Pyrolysis Kinetics of Wood and Wood Components

The kinetics of pyrolysis decomposition reactions of wood and its components are described employing mathematical modeling tools for first and n^th order reactions, autocatalytic reactions, and parallel first order and autocatalytic reactions. Secondary reactions may accelerate the reaction rate and a new autocatalytic reaction model was developed to describe this specific behavior.     

Inhomogeneities and surface structures in oscillatory catalytic kinetics

The existence of inhomogeneous surface states is investigated for three oscillatory kinetic models, using realistic values of rate and communication parameters. The analysis shows that local oscillators are effectively synchronized if the autocatalytic variable is a gas-phase concentration. If a surface concentration is the autocatalytic variable and the second variable is a gas-phase reactant, the system will admit an inhomogeneous solution in the form of two surface regions separated by a stationary or undulating front. Oscillations may be translated into either a train of trigger waves or into a stationary inhomogeneous state in a system including two surface reactants.The emergence of inhomogeneous surface states leads to falsification of the apparent reaction kinetics, and may induce dependence of the reaction rate on geometrical factors. It may also facilitate formation of permanent surface structures thereby strongly influencing evolution of the clalyst under the influence of reaction conditions.     

用非等温DSC估算硝化棉热爆炸的临界温升速率

提出了用非等温DSC估算一级自催化分解反应体系热爆炸的临界温升速率(dT／dt)Tb方法，得到了NC(11．92％N)由一级自催化分解过渡到热爆炸时的(dT／dt)Tb值。     

Required levels of catalysis for emergence of autocatalytic sets in models of chemical reaction systems

The formation of a self-sustaining autocatalytic chemical network is a necessary but not sufficient condition for the origin of life. The question of whether such a network could form "by chance" within a sufficiently complex suite of molecules and reactions is one that we have investigated for a simple chemical reaction model based on polymer ligation and cleavage. In this paper, we extend this work in several further directions. In particular, we investigate in more detail the levels of catalysis required for a self-sustaining autocatalytic network to form. We study the size of chemical networks within which we might expect to find such an autocatalytic subset, and we extend the theoretical and computational analyses to models in which catalysis requires template matching.     

Autocatalytic curing kinetics of thermosetting polymers: A new model based on temperature dependent reaction orders

Herewith we discuss a new model for thermoset polymers that follow the autocatalytic curing kinetics. This model is proposed upon investigation of the crosslinking reaction of 2,2′-Bis(4-cyanatophenyl)iso-propylidene (BACy), under isothermal conditions over a range of temperatures between 180 °C and 260 °C without catalyst. BACy undergoes crossslinking via a trimerization mechanism of the nitrile groups following an autocatalytic kinetics rather than an nth order kinetics. Comparing with other autocatalytic kinetics models, the new model takes into account that the reaction orders of the curing reactions in the polymers are temperature dependent variables rather than constants. The new model provides excellent agreement with the experimental data in a wide range of conversions and reaction temperatures.     

Concentration forcing of isothermal plug-flow reactors for autocatalytic reactions

The performance of isothermal plug-flow reactors under feed concentration forcing is analysed for improvement in average yield of product B for a homogeneous autocatalytic reaction, A →k1 A₁ →k2 A_, →k3 B, where A₁ or A₂ has an activity influence on the reaction rate constant k₁. The performance of the periodically forced reactor with the autocatalytic reaction is compared with that of the corresponding uncatalysed reaction.     

A kinetic study of the activated anionic polymerization of ε-caprolactam

The diversities existing among published kinetic studies on activated anionic polymerization of ε-caprolactam are closely examined. A kinetic model derived from a regular, linear reversible reaction mechanism is employed to explain the experimentally observed autocatalytic character of the polymerization system and to examine the dependence of the apparent activation energy on the experimental method. Several existing kinetic models tested with our experimental data show that the autocatalytic type rate equation best describes the polymerization process.     

Chemical Kinetics of the Thermal Decomposition of NTO

The kinetics of thermal decomposition of 3‐nitro‐2,4‐dihydro‐3H‐1,2,4‐triazol‐5‐one (NTO) in the temperature interval from 200 °C to 260 °C was investigated using a glass Bourdon gauge. The overall decomposition reaction includes two distinct stages: the fast first‐order decomposition and the subsequent autocatalytic reaction. The importance of the first stage increases with increasing decomposition temperature and decreasing loading density of the Bourdon gauge (m/V). A period of preliminary heating, at a lower temperature, strongly influences the autocatalytic stage when the decomposition is carried out at a higher temperature. In the temperature domain 200–220 °C, the Arrhenius constants of the decomposition reaction are found to be close to the values usually observed for nitrocompounds: E=173 kJ/mol and log₁₀ k≈12.5 (s⁻¹). It is shown that a simple model of NTO decomposition based on an autocatalytic reaction of the m‐th order can describe the course of the decomposition at high temperature but the m number appears to be excessively high, up to 4. A new model of the decomposition is developed, including an initial monomolecular reaction, decomposition of the crystalline substance, and an autocatalytic reaction of NTO dissolved in liquid decomposition products. This model gives the common order of autocatalysis, m=1.     

Hemicellulose Hydrolysis in the Presence of Heterogeneous Catalysts

A prominent autocatalytic effect in the hydrolysis of hemicelluloses was observed in the presence of heterogeneous cation-exchange catalysts, Amberlyst 15 and Smopex 101. The kinetic models proposed were based on the reactivities of the non-hydrolysed sugar monomer units and the increase of the rate constant as the reaction progresses and the degree of polymerization decreases. General kinetic models were derived and the kinetic parameters, describing the autocatalytic effect, were determined by non-linear regression analysis. The kinetic model explained very well the overall kinetics, as well as the product distribution in the hydrolysis of hemicelluloses.     

酰基供体对动态动力学拆分1-四氢萘胺的影响

采用新型消旋催化剂耦合Novozym 435成功构建1-四氢萘胺的动态动力学拆分体系用于制备光学纯(R)-1-四氢萘胺。该反应存在着自催化酰胺化反应,会降低反应的对映体选择性。从改变酰基供体结构的角度出发来抑制这种自催化酰胺化反应,考察了不同酸部以及不同醇部的酰基供体对1-四氢萘胺动态动力学拆分反应的影响,发现随着酰基供体结构变得复杂,1-四氢萘胺动态动力学拆分反应结果也相应变得越好,当采用戊酸对氯苯酯作为酰基供体时,动态动力学拆分反应结果就可达到最佳,即转化率>99%,光学纯度eeP>99%。     

Cyclization reaction in poly(acrylonitrile/itaconic acid) copolymer: An isothermal differential scanning calorimetry kinetic study

The kinetics of the cyclization reaction in the poly(acrylonitrile/itaconic acid) (PAN) copolymer in the temperature range 175–200°C was studied by isothermal differential scanning calorimetry (DSC). The reaction conforms to an n^th‐order autocatalytic model, with an overall order of 2. The kinetic parameters were derived by multiple regression analysis. The activation energy is 110.3 kJ/mol for the acid‐catalyzed reaction and 153.2 kJ/mol for the autocatalytic part. The autocatalysis indicates the catalysis of the nitrile group cyclization by the initially formed pyrimidine groups. The kinetic parameters were used to predict the reaction profile at a given temperature. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 915–920, 2003     

A model for primary and heterogeneous secondary reactions of wood pyrolysis

A chain growth model for heterogeneous secondary reactions is developed for the pyrolysis of large wood particles and the parameters determined by nonlinear optimization. The model takes both the volatile retention time and cracking and repolymerization reactions of the vapours with the decomposing solid as well as sutocatalysis into consideration. The extent of the secondary reactions is strongly influenced by the time and the ratio of the autocatalytic (propagation) reaction rate to noncatalytic (initiation) reaction rate. The wood which has a higher value of the autocatalytic/noncatalytic ratio also has a higher exothermic heat of reaction and yields a higher amount of final char residue. This fact confirms the heterogeneous secondary reactions lead to carbon enrichment of the final residue and are accompanied with an exothermic heat of reaction. The lower activation energies of the initiation and propagation reactions as compared to primary reactions (competitive reaction model consisting of weight loss and char forming reactions) confirm autocatalysis in large particles. The sealed reactor studies of small quantities of fine wood samples show that heterogeneous secondary reactions and not lower heating rates in large particles are the main source of char formed during the thermal decomposition of large wood particles. The model predictions are in agreement with the weight loss and temperature versus time curves over a wide range of particle size and furnace temperatures.     

基于数值计算方法计算最大反应速率到达时间

最大反应速率到达时间(TMR_ad)是化工工艺热风险评估中一个十分重要的参数。一般计算TMR_ad的方法是基于N级模型的分析。但对于复杂的反应过程统一采用N级模型分析计算可能会引起较大偏差甚至得到错误的评估。因此,提出运用基于反应类型的数值计算方法进行TMR_ad和T_D24的评估,通过分别代表N级反应和自催化反应的20% DTBP甲苯溶液和CHP的ARC测试分析表明：对于N级反应,该方法能可靠地用于TMR_ad和T_D24的求取;而对于自催化反应,尽管拟合效果很好,原有方法计算偏差很大,原因是不同模型下动力学参数不同,还进行偏差大小分析。由此可知该数值计算方法有广泛的适用性,对于放热曲线,需在了解其反应类型的基础上利用该方法进行TMR_ad和T_D24的评估,由此评估的结果更为可靠准确。     

Hemicellulose Hydrolysis in the Presence of Heterogeneous Catalysts

A prominent autocatalytic effect in the hydrolysis of hemicelluloses was observed in the presence of heterogeneous cation-exchange catalysts, Amberlyst 15 and Smopex 101. The kinetic models proposed were based on the reactivities of the non-hydrolysed sugar monomer units and the increase of the rate constant as the reaction progresses and the degree of polymerization decreases. General kinetic models were derived and the kinetic parameters, describing the autocatalytic effect, were determined by non-linear regression analysis. The kinetic model explained very well the overall kinetics, as well as the product distribution in the hydrolysis of hemicelluloses.

     

Comparative DSC kinetics of the reaction of DGEBA with aromatic diamines. II. Isothermal kinetic study of the reaction of DGEBA with m-phenylene diamine

In the first part of the present series the non-isothermal kinetics of the reaction of an epoxy resin based on diglycidyl ether of bis-phenol A (DGEBA) with m-phenylene diamine (mPDA) was studied. A four step kinetic analysis was applied using differential scanning calorimetry (DSC) data. It allowed us to confirm the validity of the three molecular autocatalytic model of this reaction, as well as to obtain reliable kinetic data in programmed temperature mode.The isothermal study of the DGEBA-mPDA reaction was performed applying a similar kinetic approach: (i) analysis at the peak maximum of the DSC curves; (ii) apparent activation energy analysis of the isothermal DSC data; (iii) integral and differential curve fitting methods; and (iv) modeling of the reaction and comparison of the model with the experiment.It was established that the overall kinetic parameters measured under programmed temperature conditions sufficiently well described the isothermal shift of the DSC curves along the logarithmic time scale, especially the initial stage of the reaction. A more precise analysis of the data showed that the isothermal DSC kinetics obeyed a formal model whose power exponent was approximately 2.5, or it was not well represented by the mechanistic-like three molecular autocatalytic velocity equation. Nevertheless, the activation energy of the autocatalytic rate constant determined at constant temperature mode, i.e. was found out in close agreement with the one obtained previously in programmed temperature mode, On the contrary, the ratio of the impurity catalytic to autocatalytic rate constant was slightly temperature dependent.     

Cure kinetics of aqueous phenol–formaldehyde resins used for oriented strandboard manufacturing: Analytical technique

The cure kinetics of commercial phenol–formaldehyde (PF), used as oriented strandboard face and core resins, were studied using isothermal and dynamic differential scanning calorimetry (DSC). The cure of the face resin completely followed an nth‐order reaction mechanism. The reaction order was nearly 1 with activation energy of 79.29 kJ mol^?1. The core resin showed a more complicated cure mechanism, including both nth‐order and autocatalytic reactions. The nth‐order part, with reaction order of 2.38, began at lower temperatures, but the reaction rate of the autocatalytic part increased much faster with increase in curing temperature. The total reaction order for the autocatalytic part was about 5. Cure kinetic models, for both face and core resins, were developed. It is shown that the models fitted experimental data well, and that the isothermal DSC was much more reliable than the dynamic DSC in studying the cure kinetics. Furthermore, the relationships among cure reaction conversion (curing degree), cure temperature, and cure time were predicted for both resin systems. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1642–1650, 2006     

Curing kinetics of NBR filled with coconut flour

The purpose of this investigation was to evaluate the effect of the incorporation of 20 phr of coconut woodflour of different particle sizes on the vulcanization kinetics of Nitrile Rubber (NBR) by means of rheometry and differential scanning calorimetry. The vulcanization kinetics results suggest that there might exist agglomerates of small particles (< 300 μm), which interfere with the curing reaction of the NBR. The reaction orders corresponding to the autocatalytic reaction show an increase with the extent of the reaction and with the rise on temperature. A good correlation of the vulcanization kinetics was obtained employing the generic model that involves constants associated to the reaction mechanisms, which in turn depend on the components present in the initial mix and on the autocatalytic reaction. The Osawa and Kissinger equations reproduce in a similar way the vulcanization kinetics of the compounds. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Cure kinetics of epoxy resin used for advanced composites

The cure kinetics of an epoxy resin used for the preparation of advanced polymeric composite structures was studied by isothermal differential scanning calorimetry (DSC). A series of isothermal DSC runs provided information about the kinetics of cure over a wide temperature range. According to the heat evolution behavior during the curing process, several influencing factors of isothermal curing reactions were evaluated. The results showed that the isothermal kinetic reaction of this epoxy resin followed an autocatalytic kinetic mechanism. In the latter reaction stage, the curing reaction became controlled mainly by diffusion. Cure rate was then modeled using a modified Kamal autocatalytic model that accounts for the shift from a chemically controlled reaction to a diffusion‐controlled reaction. The model parameters were determined by a nonlinear multiple regression method. Copyright © 2004 Society of Chemical Industry     

Active sites in methanol oxidation on Cu(110) determined by STM and molecular beam measurements

STM has been combined with molecular beam rate measurements to gain an understanding of the oxidative dehydrogenation of methanol to produce formaldehyde, both at the macroscopic and microscopic level. From this a model of the reaction is developed where the methanol initial reacts at very few active oxygen sites located at the short sides of oxygen islands on the Cu(110) surface. Such sites are very much rarer on a surface which is saturated with 0.5 monolayers of oxygen and the reaction rate is initially very low, but shows autocatalytic behaviour, rising in time as vacancies are created in the oxygen layer.     

2-甲基咪唑/环氧树脂体系的固化动力学研究

利用等温DSC法对E-51环氧树脂/2-甲基咪唑(2-MI)体系的固化动力学进行了研究,拟合得到n级固化模型、自催化模型及Kamal复合模型方程中的各个参数值,以确定固化模型。研究表明:E-51/2-MI体系的固化过程分为2个阶段,即诱发阶段和加成反应,其固化反应兼具n级固化与自催化的特征,符合Kamal复合模型。