Thermal explosion in semi-batch reactors

The problem of thermal explosion in a semi-batch reactor is solved. The critical conditions for the thermal explosion are studied. It is shown that the ignition resulting from the chemical reaction is possible both during the stage of feeding of the second reactant solution and after its completion. The dynamic behavior of the reactor depending on the temperature of the heat exchanger and the volumetric feed rate of the second reactant is analyzed. It is shown that ignition at the feeding stage occurs at high temperature of the heat exchanger and involves two macroscopic stages. In the first stage, the temperature increases with increasing rate (thermal explosion regime), and in the second stage, its further increase occurs in the regime of combustion of the second reactant. The critical ignition conditions are found to be independent on the feed rate of the second reactant in a particular range of its values.     

Critical Conditions of Chain Thermal Explosion

This paper reports results of experimental investigation of the critical conditions of chain thermal explosion of hydrogen–air and methane–air mixtures are introduced. The observed regularities, including the difference in critical conditions between the ignition and explosion of these combustibles, are explained by considering the chemical mechanisms of oxidation of hydrogen and methane in the context of the nonlinear theory of nonisothermal chain processes. Examples of regularities of combustion processes at atmospheric pressure are given that cannot be described without considering the leading role of the competition between the branching and termination of reaction chains. It is shown that in solving the equations used to describe combustion processes, neglect of the variation in the heat effects of reactions with temperature can lead to large errors.     

Characteristics of thermal explosion in a porous layer with diffusion of a gaseous reactant

In the context of N. N. Semenov's stationary theory, a two-temperature model is used to analyze the thermal explosion of a porous layer with diffusion of a gaseous oxidizer into it with allowance for the oxidizer distribution inside the layer. The limit of thermal explosion, the maximal preexplosion temperature rise, and distribution of the gaseous-reactant concentration are determined. The behavior of the gas and framework temperatures for various regimes of the process are studied numerically. The dependence of explosion time on diffusion and burn-up of the gaseous reactant is analyzed.Translated from Fizika Goreniya i Vzryva, Vol. 32, No. 2, pp. 100–107, March–April, 1996.     

Thermal Decomposition of Ammonium Dinitramide Vapor in a Two-Temperature Flow Reactor

The thermal decomposition of ammonium dinitramide (ADN) was studied using a two-temperature flow reactor. The vapor pressure at temperatures of 80-140°C and the heat of vaporization of ADN were determined. From the results obtained a mechanism for the thermal decomposition of ADN was proposed that includes the ADN vaporization stage, i.e., the transition from the condensed to the gaseous state in the form of the molecular complex NH$₃·HN(NO₂)₂ (ADN vapor) followed by dissociation into NH₃ and HN(NO₂)₂. The composition of the products from the thermal decomposition of ADN vapor at temperatures of 160–900°C was determined by mass spectrometry. The effective rate constant was determined for the dissociation of ADN vapor at temperatures of 160–320°C. The thermal decomposition of ADN vapor in a flow reactor was modeled using the proposed mechanism and the reaction rate constant for ADN vapor dissociation. Key words: ammonium dinitramide, dinitramide, molecular beam mass spectrometry, thermal decomposition, two-temperature flow reactor.     

Performance Evaluation of a Novel Concept “Open Plate Reactor” Applied to Highly Exothermic Reactions

The implementation of chemical reactions in batch or semibatch reactors is strongly limited by the constraints linked to the dissipation of the heat generated by the reactions. A novel concept of heat exchanger reactors offers enhanced thermal performances in a continuously operating reactor. A study program is proposed to assess the feasibility and potentialities of this concept. Two kinds of reactions are carried out to characterize simultaneously the reactor performances in terms of reaction and heat exchange: the oxidation of sodium thiosulfate by hydrogen peroxide and an acidobasic reaction (NaOH/H₂SO₄). The resultant experimental data emphasize a significant thermal efficiency: the reactant concentrations and therefore the heat generation can be increased without risk of thermal runaway.     

Effect of the initial temperature of a reactive medium on the critical conditions for thermal explosion

A nonstationary formulation of critical conditions for thermal explosion in the Thomas problem (asymmetric self-ignition) and the classical Frank-Kamenetskii problem (symmetric self-ignition) are numerically studied. By the example of the Thomas problem, it is shown that the thermal explosion limit depends on the form of the initial spatial temperature distribution of the medium. For the Frank-Kamenetskii problem, the effect of the initial spatial temperature distribution and the initial overheating of the reactant on the critical value of the Frank-Kamenetskii number is studied. The particular initial spatial temperature distribution is determined; the investigation of its evolution allows one to make a conservative estimate of the thermal explosion limit. The limitation of the classical criterion for thermal explosion is shown, and the limits of applicability of the stationary theory are numerically determined from the initial overheating. For engineering calculations, an analytical formula for conservatively estimating the critical conditions for thermal explosion as a function of the initial reactant overheating is obtained.Translated from Teoreticheskie Osnovy Khimicheskoi Tekhnologii, Vol. 39, No. 2, 2005, pp. 175–185.Original Russian Text Copyright © 2005 by Zaev, Vasilevskii, Kirillov, Mechik.     

Stationary regimes of nonisothermal chemical reactions in a porous layer

Reactions in a porous layer into which a gaseous reactant diffuses are studied. For the thermostated boundary of the layer, the effect of degeneration of a thermal explosion is observed. In the case of poor heat transfer from the layer, this effect is not observed. The functional relations between the parameters of the system are found for different cases. V. V. Kuibyshev Tomsk State University, Tomsk. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 6, pp. 63–66, November–December, 1993.     

Heat pipe controlled syntheses of ionic liquids in microstructured reactors

The combination of a heat pipe and a microstructured reactor allows to perform highly exothermal reactions under safe conditions. First experiments for the synthesis of 1,3-dimethylimidazolium-triflate from 1-methylimidazol and methyl triflate showed that at high flow rates of more than 5 ml/min through a single 300 μm × 500 μm wide and 400 mm long channel quantitative conversion could be achieved. This chemical reaction, known for extreme heat release, can be retained under thermal control even at much higher flow rates.     

Analysis of a two-phase catalytic reaction in a millimeter-scale reactor

The synthesis of a catalyst for a two-phase catalytic reaction, a millimeter-scale reactor experiment, and an analysis model for the prediction of reactor performance are presented in this paper. The catalyst nano-particulate perovskite La_0.8Sr_0.2CoO₃ was prepared by a modified sol-gel method, in which PAA (poly acrylic acid) was added to catalyst precursors. A millimeter-scale reactor experiment with the prepared catalyst was carried. Concentrated hydrogen peroxide was decomposed in the reactor and the characteristics of the reactor were measured in terms of temperature distributions and liquid production rates. The results indicated a flow regime transition, which caused the change of reactor performance. An analysis model for two-phase catalytic reaction based on the lumped flow reactor model and the diagnostic data obtained on the temperature distributions and liquid production rates is proposed. Temperature distributions and heat transfer characteristics of the reactor were predicted by a semi-empirical analysis. In this analysis, the model of the Nusselt number (Nu) was proposed as . This expression of the model reflects the effects of temperature and coordinate location on the heat transfer characteristics of the reactor. From the modeled reactor, characteristics such as the increase of heat transfer in the mid and rear parts of the reactor with the increase of reactant flow rate were obtained. With the obtained results, a tool for the design and analysis of a down-scaled catalytic reaction device was obtained.     

Conditions of the Thermal Explosion under Forced Convection of a Reacting Mixture

Conditions of origination of the thermal explosion in a cylindrical chemical reactor equipped by a certain number of symmetrically located stirrers ensuring forced convection of the reacting mixture are considered in the approximation of an infinite Peclet number and under the assumption of laminar motion of the fluid. The critical value of the thermal explosion parameter (Frank-Kamenetskii parameter) is found as a function of the number of stirrers and the distance between the centerlines of the stirrers and reactor. As the number of stirrers increases, the calculations predict a lower probability of the thermal explosion, and the critical value of the parameter can severalfold exceed its classical value. It is found that the thermal explosion parameter substantially depends on the location of the stirrers, responsible for heat removal from the reactor. __________ Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 5, pp. 49–54, September–October, 2005.     

操作参数对余热回收甲醇水蒸气重整制氢过程的影响

以模拟汽车尾气供热的甲醇水蒸气重整（MSR）制氢反应为研究对象,设计了集余热加热与MSR制氢反应于一体的肋式微反应器,考察了反应器进口热风速度、温度,反应物进口速度、温度、水醇比及顺逆流情况对MSR制氢过程的影响。计算结果表明,逆流、水醇比1.3、热风进口速度1.1 m/s、温度773 K、反应物进口速度0.1 m/s、温度493 K为该反应过程的最佳工况参数,此时甲醇转化率为99.4%,模拟汽车尾气余热的热效率为28%,反应器出口氢气的体积分数为69.6%。研究结果对开展余热综合利用及发动机尾气重整制氢掺氢燃烧的研究有借鉴意义。     

Analysis of the Thermal Explosion in Systems Porous Reagent–Active Gas–Solid Product

The specific features of dynamics of the thermal explosion in systems porous reagent–active gas–solid product under conditions where the heattransfer and masstransfer regions are separated from the ambient medium are considered. In addition to the competition of heat release and heat removal, the process of initiation of exothermal chemical interaction in these systems under normal pressures depends significantly on conditions of filtration transport of the gaseous reagent. The induction and postinduction periods of the thermal explosion are studied. The theoretical analysis of thermalexplosion issues is supplemented by an experimental study of the process for the porous titanium–nitrogen–titanium nitride system.     

Radial Reactor-Heat Exchanger for Natural Gas Combustion in a Structured Porous Metal Catalyst Bed

A compact (0.01 m³ in volume) radial reactor for deep oxidation of methane (with a heat output of 16–30 kW) that is combined with an internal water heat exchanger is designed. The reactor contains the structured porous metal catalyst 5% (0.5% Pt/γ-Al₂O₃) + 65% Ni + 5% Al. The reactor performance at different heat outputs is experimentally studied. It is demonstrated that, if catalytic methane combustion is virtually complete, the fraction of heat transferred to the internal heat exchanger (to the water) is large (31–53%) and allows the reactor to reach a specific heat output up to 130 kW/m² (on the outer surface of the catalytic heating element). The coefficients of heat transfer to the internal heat exchanger and the radial thermal conductivity of the catalyst bed are shown to theoretically strongly affect the thermal conditions in the catalyst bed. It is concluded that the proposed mathematical model fits the experimental data well.__________Translated from Teoreticheskie Osnovy Khimicheskoi Tekhnologii, Vol. 39, No. 4, 2005, pp. 432–439.Original Russian Text Copyright © 2005 by Kirillov, Kuzin, Kuz’min, Skomorokhov, Shigarov.     

Radiative transfer in a solar chemical reactor for the co-production of hydrogen and carbon by thermal decomposition of methane

Radiation heat transfer in a solar chemical reactor for the co-production of hydrogen and carbon by thermal decomposition of CH₄ is analyzed by the Monte Carlo ray-tracing method. The solar chemical reactor features a vortex flow of CH₄ confined to a cavity and laden with carbon particles that serve simultaneously as radiant absorbers and nucleation sites for the heterogeneous decomposition reaction. The reactor is treated as a 3D non-isothermal non-gray absorbing-emitting-scattering gas/particle suspension directly exposed to concentrated solar irradiation. The analysis includes coupling to conduction/convection heat transfer and chemical kinetics. Calculated temperature distribution and chemical conversion are compared with the experimentally measured values obtained with a 5 kW prototype reactor tested in a solar furnace.     

尿素合成的安全

论述了尿素厂发生化学爆炸的浓度极限、能量极限以及在高温高压条件下的爆炸范围;从物理爆炸与化学爆炸的判断、爆炸性气体的形成、爆炸火源等方面对合成塔爆炸事故进行了分析;针对造成燃爆的2个硌要条件,提出了防止合成气爆炸的基本原则和防范措施。     

Effect of material mass flows on the ignition of condensed systems containing solid-phase reaction products

We consider the problem of ignition of a preheated metallic wire covered with a layer of reagent with a low heat conductivity. Solutions are derived both by numerical and approximate analytical methods. An analysis is presented for the dependence of the critical ignition conditions on the reagent's Stefan flow and the heat release. The degeneration during the reaction of the explosive nature of the ignition related to a change in the effective heat release coefficient is described. Various criteria for defining the critical conditions are discussed.Tomsk. Translated from Fizika Goreniya i Vzryva, Vol. 28, No. 2, pp. 8–14, March–April, 1992.     

The effect of operating conditions on acylation of 2-methylnaphthalene in a microchannel reactor

Acylation of 2-methylnaphthalene(2-MN) is a very important reaction in organic synthesis,and the effiency of the continuous reactor is more than one of the batch reactor.Considering that the Friedel–Crafts acylation is a rapid exothermic reaction,in this study,we perform the acylation of 2-MN in a stainless steel microchannel flow reactor,which is characterized by high mass and heat transfer rates.The effect of reactant ratio,mixing temperature,reaction temperature,and reaction time on product yield and selectivity were investigated.Under the optimal conditions,2-methyl-6-propionylnaphthalene(2,6-MPN) was obtained in 85.8% yield with 87.5% selectivity.Compared with the conventional batch system,the continuous flow microchannel reactor provides a more efficient method for the synthesis of 2,6-MPN.     

Self-ignition of dusty media

Thermal explosions of gaseous media containing inert dust particles are examined. The critical conditions for a thermal explosion are determined using integral manifolds. An asymptotic formula for calculating the critical conditions for thermal explosions with an autocatalytic combustion reaction is obtained.Samara. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 3, pp. 133–136, May–June, 1993.     

Temperature differences between phases in a moving bed reactor

Pseudo-homogeneous models of packed bed reactors assume equal temperatures and concentrations (or chemical potentials) for the solid and the fluid phases and are simpler than heterogeneous models. An analysis is presented for the degree of temperature departure between these two models under plug flow conditions with no axial dispersion. Fixed bed, cocurrent and countercurrent flow reactors are considered. The analysis yields two important parameters: α, the ratio of solid to gas thermal capacitances, and β, which is closely related to the number of interphase heat transferase units. In most industrial reactors, where β is greater than 50, the average temperature difference between phases is small, except for countercurrent reactors where gas and solid heat capacitances are nearly equal. Within this range of α, temperature differences can persist through the reactor, even with large values of β. The maximum temperature difference between phases is attained when the reaction heat effect is released in the worst case of a localized pulse in the reactor stream with the smaller thermal capacitance. These temperature difference measures can be used to estimate the validity of a pseudo-homogeneous model. This analysis is easily extended to concentration differences between phases.