CFD modeling using heterogeneous reaction kinetics for catalytic dehydrogenation syngas reactions in a fixed-bed reactor

A comprehensive 2D computational fluid dynamics （CFD） model was developed to simulate the flow behavior and catalytic dehydrogenation reaction of syngas in a heterogenous fixed-bed reactor （FBR）. The model combined the porous medium CFD model with a reaction kinetics model. To acquire an accu- rate reaction kinetics model, a comprehensive reaction mechanism was studied for the heterogeneous catalytic dehydrogenation reaction ofsyngas over a supported metal catalyst. Based on the reaction mech- anism and a statistical test, a reliable kinetics model was proposed. The CFD model combined with the above kinetics model was validated with one set of experimental data. The CFD model was also used to predict key reaction variable distributions such as the temperature and the component concentrations in the reactor.     

Investigation of the starting process in a Ludwieg tube

The hypersonic Ludwieg tube Braunschweig (HLB) is a valve-controlled wind tunnel that has been designed for a Mach number of Ma = 5.9 and a Reynolds number range from 2.5 × 10⁶ up to 2.5 × 10⁷ 1/m. The intermittent working principle implies an unsteady onset of flow, which leads to a delay of the time frame suitable for measurements as well as to heat loads different from steady flow conditions. This work numerically simulates the starting process. It determines whether the onset of flow leads to a significant temperature rise in the model surface which in turn impacts results gathered during measurement time. The flow field in the HLB is numerically rebuilt for two operating points including valve opening. The non-stationary flow around a hyperboloid/flare configuration in the test section is calculated for one operating point including surface heating. For laminar flow it is found that due to the short duration of the starting process no significant model heating affects results obtained during measurement time.      

Modeling of pyridine synthesis process in a coupled fluidized bed reactor

To obtain the optimal operating conditions of a coupled reactor for pyridine synthesis, reactor modeling process is carried out in this paper. During the modeling process, the flow hydrodynamics, heat transfer behavior, inter-phase mass transfer behavior and reaction kinetics were taken into consideration consequently. Further, a regression program based on least square method was proposed to regress the model parameters. The prediction results agreed well with the experimental results with an average deviation of 5.9%. Finally, by setting suitable aim function, the optimal operating conditions of the coupled reactor for pyridine synthesis were determined.     

Investigation of gas flows in a radial-type reactor for GaAs epitaxy

Argon and hydrogen flows for the epitaxy of GaAs from metal-organic compounds in a radial-type reactor are visualized by means of holographic interferometry methods. On the basis of the holographic interferograms the hydrogen temperature fields are calculated for various flow velocities. It is shown that an interpretation of the phenomena taking place in the gas phase, in terms of the dimensionless numbers Re and Ra, can be useful for qualitatively estimating and comparing various flow situations; a correlation of the interference patterns with the reactor geometry and gas flow velocity, which can be used to optimize the process parameters and reactor chamber structure, is described; the instability of Ar and H₂ flows is associated with the high-velocity turbulence and natural convection, the destabilizing effect of the buoyancy forces manifesting itself significantly more strongly in the argon flow. This means that it is incorrect to predict the nature of the flows on the basis of the complex Gr/Re².Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 6, pp. 118–124, November–December, 1996.     

Dual bed reactor for the study of catalytic biomass tars conversion

A dual fixed bed laboratory scale set up has been used to compare the activity of a novel Rh/LaCoO₃/Al₂O₃ catalyst to that of dolomite, olivine and Ni/Al₂O₃, typical catalysts used in fluidized bed biomass gasification, to convert tars produced during biomass devolatilization stage. The experimental apparatus allows the catalyst to be operated under controlled conditions of temperature and with a real gas mixture obtained by the pyrolysis of the biomass carried out in a separate fixed bed reactor operated under a selected and controlled heating up rate.The proposed catalyst exhibits much better performances than conventional catalysts tested. It is able to completely convert tars and also to strongly decrease coke formation due to its good redox properties.     

A review of recent advances in selective catalytic NOx reduction reactor technologies

Research on NO_x treatment is extensive in recent years due to growing environmental awareness. Selective catalytic reduction (SCR) of NO_x, as a proven technology, offers higher NO_x control efficiency than many other NO_x treatment methods. The present work reviews the recent development of SCR reactor technologies. Firstly, catalysts and mechanism of different SCRs were briefly summarized. Different SCR reactors, e.g. structured reactor, fluidized bed reactor and moving bed reactor, were then discussed. As a more advanced technology, multifunctional reactors were also developed for SCR process and could be divided into two categories: decoupled adsorption-reaction process and combined SCR system. The mechanism and properties of these processes were discussed in detail. Some recommendations were given for the future work in SCR reactor design. SCR reactor technology for emerging energy processes was also addressed, such as oxyfuel combustion and biofuel conversion processes, which put forward new requirements for SCR technologies and also open new opportunities for advanced design of SCR reactors.     

A review of recent advances in selective catalytic NOx reduction reactor technologies

Research on NOx treatment is extensive in recent years due to growing environmental awareness. Selec- tive catalytic reduction （SCR） of NOx, as a proven technology, offers higher NOx control efficiency than many other NOx treatment methods. The present work reviews the recent development of SCR reactor technologies. Firstly, catalysts and mechanism of different SCRs were briefly summarized. Different SCR reactors, e.g. structured reactor, fluidized bed reactor and moving bed reactor, were then discussed. As a more advanced technology, multifunctional reactors were also developed for SCR process and could be divided into two categories： decoupled adsorption-reaction process and combined SCR system. The mechanism and properties of these processes were discussed in detail. Some recommendations were given for the future work in SCR reactor design. SCR reactor technology for emerging energy processes was also addressed, such as oxyfuel combustion and biofuel conversion processes, which put forward new requirements for SCR technologies and also open new opportunities for advanced design of SCR reactors.     

催化重整单元氢气气团爆炸超压分析

基于计算流体力学分析软件(FLACS),以催化重整反应单元为例建立事故模型,研究不同形状障碍物、泄漏位置,对不同泄漏时间和泄漏监测点的氢气爆炸超压的影响情况。通过研究,建立了与气体燃烧热与爆炸监测点距气团中心距离相关的最大爆炸超压模型。研究结果表明,在研究设计的遮挡物条件下,气体爆炸最大超压与折合距离在对数坐标系中均呈近似线性关系;对于不同的遮挡物,爆炸超压模型需进行修正;在反应器中部发生的事故场景,泄漏5min后最大爆炸超压明显增大。     

Investigation of the transitional process of the startup of a gas bearing

In this article we derive equations for transitional conditions of a gas lubricant under nonisothermal conditions. A numerical solution of the Reynolds equation is described for the problem of the pulsed startup of a radial bearing with a fixed geometry, and the results of this solution are given.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 156–160, September–October, 1977.     

A two-stage reduction process for the production of high-purity ultrafine Ni particles in a micro-fluidized bed reactor

A novel two-stage reduction process for synthesis of ultrafine nickel powder with a high purity and low density in a fluidized bed reactor has been developed in this work.The raw ultrafine NiO particles are first pre-reduced using hydrogen at lower temperatures(340-400 ℃),followed by further reduction at higher temperatures(500-600 ℃).The self-agglomeration of Ni particles formed during low-temperature reduction decreases the sintering activity of the newly formed ultrafine Ni particles,leading to good fluidization quality,even for the subsequent high-temperature reduction process.The agglomerated Ni particles have a high Ni content(above 99wt%),a low density(0.78 g/cm~3) and a uniform particle size(approximately 100μm).A concept design for a novel two-stage fluidized bed reactor process used to produce high-purity Ni powder was also proposed.This approach may be extended to the synthesis of other ultrafine/nanosized metals or metal oxides through a fluidization method.     

Investigation of the starting process in the shaped nozzle of a large-diameter shock tube

The characteristics of flow formation in the shock-wave starting process of a shaped nozzle with a developed subsonic section are analyzed on the basis of numerical calculations in the inviscid two-dimensional formulation. It is shown that in this case the use of steady-state boundary conditions in the throat section can lead to a significant error in determining the startup time. The results of the calculations are compared with experimental data obtained over a broad interval of the leading parameters. The results of the comparison help to explain the part played in the starting process by flow separation from the nozzle walls, which was not taken into account in the numerical investigation.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 88–95, September–October, 1986.The authors wish to express their thanks to R. I. Serikov and V. M. Khailov for the wind tunnel nozzle blowdown data and to V. P. Stulov for useful advice and discussions.     

Reaction performance of fluidized bed catalytic reactor of Group C+particles

Group C particles are often regarded as non-fluidizable but have proven to effectively fluidize with nanoparticle addition,which results in small bubbles and a high gas holdup in the dense phase during the experiments.Group C⁺particles provide an increased surface area for gas-solid contact and improve the reaction performance,especially for gas-phase catalytic reactions.On the basis of a previous study of the ozone decomposition reaction using Group C⁺particles,a two-phase model was used to evaluate the reactor contact efficiency,and was used to compare the partial oxidation performance of the n-butane to maleic anhydride reaction in fluidized-bed catalytic reactors of Group C⁺and Group A particles.The reactor with Group C⁺particles achieved a higher n-butane conversion and MAN yield compared with that using Group A particles,based on the identical catalyst quantity or on the same gas residence time.Therefore,the reactor with Group C⁺particles can achieve the same reaction conversion and yield with fewer catalysts or a smaller reactor size,or both.Therefore,the fluidized bed catalytic reactor of Group C⁺particles is expected to be of major significance in industrial processes,especially for gas-phase catalytic reactions.     

Investigation of the optimum longitudinal mixing regime in an isothermal chemical flow reactor

A one-dimensional isothermal flow reactor with longitudinal mixing is examined. The known asymptotic results are analyzed and used as a basis for formulating the conditions and initial values of the parameters for a numerical investigation by the parameter mapping method. The object of the investigation is to construct the exact boundaries of the regions of variation of the characteristic parameters in which the maximum extent of reaction is achieved in one of the limiting (Pe=0 and Pe=) or intermediate (0相似文献   

Investigation on the generation process of impact-driven high-speed liquid jets using a CFD technique

High-speed liquid jets have been applied to many fields of engineering, science and medicine. It is therefore of benefit to all these areas to investigate their characteristics by modern and inexpensive methods using a computational fluid dynamics (CFD) technique. Previously, high-speed liquid jets have been studied experimentally using a momentum exchange method, called the “impact driven method (IDM)”, by which the impact of a high-velocity projectile on the liquid package contained in the nozzle cavity produced the jet. The shock pulse reflections in the cavity caused by the impact then drove a multiple pulsed jet from the nozzle exit. In this study, a two-fluid simulation consisting of liquid and air can be successfully calculated by using a two-phase flow mixture model and a moving mesh for the projectile motion. The CFD results show good agreement to the results of previous experimental studies, both quantitatively and qualitatively. For the first time, the wave propagation within the liquid in the nozzle has been captured and analyzed, thereby demonstrating the dynamic characteristics of multiple pulsed high-speed liquid jets initiated by the IDM. This provides a breakthrough in the simulation of the supersonic injection of a liquid into air by using a well-known and user-friendly CFD software. It is useful fundamental knowledge for future studies of high-speed injection with applications in all its related fields.     

Concentration fluctuations in a stirred reactor

Turbulent mixing of tap-water with salt-water in a 6.3 dm³ Continuous Stirred Tank Reactor is analyzed from concentration measurements obtained by a conductivity micro-probe. The frequency response of the instrument has been tested previously by specific dynamic trials. As well as the field of mean concentration C and the degree of segregation \(\sqrt {c\prime ^2 } /\bar C\) the Eulerian length macroscales of concentration in the tank and characteristics of the turbulent diffusion in the discharge flow are deduced from spectral analysis of the fluctuations and from the turbulent velocity field measured by laser Doppler velocimetry.     

Investigation of the aging process of a polyvinyl chloride gel by the measurement of its dynamic moduli

Summary During the aging process of a 10%-solution of polyvinyl chloride in di-(2-ethylhexyl)phthalate at temperatures between –20 and 110°C its storage and loss moduli were measured in a frequency range from 1.6×10^–4 to 40 cycles/sec. The specially defined rate of aging plotted against the aging temperature closely resembles the crystallization curve of a normally crystallizable polymer. Although the internal structure of the system changes, when the temperature is changed, the time-temperature reduction could still be applied provided some precautions were taken.Bueche's theory for networks, combined withFlory's post-gelation treatment, has been used to calculate some gel parameters: the molecular weight between cross-links, the sol fraction, the monomeric friction coefficient etc. It is shown that the aging process strongly depends on the thermal history. The cross-links formed by crystallites, differ very much in functionality. Continued aging, after previous aging at lower temperature, causes, after a first melting of unstable crystallites, a recrystallization process. During this process entanglements are formed. This results in a pseudo-equilibrium rubber plateau at higher frequencies, followed by a dispersion region and a lower equilibrium rubber plateau at lower frequencies.

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Zusammenfassung Während der Alterung (Gelierung) einer zehnprozentigen Lösung von Polyvinylchlorid in Di-(2-Äthylhexyl)-Phthalat wurden deren Speicher- und Verlustmoduln in einem Frequenzbereich zwischen 1,610^–4 und 40 Hertz gemessen. Die speziell definierte Alterungsgeschwindigkeit wurde gegen die Alterungstemperatur aufgetragen. Die so erhaltene Kurve ist der Kristallisationskurve eines normal kristallisierenden Polymeren ähnlich. Obwohl die innere Struktur des Systems sich mit der Temperatur ändert, konnte die Zeit-Temperatur-Reduktion doch ausgeführt werden, wenn geeignete Vorsorgmaßnahmen getroffen wurden.Bueches Netzwerktheorie wurde in Kombination mitFlorys Behandlung der Nachgelierung zur Berechnung einiger Parameter des Gels verwandt. Es handelt sich hierbei um das Molekulargewicht zwischen Verknüpfungsstellen, die Solfraktion, den Reibungskoeffizienten der monomeren Einheit u.a. Es wird gezeigt, daß der Alterungsprozeß stark von der thermischen Vorgeschichte abhängt. Die Verknüpfungsstellen, die durch Kristallite gebildet werden, zeigen starke Unterschiede in der Funktionalität. Fortgesetzte Alterung bei höherer Temperatur nach vorhergegangener Alterung bei tieferer Temperatur verursacht nach anfänglichem Schmelzen unstabiler Kristallite einen Rekristallisationsprozeß. Während dieses Prozesses werden Verhakungen (entanglements) gebildet. Diese bewirken die Entstehung eines zweiten (im Pseudo-Gleichgewicht befindlichen) Gummiplateaus bei höheren Frequenzen, auf das bei niedrigeren Frequenzen nach Passieren eines Dispersionsgebietes ein echtes (im Gleichgewicht befindliches) Gummiplateau folgt.

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With 17 figures and 4 tables     

Effect of process conditions on the synthesis of carbon nanotubes by catalytic decomposition of methane

A new dual-composition catalyst based on Ni-Mo/MgO with high efficiency of producing carbon nanotubes （CNTs） from methane was reported recently. In the present article, with this type of catalyst, the impact of such experimental parameters as reaction temperature, reaction time, concentration of H2, flow rate ratio of CH4 to H2 on yield and graphitization were investigated, leading to the following optimal growth conditions： reaction time 60min, reaction temperature 900℃, CH4：H2 about 100：20mL/min, under which high-yield multi-walled CNTs bundles were synthesized. Raman measurement indicated that the as-synthesized product was well-graphitized, and the purity was estimated over 95% by TG-DSC analysis. In terms of the above results, an explanation of high-efficiency formation of CNTs bundles and the co-catalysis mechanism of Ni-Mo/MgO were suggested. 2007 Chinese Societv of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V.