Catalyst effectiveness in low-temperature water—gas shift reaction

A procedure has been developed for a priori prediction of intraparticle diffusion effects in low temperature water—gas shift reaction. Use is made of transport characteristics of the Cu/ZnO/A1₂O₃ pelleted catalyst determined independently by combination of diffusion and permeation results obtained under nonreactive conditions. The porous medium is described by the mean transport pore model and diffusional behaviour of the multicomponent reaction mixture by a modified Stefan-Maxwell equation. Using data from kinetic region obtained at 200°C, it was possible to predict the performance of a bench-scale reactor packed with pelleted catalyst at 200°C and 220°C satisfactorily.     

Strategy for predicting effective transport properties of complex porous structures

Measurements of effective transport properties of porous media, such as effective diffusivity and permeability, are well established by several experimental techniques. Effective transport properties can be also calculated from the spatially 3D reconstructed porous media, where the morphology characteristics required for the reconstruction are obtained from electron microscopy images. Here we demonstrate the reconstruction of porous alumina catalyst carrier with bimodal pore size distribution. Multi-scale concept is employed for the computation of effective diffusivity and permeability of reconstructed porous media and calculated effective transport properties are compared with transport parameters experimentally determined in Graham diffusion and simple permeation cell. The limitations of current state-of-the-art reconstruction techniques for porous media with broad pore size distribution are discussed. We show that the contribution of nano-pores towards the total diffusion flux is significant and cannot be neglected, but it is reasonable to neglect the contribution of nano-pores towards the sample permeability.     

CATALYST EFFECTIVENESS FOR MULTICOMPONENT REACTIONS WITH CONCENTRATION-DEPENDENT DIFFUSIVITIES

The diffusion coefficient,D_i has been formulated of the reacting species in the transition region of diffusion for a stoichiometricly simple isothermal reaction in a porous catalyst and its concentration dependence has been determined. The concentration dependence of D_i incorporated into the mass balance of the catalyst pellet introduces a new parameter, A (Eq. 11). The effect is demonstrated of this parameter on predicted values of effectiveness factors for three kinetic equations of the Langmuir-Hinshelwood type (Eq. 36-38). If the effectiveness factor is a monotonously decreasing function of the Thiele Modulus, or if the concentration dependence of D_i is not too strong, the effectiveness factor can be predicted from the solution of mass balance with concentration independent diffusivity. This prediction can be improved by using a mean diffusion coefficient D_i.     

Modeling cat cracker catalyst demetallization

Vanadium and nickel have been identified as the major poisons that lead to the deactivation of cracking catalysts. These metals can be demetallized by a multistage process involving gas solid non-catalytic reactions in porous particles. Mathematical models for predicting the removal rate of vanadium when diffusion, reaction, and reaction and diffusion are controlling have been developed. It is generally accepted that vanadium is deposited on specific sites in the interior of the catalyst. This is verified since a diffusion controlled model for the removal of vanadium is in close agreement with experimental results. The parameters of the diffusion controlled model were estimated by constrained nonlinear regression. A total of five lumped mass transfer parameters for vanadium removal were determined using data from a laboratory demetallization unit. These parameters were then used to predict vanadium removal and compared with experiment.     

Gas‐solid catalytic reactions with an extended DSMC model

An algorithm of diffusive gas transport in porous solids based on random collisions of molecules (DSMC) is extended to include basic heterogeneous reaction mechanisms (adsorption, coadsorption, desorption, and reaction of gas species on the surface of the solid). With this model, we study the catalytic oxidation of CO inside highly porous nanoparticle layers in the transition regime using kinetic parameters from Pd(111) surfaces at ultra high vacuum conditions. Investigation of the reaction at different temperatures reveals a clear transition between a kinetic limit (low temperatures) and a diffusion limit (high temperatures). At high temperatures and under steady‐state conditions, the porous layer shows three distinct regions with different reaction rates (reactor poisoning, an effective reaction region, and a region with CO depletion), whose extends are determined by CO concentration and mass‐transport limitation. We expect that similar investigations help optimizing the structure of gas sensor elements based on nanoparticle layers fabricated with flame spray pyrolysis. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2092–2103, 2015     

Kinetic and mass transfer in the hydrogenation of polyunsaturated organic compounds in the presence of supported Pd catalysts

The hydrogenation of 1,5-cyclo-octadiene has been studied on well-defined Pd catalysts. The reaction is affected by internal diffusion of hydrogen into the catalyst particles. Despite this limitation, the reaction orders of the reactants have been determined. All the performed kinetic runs have been simulated and the kinetic and mass-transfer parameters giving the best fit of experimental data have been evaluated. The reaction occurs in two steps, in the first, a conjugate diene (1,3-cyclo-octene) is formed via isomerization, then, hydrogenation occurs quickly forming the monoene. The hydrogenation of the obtained cyclo-octene is relatively slow and strongly inhibited by the presence of the cyclo-octadiene. This last reaction has, therefore, been used for comparing the activities of different palladium catalysts showing an exponential behaviour of the reaction rate with the metal dispersion.     

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.     

Kinetics of selective hydrogenation of pyrolysis gasoline over an egg-shell catalyst

The kinetics of liquid-phase selective hydrogenation of pyrolysis gasoline over a commercial egg-shell catalyst was investigated in a stirred semi-batch reactor in the absence of external transport limitations. Experimental observations showed that competitive hydrogenation between monoolefins and diolefins was noticeable, and that the reaction rates of diolefins were much faster than those of monoolefins. A Langmuir–Hinshelwood-type reaction mechanism was proposed for the reaction system, and then a rigorous diffusion-reaction mathematical model was developed. For the egg-shell catalyst particle, the diffusion and reaction of species were described separately in two distinct regions, i.e., the outer active region, where both diffusion and reaction occurred, and the inner inertia region, where only diffusion happened. The kinetic and adsorption parameters were estimated from the diffusion-reaction model with the experimental data. The model was able to describe the experimental observations very well. The simulation results showed that significant concentration gradients existed inside the egg-shell catalyst, which revealed that the influence of the internal diffusion resistances on the reaction kinetics was considerable.     

Porous Medium Model in Computational Fluid Dynamics Simulation of a Honeycombed SCR DeNOx Catalyst

The commercial computational fluid dynamics software FLUENT was used to simulate the flow and chemical reaction process of a honeycombed SCR DeNOx catalyst. In the calculation model, the porous medium model was applied to describe the wall body region of the honeycombed catalyst and the ordinary flow model in order to define the honeycombed channel region. From the comparison between the two gas diffusion rates within catalyst pore structure and gas medium, their effects on the SCR DeNOx reaction rate were analyzed. A correction factor of the chemical reaction rate was created to modify the porous medium model in order to accurately calculate the chemical reaction process of the catalyst model. The study results indicated that the combination between porous medium model and reaction rate correction factor could compensate the deviation of calculated reaction rate and the interfacial diffusion problems.     

MODELLING OF FORCED PERIODIC OSCILLATIONS OF CARBON MONOXIDE OXIDATION OVER PLATINUM CATALYST

The periodic switching between two feed compositions to a gradientless reactor has been found to increase significantly the average reaction rate compared with steady stale operation. Another advantage of periodic feed switching, which has not been fully exploited in catalyst studies is in kinetic model discrimination where large variations can be forced into the predicted responses of several competitive models under consideralion. The elementary step model, with parameters determined from transient experiments, is used to simulate observed transients during forced periodic feed experiments. The model is found to fit the observed behaviour well, in contrast to Langmuir-Hinshelwood models which only prove adequate for steady state behaviour. Simulation studies coupled with experimental work help to clarify the role of adsorbed species during forced cycling.     

MODELLING OF FORCED PERIODIC OSCILLATIONS OF CARBON MONOXIDE OXIDATION OVER PLATINUM CATALYST

The periodic switching between two feed compositions to a gradientless reactor has been found to increase significantly the average reaction rate compared with steady stale operation. Another advantage of periodic feed switching, which has not been fully exploited in catalyst studies is in kinetic model discrimination where large variations can be forced into the predicted responses of several competitive models under consideralion. The elementary step model, with parameters determined from transient experiments, is used to simulate observed transients during forced periodic feed experiments. The model is found to fit the observed behaviour well, in contrast to Langmuir-Hinshelwood models which only prove adequate for steady state behaviour. Simulation studies coupled with experimental work help to clarify the role of adsorbed species during forced cycling.     

Molecular Transport of Aromatic Solvents through Oil Palm Micro Fiber Filled Natural Rubber Composites: Role of Fiber Content and Interface Adhesion on Transport

Cellulose micro fiber reinforced natural rubber composites were prepared and the diffusion and transport of aromatic solvents through these composites were studied in the temperature range 30–70°C. The diffusion parameters were investigated with special reference to the effect of fiber loading, penetrant size, temperature and interphase adhesion. The effect of chemical treatments on solvent uptake was also analyzed. The transport coefficients such as diffusion, permeation and sorption coefficients were determined to evaluate the influence of interphase adhesion on transport properties. The van't Hoff relationship was used to determine the thermodynamic parameters. The first order kinetic rate constant was evaluated. Finally, experimental results of the sorption properties of the composites were compared with theoretical predictions.     

基于超临界流体密度的烷基化内扩散影响动力学

利用烷基化反应实验数据和超临界流体密度计算数据,开展了考虑内扩散影响的烷基化反应动力学研究,采用优化计算方法进行模型参数估值,确定了烷基化反应速率常数、有效扩散系数、催化剂失活速率常数.研究结果表明,基于PengRobinsn(PR)方程计算流体密度的动力学模型在显著性水平α=0.01下有较高的实验数据拟合精度和模型可信度,说明该方法是计算超临界流体密度的较好方法.从有效因子大小可以看出,烷基化反应总体上处于中孔扩散阻力区.     

Model‐Based Ex Situ Diagnostics of Water Fluxes in Catalyst Layers of Polymer Electrolyte Fuel Cells

The ability to predict the electrochemical performance of the cathode catalyst layer in a polymer electrolyte fuel cell hinges on a precise knowledge of water distribution and fluxes. Water transport mechanisms that must be accounted for include vapor diffusion, liquid water permeation and vaporization exchange. In order to facilitate experimental efforts to this effect, we propose an ex situ model of water fluxes in catalyst layers. The model formulation is similar to transmission line models that are widely used in the analysis of electrochemical impedance spectra of porous composite electrodes. Focusing in this article on steady state and isothermal conditions, we rationalize the response function between defined environmental conditions, i.e. gas pressures, partial vapor pressures and temperature, which are defined at the boundaries of the catalyst layer, and the net water flux. This response function provides diagnostic capabilities to isolate and extract water transport parameters of catalyst layers from measurements of water fluxes through membrane electrode assemblies or half cell systems. An important asset of the model is the ability to analyze catalyst layer transport properties under partial saturation.     

THE SINGLE PELLET DIFFUSION REACTOR: THEORY AND APPLICATIONS

Most heterogeneous catalysts are either supported on some kind of porous material or the catalyst itself is porous. To satisfy practical constraints such as pressure drop, handling, and separation from products, these catalysts are generally pelletized. Very often the overall chemical reaction rates within these catalyst pellets are determined by a complicated interaction of internal and external transport effects with the intrinsic kinetic rate at the active surface.     

重质油在催化剂孔道内受限扩散及其关联模型研究进展

重质油分子结构复杂、尺寸较大且容易聚集形成缔合体,致使在加氢及催化裂化催化剂孔道内存在明显的扩散阻力,显著阻碍了反应物分子与催化剂孔道内部活性位的接触,导致反应速率与转化率明显低于轻质原料。因此,研究重油分子在催化剂孔道内扩散受阻情况可以为催化剂的设计优化提供信息与指导。尽管借助仪器或膜材料能获得溶质分子的扩散性能,但与反应条件下扩散规律差别较大,因此在实际反应体系下获得重油分子在催化剂孔道中的扩散传质具有重要的现实意义。综述采用反应动力学手段获得扩散传质数据并建立受限因子关联模型及其影响因素的研究进展,从反应物选择、反应条件及催化剂等因素的优化角度对重油分子受阻扩散研究进行展望。     

The poisoning of noble metal catalysts by phosphorus compounds—II the kinetics of poisoning and a mathematical model

The kinetic data of the poisoning of a porous noble metal carrier-type catalyst by phosphorus were determined during the total oxidation of ethane with air. An appropriate mathematical model for the reaction system including internal diffusion in the porous pellet was developed. The experimentally-obtained data were used in computations to prove that the mathematical model properly describes experimental poisoning within the limits of error.     

Effect of Polymer Growth Rate and Diffusion Resistance on the Behavior of Industrial Polyethylene Fluidized Bed Reactor

The two‐phase model developed for the UNIPOL polyethylene process is improved by introducing polymer diffusion resistance, this means modelling of polyethylene fluidized bed reactors has been examined on two levels, at small scale of individual polymer particle, and macroscale of the whole reactor. The model utilizes the multigrain model that accounts for the reaction rate at catalyst surface to explore the static and dynamic bifurcation behavior of the fluidized bed catalytic reactor. Detailed bifurcation diagrams are developed and analyzed for the effect of polymer growth factor and Thiele modulus (the significance of the porous medium transport resistance is characterized by Thiele modulus) on reactor dense phase monomer concentration and reactor temperature as well as polyethylene production rate and reactor single pass conversion for the safe temperature region. The observations reveal that significant diffusion resistance to monomer transport exists, and this can mask the intrinsic rate constants of the catalyst. The investigation of polymer growth factor indicates that, the nascent stage of polymerization is highly gas phase diffusion influenced. Intraparticle temperature gradients would appear to be negligible under most normal operating conditions.     

Sorption,desorption, diffusion,and permeation of aliphatic alkanes into santoprene thermoplastic rubber

The molecular transport characteristics of Santoprene rubber in the presence of n-alkanes, cyclohexane, 2,2,4-trimethylpentane, and 1,2,3,4-tetrahydronaphthalene at 25, 40, 55, and 70°C have been studied using a sorption gravimetric technique. From the sorption and desorption studies, the diffusion coefficients have been calculated and used in the discussion of the transport results. It was observed that these data depend on the size of the penetrants. Furthermore, efforts were made to evaluate the permeation coefficients, molar mass between crosslinks, and the kinetic rate constants. These data showed a systematic dependence on the penetrant size. Activation parameters for the process of diffusion and permeation have been calculated from the temperature dependence of the transport coefficients. © 1995 John Wiley & Sons, Inc.