Initiation of chain and thermal explosions by the reactor surface. Criterion for the participation of branching chains in a thermal explosion

The combustion of hydrogen and silane is studied. It is established that the chain initiation reaction on quartz in the zone of hydrogen and silane combustion is manifested as an autocatalytic reaction which is able to initiate a chain explosion and participate in the initiation of a thermal explosion. It is shown that in the case of an oxyhydrogen gas, the assumption of a branching-chain nature of the third limit is inconsistent with Semenov’s law, which includes double exponential dependences of the chain reaction rate on time and temperature. A criterion for the participation of branching chains in complex processes is proposed based on the presence or absence of short delays of a thermal explosion (≈1 sec). According to the criterion, the explosion of an oxyhydrogen gas at atmospheric pressure with delays markedly exceeding 1 sec proceeds without the participation of branching chains and is consistently explained by the joint action of autocatalytic processes on the reactor wall and gas-phase processes. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 5, pp. 44–51, September–October, 2007.     

Dynamics of thermal explosion in the postinduction period

The dynamics of thermal self-acceleration of the reaction and heating is studied for gasless condensed compositions in the induction and postinduction periods up to complete transformation of the substance. It is shown that the propagation of the reaction over the sample is of a distinct frontal character in a broad range of values of the Biot criterion Bi and within the framework of the macrokinetics of “weak” deceleration.” Two qualitatively different mechanisms of front propagation are revealed. This is the normal propagation of the combustion front initiated by the ignition zone for large values of Bi. For small values of Bi, propagation of the front is the apparent effect, which is due to subsequent adiabatic self-ignition of separate portions of the substance uniformly heated during the induction period. In the second case, by the propagation velocity, we mean the “phase” velocity. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 6, pp. 65–70, November–December 1999.     

Macrokinetics of thermal explosion in a niobium-aluminum system. II. Phase-formation dynamics

The thermal explosion in a gas-free mixture of metal powders has been studied for the first time using the method of dynamic x-ray structural analysis. The dynamics of formation of new crystalline phases at all stages of the process is determined. It is shown that melting of aluminum is followed by dissolution of niobium in the melt and crystallization of the intermediate product NbAl₃, and the melt does not wet niobium particles. A dramatic improvement in wetting is observed at a temperature of ≈ 1040 K (apparently, due to the destruction of oxide films). This results in a thermal explosion, and the Nb₂Al phase is formed. The reaction during the thermal explosion also follows the dissolution-crystallization mechanism. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 2, pp. 45–50, March–April, 2000. This work was supported by the Russian Foundation for Fundamental Research (Grant No. 98-03-32137).     

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.     

Thermal explosion of an unbounded plate under third-kind asymmetric boundary conditions

An analytical solution is given for the problem of a thermal explosion of a plane reagent layer under third-kind asymmetric boundary conditions. The critical Frank-Kamenetskii parameters are determined for the general and particular cases. Calculation results are compared with the literature data. It is shown that the critical parameters, determined by direct and inverse methods, do not coincide for small values of the Biot criterium and depend on the scale temperature. The particular feature of the approach is the application of the inverse method to the solution of the thermal-explosion problem. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 3, pp. 88–91, May–June 1998.     

Effect of the nonuniformity of the inlet liquid distribution on the trickle-bed reactor output in an exothermic reaction accompanied by evaporation

The effect of the inlet liquid distribution on the operation of a trickle-bed reactor is experimentally investigated, and the observed phenomena are analyzed using a one-dimensional mathematical model with nonequilibrium approaches to describing phase transitions. The modeling shows that, if the fraction of the wetted surface at the reactor inlet is below unity, then, along a sufficiently long bed, there is always a stationary front of complete evaporation and complete conversion. The main conclusion within the framework of the considered one-dimensional model with separate evaporation (on completely wetted granules) and gas-phase hydrogenation (on dry granules) is that safe operation (the absence of a thermal explosion) can be ensured by complete suppression of the reaction in the gas phase.     

Superadiabatic heating in combustion chromium in nitrogen

Combustion of chromium powder in SHS reactors is examined. It is shown that the maximum temperature in the central part of the powder samples is higher than the adiabatic temperature. The observed phenomenon is due to the non-one-dimensionality of the combustion front. Overheating of the central part leads to melting of the product and a decrease in the degree of conversion, which should be taken into account in nitration of samples of large diameter. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 6, pp. 50–52, November–December 1999.     

Catalytic synthesis of triazine compounds using a flexible technology

Based on laboratory pilot studies, we have developed a flow sheet for the catalytic synthesis of triazine compounds from carbamide using a flexible technology and a catalyst for this process. The main process parameters are as follows: a carbamide melt is fed into the reactor under a pressure of 0.8 MPa at 140–160°C; the volume rate of feeding the circulating gas into the reactor is 500–750 h⁻¹, its temperature is 350–500°C, and the melt-to-gas mass ratio is 1: (7–9). The temperature of synthesis in the reactor is 350–450°C; the pressure in the reactor is 0.1–0.2 MPa. The sublimation temperature is 180–200°C. The conversion of carbamide is ∼98%. The content of the target component in the product is ∼98.8%. Depending on the composition of the circulating gas, it is possible to obtain products of melamine, cyanuric acid, or melamine cyanurate. A catalyst in the form of promoted active aluminum oxide with an inner surface of 300 to 400 m²/g and a technique for its preparation have been developed.     

Solid-State Synthesis of ZnTe in Shock Waves

Shock front velocities in a heterogeneous stoichiometric zinc-tellurium mixture in cylindrical capsules were measured at normal and elevated temperatures. In the range of preheating temperatures of 150–300°C, the velocity of a strong shock wave was found to increase by 0.91 km/sec, which is attributed to the occurrence of an exothermic reaction in the zone of high dynamic pressures with an increase in the specific volume. The average velocity of a weak shock wave increased by 0.31 km/sec in the far region of the reaction cell as the preheating temperature of the stoichiometric Zn-Te mixture increased by 150°C. X-ray structure analysis of the shock-recovered products showed almost complete transformation of the reactants with the formation of the cubic ZnTe phase. Theoretical calculations of the acceleration of the shock front velocity due to the reaction in the Zn-Te mixture were conducted. The occurrence of solid-state detonation in the tested mixture is assumed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 1, pp. 130–137, January–February, 2006.     

Gap effect on the initiation of detonation in gas mixtures in cylindrical combustion chambers. II. Triggering of detonation within a turbulent flame

Schlieren moving-picture photography is used to study the burnup of oxygen gaseous mixtures in a cylindrical chamber with a gap at its periphery. It is found that a flame penetrating from the chamber into the gap can accelerate up to detonation speeds. The reaction wave in the gap precedes the primary combustion front propagating through the chamber and the reaction products escaping the gap create secondary combustion sources in the chamber. A process occurs in which a detonation wave that appears in the gap near one flank of the flame enters the main volume through the opposite flank, first triggering an explosion in the turbulent combustion zone (“an explosion within an explosion”) and then a detonation wave in the unreacted gas charge (“knock” in an engine). An interpretation is provided for the gas-dynamic structure of the secondary combustion source which is created in the cylindrical combustion chamber by a detonation wave propagating in the gap. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 4, pp. 77–87, July–August 1998     

Steady regimes of conversion in a viscoelastic medium

The problem of propagation of a steady conversion front in a viscoelastic medium is solved by the method of matched asymptotic expansions in the approximation of low strains. The heat flux is assumed to satisfy the Fourier law, and the components of the stress and strain tensors are related by the Maxwell relations including the shear coefficient of viscosity. The temperature of the products and the velocity of propagation of the steady reaction front are found. The solution of the problem is obtained for the limiting cases of the small and large times of relaxation of viscous stresses. It is demonstrated that the model contains different regimes of reaction-front propagation, like the coupled models of solid-phase combustion for a thermoelastic body, and viscous stresses insert additional features. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 5, pp. 63–73, September–October, 2006.     

High temperature oxidation and ignition of some metallic materials in fluorine

When 3-mm-diameter cylindrical samples are heated by direct passage of an electric current through them, it is observed that titanium, molybdenum, tungsten-niobium alloy, nickel, iron, carbon steel, and stainless steel in gaseous fluorine (p=0.2 MPa) ignite through a thermal explosion mechanism, while copper, its alloys, and aluminum ignite when the melting point of the fluoride film is reached. The critical ignition temperatures of these materials are established. Experimental data on the high temperature oxidation kinetics are used to calculate the pertinent parameters of the Arrhenius for the preignition interaction of the steels and nickel with fluorine. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 4, pp. 34–42, July–August 1998.     

Macrokinetics of thermal explosion in a niobium—aluminum system. I. Basic macrokinetic stages

The basic macrokinetic stages of a gas-free thermal explosion in an Nb-Al system with different stoichiometric ratios of the reagents are determined using scanning differential calorimetry, electron metallography, and microanalysis. Chemical interaction begins when aluminum melts (993 K) and proceeds slowly up to a temperature of 1020-1040 K, at which dramatic self-heating and acceleration of the reaction (thermal explosion) occur. Migration of the melt from the center of the sample to the surface layers and growth of droplets on the sample surface are observed at the stage of slow reaction, and the drops penetrate again into the sample at the stage of thermal explosion. The phases NbAl₃ and Nb₂Al are formed in the thermal explosion regardless of the initial stoichiometry of the composition; the differences are manifested only in the ratio of these phases and in the amount of residual (nonreacted) niobium. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 2, pp. 40–44, March–April, 2000. This work was supported by the Russian Foundation for Fundamental Research (Grant No. 98-03-32137a).     

Thermal and spectroscopic analysis of worn polyoxymethylene surfaces and wear debris explaining degradation and polymerisation mechanisms

Degradation and polymerization of polyoxymethylene homopolymer (POM-H) surfaces after sliding at 8 to 150 MPa and 0.005 m/s over a total sliding distance of 3000 m is investigated by using thermal analysis (DSC, TGA, DTA) and Raman spectroscopy of worn surfaces or wear debris. There is mainly mechanical interaction and slight softening at 8 MPa (relatively high friction, low wear), softening at 16 to 55 MPa (decreasing friction and high wear) and finally melting at 150 MPa (very low friction, overload wear). At low contact pressures, wear debris remains amorphous and degradation of noncrystallised material during sliding manifests in broadening of the melting peak below the melting temperature. Degradation of C–O–C due to chain scission and radical reactions into CH₃ end groups are illustrated by Raman spectra. It is confirmed that the debris has long resident times and the maximum polymer surface temperature (T* = 93°C) is below the crystallisation temperature. At intermediate contact pressures, crystallisation results in a polymer fraction with higher thermal resistance. From the calculated temperatures T* = 120 to 150°C, crystallisation is beneficial for coherent transfer with larger particle sizes. At high contact pressures, the wear debris is immediately removed from the contact interface due to melting (T* = 200°C) and has thermal properties similar to the bulk material. There is no reaction between the debris in the interface, resulting in a thick polymer transfer film.     

化学反应系统热爆炸判据

应用摄动方法 ,研究了放热化学反应系统在发生失控 (即“热爆炸”)时 ,其化学动力学参数的临界值 ,在进行量纲 1化后 ,得到了热爆炸判据δcr的渐近分析公式 ,该公式给出了δcr随ε的变化 .     

Analysis and demonstration of a control concept for a heat integrated simulated moving bed reactor

Successful operation of a simulated moving bed reactor requires a strategy to control switching of the inlet and outlet positions between different fixed-bed segments. This is due to the narrow operation window in which an ignited state can be maintained and complete conversion is assured in exothermal irreversible reactions. This work compares three control concepts, which are either based on monitoring the reaction front or the leading thermal front. Another feature investigated is the length of the observation time considering a single switching period and a complete cycle. For the concept favored some efforts regarding parameterization are made. The wider range of temperature set-points applicable can be related to stability limits of an associated hypothetical true moving bed reactor. Responses to step inputs in feed concentration and total flow rate are investigated experimentally in order to evaluate the performance of the controller suggested. An ignited state of the reactor could be maintained successfully for 3.5 days.     

Hydrogenation of CO on supported cobalt γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst in fixed bed and slurry bubble column reactors

Fischer-Tropsch synthesis for the production of C₅+ hydrocarbons from syngas was carried out in a tubular fixed bed reactor (TFBR) and in a slurry bubble column reactor (SBCR). The Co-based catalysts for FTS were prepared by the conventional wet-impregnation of γ-Al₂O₃. Effects of operating conditions such as GHSV (1,000–4,000 ml/g·hr), reaction temperature (220–250°C) and pressure (0.5–3.0MPa) on the CO conversion and product selectivity of Co/γ-Al₂O₃ catalyst were examined in the TFBR and SBCR. The C₅+ selectivity and olefin selectivity in an SBCR were found to be higher than that in a TFBR, whereas C₂–C₄ selectivity showed a reverse trend. The CO conversion and product distribution in an SBCR were less sensitive than that in a TFBR with variations of reaction conditions.     

Solid-phase synthesis of polyamide-6

Solid-phase synthesis of PA-6, simultaneously discovered with the reaction of conversion of CL into a polymer and of practical interest primarily as a method for obtaining a polymer constructed of strictly linear macromolecules and containing a minimum amount of CL and especially cyclic oligomers, is not solid-phase in the generally understood meaning of this term. This essentially concerns a set of liquid-phase reactions which differ from ordinary reactions due to the fact that some of the participants and primarily the solvent “molecules,” which the kinetic segments of transfer chains act as, are free of translational mobility, and the entire reaction volume is in a stressed state due to differences in the coefficients of thermal expansion of amorphous and crystalline microregions of the polymer and the developed process of additional crystallization. The presence of an important reaction volume which persists in a relatively wide temperature range and the very large amplitudes of vibrations of translationally immobile reactive groups makes the process kinetically more advantageous than solid-phase polycondensation of monomeric ε-aminocaproic acid. __________ Translated from Khimicheskie Volokna, No. 4, pp. 40–48, July–August, 2006.     

THE OPTIMUM TEMPERATURE POLICY FOR A DEACTIVATING IMMOBILISED ENZYME FIXED BED REACTOR†

A criterion is developed for the optimum operation of an isothermal fixed bed reactor using a deactivating immobilised enzyme catalyst. The chemical reaction is assumed to follow a Michaelis-Menten type kinetics, and the deactivation is assumed to depend only upon the temperature of operation. An explicit equation for dT/dt is derived from the criterion which makes it possible to calculate the optimum reactor temperature as a function of time in a straightforward manner for a given initial temperature by standard numerical integration techniques. It turns out that the optimum temperature policy results in a varying conversion at the reactor outlet. A numerical example is presented which indicates that such a policy is better than both a constant temperature policy and a constant conversion policy. The techniques developed here can also be used for a gas-solid catalytic reactor involving Langmuir-Hinshelwood kinetics with concentration independent deactivation.