Single-pellet Reactor Research Articles

Oxidative coupling of biogas to ethylene over a trilobe-shaped Mn-Na2WO4/α-Al2O3 catalyst in a single-pellet reactor

Despite the development of selective catalyst formulations for oxidative coupling of methane (OCM), kinetic and reactor design studies have been hampered by a lack of data for truly isothermal reaction conditions. Because of the high exothermicity, the temperature of a fixed-bed reactor filled with Mn-Na2WO4/SiO2 catalysts will show hot-spots with a loss of selectivity. The objective of this study is to measure catalyst activity under relevant reaction conditions of pressure (3 bar), biogas composition (no dilution) and contact time, while keeping the process isothermal. To this aim, we have synthesized and shaped a trilobed Mn-Na2WO4/α-Al2O3 catalyst, which may be used as is at industrial scale, and measured its performance in a single-pellet reactor. We found that the presence of CO2 did not degrade the catalyst performance, even on the long run, which should enable the use of a CO2-rich stream such as biogas. Actually, higher methane conversions have been obtained in the presence of CO2, likely due to a possible contribution of methane dry reforming reaction. We also noticed a slow catalyst activation over the first 100 h, while selectivity was kept constant.

Ethylene Hydrogenation in Pellets with Different Pore Structures, Measured in a One-Sided Single-Pellet Reactor

Abstract The ethylene hydrogenation reaction was investigated in a kinetic turbo reactor and a one-sided single-pellet reactor. An empirical kinetic expression was fitted to experimental results taken from the turbo reactor, and the gas compositions at the catalyst centers were measured for three different pore structures by means of the single-pellet reactor. A bimodal pore model was developed and applied to the computation of the gas composition profiles inside the three pore structures. The calculated results were compared to the measurements. A distinct influence of the pore structures on the gas fluxes and concentration profiles inside the pores could be detected which demonstrates that the proper choice of the pellet pore structure is of importance for a high conversion.

Effects of Reduced Bed Temperature in Laboratory- and Full-Scale Fluidized-Bed Boilers: Particle, Deposit, and Ash Chemistry

This study focuses on how the temperature of the bed in a waste-fired fluidized-bed boiler affects the chemical composition of ash and deposits. The aim was to reduce the concentration of corrosive elements in the convection pass, which can lead to both less frequent soot blowing intervals and extended superheater lifetimes. Complementary laboratory-scale tests were carried out in a single-pellet reactor to study online alkali and Zn emissions during temperature changes, using an ICP-MS instrument. The full-scale study was based on full-scale experiments at a plant consisting of two 20 MWth fluidized-bed boilers firing a mixture of municipal solid waste and industrial waste. The boilers are normally operated at bed temperatures of about 870 °C. This normal operation was compared in this work to a test case in which the bed temperature was reduced below 720 °C by altering the air staging and flue gas recycling. The experimental work included collecting samples of fuel, ash, and particles under the two diff...

Comparison of external mass transfer approaches for heterogeneously catalyzed hydrogenation of vaporized hydrocarbons

The catalytic hydrogenation of vaporized α-methylstyrene (AMS) and 1-octene under external transfer control is tested by different interphase mass transfer approaches. The previously reported experimental data on temperature measurements in the single-pellet reactor and the new experimental data in a fixed-bed reactor are analyzed theoretically. The independent diffusion (Fick) approach absolutely failed to fit the experiments. Three methods that take into account multicomponent diffusion (and are based on the Maxwell–Stefan equations) give close results and agree well with the experimental data. Nevertheless, for hydrogenation (or dehydrogenation) of vaporized hydrocarbons the pseudo-binary approach considered in this study is recommended. It is very simple to use even in a case of complex mixture of hydrocarbons and/or several hydrogenation reactions, when general approaches demanded cumbersome matrix operations. Another more fundamental advantage of the pseudo-binary treatment is that the usage of traditional empirical correlations Sh( Re, Sc), Nu( Re, Pr) for gas–solid transfer (which, as a rule, are based on the Chilton–Colburn analogy) gets the solid theoretical background.

Simultaneous FTIR/UV-Vis study of reactions over metallo-zeolites: Approach to quantitative in situ studies

A new quantitative approach to in situ experiments is proposed based on consistent combination of modeling of the relevant processes in the catalyst pellet with realization of the multi-spectroscopic experiment under dynamic regimes. Using well defined Co- and Fe-FER samples the approach is illustrated by three case studies showing potential of the approach for detail analysis of the processes over metallo-zeolites, i.e. adsorption of NH 3 on CoNa-FER, and reactions of selective catalytic reduction of NO by NH 3, and N 2O decomposition over Fe-FER. Combination of simultaneous in situ FTIR and UV-Vis spectroscopies under transient conditions and a single-pellet reactor model of the spectroscopic cell are used for obtaining some quantitative information on the transport, adsorption/desorption phenomena, and surface species transformation over transition metal ion/zeolite system under such conditions.

In situ MRI of the structure and function of multiphase catalytic reactors

NMR imaging (MRI) was used to study the distribution of the liquid phase in an operating trickle bed reactor using hydrogenation of α-methylstyrene or n-octene-1 as representative examples. In a single pellet reactor, the existence of oscillating regimes under unchanged external conditions was shown. The experiments with packed beds have demonstrated the non-uniform distribution of the liquid phase over the bed, the presence of partially liquid-filled or completely dry catalyst particles in the operating reactor, and the existence of liquid phase transport between liquid-filled and dry catalyst particles. Detection of spatially resolved NMR spectra was used to characterize chemical conversion variations within the operating reactor. Preliminary MRI results for an operating monolithic reactor were obtained. It was found that MRI can be used to directly image solid materials using NMR signal detection of nuclei other than 1H. In particular, imaging of alumina using 27Al NMR signal appears highly promising for the development of novel MRI applications in chemical engineering and catalysis, including spatially resolved NMR thermometry.

Spatiotemporal patterns within a porous catalyst: dynamic carbon monoxide oxidation in a single-pellet reactor

Competitive adsorption and reactant inhibition are expected to couple with diffusion resistance to form complex patterns within porous adsorbents and catalysts, especially under dynamic conditions. CO oxidation over Pt/Al 2O 3, which exhibits these surface processes, was studied in this work. A miniaturized single-pellet reactor was used to continuously and directly measure gas concentration gradients within a porous pellet as feed composition was varied. Responses of nonadsorbing species were analyzed in order to determine diffusion coefficients. Single-species responses provided starting estimates of CO adsorption parameters. Reaction experiments were performed at 398– 448 K under maximum CO and O 2 pressures of 16 and 110 Pa , respectively. An elementary-step diffusion–reaction model was used to simulate experimental results. A bimodal distribution of active sites was proposed in order to explain the transient responses. Analysis showed that the bimodal distribution could not have been detected by steady-state experiments. The intrapellet spatiotemporal patterns predicted by the model, and confirmed by comparison to measurements of concentration gradients across the pellet, provide understanding of the dynamic coupling between diffusion, adsorption, and reaction within the pellet. Methods are discussed to reduce computational requirements, including assumption of quasi-equilibrium adsorption, and reduction of the surface-to-gas capacity ratio.

Effects of 1,2 Dichloroethane Addition on the Optimal Silver Catalyst Distribution in Pellets for Epoxidation of Ethylene

The issue of optimal catalyst distribution was studied experimentally for the ethylene epoxidation reaction network on a Ag/α-Al2O3catalyst in the presence of 1,2-dichloroethane (DCE). For a fixed amount of silver, the influence of location and width of the catalytic layer on the conversion of ethylene, and on the selectivity and yield to ethylene oxide, was investigated for different concentrations of DCE (0–2 ppm) using a single-pellet reactor. Two levels of ethane in the feed (0 and 0.5 vol%) and a temperature range between 210 and 270°C were studied. Depending on the experimental conditions, promotion, inhibition, and poisoning effects of DCE were observed. The catalyst distribution and the concentrations of DCE and ethane in the feed, which provide high selectivity and yield, were identified. XPS analysis was also conducted to determine the state and composition of the silver catalyst, both prior and subsequent to reaction.

Single pellet reactor for the dynamic analysis of gas-solid reactions “reaction of SO 2 with activated soda”

Single-pellet moment technique which was initially proposed for the measurement of effective diffusivities in porous catalysts and adsorption rate and equilibrium constants was modified for the analysis of gas-solid reactions. In this technique dynamic version of the Wicke-Kallenbach single-pellet cell was used. A pulse of a tracer gas was introduced into stream flowing over one end face of the pellet. In the modified pulse-double response procedure, experimental moments of the response peaks of both reactant and product gases measured in both top and bottom outlet streams of the single-pellet reactor were used for the evaluation of intrapellet rate and equilibrium parameters. Application of the technique to the reaction of SO 2 with activated soda indicated that the value of Thiele modulus decreased from 6.74 to 0.33 by the increase of fractional conversion of Na 2CO 3 to Na 2SO 3 from zero to 0.63. In this range of conversion values a significant variation in pore structure and also a decrease of effective diffusivity of SO 2 from 0.045 cm 2/s to 0.025 cm 2/s was observed. A cell model which considered the changes in pore length and radius with reaction extent and variations in product layer diffusion resistance was shown to be successfully used in the analysis of experimental observations. A review of single-pellet dynamic reactor studies for catalytic and noncatalytic gas-solid reactions was also given.

ACTIVITY AND SELECTIVITY VARIATIONS IN THE OXIDATIVE DEHYDROGENATION OF ETHANE OVER A MOLYBDENUM - VANADIUM - NIOBIUM MIXED OXIDE CATALYST

A single pellet reactor was used to investigate the activity and selectivity variations with time in the oxidative dehydrogenation of ethane with a mixed oxide catalyst having a composition of Mo0.71V0.2Nb0.09. Experiments conducted at 450°C in the absence of oxygen showed rapid deactivation. In these experiments very high selectivities, reaching to unity, were observed at fractional conversions of 0.1 or less. An increase in conversion and a decrease in selectivity was observed with a decrease in catalyst porosity and also with an increase in pellet length, indicating the significance of diffusion time. This effect is found to be especially significant at the initial stages of reaction (fresh catalyst). In the absence of oxygen a large amount of CO was produced during the first couple of minutes of the reaction. On the other hand, in the presence of oxygen, the major side product is CO2. It was also concluded that gas phase oxidation of ethane to CO2 is not significant.

Catalyst pellet with unsymmetrical shell-progressive deactivation

Activity factors that delineate the effect of shell-progressive, independent deactivation on global rate are given for slab-like pellets with unequal surface concentrations for both reactor and poisoning species. It is shown that an increase in the global rate can be achieved over the entire lifetime of the catalyst by properly selecting the surface concentration ratio. Use of the asymmetry in single-pellet reactor arrangement should provide more information on catalyst deactivation. The activity factors for partially wetted catalyst pellets, typical in trickle-bed reactors, are also given.

Estimation of the catalyst pellet activity distribution from experimental kinetic data

In this paper a method of catalyst pellet activity distribution estimation from experimental kinetic data is described. Measurements of outlet concentration from a laboratory continuous stirred single-pellet reactor vs feed rate for ethylene hydrogenation on Pt/alumina pellet are used as kinetic data. To find the best estimate the gradient method is employed and the gradient is computed with the help of the adjoint equation. The estimated activity distribution is compared with the Pt distribution obtained by the scanning electron microscope.

Determination of binary gas diffusion coefficients in spherical porous media by steady and unsteady-state analysis of single pellet reactor data

A mathematical model that describes both the transient and steady-state response of a single spherical porous catalyst pellet to a step input in concentration of a dilute binary gas mixture is presented. The model equations, which contain the effective diffusion coefficient of the dilute binary as the only parameter, are reduced to a set of triple-series equations that are solved using the least square method of weighted residuals. To test the theory, an experimental apparatus was constructed and used to collect both transient response and steady-state data for a helium-nitrogen binary gas pair. Effective diffusion coefficients determined from these measurements using time-domain and Laplace transform domain parameter estimation are shown to be within the correct order-of-magnitude and agree within 25% of each other. Some possible extensions of the model are also suggested.

THE ROLE OF LATTICE OXYGEN IN THE DYNAMIC BEHAVIOR OF OXIDE CATALYSTS

The lattice of an oxide catalyst used for oxidation reactions can act as a reservoir for oxygen, storing and releasing it for reactions at the catalyst surface under appropriate conditions. The implication of this oxygen storage property of an oxide catalyst on its dynamic response characteristics has been investigated through an experimental study of 2-butene oxidation over vanadium oxide as a model reaction. Isothermal reaction rate measurements in a differential reactor and nonisothermal studies in a single pellet reactor have been carried out. Following a step increase in the feed butene concentration, isothermal reaction rate overshoot and pellet temperature overshoot were observed. These observations could be modelled in a qualitatively correct way by a very simple model accounting for the participation of lattice oxygen in the catalytic reactions under dynamic conditions. It is demonstrated through model simulations that the ignition characteristics of a catalyst pellet are significantly affected b...

A novel reactor system that permits the direct determination of deactivation kinetics for a heterogeneous catalyst

This reactor system consists of a differential reactor and a single-pellet reactor both with external recycle of the inlet feed. The design and operation of these two reactors permit direct determination of the deactivation kinetics for a heterogeneous catalyst and a better value for the initial reaction rate. Direct determination of deactivation kinetics is not possible in systems operating at constant space velocity because the reaction mixture composition varies with time. For the system presented in this paper deactivation data are obtained at constant reaction mixture composition by varying the space velocity to compensate for catalyst deactivation. Feed rates, recycle rates, and reactor system volumes in our system are designed to minimize system response time which for this system is on the order of a few seconds. To demonstrate the operation of this novel reactor system, the fouling of a platinum catalyst was studied using a model reaction.

Gas-solid reactions: Experimental evaluation of the zone model

Zinc sulfide pellets sintered at 1200°C, 1000°C, 800°C and 600°C were partially oxidized in a single pellet reactor. Sulfur profiles of these samples were measured using Auger electron spectroscopy (AES) and electron probe microanalysis (EPMA). These profiles showed the coexistence of three zones: ash layer, reaction zone, and unreacted core. The zone thickness was found to be approximately the same for samples of comparable porosities. The effective diffusivities of sintered zinc sulfide pellets calculated by using the values of zone thickness measured in the present study compare well with the literature values reported from kinetic studies.

An analytical solution for the transient response in a diffusion cell of the Wicke-Kallenbach type

An analytical solution is developed for the problem of transient diffusion and first order reaction in a solid flanked by two well mixed fluid compartments. This solution is necessary and useful in describing unsteady-state measurements in a Wicke-Kallenbach diffusion cell or reaction studies in a single pellet reactor. It is shown that a proper choice of an inner product vector space, following the methodology developed by Ramkrishna and Amundson [14, 15], leads readily to the desired solution and guarantees its completeness. This solution is valid for the equivalent heat transfer problem also.

Oscillatory phenomena during ammonia oxidation in the presence of a PtRh catalyst

The paper describes oscillatory phenomena observed during ammonia oxidation on a PtRh catalyst in a single pellet reactor. The phenomena consist in periodical changes of the catalyst temperature and a periodical initiation of the reaction in the gas phase. They are interpreted via coupling of the catalytic dissociation and homogeneous oxidation of ammonia.

Experimental study of the poisoning of a single catalyst pellet in a diffusion reactor

Catalyst poisoning experiments were carried out in an isothermal single-pellet reactor under conditions where both kinetic and diffusion processes contribute to the overall reaction rate (Thiele parameter between about 0·5 and 5). The concentration of the reacting species was monitored at the pellet's external surface and at its center plane. Impurity poisoning and parallel, series and triangular self-poisoning processes could be qualitatively discriminate by a sequence of three experiments, once the order of the main reaction was established. It was found that the hydrogenolysis of cyclopropane to propane between 50 and 75°C over an ν-Al 2O 3 supported Pt catalyst is simultaneously poisoned by the reactant cyclopropane and the product propane (triangular self-poisoning). Quantitative values of the poisoning kinetic exponents were evaluated by decoupling the time variable from the poisoning process, using relative overall rate versus normalized center-plane concentration data. Superimposing time on the results (center-plane concentration versus time and relative overall rate versus time data) detected an initial time-delay in the poisoning process. This time-delay was shown to depend upon the hydrogen-pretreatment of the catalyst. Conventional bulk rate-time studies would have been seriously hampered by the complicated time-behavior of the system, while the approach presented in this paper allowed the resolution of the mechanism and kinetics of the poisoning process.

An Improved Single-Pellet Reactor to Study the Interaction of Kinetics with Mass Transfer Effects in Heterogeneous Catalysis

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTAn Improved Single-Pellet Reactor to Study the Interaction of Kinetics with Mass Transfer Effects in Heterogeneous CatalysisL. Louis Hegedus and Eugene E. PetersenCite this: Ind. Eng. Chem. Fundamen. 1972, 11, 4, 579–584Publication Date (Print):November 1, 1972Publication History Published online1 May 2002Published inissue 1 November 1972https://doi.org/10.1021/i160044a025RIGHTS & PERMISSIONSArticle Views158Altmetric-Citations15LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (616 KB) Get e-Alerts