Monolithic Reactor Research Articles

Dynamic hysteresis in monolith reactors and hysteresis effects during co-oxidation of CO and [formula omitted

In many laboratory catalytic reactors light-off curves describing the exit conversion versus inlet gas temperature do not follow the same path during ramp up and ramp down and this leads to dynamic hysteresis. This dynamic hysteresis, which may occur even under ideal conditions (e.g. plug flow and negligible heat effects) is fundamentally different from the steady-state hysteresis which is due to heat effects and thermal feedback. We present expressions for the width of the dynamic hysteresis loop as a function of the ramp rate, solid to gas heat capacity ratio, space time and the heat Peclet number, for the limiting cases of pseudo-homogeneous and two phase plug flow models with negligible heat effects (ΔTad=0). We review the conditions under which steady-state hysteresis can exist for a finite ΔTad and demonstrate the combined impact of steady-state and dynamic hysteresis effects. We study the light-off behavior and dynamic hysteresis during the co-oxidation of CO+C2H6 mixture on Pt/Al2O3. It is found that 1% of CO alone ignites at 473K whereas 500ppm of C2H6 alone ignites at 723K. The exotherm generated by the CO oxidation can reduce the ignition temperature of C2H6 during the co-oxidation of CO and C2H6. Bifurcation diagrams of solid exit temperature versus fluid inlet temperature for different inlet compositions illustrate the simultaneous as well as separate ignitions for CO and C2H6. Each 1% increase in the CO feed mole percentage decreases C2H6 ignition temperature by about 80°C. A feed of 3% CO is required for simultaneous ignition of a mixture of CO and 500ppm ofC2H6.

Microfibrous entrapped hybrid iron-based catalysts for Fischer–Tropsch synthesis

Fischer–Tropsch synthesis (FTS) is a highly exothermic reaction, and a FTS reactor must have an efficient heat transfer mechanism in order to maintain a stable reaction temperature. Copper microfibrous entrapped catalyst (Cu MFEC) has demonstrated an excellent intra-bed heat transfer ability for FTS thermal management. The porous structure of Cu MFEC can entrap any pre-manufactured catalyst particles, while its counterpart approaches such as metallic foams or monolith reactor structures are typically based on washcoating process and require the details about catalyst formulation and preparation procedure to proceed. In this investigation, a highly active and stable iron-based FTS (Fe-FTS) catalyst, i.e. iron supported on γ-alumina and promoted with copper and potassium, has been developed. The apparent reaction rate constant of Fe-FTS and its productivity at 27.7% of CO conversion are 167mmol (H2+CO)/gcat/h and 0.30gC5+/gcat/h, respectively, which are comparable to the most active unsupported Fe-based catalyst. When Fe-FTS was physically mixed with equal mass of mesoporous aluminosilicate (MAS) loaded with 0.3wt% Pt, a hybrid catalyst was formed. This hybrid catalyst demonstrated an extremely low CO2 selectivity (27.7 C%) and a very high selectivity (38.1 C%) to liquid products (C5–C20). The result suggested that FTS and hydrocracking process simultaneously took place within a single reactor. Furthermore, FTS was carried out in a larger tubular reactor (34.0mm I.D.) packed with Cu MFEC. It demonstrated a radial temperature gradient less than 5°C and similar reactivity and product selectivity as those obtained in a small FTS reactor (9.5mm I.D.). While with the same catalyst loading density and the same test conditions, the comparative packed bed (34.0mm I.D.) reached radial temperature gradient around 54°C. It was also found that the FTS based Cu MFEC required negligible amount of time to reach its steady state compared with the packed bed. With these attributes and advantages, Cu MFEC approach is a promising alternative to the traditional packed bed for exothermic reactions such as FTS.

Dynamic photocatalytic reduction of CO2 to CO in a honeycomb monolith reactor loaded with Cu and N doped TiO2 nanocatalysts

Cordierite honeycomb monoliths loaded with N/TiO2 and Cu/TiO2 nanocatalysts for dynamic photocatalytic CO2 reduction with H2 to CO in a continuous photoreactor illuminated with UV-light irradiations have been investigated. The nanocatalysts, loaded over the monoliths channels using sol-gel dip-coating method, were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), N2 adsorption-desorption, X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance and photoluminescence (PL) analysis. Crystalline and anatase TiO2 structure with nanoparticles evenly supported over the cordierite monolith channels were observed. Cu and N presented over TiO2, shifted band gap energy towards visible region and hindered charges recombination rate. Loading Cu and N greatly improved TiO2 photoactivity for dynamic CO2 reduction to CO. Due to high photoactivity and selectivity, Cu/TiO2 assisted system yielded 14 times higher CO than the N/TiO2 and 64 times the amount of copper observed over pure TiO2 in a continuous operation of photoreactor. This significant improvement in Cu/TiO2 activity was noticeable due to efficient trapping and transport of electrons by Cu-metal. With unique properties, N/TiO2 showed good activity for continuous CO2 reduction to CH4. In addition, a photocatalytic reaction mechanism is proposed to understand the experimental results over Cu and N modified TiO2 catalysts in a continuous operation of photoreactor.

Ethylene epoxidation in microwave heated structured reactors

In the present work we show the microwave-induced heating of monolithic reactors containing a thin-layered catalyst that exhibits a strong and selective heating susceptibility under microwave irradiation. The combination of microwave radiation and structured reactors has been successfully applied for the intensification of the selective oxidation of ethylene to ethylene oxide (epoxidation) while operating at lower power consumptions and with higher energy efficiencies than in conventional heating conditions. The microwave radiation selectively heats the catalyst and the monolith walls while maintaining a relatively colder gas stream thereby creating a gas/solid temperature gradient of up to ∼70°C at a reaction temperature of 225°C. Moreover, the influence of different parameters such as the distribution of the catalyst onto the structured monoliths or the temperature measurement techniques employed to determine the heating profiles (Optic Fibers and/or IR thermography) have been also thoroughly evaluated to justify the obtained catalytic results.

Effective Diffusivity Measurement on a Monolithic Reactor using SPACI-MS Technique

Measuring Effective Diffusivity in the pores of the catalytic washcoat of a monolith reactor is a challenging task. In this work, Spaci-MS technique has been used to measure effective diffusivity in the porous washcoat, and through the porous walls of the cordierite monolith substrate. A mixture of a tracer gas (in this case O2) was introduced in a center (feed) channel along with a carrier gas, and the axial profile of tracer gas was measured in the neighboring channels. The axial profile of the tracer gas in the neighboring channels depends on the diffusivities in the gas phase, in the washcoat layer and through the substrate wall. A mass balance was made around the 1-adjacent channel to compare the calculated tracer profile with the measured profile, and used to get the value of effective diffusivity. Experiments were done on an uncoated (blank, substrate only) monolith to measure the effective diffusivity in the substrate, and then experiments were repeated for a washcoated monolith to measure the effective diffusivity in the catalytic washcoat layer. The experiments were also repeated at different temperatures to get the temperature dependence of the observed effective diffusivity. The results showed that the monolith substrate was in the bulk diffusion region and the washcoat exhibited Knudsen diffusion. Effective diffusivity of O2 in substrate was calculated to be 9.28×10−6m2/s and in washcoat to be 2.2×10−6m2/s at 100°C.

Ultrafine palladium nanoparticles immobilized into poly(4-vinylpyridine)-based porous monolith for continuous-flow Mizoroki–Heck reaction

Ultrafine Pd nanoparticles (dm=2.3nm), obtained by metal vapor synthesis technique, were immobilized into a poly(4-vinylpyridine)-based porous monolith by means of a new synthetic approach. The synthesis involves stabilization of Pd nanoparticles with 4-vinylpyridine ligand and their subsequent immobilization into the monolith by radical co-polymerization of the resulting metal-embedding monomer with ethylene glycol dimethacrylate in presence of porogenic agents (i.e. DMF and PEG-400) inside stainless-steel columns (HPLC type). The hybrid monolithic reactors containing highly dispersed Pd nanoparticles were effectively used as catalyst for Mizoroki–Heck cross-coupling reactions carried out under continuous-flow conditions. The devices showed long life-time (>65h) and very low Pd leaching (<2ppm).

Structuring hybrid palladium nanoparticles in metallic monolithic reactors for continuous-flow three-phase alkyne hydrogenation

Ligand-modified palladium nanoparticles are stabilised in the microchannels of an alumina-coated metallic monolith and advantageously used in the continuous-flow three-phase semi-hydrogenation of acetylenic compounds.

Mass transfer limitations in a monolithic reactor for the catalytic oxidation of ethanol

A theoretical study of a monolithic reactor for the catalytic oxidation of Volatile Organic Compounds (VOC) over manganese–copper mixed oxide catalysts is presented. An isothermal one-dimensional heterogeneous model is selected to account for the external (gas–solid) and internal (washcoat) mass transport limitations.The results reveal that under certain operating conditions, e.g. thick washcoats and high conversions of VOCs (high temperatures), the overall process rate is reduced by internal diffusion inside the catalyst. At high temperatures and/or monoliths with poor interfacial areas, the overall process rate is limited by external mass transfer resistance to the catalyst. Proper quantification of both internal and external mass transfer resistances by means of a heterogeneous model can result in avoiding situations of incomplete VOC abatement under conditions of high VOC dilution.

Nitrogen-doped-TiO2 nanocatalyst for selective photocatalytic CO2 reduction to fuels in a monolith reactor

In this study, photocatalytic CO2 reduction with H2 over nitrogen (N)-doped TiO2 nanocatalyst in a monolith photoreactor has been investigated. The N-doped TiO2 nanocatalyst was synthesized by sol-gel method, dip-coated over the monolith channels, and characterized by XRD, SEM and N2 adsorption-desorption. Highly crystalline and anatase phase TiO2 was produced in the N-doped TiO2 samples with increased surface area and reduced crystallite size. The N-doped TiO2 nanocatalyst demonstrated excellent photoactivity for selective CO2 reduction to CO in a continuous monolith photoreactor. The 3 wt. % N-doped TiO2 was found to be the most optimal, giving maximum CO yield rate of 56.30 µmole g-catal.-1 h-1 with selectivity of 96.3% at CO2/H2 feed ratio 1 and feed flow rate 20 mL/min. The performance of monolithic N-doped TiO2 nanocatalyst for selective and continuous CO production was 4.7 fold higher than un-doped TiO2. The significantly enhanced TiO2 activity was evidently due to hindered charges recombination rate due to N2 doping. The N-doped TO2 gave prolonged stability for continuous CO and CH4 production over the irradiation time.

Modeling of Square Channel Reactor for Optimum Length Prediction

The optimal dimensions of square channel reactor are basically information when microreactor will be assigned. To obtain good geometries of channel reactors, general mathematic models for optimum length prediction should be developed before performing details design and optimization. In this context, analytical models for the square channel of monolith reactor were presented. The models were developed following the work done by Francois Mathieu-Potvin and Louis Gosselin in order to predict an optimal length of the square channel of monolith reactor resulting in the maximum reaction rate. The models were constructed based on fully developed laminar flow, isothermal and gas phase system. The steam methane reforming reaction was used as a case study for estimation of the reactor length. The results obtained from this analytical technique had been compared with that obtained by Computational Fluid Dynamic using FEM software (COMSOL Multiphysic version 3.5a) and experimental works.

ChemInform Abstract: Nanocatalysis in Flow

AbstractReview: [110 refs.

Impact of heat and mass dispersion and thermal effects on the scale-up of monolith reactors

Monolith reactors used in catalytic after-treatment systems (e.g. TWC, DOC, LNT and SCR) are often studied in laboratory experiments using monolith samples of the same cell density, washcoat thickness, and catalyst loading but with a smaller length compared to the full-scale reactor. The flow rate in the laboratory scale operation is selected so that the same space velocity is maintained as in the full-scale system. However, matching space velocity between the laboratory-scale and full-scale reactor leads to similar performance only under ideal conditions such as isothermal or adiabatic operation with negligible heat and mass dispersion (i.e. plug flow conditions) and small adiabatic temperature rise. In this work, it is shown that when thermal effects are significant (or adiabatic temperature rise is large) and heat and mass transfer and dispersion effects are not negligible, similarity may not exist and the ignition/extinction behavior of the two systems can be qualitatively different. Further, when laboratory-scale data is used for kinetic parameter estimation, failure to consider these effects in the reactor model can lead to false kinetic parameters and inaccurate prediction of reactor performances. Another major reason for possible discrepancy between laboratory-scale and full-scale reactors, external heat exchange with furnace (surroundings), is also investigated. We present analysis, simulations, and analytical results quantifying the impact of these effects on the scale-up of monolith reactors.

Monolithic Honeycombs in Loop Reactor Configuration for Intensification of Multiphase Processes

AbstractMonolithic honeycomb reactors are characterized by small parallel channels, which make them an interesting approach to be applied as microstructured reactors. Besides the established manufacturing of ceramic honeycombs, these structures exhibit excellent mass transfer characteristics for multiphase reactions combined with degrees of freedom in the structural design. The potential of monolithic honeycomb reactors operated in the beneficial slug flow regime is evaluated for heterogeneously catalyzed gas‐liquid reactions. Results of the Fischer‐Tropsch synthesis and hydrogenation of glucose to sorbitol were analyzed in order to obtain a more generalized picture of the interaction between mass transfer and reaction in small channels operated in multiphase flow.

A one-step preparation method of monolithic enzyme reactor for highly efficient sample preparation coupled to mass spectrometry-based proteomics studies

Mass spectrometry (MS) coupled to sample preparation and separation techniques has become a primary tool for proteomics studies. However, due to sample complexity, it is often challenging to achieve fast and efficient sample preparation prior to MS analysis. In recent decades, monolithic materials have been developed not only as chromatographic media, but also as efficient solid supports for immobilizing multiple types of affinity reagents. Herein, the N-acryloxysuccinimide-co-acrylamide-co-N,N′-methylenebisacrylamide (NAS-AAm-Bis) monolith was fabricated within silanized 200μm i.d. fused-silica capillaries and was used as an immobilized enzyme reactor (IMER). The column was conjugated with trypsin/Lys-C and Lys-N enzymes to allow enzymatic digestions to occur while protein mixture was loaded onto the IMER column followed by MS-based proteomics analysis. Similar MS signal and protein sequence coverage were observed using protein standard bovine serum albumin (BSA) compared to in-solution digestion. Furthermore, mouse serum, yeast, and human cell lysate samples were also subjected to enzymatic digestion by both IMER (in seconds to minutes) and conventional in solution digestion (overnight) for comparison in large-scale proteomics studies. Comparable protein identification results obtained by the two methods highlighted the potential of employing NAS-based IMER column for fast and highly efficient sample preparation for MS analysis in proteomics studies.

Nanocatalysis in Flow.

Nanocatalysis in flow is catalysis by metallic nanoparticles (NPs; 1-50 nm) performed in microstructured reactors. These catalytic processes make use of the enhanced catalytic activity and selectivity of NPs and fulfill the requirements of green chemistry. Anchoring catalytically active metal NPs within a microfluidic reactor enhances the reagent/catalyst interaction, while avoiding diffusion limitations experienced in classical approaches. Different strategies for supporting NPs are reviewed herein, namely, packed-bed reactors, monolithic flow-through reactors, wall catalysts, and a selection of novel approaches (NPs embedded on nanotubes, nanowires, catalytic membranes, and magnetic NPs). Through a number of catalytic reactions, such as hydrogenations, oxidations, and cross-coupling reactions, the advantages and possible drawbacks of each approach are illustrated.

Investigation of Coke Formation in Dry Methane Reforming over Nickel-based Monolithic Catalysts

Coking accumulations via dry methane reforming (DMR) over 10NAM monolithic catalyst and pelletized catalyst was investigated. 10NAM catalyst was synthesized and coated on a wall of monolithic reactor. Pelletized catalyst of 10NAM was also prepared for the comparison. Consequently, catalyst was characterized by BET, <TEX>$H_2-TPR$</TEX> and <TEX>$H_2-TPD$</TEX>. The catalytic reaction was undergone at <TEX>$600^{\circ}C$</TEX> under atmospheric pressure and <TEX>$CH_4$</TEX> to <TEX>$CO_2$</TEX> reactant ratio of 1:2. The coking formation over spent catalyst was then carried out in the hydrogen flow using temperature programmed technique (TPH). According to the results, DMR over 10NAM monolithic catalyst exhibits a minimized coking formation comparing to the use of pelletized catalyst. This could be attributed to a prominent heat transfer efficiency of the monolithic catalyst.

Steady-state and dynamic hysteresis effects during lean co-oxidation of CO and C3H6 over Pt/Al2O3 monolithic catalyst

A global kinetic model is developed for the co-oxidation of CO and C3H6 over a Pt/Al2O3 diesel oxidation monolithic catalyst based on a recent bench flow reactor study (Abedi et al., 2012). A monolith reactor model containing these kinetics is used to elucidate the dynamic and steady state hysteresis behavior observed during oxidation of a CO+C3H6 mixture on Pt/Al2O3. Dynamic hysteresis is observed during temperature-programmed oxidation which involves ramping the feed gas temperature up and down at a constant ramp rate. The predicted solid temperature spatial profile during ramp-up and ramp-down shows how the catalyst temperature lags behind the inlet temperature change, especially during the ramp-down. The dynamic hysteresis loop, whose width increases with the ramp rate and effective heat Peclet number (Peh,eff), is mainly a result of disparate time scales of thermal front propagation and temperature ramp and will be present even for reactions with negligible heat release. Steady state hysteresis can also be explained based on adiabatic temperature rise (ΔTad) and Peh,eff, i.e. thermokinetic multiplicity. The region of steady state hysteresis loop expands with an increasing ΔTad which is proportional to the feed concentrations of CO and C3H6. For fixed ΔTad, the steady state multiplicity disappears as Peh,eff increases or when there is negligible thermal feedback due to intra or interphase heat transfer. Finally, the model is used to construct a map that may be used to assess the light-off features of multiple reaction systems in monolith reactors.

Bifurcation analysis of thermally coupled homogeneous–heterogeneous combustion

Using a two-mode lumped model, we present theory and comprehensive bifurcation analysis of thermally coupled homogeneous–heterogeneous combustion of propane and methane in short monolith, fibermat or gauze type reactors with a focus on the dependence of the ignition, extinction, hysteresis, double and boundary limit loci on the various design and operating parameters. We analyze the impact of inlet fuel mole fraction, inlet temperature, residence time and channel hydraulic radius on the relative position of the homogeneous and catalytic ignition and extinction points and identify the parameter regions in which either catalytic or homogeneous reaction dominates. We also identify the regions in which catalytic ignition leads either to an intermediate branch on which the homogeneous reaction rate is negligible or directly to a high conversion and temperature state thereby facilitating homogeneous ignition. For the case of methane oxidation, we examine both the lean and rich feeds with the operating pressure as the bifurcation variable and compare the predicted results with available experimental data and numerical simulations using detailed CFD models.

Design of a “high-efficiency” NH3-SCR reactor for stationary applications. A kinetic study of NH3 oxidation and NH3-SCR over V-based catalysts

In this work, the kinetics of the NH3/NO/O2 reacting system over V2O5–WO3/TiO2 catalysts was studied under conditions of NH3/NO ratio≥1 and temperature up to 400°C, where some ammonia oxidation is expected. NH3-SCR and NH3-oxidation tests were performed over powdered catalysts, at varying V-loads.Over a high V-load home-made catalyst, NO conversion passed through a maximum at increasing temperature, as commonly observed in SCR tests at the onset of ammonia oxidation; however, since the experiments were performed under excess ammonia, the high temperature decrease of NO conversion could not be associated with a lack of surface NH3. The experiments were interpreted as the evidence of an unselective NH3-oxidation route, giving rise to some NO production. NH3-oxidation tests, though, showed low selectivity to NO (which is in line with the current literature); however, a quantitative analysis of data showed that the observed product distribution is fully in line with an indirect-consecutive reaction scheme from NH3 to N2, wherein NO is formed by NH3 oxidation and is then rapidly converted to N2 via NH3-SCR.The kinetic investigation was extended to commercial catalysts with low and medium V-loads. NH3-SCR tests showed increasing trends of NO conversion with temperature within the range 250–400°C, thus they did not show any clear evidence for NH3 oxidation. However, NH3-oxidation tests showed that the reaction was active above 350°C; notably, over 95% selectivity to N2 was observed. A quantitative analysis of the NH3-oxidation tests, based on the independent estimate of the intrinsic rate of NH3-SCR, showed that the observation of few ppm NO slip is fully in line with the assumption of a two-step kinetic scheme.A reaction path analysis shows that by neglecting the unselective nature of ammonia oxidation on V-catalysts the rate of this reaction is overestimated by a factor of 2.Preliminary evaluations suggest that the impact of NH3-oxidation on the integral performance of monolith reactors (operating at NH3/NO ratios close to 1) is non negligible, so that the correct identification of the reaction stoichiometry is crucial.

Steady-State Modeling of Pt-Catalyzed H2 Combustion in a Monolithic Reactor: From Micro- to Macro-kinetics

Abstract Micro-kinetic modeling of Pt-catalyzed hydrogen oxidation in a monolithic reactor was carried out for various feed temperatures. The monolith behavior was described by a simplified steady-state model that avoids surface species enthalpy evaluation and does not have temperatures as unknowns. A simplified pseudoarch length method was used to obtain the multiple solution branches for this typical exothermic reaction system. Based on parametric sensitivity and surface species distribution analyses of the micro-kinetic modeling results, macro-kinetic models that involve only gas concentrations were derived and were used to explain the composition effects on ignition temperature.