Catalyst Support Pellets Research Articles

The Effect of Peptizing Agent Concentration on Processing and Properties of Alumina Based Catalyst Support

Gamma alumina (γ-Al2O3) has drawn attention as a support material for heterogeneous catalysts extensively used in the oil and gas industry due to its superior properties. Boehmite (AlOOH) is an industrially accepted and well-known precursor to produce gamma alumina-based support. The process additions such as nitric acid (HNO3) are commonly used in shaping process of alumina to control textural, mechanical and structural properties of final catalyst support. In this work, gamma alumina precursor boehmite was peptized by nitric acid at molar ratio of HNO3/AlOOH ranging from 0 to 0.054. With addition of nitric acid, extrusion paste showed higher plasticity up to 0.017 mole of HNO3/AlOOH. The addition of more than this point led to an inhomogenity of extrusion paste caused by hard and brittle structure caused by the formation of aluminum nitrate salts. Also, higher mechanical strength was observed for samples peptized with lower nitric acid concentration because of effective deagglomeration by peptization. The pore size of catalyst support pellets increased up to an acid/ boehmite molar ratio of 0.017 by peptizing of boehmite. However, beyond this point pore size decreased due to dissolution of boehmite.

Magnetic resonance velocity imaging of turbulent gas flow in a packed bed of catalyst support pellets

Compressed sensing magnetic resonance methods have been used to image the time-averaged velocity and turbulent kinetic energy in 3D for turbulent gas flowing through a bed of porous, hollow cylindrical catalyst support pellets. Velocity and turbulent kinetic energy images were acquired at a spatial resolution of 0.70 mm (x) × 0.70 mm (y) × 1.0 mm (z) for particle Reynolds numbers, Rep, of 500, 2500 and 6500 in a bed with a tube-to-particle diameter ratio of 4.7. These data represent the first full-field measurements of turbulent gas flow in packed beds of non-spherical pellets. The resulting images reveal several interesting features of the hydrodynamics in this system. A large degree of flow heterogeneity is observed in the bed, with regions of high-speed fluid observed near the walls and in large voids, and regions of backflow observed in the wake of pellets, between pellets, and within the pellet holes. For increasing Rep, the normalized axial velocity at the wall is found to increase, and the normalized turbulent kinetic energy becomes more homogeneous throughout the bed. The correlation between the turbulent kinetic energy and time-averaged velocity shows that the highest turbulent kinetic energy occurs in regions of intermediate time-averaged velocity. Further, the turbulent kinetic energy profile at the pellet-scale is substantially different from the case of simple channel flow for Rep≥ 2500. Overall, these measurements clearly demonstrate the ability of magnetic resonance methods for acquiring full-field flow data in packed bed systems using commercially-relevant pellets and flow conditions.

Investigation of gas diffusion phenomena in porous catalyst support pellets based on microstructure

Catalytic performance is influenced by the effective gas diffusivity in a heterogeneous catalyst. The diffusivity is dependent on the microstructure of the catalyst. Therefore, the microstructure of a catalyst must be characterized, and the effective gas diffusivity should be predicted using the microstructural properties. Mercury intrusion porosimetry (MIP) is a popular technique to characterize microstructure and subsequently predict the effective gas diffusivity with the random pore model (RPM) using the MIP results. However, this methodology cannot be used for multilayered catalysts such as automotive catalysts due to the difficulty of microstructure characterization. Three-dimensional (3D) image analysis is expected to be an informative technique to characterize the porous structure of multilayered catalysts because each layer can be analyzed independently. In this research, X-ray nano computed tomography (nano-CT), 3D-transmission electron microscopy (TEM) and MIP were employed to characterize porous Al2O3 pellet samples. A series of samples with different microstructures were prepared. The effective diffusion coefficient of CO2 was evaluated experimentally for each sample. The effective diffusion coefficient was also predicted with the RPM or by simulation. Pore properties obtained by MIP were applied for the RPM. 3D images obtained by nano-CT and 3D-TEM were used for application of the simulation. A strong correlation between the experimentally evaluated effective diffusion coefficient and the predicted diffusion coefficients was thus confirmed.

A class of particulate problems suited to FDEM requiring accurate simulation of shape effects in packed granular structures

In many granular material simulation applications, DEM capability is focused on the dynamic solid particulate flow properties and on systems in which millions of particles are involved. The time of relevance is many seconds or even minutes of real time. Simplifying assumptions are made to achieve run completion in practical timescales. There are certain applications, typically involving manufactured particles, where a representative pack is of the order of a thousand particles. More accurate capturing of the influence of complex shape is then often possible. Higher accuracies are necessary to model the topology of the void space, for example, for further CFD simulation and optimisation of fluid flow properties. Alternatively, the accuracy may be critical for structural performance and the force or stress transmission through the contact points is to be controlled to avoid material damage and poor function. This paper briefly summarises methods for simulation of shape effects on packing structures in the granular community and narrows the scope to problems where shape effects are of overriding concern. Two applications of mono-sized, mono-shaped packing problems are highlighted: catalyst support pellets in gas reforming and concrete armour units in breakwater structures. The clear advantages of FDEM for complex-shaped particle interactions in packed systems with relatively few particles are discussed. A class of particulate problems, ‘FDEM-suited’ problems, ones that are ideal to be solved by FDEM rather than by DEM, is proposed for science and engineering use.

Discovery of Unprecedented Ion–exchange Behavior of Nanometric Scheelite Prepared from Single‐Source Precursors

AbstractWater‐soluble precursors for nanometric calcium tungstate (CaWO4) were prepared from calcium 2‐hydroxycarboxylate salts. The precursors decompose as low as 425 °C to yield nanocrystalline scheelite. Supported CaWO4 in a convenient form for water treatment processes was prepared by impregnation of high surface area catalyst support pellets with an aqueous solution of a single‐source precursor followed by drying and calcining. Both the powder and supported nanocrystalline forms of CaWO4 underwent ion‐exchange reaction with aqueous lead ions whereby Ca2+ was released and Pb2+ was adsorbed to produce crystalline stolzite (PbWO4). Electron microscopy demonstrated that little morphological change of the particles occurred upon reaction with excess Pb2+. The process appears to occur via an ion‐exchange route. The nonlinear pseudo first‐order kinetic model gave the best fit for both the powder and supported forms of CaWO4, allowing the kinetic results from the two materials to be directly compared to each other. Supporting of CaWO4 improves the kinetics at higher CaWO4:Pb2+ ratios while at lower ratios diffusion of the Pb2+ ions into the porous pellets likely becomes the controlling factor and the powder form uptakes Pb2+ faster. The molar capacity of the material for Pb2+ uptake improved from 70 % to 100 % upon supporting the CaWO4. The novel ion‐exchange materials were demonstrated to be useful for the purification of industrial wastewaters and juice.

Nanocasting of novel, designer-structured catalyst supports

Highly structured catalyst support pellets have been produced that possess multiple, but regiospecific and well-defined, pore geometries. Mesoporous silica pellets with controlled pore sizes have been synthesised using one type of pore template for the pellet core zone, whilst using a different type of template for the surrounding shell region. These materials offer the potential of adding precisely engineered diffusion barriers to the outside of structured catalyst supports, or providing different environments for pore structure-sensitive reactions in different parts of a catalyst pellet. The pore structures of the newly synthesised materials have been characterised using the recently introduced, integrated nitrogen sorption and mercury porosimetry technique. The data arising from the latter type of experiments on materials with cylindrical channels has been analysed using both the conventional BJH pore diameter technique, and also a new analysis method that can determine both the pore length distribution and interconnectivity. The transport properties of the new materials have also been assessed.

A nuclear magnetic resonance microscopy study of mass transport in porous materials

The1H nuclear magnetic resonance (NMR) microimaging is employed to study the mass transport processes in porous materials, including individual catalyst support pellets and beds comprised of porous grains. Drying and adsorption are investigated by detecting the temporal evolution of the one-dimensional spatial profiles or two-dimensional maps of liquid content without interrupting the process under study. The characteristic features of these processes, such as the main mechanisms and the limiting stages of mass transport, and some factors which can alter the efficiency of mass transport are considered. The problems associated with the relaxation weighting of the NMR signals, often unavoidable for liquids permeating mesoporous solids, and the possibilities to overcome these problems are discussed.

Theoretical Aspects of the Estimation of Pore and Mass Fractal Dimensions of Porous Media on the Macroscopic Scale using NMR Imaging

NMR imaging studies, employing spin density and spin–lattice relaxation time ( T 1) contrasted images, have been used in the past to probe the spatial variation of voidage and pore size in porous materials such as porous catalyst support pellets. A method has been suggested to estimate the values of the mass and pore fractal dimensions of the structures underlying NMR spin density images. The method has been tested on simulated images of model structures of known geometry and shown to be a successful probe of the nature of the structure underlying an image. This method is suggested as a way of selecting a suitable model to simulate the structural properties of real porous materials over length scales from 0.01–10 mm.

In Situ NMR Imaging Studies of the Drying Kinetics of Porous Catalyst Support Pellets

The proton NMR imaging (microscopy) technique is employed to study the process of drying of alumina and titania porous catalyst support pellets impregnated with various liquids. Despite the short spin−spin relaxation times usually exhibited by liquids that permeate porous solids, the possibility to monitor their evaporation in real time by detecting one-dimensional liquid concentration profiles along the diameter of a cylindrical sample is demonstrated. In the qualitative discussion of the results, it is shown that the experiment is sensitive to the fine details of the drying process, such as the efficiency of liquid transport and peculiar profile shape transformations in the course of drying. A simple experiment is proposed that yields an empirical relation between the detected profile intensity and the actual liquid content, which allows one to correct the detected profiles for the inevitable T2-weighting effects.

The Use of Magnetic Resonance Images in the Simulation of Diffusion in Porous Catalyst Support Pellets

Previous work on the simulation of diffusion in porous materials and, in particular, catalyst support pellets has, in general, made use of abstract mathematical models to represent the structure of those materials. In this work a magnetic resonance image of the spatial distribution of voidage within a thin slice through a porous catalyst support pellet is used as the basis for a potentially more realistic representation of the porous structure. Simulations of diffusion are performed using a simulation lattice derived from the image and the resulting predictions of tortuosity are compared with experimentally determined values obtained using PGSE NMR and magnetic resonance imaging techniques. This approach is applied to the study of five different types of catalyst support pellet.

Production, Consolidation, Sintering, and Stability of Novel Nano-Crystalline Zirconia Catalyst Support Powders

AbstractNano-crystalline, zirconium oxide/oxyhydroxide particulates were produced using a novel, continuous, flow-through hydrothermal technology developed at the Pacific Northwest National Laboratory. Homogeneous aqueous solutions containing ZrO(NO 3)2 and urea were pumped at pressures of 6000-8000 psi through a heated stainless steel reaction tube at 340-350°C. Flow at pressure was maintained using a nozzle at the down-stream end of the reaction tube. In this process, termed Rapid Thermal Decomposition of precursors in Solution (RTDS), the formation of zirconium oxide/oxyhydroxide nano-crystals (diameters of generally less than 15-nm) occurred during the solution's brief residence time (<15 seconds) in the reaction tube, yielding aqueous slurries of agglomerated particulates. Powders were separated from these slurries, washed, then dried. RTDS powder prepared from 0.3M ZrO(NO3)2 and 0.6M urea were mixed with binder and formed into pellets using a commercial screw extruder. These extrudates were subsequently sintered at 500°C, yielding zirconia catalyst support pellets with a measured surface area of 113 m2/g. After application of 3% ruthenium metal to the sintered RTDS zirconia extrudates, the resulting catalyst was evaluated for catalytic activity using the hydrothermal hydrogenation of the sugar xylose to xylitol as a model reaction. After 3 hours at 140°C, the RTDS catalyst exhibited useful activity with 92% conversion and 66% selectivity to xylitol formation.

Characterization of structural inhomogeneities in porous media

AbstractRecent studies have demonstrated that spatial heterogeneities in voidage and pore size, over lengthscales of 0.1–1 mm, significantly affect the transport of liquids within a given porous medium. Nuclear magnetic resonance imaging is used to probe the structure of porous catalyst support pellets, and four image analysis techniques characterize the degree and nature of heterogeneities observed in the spin‐lattice relaxation‐time images of six pellets taken from the same batch. Power‐spectrum, cluster‐size, and percolation analysis reveal significant differences in spatial correlations and topological characteristics of the six images. The fractal dimension of all the images, however, is the same suggesting that this quantity characterizes the intrinsic heterogeneities associated with the particular batch of porous pellets studied in this work. The results suggest that structural models of such pellets should account for these macroscopic variations in pellet structure, as characterized here.

Transport heterogeneity in porous pellets—I. PGSE NMR studies

This paper is the first in a series of reports describing Nuclear Magnetic Resonance (NMR) and numerical modelling studies of the representation of the structure of porous media and associated transport phenomena. In this paper the interpretation of Pulsed Gradient Spin Echo (PGSE) NMR data are considered. In extending earlier studies in which the porous medium is assumed to be macroscopically homogeneous, we have considered the effect of spatial variations in both the diffusivity and relaxation properties within the porous medium, occurring over lengthscales of order > 100 μm. Expressions for the log-attenuation plot and the diffusivity are derived. The theory gives excellent agreement with experimental PGSE data obtained for a variety of porous catalyst support pellets, and both the effective diffusivity of the materials and an indication of the degree of heterogeneity of diffusion are obtained. Sample tortuosities are found to be in the range 1.3–4.3. The results suggest that macroscopically heterogeneous materials have a greater tortuosity than more homogeneous materials.

Transport heterogeneity in porous pellets—II. NMR imaging studies under transient and steady-state conditions

In the first paper of this study, PGSE NMR was used to measure liquid diffusion in porous catalyst support pellets. The results were explained in terms of heterogeneity in the structure and transport properties of these materials. In this part of the study, NMR imaging is used to study diffusion within the same samples. By acquiring a series of images during the course of a transient diffusion process, it is possible to estimate the effective diffusivity of water confined within the pore space of the pellets. NMR imaging is also used to produce two-dimensional images which show clearly that porous catalyst support pellets contain significant heterogeneities, as suggested by the spatially non-resolved PGSE NMR experiments presented in Paper I (Hollewand and Gladden, Chem. Eng. Sci. 50, 309–326, 1995). Finally, the PGSE and imaging NMR experiments are combined to produce images of the local diffusivity of water contained within individual pellets. These images allow the degree of diffusional heterogeneity to be quantified and give results in agreement with the PGSE NMR study. The results demonstrate that it is necessary to consider heterogeneities in the porous structure over different lengthscales in order to be able to understand the relationship between structure and transport in porous solids.

Probing the structure of porous pellets: An NMR study of drying

Partially saturated porous catalyst support pellets have been studied using NMR relaxation, PGSE, and imaging techniques. Spatially nonresolved PGSE data were acquired as a function of the fractional saturation of the pellets during drying. Combination of these data with an estimate of the width of the pore-size distribution, as determined by mercury porosimetry, and the results of numerical simulations of diffusion in a random pore network enabled predictions of the connectivity of the pore structure to be determined. NMR imaging was shown to be effective in discriminating between different models of drying; even when the bulk drying curves are similar, significantly different moisture redistribution phenomena are seen to occur.

Heterogeneities in Structure and Diffusion within Porous Catalyst Support Pellets Observed by NMR Imaging

The pore structure of porous oxide pellets commonly used as catalyst supports or as adsorbents in separation processes continues to attract much attention. This paper presents the results of a nuclear magnetic resonance (NMR) imaging study of various alumina and silica pellets. Both spin-echo and gradient-reversal techniques have been employed to produce two-dimensional maps of the spin density, spin-lattice relaxation time, and self-diffusion coefficient in the samples. NMR images show that the porous pellets contain significant structural heterogeneity, and this is reflected in the spatially resolved liquid-phase diffusion coefficients. The work suggests that NMR imaging techniques provide additional information which may be used to complement existing porosimetry and adsorption methods in characterising pore structures. These data will aid in the development of more realistic models of porous solids, which may then be used to predict transport phenomena. The advantage of NMR imaging methods is that they allow the characterisation of pore structure and transport processes within the same experimental environment. The NMR images presented in this paper represent the first application of this technique to visualize the internal structure of and diffusion within porous catalyst support pellets.

Concentration profiles in impregnation of porous catalysts*1

Regulation of the depth and uniformity of the impregnation of catalyst support pellets with an active ingredient is of considerable industrial importance. A model for the time-dependent flow of impregnating solution into a dry pellet and interior dispersal of impregnant is presented. It consists of plug flow into a single pore with either mass transfer across the liquid-solid interface or adsorption kinetics providing resistance to the removal of impregnant from solution. Predictions based on the model are consistent with laboratory impregnations. Of three independent parameters, the relative capacity for adsorption is both the most important in regulating dispersal and the easiest to adjust in practice.