Conversion of tar in a counter flow of supercritical water at a temperature varying along the reactor axis

The article reports the conversion of tar (empirical formula CH_1.47N_0.01S_0.007) continuously introduced into a counter flow of supercritical water (SCW) at 30 MPa in a tubular reactor with a temperature gradient along its vertical axis (450°C at the top and 650°C at the bottom). The yields of liquid products and volatile (C₁–C₉) hydrocarbons are 41.4 and 28.4%, respectively, relative to the weight of tar supplied into the reactor. Methane is the major component (40.5 mol %) of the volatile products, and the liquid products are dominated by oils (74.4 wt %). Deasphaltization and desulfurization of tar conversion products are observed. The average rate of water decomposition calculated from the quantity of O atoms in the conversion products is 0.24 g/min. Use of counter flows of the reactants in combination with a temperature gradient along the reactor axis affords a higher yield of low-boiling hydrocarbons than in the case of SCW pumping through a tar layer.     

Soot particle size distributions in a well-stirred reactor/plug flow reactor

A well-stirred reactor (WSR) followed by a plug flow reactor (PFR) is being used to study polycyclic aromatic hydrocarbon (PAH) growth and soot inception. Soot size distributions were measured using a dilution probe followed by a nano-differential mobility analyzer (Nano-DMA). A rapid insertion probe was fabricated to thermophoretically collect particles from the reactor for transmission electron microscopy (TEM) imaging. Results are presented on the effect of equivalence ratio on the soot size distributions obtained for fixed dilution ratio, the effect of dilution ratio on the soot size distributions obtained for fixed equivalence ratio, and the effect of temperature on the soot size distributions obtained for fixed equivalence ratio. In addition to particle sizing measurements, gas samples were analyzed by a gas chromatograph to determine the concentration of gaseous species in the PFR thought to be important in soot formation. Our soot size distribution measurements demonstrate that the mixing conditions in the flame zone affect whether or not a nucleation mode was detected in the size distribution.     

On the mechanism of influence of a low-frequency energy source on shock wave structure in transonic profile flow

The mechanism of interaction between a pulsed-periodic source of low energy and a closing shock arising near airfoils in transonic flight is studied. Based on the numerical solution of 2D nonstationary equations of gas dynamics, the evolution of the shock-wave pattern in the flow about a symmetric airfoil at low-frequency energy input is investigated and the mechanism of interaction is determined that differs from the generally accepted one, which considers a low-density wake. The new mechanism is not observed at single input of energy.     

Some results of experiments on discharges in shock tubes with parallel conductors

The paper presents some results of measurements on shock tubes with electrodes in the form of parallel conductors. It deals primarily with the shape of plasma moving in the said tube, the dependence of the plasma velocity on the pressure of air, helium and hydrogen filling the tube; further, it gives the results of measurements of the dependence of the plasma velocity on the distance from the discharge origin and on the voltage across the condenser battery.The author believes that the experimental results given in this paper could be of use for certain theoretical discussions of plasma behaviour in shock tubes with parallel conductors, as well as for an eventual plasma acceleration by means of an external magnetic field. Finally, he wishes to express his thanks to Ing. J. Hyneek and the staff of the Physics Department for their unselfish assistance in the construction of the apparatus.     

Salt deposition from hydrothermal solutions in a flow reactor

A flow hydrothermal setup with a tubular reactor equipped with a plunger pump and back pressure valves is used to study the effects of scaling in the K₂SO₄-KCl-H₂O, K₂SO₄-K₂CO₃-H₂O, and Na₂SO₄-NaCl-H₂O systems at pressures of up to 270–340 kg/cm², temperatures of 400–600°C, and flow rates of 5.0 and 2.5 ml/min in order to establish conditions for the formation of salt plugs of type 2 (K₂SO₄, Na₂SO₄) in the flow mode at supercritical (SC) state parameters and to explore ways of eliminating such salt deposits by means of hydrothermal solvents, more specifically, high-temperature aqueous solutions of salts of type 1 (KCl, K₂CO₃, and NaCl). The concentrations of hydrothermal solvents sufficient to prevent the plugging of flow systems with solutions containing 0.26–0.27 mol % K₂SO₄ or Na₂SO₄ are determined, and the effects of the flow rate and chemical composition of type 1 salts on this process are studied. The results show that the phenomenon of scaling with the formation of salt plugs, which hinders the practical use of supercritical water oxidation technology, can be eliminated by adding readily soluble electrolytes, salts of type 1, to initial aqueous solution of type 2 salts.     

Thermophoresis and the Brownian diffusion of nanoparticles in a flow reactor

The influence of thermophoresis and Brownian diffusion on deposition of the nanoparticles from a laminar gas flow on adsorbing walls of a flow reactor is investigated theoretically. Two similarity criteria characterizing the process of deposition of nanoparticles under nonisothermal conditions are formulated. It is shown that the influence of thermophoresis is significant only at the inlet area of the reactor, while Brownian diffusion acts over its entire length. To describe the interaction between the gas flow and the nanoparticles, the free-molecular approximation is used. The results of numerical calculations are given.     

Probing PAH formation chemical kinetics from benzene and toluene pyrolysis in a single-pulse shock tube

Benzene and toluene were pyrolyzed under highly argon-diluted conditions at a nominal pressure of 20 bar in a single-pulse shock tube coupled to gas chromatography/gas chromatography–mass spectrometry (GC/GC–MS) diagnostics. Concentration evolutions of polycyclic aromatic hydrocarbon (PAH) intermediates were measured in a temperature range of 1100–1800 K by analyzing the post-shock gas mixtures. Different PAH speciation behaviors, regarding types, concentrations and formation temperature windows, were observed in the two reaction systems. A kinetic model was proposed to predict and interpret the measurements. Through a combination of experimental and modeling efforts, PAH formation patterns from species pools of benzene and toluene pyrolysis were illustrated. In both cases, channels leading to PAHs basically originate from the respective fuel radicals, phenyl and benzyl. Due to the higher thermal stability of benzene, the production of phenyl, and thus most PAH species, occur in higher temperature windows, in comparison to the case of toluene. In benzene pyrolysis, benzyne participates in the formation of crucial PAH species such as naphthalene and acenaphthylene. Phenyl self-recombination takes considerable carbon flux into biphenyl, which serves as an important intermediate leading to acenaphthylene through hydrogen loss and ring closure. The resonantly-stabilized benzyl is abundant in toluene pyrolysis, and its decomposition further produces other resonantly-stabilized radicals such as fulvenallenyl and propargyl. Barrierless addition reactions among these radicals are found to be important sources of PAHs. Fuel-specific pathways have pronounced effects on PAH speciation behaviors, particularly at lower temperatures where fuel depletion is not completed within the reaction time of 4.0 ms. Contributions from the commonly existing Hydrogen-Abstraction-Carbon-Addition (HACA) routes increase with the temperature in both cases.     

Effects of benzene and naphthalene addition on soot inception in a well-stirred reactor/plug flow reactor

A well-stirred reactor (WSR) followed by a plug flow reactor (PFR) is being used to study soot inception. Soot size distributions were measured using two different dilution probes followed by a nano-differential mobility analyzer (nano-DMA). One of the dilution probes was developed for the PFR section, while the second probe was specifically developed for use in the WSR section. Results are presented on the effect of residence time on the soot size distributions obtained for fixed dilution ratio and equivalence ratio. In addition, a technique to inject aromatics and PAH species in the transition region between the WSR and PFR was developed. Results are presented on the effect of benzene and naphthalene on the soot size distributions obtained for differing seeding concentrations and residence times. The results demonstrate for the first time the sensitivity of the soot particle size distribution to the seeding of aromatic species in a WSR/PFR.     

Pressure fluctuations within a turbulent gas flow and their interaction with a shock wave

The interaction of a shock wave with a turbulent air flow is investigated experimentally. The turbulence was created with the aid of a grid. On its reflection from a perforated disc the wave propagated through a turbulent flow. The Mach number of the incident shock was equal to 1.9–4, the Mach number of the reflected wave was equal to 1.6–2.5. We found the autocorrelation functions of pressure fluctuations and their phase diagrams. The turbulent length scale of pressure fluctuations behind the incident shock was determined. The appropriate quantity behind the reflected wave is less of an order as compared with the previous case. It is established that the pressure behind the reflected wave in the turbulent flow is 7–8% higher as compared with the pressure in the laminar flow, if other conditions are the same.     

Fluorescent Properties of a Hybrid Cadmium Sulfide-Dendrimer Nanocomposite and its Quenching with Nitromethane

A fluorescent hybrid cadmium sulphide quantum dots (QDs) dendrimer nanocomposite (DAB-CdS) synthesised in water and stable in aqueous solution is described. The dendrimer, DAB-G5 dendrimer (polypropylenimine tetrahexacontaamine) generation 5, a diaminobutene core with 64 amine terminal primary groups. The maximum of the excitation and emission spectra, Stokes’ shift and the emission full width of half maximum of this nanocomposite are, respectively: 351, 535, 204 and 212 nm. The fluorescence time decay was complex and a four component decay time model originated a good fit (χ = 1.20) with the following lifetimes: τ ₁ = 657 ps; τ ₂ = 10.0 ns; τ ₃ = 59.42 ns; and τ ₄ = 265 ns. The fluorescence intensity of the nanocomposite is markedly quenched by the presence of nitromethane with a dynamic Stern-Volmer constant of 25 M⁻¹. The quenching profiles show that about 81% of the CdS QDs are located in the external layer of the dendrimer accessible to the quencher. PARAFAC analysis of the excitation emission matrices (EEM) acquired as function of the nitromethane concentration showed a trilinear data structure with only one linearly independent component describing the quenching which allows robust estimation of the excitation and emission spectra and of the quenching profiles. This water soluble and fluorescent nanocomposite shows a set of favourable properties to its use in sensor applications.     

Unidimensional steady flow in collapsible tubes with applications to blood vessels

Summary A simple unidimensional model for the steady flow of a viscous incompressible fluid in collapsible tubes is proposed. The viscosity is introduced in a phenomenological way. The matching with the structural mechanics of the wall is given by a simple local way. The predictions of the model are applied to some of more common experimental devices. The agreement with the measured values is very satisfactory. This work was partially supported by Ministero della Pubblica Istruzione.     

Influence of dissolved gases on sonochemistry and sonoluminescence in a flow reactor

In the present work, the influence of gas addition is investigated on both sonoluminescence (SL) and radical formation at 47 and 248 kHz. The frequencies chosen in this study generate two distinct bubble types, allowing to generalize the conclusions for other ultrasonic reactors. In this case, 47 kHz provides transient bubbles, while stable ones dominate at 248 kHz. For both bubble types, the hydroxyl radical and SL yield under gas addition followed the sequence: Ar > Air > N₂ >> CO₂. A comprehensive interpretation is given for these results, based on a combination of thermal gas properties, chemical reactions occurring within the cavitation bubble, and the amount of bubbles. Furthermore, in the cases where argon, air and nitrogen were bubbled, a reasonable correlation existed between the OH-radical yield and the SL signal, being most pronounced under stable cavitation at 248 kHz. Presuming that SL and OH originate from different bubble populations, the results indicate that both populations respond similarly to a change in acoustic power and dissolved gas. Consequently, in the presence of non-volatile pollutants that do not quench SL, sonoluminescence can be used as an online tool to qualitatively monitor radical formation.     

Formation of reaction intermediates and primary volatiles during acid-catalysed fast pyrolysis of cellulose in a wire-mesh reactor

This study aims to understand the fundamental reaction mechanisms during fast pyrolysis of the acid-impregnated cellulose in a wire-mesh reactor at 40–450 °C and 20 °C/s, via quantifying key compounds in the reaction intermediates and primary volatiles. Acid impregnation reduces the onset reaction temperature of cellulose pyrolysis. During acid-catalysed cellulose pyrolysis, 1,6-anhydro-β-d-glucofuranose (AGF), levoglucosenone (LGO) and 5-hydroxymethylfurfural (5-HMF) are identified as major products in the primary volatiles, and the formation of levoglucosan is greatly suppressed. At temperatures < 100 °C, acid catalyses hydrolysis reactions to produce glucose, which is further dehydrated to AGF at 120 °C. At temperatures > 160 °C, acid enhances the dehydration of glucose, levoglucosan and AGF to produce 5-HMF and LGO as major primary products. Once produced, those products can be easily released into the vapour phase, as either aerosols via thermal ejection or vapours via evaporation. As the pyrolysis temperature increases to 240 °C, aromatic compounds can be identified in the primary volatiles, indicating condensation reactions also play important roles during acid-catalysed cellulose pyrolysis under the conditions. As a result, char formation becomes the favoured pathway during acid-catalysed cellulose pyrolysis at temperatures > 300 °C.     

Statistics of shock waves in a two-dimensional granular flow

We have investigated the dynamics of shock waves in a single layer of uniform balls in a small-angle two-dimensional funnel. When the funnel half-angle 0 degrees < or approximately beta < or approximately 2 degrees, the flow is intermittent and kinematic shock waves are observed to propagate against the flow. We have used fast video equipment and image analysis methods to study the statistics of the shock waves. It is found that their speed and frequency increase with the distance from the outlet. In particular, the shock speed scales as the ratio of the local funnel width to the width of the funnel outlet. Various kinds of interactions between shock waves are observed, including repulsion. New shock waves are only created at those sites where a close-packed triangular packing of the monodisperse balls fits across the funnel.     

Spectral flow and vortex dynamics in a temperature gradient

In the mixed state of superconductors the spectral flow of fermion zero modes in the vortex core couples the motion of vortices to that of the normal fluid. This gives rise to a heat current perpendicular to the direction of vortex motion and therefore to longitudinal thermomagnetic effects like the thermopower and the Peltier effect. Analysis of vortex motion in a temperature gradient on this basis yields excellent agreement with experimental results.     

Size controlled synthesis of semiconductor nanocrystals in a continuous-flow mode microcapillary reactor

A microcapillary reactor with 320 μm inner diameter was utilized for CdSe nanoparticle synthesis. The influence of the reaction temperature and flow rate of precursors on the size and size distribution of prepared CdSe nanoparticles was systematically studied. The as-prepared nanoparticles exhibit sharp excitonic absorption and photoluminescence peak (FWHM 30 nm) with a quantum-yield around 10–40%. The microcapillary reactor was also used for CdSe/ZnS core-shell nanoparticle synthesis in continuous-flow mode. The quantum yield of the core-shell nanoparticles was found to be considerably influenced by the reactor temperature and have a close correlation with the thickness of ZnS shell under growth. An optimized quantum yield up to 70% was obtained for the CdSe/ZnS core-shell nanoparticles.