SO2催化氧化过程的几个理论问题

综合介绍了ＳＯ２催化反应过程中的几个理论问题。认为ＳＯ２在钒催化剂上的液相催化反应可用双钒两段反应机理表征：小颗粒钒催化剂上ＳＯ２的催化反应受液相扩散的影响；钒催化剂的曲节因子在转化率不太高时为４左右，高转化率时曲节因子随转化率增大而在，相应地效率因子下降；低温区钒催化剂活性下降是由于熔盐中析出不溶的无活性的四价钒化合物造成的。并由此提出了考虑多种因素影响的动力学模型。     

表面中毒C207催化剂的曲节因子

采用N_2-CO_2气体稳态法测定了表面中毒C207催化剂的扩散通量,应用平行交联孔模型求得催化剂的曲节因子,按平均孔径计算的曲节因子δ_F:未中毒催化剂3.65,中毒催化剂2.99至3.11;按孔径分布计算的曲节因子δ:未中毒催化剂4.17,中毒催化剂3.45至3.71。     

SB—5型钴钼耐硫变换催化剂反应动力学的研究

本文对ＳＢ－５型钴钼耐硫变换催化剂进行了系统的动力学研究，获得了可信的本征动力学方程和催化剂的曲节因子，并在常压下测定了工业颗粒催化剂的宏观反应速率，所建立的内扩散效率因子模型的计算值与实验值相吻合。     

SB—5型钴钼耐硫变换催化剂反应动力学的研究

本文对ＳＢ－５型钴钼耐硫变换催化剂进行了系统的动力学研究，获得了可信的本征动力学方程和催化剂的曲节因子，并在常压下测定了工业颗粒催化剂的宏观反应速率，所建立的内扩散效率因子模型的计算值与实验值相吻合。     

反应条件下多孔催化剂上有效扩散系数及曲节因子的分析计算

采用CDS－810微反系统,在实验条件为：常压、氢苯比3.0-8.0、温度363.15K-453.15K时,进行了在四种不同粒度Ni/Al2O3催化剂上的气相苯加氢反应,利用实验数据计算了不同反应组成、不同反应速率时苯的有效扩散系数、反应死区及催化剂的曲节因子。计算结果表明：有效扩散系数不随反应速率改变,多孔催化剂的曲节因子可以看作多孔催化剂的结构参数,不随反应条件变化。     

多孔催化剂效率因子的多组分扩散模型 (Ⅱ)B109中温变换催化剂的效率因子

按照本文(Ⅰ)中提出的多孔催化剂效率因子的多组分扩散模型及其数值计算方法,本报计算了B109中温变换催化剂的效率因子,并与实验测定值进行了比较.测试了B109变换催化剂的孔隙率、孔径分布、曲节因子和常压下的本征动力学,并在内循环无梯度反应器中测试了常压下φ9.8×8.3mm圆柱状颗粒B109催化剂于各种气体组成和温度条件下只计入内扩散过程的宏观反应速率.由此获得十六种情况下效率因子的实验观察值为0.142至0.455,相同反应条件下模型预计值与实验观察值的相对误差为-0.25至0.06.比较的结果令人满意.     

铁铬变换催化剂物化性能研究

系统地测定了国内不同型号的铁铬变换催化剂在还原前、后的比表面积、孔容变化情况;测定了变换催化剂经不同处理后的比表面积、孔容、孔分布、曲节因子以及晶相结构等重要特征.研究结果表明,在铁铬催化剂上,用不同硫化氢浓度(0~500~2000ppm）半水煤气进行变换反应后,其比表面积、孔容、平均孔半径、孔分布、晶相结构以及曲节因子等,无明显变化。     

C207铜基催化剂常压甲醇分解反应动力学(Ⅱ)催化剂效率因子正交配置解

在内循环无梯度反应器中常压下测定了工业颗粒C207催化剂甲醇分解反应宏观速率,测定了催化剂孔径分布和曲节因子,获得了催化剂效率因子的实测值。探讨了计算催化剂效率因子的甲醇单组分模型和多组分模型的正交配置解,颗粒催化剂存在中心平衡死区。在相同反应条件下,催化剂效率因子模型计算值与实测值的相对误差的绝对值的平均值在11%以内,结果表明计算模型是可行的。     

LB型节能高温变换催化剂曲节因子测定

采用改进的Wicke-Kallenbach扩散池,用稳态法在常压40 ℃和60 ℃下测定了LB型节能高温变换催化剂曲节因子和气体有效扩散系数,测定中各实验点氮气与二氧化碳的摩尔通量之比符合Graham理论,与理论值误差在5.0%之内,计算得到了曲节因子为2.603,以平均孔径计算的曲节因子为1.788,它们与气体流量和实验温度无关.     

烃类转化催化剂以及碳纤维载体孔结构表征参数的测定

在害功能扩散他装置上, 用定压稳态法, 以CH₄、N₂为扩散气体, 在实验条件下测得了天然气蒸汽转化催化剂及碳纤维载体的孔结构表征参数曲节因子。在常压及30～500 ℃范围内, 催化剂的曲节因子τ值为1.8~3.5;碳纤维载体的τ值为(5~9)×10^-3～(5~9)×10^-4。由本实验数据计算得到催化剂的有效因子是:C119-02(η)=0.165，Z107(η)=0.207，Z110y(η)=0.208,川Ⅲ型(η)=0.215，C10(η)=0.223，CN-18(η)=0.299。文中对所得结果进行了简要分析。碳纤维载体中所进行的扩散接近对流扩散状态, 表明它是一种理想的催化剂载体。     

X-ray microtomography shows pore structure and tortuosity in alkali-activated binders

Durability of alkali-activated binders is of vital importance in their commercial application, and depends strongly on microstructure and pore network characteristics. X-ray microtomography (μCT) offers, for the first time, direct insight into microstructural and pore structure characteristics in three dimensions. Here, μCT is performed on a set of sodium metasilicate-activated fly ash/slag blends, using a synchrotron beamline instrument. Segmentation of the samples into pore and solid regions is then conducted, and pore tortuosity is calculated by a random walker method. Segmented porosity and diffusion tortuosity are correlated, and vary as a function of slag content (slag addition reduces porosity and increases tortuosity), and sample age (extended curing gives lower porosity and higher tortuosity). This is particularly notable for samples with ≥ 50% slag content, where a space-filling calcium (alumino)silicate hydrate gel provides porosity reductions which are not observed for the sodium aluminosilicate (‘geopolymer’) gels which do not chemically bind water of hydration.     

混凝土孔结构及其分形维数与氯离子扩散性能的关系

氯离子扩散系数是研究海洋环境下混凝土结构耐久性的重要参数之一。通过开展不同水胶比混凝土的压汞试验和盐雾扩散试验,研究了混凝土内部孔隙率、孔径分布及临界孔径对氯离子扩散系数的影响规律。结合Menger海绵体模型,建立孔体积分形维数与氯离子扩散系数的关系。结果表明:孔隙率和临界孔径与无量纲化氯离子扩散系数的相关性很高,可作为反映混凝土氯离子扩散性能的重要参数;通过数学分析计算得到的孔表面分形维数分布在2.56~3.86之间,孔体积分形维数分布在2.85~2.98之间;基于压汞法和分形理论计算得到的孔体积分形维数可以作为评价氯离子扩散系数的指标,在孔径小于10 nm、10～100 nm、100～1 000 nm以及大于1 000 nm四类区间,氯离子扩散系数随孔体积分形维数的增加而下降。     

Effective diffusivity and optimum apparent density of vanadia catalysts for sulfur dioxide oxidation

The effective diffusivities of air and SO₂ in four industrial vanadium pentoxide catalysts were measured at steady-state using helium as the counter diffusing gas. An improved catalyst mounting technique and diffusion cell were employed. The nonsurface component of diffusion was successfully correlated using Bruggeman's model for tortuosity. and ¯a based on pore size distribution data or calculated from specific pore volume and surface. However, it was necessary to use flow porosity in place of open porosity. Since the same pore model can be used for the catalytic oxidation So₂, non-reacting flow measurements can be employed to predict effective diffusivities under reaction conditions in this case.With models for the effective diffusivity and the kinetics of the catalytic oxidation of SO₂, an optimum apparent density of the catalyst may be determined which gives the maximum rate of reaction per unit volume of catalyst. Calculations are given for the SVD catalyst.     

Bounds for effective diffusivities using moment and transport constraints

Upper and lower bounds for an effective diffusivity characteristic of binary diffusion through a heterogeneous medium are calculated by imposing geometric and transport constraints on the pore size distribution. The method relies upon identifying moment spaces induced by Tchebycheff systems closely related to diffusion experiments and a priori information about the geometry. If statistics beyond the porosity and average pore size are included, temperature dependent tortuosity factors result from the strong coupling between the diffusive flux within a capillary and the pore size distribution. It is shown that hybrid statistical information, such as the porosity and a single diffusion experiment, can delimit significantly the allowed region for extrapolated data.     

Modeling residue hydrotreating

In this paper a kinetic model for the residue hydrotreating is presented. The model considers kinetics, adsorption and diffusion inside the hydrodemetallization catalyst pellets. The diffusion is described by the Stefan–Maxwell equations extended for starkly different sized molecules, due to molecular size of residue molecules large distribution. Effective diffusion coefficients vary with porosity and tortuosity evolution due to volume constraints in the catalyst pellets. The model's kinetic and thermodynamic parameters were estimated from a set of experiments carried out in a batch reactor with a Middle East vacuum residue. A comparison between experimental and simulated metal profiles inside the catalyst exhibit the slow diffusion, validating the importance of adding mass transfer phenomena and considering volume constraints even in a macroporous demetallization catalyst.The model was validated by other tests performed in the same reactor using the same vacuum residue, but with different catalysts. The differences in the catalyst pore size allowed to predict all the hydrotreatment yields, showing diffusion is a crucial factor.     

Mass transfer for dissolving solids in supercritical carbon dioxide Part II: Resistance in the porous layer

The internal mass transfer resistance was measured by dissolving naphthalene from a porous insoluble rod in a stream of supercritical carbon dioxide. Initially the pores of the porous medium (sintered bronze) were filled with naphthalene. During dissolution the naphthalene surface retreats into the porous rod and the length of the diffusion path increases. The tortuosity factor for the porous material was determined from experimental data. The temperature was varied from 35 to 55 °C, the pressure from 118 to 226 bar and the mean pore diameter was either 8 or 20 μm. The results show that the tortuosity factor does not depend on the pressure and pore diameter. This result was to be expected since the mean free path of the molecules is much smaller than the pore diameter. The average tortuosity factor, however, decreased with increase of temperature. From this temperature dependence surface diffusion of the solute absorbed on the pore wall is concluded to take place and thereby to contribute to the observed overall rate of mass transfer.     

气体、溶液中的非电解质和电解质在多孔颗粒内扩散的对比研究

实验测量了气体、溶液中的非电解质以及电解质三类性质相差较大的体系在相同的多孔颗粒内(孔隙率0.234～0.632)的有效扩散系数,并计算出曲折因子.探讨了由D_(?)=D_oε/τ定义的曲折因子对孔隙结构和扩散组分特性的依赖关系.     

The hydrotreatment of n-C16 over Pt/HPMo/SBA-15 and the investigation of diffusion effect using a novel W-P criterion

A series of Pt/HPMo/SBA-15 catalysts with the pore size as a single variable were prepared to elucidate the internal diffusion effect on the hydrotreatment of n-C16. A novel model was developed to calculate the Weisz-Prater number (N_W-P) because the traditional model relies on excessive empirical reference values and affects the accuracy remarkably. The utilization of the internal effectiveness factor, Thiele modulus, and the apparent activities over catalysts with different diffusion capacity gave rise to N_W-P with improved accuracy, which can also be extended to other heterogeneous catalytic systems with macromolecules as reactants. Combining the novel model with the diffusion–reaction kinetic method, a conclusion has been drawn that the formation of m-i-C16 is more susceptible to internal diffusion than the consumption of n-C16. Besides, the pore size should be not less than 10 nm to avoid the step-limitation of internal diffusion, in the premise of adequate acid sites.     

Application of synchrotron microtomography for pore structure characterization of deteriorated cementitious materials due to leaching

Deteriorated mortars and cement pastes (w/c = 0.50) were prepared by an accelerated leaching test using electrochemical migration technique. This technique enabled the reduction of the CaO/SiO₂ molar ratio to less than 2.0. Non-destructive three-dimensional imaging of the internal microstructure of the deteriorated cement matrix in hardened cement paste and mortar was performed using synchrotron X-ray computed microtomography at SPring-8, Japan. After image analysis at a spatial resolution of 0.5 μm/voxel the microtomographic images successfully visualized increased pore spaces in the deteriorated cement matrix with the effective porosity ranging from 0.31 to 0.38. In addition the diffusion tortuosity in the pore space derived from random walk simulation was also evaluated as a pore structure-transport parameter. Indications suggest that the deterioration of the cement matrix due primarily to the dissolution of portlandite decreases the diffusion tortuosity to a single digit as the degree of pore connectivity becomes larger at the submicron scale.