Biblioteca Digital

New developments in the kinetic modelling of complex non-linear polymerizations

A New Approach for the Kinetic Modelling of Non-Linear Free-Radical Polymerizations and its Application to Poly(Vinyl-Acetate) Bulk and Solution Manufacturing Processes

Kinetic Modelling of Semi-Batch Radical Crosslinking Copolymerizations

Specific marine macro algae species abundant at the Portuguese coast (Laminaria hiperborea, Bifurcaria bifurcata, Sargassum muticum and Fucus spiralis) were shown to be effective for removing toxic metals (Cd(II), Zn(II) and Pb(II)) from aqueous solutions. The initial metal concentrations in solution were about 75-100 mg L-1. The observed biosorption capacities for cadmium, zinc and lead ions were in the ranges of 23.9-39.5 mg g-1, 18.6-32.0 mg g-1 and 32.3-50.4 mg g-1, respectively. Kinetic studies revealed that the metal uptake rate was rather fast, with 75% of the total amount occurring in the first 10 min for all algal species. Experimental data were well fitted by a pseudo-second order rate equation. The contribution of internal diffusion mechanism was significant only to the initial biosorption stage. Results indicate that all the studied macro algae species can provide an efficient and cost-effective technology for eliminating heavy metals from industrial effluents.

Because biosorption is a low cost and effective method for treating metal-bearing wastewaters, understanding the process kinetics is relevant for design purposes. In the present study, the performance of the aquatic moss Fontinalis antipyretica for removing cadmium and lead from simulated wastewaters has been evaluated. Five kinetic models (first-order, pseudo-first-order, Elovich, modified Ritchie second-order and pseudo-second-order) were fitted to the experimental data and compared. Previously, the effect of parameters such as the initial solution pH, contact time, and initial metal ion concentration on biosorption was investigated. The initial pH of the solution was found to have an optimum value in the range of 4.0-6.0. The equilibrium sorption capacity of cadmium and lead by Fontinalis antipyretica increased with the initial metal concentration. For an initial metal concentration of 10 mg L-1, the uptake capacity of the moss, at equilibrium, is the same for both metals (4.8 mg g-1). Nevertheless, when the initial concentration increases up to 100 mg L-1, the uptake of Pb(II) was higher than 78%. The pseudo-second order biosorption kinetics provided the better correlation with the experimental data (R2 ≥ 0.999).

Os frutooligossacarídeos são considerados prebióticos, uma vez que promovem seletivamente o crescimento de micro-organismos probióticos como Lactobacillus acidophillus e Bifidobacterium bifidus. Novas enzimas, na forma livre ou imobilizada, representam uma das possibilidades para síntese destes compostos. Neste trabalho procedeu-se ao estudo da modelagem cinética e simulação da síntese de frutooligossacarídeos a partir de sacarose em diferentes tipos de reatores, pela enzima frutosiltransferase produzida pela levedura do gênero Rhodotorula, isolada em trabalhos prévios. Os estudos foram realizados sob condições de pH 4,5, 50°C e 5 UTF/mL de concentração da enzima. Tanto a enzima livre quanto a imobilizada mostraram seguir a cinética de Michaelis-Menten com inibição pelo substrato para concentrações acima de 70% e 60% (p/v), respectivamente. Observou-se inibição competitiva da glicose para os substratos sacarose, kestose e nistose. Por outro lado, considerou-se significativa a atividade hidrolítica da nistose, sendo incluída no modelo. Após a análise de sensibilidade dos parâmetros cinéticos, estes foram ajustados por simulação, e determinou-se seus valores intrínsecos. O modelo mostrou-se válido com desvios menores que 4% para a enzima livre (57% de FOS) e de 5% para enzima imobilizada (46% de FOS)...

Understanding the dynamic behavior of living organisms is a great challenge in systems biology. To address this, computational dynamic modeling of metabolic networks is essential to guide experimentation and to explain properties of complex biological systems. Large-scale kinetic models at the reaction network level are usually constructed using mechanistic enzymatic rate equations and a large number of kinetic parameters. However, two of the biggest obstacles to construct accurate dynamic models are model complexity and limited in vivo kinetic information. In the present work, we test an alternative strategy with a relatively small number of kinetic parameters composed by the approximated lin-log kinetics, coupled with a constraint-based method and a priori model reduction based on time scale analysis and a conjunctive fusion approach (Machado et al., 2010).. This workflow was evaluated for the condensed version of a genome-scale kinetic model of Escherichia coli metabolism (Orth et al., 2010). The presented approach seems to be a promising mechanism for detailed kinetic modeling even at the genome-scale of the metabolism of other organisms.

The paper presents comparative kinetic modelling of nitrogen oxides (NOx) removal from flue gases by selective non-catalytic reduction process using ammonia as reducing agent. The computer code SENKIN is used in this study with the three published chemical kinetic mechanisms; Zanoelo, Kilpinen and Skreiberg. Kinetic modeling was performed for an isothermal plug flow reactor at atmospheric pressure so as to compare it with the experimental results. A 500 ppm NOx background in the flue gas is considered and kept constant throughout the investigation. The ammonia performance was modeled in the range of 750 to 1250 ºC using the molar ratios NH3/NOx from 0.25 to 3.0 and residence times up to 1.5 seconds. The modeling using all the mechanisms exhibits and confirms a temperature window of NOx reduction with ammonia. It was observed that 80% of NOx reduction efficiency could be achieved if the flue gas is given 300 msec to react with ammonia, while it is passing through a section within a temperature range of 910 to 1060 ºC (Kilpinen mechanism) or within a temperature range of 925 to 1030 ºC (Zanoelo mechanism) or within a temperature range of 890 to 1090 ºC (Skreiberg mechanism).

Because biosorption is a low cost and effective method for treating metal-bearing wastewaters, understanding the process kinetics is relevant for design purposes. In the present study, the performance of the aquatic moss Fontinalis antipyretica for removing cadmium and lead from simulated wastewaters has been evaluated. Five kinetic models (first-order, pseudo-first-order, Elovich, modified Ritchie second-order and pseudo-second-order) were fitted to the experimental data and compared. Previously, the effect of parameters such as the initial solution pH, contact time, and initial metal ion concentration on biosorption was investigated. The initial pH of the solution was found to have an optimum value in the range of 4.0-6.0. The equilibrium sorption capacity of cadmium and lead by Fontinalis antipyretica increased with the initial metal concentration. For an initial metal concentration of 10 mg L-1, the uptake capacity of the moss, at equilibrium, is the same for both metals (4.8 mg g-1). Nevertheless, when the initial concentration increases up to 100 mg L-1, the uptake of Pb(II) was higher than 78%. The pseudo-second order biosorption kinetics provided the better correlation with the experimental data (R² ≥ 0.999).

Mycobacterium tuberculosis(Mtu), a successful pathogen, has developed resistance against the existing anti-tubercular drugs necessitating discovery of drugs with novel action. Enzymes involved in peptidoglycan biosynthesis are attractive targets for antibacterial drug discovery. The bifunctional enzyme mycobacterial GlmU (Glucosamine 1-phosphate N-acetyltransferase/ N-acetylglucosamine-1-phosphate uridyltransferase) has been a target enzyme for drug discovery. Its C- and N- terminal domains catalyze acetyltransferase (rxn-1) and uridyltransferase (rxn-2) activities respectively and the final product is involved in peptidoglycan synthesis. However, the bifunctional nature of GlmU poses difficulty in deciding which function to be intervened for therapeutic advantage. Genetic analysis showed this as an essential gene but it is still unclear whether any one or both of the activities are critical for cell survival. Often enzymatic activity with suitable high-throughput assay is chosen for random screening, which may not be the appropriate biological function inhibited for maximal effect. Prediction of rate-limiting function by dynamic network analysis of reactions could be an option to identify the appropriate function. With a view to provide insights into biochemical assays with appropriate activity for inhibitor screening...

The traditional technology for synthetic-gas (H2 and CO) production from natural gas is steam reforming. A major drawback of this technology is the intensive energy requirement due to the high endothermicity of the reforming reactions. A possible alternative is methane partial oxidation, or in a combination with steam or CO2, which could offer an advantage of vastly reduced energy requirement of the reforming process. This paper reports on a feasibility study of CH4 partial oxidation into H2 and CO by means of thermodynamic and kinetic analyses. The thermodynamic analysis has been performed using the Gibbs free energy minimisation method, and the kinetic modelling has employed the CHEMKIN package incorporating the GRI 1.2 mechanisms of CH4 oxidation. The effects of initial O2/CH4 ratio, temperature and pressure are examined. The thermodynamic analysis indicates that the synthetic-gas yields are strongly dependent on the initial O2/CH4 ratio with maxima occurring at an optimal initial O2/CH4 ratio varying with temperature. The optimal O2/CH4 ratio decreases with increasing temperature and approaches 0.5 at temperatures greater than 1073 K. The synthetic-gas yields also increase with increasing temperature but with decreasing pressure...

n a photocatalytic reduction process when products formed are not effectively desorbed, they could hinder the diffusion of intermediates on the surface of the catalyst, as well as increase the chance of collisions among the products, resulting photo-oxidation in a reserve reaction on the surface. This paper analyses a simple kinetic model incorporating the coupled effect of the adsorptive photocatalytic reduction and oxidation. The development is based on Langmuir–Hinshelwood mechanism to model the formation rates of hydrogen and methane through photocatalytic reduction of carbon dioxide with water vapour. Experimental data obtained from literatures have achieved a very good fit. Such model could aid as a tool for related areas of studies. A comparative study using the model developed, showed that product concentration in term of ppm would be an effective measurement of product yields through photocatalytic reduction of carbon dioxide with water vapour; Seng Sing Tan, Linda Zou and Eric Hu

This is the 63rd report of a series of workshops organised by the European Centre for the Validation of Alternative Methods (ECVAM), within the Institute for Health and Consumer Protection (IHCP). The workshop, entitled ¿Physiologically based Kinetic Modelling (PBK): Meeting the 3Rs Agenda¿, was held at ECVAM (Ispra, Italy) on October 10-12, 2005. Participants included international experts in PBK modelling and in the risk assessment of chemical products, from academia, regulatory or risk assessment advisory bodies, and industry.The objectives of the workshop were: 1) to better define the potential role of PBK modelling, as a set of techniques capable of contributing to the reduction, refinement and replacement of the use laboratory animals in the risk assessment process of potentially toxic chemicals; 2) to identify the needs for technical improvements in PBK modelling and its applications; 3) to increase the understanding and acceptance of the capabilities and limitations of PBK modelling techniques in toxicological risk assessment by regulatory authorities. This consensus report describes the strategy proposed by the participants to meet these objectives. It is also intended to provide the non-expert reader with an overview of the field in relation to the 3Rs and with some key references.; JRC.I.2-Validation of biomedical testing methods

This article deals with polypropylene stabilization by hindered 2,6-di-tert-butylphenols. Two aspects are mainly considered: the influence of stabilizer structure, of its concentration and temperature on induction period duration through a literature compilation completed by results obtained on PP samples stabilized by Irganox 1010 in conditions in which physical loss was negligible. Results show that the induction period duration is almost proportional to the phenol concentration and that the proportionality ratio is almost independent of the stabilizer structure in the investigated phenol family. A unique set of kinetic parameters can be therefore used to model the kinetic behaviour of all the family members. The kinetic approach can be more or less complex depending on the number of secondary processes taken into account. The results of simulations indicate that a two steps process allows generation of a kinetic behaviour in good agreement with experimental trends regarding effects of both stabilizer concentration and temperature on induction time.

PA 6-6 hydrolysis at 60, 70, 80 and 90 °C in distilled water has been studied by Fourier transform infrared spectroscopy, viscometry in molten state, differential scanning calorimetry and uniaxial tensile testing. The molar mass decreases sharply from the early periods of exposure to reach an equilibrium value of about MnE ≈ 10−11 kg mol−1 almost temperature independent. Hydrolytic chain scissions destroy the entanglement network in the amorphous phase and liberate small macromolecular segments which rearrange locally and initiate a chemicrystallisation. As expected, the embrittlement occurs at a very low conversion of the hydrolysis, in particular when the number average molar mass becomes lower than a critical value of about MnF ≈ 17 kg mol−1, i.e. very close to its initial value. A new kinetic model has been derived from the classical mechanistic scheme of reversible hydrolysis. This model describes satisfyingly all the kinetic characteristics of the reversible hydrolysis of PA 6-6 not controlled by water diffusion: decrease in molar mass, increase in crystallinity ratio and decrease in ultimate elongation, but also of other types of polyamides previously studied, such as PA 11. Moreover, when it is used as an inverse method...

Irganox 1010 stabilized PE was monitored by carbonyl build-up and DSC under oxygen. A scheme for PE stabilization by phenols was implemented and its kinetic parameters were calculated from experimental results. This model was validated from its ability to simulate kinetics curves for carbonyl build up, induction period changes with stabilizer concentration, and stabilizer depletion curve in thermal ageing. The use of OIT measurement for quantifying stabilizer is also discussed. Kinetic analysis showed that OIT is actually proportional to stabilizer concentration in virgin samples but this is not true for aged samples because of negative influence of oxidation unstable by-products. The model was also employed for discussing some scenarii proposed as explanation of heterogeneity observed during thermal oxidation of stabilized polyolefins.

Specific marine macro algae species abundant at the Portuguese coast (Laminaria hyperborea, Bifurcaria bifurcata, Sargassum muticum and Fucus spiralis) were shown to be effective for removing toxic metals (Cd(II), Zn(II) and Pb(II)) from aqueous solutions. The initial metal concentrations in solution were about 75–100 mg L−1. The observed biosorption capacities for cadmium, zinc and lead ions were in the ranges of 23.9–39.5, 18.6–32.0 and 32.3–50.4 mg g−1, respectively. Kinetic studies revealed that the metal uptake rate was rather fast, with 75% of the total amount occurring in the first 10 min for all algal species. Experimental data were well fitted by a pseudo-second order rate equation. The contribution of internal diffusion mechanism was significant only to the initial biosorption stage. Results indicate that all the studied macro algae species can provide an efficient and cost-effective technology for eliminating heavy metals from industrial effluents.

We describe a new rationale to kinetic modelling in which adjustable parameters are avoided through the use of quantum chemistry. This new approach reverses the standard modelling approach in which, having first assumed a kinetic model, it is then fitted

We present the results of kinetic modelling of quantum effects in laser-beam interaction. In the developed numerical model, electron-positron pair production by hard photons, hard photon emission and the electromagnetic fields generated by the created charged particles are taken into account. Interaction of a relativistic electron beam with a strong laser pulse is analyzed. It is shown that the quantum effects can be important even for moderately intense laser pulses when the number of emitted photons by single electron is not large. Electron-positron pair plasma production in extremely-intense laser field via development of electromagnetic cascades is also studied. The simulation results confirm the prediction of strong laser field absorption in the self-generated electron-positron plasma. It is shown that the self-generated electron-positron plasma can be an efficient source of energetic gamma-quanta.; Comment: Submitted to Nuclear Instruments and Methods in Physics Research A

Hydrogen embrittlement is a problem that offers challenges both to technology and to the theory of metallurgy. In the presence of a hydrogen rich environment, applications such as rolling bearings display a significant decrease in alloy strength and accelerated failure due to rolling contact fatigue. In spite of these problems being well recognised, there is little understanding as to which mechanisms are present in hydrogen induced bearing failure. The objective of this thesis are twofold. First, a novel alloy combining the excellent hardness of bearing steels, and resistance to hydrogen embrittlement, is proposed. Second, a new technique to identify the nature of hydrogen embrittlement in bearing steels is suggested. The new alloy was a successful result of computer aided alloy design; thermodynamic and kinetic modelling were employed to design a composition and heat treatment combining (1) fine cementite providing a strong and ductile microstructure, and (2) nano-sized vanadium carbide precipitates acting as hydrogen traps. A novel technique is proposed to visualise the migration of hydrogen to indentation-induced cracks. The observations employing this technique strongly suggest that hydrogen enhanced localised plasticity prevails in bearing steels. While proposing a hydrogen tolerant bearing steel grade...