning Calorimetry (DSC) for heat of pyrolysis. This study showed that compensation effect existed, but found that the enthalpy of transition held a stronger proposition on the transition kinetics.

Effect of silver oxide doping on surface and catalytic properties of Co 3O 4/Al 2O 3 system

Cobaltic oxide catalyst supported on gamma-alumina having the formula 0.1 Co 3O 4/Al 2O 3 was prepared by wet impregnation method using finely powdered Al(OH) 3 and cobalt nitrate dissolved in the least amount of distilled water. Four silver oxide-doped samples were prepared by impregnating Al(OH) 3 solid with calculated amounts of silver nitrate dissolved also in the least amount of distilled water prior to impregnation with cobalt nitrate solution. The amounts of Ag 2O were 0.2, 0.4, 0.8 and 1.6 wt.%. Pure and doped solids were subjected to heat treatment at 400, 600 and 800 °C. Diffraction lines of Co 3O 4 phase were detected in the XRD patterns of pure and doped solids precalcined at different temperatures. However, the doping process conducted at different temperatures brought about a progressive significant increase in the particle size and degree of ordering of Co 3O 4 phase. Ag 2O-doping of the investigated system affected a measurable decrease in its specific surface areas. The catalytic activities of different solids, in CO-oxidation by O 2 conducted at 125–225 °C, were found to increase significantly by increasing the amount of dopant added reaching to a maximum limit at 0.8 wt.% Ag 2O. The maximum increase in the catalytic activity expressed as reaction rate constant k measured at 150 °C over the solids precalcined at 400 °C and at 175 °C for the solids precalcined at 600 and 800 °C attained 337%, 118% and 219%, respectively. The doping process did not modify the activation energy ( E a) of the catalyzed reaction, and the observed apparent changes in E a values resulted from a compensation effect as evidenced from almost same changes in the values of pre-exponential factor of Arrhenius equation. Therefore, the observed increase in the catalytic activity of the investigated system due to Ag 2O treatment, which did not change the mechanism of the catalyzed reaction, could be attributed to an effective increase in the concentration of active sites contributing in chemisorption and catalysis of CO oxidation by O 2.

Thermal degradation studies on rigid polyurethane foams blown with pentane

AbstractThe effect of sodium dihydrogenphosphate, trisodium pyrophosphate, and sodium aluminocarbonate on the thermal decomposition of rigid polyurethane (PUR) foams, based on diphenylmethane‐4,4‐diisocyanate, diphenyl‐2,2‐propane‐4,4‐dioxyoligo(ethylene oxide), and oxyalkylenated toluene‐2,6‐diamine, blown with pentane, was studied. Thermogravimetric (TG) data have shown that there is a stabilization effect of additives in the initial stage of degradation, both in nitrogen and air atmosphere, and the decomposition proceeded in two steps up to 600°C. Results of the kinetic analysis by the isoconversional methods of Ozawa–Flynn–Wall and Friedman yielded values of (apparent) activation energy (Ea) and preexponential factor (A). For phosphate‐stabilized PUR samples, Ea remained stable over a broad area of the degree of conversion, while for carbonate‐containing sample two regions of Ea were observed. Further advanced kinetic analysis by a nonlinear regression method revealed the form of kinetic function that was the best approximation for experimental data—for a two‐stage consecutive reaction the first step was the Avrami–Erofeev nucleation‐dependent model, and the second step was a chemical reaction (1st or nth order) model. The integrated thermogravimetric (TG)/Fourier transform infrared (FTIR) technique probed the thermal degradation of modified PURs by analyzing the evolved gases. The solid residue remaining at different temperatures was identified by diffuse reflection FTIR (Kubelka–Munk format). The complex thermal behavior was discussed on the basis of the obtained results—it can be shown that the global stabilization effect is a multistage process whose initial conditions are of critical importance in governing the nature of the entire process. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2319–2330, 2003

Self-diffusion of iron in amorphous iron nitride

The measurement of self-diffusion of iron in amorphous ${\mathrm{FeN}}_{0.7}$ using secondary-ion mass spectroscopy is reported. Diffusion broadening of tracer layers of ${}^{57}{\mathrm{FeN}}_{0.7}$ was observed after isothermal vacuum annealing of the films at different temperatures. Strong structural relaxation effects on diffusion coefficient were observed below crystallization temperature of the amorphous phase. In the well-relaxed state, the values of preexponential factor ${D}_{0}$ and activation energy q are given by $\mathrm{ln}{D}_{0}=\ensuremath{-}16.6\ifmmode\pm\else\textpm\fi{}2{\mathrm{m}}^{2}/\mathrm{s}$ and $q=1.3\ifmmode\pm\else\textpm\fi{}0.2\mathrm{eV}.$ On the basis of correlation between $\mathrm{ln}{D}_{0}$ and q, it is suggested that the mechanism of self-diffusion of iron in amorphous iron nitride is very similar to that in metallic glasses.

Decomposition behavior of vinyl ester resins prepared in presence of tertiary amines

Vinyl ester resins V1, V2, and V3of acid value (∼6 mg KOH/g solid) were synthesized using bisphenol-A epoxy and acrylic acid in the presence of triethyl, tripropyl, and tributyl amines, respectively. The synthesized resins were characterized by Fourier transform infrared spectroscopy and a new peak at 2360 cm−1was observed due to attachment of amines to resin structure by a hydrogen bond. All the resins containing styrene (40% w/w) as reactive diluent were cured with benzoyl peroxide (2 phr) at 90°C for 1 hr. Decomposition behavior of the samples was studied by dynamic thermogravimetry at a heating rate of 10°C min−1under nitrogen atmosphere. For all the samples, the order of reaction was found to be 1. This was calculated using Coats and Redfern equation in accordance with best-fit analysis and was confirmed further by linear regression analysis. From this value of reaction order, activation energy (E) and pre-exponential factor (Z) were calculated. The activation energy increases from 25.90 to 30.72 kcal mol−1for V1to V3. The validity of data was checked by t-test statistical analysis.

Hybrid SNCR‐SCR technologies for NOx control: Modeling and experiment

AbstractThe hybrid process of homogeneous gas‐phase selective noncatalytic reduction (SNCR) followed by selective catalytic reduction (SCR) of nitric oxide (NO) was investigated through experimentation and modeling. Measurements, using NO‐doped flue gas from a gas‐fired 29 kW test combustor, provided data that allowed a fundamentally based catalyst monolith model, adapted from the literature, to be calibrated. The SCR model, which included pore and gas film diffusion (molecular and Knudsen) and Langmuir‐Hinshelwood surface reaction kinetics, used published values for the catalyst pre‐exponential factor and activation energy, and experimental measurements to determine the previously unavailable ammonia (NH3) adsorption coefficient on the catalyst surface. Then, with no further adjustment, a combined SNCR‐SCR model was developed using literature values for the reaction rate coefficients of 112 elementary homogeneous reactions. This combined model agreed well with combined SNCR‐SCR data. The combined model was then extrapolated to practical coal combustion conditions, and the economic benefits of a combined SNCR‐SCR process were explored. With a desired total NO reduction of 95% and NH3 slip in the exhaust constrained not to exceed 5 ppm, adding SNCR led to only modest decreases in the required SCR catalyst volume. Estimated system capital and annualized costs were not economically viable unless the SNCR portion by itself reduced more than 60% of the NO.