Catalyst Particle Diameter Research Articles

Intra-particle diffusion limitation for steam methane reforming over a Ni-based catalyst

This work aims to evaluate the diffusion limitation for the steam methane reforming process over spherical catalyst particles (Ni/α-Al2O). Numerical calculations based on a CFD model were conducted to determine the intrinsic and actual reaction rates. The CFD model was validated using experimental results, and good agreement was achieved. The effectiveness factor for the reaction rates was used to express the effect of diffusion limitation within the porous catalyst particle. The effectiveness factor was determined for a wide range of operational parameters, including a temperature range of 800–1300 K and a steam-to-methane ratio of 1–5. The analysis was performed for spherical catalyst particles with diameters up to 3.42 mm, and the effectiveness factor as a function of catalyst particle diameter, temperature, and steam-to-methane ratio was established. The results show that the effectiveness factor is inversely proportional to the diameter of the catalyst particle and that an increase in reaction temperature and steam-to-methane ratio leads to an increase in diffusion limitation. The limitations of the analysis are discussed and justified. The final expression for the effectiveness factor can be used in a pseudo-homogeneous steam methane reformer model where the effectiveness factor varies depending on the position in the packed bed.

Modelling of Jatropha Oil Hydrocracking in a Trickle-Bed Reactor to Produce Green Fuel

Trickle-bed reactor (TBR) modelling to produce green fuel via hydrocracking of jatropha oil using silica-alumina-supported Ni-W catalysts was performed in this research. The objectives of this study are to obtain a TBR with good heat transfer and the optimum condition for high purities of products. A two-dimensional axisymmetric model with a diameter of 0.1 m and a length of 10 m was used as a representative of the actual TBR system. Heterogeneous phenomenological models were developed considering mass, energy, and momentum transfers. The optimisation was conducted to obtain the highest green fuel purity by varying catalyst particle diameter, inlet gas velocity, feed molar ratio, and inlet temperature. The simulation shows that a TBR with an aspect ratio of 100 has achieved a good heat transfer. The diesel purity reaches 44.22% at 420°C, kerosene purity reaches 21.39% at 500°C, and naphtha purity reaches 25.30% at 500°C. The optimum condition is reached at the catalyst diameter of 1 mm, the inlet gas velocity of 1 cm/s, the feed molar ratio of 105.5, and the inlet temperature at 500°C with the green fuel purity of 69.4%.

Delay time of gases's catalytic heterogeneous ignition on various sizes's catalyst particles

In this work, catalytic ignition delay time of combustible gas's small impurities in air on a spherical metal particle of various diameters is analytically determined by the example of gas-air mixtures's flameless combustion with hydrogen impurities on a platinum particle. It is shown that stable flameless combustion is observed after an induction period for particles of a certain range. It has been established that catalytic ignition time of gases is divided into three stages: 1. inert heating, the duration of which still depends on the combustible gas concentration; 2. the stage of self-acceleration and catalyst temperature rise during the course of the catalytic reaction in the transition region; 3. stage of diffusion inhibition and reaching stable catalytic combustion. The characteristic relaxation time was used in a dimensionless form. To determine the duration of the second stage, a modified Frank-Kamenetsky approach is applied. The duration of diffusion inhibition stage in the dimensionless form is practically independent of catalyst particle's diameter, although the catalytic combustion temperature decreases with an increase in the catalyst diameter. Heat transfer by radiation, the role of which increases with the growth of the catalyst size, is included in the effective heat transfer coefficient, which allows maintaining the classical ideology to solving the problem of the induction period.

Isobutyl acetate by reactive distillation. Part II. Kinetic study

This work deals with the kinetic modeling of the synthesis of isobutyl acetate by liquid-phase esterification of isobutanol with acetic acid, in presence of the heterogeneous catalyst Amberlyst 15. Based upon a statistical experimental design, isothermal batch experiments were performed in the temperature range of 343.15–373.15 K, at three different catalyst loadings, and at different acid to alcohol mole ratios (1:1, 2:1 and 1:2). Preliminary experiments were run to assess the effects of internal and external mass transfer phenomena, and they were ruled out based upon the Weisz–Prater criterion. Thus, further experiments were run at stirring rates of 800 RPM, and catalyst particle diameters under 300 μm to ensure a kinetic regime. Independent experiments were run to obtain the chemical equilibrium constant at different temperatures, and by mean of a van’t Hoff equation, a ΔHR of 16.47 kJ/mol was obtained. Incorporating the equilibrium constant, the kinetic experimental data were regressed with a molar fraction-based pseudo homogeneous model. The obtained model accurately fits experiments and it can be used for further simulation of reactive distillation processes for isobutyl acetate production.

Performance of a novel single-tubular ammonia-based reactor driven by concentrated solar power

Because of high concentrating ratios of Concentrated Solar Power Technologies (CSP), and maturity of industrial ammonia synthesis process, the high-temperature ammonia thermochemical energy storage based on CSP technology has a promising potential. As a prototype of a tubular ammonia reactor under un-uniform heat flux originating from a sunlight spot or line focusing, it is assumed that catalyst particles packed in it is isotropic, and the heat transfer and mass diffusion in the axial direction of the tube were ignored since the length-diameter aspect ratio of the tube is high. Consequently, employing the local thermal non-equilibrium method, a 3-dimensional model for an ammonia decomposition reaction in porous media was developed with semi-perimeter un-uniform heat flux. For further research, the diameter of catalyst particles, the porosity of packed particles and inflow modes were recognized as important factors. The results show that the particular axial or radial distributions of the porosity significantly affect the reaction characteristics. Especially, the distribution of radial linear reduction of porosity increases the ammonia conversion rate and reduced maximum wall temperature. At the same time, the smaller catalyst particles and the inflow mode of one-side in and one-side out all contribute to improving the performance of the tubular reactor. In general, the following measures can effectively improve the reactor performance: choose a smaller catalyst particle diameter, the porosity of catalyst particles packed decreases linearly along the radial direction from the center of the tube, and the inflow mode in the tube is one-side in and one-side out.

A fluidized-bed model for NiMgW-catalyzed CO2 methanation

The reduction of carbon dioxide to methane by hydrogen (“CO2 methanation”) using renewable energy is a promising process for recycling CO2. Better catalysts and better reactors are both required for the practical application of CO2 methanation. This study examines how the operating parameters affect CO2 methanation in a highly efficient fluidized-bed reactor. We first measured the kinetics of the CO2 methanation reaction using an NiMgW catalyst, which has been reported to exhibit superior catalytic performance. We then developed a fluidized-bed reactor model based on an earlier model for CO2 methanation. The fluidized bed model indicated that the NiMgW was indeed superior to two other previously studied catalysts in terms of faster conversion of reactants and higher concentrations of product CH4 throughout the reactor. The overall rate of production of CH4 increased with temperature and H2/CO2 ratio and decreased as the inlet reactant flow rate, catalyst particle diameter, and catalyst particle sphericity increased.

CFD Simulation of Porosity and Particle Diameter Influence on Wall-to-Bed Heat Transfer in Trickle Bed Reactors

Wall-to-bed (or wall-to-fluid) heat transfer issues in trickle bed reactors (TBR) has an important impact on operation and efficiency in this category of reactors. In this study, the hydrodynamic and thermal behavior of trickle bed reactors was simulated by means of computational fluid dynamics (CFD) technique. The multiphase behavior of trickle bed reactor was studied by the implementation of the Eulerian-Eulerian multiphase approach. Also, bed porosity effect was modeled by porosity function method. In order to study the effect of operating parameters on wall-to-bed heat transfer, the influence of catalyst particle diameter and catalytic bed porosity was investigated on wall-to-bed Nu number. The results showed that the enhancement of catalytic bed porosity from 0.36 to 0.5 decreases the Nu number about 15% due to a reduction of liquid velocity adjacent to the reactor wall. Also, the increase of particle diameter from 4 to 6 millimeter decreases wall-to-bed Nu number about 15% owing to a reduction in liquid phase volume fraction.

Surface Interaction Between Carbon Patches and Catalyst Nanoparticle as the Key Factor in Aligned Carbon Nanotube Growth Using Alcohol Catalytic CVD

Aligned carbon nanotubes (ACNTs) have very unique properties that made them applicable as electrode material for energy storage devices. ACNTs were grown directly on substrates by catalytic chemical vapor deposition (CVD). Cobalt, thermally-oxidized aluminum and ethanol were used as catalyst, catalyst-support and carbon feedstock, respectively. Optimum catalytic and reaction conditions were investigated by varying the CVD processing temperature and time. CVD processing temperature of 725[Formula: see text]C and processing time at 10[Formula: see text]min showed the optimum CNT alignment with thickness of above 2[Formula: see text][Formula: see text]m. Other than the effect of catalyst particle diameter, formation of amorphous carbon at the top of CNTs observed by SEM is suggested as another factor that influences the growth mechanism of ACNTs. The CVD processing parameter and the interaction between small carbon patches are supposed to be built after dehydrogenation of the first carbonaceous molecules and the catalyst nanoparticle surface considered as the hidden factors behind the growth results.

Temperature Dependence of Effective Thickness of Ni-GDC Anode for SOFC

ABSTRACTS Solid oxide fuel cell (SOFC) is highly efficient due to its high operating temperature (700 ˚C ~ 1000 ˚C) and therefore is expected as one of the major power generation systems in the near future. However, the high temperature restricts the material that can be used, and causes degradation of performance due to heat cycle. Therefore, it is required to lower the operating temperature, which leads to high overvoltage. To lower the overpotential, not only the material but also the microstructure of the electrode is important. The anode of SOFC is generally made from a so-called cermet, which is a composite of ceramic and metal. Ni-YSZ (yttria stabilized zirconia) has been widely used as cermet, but GDC (Gadolinium Doped Ceria), which shows higher ionic conductivity than YSZ at low temperatures, is recently been used[1]. We have reported the effects of anode thickness and particle diameter of Ni particles catalyst on electrode performance using Ni-GDC cermet anode. When the diameter of Ni particles was large, anode thickness dependence of overvoltage was larger than when its diameter was small, indicating an increase of active reaction zone [2]. However, the structure of the examined electrode was not uniform, because the number of times the electrode was sintered was different for the anode with different thickness. Therefore, in this study, the anodes were made by using uniaxial pressing to construct a uniform anode structure. EXPERIMENTAL Powders of NiO, Gd0.1Ce0.9O1.9 and pore forming agent were used to fabricate the anode. The powders were mixed and molded by hand press, and then the obtained pellet was sintered. GDC was used as the electrolyte, and Sm0.5Sr0.5CoO3 (SSC) was used as the cathode. The operating temperature of SOFC was 900 ˚C, 800 ˚C and 700 ˚C. The anode and the cathode were supplied with H2 and O2, respectively. Current density vs. voltage was measured using a galvanostat. RESULTS and DISCUSSIONS The performance of the cell with thick anode and thin anode were comparable at high temperature. In comparison, at low temperature, maximum current density of the cell with thicker anode was higher. The results indicate that the effective reaction zone was comparable at high temperature, irrespective of the anode thickness, and that the effective reaction zone of the thick anode was wider than the thin anode at low temperature. At high temperature, the resistance due to the reaction is expected to be relatively small, therefore, the reaction occured only in the vicinity of the electrolyte, and effective reaction zone was suppressed irrespective of the anode thickness. On the other hand, at low temperature, the resistance due to the reaction is relatively large, therefore, the entire electrode is effectively used. Thus, the thick anode showed lower overpotential at low temperature. REFERENCES [1] B. Zhu, Albimsson, C. Andersson, K. Borsand, M. Mellander, J. Electrochem. Soc., 97, (40), 391-399 (1997) [2] H. Fukunaga, T. Ohno, C. Arai, T. Takatsuka, K. Yamada, J. Electrochem.Soc.,7,(1),1527-1531 (2007)

Experimental studies and two-dimensional modelling of a packed bed bioreactor used for production of galacto-oligosaccharides (GOS) from milk whey.

In the present study, extensive experimental investigations and detailed theoretical analysis of a two-dimensional packed bed bioreactor, employed for the production of galacto-oligosaccharides (GOS) from milk whey were performed. Model equations, in one- and two-dimensions, capable of predicting the substrate concentration distribution in the bioreactor were developed by coupling mass balance equation with appropriate velocity distribution equation and solved numerically. Validation of the proposed model equations was done by a set of experimental data obtained from the bioreactor. The effects of reactor to catalyst particle diameter ratio (d t/d p), feed flowrate (10(-6)-10(-9)m(3)s(-1)), and initial lactose concentration (50-200kgm(-3)) on substrate concentration distribution were investigated in detail. While, the distribution of substrate concentration in axial direction was independent of d t/d p, it was observed that for d t/d p <40, significant radial concentration distribution existed. It was further observed that the substrate conversion and product yield obtained experimentally showed an excellent agreement (97±2%) with the results predicted by the two-dimensional model equation, whereas, the results predicted by the one-dimensional model equation did not lie within the desired confidence level (<90%). The results were confirmed by both curve fitting and statistical analysis. The prediction of substrate concentration distribution in axial and radial directions using the developed two-dimensional model equation is necessary for computing the bioreactor volume to achieve the desired GOS yield.

High-temperature ammonia oxidation over a platinum catalyst under conditions of the parallel formation of nitrogen-containing products

Experimental data on the effect of the temperature of air containing low ammonia concentrations on the temperature and rate of catalytic ammonia combustion over a platinum catalyst as a function of the NH3 concentration and catalyst particle diameter are analyzed with account taken of thermal radiation under the assumption that nitrogen and nitric oxide form in parallel. At relatively low air temperatures (near the combustion extinction point), the ammonia combustion temperature is determined by the nitrogen formation reaction (which releases a larger amount of heat and is characterized by a lower activation energy). At higher air temperatures and ammonia concentrations, the combustion temperature is determined by the nitric oxide formation reaction (which releases a smaller amount of heat and is characterized by a higher activation energy). The dependence of the critical temperature of the gas mixture on the catalyst particle diameter under conditions of the catalytic autoignition of the combustible gas and combustion extinction (at low ammonia concentrations) has been determined and analyzed for the case in which there is radiative heat transfer between the gas and the cold walls of the reactor.

Acetone hydrogenation in exothermic reactor of an isopropanol–acetone–hydrogen chemical heat pump: effect of intra-particle mass and heat transfer

Intra-particle mass and heat transfer plays an important role in performance of the exothermic fixed-bed reactor for an isopropanol–acetone–hydrogen chemical heat pump. In this work, an exothermic fixed-bed reactor model, taking into account the actual packing structure, is established in the commercial software Fluent. A 120° segment of a tube with tube-to-particle diameter ratio (n) of 4, where realistic particles are packed and set to porous media, is used to simulate the 3D external flow, concentration and temperature fields in the exothermic packed-bed reactor. The influence of catalyst particle diameter (d p) and micropore diameter (d 0) on the intra-particle temperature, species distribution, reaction rate and selectivity is discussed. The appropriate d p and d 0 are obtained. Simulation results showed that intra-particle temperature gradient is not obvious. Large d p and small d 0 lead to remarkable gradient of reaction rate inside the catalyst particle and the decrease in the catalyst efficiency and reduce the acetone conversion and the selectivity in isopropanol. The optimal results reveal that the spherical catalyst with d p of 1 mm and d pore of 10 nm is appropriate for high-temperature acetone hydrogenation.

The Simulation of an Industrial Fixed Bed Reactor for Methanol Dehydration to Dimethyl Ether

Compared to methane and methanol, dimethyl ether seems to be a superior candidate for high quality diesel fuel in the near future. To meet the continuously increasing global requirements, a larger scale reactor is needed to produce more dimethyl ether. A heterogeneous one-dimensional model was developed to simulate a staged adiabatic fixed bed reactor for the catalytic dehydration of methanol to dimethyl ether. To verify the proposed model, the simulated catalyst bed temperature has been compared to available data from an industrial reactor. A good agreement has been found between the simulation and plant data. Also, the effects of some operational parameters, such as feed flow rates, the inlet temperatures of methanol, pressure, and catalyst particle diameter on reactor behavior were investigated. The simulated results show that this model is reasonable and reliable.

Nano-Sized Titanium Oxides as Electrocatalyst for Oxygen Reduction Reaction toward PEFC

Platinum, which is used as cathode electrocatalysts for polymer electrolyte fuel cell, have many problems such as its high cost, small amount of resources, insufficient activity for oxygen reduction reaction (ORR) and instability. In this study, we focused on titanium which had larger amount of resources. Although we prepared titanium oxide-based cathodes by oxidation of titanium carbonitrides, the ORR activity was lower due to its large primary particle size. Therefore, we tried to prepare fine primary particles by oxidation of oxytitanium phthalocyanine in low p O2 atmosphere and fine secondary particles by planetary ball milling in dry process. We obtained the primary and secondary particle diameter of titanium oxide-based catalyst was 10 nm and ~50 mm, respectively. The ORR activity was 140 times larger than that of oxide-based compounds prepared from carbonitrides. This result indicated that titanium oxide-based compounds had great potential as alternative cathodes for platinum.

Cr as a key factor for direct synthesis of multi-walled carbon nanotubes on industrial alloys

Six kinds of industrial alloys, SUS316, FCH2, Invar, Permalloy, Inconel, and Nichrome, and three kinds of pure metals, Fe, Ni, and Cr, were used for substrates on which multi-walled carbon nanotubes (MWCNTs) were directly synthesized from ethylene. To activate their surfaces for the MWCNT formation, their surfaces were modified by two steps, (1) oxidation step in Ar–O2 mixture gas and (1) following reduction step in Ar–H2 mixture gas. It was discovered that Cr is a key component to realize the catalytic growth of MWCNTs on Fe and Ni emerged from the alloys. Using SUS316 to observe the influence of oxidation duration on MWCNT diameter, it was seen that increasing the oxidation duration resulted in the increase of the MWCNT diameter. This tendency can be explained by diffusion of Fe through Cr-rich layer, which causes the increase of catalyst particle diameters. The excess Fe diffusion through the Cr-rich layer resulted in the formation of a unique mushroom structure. These effects seen in the observation on the oxidation duration did not appear when reduction duration was prolonged because the Cr-rich layer could inhibit the Fe diffusion. Instead, the length of the MWCNTs can became maximum by employing an appropriate reduction duration.

Pore diffusion model to predict the kinetics of heterogeneous catalytic esterification of acetic acid and methanol

A novel pore diffusion model in combination with a homogeneous reaction in the bulk solution is proposed to model the kinetic behavior of conversion of acetic acid during its esterification with methanol using ion exchange resin catalyst. The model parameters are determined based on the experimental data of a batch reactor with reactant quantities in stoichiometric ratio. Experimental kinetic data of conversion of acetic acid is obtained for various ranges of parameters such as catalyst loading in g/cc of reactant mixture volume, temperature in the range of 323.15–353.15K, average diameter of catalyst particle varied from 425μm to 925μm and RPM of magnetic stirrer. A multi time scale model is adopted in updating the conversion of acetic acid in the bulk liquid where the measurement is carried out. Since the homogeneous part of catalytic esterification is slow reaction, a large time step is used for updating bulk concentration or conversion, whereas a small time step is used to find the concentration distribution and inward flux of acetic acid inside the catalyst particle in a quasi steady state mode. This multi time scale approach leads to utilizing only the initial time kinetics of reaction for predicting two reaction constants namely kf1, the forward reaction rate constant for homogeneous reaction in bulk solution and kf2, the forward reaction rate constant for reaction inside the catalyst particle. The equilibrium conversion data provides the reaction equilibrium constant. The model predicts the overall kinetics of heterogeneous esterification reaction accurately.

Degradation of Organophosphorus Pesticide in a Packed-Bed Plasma Reactor: Effects of Operating Parameters and Kinetics Study

The degradation effectiveness and reaction kinetics of representative organophosphorus (OP) pesticide in a packed-bed plasma reactor have been studied. Important parameters, including peak voltage, pulse frequency, gas-flow rate, initial concentration, diameter of catalyst particles, and thickness of catalyst bed which influences the removal efficiency, were investigated. Experimental results indicated that rogor removal efficiency as high as 80% can be achieved at 35 kV with the gas flow rate of 800 mL/min and initial concentration of 11.2 mg/m3.The removal efficiency increased with the increase of pulsed high voltage, and pulse frequency, the decrease of the diameter of catalyst particles and the thickness of catalyst bed. Finally, a model was established to predict the degradation of the rogor, which generally can simulate the experimental measurements to some degree.

In situ preparation of nickel/carbon core–shell structure by chemical vapor deposition

Nickel/carbon core–shell structure with uniform diameter has been synthesized by galvanostatic electrodeposition nickel on its surface followed by chemical vapor deposition. We proposed the growth mechanism of the core–shell structure that the precipitation of carbon from metal catalysts during the high temperature growth period lifts up metal particles leading to the formation of core–shell structure or carbon nanotubes with respect to the diameter of catalyst particles. The substrate with deposited nickel was characterized by optical microscope. The elements and features of the substrate were studied by energy dispersive X-ray spectroscopy and X-ray diffraction respectively. The morphology of the resulting material was examined by field emitting scanning electron microscopy. In addition, the electrochemical performance of the core–shell structure modified electrodes was also investigated. The result shows that electrodes modified with core structure have better electrochemical property than the bare electrodes in the [Fe(CN)6]3−/[Fe(CN)6]4− solution at a scan rate of 20mVs−1.

Analysis of External and Internal Mass Transfer at Low Reynolds Numbers in a Multiple-Slit Packed Bed Microstructured Reactor for Synthesis of Methanol from Syngas

The possibility of mass transfer limitations in an integrated micro packed bed reactor–heat exchanger (IMPBRHE) for methanol synthesis was experimentally investigated. Experiments were performed with three different particle size distributions (50–200 μm) of a Cu-based catalyst at 80 bar and 215–270 °C. Negligible effects of pore diffusion limitations on the performance of the reactor under methanol synthesis conditions for catalyst particle diameters up to 125 μm were found. Due to a very low Reynolds numbers (∼1) and dominance of molecular diffusion, variation of the total pressure was applied as a suitable technique to alter the diffusivities of reactants in the gas mixture by dilution, while keeping the reactant flow and partial pressure constant. No significant change in the CO conversion was observed in the temperature range 235–255 °C, pressure range 50–90 bar, and for reactant contact times of 105–308 ms·g/mL. The same procedure was applied to a laboratory fixed bed reactor with similar results. P...

Simulation and kinetic study of transesterification of triolein to biodiesel using modular reactors

A laboratory pilot unit has been developed to solve the issues of thermodynamic equilibrium shift for the transesterification of vegetable oil into biodiesel. It consists in a two stages continuous heterogeneous catalytic reactor operating at 50bar and 175°C. Under the working conditions, only one homogeneous phase can be assumed. The reactions of transesterification need a large excess of methanol implying an additional energy consumption and higher capital expenditure because of larger size of equipments, and operating pressure.In this work, a numerical model has been developed in order to better understand the different coupled phenomena (thermodynamic equilibrium, kinetics, mass transfer limitations, etc.) governing biodiesel production. The model is based on a heterogeneous plug flow reactor with axial dispersion taking into account mass transfer in the catalyst and dynamic aspects. The reactor is supposed isothermal since transesterification reactions are almost athermic.Kinetic parameters have been optimized by comparison with experimental results previously obtained by IFPEn. Simulation results are used to establish the future experimental conditions that will be used on the pilot unit. Mass transfer limitations are characterized by changing the diameter of catalyst particle. Concentration and reaction rate profiles are depicted in the fluid phase and in the catalyst. The simulations indicate that a conversion of triolein of 87% can be reached with a methanol to triolein molar ratio of 36 at reactor inlet and with a mean residence time of 1h.