Controlling low-pressure flame conditions by affecting the cold service lines located far from the heated reactor

A study was performed of a low-pressure oxyhydrogen flame in an isothermal regime in the diffusion-controlled region of heterogeneous chain termination. The chain propagation and termination sites were widely separated in space. Controlling the chain loss conditions made it possible to carry out isothermal combustion of the mixture in the diffusion-controlled region and to verify the proposition of the Semenov theory that the propagation of a chain explosion in time near the limit can be described by a linear model. From a comparison of calculated and experimental data, gas-surface interaction with a variable rate constant was found and the range of variation of this constant during one ignition was determined. The large differences between the kinetic curves obtained previously for various modes of chain termination on the wall were found to be related primarily to heterogeneous chain interaction, which governs the gas-phase process in the kinetic region and has a weak effect on this process in the diffusion-controlled region of chain termination. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 2, pp. 32–42, March–April, 2007.     

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.     

Heterogeneous chain propagation on a quartz surface

Results of experimental and analytical studies of heterogeneous chain processes on quartz are given. A calculation is made of the ignition of an oxyhydrogen mixture in a quartz reactor under conditions of intense heterogeneous negative interaction of chains in the kinetic and diffusion regions. An energy criterion was developed experimentally that allows one to distinguish the range of reactions with an elevated probability of heterogeneous chain propagation on quartz. The possibility of the formation of atoms and radicals on the surface of quartz and conditions for their passage into the bulk are studied. The possibility of passage of hydrogen atoms from a microporous SiO₂ surface into the bulk is analyzed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 3, pp. 42–52, May–June, 2006.     

Thermal explosion in self-propagating high-temperature synthesis systems with allowance for chemical gas transport

On the basis of a model of the reaction of components, taking the change in the contact surface into account, we analyze how the transport of one component to the surface of the other affects the ignition temperature. We have found that chemical gas transport converts the reaction process from ignition at the center of the space to ignition from the reactor wall. In the reactor are two regions, delimited by a curve of equilibrium concentrations of the carrier compound transporting the other component. The carrier compound forms and builds up in one region and in the other region separates out on the surface of the other. The fact that the reactor is closed and that diffusion occurs from one region to the other make it possible to produce superequilibrium concentrations of the carrier compounds in the region of deposition. Chernogolovka. Translated from Fizika Goreniya i Vzyrva, Vol. 29, No. 5, pp. 45–51, September–October, 1993.     

Specific inhibition as the kinetic principle of the Fischer-Tropsch synthesis

FT CO-hydrogenation on cobalt catalysts has been performed in a well stirred continuously operated slurry reactor with and without addition ofn-octene-1 at different CO partial pressures. Kinetic data have been determined with the help of a mechanistic model. Increasing CO partial pressure inhibits individual reaction steps selectively, in particular associative methane desorption, the steps of chain termination (as paraffins and olefins likewise), whereas in secondary olefin conversion hydrogenation is inhibited drastically, however double bond shift is even promoted. Secondary hydrogenation is concluded to proceed on different sites than chain growth.     

Study of polystyrene degradation using continuous distribution kinetics in a bubbling reactor

A bubbling reactor for pyrolysis of a polystyrene melt stirred by bubbles of flowing nitrogen gas at atmospheric pressure permits uniform-temperature distribution. Sweep-gas experiments at temperatures 340–370 ‡C allowed pyrolysis products to be collected separately as reactor residue(solidified polystyrene melt), condensed vapor, and uncondensed gas products. Molecular-weight distributions (MWDs) were determined by gel permeation chromatography that indicated random and chain scission. The mathematical model accounts for the mass transfer of vaporized products from the polymer melt to gas bubbles. The driving force for mass transfer is the interphase difference of MWDs based on equilibrium at the vapor-liquid interface. The activation energy and pre-exponential of chain scission were determined to be 49 kcal/mol and 8.94x10¹³s^-1, respectively. Presented at the Int’l Symp. on Chem. Eng. (Cheju, Feb. 8–10, 2001), dedicated to Prof. H. S. Chun on the occasion of his retirement from Korea University.     

Continuous precipitation polymerization of vinylidene fluoride in supercritical carbon dioxide: A model for understanding the molecular-weight distribution

Poly(vinylidene fluoride) (PVDF) that is synthesized by precipitation polymerization in supercritical carbon dioxide (scCO₂) has a bimodal molecular weight distribution (MWD) and a very broad polydispersity index (PDI) under certain reaction conditions. Different models have been formulated to account for this behavior. This paper presents a homogenous model for a continuous stirred-tank reactor (CSTR) that includes the change of the termination reaction from kinetic control to diffusion control as the chain length of the polymeric radicals increases, and accounts for the change in the termination rate constant with macroradical chain length in the diffusion-controlled region. The model also includes the chain transfer to polymer reaction. Comparison of the model output with experimental data demonstrates that changes of the MWD, including the development of a bimodal distribution, with such reaction conditions as monomer concentration and average residence time are successfully predicted. In addition, the model can capture the occurrence of gelation, which appears to be responsible for a region of inoperability that was observed in the polymerization experiments. The success of this homogeneous model is consistent with recent research demonstrating that the CO₂-rich phase is the main locus of polymerization for the precipitation polymerization of vinylidene fluoride and vinylidene fluoride/hexafluoropropylene mixtures in scCO₂, at the conditions that have been studied to date.     

Extraction of bitumen with sub- and supercritical water

The sub- and supercritical water extractions of Athabasca oil sand bitumens were studied using a micro reactor. The experiments were carried out in the temperature range of 360–380 °C, pressure 15–30 MPa and water density 0.07–0.65 g/cm³ for 0–2 hrs. The extraction conversion of bitumens increased with solvent power and temperature. A maximum conversion of 24% was obtained after 90 min extraction at the supercritical condition. Hydrogen and carbon mono-oxide were not detected in sub-critical region but in the supercritical region. The supercritical condition was favorable to the hydrogen formation for bitumen extraction. The extraction products were upgraded relative to the original bitumens due to direct hydrolysis of low-energy linkage and H₂ formed by water gas shift reaction in supercritical condition. 18% of initial sulfur in bitumen can be removed at maximum conversion condition. The asphaltene contents of the residue were significantly higher than that of original bitumen due to preferential extraction of aromatic compounds in supercritical condition.     

Monomolecular chain termination in the kinetics of dimethacrylate postpolymerization

The kinetics of postpolymerization (after ultraviolet illumination was stopped) for a number of dimethacrylates that differed by nature and molecular mass was experimentally studied over a wide range of temperatures. A series of kinetic curves that differed by the starting conversion of the dark period of time was obtained for every temperature. The proposed kinetic model of the process is based on the following main principles: (1) the process at an interface on the liquid monomer–solid polymer (micrograins) boundary takes a main share of the kinetics of postpolymerization; (2) chain termination at an interface is monomolecular, is controlled by the chain propagation rate, and represents by itself the self‐burial act of active radicals in the conformation trap; and (3) monomolecular chain termination is characterized by a wide spectrum of characteristic times and that is why the function of the relaxation is described by Kohlrausch's stretched exponential law. The obtained kinetic equation was in good agreement with all of the sets of experimental data. This permitted us to estimate the rate constant of chain termination (k_t) and to determine the scaling dependence of k_t on the molar‐volumetric concentration of the monomer in bulk [M₀]. We assumed that the stretched exponential law and scaling dependence k_t from [M₀] were characterized by common peculiarities, namely, a wide range of characteristic times of relaxation possessed by a property of the fractal set. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2376–2382, 2004     

Kinetics of all-trans-β-carotene degradation on heating with and without phenylalanine

All-trans-β-carotene was heated in liquid paraffin at 210°C for 15 min in the presence and the absence of phenylalanine to assess the effect of the amino acid on the rate of degradation of all-trans-β-carotene. The curve that represents all-trans-β-carotene degradation in both model systems is formed of two distinct parts that correspond, respectively, to the propagation and termination phases of an autocatalytic reaction. The reaction over 1–15 min followed first-order reaction kinetics in both systems, and the rate constant obtained was 2.8 times lower in the presence of phenylalanine. The kinetic behavior and the rate constant for color loss were similar to those for all-trans-β-carotene degradation for each model system.     

Influence of structural properties of catalysts at various stages of selective oxidation: from catalyst preparation to catalytic reactors

Solid oxides selective in mild oxidation exhibit a definite crystal morphology. Each crystal face displays a surface crystal field which rules the transformation of the reactant to a given product. The catalytic metastability of the surface which has to be maintained in the operating conditions prevailing in reactors depends on the reactivity of the bulk, because of oxygen diffusion process. Application to VPO, the catalyst for oxidation of n-butane to maleic anhydride, gives the opportunity to examine these properties in the series preparation–activation–catalysis–ageing, in relation to the type of catalytic reactor. This revised version was published online in August 2006 with corrections to the Cover Date.     

Investigation of La0.8Sr0.2CoO3/Ce0.85Sm0.15O2-x cathode performance of solid oxide fuel cell by electrochemical impedance spectroscopy: Effect of firing temperature

Perovskite type complex oxide L_0.8Sr_0.2CoO_3-δ symmetrical cells were prepared on Samaria doped ceria electrolyte Ce_0.85Sm_0.15O_2-x by using the screen-printing method in a laboratory scale. The performance of the symmetrical cell was investigated by using electrochemical spectroscopy at frequency ranging from 0.1–300 kHz. Effect of firing temperature from 975–1,050 °C was investigated under the controlled oxygen pressure from 0.002–0.21 atm and controlled measuring temperature from 635–782 °C. The preliminary results indicated that, for all cells prepared at different firing temperatures, the SEM and XRD did not indicate any differences between them. By using EIS, however, two impedance arcs were obviously observed. This first arc was found at high frequency region (<1,000 Hz) and the second one was observed at low frequency region (>10 Hz). The high frequency arc corresponded to the impedance of electron-transfer and ion-transfer processes occurring at the current collector/electrode and electrode/electrolyte interfaces. The low frequency arc was the convoluted contribution of the diffusion processes (non-charge transfer processes). Changing firing temperature, measuring temperature and oxygen pressure leads to changing of symmetrical cell performances. The activation energy of these symmetrical cells was around 1.5–2.0 eV depending on the firing temperature and oxygen pressure.     

Intrinsic kinetics of one-step dimethyl ether synthesis from hydrogen-rich synthesis gas over bi-functional catalyst

The reaction kinetics of the dimethyl ether synthesis from hydrogen-rich synthesis gas over bi-functional catalyst was investigated using an isothermal integral reactor at 220–260°C temperature, 3–7 MPa pressure, and 1,000–2,500 mL/g·h space velocity. The H₂/CO ratio of the synthetic gas was chosen between 3 : 1 and 6 : 1. The bi-functional catalyst was prepared by physically mixing commercial CuO/ZnO/Al₂O₃ and γ-alumina, which act as methanol synthesis catalyst and dehydration catalyst, respectively. The three reactions, including methanol synthesis from CO and CO₂ as well as methanol dehydration, were chosen as independent reactions. The Langmuir-Hinshelwood kinetic models for dimethyl ether synthesis were adopted. Kinetics parameters were obtained using the Levenberg-Marquardt mathematical method. The model was reliable according to statistical and residual error analyses. The effects of different process conditions on the reactor performance were also investigated.     

Preexplosion phenomena in heavy metal azides

The paper reports results of investigation of the explosive decomposition of heavy metal azides in real time. The characteristics of the detected predetonation effects — the preexplosion conductance and luminescence of heavy metal azides — are described. The obtained value of the preexplosion conductivity of silver azide indicates that the process is of a chain nature. A model for the development of explosion of heavy metal azides is developed including multiplication of active particles (holes) by a first-order reaction and chain termination by a second-order reaction. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 5, pp. 78–89, September–October, 2000. This work was supported by the Russian Foundation of Fundamental Research (Grant No. 98-03-32001a) and the Foundation of the Ministry of General and Professional Education of the Russian Federation.     

Performance studies of various cracking catalysts in the conversion of canola oil to fuels and chemicals in a fluidized-bed reactor

Studies were conducted at atmospheric pressure at temperatures in the range of 400–500°C and fluidizing gas velocities in the range of 0.37–0.58 m/min (at standard temperature and pressure) to evaluate the performance of various cracking catalysts for canola oil conversion in a fluidized-bed reactor. Results show that canola oil conversions were high (in the range of 78–98 wt%) and increased with an increase in both temperature and catalyst acid site density and with a decrease in fluidizing gas velocity. The product distribution mostly consisted of hydrocarbon gases in the C₁–C₅ range, a mixture of aromatic and aliphatic hydrocarbons in the organic liquid product (OLP) and coke. The yields of C₄ hydrocarbons, aromatic hydrocarbons and C₂–C₄ olefins increased with both temperature and catalyst acid site density but decreased with an increase in fluidizing gas velocity. In contrast, the yields of aliphatic and C₅ hydrocarbons followed trends completely opposite to those of C₂–C₄ olefins and aromatic hydrocarbons. A comparison of performance of the catalysts in a fluidized-bed reactor with earlier work in a fixed-bed reactor showed that selectivities for formation of both C₅ and iso-C₄ hydrocarbons in a fluidized-bed reactor were extremely high (maximum of 68.7 and 18 wt% of the gas product) as compared to maximum selectivities of 18 and 16 wt% of the gas product, respectively, in the fixed-bed reactor. Also, selectivity for formation of gas products was higher for runs with the fluidized-bed reactor than for those with the fixed-bed reactor, whereas the selectivity for OLP was higher with the fixed-bed reactor. Furthermore, both temperature and catalyst determined whether the fractions of aromatic hydrocarbons in the OLP were higher in the fluidized-bed or fixed-bed reactor.     

Two phase (air-molten carbonate salt) flow characteristics in a molten salt oxidation reactor

Molten salt oxidation process is one of the most promising alternatives to incineration that can be used to effectively destroy the organic components of mixed and hazardous wastes. To detect the flow characteristics of the molten salt oxidation process (air-molten carbonate salt two-phase flow), differential pressure fluctuation signals from a molten salt oxidation process have been analyzed by adopting the stochastic methods. Effects of the input air flow rate (0.05–0.22 m/sec) and the molten salt temperature (870–970 °C) on the phase holdup and flow characteristics are studied. The gas holdup increases with an increasing molten salt temperature due to the decrease of the viscosity and surface tension of the molten carbonate salt. It is found that a stochastic analysis of the differential pressure signals enables us to obtain the flow characteristics in the molten salt oxidation process. The experimentally obtained gas holdup data in the molten salt reactor were well described and characterized by means of the drift-flux model. This work was presented at the 7th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

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.     

Effect of heat transfer on the distributions of OH and CH radicals in the boundary layer with ethanol combustion

The reasons for formation of superequilibrium concentrations of radicals are studied by means of joint consideration of experimental data on the distributions of CH and OH molecules formed during diffusion combustion of ethanol and data on heat transfer in the chemical reaction region. The air flow velocity near the stagnation point in experiments with combustion is 0.7 m/sec, and the flow velocity along a flat plate is 10 m/sec (the turbulence levels are 1 and 18%). Mutual locations of specific features in the distributions of the heat-release rate and temperature are analyzed and compared with the distributions of OH and CH radicals. For all turbulence levels and flow velocities considered, the maximum concentration of radicals is reached on the boundaries of the heat-release region, whose locations are determined by molecular transport mechanisms. It is demonstrated that this conclusion is applicable to experimental data on diffusion combustion of a submerged hydrogen jet in air. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 3–11, November–December, 2008.