Experimental study of mass transfer limitations in Fe- and Cu-zeolite-based NH3-SCR monolithic catalysts

An experimental study of steady-state selective catalytic reduction (SCR) of NO_x with NH₃ on both Fe-ZSM-5 and Cu-ZSM-5 monolithic catalysts was carried out to investigate the extent of mass transfer limitations in various SCR reactions. Catalysts with different washcoat loadings, washcoat thicknesses and lengths were synthesized for this purpose. SCR system reactions examined included NO oxidation, NH₃ oxidation, standard SCR, fast SCR and NO₂ SCR. Comparisons of conversions obtained on catalysts with the same washcoat volumes but different washcoat thicknesses indicated the presence of washcoat diffusion limitations. NH₃ oxidation, an important side reaction in SCR system, showed the presence of washcoat diffusion limitations starting at 350 °C on Fe-zeolite and 300 °C on Cu-zeolite catalysts. Washcoat diffusion limitations were observed for the standard SCR reaction (NH₃+NO+O₂) on both Fe-zeolite (≥350 °C) and Cu-zeolite (≥250 °C). For the fast (NH₃+NO+NO₂) and NO₂ SCR (NH₃+NO₂) reactions, diffusion limitations were observed throughout the temperature range explored (200–550 °C). The experimental findings are corroborated by theoretical analyses. Even though the experimentally observed differences in conversions clearly indicate the presence of washcoat diffusion limitations, the contribution of external mass transfer was also found to be important under certain conditions. The transition temperatures for shifts in controlling regimes from kinetic to washcoat diffusion to external mass transfer are determined using simplified kinetics. The findings indicate the necessity of inclusion of mass transfer limitations in SCR modeling, catalyst design and optimization.     

气液传质理论研究进展

文内对气液传质的理论研究进行了评述，并预测了未来的发展方向     

气体折流塔板流体力学和传质实验研究

对气体折流塔板的流体力学和传质进行了实验,找出了气液流动对塔板压降和传质的影响规律,通过气体折流塔板与传统筛板在流体力学与传质等方面的比较,找到了气体折流塔板的适用范围及相对于传统筛板的优点,为以后此塔板的应用提供了有效数据。     

Honeycomb supports with high thermal conductivity for gas/solid chemical processes

The scope of this review article is to address the use of novel monolithic catalysts with high thermal conductivity in externally cooled tubular reactors for gas/solid exothermic chemical processes in place of conventional packed beds of catalyst pellets.

After discussing the analysis and the implications of heat conduction in honeycomb monolith structures, we review herein simulation studies and experimental investigations showing that near-isothermal reactor operation can be achieved even under very high thermal loads by adopting specific materials and designs of the honeycomb supports associated with high effective radial thermal conductivities. For such monoliths, the limiting thermal resistance is located at the interface between the monolith and the inner tube wall (“gap resistance”). Recent measurements of the “gap” heat transfer coefficient point to very large values (>400 W/(m² K)), which are controlled both by the tube–monolith clearance at the actual operating conditions and by the thermal conductivity of the process gas.     

Jet pulsed filters as chemical gas/solid reactors—Experiment and model prediction

A recently introduced generations filter model [A. Kavouras, G. Krammer, Distributions of age, thickness and gas velocity in the cake of jet pulsed filters—application and validation of a generations filter model, Chem. Eng. Sci. 58 (2003) 223–238] allows one to determine from macroscopic experimental data the distribution of cake thickness versus filter area and in consequence the complete filter behaviour. Based on a simplified assumption this generations filter model is employed in a predictive way to estimate operating points of imperfectly cleaned filters. However, filter behaviour in terms of the fraction of the filter area cleaned when jet pulsed cannot be predicted yet without experimental data. This is due to the variable cleaning properties of the cake, which are dependent on the filter operating parameters.

Combining the predictive filter model and a model for the chemical reactions in the fixed bed of the filter cake [A. Kavouras, B. Breitschaedel, G. Krammer, A. Garea, J.A. Marques, A. Irabien, SO₂ removal in the filter cake of a jet pulsed filter: a combined filter and fixed bed reaction model, Ind. Eng. Chem. Res. 41 (2002) 5459–5469], the filter performance as a gas/solid reactor can also be simulated in a predictive way. It is found that the pressure drop across the filter has a weak influence on filter performance as a gas/solid reactor.     

用于机动车排气净化的含Pd催化剂初步研究

本文制备出一种含Pd催化剂,用于治理机动车排气中的CO、HC和NOx的排放,初步研究了其催化效果,包括空燃比特性和温度特性。实验结果表明,该催化剂具有较好的催化能力。     

Experimental and theoretical (ReaxFF) study of manganese-based catalysts for low-temperature toluene oxidation

This study presents detailed experimental and theoretical investigation of manganese-based metal oxides, MnMO_x (M: Fe, Ni, Cu) as potential catalysts for the low-temperature toluene oxidation. The first part of the paper deals with the detailed characterization of the prepared catalysts and testing of their catalytic activity and stability in the fixed-bed reactor. The MnFeO_x exhibited superior and stable catalytic activity for toluene oxidation (T₉₀ = 419–446 K), comparable with the activity of the commercial Pt–Al₂O₃ catalyst (T₉₀ = 393–423 K). Among the studied catalysts the following order of catalytic activity was determined: MnFeO_x > MnNiO_x ≈ MnCuO_x > MnO_x. The one-dimensional (1D) pseudo-homogeneous model was applied to describe behavior of the fixed bed reactor for the low temperature toluene oxidation over prepared MnFeO_x catalysts. The second part of the paper is focused on theoretical investigation of toluene interaction on the surface of the single metal oxides (Mn₂O₃, MnO₂, Fe₂O₃, NiO and CuO) in the oxygen atmosphere using the ReaxFF method, since they were individual dominant phases in the prepared catalysts. A good correlation between the predicted binding energy of toluene adsorption on the surface of studied metal oxide phases and experimentally determined catalytic activities was observed.     

A theoretical study of mass transfer with chemical reaction in drops

A model has been developed for the preparation of mass transfer by forced convection inside drops for mass transfer with and without simultaneous first-order chemical reaction. Solutions of the forced convection equation have been obtained using finite difference methods for Reynolds number and viscosity ratio ranges of 0 ≤ Re ≤ 90 and 0 ≤ μ₁/μ_o ≤ ∞, respectively. The effect of circulation rate inside the drop on the mass transfer rate has also been studied. The model is believed to be generally applicable as evidenced by a comparison with existing analytical models. Thus the results for physical mass transfer approached the Kronig and Brink solution when the viscosity ratio tended towards zero in the Stokes' flow regime. However, as the viscosity ratio increased, the results approached the Newman solution. The model also agreed with those of Johns and Beckmann for physical mass transfer. Similarly, the results for mass transfer with first-order chemical reaction agreed with Dankwerts' generalization of the Kronig and Brink and the Newman models.     

Preparation and characterization of Ir-based catalysts on metallic supports for high-temperature steam reforming of methanol

Ir-based catalysts on heat-resisting foil supports with different washcoats were investigated for hydrogen production by high-temperature steam reforming of methanol. Al₂O₃, Ce_0.8Zr_0.2O₂–Al₂O₃, Ce_0.8Zr_0.2O₂/Al₂O₃ and Ce_0.8Zr_0.2O₂ coatings were prepared on the metallic supports and iridium was deposited on them as the active component. The samples were characterized by X-ray powder diffraction (XRD), ultrasonic vibration test, scanning electron microscope (SEM) and temperature-programmed reduction (TPR). The performance of the catalysts for steam reforming of methanol was evaluated with a fixed-bed reactor. It was found that the phase structure, the shape of the surface particles and the coating adherence were different from each other for the four kinds of coatings. The activities, selectivities and stabilities of these Ir-based catalysts on metallic supports were compared to select the optimal one for use in high-temperature methanol steam reforming. The results indicated that the Ir/Ce_0.8Zr_0.2O₂/Al₂O₃/FeCrAl catalyst showed better performance than the other catalysts, which is a promising candidate for hydrogen production via the methanol steam reforming process in Pd membrane reactors.     

负载固相的铑基催化剂应用于烯烃氢甲酰化反应的研究进展

负载固相的催化剂因其简便的分离循环操作以及可观的催化性能而广受关注,但存在反应活性较差、金属流失量较大、催化剂制备成本较高等问题。本文首先从不同负载材料的角度综述了近年来该类催化剂最新的研究进展,主要探讨了载体的表面性质、催化剂的制备方法、膦配体等对催化性能的影响;最后介绍了新型的单原子催化剂所取得的突破性进展。分析表明：具有"类均相"特点的多孔有机聚合物的催化活性很好,而超支化聚合物功能化的磁性纳米催化剂的稳定性更佳。另外还对负载型铑催化剂未来的研究方向进行了展望：需要进一步加深对多孔有机聚合物的化学结构的理解,以便对其更好地表征;借助一些先进的表征技术如高角环状暗场扫描透射电镜和密度泛函理论的计算来深入研究载体结构对单原子催化剂的催化性能的影响。     

Ceramic catalyst supports and catalysts for chemical processes and scrubbing of gas discharges

Translated from Steklo i Keramika, No. 1, pp. 3–6, January, 1992.     

A comparative study of gas phase esterification on solid acid catalysts

For the first time, a comprehensive comparison of the intrinsic activities of solid acid catalysts in terms of turnover frequency (TOF) is reported for the gas-phase esterification of acetic acid with methanol. The catalysts studied included a zeolite (Hβ), two modified zirconias (sulfated zirconia, SZ; and tungstated zirconia, WZ), and an acidic resin-silica composite (Nafion/silica, SAC-13). Activities on a per weight basis decreased in the following order: Hβ ～ SAC-13 ? SZ >  WZ at 130 °C. However, on a rate-per-site basis (TOF), all catalysts showed comparable activities. The TOF results suggest that the acid sites of these catalysts have similar capacity for effectively catalyzing esterification. All catalysts deactivated to a quasi-steady-state rate with TOS. Regeneration experiments suggested that catalyst deactivation was due mainly to site blockage by carbonaceous deposits. Selective poisoning experiments showed that the reaction predominately took place on Brønsted acid sites.     

Heat and mass transfer limitations in pre-turbocharger catalysts

The use of the Diesel engine is increasing, while at the same time emission limits are becoming more stringent. To reduce emissions, novel catalyst converter designs have been proposed, including the placement of a small converter before the turbocharger. The role of this catalyst is to provide some reduction of the pollutant level prior to the main catalyst. This catalyst is typified by high flow rates. In this paper the performance of small catalytic converters operated at high flowrate is examined using computer simulation. The oxidation of CO is used as a model reaction. It is shown that for a typical oxidation catalyst, conversions of the order of 30% can be achieved at typical temperatures and a GHSV of 5 million h⁻¹.     

Experimental study of mass transfer in a dense bubble swarm

We consider the liquid-side mass transfer coefficient k_L in a dense bubble swarm for a wide range of gas volume fraction (0.45%≤α_G≤16.5%). The study is performed for an air–water system in a square column. Bubble size, shape and velocity have been measured for different gas flow rates by means of a high speed camera. Gas volume fraction and bubble velocity have also been measured by a dual-tip optical probe. Both of these measurements show that the bubble vertical velocity decreases when increasing α_G in agreement with previous investigations. The mass transfer is measured from the time evolution of the dissolved oxygen concentration, which is obtained by the gassing-out method. The mass transfer coefficient is found to be very close to that of a single bubble provided the bubble Reynolds number is based on the average equivalent diameter 〈d_eq〉 and the vertical slip velocity 〈V_z〉.     

上流式反应器气液相间传质特性的实验研究

上流式反应器设置在固定床渣油加氢反应器前有利于提高渣油原料适用性,延长装置运行时间。实验研究了上流式反应器气液相间传质,采用五齿柱形氧化铝催化剂模拟工业催化剂颗粒,水溶液模拟渣油,空气模拟氢气,采用无氧水物理吸收和亚硫酸钠化学吸收的方法,测定了在高气液比的条件下上流式反应器床层气液相间传质特性实验。考察了表观气速、表观液速、填料粒径、内构件、催化剂级配和床层高径比对液相体积传质系数和气液相界比表面积的影响规律。实验数据表明,液相体积传质系数随着气、液速的增大而增大;随填料颗粒增大而减小;在床层内安装合适的内构件或增大反应器高径比,能够促进气液相间传质。基于实验数据拟合了适合上流式反应器液相体积传质系数和气液相界比表面积的经验关联式,拟合误差最大分别为12%和24%;表明所建气液相间传质的经验关联式能更好地预测上流式反应器中的气液相间传质特性。     

Heat and mass transfer in honeycomb catalysts—II

Heat and mass gas-to-solid coefficients associated with the vaporisation of water and some hydrocarbons from the surface of a porous, monolithic structure were experimentally established. The mass transfer results are correlated using Reynolds and Schmidt number and the length-to-diameter ratio. The data for heat transfer are correlated on the Nu-Re(d/L) plot. The experimental results can be used for design of afterburner reactors utilizing monolithic structures.     

A study of local liquid/solid mass transfer in packed beds under trickling and induced pulsing flow

Induced pulsing flow (by cyclic liquid feeding) in packed beds, operated in the trickling flow regime, is studied as a method of overall improvement of catalytic reactor operation. In this paper results are reported of experiments aimed at determining local and global liquid/solid mass transfer rates, mainly for the so-called fast mode of ON-OFF periodic liquid feeding, with frequencies of order 0.1 Hz. Such mass transfer data for the fast mode of induced pulsing are not available in the literature. Uniform 6 mm glass spheres and alumina cylindrical extrudates, of 1.5 mm diameter and a narrow distribution of lengths, are employed in the tests. For completeness, results are also reported for single-phase (liquid) and trickling flow through the same packed beds. A well-known electrochemical technique is employed to measure instantaneous local mass transfer coefficients by means of quite a few probes distributed throughout the bed. The hydrodynamic characteristics under the above conditions, reported in companion papers, are helpful in interpreting the new mass transfer data.There is a wide spread of the time-averaged local mass transfer rates, in all cases tested, apparently due to packing and flow non-uniformities. This spread is much smaller in the case of packed uniform spheres. In general, the benefits of cyclic liquid feeding are more evident in the packed bed of spheres than in that of cylindrical extrudates; for instance, with increasing mean liquid rate, induced pulsing tends to reduce the spread of local mass transfer coefficients, which suggests that more uniform fluids distribution is promoted. The imposed liquid pulses are reflected in the observed periodic variation of local mass transfer coefficients; the latter appear to decay along the bed in the same manner as the liquid pulses. Other trends of local mass transfer rates are identified and discussed in relation to measured variation of liquid holdup, under the same conditions. For packed spheres, the measured global mass transfer rates are in fair agreement with literature correlations obtained for the trickling flow regime, unlike the case of packed extrudates where significant deviation is observed.     

Hexagonal mesoporous silicas with and without Zr as supports for HDS catalysts

Hexagonal mesoporous materials (HMS) with and without zirconium were used as supports for preparation of HDS catalysts. The catalysts prepared by modification of the supports with 12-phosphomolybdic acid (HPMo) were characterised by nitrogen adsorption, IR spectroscopy, TPD of ammonia, TPR and catalytic activity in thiophene hydrodesulphurisation. Parent silicas showed mesoporous structure with BET surface area between 1200 and 800 m² g⁻¹, pore size diameter around 3.3 nm and acidity around 0.3 mmol NH₃ g⁻¹. Molybdenum catalysts possess stronger acidity than the supports used. The catalytic activities in hydrodesulphurisation of thiophene of the molybdenum containing catalysts prepared with HMS were higher than the activity of the catalyst prepared with amorphous silica. Higher acidity of Zr–HMS supports lead to lower stability in the thiophene conversion.     

干熄炉焦炭床层换热的实验和理论研究

干法熄焦工艺的技术核心是干熄炉内焦炭与循环气体的热交换.本文根据干熄炉内焦炭床层换热的特点,建立了固定床干熄炉传热模拟实验装置,针对焦炭粒度、冷却气体流量等关键参数进行了实验研究.为了有效地处理实验数据,重新定义、推导了平均换热系数的计算公式,得到了干熄炉冷却段平均换热系数及其相关准则数关系;并利用导热反问题原理得到了焦炭床层的局部换热系数.研究结果表明,局部换热系数和平均换热系数的变化规律相似,冷却气体流量的增加有利于提高换热系数;换热系数对焦炭的粒度较敏感,焦炭粒度变小时,换热系数增加.     

Liquid/solid mass transfer in an air-lift loop reactor with a dispersed solid phase

In processes with immobilized cells mass transfer across the boundary layer surrounding the support often plays an important role. Relatively little is known about external mass transfer as a function of the superficial gas velocity in bioreactors such as air-lift loop reactors. In this work ion-exchange resins were used as a solid phase to determine the mass-transfer coefficient in such a reactor. Relations between the Sherwood number and the superficial gas velocity were derived and compared with relations from the literature. Relations in which the Sherwood number is a function of the energy-dissipation rate and relations in which the relative particle velocity is calculated from the rate of free fall of the particle were compared. It was shown that the Sherwood numbers that were functions of the energy-dissipation rates were higher than could be calculated on the basis of the rate of free fall. The Sherwood number obtained was used to calculate the k_l,s of carrageenan gel beads as a solid phase in an air-lift loop reactor. © 1998 SCI.