Catalyst Pellets Research Articles

Novel resource utilization approach of spent catalyst from flue gas denitrification: Preparing high quality titanium-bearing pellets

Spent titanium-based catalyst is a waste generated by the selective catalytic reduction (SCR) denitrification process, whose generation is dramatically increasing in the iron and steel industry year by year. A novel approach was proposed in this work to prepare titanium-bearing pellets by using spent catalyst. The influences of spent catalyst addition on the comprehensive properties of pellets were studied. The corresponding relationship between the change of phase structure and the spent catalyst pellets' strength was investigated. The results demonstrated that the spent catalyst could be adsorbed on the surface of ore particles physically and chemically, improving the adhesion of particles inside the pellet and the hydrophilicity, thus obtaining good comprehensive performance of the green pellets. When adding 5.0 wt% spent catalyst, the bentonite dosage could be reduced by 50%, and the strength of roasted pellets could meet the requirement of large blast furnaces. The consolidation mechanism of the spent catalyst pellets indicated that TiO2 reacted with FeO first to generate FeTiO3 in the preheating process and further oxidized to form Fe2TiO5 in the roasting process. The unreacted TiO2 would appear in the roasted pellets and inhibit recrystallization and polycrystalline growth of Fe2O3 grains, resulting in the reduction of compressive strength. Overall, high-quality titanium-bearing pellets could be obtained when the amount of spent catalyst was less than 5.0 wt%. This study thus provided an alternative route to resource utilization of spent catalysts in iron and steel enterprises.

Evaluation of the packed state of catalyst pellets in the gas phase reactor (Effect of catalyst pellet shape on its packed state)

In this paper, we propose a simulation method to evaluate the packed state of catalyst pellets in a pipe reactor for gas phase reaction. This simulation method is based on dynamic explicit FEM (finite element method) and consists of three steps. Specifically, first, the catalyst pellets freely fall under gravity and flow into a packing hose with a hopper-like inlet. Then, after the catalyst pellets reach the bottom of the pipe reactor, the packing hose is pulled up to create a random packing of the catalyst pellets in the pipe reactor. Finally, the bulk density and stress distribution of the catalyst pellets randomly packed in the pipe reactor are calculated, and the packed state of the catalyst pellets is evaluated. Using this simulation method, we simulated the packing of cylindrical catalyst pellets into the pipe reactor and evaluated the packed state of the catalyst pellets in the pipe reactor. As a result, it is clarified that the maximum equivalent stress generated at the bottom of the catalyst pellets packed in the pipe reactor approaches a constant value as the ratio of the packed height of the catalyst pellets to the diameter of the pipe reactor increases. In addition, the effects of the shape of the catalyst pellets on the bulk density and average equivalent stress of the catalyst pellets in the pipe reactor are elucidated, and the optimal shape of the catalyst pellets that can increase the bulk density and reduces the stress is found. The optimally shaped catalyst pellets are expected to improve their packed state in the pipe reactor, extend the durability of the catalyst pellets and increase the efficiency of production. In this paper, it is assumed that the packed state and durability of the catalyst pellets are evaluated based on the bulk density and stress distribution of the catalyst pellets in the pipe reactor.

Multiphase segmentation of digital material images

Abstract Multiphase segmentation of pore-scale features and identification of mineralogy from digital images of materials is critical for many applications in the natural resources sector. However, the materials involved (rocks, catalyst pellets, and synthetic alloys) have complex and unpredictable composition. Algorithms that can be extended for multiphase segmentation of images of these materials are relatively few and very human-intensive. Challenges lie in designing algorithms that are context free, can function with less training data, and can handle the unpredictability of material composition. Semisupervised algorithms have shown success in classification in situations characterized by limited training data; they use unlabeled data in addition to labeled data to produce classification. The segmentation obtained can be more accurate than fully supervised learning approaches. This work proposes using a semisupervised clustering algorithm named Continuous Iterative Guided Spectral Class Rejection (CIGSCR) toward multiphase segmentation of digital scans of materials. CIGSCR harnesses spectral cohesion, splitting the intensity histogram of the input image down into clusters. This splitting provides the foundation for classification strategies that can be implemented as postprocessing steps to get the final segmentation. One classification strategy is presented. Micro-computed tomography scans of rocks are used to present the results. It is demonstrated that CIGSCR successfully enables distinguishing features up to the uniqueness of grayscale values, and extracting features present in full image stacks (3D), including features not presented in the training data. Results including instances of success and limitations are presented. Scalability to data sizes $ \mathcal{O}\left({10}^9\right) $ voxels is briefly discussed.

Engineering an egg-shell structure for the Ag/SiO2 pellet catalyst for selective hydrogenation of dimethyl oxalate to methyl glycolate

An egg-shell-type Ag/SiO2 pellet catalyst with an optimized thickness of the Ag-shell exhibits both enhanced activity and selectivity for DMO hydrogenation to MG.

Core–shell catalyst pellets for effective reaction heat management

Catalyst research is concerned with synthesizing increasingly active materials, leading to safety issues at reactor scale, unless the reaction heat release is controllable. Computational studies predict that core–shell pellets with catalytically active core and inert shell are beneficial for this purpose, compared to established concepts such as catalyst pellet dilution. At high temperatures, reactant diffusion through the shell becomes rate-determining, resulting in a well-controllable heat release rate, which prevents further temperature increase. Here, industrial catalyst pellets were coated in a fluidized-bed pilot plant, demonstrating large-scale production feasibility. The obtained pellets were characterized via Dynamic Image Analysis, Scanning Electron Microscopy and X-ray Computed Tomography. Conducted CO2 methanation experiments confirm the predicted trends, if the applied shell is fully closed. Furthermore, mathematical and experimental studies demonstrate, that the inert shell shifts selectivity. Based on this work, safer and yet economical reactor operation is anticipated also for other reaction systems.

The influence of dissociative adsorption on effectiveness factors of porous catalysts

AbstractThe actual concentration profile of a reacting fluid inside a porous catalyst is something of a mystery. However, there are powerful models to predict the concentration distribution, and effectiveness factor as a function of the Thiele modulus. This work provides a model for the porous catalyst particle considering heterogeneous kinetics for steady‐state Langmuir‐Hinshelwood rate expressions, including the cases when adsorption stoichiometry is other than 1:1, as for dissociative adsorption (1:2), which have not been covered by the preceding literature. The isothermal and the non‐isothermal particles are considered. Numerical solutions for the concentration profile of a limiting reactant were calculated by three methods: finite differences with Newton‐Raphson recursion, Chebyshev orthogonal collocation, and integration by Adams method as done by Weisz and Hicks. The Weisz‐Hicks method outperformed the other two. Plots show the results for specific cases, which are briefly discussed. Adsorption stoichiometry and pellet geometry are of paramount importance for the assessment of effectiveness factors. The results demonstrate that the effectiveness of a catalyst pellet with prevalent dissociative adsorption of a reactant is always inferior to the effectiveness with prevalent simple adsorption.

Trans-level multi-scale simulation of porous catalytic systems: Bridging reaction kinetics and reactor performance

Multi-scale porous structures inside and/or between the catalyst pellets or particles are found in many chemical processes, where strong coupling of reaction and transport results in complex apparent reaction kinetics influential to the reactor performance. Traditional continuum-based porous media models and simulation methods can hardly describe such structures and their scale effects faithfully. A trans-level multi-scale discrete computational framework is hence proposed to address this complexity and implemented for an olefin catalytic cracking (OCC) process. The apparent reaction kinetics at the REV (representative elementary volume) scale is obtained by hard-sphere pseudo-particle modeling (HS-PPM), and coupled with computational fluid dynamics / discrete element method (CFD-DEM) for the reactor-level hydrodynamics via a one-dimensional (1D) finite difference scheme for particle-level diffusion. The mesoscales of the REVs and the flow networks between the particles are thus covered by the framework, which are previously described by simple average quantities in the continuum methods. The reactant conversion rate and target product selectivity obtained agree well with experimental results, while a continuum approach may give significantly different and unreasonable results. The multi-scale method is, therefore, demonstrated to be necessary and effective for bridging the intrinsic reaction kinetics with the performance of porous catalytic reactors.

Egg-Shell-Type MgAl2O4 Pellet Catalyst for Steam Methane Reforming Reaction Activity: Effect of Pellet Preparation Temperature

A pellet catalyst was prepared to be used in a large-scale steam methane reformer. Hydrotalcite powder (MG30) was used as a precursor to prepare MgAl2O4 pellet supports at different calcination temperatures. Ni-supported catalysts with egg-shell-type distribution were prepared on these pellet supports: Ni/sup-x (where x is the calcination temperature of the support with x = 1273, 1373, and 1473 K). Among them, Ni/sup-1473, which experienced the highest calcination temperature (1473 K), showed the highest methane conversion and lowest weight loss owing to carbon deposition. As a result, when the calcination temperature increased, the egg-shell thickness decreased, and the reducibility of the catalyst was enhanced. Although a small amount of Ni (3.5 wt%) was used, the egg-shell-type catalyst had superior catalytic activity and coke resistance. Therefore, the egg-shell-type catalyst using Ni as the active material and MgAl2O4 calcined at high temperature as the support is expected to be appropriate for large-scale industrial steam methane reforming reactions.

Evaporating water-cooled methanation reactor for solid-oxide stack-based power-to-methane systems: Design, experiment and modeling

The power-to-methane process using a solid-oxide electrolyser, followed by an exothermic methanation reactor, is effective for the production of an energy carrier (synthesis methane) that can be readily stored and transported at a large scale. A significant gain of the coupling is the steam generation from the heat produced by the methanation reaction for the more effective steam electrolysis (compared to liquid water electrolysis). This paper investigates, both theoretically and experimentally, the system integrability and operability of an evaporating water-cooled methanation reactor designed for thermal coupling with a solid-oxide electrolyser. The results show that the pressurized steam generation directly by the reactor’s cooling system improves the flexibility of the system design. Under certain conditions, an increase in overall power-to-methane efficiency of more than 3 % can be obtained from the use of a superheater and a turbine located after the evaporation process. The parametric study of the methanation reactor indicates that decreasing the flowrate of the H2 and CO2 mixture, increasing the reaction gas pressure and increasing the cooling system pressure (saturation temperature) shifts the hot spot towards the inlet and improves the H2 conversion rate, which can reach above 97 %. It was determined that, under steady-state operation, the steam generation can be sufficiently stable for injection into the solid-oxide electrolyser; rapid pulsations with a standard deviation under 10% were measured. The calibrated 1D pseudo-homogeneous plug flow reactor model captures many of the trends, though, the assessment is limited without the exact kinetic model of the catalyst pellets.

Direct production of olefins from syngas with ultrahigh carbon efficiency

Syngas conversion serves as a competitive strategy to produce olefins chemicals from nonpetroleum resources. However, the goal to achieve desirable olefins selectivity with limited undesired C1 by-products remains a grand challenge. Herein, we present a non-classical Fischer-Tropsch to olefins process featuring high carbon efficiency that realizes 80.1% olefins selectivity with ultralow total selectivity of CH4 and CO2 (<5%) at CO conversion of 45.8%. This is enabled by sodium-promoted metallic ruthenium (Ru) nanoparticles with negligible water-gas-shift reactivity. Change in the local electronic structure and the decreased reactivity of chemisorbed H species on Ru surfaces tailor the reaction pathway to favor olefins production. No obvious deactivation is observed within 550 hours and the pellet catalyst also exhibits excellent catalytic performance in a pilot-scale reactor, suggesting promising practical applications.

Green process of fuel production under porous γ-Al2O3 catalyst: Study of activation and deactivation kinetic for MTD process

One of the methods of industrial dimethyl ether production is the catalytic dehydration of methanol. In this research work, methanol dehydration reactor has been modeled using continuous model and its results have been compared with experimental works and Voronoi pore network model. A 1D heterogeneous dispersed plug flow model was utilized to model an adiabatic fixed-bed reactor for the catalytic dehydration of methanol to dimethyl ether. The mass and heat transfer equations are numerically solved for the reactor. The concentration of the reactant and products and also the temperature varies along the reactor, therefore the effectiveness factor would also change in the reactor. We used the the effectiveness factor that was simulated according to the diffusion and reaction in the catalyst pellet as a Voronoi pore network model. Sensitivity analysis was performed to determine the influence of T, P and weight hourly space velocity on performance of the chemical reactor. Acceptable agreement was reached between the measured and the model data. The results showed that the maximum reaction conversion was obtained about 90 % at WHSV = 10 h−1 and T = 560 K, while the inlet temperature (Tinlet) had a greater effect on methanol conversion. In addition, the effect of water in the feed on methanol conversion was quantitatively studied. Also, the deactivation kinetics of γ-Al2O3 heterogeneous-acidic catalyst in methanol to dimethyl ether dehydration process was studied using integral analysis method. Based on independent deactivation kinetics, a second order was found that accurately fitted the experimental conversion time data. The main reaction activation energies and catalyst deactivation energies were 143.1 and −102.1 kJ/mol, respectively.

Non-thermal plasma synthesis of supported Cu-Mn-Ce mixed oxide catalyst towards highly improved catalytic performance for volatile organic compound oxidation.

Compared with that of the transition metal mixed oxide pellet catalyst, the catalytic activity of the supported mixed oxide catalyst was significantly reduced, which was limited in practical industrial applications. In this work, supported Cu-Mn-Ce mixed oxide catalysts were prepared by non-thermal plasma. Catalyst characterization result demonstrated that plasma treatment could promote the proportion of oxygen vacancy and enhance the adsorptive strength of VOCs on the surface of catalyst. Meanwhile, plasma treatment process exerted a slight influence on the pore structure and morphological property of the catalyst. Consequently, CMC/SiO2-P exhibited much higher catalytic activity than CMC/SiO2 prepared by the incipient wetness impregnation method for the catalytic oxidation of toluene and n-hexane. Among the catalysts prepared, the 15%CMC/SiO2-P catalyst even exhibited a high catalytic activity comparable to that of the supported noble metal catalyst for the oxidation of the inert hexane. The T98 of toluene and n-hexane over 15%CMC/SiO2-P was 260 and 280°C under the conditions of VOC concentration at 1000 ppm and WHSV at 20,000 mL·g-1·h-1, respectively. This work provided a novel method for the preparation of the supported transition metal mixed oxide catalyst.

Integration of multi-scale porosimetry and multi-modal imaging in the study of structure-transport relationships in porous catalyst pellets

The forming process for heterogeneous catalyst pellets impacts on reactor performance. In order to intelligently optimise the product formulation and manufacturing procedure, the impact of fabrication options on the resultant pore structure and mass transfer properties must be fully understood. In this work, the more direct information possible from multi-modal imaging methods has been combined with the more statistically-representative, multi-scale data from porosimetry, including the rarely-used gas overcondensation technique, to characterise batches of methanol synthesis catalyst tablets made using different feed types. Despite the hierarchies of complexity of the porous pellets revealed by the computerised X-ray tomography and FIB-SEM images, the impact on mass transfer of controlled modifications to the void space, achieved through mercury porosimetry, could be modelled using a relatively simple random pore-bond network. The characteristic parameters for the model were obtained from a percolation theory-based analysis of the overcondensation data. A quantitative relationship was thereby obtained between the structural information contained within the overcondensation isotherms and the rate of mass uptake of gas into the porous pellets. This revealed the differential importance to mass transfer of particular sets of pores, associated with certain pellet structural features, and the impact on tortuosity of pellet fabrication parameters such as particle feed size.

Computational Design of Pore Structure in Heavy Paraffin Dehydrogenation Catalyst Pellets

Computational Design of Pore Structure in Heavy Paraffin Dehydrogenation Catalyst Pellets

Considerations when determining Counter-diffusion constants in liquid phase catalytic reactions using the Zero Length column (ZLC) method

The confusion with diffusion: In heterogeneous catalysis often, the catalytic material is housed within a porous support structure. It is important therefore to determine whether this support structure exerts limiting behaviors. In this paper, desorption and diffusion effects are separated under reactions conditions, to provide accurate effective diffusion constants in swelling and gate-opening catalysts. • A Zero Length Column (ZLC) method was developed using liquid chromatography. • Diffusion in porous catalyst pellets as small as 5 mg can be directly characterised. • Reactant counter-diffusion coefficients were determined for the Heck reaction. • Clear differences in diffusion rates for reactants versus non-reactant species. A Zero Length Column (ZLC) method was developed using a liquid chromatographic system to calculate reactant counter-diffusion coefficients in porous, solvent swollen catalytic pellets and gate-opening support materials. Reactant diffusivities within these porous materials were determined at the reaction conditions of 80 °C and 0.3–0.75 mL min −1 . For all materials, molecules of a similar size to the pore apertures, or molecules with strong interactions with the material surfaces, catalysts, were observable by the ZLC method, with both micro and macropore diffusion observed. Differences between Pd(II) and Pd(0) forms of the EnCat30 catalyst were examined to determine the effects of catalytic deactivation. The deactivated catalyst showed iodobenzene macroporous diffusion constants similar to the relatively inert toluene molecules. Finally, pulse field gradient NMR was utilised to compare and validate ZLC diffusion measurements for solvent swollen reaction systems. This study presents much needed novel findings on diffusivity data for liquid phase catalytic systems.

Radially layered configuration for improved performance of packed bed reactors

A particle-resolved computational fluid dynamics model is built and validated to investigate the coupled transfer processes and reactions in radially layered packed beds (RLPBs), with dry reforming of methane as a model reaction. RLPBs are 53.3–69.8% lower in pressure drop and 5.4–21.6 K higher in cold spot temperature than randomly packed beds (RPBs). The methane conversions for RLPBs are slightly lower than these for RPBs, but using smaller catalyst pellets in a RLPB can balance methane conversion and pressure drop. Besides, the diameter of separating wall in RLPBs can be adjusted to meet different requirements. The existence of separating wall in RLPBs makes catalyst pellets orderly distributed and then makes flow channels less tortuous, which leads to low flow resistances and good axial convective heat transfer. This work shows that RLPBs are advantageous in enhancing transfer processes and the results should serve to guide the optimal design of RLPBs.

3D printed co-precipitated Ni-Al CO2 methanation catalysts by Binder Jetting: Fabrication, characterization and test in a single pellet string reactor

The Binder Jetting 3D printing technique was used to fabricate spherical Ni-Al CO2 methanation catalysts from co-precipitated catalyst precursor powder. Catalysts with different nickel loadings were prepared by varying the molar Ni/Al ratio (1/1, 2/1, 3/1, 5/1) during precipitation or the amount of admixed precursor powder (10, 20, 30 wt.%) for printing. XRD and TG analysis revealed phase composition and reducibility characteristics analogous to conventionally prepared Ni-Al catalysts. TEM displayed nickel particle sizes as small as 3.9 nm with a Ni dispersion of up to 13.4 % determined by H2 chemisorption. Moreover, meso- and macroporous catalyst pellets were generated with a specific surface area of up to 192 m2∕gcat. The catalysts NiAl11–20, NiAl21–20 and NiAl51–20 were exemplarily tested in a single pellet string reactor where CO2 conversions of up to 99 % and a high selectivity towards CH4 was observed at ptot = 9 bar upon reaching thermodynamic equilibrium.

Probing deactivation by coking in catalyst pellets for dry reforming of methane using a pore network model

Dry reforming of methane (DRM) is an attractive technology for utilizing the greenhouse gases (CO2 and CH4) to produce syngas. However, the catalyst pellets for DRM are heavily plagued by deactivation by coking, which prevents this technology from commercialization. In this work, a pore network model is developed to probe the catalyst deactivation by coking in a Ni/Al2O3 catalyst pellet for DRM. The reaction conditions can significantly change the coking rate and then affect the catalyst deactivation. The catalyst lifetime is higher under lower temperature, pressure, and CH4/CO2 molar ratio, but the maximum coke content in a catalyst pellet is independent of these reaction conditions. The catalyst pellet with larger pore diameter, narrower pore size distribution and higher pore connectivity is more robust against catalyst deactivation by coking, as the pores in this pellet are more difficult to be plugged or inaccessible. The maximum coke content is also higher for narrower pore size distribution and higher pore connectivity, as the number of inaccessible pores is lower. Besides, the catalyst pellet radius only slightly affects the coke content, although the diffusion limitation increases with the pellet radius. These results should serve to guide the rational design of robust DRM catalyst pellets against deactivation by coking.

Continuum multiscale modeling of absorption processes in micro- and nanocatalysts

In this paper, we propose a novel, semi-analytic approach for the two-scale, computational modeling of concentration transport in packed bed reactors. Within the reactor, catalytic pellets are stacked, which alter the concentration evolution. Firstly, the considered experimental setup is discussed and a naive one-scale approach is presented. This one-scale model motivates, due to unphysical fitted values, to enrich the computational procedure by another scale. The computations on the second scale, here referred to as microscale, are based on a proper investigation of the diffusion process in the catalytic pellets from which, after continuum-consistent considerations, a sink term for the macroscopic advection–diffusion–reaction process can be identified. For the special case of a spherical catalyst pellet, the parabolic partial differential equation at the microscale can be reduced to a single ordinary differential equation in time through a semi-analytic approach. After the presentation of our model, we show results for its calibration against the macroscopic response of a simple standard mass transport experiment. Based thereon, the effective diffusion parameters of the catalyst pellets can be identified.

Fine-tuned fabrication parameters of CaO catalyst pellets for transesterification of palm oil to biodiesel

In recent years, the use of heterogeneous catalysts has gained increasing attention for biodiesel production. Among metal oxide catalysts, CaO powder is widely explored and considered to be a promising catalyst. However, the powder catalyst is inappropriate for continuous biodiesel production. Therefore, the catalyst pellet fabrication parameters related to its properties and biodiesel yield were investigated. Key results included the observation that catalysts produced using an Al2O3 binder possessed the highest catalytic activity compared to kaolin and diatomite supported catalysts, for calcination temperatures of 800 °C, possessing a comparable catalytic activity to the CaO powder. The CaO to Al2O3 mass ratio exhibited a significant effect on the surface area and total basicity. The highest biodiesel yield was obtained using the CaO-A(800)-3.5:1 catalyst due to the presence of several active crystal phases including CaO, Ca(OH)2 and CaSiO3 phases to accelerate the transesterification rate. These fabricated CaO catalyst pellets can be utilized as a catalyst for the continuous biodiesel production process.