Catalytic combustion assisted methane steam reforming in a catalytic plate reactor

A theoretical study of methane steam reforming coupled with methane catalytic combustion in a catalytic plate reactor (CPR) based on a two-dimensional model is presented. Plates with coated catalyst layers of order of micrometers at distances of order of millimetres offer a high degree of compactness and minimise heat and mass transport resistances. Choosing similar operating conditions in terms of inlet composition and temperature as in industrial reformer allows a direct comparison of CPRs with the latter. It is shown that short distance between heat source and heat sink increases the efficiency of heat exchange. Transverse temperature gradients do not exceed across the wall and across the gas-phase, in contrast to difference in temperature of outside wall and mean gas phase temperature inside the tube usually observed in conventional reformers. The effectiveness factors for the reforming chemical reactions are about one order of magnitude higher than in conventional processes. Minimisation of heat and mass transfer resistances results in reduction of reactor volume and catalyst weight by two orders of magnitude as compared to industrial reformer. Alteration of distance between plates in the range 1- does not result in significant difference in reactor performance, if made at constant inlet flowrates. However, if such modifications are made at constant inlet velocities, conversion and temperature profiles are considerably affected. Similar effects are observed when catalyst layer thicknesses are increased.     

Stability and performance of catalytic microreactors: Simulations of propane catalytic combustion on Pt

A pseudo-two-dimensional (2D) model is developed to analyze the operation of platinum-catalyzed microburners for lean propane-air combustion. Comparison with computational fluid dynamics (CFD) simulations reveals that the transverse heat and mass transfer is reasonably captured using constant values of Nusselt and Sherwood numbers in the pseudo-2D model. The model also reasonably captures the axial variations in temperatures observed experimentally in a microburner with a gap size. It is found that the transverse heat and mass transport strongly depend on the inlet flow rate and the thermal conductivity of the burner solid structure. The microburner is surface reaction limited at very low velocities and mass transfer limited at high velocities. At intermediate range of velocities (preferred range of reactor operation), mass transfer affects the microburner performance strongly at low wall conductivities, whereas transverse heat transfer affects stability under most conditions and has a greater influence at high wall conductivities. At sufficiently low flow rates, complete fuel conversion occurs and reactor size has a slight effect on operation (conversion and temperature). For fast flows, propane conversion strongly depends on residence time; for a reactor with gap size of , a residence time higher than 6 ms is required to prevent propane breakthrough. The effect of reactor size on stability depends on whether the residence time or flow rate is kept constant as the size varies. Comparisons to homogeneous burners are also presented.     

Trickle-bed CFD studies in the catalytic wet oxidation of phenolic acids

An Euler-Euler computational fluid model was developed successfully for the hydrodynamic prediction of a trickle-bed reactor (TBR) designed for advanced wastewater treatment facilities. Catalytic wet air oxidation of phenolic acids was simulated in a TBR by means of computational fluid dynamic (CFD) in the temperature range and pressures . The hydrodynamic model validation was accomplished through the comparison of simulated pressure drop and liquid holdup with experimental data from the literature. In a broad range of gas and liquid flows studied (G=0.10-0.70 and ) at different operation conditions, CFD demonstrated the considerable effect of operating pressure in pressure drop, whereas a minor influence was detected for the liquid holdup. CFD runs were then performed for the catalytic wet air oxidation of aqueous phenolic acids solution. The reactor behaviour was analysed by means of total organic carbon profiles which reflected the influence of temperature, pressure, gas-liquid flows and initial pollutant concentration.     

Effectiveness in catalytic washcoats with multi-step mechanisms for catalytic combustion of hydrogen

The effect of intra-phase diffusion for channel-flow oxidation reactors with washcoats ranging in thickness from 10 to is explored in combination with a detailed surface chemistry for low-temperature H₂ oxidation over supported Pd/PdO_x catalysts. A numerical model of a porous catalyst washcoat is developed to assess how local conditions influence catalyst effectiveness when considering a detailed multi-step surface mechanism, and this washcoat model is integrated into a channel flow reactor model to assess if and when effectiveness correlations may apply for channel flow reactors. The Pd-H₂-O₂ surface chemistry mechanism, which is validated against experimental measurements in an annular flow reactor, implements thermodynamically consistent interaction potentials of surface species and predicts non-linear behavior of conversion with respect to H₂ concentrations at the low equivalence ratios of the current study, particularly at lower temperatures where surface chemistry dominates overall reaction rates. The catalytic washcoat model further indicates that conversion and similarly catalyst effectiveness are strongly dependent upon the site fractions of vacancies available for H₂ adsorption, which vary strongly with flow conditions and at higher conversions with depth in the porous washcoat. This leads to difficulty in developing simple models for catalyst washcoat effectiveness based upon any parameter such as a Thiele modulus. Furthermore the results suggest that care should be taken in interpreting kinetic data for oxidation reactions even when relatively thin washcoats are employed for reaction rate studies.     

Light-off criterion and transient analysis of catalytic monoliths

A one-dimensional two-phase model is used to derive an analytical light-off criterion for a straight channeled catalytic monolith with washcoat, in which the flow is laminar. For the case of uniform catalyst loading and a first order reaction, the light-off criterion is given by

     

DEM-LES simulation of coal combustion in a bubbling fluidized bed Part II: coal combustion at the particle level

The discrete element method-large eddy simulation (DEM-LES) is used to model coal combustion at the particle level in a bubbling fluidized bed. The gas phase is modelled as a continuum and the solid phase is modeled by DEM. Chemical reactions consist in the heterogeneous reactions of char with O₂, CO, CO₂, NO, and N₂O, and in the homogeneous reactions involving CO, O₂, NO, and N₂O. The colliding particle-particle heat transfer is based on the analysis of the elastic deformation of the spheres during their contact. The model predicts the effects of the particle heterogeneous flow structure on the thermal characteristics of coal particles when heating and burning, and the gaseous emissions from a fluidized sand-coal binary mixture. The heating rates are 1627 and for, respectively, 0.8 and diameter coal particles fed into the fluidized bed. The instantaneous contribution of the collision heat transfer is weak, less than 5.0% of the total power exchanges (coal combustion, radiation, convection and collision) during the heating and 1.5% during the combustion. The temperature of the coal particles exceeds the bed temperature, which is in qualitative agreement with experimental data from literature. The effects of the diameter of coal particles, of the bed temperature, and of the inlet gas velocity on the thermal characteristics are also studied.     

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.     

Permeation behavior during the catalytic oxidation of CO in a Pt-loaded Y-type zeolite membrane

A Pt-loaded Y-type zeolite (Pt/NaY) membrane was prepared on the surface of a porous α-Al₂O₃ support tube by hydrothermal synthesis and then ion-exchanged with platinum. The thickness of the zeolite layer and the amount of Pt loaded were ca. and , respectively. The membrane was employed in the form of a cylindrical thin catalyst for the selective oxidation of CO in an H₂-rich mixture. A mixture of CO, O₂ and H₂ was fed to the outer surface of the membrane, and CO was selectively oxidized during its permeation through the thin layer. The permeation fluxes for H₂ and CO were determined at 423-. Permeation fluxes also calculated by means of a mathematical model using effective diffusion coefficients and reaction kinetics. The effective diffusion coefficients through the zeolite membrane were estimated from gas permeation test data, obtained at 423-, and the oxidation rates of CO were determined over a particulate catalyst that had the same composition as the Pt/NaY membrane. As a result, the diffusion coefficients of O₂, N₂ and CO were determined to be (0.7-1.0) at 423-, and the activation energies for the rate constants for CO oxidation were 61-. The predicted permeation fluxes of H₂ and CO using the mathematical model were in good agreement with the experimental data, when the oxidation selectivity of CO to H₂ was assumed to be 80% in the model calculation.     

Propyne hydrogenation in a continuous polymeric catalytic membrane reactor

Propyne hydrogenation was studied in a continuous polymeric catalytic membrane reactor (pCMR), both experimentally and theoretically. It was used a poly(dimethylsiloxane) (PDMS) composite membrane with an average thickness of , loaded with 5 wt% of 9 nm diameter Pd clusters. The reaction was conducted at 308 K and several feed compositions at a fixed flow rate were tested.The mathematical model proposed includes the mass balances to the retentate and permeate chambers and the mass balance and transport kinetics through the catalytic membrane. The pCMR model also considers a reaction rate equation composed of two terms: the propyne to propylene and the propylene to propane hydrogenations. The selectivity between these two reactions is described by the bicomponent adsorption of propyne and propylene obtained by the IAST model (thermodynamic selectivity). The proposed model represents quite well the experimental data regarding the flow rates and mixture compositions of the permeate and retentate streams.     

Transient microkinetic modelling of n-heptane catalytic cracking over H-USY zeolite

A microkinetic model for the catalytic cracking of n-heptane is proposed comprising a set of significant elementary steps which generate a complex reaction network. This approach constitutes a compromise between fundamental and lumped models since the reaction scheme is detailed to the carbon atom level by considering separately olefins, paraffins and adsorbed carbenium ions with the same carbon atom number. Elementary rate constants are estimated through expressions relating species reactivity with their carbon atom number. Fitting the model against experimental data over a large parameter space was performed using a micro-genetic algorithm with binary encoding and logarithmic distribution. The advantage of this approach is that it allows modelling the reaction network without supposition of rate determining steps. The model predicts very well the observed transient activity for n-heptane cracking over H-USY zeolite at and in general, it reasonably predicts the experimental trends for the products distribution.     

Kinetic modelling of VOC catalytic steam pyrolysis for tar abatement phenomena in gasification/pyrolysis technologies

Tar elimination and hot-gas conditioning in thermochemical conversion processes, i.e. thermal gasification, pyrolysis of heterogeneous materials involved two main classes of catalysts and/or additives: metallic and mineral oxides. This communication focused on the experimental kinetic data on catalytic steam cracking of vaporized toluene ( space-time ) as a tar-derived species and/or Volatile Organic Compound (VOC). Toluene (C₇H₈) has been chosen as a model formula for reactive tar-derived one-ring species determined from tar constituents. Gaseous product distribution data were obtained at atmospheric pressure steam pyrolysis temperature range of 923-1223 K and GHSV 1200-2300 Nm³ (m³ h)⁻¹. The overall catalytic pyrolysis of toluene over a commercial available metal based catalyst NiMo/γ-Al₂O₃ was compared to the pyrolysis in presence of basic non-metallic mineral additives, i.e. Norwegian (Norsk Hydro) dolomitic magnesium oxide [MgO], Swedish low surface quicklime [CaO], and calcined dolomite [CaMg(O)₂]. The operational conditions were applied without internal or external mass-transfer limitations. Kinetics for the pyrolysis could be described by first-order reactions for all the studied additives. The influence of hydrogen gas (30 vol%, ) and water vapor () in vaporized toluene cracking runs over low surface quicklime [CaO] was determined. A mechanistic model of the Langmuir-Hinshelwood type describing toluene decomposition was also developed.     

The development of high performance P84 co-polyimide hollow fibers for pervaporation dehydration of isopropanol

Probably because of material and fabrication limitations, most previously developed hollow fibers were lack of high performance for pervaporation dehydration applications. In this paper, we have successfully developed integrally skinned BTDA-TDI/MDI (P84) co-polyimide hollow fibers for pervaporation dehydration of isopropanol (IPA), which have impressive flux and selectivity towards water. The effects of spinning conditions such as air gap distance, coagulation temperature, and dope/bore fluid flow rates on membrane formation, morphology and pervaporation performance have been determined. Even though spinning conditions affect membrane separation performance, it is found that silicone rubber coating and heat treatment play much more important roles on performance enhancement. Not only can the silicone rubber coating effectively seal the membrane defects, but also triple the selectivity because of its nature of high water permeability while its hydrophobic property does not dominate. A tremendous increase in separation factor/selectivity (20-100 times) is observed after a heat treatment at . The newly developed P84 hollow fibers after heat treatment have a flux of and a separation factor of 10 585 for dehydration of 85/15 (w/w) IPA/water mixture at .     

开缝翅片换热器三维流动传热特性数值研究

建立了开缝翅片换热器三维流动传热物理模型和数学模型,针对不同开缝位置的换热单元进行数值计算。并应用场协同理论分析了不同入口速度对换热特性的影响。研究表明：对翅片进行开缝能够有效提高翅片管的换热性能、强化传热,且下游开缝比上游开缝换热效果好,全部开缝换热效果最好。随着入口速度的增大4种翅片换热量明显增强,增加入口流速也可提高换热效率,但压力损失较大。计算结果可为工程实际应用提供一定的理论指导。     

Liquid phase catalytic hydrodechlorination of 2,4-dichlorophenol over carbon supported palladium: an evaluation of transport limitations

The catalytic hydrodechlorination (HDC) of aqueous 2,4-dichlorophenol (2,4-DCP) solutions over Pd/C catalysts (1-10% w/w Pd) has been investigated at 303 K in a stirred slurry reactor. The experimental results have shown that 2,4-DCP is converted to phenol quantitatively and 2-chlorophenol (2-CP) is the only intermediate product within detect limitations . The system is 100% selective in terms of dechlorination and phenol hydrogenation only proceeds once complete dechlorination has been attained. The reaction pathway is illustrated and HDC progress is related to pH changes in solution. The mass-transfer limitations have been evaluated experimentally using the diagnostic criteria associated with varying hydrogen flow rate, stirring speed, catalyst concentration and particle size. Experimental results combined with parameter estimation have revealed the influence of mass transfer at the liquid/solid interface and intraparticle diffusion in limiting HDC rate. These effects can be minimized for the less active 1% w/w Pd/C catalysts where the stirring speed , hydrogen flow , catalyst concentration and particle sizes . The selectivity trends associated with 1% w/w Pd/C were the same whether the system operated under physical transport or chemical control. The selectivity with respect to 2-CP was however limited by mass-transfer processes in the HDC reaction using higher Pd loadings.     

Electrochemically mediated oxidation of glutathione and N-acetylcysteine with 4,4′-biphenol

The electrochemical oxidation of 4,4′-biphenol was studied at a glassy carbon electrode in the presence of glutathione and N-acetylcysteine, using cyclic voltammetry as a diagnostic technique. The results indicate a complicated catalytic system. The catalytic current depends on the concentration of glutathione (or N-acetylcysteine) and solution's pH. In this work a unique kinetic zone diagram for catalytic behavior of 4,4′-biphenol in the presence of glutathione is constructed using the variables “excess factor” and “kinetic parameter” (λ=pH/v). This paper also describes the elucidation of second order homogeneous catalytic reaction (EC′) of 4,4′-biphenol with glutathione and N-acetylcysteine. In addition, the observed homogeneous rate constant of catalytic reaction of electrochemically generated 4,4′-diphenoquinone with glutathione (or N-acetylcysteine) was investigated using digital simulation method.     

Discrete element simulation of a flat-bottomed spouted bed in the 3-D cylindrical coordinate system

A spouted bed is simulated in three dimensions by a discrete element method (DEM) in a cylindrical coordinate system. The numerical scheme is based on a second order finite difference method in space and a second order Adams-Bashforth method for time advancement. Gas-particle interaction is assumed to obey the Ergun equation (for void fraction less than 0.8) and its corrected model by Wen and Yu (for void fraction greater than 0.8). The spouted bed vessel is a flat-bottomed cylinder in height and in diameter. The gas inlet diameter is . Three hundred thousand monosized spheres of diameter are used in the simulation. The typical characteristics of spouted beds, such as spout, annulus and fountain, are reproduced. Particle velocity profiles show good agreement with experimental results and self-similarity of the radial distribution of axial particle velocities is reported. Gas flow patterns are also studied and the effect a vortex ring fixed at the bottom of the vessel is investigated. The simulation is validated through comparisons with results reported in the literature.     

板翅式换热器平直翅片表面流动及传热特性

为了提高板翅式换热器的换热性能,采用CFD数值模拟方法,研究了翅片结构参数和入口Re数对板翅式换热器平直翅片的表面传热与流动阻力特性的影响。研究结果表明:当流体被加热时,翅片通道内部靠近固体壁面的流体温度较高,通道中心主流体区温度较低。流体在翅片通道内的温度分布呈一定梯度,靠近一次表面的流体温度梯度较大,而靠近二次表面的流体温度梯度较小。随着翅片高度和翅片间距的增加,平直翅片的表面传热因子和摩擦因子增大。而且,增加翅片的厚度,可在一定程度提高其换热性能,但翅片厚度存在一个最优值。研究结果可为板翅式换热器的优化设计提供理论指导。