Modelling of catalyst pellet poisoning for benzene hydrogenation

Based on the kinetics of benzene hydrogenation, pore diffusion and catalyst deactivation, time-dependent effectiveness behaviour of a single Ni? SiO₂? Al₂O₃ catalyst pellet where the chemical reaction rate is determined by pore diffusion was simulated for different conditions of operation. Poisoning kinetics were measured in a series of differential reactor experiments at atmospheric total pressure at temperatures ranging from 403 to 473 K. A computed effectiveness factor has been compared with experimental values for a catalyst pellet of industrial size. A good degree of correlation between theoretical prediction and the experimental results was found.     

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.     

Effectiveness of reaction proceeding with degree kinetics in a plate catalyst grain

The dependences of the effectiveness factor of a plate catalyst grain on the Thiele parameter for reactions with degree kinetics in isothermal and non-isothermal conditions were investigated. Analysis of a possible stationary distribution of concentration in the thickness of a catalyst grain was performed. The condition under which “dead zones” arise and their sizes in the central part of a catalyst grain depending on the reaction order were established.     

C301铜基催化剂甲醇合成反应动力学(Ⅱ)宏观动力学模型

本文以工业应用为目的,采用内循环无梯度反应器,在工业甲醇合成操作条件下研究了工业粒度C301型铜基催化剂上一氧化碳、二氧化碳与氢合成甲醇的宏观动力学.选择以下二个反应:CO+2H_2(?)CH_3OH CO_2+3H_2(?)CH_3OH+H_2O为系统的独立反应,获得了可供实用的宏观动力学方程,并讨论了反应条件对甲醇合成效率因子的影响.     

对苯二甲酸精制钯炭催化剂内扩散及有效因子的研究

在高压间歇反应釜研究了对苯二甲酸中杂质对羧基苯甲醛（4-CBA）在不同粒径的钯炭催化剂上进行加氢反应的特性。结果表明,催化剂粒径对4-CBA的加氢反应影响较大,催化剂的粒径减小,有效因子增大,反应速率增大,反应为内扩散控制。在反应温度150 ℃和240 ℃条件下,测定了钯炭催化剂内扩散有效因子,估算4-CBA在钯炭催化剂内的有效扩散系数Deff和催化剂的曲折因子,并提出了钯炭催化剂开发的改进方向。     

Catalytic combustion of methane in a monolith washcoat: Effect of water inhibition on the effectiveness factor

This paper reports the results of a numerical investigation of the diffusion and reaction of methane in the washcoat of a catalytic monolith reactor. The kinetic rate expression used is an empirical equation determined experimentally for a palladium oxide catalyst. The effect of water inhibition on the reaction rate is included in the model. A multi-species diffusion and reaction model is used to simulate the process. The model is solved in a 2-D space using a finite element method. It is observed that the inhibiting effect of water tends to lower the observed reaction rate and that a higher surface water concentration results in an increase in the observed effectiveness factor. The effectiveness factor depends on three dimensionless parameters. Strong diffusion limitation can lead to high water concentrations at the interior of the catalytic washcoat.     

Liquid phase synthesis of methyl tert-butyl ether catalyzed by ion exchange resin

Synthesis of methyl tertiary butyl ether (MTBE) from methanol and isobutene was studied using macroporous cation exchange resin, Amberlyst 15 in the hydrogen form, as a catalyst in the temperature range of 313-328 K. The reaction was carried out in a batch reactor at a pressure of one atmosphere in the liquid phase. A high degree of agitation was maintained in order to eliminate film diffusion resistance. The effect of catalyst loading, catalyst particle size and reaction temperature on reaction rate were studied. The reaction rate increased with increase in catalyst concentration and reaction temperature. Resin particle size had virtually no effect on the rate under the experimental conditions. The reaction rate data were analysed using homogeneous kinetics and heterogeneous models based upon Langmuir-Hinshelwood rate expressions. The apparent activation energies using homogeneous and heterogeneous models were determined and found as 79.0 kJ/mol and 76.7 kJ/mol respectively.     

MASS TRANSFER WITH CHEMICAL REACTION IN PARTIALLY WETTED FLAT PLATE CATALYST PELLETS: RIVULET FLOW ANALYSIS

Mass transfer with chemical reaction is analyzed in a system formed by a flat plate solid catalyst, partially wetted by a flowing rivulet of a liquid in contact with a stagnant pure gas. The paper solves the fluid dynamic problem of the liquid phase first, and afterwards incorporates the mass transfer and the chemical reaction. The system is assumed to be isothermal and at steady state, with a first order kinetics whose limiting reactant is in the gas phase. This work studies the influence of the gas-liquid surface tension, the liquid reactant flow rate, the liquid viscosity and the angle of inclination of the solid, upon the wetting factor. The model proposed also predicts the effect of these parameters and the Thiele modulus on the overall effectiveness factor and the molar flux of the limiting gaseous reactant at the catalytic solid-liquid interface in a direct way. This approach makes the wetting factor a non-manipulated variable.     

表面中毒甲醇成催化剂效率因子

对ＣＯ、ＣＯ２加氢合成甲醇平行反应提出了表面中毒催化剂效率因子的关键组分计算模型，并用正交配置法求解．硫化氢表面中毒深度由失活速率方程求得．计算结果表明。随Ｈ２Ｓ含量增加和使用时间增加，催化剂效率因子减小；随反应温度升高，表面中毒深度增加，又受内扩散影响，催化剂效率因子明显减小．     

无锑催化剂对PET缩聚反应动力学的影响

用PET缩聚反应动力学方法研究自制无锑催化剂的催化效果,无锑催化剂对缩聚反应速率常数及 反应时间的影响。缩聚反应中,使用无锑催化剂(相对TPA质量分数为0.02％)与常规三氧化二锑催化剂 (相对TPA质量分数为0.04％)相比,缩聚反应速率常数可提高1～2倍以上,反应时间可缩短23～40min。     

新型无锑催化剂在聚酯合成中酯化反应动力学研究

用自制的新型无锑催化剂,主要含有TiO2、钛酸四丁酯,研究了其催化酯化反应的动力学。实验结果表明,在同样的反应条件下,加入自制催化剂与不加催化剂的反应相比,酯化反应活化能有大幅度的降低;酯化反应时间也可缩短39%。     

Use of stirred batch reactors for the assessment of adsorption constants in porous solid catalysts with simultaneous diffusion and reaction. Theoretical analysis

A simple, pseudo-equilibrium model was derived for a catalytic system with a first order chemical reaction and simultaneous diffusive and adsorptive processes, in order to assess the corresponding kinetics and Henry law's-type adsorption parameters. Solutions from this model were compared to exact solutions from a more detailed, general model. It was shown that under most of the experimental conditions used in stirred batch reactors and the usual model considerations, it is only possible to assess apparent adsorption parameters. Also, we observed that a stable relationship between the concentrations in the gas and solid phases is reached. The error produced in assuming that the apparent adsorption constant is the real one was calculated to be very important. The value of the apparent adsorption constant depends on various system properties and experimental conditions, such as the Thiele modulus, the amount of catalyst and the contact time. The ratio between the apparent and real adsorption constants was shown to be the transient effectiveness factor at any moment. This ratio reaches a maximum value for the pseudo-equilibrium state, that is always larger than the steady-state effectiveness factor, becoming closer as long as the system's adsorption capacity decreases. The analysis determines the operative conditions to reduce the parametric correlation. Also a criterion for the applicability of usual approximations in the assessment of kinetics and equilibrium adsorption parameters in porous solid catalysts by means of pulse injection methods is established.     

在Pb-Bi-La-Mo/SiO_2催化剂上甲醇氧化的内扩散影响及甲醇扩散系数和催化剂曲折因子的测定

在30-40目的Pb_(0.88)Bi_(0.06)La_(0.02)Mo/SiO_2催化剂上甲醇氧化制甲醛的反应在动力学区域进行,其速度规律服从二步骤Redox机理动力学方程.当催化剂增大到3mm时,其动力学方程受内扩散影响严重,实验上测定了催化剂有效因子在0.28—0.12之间.作者对甲醇内扩散控制时的Redox机理动力学方程进行了理论上的推导和实验上的验证.实验上测定了受内扩散控制时的反应活化能,并从理论上指出动力学区域与内扩散区域活化能的关系.用动力学方法测定了甲醇在给定反应条件下的扩散系数和曲折因子.     

Catalyst particle shapes and pore structure engineering for hydrodesulfurization and hydrodenitrogenation reactions

Catalyst particle shapes and pore structure engineering are crucial for alleviating internal diffusion limitations in the hydrodesulfurization (HDS)/hydrodenitrogenation (HDN) of gas oil. The effects of catalyst particle shapes (sphere, cylinder, trilobe, and tetralobe) and pore structures (pore diameter and porosity) on HDS/HDN performance at the particle scale are investigated via mathematical modeling. The relationship between particle shape and effectiveness factor is first established, and the specific surface areas of different catalyst particles show a positive correlation with the average HDS/HDN reaction rates. The catalyst particle shapes primarily alter the average HDS/HDN reaction rate to adjust the HDS/HDN effectiveness factor. An optimal average HDS/HDN reaction rate exists as the catalyst pore diameter and porosity increase, and this optimum value indicates a tradeoff between diffusion and reaction. In contrast to catalyst particle shapes, the catalyst pore diameter and the porosity of catalyst particles primarily alter the surface HDS/HDN reaction rate to adjust the HDS/HDN effectiveness factor. This study provides insights into the engineering of catalyst particle shapes and pore structures for improving HDS/HDN catalyst particle efficiency.     

Reactivation of partially poisoned nickel catalyst by bleaching earth

The effect of activated bleaching earth on the hydrogenation kinetics of fats has been studied in a model system containing sodium soaps, phospholipids and allyl isothiocyanate. Both unsupported and supported catalysts were inactivated by these inhibitors. The optimal quantity of adsorbent, when added to the reaction system, caused a maximal increase of hydrogenation rate in the presence of these inhibitors. In the case of partial inactivation of a catalyst with these inhibitors, the addition of bleaching earth resulted in a regeneration of the catalyst’s active surface and an immediate increase in the hydrogenation rate. The effectiveness of this interaction depended upon the inhibitor type, catalyst type and time of contact with catalyst inactivators. The results suggest the positive effect of activated bleaching earth during hydrogenation results from the sorption processes, inhibitor transformations on the adsorbent and direct interaction with the nickel catalyst.     

Optimization of catalyst pellet structures and operation conditions for CO methanation

A fundamental understanding of the effects of catalyst pellet structures and operation conditions on catalytic performance is crucial for the reactions limited by diffusion mass transfer. In this work, a numerical investigation has been carried out to understand the effect of catalyst pellet shapes (sphere, cylinder, trilobe and tetralobe) on the reaction-diffusion behaviors of CO methanation. The results reveal that the poly-lobe pellets with larger external specific surface area have shorter diffusion path, and thus result in higher effectiveness factors and CO conversion rates in comparison with the spherical and cylindrical pellets. The effects of operating conditions and pore structures on the trilobular catalyst pellet with high performance are further probed. Though lower temperature can contribute to larger effectiveness factors of pellets, it also brings about lower reaction rates, and pressure has little impact on the effectiveness factors of the pellets. The increase in porosity can reduce the pellet internal diffusion limitations effectively and there exists an optimal porosity for the methanation reaction. Finally, the height of the trilobular pellet is optimized under the given geometric volume, and the results demonstrate that the higher the trilobular catalyst, the better the reaction performance within the allowable mechanical strength range.     

Evaluating the effectiveness factor from a 1D approximation fitted at high Thiele modulus: Spanning commercial pellet shapes with linear kinetics

In the present contribution a one-dimensional model (1D model) is employed to account for the effect of pellet shape in the solution of diffusion-catalytic reaction problems. The 1D model free parameter (σ) is fitted so as to match the second term of the series for the effective reaction rate at fast kinetic conditions. The performance of the model was evaluated for a large collection of pellets available commercially, covering almost all the main shapes offered by catalyst manufacturers. For isothermal linear kinetics the effectiveness factors calculated by the 1D model differ at most by 3% from those of the actual 3D pellets.     

A robust iterative method of computing effectiveness factors in porous catalysts

A robust, efficient numerical method for computing the effectiveness factor of a heterogeneous reaction in a catalyst is developed in this study. The method is based on shooting at the outer surface of the catalyst and is optimized for an accurate estimation of the concentration gradient at the outer surface. The shooting at the outer surface, however, is inherently unstable for a cylindrical or a spherical catalyst, and it also can become unstable with an increasing Thiele modulus even for a slab catalyst. From an analysis of the governing equation, however, three criteria are developed to make the method work efficiently in the presence of instability. The numerical method is shown to be effective for diverse kinetic expressions, from simple power-law to sophisticated Langmiur-Hinselwood kinetics, and for isothermal or non-isothermal catalysts. The method is also shown to be easily applicable to a more complex case of multiple steady states, estimating all the corresponding effectiveness factors. In this method, a simple rule that determines the stability of each steady state is proposed.     

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.     

Diffusional effects in wetproofed catalysts for isotopic exchange between hydrogen gas and water vapour

Deuterium transfer rates between water vapour and hydrogen gas have been obtained for platinized carbon powder (Pt-C) and Teflon wetproofed platinized carbon films. The pore diffusional resistance in the catalyst films was assessed by calculating the effectiveness factor, ?, from a mathematical model for simultaneous diffusion and first order reversible reaction kinetics. Wetproofing the Pt-C catalyst with Teflon increased the intrinsic rate of reaction, but the effectiveness factor decreased with increasing catalyst layer thickness. Pore size and surface area distributions and water vapour adsorption isotherms of the Pt-C/Teflon catalyst films were similar to those of the Pt-C itself. Since Teflon had negligibly small surface area and very large pores compared to the carbon support, wetproofing did not cause significant changes to the physical characteristics of the platinized carbon.