氧燃烧方式下石灰石直接固硫反应模拟

氧燃烧方式下高浓度CO2气氛使得石灰石与SO2的气固反应存在直接固硫和间接固硫两种方式。在热重分析仪上进行了石灰石直接硫化的实验,考察了温度、SO2浓度对直接固硫反应的影响。基于球形颗粒气体扩散理论,在未反应收缩核模型的基础上推导出一种从实验数据计算化学反应速率常数和SO2反应级数的新方法。同时在已有研究的基础上改进了产物层扩散系数的计算方法,并采用未反应收缩核模型对不同温度、SO2浓度条件下石灰石直接固硫反应进行模拟,模拟结果与实验结果较为吻合。在所建立模型的基础上定性讨论了温度、孔隙率、平均孔径对产物层有效扩散系数的影响,发现温度对有效扩散系数影响很显著,而孔隙率、平均孔径的影响较小。     

The role of different nitrogen functional groups on the removal of SO2 from flue gases by N-doped activated carbon powders and fibres

SO₂ removal from flue gases by carbonaceous materials is determined by their behaviour as catalysts for SO₂ oxidation into SO₃ or H₂SO₄ in the presence of O₂ or O₂ and steam, respectively. Previous studies have demonstrated that nitrogen (N) functional groups are active sites for the adsorption and oxidation of SO₂, although the nature of the N groups with the higher activity had not been established yet. For this reason, in the present work a number of activated carbons (AC) and activated carbon fibres (ACF) doped with N atoms have been prepared using different methods. The number and nature of these N groups have been assessed by XPS. The materials prepared have a wide range of nitrogen content, which is distributed into different chemical species. In this way, we were able to determine the effect of the N content and the role of the different N-containing functional groups on the catalytic activity for SO₂ oxidation. The results confirm that, although the pore volume and the pore size distribution strongly influence the catalytic activity, the presence of N species at the surface increases the catalytic activity. They also demonstrate that, among the different N functional groups, pyridinic nitrogen is the most active for this reaction.     

Carbon materials as catalyst supports for SO2 oxidation: catalytic activity of CuO-AC

Copper catalysts supported on acid treated activated carbon (AC) were prepared, characterized and tested in terms of their SO₂ oxidation activity. Reactions of CuO-AC in flow systems with sulfur dioxide, oxygen and nitrogen streams were investigated to determine the types of chemical interactions that occur on the sorbent surface. The effects of reaction temperature, acid treatment, metal loading, support particle size, SO₂ concentration and O₂ concentration on SO₂ oxidation activity were evaluated. It was found that carbon materials used as catalyst supports for copper oxide catalysts provided a high catalytic activity for adsorbing SO₂ from flue gas and oxidizing it. Acid pretreatment of the carbon supports increased the content of specific surface chemical groups to enhance the catalytic activity for SO₂ oxidation. Metal loading, as well as support particle size, have a significant influence on the SO₂ activity. The supported metals rather than surface oxygen functional groups on AC may be the active sites for adsorbing SO₂.     

Optimizing catalyst pore network structure in the presence of deactivation by coking

Designing the pore network structure is an effective approach to improve the performance of industrial catalyst particles, although it receives less attention than designing catalytic surfaces or active sites. This work presents a first example of the optimization of catalyst pore network structures in the presence of deactivation by coke formation, using a three-dimensional pore network model. Propane dehydrogenation in a Pt-Sn/Al₂O₃ catalyst particle is taken as the model reaction system. Catalyst particles with unimodal and bimodal pore-size distributions are investigated, both being commonly used in industry. The porosity, connectivity, pore size, and their spatial distributions are optimized under two separate assumptions: constant intrinsic activity per unit catalyst weight and constant intrinsic activity per unit internal surface area. The optimized catalyst shows up to 14-fold improvement in the time-averaged propene formation rate, when compared to a benchmark catalyst. This significant improvement is primarily because of reductions in diffusion resistance and pore blockage.     

Activity and durability of iron-exchanged mordenite-type zeolite catalyst for the reduction of NO by NH3

NO removal activity and the durability of iron-exchanged mordenite type zeolite catalyst (FeHM) have been examined in a continuous fixed bed flow reactor. The catalytic activity for NO reduction by NH₃ in the presence of oxygen was much higher than that in the absence of oxygen, and it was fully reversible with respect to the presence of oxygen in the feed gas stream. The oxidation ability of SCR catalysts including FeHM was critical for both reactions of NH₃ and SO₂ oxidation, thus for the NO removal activity and its sulfur tolerance. The maximum conversion of NO for FeHM catalyst with respect to the reaction temperature shifted to the higher temperature due to its mild oxidation ability. The deactivation behaviors such as the changes of the physicochemical properties of the catalyst and the loss of NO removal activity induced by SO₂ could not be distinguished, regardless of the metals exchanged in zeolite. However, the amount of deactivating agents deposited on the catalyst surface depended on the species of metals exchanged on the mordenite type zeolite, which was mainly attributed to the oxidation ability of metals for SO₂ conversion to SO₃.     

Kinetics and effectiveness factor for SO2 oxidation on an industrial vanadium catalyst

An experimental study of the reaction rate for SO₂ oxidation on a commercial V₂O₅ catalyst is presented. Results in the purely kinetic region are correlated by means of 12 published rate expressions. It is concluded that most rate expressions are adequate only in a narrow temperature and composition range and that probably no single rate expression can be applied in the whole range of industrial operating conditions. Experiments with large size pellets are used to determine catalyst effectiveness as a function of temperature and conversion. It is found that a serious transport resistance occurs for T ? 450°C and that the effectiveness decreases somewhat at high conversion. The effective diffusivity of SO₂ is determined and finally the tortuosity of the catalyst is calculated. The results show that the effective diffusivity can be determined on the basis of a gas-phase diffusion model and that the tortuosity factor falls within the range of previously published values with a slight increase for high conversion and possibly also for low temperature.     

Simultaneous removal of SO2 and NOx by microwave with potassium permanganate over zeolite

Simultaneous sulfur dioxide (SO₂) and nitrogen oxides (NO_x) removal from flue gas can be achieved with high efficiency by microwave with potassium permanganate (KMnO₄) over zeolite. The experimental results showed that the microwave reactor could be used to oxidation of SO₂ to sulfate with the best desulfurization efficiency of 96.8% and oxidize NO_x to nitrates with the best NO_x removal efficiency of 98.4%. Microwave accentuates catalytic oxidation treatment, and microwave addition can increase the SO₂ and NO_x removal efficiency by 7.2% and 12.2% separately. The addition of zeolite to microwave potassium permanganate increases from 16.5% to 43.5% the microwave removal efficiency for SO₂, and the NO_x removal efficiency from 85.6% to 98.2%. The additional use of potassium permanganate to the microwave zeolite leads to the enhancement of SO₂ removal efficiency up from 53.9% to 95%, and denitrification efficiency up from 85.6% to 98.2%. The optimal microwave power and empty bed residence time (EBRT) on simultaneous desulfurization and denitrification are 259 W and 0.357 s, respectively. SO₂ and NO_x were rapidly oxidized in microwave induced catalytic oxidation reaction using potassium permanganate with zeolite being the catalyst and microwave absorbent.     

The active phase of Au–Pd/Al2O3 for CO oxidation

Au–Pd/Al₂O₃ catalyst was prepared by modified impregnation method. It was found that the catalyst calcined in air at 473 K showed higher CO oxidation activity in comparison with the catalysts treated at other temperature. Nitrogen adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure spectroscopy (XANES) techniques were employed to study the relationship between the surface/bulk structures of these catalysts and their catalytic performance. The results indicated the higher activity was attributed to the smaller pore volume and co-existence of PdO and Au⁰ in their surface. The formation of Au_xPd_y alloy was unfavorable for the catalytic reaction.     

Transient diffusion,adsorption and reaction in porous catalysts: The reaction controlled,quasi-steady catalytic surface

The process of transient diffusion, adsorption and heterogeneous reaction in porous catalysts is analysed using the method of volume averaging. The analysis illustrates under what circumstances the effective diffusivity tensor is independent of the adsorption and reaction process, and thus indicates when effective diffusivities determined in passive systems can be used in the analysis of active systems. Normal mode analysis is used to determine under what conditions the process can be described in terms of a reaction controlled, quasi-steady catalytic surface.     

Microwave catalytic NOx and SO2 removal using FeCu/zeolite as catalyst

Non-thermal plasma technology is a promising process for flue gas treatment. Microwave catalytic NO_x and SO₂ removal simultaneously has been investigated using FeCu/zeolite as catalyst. The experimental results showed that a microwave reactor with FeCu/zeolite only could be used to microwave catalytic oxidative 91.7% NO_x to nitrates and 79.6% SO₂ to sulfate; the reaction efficiencies of microwave catalytic reduction of NO_x and SO₂ in a microwave reactor with FeCu/zeolite and ammonium bicarbonate (NH₄HCO₃) as a reducing agent could be up to 95.8% and 93.4% respectively. Microwave irradiation accentuates catalytic reduction of SO₂ and NO_x treatment, and microwave addition can increases SO₂ removal efficiency from 14.5% to 18.7%, and NO_x removal efficiency from 13.4% to 18.7%, separately. FeCu/zeolite catalyst was characterized by X-ray diffraction (XRD), X-ray photoelectron spectrum analysis (XPS), scanning electron microscopy (SEM) and the Brunauer Emmett Teller (BET) method. Microwave catalytic NO_x and SO₂ removal follows Langmuir-Hinshelwood (L-H) kinetics.     

Effect of sol–gel conditions on BET surface area,pore volume,mean pore radius,palladium dispersion,palladium particle size,and catalytic CO oxidation activity of Pd/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> cryogels

Porous and homogeneous palladium-alumina cryogels were synthesized from aluminium sec-butoxide (ASB) and palladium nitrate through one-pot sol–gel processing in an aqueous system and subsequent freeze drying. In order to optimize the sol–gel conditions so that higher catalytic oxidation activity could be acquired after high temperatures heating, effects of H₂O/ASB, HNO₃/ASB, ethylenediamine/Pd, and urea/ASB ratios on BET surface area, pore volume, mean pore radius, Pd dispersion, Pd diameter, and catalytic CO oxidation activity of the Pd/Al₂O₃ cryogels were examined. It was revealed that optimized molar ratios were ASB:H₂O:HNO₃:urea?=?1:76:0.26:0.29 and ethylenediamine:Pd?=?3.4:1, at the ratios of which higher Pd dispersion and more superior catalytic activity were obtained. It was suggested that maintenance of as large BET surface area and pore volume as possible even after the heating was important to obtain high Pd dispersion, which consequently brought about superior catalytic activity. It was also suggested that porosity of the cryogel also played an important role in suppressing the sintering of palladium. The palladium-alumina cryogel prepared under the optimized sol–gel conditions was compared with corresponding commercial catalyst in regard to Pd particle size, Pd dispersion, PdO reducibility, catalytic CO oxidation activity. It was shown, after heating the cryogel at 800 °C for 5 h, that finer palladium particles with ca. 3.5 nm diameter were dispersed throughout alumina support with higher dispersion (ca. 32%), which was primarily responsible for higher catalytic oxidation activity on the cryogel.     

Rational design of hierarchically structured porous catalysts for autothermal reforming of methane

It is shown that the performance of a commercial, meso-macroporous catalyst for the autothermal reforming of methane on Ni/Al₂O₃ could be improved significantly by optimizing the macroporosity and the size of the macropores. The commercial catalyst, taken as a base case, contains macropores with an average diameter of 2 μm and mesopores with an average diameter of 20 nm. The kinetics of Xu and Froment (1989a) for steam reforming and the water gas shift reaction were employed, in combination with kinetics for the total oxidation of methane. Multicomponent molecular diffusion, Knudsen diffusion and viscous flow were accounted for in the modeling of transport in the macropores. At typical reaction conditions, Knudsen diffusion dominates transport in the mesopores; the effect of pore surface roughness on Knudsen diffusion was included in the simulations. Both the macropore size and the macroporosity influence the overall conversion; increases of up to 40–300% with respect to a commercial catalyst are possible. A larger macroporosity typically favors a lower CO/H₂ ratio, that is, a higher selectivity toward hydrogen, when the reverse reaction of the water gas shift reaction dominates, and vice versa. Temperature gradients in the catalyst increase with macroporosity, as a result of the lower thermal conductivity of the solid porous material, but the maximum temperature in the catalyst was around 10 K above that at the outer surface at the investigated operating conditions.     

Symposium on application of pulse techniques to kinetic measurements: Influence of cycling on the rate of oxidation of SO2 over a vanadia catalyst

Feed composition cycling of the catalytic oxidation of SO₂ to SO₃ in an isothermal, differential reactor was studied using an industrial vanadia catalyst. Reaction was carried out at 405°C using square-wave cycles of SO₂, O₂ and N₂ at pressures close to atmospheric and at constant total molar flow through the reactor. Significant increases were found in the rate of oxidation of SO₂ over steady-state values, depending upon the choice of both amplitude and period. Some preliminary interpretation of these results is provided.     

Dynamic analysis of changes in diffusion and sorption parameters in the reaction of SO2 with activated soda ash

Variations in pore structure cause significant changes in diffusion resistance and adsorption characteristics during the reaction of SO₂ with activated soda. Breakthrough and moment analysis of the SO₂-activated soda ash reaction showed that the effective diffusion coefficient of SO₂ in the solid product (mostly Na₂SO₃) was about three times smaller than its value in the original activated soda. About 30% of the diffusion flux of SO₂ in the solid product was found to be due to surface diffusion. The adsorption equilibrium constant of SO₂ on the solid product was found to be about half the value of the adsorption equilibrium constant on activated soda. It was also determined that SO₂ was irreversibly adsorbed on the solid product Na₂SO₃ at 200°C and 3.7 atm. The gaseous product CO₂ was not adsorbed on the solid product while it was slightly adsorbed on activated soda.     

Development and reactivity tests of Ce-Zr-based Claus catalysts for coal gas cleanup

Claus reaction (2H₂S + SO₂ ↔ 3/nS_n + 2H₂O) was used to clean the gasified coal gas and the reactivity of several metal oxide-based catalysts on Claus reaction was investigated at various operating conditions. In order to convert H₂S contained in the gasified coal gas to elemental sulfur during Claus reaction, the catalysts having the high activity under the highly reducing condition with the moisture should be developed. CeO₂, ZrO₂, and Ce_1−xZr_xO₂ catalysts were prepared for Claus reaction and their reactivity changes due to the existence of the reducing gases and H₂O in the fuel gas was investigated in this study. The Ce-based catalysts shows that their activity was deteriorated by the reduction of the catalyst due to the reducing gases at higher than 220 °C. Meanwhile, the effect of the reducing gases on the catalytic activity was not considerable at low temperature. The activities of all three catalysts were degraded on the condition that the moisture existed in the test gas. Specifically, the Ce-based catalysts were remarkably deactivated by their sulfation. The Ce-Zr-based catalyst had a high catalytic activity when the reducing gases and the moisture co-existed in the simulated fuel gas. The deactivation of the Ce-Zr-based catalyst was not observed in this study. The lattice oxygen of the Ce-based catalyst was used for the oxidation of H₂S and the lattice oxygen vacancy on the catalyst was contributed to the reduction of SO₂. ZrO₂ added to the Ce-Zr-based catalyst improved the redox properties of the catalyst in Claus reaction by increasing the mobility of the lattice oxygen of CeO₂.     

Composition cycling of an SO2 oxidation reactor

Experiments are described in which the second stage of a two-bed catalytic reactor is used to study the catalytic oxidation of SO₂ over a potassiumWhen the feed to Stage 2 is continuously cycled the rate of oxidation SO₂ in Stage 2 is higher than when Stage 2 is operated continuously.Mass spectrometric measurements of the time-resolved relative ratio of SO₃ in the output from the Stage 2 reactor, coupled with measurements of theA tentative explanation for the increased rate of SO₂ oxidation observed is presented. It is suggested that SO₃ in the catalyst phase or potass     

高活性隐钾锰矿分子筛K-OMS-2的制备及其催化氧化苯甲醇制苯甲醛反应性能的研究

采用醋酸丁酯处理后固相合成的方法制备了一种隐钾锰矿分子筛催化剂K-OMS-2。相对于传统固相法制备的样品,此法所制备催化剂的比表面积略有下降,但孔容和孔径明显增大,其原因可能是在焙烧过程中醋酸丁酯的挥发分解形成了造孔效应。在苯甲醇选择性氧化制苯甲醛反应中,所制备K-OMS-2催化剂的活性相对传统固相法制备的样品有显著提高。经413 K合成、433 K焙烧的催化剂反应转化率达到79.6%,苯甲醛选择性维持在100%。催化剂活性提高的主要原因可能是催化剂较大的孔容、孔径促进了苯甲醇氧化反应中分子的扩散效率,因而提高了总体催化反应速率。     

Controlling the Sulfur Poisoning of Ag/Al2O3 Catalysts for the Hydrocarbon SCR Reaction by Using a Regenerable SOx Trap

The deactivation of a silver-based hydrocarbon selective catalytic reduction catalyst by SO_x and the subsequent regeneration under various operating conditions has been investigated. Using a sulfur trap based on a silica-supported catalyst it was found that, for a Ag/SiO₂ + Ag/Al₂O₃ combination, the negative effect of SO₂ on the n-octane-SCR reaction can be eliminated under normal operating conditions. The trap can be regenerated by hydrogen at low temperatures or at higher temperatures using a hydrocarbon reductant.     

Restricted diffusion of residual molecules in catalyst pores under reactive conditions

The restricted diffusion of residual molecules under catalytic conversion conditions was investigated using commercial catalysts. The effectiveness factor, η, and the effective diffusion coefficient, D_e, for residual molecules were evaluated and explicitly compared based on a pseudo-first-order reaction kinetic model and the Thiele modulus. The experimental results showed that the restrictive diffusion of heavy oil molecules is affected by the joint functions of several factors, such as the operating conditions, the size and configuration of the reactant molecules, and the average pore diameter of the catalyst. The reaction temperature has a greater influence on restrictive diffusion than the other operating factors, and the ratio of reactant molecular size to catalyst pore size is the most critical factor that determines the degree of restrictive diffusion. Moreover, a mathematical expression was derived for the restrictive factors in order to describe the relationship between the effective diffusivity and ratio of molecule-to-pore size.