Effect of supports on the redox performance of pyrite cinder in chemical looping combustion

Chemical looping combustion (CLC) is a clean and efficient flame-free combustion technology,which combust the fuels by lattice oxygen from a solid oxygen carrier with inherent CO2 capture.The develop-ment of oxygen carriers with low cost and high redox performance is crucial to the whole efficiency of CLC process.As the solid by-product from the sulfuric acid production,pyrite cinder presented excellent redox performance as an oxygen carrier in CLC process.The main components in pyrite cinder are Fe2O3,CaSO4,Al2O3 and SiO2 in which Fe2O3 is the active component to provide lattice oxygen.In order to sys-tematic investigate the functions of supports (CaSO4,Al2O3 and SiO2) in pyrite cinder,three oxygen car-riers (Fe2O3-CaSO4,Fe2O3-Al2O3 and Fe2O3-SiO2) were prepared and evaluated in this study.The results showed that Fe2O3-CaSO4 displayed high redox activity and cycling stability in the multiple redox cycles.However,both Fe2O3-Al2O3 and Fe2O3-SiO2 experienced serious deactivation during redox reactions.It indicated that the inert Fe-Si solid solution (Fe2SiO4) was formed in the spent Fe2O3-SiO2 sample,which decreased the oxygen carrying capacity of this sample.The XPS results showed that the oxygen species on the surface of Fe2O3-CaSO4 could be fully recovered after the 20 redox cycles.It can be concluded that CaSO4 is the key to the high redox activity and cycling stability of pyrite cinder.     

Study of the copper leaching in the wet oxidation of phenol with CuO-based catalysts: Causes and effects

Catalytic wet oxidation of phenol as a model pollutant has been performed in a three phase fixed-bed reactor (FBR) by using a commercial catalyst based on copper oxide in order to analyze the variables affecting significantly the copper leaching. It has been found that temperature has an almost negligible influence in the range studied (70–160 °C). On the contrary, an important effect of the pH value was noticed. The copper leaching reduces when the pH of the solution fed to the reactor increases, being almost negligible at pH ≥ 5. Moreover, the composition of the reaction media also influences the leaching. Higher copper concentrations than those expected by the effect of the acid aqueous media have been measured in the reactor effluent when phenol, catechol, hydroquinone, p-benzoquinone and maleic acid are present in the reaction media. On the contrary, oxalic acid has a negative influence on the leaching, since it captures the copper in solution to form copper oxalate which precipitates on the catalyst surface. For a previously acidified medium, the acetic and formic acids do not have any other effect on the copper leaching. It has been also demonstrated that as copper in solution decreases, so does phenol conversion, because the homogeneous catalysis contributes significantly to the oxidation reactions even in fixed-bed reactors.     

固定床反应器中扩散反应问题的实验测定

为研究反应器尺度上的扩散反应问题，建立了一个可同时在轴向和径向进行温度和浓度测定的二维壁冷却式固定床反应器。选择３种操作条件进行了实验测定，利用所测实验数据对拟均相二维平推流模型进行了参数估计，发现按平推流模型得到的径向温度曲线在壁面附近同实验测定有很大偏差，本文认为是由于在颗粒大小不容忽视的条件下平推流假定过于简单之故。应当加以改进。     

The effect of non-uniform temperature on the sorption-enhanced steam methane reforming in a tubular fixed-bed reactor

The effect of non-uniform temperature on the sorption-enhanced steam methane reforming (SE-SMR) in a tubular fixed-bed reactor with a constant wall temperature of 600 °C is investigated numerically by an experimentally verified unsteady two-dimensional model. The reactor uses Ni/Al₂O₃ as the reforming catalyst and CaO as the sorbent. The reaction of SMR is enhanced by removing the CO₂ through the reaction of CaO + CO₂ → CaCO₃ based on the Le Chatelier's principle. A non-uniform temperature distribution instead of a uniform temperature in the reactor appears due to the rapid endothermic reaction of SMR followed by an exothermic reaction of CO₂ sorption. For a small weight hourly space velocity (WHSV) of 0.67 h^?1 before the CO₂ breakthrough, both a low and a high temperature regions exist simultaneously in the catalyst/sorbent bed, and their sizes are enlarged and the temperature distribution is more non-uniform for a larger tube diameter (D). Both the CH₄ conversion and the H₂ molar fraction are slightly increased with the increase of D. Based on the parameters adopted in this work, the CH₄ conversion, the H₂ and CO molar fractions at D = 60 mm are 84.6%, 94.4%, and 0.63%, respectively. After CO₂ breakthrough, the reaction of SMR dominates, and the reactor performance is remarkably reduced due to low reactor temperature.For a higher value of WHSV (4.03 h^?1) before CO₂ breakthrough, both the reaction times for SMR and CO₂ sorption become much shorter. The size of low temperature region becomes larger, and the high temperature region inside the catalyst/sorbent bed doesn't exist for D ≥ 30 mm. The maximum temperature difference inside the catalyst/sorbent bed is greater than 67 °C. Both the CH₄ conversion and H₂ molar fraction are slightly decreased with the increase of D. However, this phenomenon is qualitatively opposite to that for small WHSV of 0.67 h^?1. The CH₄ conversion and H₂ molar fraction at D = 60 mm are 52.6% and 78.7%, respectively, which are much lower than those for WHSV = 0.67 h^?1.     

Novel quasi-autothermal hydrogen production process in a fixed-bed using a chemical looping approach: A numerical study

This paper presents a novel quasi-autothermal hydrogen production process. The proposed layout couples a Chemical Looping Combustion (CLC) section and a Steam Methane Reforming (SMR) one. In CLC section, four packed-beds are operated using Ni as oxygen carrier and CH₄ as fuel to continuously produce a hot gaseous mixture of H₂O and CO₂. In SMR section, two fixed-beds filled with Ni-based catalyst convert CH₄ and H₂O into a H₂-rich syngas. Four heat exchangers were employed to recover residual heat content of all the exhaust gas currents. By means of a previously developed 1D numerical model, a dynamic simulation study was carried out to evaluate feasibility of the proposed system in terms of methane conversion (100% circa), hydrogen yield (about 0.65 mol_H2/mol_CH4) and selectivity (about 70%), and syngas ratio (about 2.3 mol_H2/mol_CO). Energetic and environmental analyses of the system performed with respect to conventional steam methane reforming, highlights an energy saving of about 98% and avoided CO₂ emission of about 99%.     

Pattern formation in models of fixed-bed reactors

This work reviews and compares spatiotemporal patterns in three models of adiabatic fixed catalytic beds for reactions with oscillatory kinetics: homogeneous and heterogeneous models, which are studied using generic first-order kinetics, and a detailed model of CO oxidation in the monolithic reactor. These three models describe reactors with one, two or all three phases (fluid-, solid- and adsorbed-phases), respectively. Pattern selection is based on the oscillatory or bistable nature of the kinetics and on the nature of fronts. The heterogenous and detailed models may exhibit local bistability while the homogeneous model does not admit this property: a simple conversion between the parameters of the homogeneous and heterogeneous models is suggested.

The spatiotemporal patterns in the reactor can be predicted from the sequence of phase planes spanned by the reactor. Stationary or oscillatory front solutions, oscillatory states that sweep the whole surface or excitation fronts may be realized in the homogeneous and heterogeneous models. The detailed model of the converter may exhibit oscillatory motion, which may be periodic or chaotic, in which typically a hot domain enters the reactor exit and moves quickly upstream; the following extinction occurs almost simultaneously due to strong coupling by convection.     

固定床反应器拟均相轴向扩散模型通用软件的开发

本文对求解化学反应器轴向扩散模型的四种数值方法—迭代法、打靶法、有限差分法和正交配置法进行了比较,对迭代法做了改进。利用改进的迭代法开发了求解化学反应器轴向扩散模型的通用程序。     

Catalytic hydrogenation of o-nitroanisole in a microreactor: Reactor performance and kinetic studies

Hydrogenation of o-nitroanisole to o-anisidine was conducted in a packed-bed microreactor as a model hydrogenation reaction of importance to the pharmaceutical and fine chemicals industries with the aim of investigating the reactor performance and kinetics of the reaction. The effects of different processing conditions viz. hydrogen pressure, o-nitroanisole concentration, temperature, and residence time on the conversion of o-nitroanisole, space-time yield (STY), and selectivity of o-anisidine were studied using 2% Pd/zeolite catalyst. The kinetic study was undertaken in a differential reactor mode keeping the conversion of o-nitroanisole at less than 10%. During the kinetic study, it was observed that the intermediate 2-methoxynitrosobenzene was present in the reactor at low catalyst loading and low conversions because of short residence time in the reactor. Therefore, for the kinetics study, the overall reaction was treated as comprising two separate reactions: first the reduction of o-nitroanisole to 2-methoxynitrosobenzene and then, the reduction of 2-methoxynitrosobenzene to o-anisidine. Internal and external mass and heat transfer limitations in the microreactor were examined. Different rate laws using different mechanisms from the literature were considered to fit the experimental data. Two rate equations for the two consecutive reactions assuming Langmuir-Hinshelwood mechanism provided the best fit to the experimental data. These two rate equations predicted the experimental rates within 10% error. Experiments were also carried out in an integral reactor, and the reactor performance data were found to be in agreement with the predictions of the theoretical models.     

Effect of the reaction bath temperature in a fixed-bed reactor for oxidation of oxylene over V2O5/TiO2 catalysts

The effects of the reaction variables in the operation of a fixed-bed reactor for oxidation ofo-xylene over V₂O₅/TiO₂ catalysts were studied experimentally using a bench reactor. Reaction temperature, feed flow rate and feed concentration ofo-xylene were found to have significant effects on the product distribution and the temperature profile in the reactor. Drastic enhancements ofo-xylene oxidation reaction were observed at some conditions, which was ascribed to the effect of heat accumulated in the bed and indicated a possible way to increase the productivity in the industrial condition. This paper was presented at the 8th APCChE (Asia Pacific Confederation of Chemical Engineering) Congress held at Seoul between August 16 and 19, 1999.     

H2S retention with Ca-based sorbents in a pressurized fixed-bed reactor: application to moving-bed design

The H₂S retention with Ca-based sorbents in a pressurized fixed-bed reactor (1 MPa) has been analyzed, obtaining the breakthrough curves with a dolomite and two different limestones, different particle size (+0.8-1.0, +1.25-1.6, and +1.6-2.0 mm), and both at calcining (1173 K) and non-calcining conditions (1123 K). The effect of the stoichiometric time in the breakthrough curves has been analyzed varying the bed length, the gas velocity and the sorbent fraction in the bed. From these results, the conversion and H₂S concentration profiles in the transition zone and the length of unused bed (LUB) have been determined. H₂S retention in fixed-bed until concentration close to the given by the thermodynamic equilibrium was obtained using dolomite or limestone at calcining conditions, and dolomite at non-calcining conditions. The results of H₂S retention in a fixed-bed reactor has been applied to the calculus of the minimum height of a countercurrent moving-bed reactor to obtain the maximum H₂S retention with the minimum amount of sorbent. A mathematical model was developed to predict the experimental results obtained in the fixed-bed reactor, which was also valid for the design of countercurrent moving-bed reactors for gas desulphurization.