•Literature on hydrodynamics and mass transfer in minichannels is reviewed.•Focus is given to open channel structures.•Available correlations to predict Taylor flow characteristics are ...discussed.•Mass transfer via different routes in Taylor flow mode is analysed in detail.•The interaction of different mass transfer steps in reaction scenarios is studied.
Over the past decades, Taylor flow has got an increasing interest due to its potential to intensify reaction processes with fast kinetics which are usually controlled by mass transfer processes. Besides high mass transfer rates, the segmented flow regime offers a sharp residence time distribution of the liquid phase and a low pressure drop. Taylor flow appears in micro and minichannels whereas the terms have not always been used with a clear distinction. Minichannels are channels with characteristic diameters between 400μm and about 1mm, which will be in the centre of this work. These channel dimensions appear in monolithic reactors also known as reactors with honeycomb catalyst packings, in concepts using bundles of capillary tubes, as well as in microchannel plate reactors.
This article presents a comprehensive overview of hydrodynamics and mass transfer in the minichannels of an open flow structure, i.e. in channels without internals, operated in the Taylor flow regime. The review summarises available correlations to predict Taylor flow characteristics, as well as mass transfer coefficients between all involved phases (gas–liquid, liquid–solid, gas–solid). Within this scope, the impact of operational and design parameters is critically discussed and limits of application for the individual correlations are defined. Special attention is given to the interaction of these mass transfer steps in heterogeneously catalysed chemical reactions.
Recent developments in transformations of biobased 5‐hydroxymethylfurfural to 2,5‐dimethylfuran, a potential liquid fuel, are critically summarized. The highest yield of 2,5‐dimethylfuran (more than ...98 %) from 5‐hydroxymethylfurfural are obtained over bimetallic Cu−Co supported on carbon at 180 °C under 5 bar hydrogen in 2‐propanol and over Ni supported on mesoporous carbon at 200 °C under 30 bar hydrogen in water in a batch reactor. The desired catalyst should have relatively high metal dispersion and some acidity to facilitate both hydrogenation and hydrogenolysis. However, overhydrogenation and overhydrogenolysis forming 2,5‐dimethyltetrahydrofuran and methylfuran, respectively, should be suppressed. Furthermore, a hydrophobic support is more selective than oxide‐based support. After a careful adjustment of the residence time in a continuous reactor it is also possible to produce high yields of 2,5‐dimethylfuran even over Pt/C. The main challenges limiting the industrial feasibility of these reactions are relatively low initial reactant concentration, catalyst deactivation by sintering, leaching and coking. In addition to selection of optimum reaction conditions and catalyst properties, kinetic modelling was also summarized.
Biomass to fuel: Recent developments in transformations of biomass‐derived 5‐hydroxymethylfurfural to 2,5‐dimethylfuran acid, a potential liquid fuel, are critically summarized. The main challenges in transformation of 5‐hydroxymethylfurfural to 2,5‐dimethylfuran are low reactant concentration, overhydrogenation and hydrogenolysis, and catalyst deactivation. A proper catalyst should have optimized metal dispersion and number of acidic sites. Results from continuous operation and kinetic modelling in transformation of 5‐hydroxymethylfurfural to 2,5‐dimethylfuran are also summarized.
The notion of apparent activation energy is discussed in relation to structure sensitivity of complex heterogeneous catalytic reactions comprising several steps. The cases of nonuniform surfaces ...(intrinsic and induced) are considered, showing that the apparent activation energy depends on the cluster size and surface coverage. The theoretical analysis of the apparent activation energy was also extended for the two step sequence with deactivating catalyst by poisoning or when coking occurs as a result of the reactant interactions with an intermediate.
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Hydrodeoxygenation (HDO) of bio-oils, lignin and their model compounds is summarized in this review. The main emphasis is put on elucidating the reaction network, catalyst stability and ...time-on-stream behavior, in order to better understand the prerequisite for industrial utilization of biomass in HDO to produce fuels and chemicals. The results have shown that more oxygenated feedstock, selection of temperature and pressure as well as presence of certain catalyst poisons or co-feed have a prominent role in the HDO of real biomass. Theoretical considerations, such as density function theory (DFT) calculations, were also considered, giving scientific background for the further development of HDO of real biomass.
Analysis of apparent activation energy is presented for different heterogeneous catalytic reactions with parallel reaction routes. In the case of kinetic coupling between catalytic cycles the ...activation energy in a particular route depends not only on the activation energies of the elementary steps comprising this route, but also on the frequency of the steps in a parallel route. Expressions were derived for coupling between routes through irreversible adsorption of the substrate, quasi-equilibrated binding as well as different substrate adsorption modes. Theoretical analysis of the apparent activation energy was extended for the reaction network with two routes possessing mechanistically different rate determining steps (i.e. monomolecular vs bimolecular). For structure sensitive reactions an expression for the apparent activation energy for parallel reactions was developed for cases with a continuous distribution of active centers and a cubo-octahedral representation of the metal clusters. A comparison between the theoretical analysis and experimental data on transformations of furfural to furfuryl alcohol and furan on ruthenium clusters shows applicability of the developed theoretical framework.
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•Kinetics of fatty acids methyl esters and triglycerides hydrodeoxygenation.•Pd/C and sulfur free 5 wt% Ni/H-Y-80 catalysts.•Reaction network involves hydrogenation, deoxygenation and direct ...hydrodeoxygenation.•Adequate fitting for both studied substrates.•Rate constants show dependence on the fatty acids carbon chain length.
The kinetics of fatty acids methyl esters (FAME) and triglycerides (TG) hydrodeoxygenation (HDO) over 5 wt% Ni/H-Y-80 and 5 wt% Pd/C catalysts into green-diesel range hydrocarbons was studied experimentally and modeled numerically. The liquid-phase HDO was performed in a semi-batch reactor at the reaction temperature of 300 °C and pressure 30 bar on a sulfur free nickel supported catalyst and a palladium catalyst for comparison.
The fit of the model was evaluated by comparing the concentration profiles obtained from the model with the experimental data. The model confirmed that a complete reaction network involves hydrogenation, decarboxylation/decarbonylation and the direct hydrodeoxygenation of FA. Overall, the model displayed a good fitting for both studied substrates. Moreover, the model is expected to be applicable to different fatty acids. The rate constants for the conversion of FAME and TG, containing mixture of two groups of molecules C:16 and C:18, show dependence on the fatty acids carbon chain length.
The development of magnetic field sensors for biomedical applications primarily focuses on equivalent magnetic noise reduction or overall design improvement in order to make them smaller and cheaper ...while keeping the required values of a limit of detection. One of the cutting-edge topics today is the use of magnetic field sensors for applications such as magnetocardiography, magnetotomography, magnetomyography, magnetoneurography, or their application in point-of-care devices. This introductory review focuses on modern magnetic field sensors suitable for biomedicine applications from a physical point of view and provides an overview of recent studies in this field. Types of magnetic field sensors include direct current superconducting quantum interference devices, search coil, fluxgate, magnetoelectric, giant magneto-impedance, anisotropic/giant/tunneling magnetoresistance, optically pumped, cavity optomechanical, Hall effect, magnetoelastic, spin wave interferometry, and those based on the behavior of nitrogen-vacancy centers in the atomic lattice of diamond.
Diversity of heterogeneous catalysis as a discipline spanning many scales resulted in less attention to the meso-scale. An important issue in catalysis research, often overlooked by the academic ...community, is preparation of shaped bodies in a systematic way unraveling the scientific basis of such preparation. In particular presence of binders in shaped catalysts could influence reactions on bifunctional catalysts. Several challenges related to efficient spatial arrangement of active catalytic phases with a controlled distance between active sites are discussed.
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Recent developments in sugar transformations to methyl lactate and lactic acid are critically summarized. The highest yield of methyl lactate from glucose obtained over Sn(salen)/octylmethyl ...imidazolium bromide catalyst was 68 % at 160 °C whereas the highest yield of lactic acid of 58 % was achieved over hierarchical Lewis acidic Sn‐Beta catalysts at 200 °C under inert atmosphere. In addition to the desired products also humins are formed in water whereas in methanol alkyl glucosides‐ and ‐fructosides as well as acetals were generated, especially in the presence of Brønsted‐acidic sites. The main challenges limiting the industrial feasibility of these reactions are incomplete liquid phase mass balance closure, complicated product analysis and a lack of kinetic data. In addition to reporting optimized reaction conditions and catalyst properties also catalyst reuse and regeneration as well as kinetic modelling and continuous operation are summarized.
Fast as sugar? Recent developments in sugar transformations to methyl lactate and lactic acid are summarized. The highest yields of methyl lactate and lactic acid are in the range of 58–68 %. In addition to the desired products also humins are formed in water. The main challenges limiting industrial feasibility of these reactions are incomplete liquid‐phase mass balance closure, complicated product analysis and a lack of kinetic data.
•Kinetic modeling was done for isobutane dehydrogenation over Ga2O3/Al2O3.•Reaction network includes the main and by-products.•MCMC analysis was used to elucidate potential correlations between ...parameters.•Kinetic model adequately described experimental data.
Kinetic modeling was done for isobutane dehydrogenation using an alumina supported gallia catalyst in a fixed bed reactor at atmosphere pressure, 520–580 °C and a short residence time (ca. 0.06–0.3 s). A complex reaction network was taken into account considering formation of not only isobutane, but also a range of other hydrocarbons in various side reactions. Parameter investigation revealed an adequate description of the experimental data. The apparent activation energy of isobutane dehydrogenation was estimated to be 195 kJ/mol.
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