A simple model of a kinetic heterogeneous catalytic oscillator

A model of a heterogeneous catalytic oscillator with minimum number of kinetic parameters is proposed. Feedback in the oscillator is the dependence of the rate constant of one of the steps on the filling of the surface of the catalyst by adsorbed particles.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 2, pp. 152–154, March–April, 1992.     

Overall kinetic model for oscillating catalytic oxidation of methanol

An overall kinetic model for the deep catalytic oxidation of methanol is presented. It is based on the balance of chemical species and could described self-sustained oscillations.

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Remarks on the relationship between isothermal and non-isothermal kinetic results. A new method for the kinetic analysis of isothermal data

Addition of 10mg/l of the haevy metal ions Zn⁺⁺, Cu⁺⁺, Pb⁺⁺, Cd⁺⁺ and Cr⁺⁺ to nongrowing E.coli cells shows a decrease in heat production by aerobic and anaerobic glycolysis up to 80 %. The heat production of cells incubated without glucose was decreased drastically too, after addition of 10mg/l of these heavy metal ions.The pattern of heat released during active transport of α MG was influenced by these haevy metal ions in a way, that there was a decrease in maximum heat flow, whereas the total heat production seemed to be constant.After increasing the metal ion concentration up to 100mg/l no endogenous heat production could be detected. The inhibition of heat production after addition of glucose was higher than 90 % in this case and there was a strong inhibition in α MG uptake.The results of these investigations indicate that the basic action of all tested metal ions is the energy metabolism and not the transport as stated earlier.     

Equivalent isothermal curves for kinetic analysis of non-stoichiometry

The author proposed a method for analyzing kinetics of non-stoichiometry by observing cyclic mass change behavior under cyclic temperature change. Because relationship between the mass and the mass change rate is independent on the previous thermal history of the specimen, we can get equivalent isothermal curves (synthesized isothermal curves) by extracting datum pairs of mass and mass change rate at a given temperature and many conversions, changing the frequencies. The equivalent isothermal curves are really the same as curves of conversion and rate of conversion observed isothermally, so that conventional methods for kinetic analysis can be similarly applied to the equivalent isothermal curves. When multiple elementary processes are proceeding, they can be separately observed by changing the frequency, so that, the method proposed in this short communication is useful and effective to elucidate kinetics of non-stoichiometry as well as reversible reactions.     

A kinetic model for predicting biodegradation

Biodegradation plays a key role in the environmental risk assessment of organic chemicals. The need to assess biodegradability of a chemical for regulatory purposes supports the development of a model for predicting the extent of biodegradation at different time frames, in particular the extent of ultimate biodegradation within a ‘10?day window’ criterion as well as estimating biodegradation half-lives. Conceptually this implies expressing the rate of catabolic transformations as a function of time. An attempt to correlate the kinetics of biodegradation with molecular structure of chemicals is presented. A simplified biodegradation kinetic model was formulated by combining the probabilistic approach of the original formulation of the CATABOL model with the assumption of first order kinetics of catabolic transformations. Nonlinear regression analysis was used to fit the model parameters to OECD 301F biodegradation kinetic data for a set of 208 chemicals. The new model allows the prediction of biodegradation multi-pathways, primary and ultimate half-lives and simulation of related kinetic biodegradation parameters such as biological oxygen demand (BOD), carbon dioxide production, and the nature and amount of metabolites as a function of time. The model may also be used for evaluating the OECD ready biodegradability potential of a chemical within the ‘10-day window’ criterion.     

A kinetic model for limonene oxidation

The kinetic regularities of the autooxidation of R(+)−limonene (LH) were experimentally studied at various temperatures. A kinetic model including the main elementary reactions and the corresponding rate constants was developed. The degenerate chain branching during limonene oxidation proceeds via bimolecular reactions and involves hydroperoxide LOOH + LH → L· + LO· + H₂O and LOOH + LOOH → LO· + LO₂ · + H₂O. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 80–86, January, 2008.     

A kinetic model for predicting biodegradation

Biodegradation plays a key role in the environmental risk assessment of organic chemicals. The need to assess biodegradability of a chemical for regulatory purposes supports the development of a model for predicting the extent of biodegradation at different time frames, in particular the extent of ultimate biodegradation within a '10 day window' criterion as well as estimating biodegradation half-lives. Conceptually this implies expressing the rate of catabolic transformations as a function of time. An attempt to correlate the kinetics of biodegradation with molecular structure of chemicals is presented. A simplified biodegradation kinetic model was formulated by combining the probabilistic approach of the original formulation of the CATABOL model with the assumption of first order kinetics of catabolic transformations. Nonlinear regression analysis was used to fit the model parameters to OECD 301F biodegradation kinetic data for a set of 208 chemicals. The new model allows the prediction of biodegradation multi-pathways, primary and ultimate half-lives and simulation of related kinetic biodegradation parameters such as biological oxygen demand (BOD), carbon dioxide production, and the nature and amount of metabolites as a function of time. The model may also be used for evaluating the OECD ready biodegradability potential of a chemical within the '10-day window' criterion.     

A new catalytic kinetic spectrophotometric method for determination of iron

The catalytic effect of iron(III) on the oxidation of reduced Rhodamine B with hydrogen peroxide in acetic acid medium has been studied. The reaction is highly accelerated by potassium thiocyanate. A new catalytic kinetic spectrophotometric method for the determination of iron has been developed. Iron(III) can be determined by the fixed time method with a detection limit of 4 x 10(-11) g/ml.     

Evaluation of isothermal kinetic of sweetener

Summary Sulfonylureas are widely used for the treatment of non-insulin dependent diabetes mellitus. Glibenclamide belongs to the group of substituted arylsulfonylureas. Many representative of this group shows polymorphism. The purpose of this work was to investigate the thermal behaviour and compatibility between glibenclamide and some excipients using thermoanalytical techniques (TG-DTG/DSC). The thermal and isothermal kinetics data showed incompatibility between glibenclamide and magnesium stearate.     

A kinetic model for nonisothermic homogeneous nucleation

A model for isothermal homogeneous nucleation is proposed that improves the classical model. A quasiequilibrium distribution of clusters was calculated on a basis of the Frenkel’-Lothe-Pound theory. The dependence of the free energy of clusters on their size was represented by an interpolation formula relating the free energy of dimers and large clusters to which a notion of macroscopic surface tension is applicable. The nucleation rate and the dependence of the cluster temperature on their size were calculated by balance equations describing the heating of from a cluster due to the condensation of monomers and its cooling due to collisions with an ambient gas. It is shown that the nucleation rate in excess buffer gas is higher than for the pure condensing gas by approximately two orders of magnitude. The model adequately describes the experimental data for the nucleation of methanol supersaturated vapor.     

裂化催化剂水热失活动力学及装置平衡活性模型

根据裂化催化剂水热失活过程伴随着超稳化过程的特点，确定了对应不同自抑制函数的催化剂水热失活动力学模型方程。利用裂化催化剂水热失活实验数据进行参数估值，建立了裂化催化剂水热失活动力学模型。统计检验结果表明，二级自抑制的一级水热失活模型能很好地模拟实验数据，是较理想的水热失活动力学模型。考虑工业装置中裂化催化剂呈全混流，建立了裂化催化剂平衡活性模型方程，并且装置平衡催化剂微反活性的模型计算值与实测值相当吻合。该模型的预测结果表明，随着再生器温度或催化剂藏量的提高，平衡剂的微反活性逐步降低；平衡剂微反活性随催化剂单耗的提高先快速提高，然后缓慢提高。     

A simplified model for the predissociation of diatomic molecules

A model consisting of a harmonic oscillator well and a repulsive inverse square potential, coupled by a delta function, is solved. We find the S-function for this case and study its poles as a function of the coupling strength. These poles show how the harmonic levels shift and broaden as the two potential curves couple and predissociation occurs. A “new state” is found when the energy threshold is just below the first excited state of the harmonic oscillator.     

A simplified model of oscillating electrolytes

Unstable electrophoretic transport leading to oscillations in concentration profiles occur in certain electrolyte systems known as oscillating electrolytes whose eigenmobilities are complex valued. The study of the nonlinear behavior of such systems is of great interest but is constrained due to a high degree of complexity in the governing equations. Here we present a simplified model of unstable electrophoretic transport in a binary system that reduces the governing equations to two partial differential equations only and does away with other equations that characterize acid–base dissociation reactions and electroneutrality. We present analytical expressions for electromigration fluxes and validate the model with full nonlinear simulations. The model exhibits similar nonlinear behavior as the actual unstable electrophoretic system under various initial disturbances. For comparison, we also show that similar modeling for a stable system predicts concentration profiles that quantitatively agree with its nonoscillating dynamics. Moreover, the unique feature of electromigration flux in oscillating electrolytes that unfolds from the modeling led us to find an elegant explanation of the instability mechanism. Our theory gives a qualitative understanding of the existence and growth of large oscillation patterns in oscillating electrolytes.     

A three-phase nonequilibrium model for catalytic distillation

Nonequilibrium model for steady state simulation of catalytic distillation is presented. Mathematical model takes into account both mass and heat transfers across the gas liquid interface and through the liquid-solid (catalyst) interface. Equations describing the mentioned phenomena are based on the effective diffusivity approach. The resulting system of nonlinear algebraic equations was implemented in the FORTRAN programming language and solved by the BUNLSI (Ferraris & Tronconi, 1986) solver. The described model was verified using the experimental data obtained from a continuous distillation column equipped with catalytic packing. As an experimental model system, synthesis of propyl propionate from propan-1-ol and propionic acid was chosen. Comparison of experimental and simulation data is presented, and appropriateness of the developed model for other types of catalytic distillation processes is discussed. Presented at the 35th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 26–30 May 2008.     

A simplified model of the ECD

A simplified model of the ECD is presented, which is based on the assumption that only a change in the concentration of electron is generating a signal. The model allows to determine four different time constants related to: the collection of electrons (₁), the loss of electrons in the capture process by the impurity molecules (₂), the loss of sample molecules by electron capture (_n) and the removing rate of molecules from the detector volume by the carrier gas (_v). The values of these time constants have been estimated to be in the range of s for ₁, ms for ₂, a part of a second for _n and a few seconds for _v. The electron capture efficiency coefficient (p) and the detection coefficient (S_d) have been defined. These coefficients serve in the model for the coulometric calculation of the mass of analyzed compounds, if the detector works using the conditions described.     

A simplified model of the ECD

A simplified model of the ECD is presented, which is based on the assumption that only a change in the concentration of electron is generating a signal. The model allows to determine four different time constants related to: the collection of electrons (tau(1)), the loss of electrons in the capture process by the impurity molecules (tau(2)), the loss of sample molecules by electron capture (tau(n)) and the removing rate of molecules from the detector volume by the carrier gas (tau(v)). The values of these time constants have been estimated to be in the range of micros for tau(1), ms for tau(2), a part of a second for tau(n) and a few seconds for tau(v). The electron capture efficiency coefficient (p) and the detection coefficient (S(d)) have been defined. These coefficients serve in the model for the coulometric calculation of the mass of analyzed compounds, if the detector works using the conditions described.     

A kinetic model for nanocrystal morphology evolution.

A kinetic model is developed with the goal of understanding and predicting the morphology evolution of nanocrystals in nonequilibrium growth conditions. The model is based on the assumption that under such conditions, different crystal planes have different kinetic parameters. This model focuses on the morphology-developing stage and is successfully related to the nucleation process and other crystal evolution mechanisms. It is believed to be a universal model and is applied to discuss the morphology evolution of CdSe nanocrystals, including the aspect ratio, injection schemes, ligands effect and morphology distribution.     

A generalized kinetic model for heterogeneous gas-solid reactions

We present a generalized kinetic model for gas-solid heterogeneous reactions taking place at the interface between two phases. The model studies the reaction kinetics by taking into account the reactions at the interface, as well as the transport process within the product layer. The standard unreacted shrinking core model relies on the assumption of quasi-static diffusion that results in a steady-state concentration profile of gas reactant in the product layer. By relaxing this assumption and resolving the entire problem, general solutions can be obtained for reaction kinetics, including the reaction front velocity and the conversion (volume fraction of reacted solid). The unreacted shrinking core model is shown to be accurate and in agreement with the generalized model for slow reaction (or fast diffusion), low concentration of gas reactant, and small solid size. Otherwise, a generalized kinetic model should be used.     

A kinetic model for an atomic absorption signal

A kinetic model is proposed for describing the shape of an atomic absorption signal in the case of first-order kinetics. The formation energy of free copper ions (87 x 7 kJ/mol) and other kinetic parameters of an atomic absorption signal of copper at 2273–3073 K were determined from the experimental data.