Theoretical Kinetic Model Research Articles

Kinetic analysis of nicotine desorption from silicon dioxide under non-isothermal conditions

Abstract The non-isothermal kinetics of nicotine desorption from precipitated silicon dioxide was investigated. The non-isothermal thermogravimetric curves of nicotine desorption at different heating rates (β = 5, 10 and 20 K/min) were recorded. The kinetics model of non-isothermal kinetics of nicotine desorption was determined by using the model-fitting and master-plot methods. It was found that non-isothermal nicotine desorption can mathematically describe by the theoretical kinetics model of phase – boundary controlled reaction (contracting volume, i.e., three dimensional shape) – R3. By applying the method of Kissinger and method of invariant kinetic parameters the values of kinetic parameters were determined. It was established that both applied method give almost identical kinetic parameters regardless their theoretical limitations. The dependence of activation energy on the degree of nicotine desorption was determined using the KAS method. It was found that the value of activation energy is independent on the degree of nicotine desorption. The effect of heating mode on the kinetic of nicotine desorption was analyzed. It was established that the type of heating mode (isothermal and non-isothermal) does not influence the mechanism of the formation of the activation complex nor the kinetics model of desorption. The increase in the values of kinetic parameters obtained for non-isothermal heating in comparison to the isothermal ones are explained with the Larson's model of selective transfer of energy.

Incompatible reaction for (3-4-epoxycyclohexane) methyl-3′-4′-epoxycyclohexyl-carboxylate (EEC) by calorimetric technology and theoretical kinetic model

(3-4-Epoxycyclohexane) methyl-3′-4′-epoxycyclohexyl-carboxylate (EEC) is a typical epoxy resin (EP). In Asia, due to the unstable reactive natures of EP, various thermal hazard and runaway reaction incidents have been occasioned by EP in the manufacturing process, such as fire, explosion, and toxic release, resulting in loss of life as well financial catastrophes and social outcries. Certain catalysis substances, H2SO4, acetic acid, or NaOH, may accelerate the reaction or curing rate for EP. However, an incompatible reaction with these chemical substances may induce a thermal hazard, causing a runaway excursion during the last stage. We employed thermogravimetry (TG) to obtain thermal stability parameters under non-isothermal conditions to evaluate the runaway reactions for EEC. The experimental data were compared with kinetics-based curve fitting to assess thermally hazardous phenomena by optimizing curve fitting on the kinetic parameters. The aim of this study was to estimate the incompatible hazards for EEC, provide thermal hazard information in order to determine the optimum operation conditions, and diminish the likelihood of fire and explosion accidents incurred by EP.

High-pressure Processing: Kinetic Models for Microbial and Enzyme Inactivation

High-pressure processing (HPP) has become the most widely accepted nonthermal food preservation technology. The pressure range for commercial processes is typically around 100–600 MPa, whereas moderate temperature (up to 65 °C) may be used to increase microbial and enzymatic inactivation levels. However, these industrial processing conditions are insufficient to achieve sterilization since much higher pressure levels (>1,000 MPa) would be required to inactivate bacterial endospores and enzymes of importance in food preservation. The next generation of commercial pressure processing units will operate at about 90–120 °C and 600–800 MPa for treatments defined as pressure-assisted thermal processing or pressure-assisted thermal sterilization if the commercial food sterilization level required is achieved. Most published HPP kinetic studies have focused only on pressure effects on the microbial load and enzyme activity in foods and model systems. Published work on primary and secondary models to predict simultaneously the effect of pressure and temperature on microbial and enzymatic inactivation kinetics is still incomplete. Moreover, few references provide a detailed and complete analysis of the theoretical, empirical, and semiempirical kinetic models proposed to predict the level of microbial and enzyme inactivation achieved. This review organizes these published kinetic models according to the approach used and then presents an in-depth and critical revision to define the modeling research needed to provide commercial users with the computational tools needed to develop and optimize pasteurization and sterilization pressure treatments.

Atmospheric Chemistry Modelling of Amine Emissions from Post Combustion CO2 Capture Technology

Abstract Emissions from post combustion CO 2 capture plants using amine solvents are of concern due to their adverse impacts on the human health and environment. Potent carcinogens such as nitrosamines and nitramines resulting from the degradation of the amine emissions in the atmosphere have not been fully investigated. It is, therefore, imperative to determine the atmospheric fate of these amine emissions, such as their chemical transformation, deposition and transport pathways away from the emitting facility so as to perform essential risk assessments. More importantly, there is a lack of integration of amine atmospheric chemistry with dispersion studies. In this work, the atmospheric chemistry of the reference solvent for CO 2 capture, monoethanolamine, and the most common degradation amines, methylamine and dimethylamine, formed as part of the post combustion capture process are considered along with dispersion calculations. Rate constants describing the atmospheric chemistry reactions of the amines of interest are obtained using theoretical quantum chemistry methods and kinetic modeling. The dispersion of these amines in the atmosphere is modeled using an air-dispersion model, ADMS 5. A worst case study on the UK's largest CO 2 capture pilot plant, Ferrybridge, is carried out to estimate the maximum tolerable emissions of these amines into the atmosphere so that the calculated concentrations do not exceed guideline values and that the risk is acceptable.

A mathematical model to describe the photocatalytic reality: What is the probability that a photon does its job?

The purpose of this study is to present a simple methodology for predicting the performance of degradation of a photocatalytic system. The proposed model has the advantages of a minimal use of experimental data, and no introduction of adjustment variables. In our simulation we introduced a new parameter ψ reflecting the probability that a photon absorbed by the photocatalytic suspension gives an oxidizing agent. The first step in building our predictive model is to calculate, using Monte Carlo simulation, the absorbed energy per unit time and volume irradiated in space “LVRPA” for both types of selected photocatalysts (P25 and UV100). The second stage is to find an adequate theoretical kinetic model for the degradation of the primary pollutant (phenol) in different intermediates and ultimately to CO2 and H2O. The probability ψ serves to link the two previous steps in the selected mathematical environment. Our model is proven effective for predicting the kinetics of phenol degradation and its two intermediates: hydroquinone and benzoquinone. The global deviation between experimental data and model outputs were, for the primary pollutant, 12.18% for P25 and 9.63% for UV100, with ψP25=1.16% and ψUV100=0.88%.

Role of Vibrational Excitation of the Adlayer in 'Precursor-Mediated' Sticking Probabilities of CO on Metal Surfaces: A Kinetic Model

Abstract The available experimental and theoretical evidence for vibrational excitation of the adlayer in CO chemisorption on metal surfaces raises the question on the validity of the generally accepted model of 'precursor-mediated' adsorption in the presence of adsorption–stimulated desorption (asd) as triggered by the hyperthermal conditions of the adlayer. The analysis shows that a 'precursor-mediated' kinetics still describes the total rate of adsorption, but that sticking probability measurements carried out in the presence of asd might actually measure the net adsorption rate, that is a lower sticking probability. Literature data, on the decrease of the sticking probability at increasing energies of the impinging CO molecular beams, have tentatively been interpreted by assuming an enhancement of the desorption rate caused by the energy transfer from the beam. The analysis is supported by a theoretical kinetic model of 'precursor- mediated' adsorption which takes into account, explicitly, the vibrational excitation of the adspecies.

Theoretical Kinetic Analysis of Heterogeneous Photocatalysis by TiO2 Nanotube Arrays: the Effects of Nanotube Geometry on Photocatalytic Activity

TiO2 nanotube arrays are important functional materials in photocatalysis. Compared with other TiO2 materials, the geometrical parameters of the nanotubes in an array significantly affect photocatalytic activity, but how they do this remains unclear. In the present work, a simple theoretical kinetic model to study the effects of nanotube diameter, wall thickness, and length on the photocatalytic activity of TiO2 nanotube arrays is developed, in which reactant (O2) transport is considered. The photocatalytic activity first increases and then decreases as the diameter and wall thickness of the nanotubes increase because of changes in light absorption, surface area, and reactant transport. The photocatalytic activity increases, and then reaches saturation as the nanotube length increases, which is mainly influenced by the change of light absorption along the nanotube. The present kinetic model agrees well with experimental results and clearly explains the photocatalytic activity of TiO2 nanotubes, helping us...

A theoretical kinetic model of the temperature and pH dependent dimerization of orthosilicic acid in aqueous solution

The first steps in a pH- and temperature-dependent theoretical kinetic model of silicate polymerization and dissolution are examined in this work with a combined ab initio and transition state theory based study of the dimerization of H(4)SiO(4). The role of solvation has been of primary concern in this work, and its influence on theoretical activation energies and pre-exponential factors has been thoroughly benchmarked. Relatively inexpensive MP2/6-31+G(d)//HF/6-31+G(d) calculations of octahydrate clusters, with conductor-like polarizable continuum model corrections obtained in the MP2-level single-point calculations, have been shown to lead to a good description of the limited experimentally determined energetics of dimerization for most elementary reactions. Pre-exponential factors computed from this level of theory are found to be relatively insensitive to the level of theory utilized for geometry optimizations, the number of explicit waters, hindered rotor corrections, and variational effects arising from the minimization of rate constants. Within this framework, a kinetic model of the chemistry of H(4)SiO(4) and H(3)SiO(4)(-), forming H(6)Si(2)O(7) and H(5)Si(2)O(7)(-), has been compiled. Numerical simulations over pH = 3-12 show that a number of pH- and temperature dependent trends in reaction rates and positions of equilibrium are well described with this simple dimerization model. More specifically to the dimerization process, we obtain dimerization constants, log K(dim), of 1.85 and -7.15 for the formation of H(6)Si(2)O(7) and H(5)Si(2)O(7)(-) respectively, which compare well with experimentally determined values of 1.2 and -8.5, respectively.

Anodic oxidation of a biologically treated leachate on a boron-doped diamond anode

In the present study, the anodic oxidation of a leachate from an intermunicipal sanitary landfill was evaluated using a boron-doped diamond anode as a possible post-treatment to the biological one, in order to diminish the remaining bio-refractory organic matter. The influence of the dilution of the leachate sample and the applied current density on the performance of electrochemical oxidation was investigated. For the different assays performed, a comparison between the theoretical kinetic model for organic mineralization, proposed in the literature, and experimental data was done and the best correspondences were attained at lower applied current density for sample without dilution and at higher applied current density for the more diluted sample. However, the energetic most favourable treatment is attained for sample without dilution at very low current density. The DOC/COD ratio was also analysed and apparently, the mineralization of the organic matter improves with the dilution of the leachate samples.

Theoretical Kinetic Analysis of Heterogeneous Photocatalysis: The Effects of Surface Trapping and Bulk Recombination through Defects

A theoretical kinetic model is developed to study the functions of defects in photocatalysis, and the kinetic courses that are related to the defects on the surface and in the bulk are systematically analyzed. The analysis gives a description of the functions of the defects on the surface. It is found that increased speed of surface trapping leads to decreased photocatalytic performance. Surface trapping of electrons at defects is not sufficient to increase the photocatalytic activity, which is dependent on the kinetic courses that follow surface trapping, namely, surface recombination and interfacial transfer. The role of bulk defects as recombination centers is considered. The model gives a simple and general physical and mathematical treatment taking bulk recombination into consideration, based on which the effect of the bulk defects is analyzed.

A unified computational model of the genetic regulatory networks underlying synaptic, intrinsic and homeostatic plasticity

It is well established that the phenomena of synaptic, intrinsic and homeostatic plasticity are mediated – at least in part – by a multitude of activity-dependent gene transcription and translation processes [1-3]. Various isolated aspects of the complex genetic regulatory network (GRN) underlying these interconnected plasticity mechanisms have been examined previously in detailed computational models [4,5]. However, no study has yet taken an integrated, systems biology approach to examining the emergent dynamics of these interacting elements over longer timescales. Here, we present theoretical descriptions and kinetic models of the principle mechanisms responsible for synaptic and neuronal plasticity within a single simulated Hodgkin-Huxley neuron. We describe how intracellular Calcium dynamics and neural activity mediate synaptic tagging and capture (STC), bistable CaMKII auto-phosphorylation, nuclear CREB activation via multiple converging secondary messenger pathways, and the activity-dependent accumulation of immediate early genes (IEGs) controlling homeostatic plasticity. We then demonstrate that this unified model allows a wide range of experimental plasticity data to be replicated. Furthermore, we describe how this model can be used to examine the cell-wide and synapse-specific effects of various activity regimes and putative pharmacological manipulations on neural processing over short and long timescales. These include an examination of the interaction between intrinsic and synaptic plasticity, each dictated by the level of activated CREB; and the differences in functionality generated by STC under regimes of reduced protein synthesis [2,6]. Finally, we discuss how these processes might contribute to maintaining an appropriate regime for transient dynamics in putative cell assemblies within contemporary neural network models of cognitive processing [7,8].

Continuous hydrolysis of modified wheat gluten in an enzymatic membrane reactor

The low solubility of wheat gluten is one of the major limitations to its use in food processing, and enzymatic hydrolysis has been found to be an effective way to prepare more soluble bioactive peptides from gluten. The aim of this study was to prepare bioactive peptides from modified wheat gluten (MWG) in a continuous enzymatic membrane reactor (EMR) that allowed rapid separation of low-molecular-weight peptides from hydrolysates, thus avoiding the disadvantages of batch reaction such as inefficient use of enzymes, inconsistent products due to batch-to-batch variation, substrate-product inhibition, low productivity and excessive hydrolysis. Wheat gluten was modified to decrease its lipid and starch contents in order to prevent membrane fouling. The optimal working conditions for Alcalase to hydrolyse MWG in the EMR were a substrate concentration of 20 g L(-1) , an enzyme/substrate ratio of 0.03, an operating pressure of 0.04 MPa, a temperature of 40 °C and a pH of 9. The operating stability of the EMR (including residual enzyme activity, productivity and capacity) was high. The permeate fractions showed antioxidant activities that were mostly due to low-molecular-weight peptides. A simple theoretical kinetic model was successfully applied to the enzymatic hydrolysis of MWG in the EMR. Modification of wheat gluten made the continuous enzymatic membrane reaction more efficient and the EMR proved to be an effective means of producing peptides with particular properties and bioactivities. The permeate fractions (mainly < 1000 Da) were homogeneous and stable and also showed strong antioxidant activities.

Changes in activation energy and kinetic mechanism during EVA crosslinking

The activation energy and kinetic mechanism throughout the crosslinking process of copolymer ethylene-vinyl acetate (EVA), initiated by dicumyl peroxide (DCP), were determined in the framework of a multi-step solid-state process. Crosslinking was performed in a differential scanning calorimeter (DSC) under non-isothermal conditions. The activation energy (E) was determined by means of the isoconversional integral method using the Cai approach. The E values (87–105 kJ mol−1) were found to be dependent on the crosslinking conversion (α) and DCP content. Kinetic mechanisms were determined by the Criado method from Eα(T) data through the use of the master plots of theoretical kinetic models. At 0.1 < α < 0.5, the EVA crosslinking obeys homogeneous kinetics with rate-controlled reactions (Fn mechanisms). At α ≥ 0.5, the medium becomes heterogeneous and EVA crosslinking occurs through a diffusion-controlled reaction (Dn mechanisms). At α ≅ 0.9, the temperature increases and the EVA crosslinking follows homogeneous kinetics with Fn mechanisms.

Kinetic modeling of the peroxide cross-linking of polymers: From a theoretical model framework to its application for a complex polymer system

Peroxides react with polymers in a variety of ways. The fundamental comprehension and prediction of pertinent kinetics of reactions is consequently indispensable. Based on the mechanisms of reactions of cross-linking process, a new theoretical kinetic model framework was developed. The kinetic model was then applied to the reactions of cross-linking process of various peroxides and a chosen complex polymer compound, namely partially hydrogenated poly(acrylonitrile- co-1,3-butadiene). Whereas the initial macromolecular backbone structure was determined utilizing attached proton test carbon nuclear magnetic resonance, the evolution of overall concentration of cross-links was monitored through viscoelastic characteristics of the system. The model demonstrated good agreement with experimentally measured data and, moreover; the evolution of concentrations of various crucial species inherent to the cross-linking process were predicted. The most significant advantage of the developed kinetic model is that it may be readily applied to an assortment of polymer/peroxide systems.

A kinetics model for martensite variants rearrangement in ferromagnetic shape memory alloys

A kinetics model describing macroscopic behavior of martensitic variants rearrangement in ferromagnetic shape memory alloys is proposed. The model is based on a tensor description of thermodynamic continuum mechanics taking into account magnetomechanical coupling on the rearrangement process. A generalized thermodynamic driving force which rearranges the martensitic variants is derived. The thermodynamic evolution equations of the kinetics are obtained by means of the dissipation, which is induced by the twin boundary motion during rearrangement process, reaching a maximum. The ability of the model to characterize the macroscopic stress-strain behavior and magnetic field-induced strain of a ferromagnetic shape memory alloy rod under both magnetic field and stress field is demonstrated. The application of the theoretical kinetics model shows the good agreement with experimental observation.

Kinetic modeling of the peroxide cross-linking of polymer/monomer blends: From a theoretical model framework to its application for a complex polymer/monomer dispersion system

Multi-functional vinyl monomers, which are highly reactive towards free radicals and are readily graft to polymer chains, ordinarily form a complex cross-linked network with the latter. Initial phase distribution within the polymer matrix is of a great importance for the final user properties of a composite material. Based on the mechanisms of cross-linking process’ reactions, a new theoretical kinetic model framework was developed incorporating blend morphology. The kinetic model was then applied to the cross-linking process’ reactions of various monomer moieties and a chosen complex polymer compound, namely partially hydrogenated poly(acrylonitrile- co-1,3-butadiene) alongside with an organic peroxide. Whereas the blend morphology was determined utilizing field emission scanning electron microscopy, the overall concentration of cross-links’ evolution was monitored through viscoelastic characteristics of the system. The model demonstrated good agreement with experimentally measured data and, moreover; the evolution of concentrations of various crucial species inherent to the cross-linking process were predicted in homogeneous, heterogeneous, and interfacial phase. The most significant advantage of the developed kinetic model is that it may be readily applied to an assortment of polymer/monomer/peroxide systems.

Relativistic effects on the collisionless–collisional transition of the filamentation instability in fast ignition

AbstractRelativistic collisional effects on the filamentation instability are analytically and numerically investigated by comparing collisionless and collisional scenarios for a fast ignition (FI) configuration. The theoretical kinetic model, including warm species and space charge effects, predicts the preferential formation of larger filaments and the inhibition/enhancement of the instability when collisions are accounted for. These collisional effects are qualitatively and quantitatively confirmed by 1D and 2D particle-in-cell (PIC) simulations, also providing a physical picture for the inhibition/enhancement regime due to collisions, based on the electron beam slowdown. By plugging typical FI parameters in the dispersion relation, the theoretical model predicts significant growth rates of the instability deep inside the FI target, thus showing the potential role of the filamentation instability as a mechanism for energy deposition into the pellet core.

Modeling of catalytic gasification kinetics of coal char and carbon

Calcium- and potassium-catalyzed gasification reactions of coal char and carbon by CO 2 are conducted, and the common theoretical kinetic models for gas–carbon (or char) reaction are reviewed. The obtained experimental reactivities as a function of conversion are compared with those calculated based on the random pore model (RPM), and great deviations are found at low or high conversion levels as predicted by theory. Namely, calcium-catalyzed gasification shows enhanced reactivity at low conversion levels of <0.4, whereas potassium-catalyzed gasification indicated a peculiarity that the reactivity increases with conversion. CO 2 chemisorption analysis received satisfactory successes in both interpreting catalytic effects and correlating the gasification reactivity with irreversible CO 2 chemical uptakes (CCU ir) of char and carbon at 300 °C. In details, calcium and potassium additions led to significant increases in CCU ir and correspondent high reactivities of the char and carbon. Furthermore, CCU ir of char and carbon decreased with conversion for calcium-catalyzed reaction but increased for potassium-catalyzed one, corresponded to the tendency of their reactivity. The RPM is extended and applied to these catalytic gasification systems. It is found that the extended RPM predicts the experimental reactivity satisfactorily. The most important finding of this paper is that the empirical constants in the extended RPM correlate well with catalyst loadings on coal.

Degradation of PEO in the Solid State: A Theoretical Kinetic Model

The quantitative kinetic model presented in this work provides insights of the inner mechanisms that play a key role in the natural oxidative degradation (photoinduced and thermal) of a model polymer, polyethylene oxide (PEO). A set of key reactions is selected from ab initio data, theoretical rate constant calculations, and experimental rate constants. Experimental results, i.e., induction times and quantities of carbonyl end-products, are accurately reproduced by this model. While this study grasps the complexity of the degradation mechanism, results depend mainly on the reactivity of peroxy radicals and that of hydroperoxide groups. Three pathways are available to the peroxy species, i.e., intermolecular and γ-intramolecular H-abstraction reactions, and the termination reaction. The competition between these processes is the driving force of the degradation mechanism. Moreover, in thermo-oxidative conditions, the relative quantities of esters and formates depend strongly on four competitive hydroperoxi...

Computer aided modelling of friction in rubber-pad forming process

Rubber-pad forming process is widely used in aerospace components manufacturing. The problems associated in modelling this process is friction forces between sheet and forming tools play an important role because of their influence on the process performance and on the final product properties. This frictional behaviour is often taken into account by using a constant coefficient of friction in the finite element simulations of sheet metal forming processes. This is a poor description of the real frictional behaviour and therefore theoretical static and kinetic friction models are presented in this paper for an axisymmetric rubber-pad forming process. Both models are based on the local contact conditions. Furthermore, rubber-pad forming experiments and simulations were performed. The measured friction curves using the new friction models were implemented in the finite element code ABAQUS. From the results of simulations it was found that the new friction models have better correlation with experimental results compare to traditional Coulomb friction model.