Axial CO Research Articles

A Study of Gas Flow in a Slurry Bubble Column Reactor for the DME Direct Synthesis: Mathematical Modeling from Homogeneity vs. Heterogeneity Point of View

In the present study, a heterogeneous and homogeneous gas flow dispersion model for simulation and optimization of a large-scale catalytic slurry reactor for the direct synthesis of dimethyl ether (DME) from synthesis gas (syngas) and CO2, using a churn-turbulent regime was developed. In the heterogeneous flow model, the gas phase was distributed into two bubble phases including small and large while in the homogeneous one, the gas phase was distributed into only one large bubble phase. The results indicated that the heterogeneous gas flow model was in a better agreement with experimental pilot-plant data compared with that of the homogeneous one. Also, through investigating the heterogeneous gas flow for small bubbles as well as the large bubbles in the slurry phase (i.e.; including paraffins and the catalyst), the temperature profile along the reactor was obtained. The optimum value of rector diameter and height obtained at 3.2 and 20 meters respectively. The effects of operating variables on the axial catalyst distribution, DME productivity and CO conversion were also understudied in this research.

Generation of azimuthally polarized beams in fast axial flow CO2 laser with hybrid circular subwavelength grating mirror

A hybrid circular subwavelength grating mirror is proposed and fabricated as a rear mirror in a fast axial flow CO2 laser system to generate azimuthally polarized beams (APBs). This grating mirror, with particular gold-covered ridges and nanopillar-stuffed grooves, performs wideband TE wave reflectivity and high polarization selectivity. It shows that the polarization selectivity mechanism lies in the gold ridge's high reflectivity to the TE wave and the lower TM wave reflectivity, which are the result of the mode leaking into substrate through the dielectric-like nanopillar layer. Finally, a high-quality 550 W APB is obtained in subsequent experiments, which provides potential applications in drilling and welding.

A quantum chemical calculation on Fe(CO)5 revealing the operation of the Dewar–Chatt–Duncanson model

A nonstandard computational scheme has been applied to calculate Fe(CO)(5) with the aim to illustrate the operation of the Dewar-Chatt-Duncanson model by computation. A full configuration interaction (CI) calculation in an active space has been performed. The active space is built from naturally localized molecular orbitals (NLMOs) localized in bond regions or forming lone pairs. For selecting this active space, Weinhold's perturbation theory formulated in the natural bond orbital (NBO) space has been applied. Bonding, lone pair, and antibond NBOs exhibiting large interaction energies serve to define the active space. The actually applied active space, however, comprises NLMOs that are close in shape to the NBOs indicated by perturbation theory. Thus, a CI calculation with localized orbitals has been performed meeting the classical reasoning of chemists that is often based on local bonding concepts. The computational scheme yields the Lewis structure for Fe(CO)(5) whose energy is identical to the Hartree-Fock energy. The Lewis energy comprises CO → Fe σ-electron transfer (ET) and CO ← Fe electron back donation (BD). This Lewis energy gets lowered by localized correlation energy contributions caused by ET processes where electrons are back donated from the Fe d-lone pairs into the CO ligands. Thus, electron correlation within the selected active space is dominated by electron BD. Energies and electron populations of the NBOs support the notion that electrons are preferentially back donated into the equatorial CO ligands. Weights of local Slater determinants, determining the correlation energy, also point to a predominant BD into the equatorial CO ligands. Correlation energy increments resulting from electron BD into single antibond orbitals of the CO ligands have been calculated. These energy quantities also demonstrate that BD into the equatorial CO ligands is more energy lowering than BD into the axial CO ligands.

Mathematical modeling of a slurry reactor for DME direct synthesis from syngas

In this paper, an axial dispersion mathematical model is developed to simulate a three-phase slurry bubble column reactor for direct synthesis of dimethyl ether (DME) from syngas. This large-scale reactor is modeled using mass and energy balances, catalyst sedimentation and single-bubble as well as two-bubbles class flow hydrodynamics. A comparison between the two hydrodynamic models through pilot plant experimental data from the literature shows that heterogeneous two-bubbles flow model is in better agreement with the experimental data than homogeneous single-bubble gas flow model. Also, by investigating the heterogeneous gas flow and axial dispersion model for small bubbles as well as the large bubbles and slurry (i.e. including paraffins and the catalyst) phase, the temperature profile along the reactor is obtained. A comparison between isothermal and non-isothermal reactors reveals no obvious performance difference between them. The optimum values of reactor diameter and height were obtained at 7 m and 50 m, respectively. The effects of operating variables on the axial catalyst distribution, DME productivity and CO conversion are also investigated in this research.

Fine tuning of the redox function of Pseudomonas aeruginosa cytochrome c551 through structural properties of a polypeptide loop bearing an axial Met residue

Pseudomonas aeruginosa cytochrome c551 (PA) possesses a long polypeptide loop near its heme, and a unique hydrogen bond network among Ser52, axial Met61, and the heme 13-propionate side chain, i.e., Ser52 amide NH is hydrogen bonded to axial Met61 carbonyl CO, Met61 amide NH to Ser52 carbonyl CO, and Ser52 side chain OH to the heme 13-propionate side chain, contributes to stabilization of the structure of the loop [Y. Matsuura, T. Takano, R.E. Dickerson, J. Mol. Biol. 156 (1982) 389–409]. In this study, the structure and redox function of S52N and S52G mutants were characterized in order to elucidate the role of Ser52 in functional regulation of the protein. We found that the redox function of PA was hardly affected by an S52N mutation, but was slightly by an S52G one. The functional similarity between the wild-type protein and the S52N mutant demonstrated that Asn52 in the mutant plays a similar pivotal role in the formation of the unique hydrogen bond network that stabilizes the structure of the loop as Ser52 in the wild-type protein does. On the other hand, the functional alteration induced by the S52G mutation can be attributed to a structural change of the loop due to the lack of the hydrogen bond between the Gly52 and heme 13-propionate side chain in the mutant. Thus, this study demonstrated that the function of the protein can be tuned through the structural properties of the polypeptide loop near its heme.

Biosyngas production by autothermal reforming of waste cooking oil with propane using a plasma-assisted gliding arc reactor

Biosyngas production from renewable sources such as biomass has no impact on atmospheric CO 2 levels. In this work, the initial tests results are presented for the conversion of waste cooking oil (WCO) to biosyngas by catalytic partial oxidation over a granular Ni-based catalyst. In addition, autothermal reforming of propane with water and normal air was also carried out. The investigations were performed in a partially adiabatic plasma-assisted (non-thermal) gliding arc (GlidArc) reactor at fixed pressure (1 bar) and electric power (0.3 kW). Detailed axial temperature distributions, product concentrations, reactant conversions, H 2 and CO yields, H 2/CO ratio and thermal efficiency, as a function of the cold and hot WCO flow rate, the water flow rate and the time on stream were studied. Propane and normal air were used as oxidizing components to maintain autothermal operation.

Decomposition of beta-ray induced ESR spectra of fossil tooth enamel

Two fossil tooth enamel fragments were irradiated with beta rays, one through the outer surface, the other through the dentine–enamel junction. The angular ESR spectra of the two fragments were decomposed using an automated simulated annealing (SA) procedure, which is particularly well suited to separate overlapping signals. Beta irradiation generated different qualitative and quantitative responses to previous gamma irradiation experiments. Similar to gamma rays, the beta irradiation created both non-oriented and oriented CO 2 − radicals. In contrast to gamma irradiation, which only created orthorhombic oriented CO 2 − radicals, both axial and orthorhombic CO 2 − radicals were extracted after beta irradiation. Furthermore, gamma irradiation created significantly more non-oriented radicals than beta irradiation. Altogether, the radical distribution created by beta irradiation resembled that of the natural sample, which had been exposed to environmental irradiation over several hundreds of thousands of years. The natural sample contained 9% non-orientated CO 2 − radicals and a mix of orthorhombic to axial CO 2 − radicals in the ratio of 35:65. The beta induced spectra of the fragment irradiated through the outer surface contained 9% non-orientated CO 2 − radicals and a mix of orthorhombic to axial CO 2 − radicals in the ratio of 45:55, while for the other sample these values were 19% and 59:41, respectively. The angle between the axial and orthorhombic CO 2 − radicals is around 23° in both natural and beta irradiation components. This indicates that the radicals produced by the different irradiation modes are located in the same positions in the hydroxyapatite crystals. The higher percentage of non-oriented CO 2 − radicals closer to the dentine–enamel junction points to interprismatic zones for their possible location.

ENDOR study of CO2− radicals in hydroxyapatite of γ-irradiated bone

ENDOR spectra and ENDOR-induced EPR spectra (dependences of ENDOR signals vs. the value of magnetic field B ) of γ-irradiated bone plates and powders are studied. Typical ENDOR spectrum consisted of two signals centred at the Larmor frequencies of 1 H and 31 P nuclei, respectively. The ENDOR signals were found to be the superposition of the signals of axial (oriented) and orthorhombic (disordered) CO 2 − radicals. Changes of the ratio between the ENDOR signal intensities for g ‖ and g ⊥ at the rotation of the plate in the magnetic field is explained by the orientation-dependent redistribution of the EPR spectral density. EI-EPR spectra satisfactorily coincide with the spectra of conventional EPR absorption. Changes of ENDOR spectrum at the annealing of a sample before γ-irradiation ( T ann = 400 °C) are explained by Larmor ENDOR mechanism. Larmor ENDOR mechanism is associated with diffusive polarization of distant (far from a paramagnetic center) “free” nuclei under the conditions of EPR signal saturation and their depolarization by radio-frequency field in ENDOR.

Syntheses, structure characterisation and X-ray structures of the complexes Ru 3(CO) 11L [L = Ph 2P(C 6H 4Me- p), Ph 2PC 6F 5, P(C 6H 4Cl- p) 3, P(3,5-CF 3–C 6H 3) 3 and P(C 6H 4Me- p) 3

Five trinuclear monosubstituted complexes of the type Ru 3(CO) 11L were synthesised by the reaction of Ru 3(CO) 12 with phosphine ligands (L) using the radical anion catalysed method. The structures of the resulting clusters were elucidated by means of elemental analyses and spectroscopic methods, including IR, 1H NMR, 13C NMR and 31P NMR spectroscopy. 31P NMR spectra of the complexes Ru 3(CO) 11P(C 6H 4Me- p) 3 and Ru 3(CO) 11P(C 6H 4Cl- p) 3 showed splitting of the phosphorus signals into triplets. X-ray crystallographic studies of all five complexes were carried out. Out of the three unidentate tertiary phosphine complexes, Ru 3(CO) 11P(C 6H 4Me- p) 3 and Ru 3(CO) 11P(C 6H 4Cl- p) 3 are ordered while the complex Ru 3(CO) 11P(3,5-CF 3–C 6H 3) 3 exhibits disorder with respect to the trifluoromethyl groups. In all the five monosubstituted complexes, the ligand occupies the equatorial position due to steric reasons and coordination of the ligands are only at the phosphorus atom. The effect of substitution resulted in significant differences in the Ru–Ru distances. Out of the three Ru–Ru bonds, the one which is cis to the ligand is noticeably longer and the mean value for this longest Ru–Ru bond is 2.889 Å, while the mean value for the two shorter Ru–Ru bonds is 2.841 Å. The P–C distances are in the range 2.333(6)–2.354(6) Å. The equatorial Ru–CO moieties are almost linear and the average value for all the five complexes studied range from 176.3 to 177.6°, while the axial CO groups are slightly bent, ranging from 173.8 to 174.1°.

Decomposition of the laboratory gamma irradiation component of angular ESR spectra of fossil tooth enamel fragments

Spectrum decomposition of the angular measurements of fossil tooth enamel fragments using an automated simulated annealing (SA) procedure shows that the mix CO(2)(-) radicals generated by laboratory irradiation is significantly different to that of the natural sample. The naturally irradiated sample contains about 10% of non-oriented CO(2)(-) radicals and a mix of 35:65 orthorhombic to axial CO(2)(-) radicals. In contrast, laboratory irradiation generated about 40% of non-oriented radicals and a large amount of orthorhombic CO(2)(-) radicals, while we failed to detect any axial CO(2)(-) radicals. The results indicate that geological aging of the sample incurs various annealing and transfer processes; their precise nature is yet unknown. Nevertheless, the understanding of the formation and transfer processes that leads to the observed mix of CO(2)(-) radicals in fossil tooth enamel is essential for the reliable application of ESR dating.

Valence ionized states of iron pentacarbonyl and η5-cyclopentadienyl cobalt dicarbonyl studied by symmetry-adapted cluster-configuration interaction calculation and collision-energy resolved Penning ionization electron spectroscopy

Valence ionized states of iron pentacarbonyl Fe(CO)(5) and eta(5)-cyclopentadienyl cobalt dicarbonyl Co(eta(5)-C(5)H(5))(CO)(2) have been studied by ultraviolet photoelectron spectroscopy, two-dimensional Penning ionization electron spectroscopy (2D-PIES), and symmetry-adapted cluster-configuration interaction calculations. Theory provided reliable assignments for the complex ionization spectra of these molecules, which have metal-carbonyl bonds. Theoretical ionization energies agreed well with experimental observations and the calculated wave functions could explain the relative intensities of PIES spectra. The collision-energy dependence of partial ionization cross sections (CEDPICS) was obtained by 2D-PIES. To interpret these CEDPICS, the interaction potentials between the molecules and a Li atom were examined in several coordinates by calculations. The relation between the slope of the CEDPICS and the electronic structure of the ionized states, such as molecular symmetry and the spatial distribution of ionizing orbitals, was analyzed. In Fe(CO)(5), an attractive interaction was obtained for the equatorial CO, while the interaction for the axial CO direction was repulsive. For Co(eta(5)-C(5)H(5))(CO)(2), the interaction potential in the direction of both Co-C-O and Co-Cp ring was attractive. These anisotropic interactions and ionizing orbital distributions consistently explain the relative slopes of the CEDPICS.

CFD Simulation of the Effect of Monolith Wall Thickness on the Light off Performance of a Catalytic Converter

CFD (computational fluid dynamic) simulation that is based on finite volume method is used to study the light off performance of automotive catalytic converter. The flow distribution and the conversion efficiency of CO are achieved first, and the predictions are in good agreement with the experiment and finite difference method reported before. The present work investigates the effect of ceramic monolith wall thickness on the temporal variations of the radial solid temperature, mean solid temperature, axial flow temperature and CO conversion efficiencies in monolithic reactors during the light-off phase. It is found that the wall thickness can have a significant influence on the light off performance. When the monolith wall thickness is thinner, the light off performance of automotive catalytic converter is better. These studies help to understand better the performance of the catalytic converter in order to optimize the converter design.

Dynamics and Equilibrium of Heme Axial Ligation in Mesoporous Nanocrystalline TiO2 Thin Films

Comparative studies of axial CO and solvent coordination to iron(II) protoporphyrin IX (FeIIPPIX) anchored to the surface of mesoporous nanocrystalline (anatase) TiO2 thin films (FeIIPPIX/TiO2) immersed in dimethyl sulfoxide (DMSO), pyridine (py), and methanol (MeOH) and to FeIIPPIX in fluid DMSO and py solution are reported. The equilibrium constants, KeqCOS, for CO coordination to FeIIPPIX/TiO2 immersed in py (2.4 x 10(3) M-1) < DMSO (6 x 10(4) M-1) < MeOH (2.3 x 10(5) M-1) were quantified. The corresponding values in fluid py or DMSO solution were 2 times larger (4.5 x 10(3) and 1 x 10(5) M-1, respectively). The observed ligand exchange rates (kobs) measured after pulsed 532 nm laser excitation (5-6 ns fwhm, 1-3 mJ/pulse) of (S)(CO)FeIIPPIX/TiO2, where S is solvent, in saturated CO solutions were measured: py (2.2 s-1), DMSO (460 s-1), MeOH (2.09 x 10(5) s-1). The corresponding values in fluid solution were 2.0 s-1 (py) and 230 s-1 (DMSO). The observed ligand exchange rate varied linearly with [CO], and second-order rate constants were determined for FeIIPPIX/TiO2 immersed in DMSO (3.1 x 10(5) M-1 s-1) and MeOH (1.5 x 10(7) M-1 s-1). The observed rate for CO addition to (py)2FeIIPPIX/TiO2 immersed in py did not vary linearly with [CO]. The relevance of the measured kinetics and thermodynamics to a dissociative mechanism for ligand exchange is discussed.

Synthesis, Characterization, and Photoinduced Electron Transfer Processes of Orthogonal Ruthenium Phthalocyanine−Fullerene Assemblies

The convergent synthesis, electrochemical characterization, and photophysical studies of phthalocyanine-fullerene hybrids 3-5 bearing an orthogonal geometry (Chart ) are reported. These donor-acceptor arrays have been assembled through metal coordination of linear fullerene mono- and bispyridyl ligands to ruthenium(II) phthalocyanines. The hybrid [Ru(CO)(C(60)Py)Pc] (3) and the triad [Ru(2)(CO)(2)(C(60)Py(2))Pc(2)] (5) were prepared by treatment of the phthalocyanine 6 with the mono- and hexakis-substituted C(60)-pyridyl ligands 1 and 2, respectively. The triad [Ru(C(60)Py)(2)Pc] (4) was prepared in a similar manner from the monosubstituted C(60)-pyridyl ligand 1 and the phthalocyanine precursor 7. The simplicity of this versatile synthetic approach allows to determine the influence of the donor and acceptor ratio in the radical ion pair state lifetime. The chemical, electrochemical, and photophysical characterization of the phthalocyanine-fullerene hybrids 3-5 was conducted using (1)H and (13)C NMR, UV/vis, and IR spectroscopies, as well as mass spectrometry, cyclic voltammetry, femtosecond transient absorption studies, and nanosecond laser flash photolysis experiments. Arrays 3-5 exhibit electronic coupling between the two electroactive components in the ground state, which is modulated by the axial CO and 4-pyridylfulleropyrrolidine ligands. With respect to the excited state, we have demonstrated that RuPc/C(60) electron donor-acceptor hybrids are a versatile platform to fine-tune the outcome and dynamics of charge transfer processes. The use of ruthenium(II) phthalocyanines instead of the corresponding zinc(II) complexes allows the suppression of energy wasting and unwanted charge recombination, affording radical ion pair state lifetimes on the order of hundreds of nanoseconds for the C(60)-monoadduct-based complexes 3 and 4. For the hexakis-substituted C(60) unit 2, the reduction potential is shifted cathodically, thus raising the radical ion pair state energy. However, the location of the RuPc triplet excited state is not high enough, and still offers a rapid deactivation of the radical ion pair state.

Thermally stimulated transformation of the EPR spectra in γ-irradiated bone tissue

γ-irradiated bone tissue annealed at different temperatures is studied by EPR. In powder samples the annealing-induced changes of the EPR spectra are mainly determined by sharp growth of a signal caused by the products of thermal decay of an organic component in the tissue. This signal considerably complicates the analysis of the EPR spectrum shape for the annealed samples. On the other hand, in the spectra of the plates of bone tissue a change in the lineshape of the EPR signal from CO 2 − radicals after annealing is more pronounced than in powder, and it is attributed to the change in the contributions from axial and orthorhombic CO 2 − radicals. The analysis of different radicals concentrations vs. annealing temperature revealed above 210 °C the temperature-induced transformation of orthorhombic CO 2 − radicals into axial ones in bone tissue which (together with organic component) is responsible for the changes of EPR lineshape at annealing.

Effect of Lewis acid on the structure of a diiron dithiolate complex based on the active site of [FeFe]-hydrogenase assessed by density functional theory

The effect of Lewis acid on the structure and H2 productivity of a diiron dithiolate complex was investigated by using density functional theory (DFT) calculations. When a model molecule of [(CH3SH)(CO)2Fe(p)(mu-SCH2NHCH2S)Fe(d)(CO)3] was geometrically optimized, two isomers were found: one is the unrotated structure (1) with no ligand between two Fe atoms and the other is the rotated structure (1*) with one CO ligand between two Fe atoms. The energy of 1* was higher than 1 by 6.4 kcal/mol in a vacuum. DFT calculations also revealed that all Lewis acids bound to the rotated structure more strongly than to the unrotated structure, leading to the stabilization of the rotated structure. In particular, when AlCl3 is used, the rotated structure (1*/AlCl3) is more stable than the unrotated one (1/AlCl3) by 1.2 kcal/mol in a vacuum. The stabilization of the rotated structure arises from both the stronger basicity of the mu-CO ligand than the axial CO ligand and the increase of the bond strength between the mu-CO ligand and Fe(p) atom upon binding of Lewis acid to 1*. Calculation of energy barriers during electrocatalytic H2 production revealed that 1*/AlCl3 could efficiently produce H2via a chemical-electrochemical-chemical-electrochemical mechanism. The analysis of the energy level of the lowest unoccupied molecular orbital showed that 1*/AlCl3 may produce H2 at significantly lower reduction potential as compared with 1*. It is also found that the catalytic activity decreases with increasing polarity of the medium.

CO 2 − radicals in synthetic hydroxyapatite

Powders of the B-type synthetic apatite exposed to gamma or ultraviolet irradiation were investigated using EPR spectroscopy. It was shown that ultraviolet irradiation leads to the appearance of the EPR spectrum near g = 2, which is similar to the spectrum observed upon gamma irradiation. The decomposition of the EPR spectra into components and the simulation of the shape of the experimental EPR signals revealed that these signals are associated primarily with two types of CO 2 − radicals, namely, the axial CO 2 − radicals and the orthorhombic CO 2 − radicals. The differences in the shapes of the EPR spectra of the samples exposed to gamma and ultraviolet irradiation were explained by different ratios between the axial and orthorhombic CO 2 − radicals. It was established that thermal annealing results in an increase in the relative contribution to the total EPR spectrum. This increase was explained by the transformation of the orthorhombic radicals into the axial radicals.

Optimization of the operational parameters in a fast axial flow CW CO 2 laser using artificial neural networks and genetic algorithms

This paper presents an artificial intelligence approach for optimization of the operational parameters such as gas pressure ratio and discharge current in a fast-axial-flow CW CO 2 laser by coupling artificial neural networks and genetic algorithm. First, a series of experiments were used as the learning data for artificial neural networks. The best-trained network was connected to genetic algorithm as a fitness function to find the optimum parameters. After the optimization, the calculated laser power increases by 33% and the measured value increases by 21% in an experiment as compared to a non-optimized case.

Determining Transition-State Geometries in Liquids Using 2D-IR

Many properties of chemical reactions are determined by the transition state connecting reactant and product, yet it is difficult to directly obtain any information about these short-lived structures in liquids. We show that two-dimensional infrared (2D-IR) spectroscopy can provide direct information about transition states by tracking the transformation of vibrational modes as a molecule crossed a transition state. We successfully monitored a simple chemical reaction, the fluxional rearrangement of Fe(CO)5, in which the exchange of axial and equatorial CO ligands causes an exchange of vibrational energy between the normal modes of the molecule. This energy transfer provides direct evidence regarding the time scale, transition state, and mechanism of the reaction.

Major gaseous and PAH emissions from a fluidized-bed combustor firing rice husk with high combustion efficiency

This experimental work investigated major gaseous (CO and NO x ) and PAH emissions from a 400 kW th fluidized-bed combustor with a cone-shaped bed (referred to as ‘conical FBC’) firing rice husk with high, over 99%, combustion efficiency. Experimental tests were carried out at the fuel feed rate of 80 kg/h for different values of excess air (EA). As revealed by the experimental results, EA had substantial effects on the axial CO and NO x concentration profiles and corresponding emissions from the combustor. The concentration (mg/kg-ash) and specific emission (μg/kW h) of twelve polycyclic aromatic hydrocarbons (PAHs), acenaphthylene, fluorene, phenanthrene, fluoranthene, pyrene, benz[ a]anthracene, chrysene, benzo[ b]fluoranthene, benzo[ k]fluoranthene, benzo[ a]pyrene, dibenz[ a, h]anthracene and indeno[ 1,2,3-cd]pyrene, were quantified in this work for different size fractions of ash emitted from the conical FBC firing rice husk at EA = 20.9%. The total PAHs emission was found to be predominant for the coarsest ash particles, due to the effects of a highly developed internal surface in a particle volume. The highest emission was shown by acenaphthylene, 4.1 μg/kW h, when the total yield of PAHs via fly ash was about 10 μg/kW h.