Bed-to-wall heat transfer characteristics in a circulating fluidized bed

The bed-to-wall heat transfer coefficients were measured in a circulating fluidized bed of FCC particles (d_p = 65 μm). The effects of gas velocity (1.0–4.0 m/s), solid circulation rate (10–50 kg/m²s) and particle suspension density (15–100 kg/m³) on the bed-to-wall heat transfer coefficient have been determined in a circulating fluidized bed (0.1 m-ID x 5.3 rn-high). The heat transfer coefficient strongly depends on particle suspension density, solid circulation rate, and gas velocity. The axial variation of heat transfer coefficients is a strong function of the axial solid holdup profile in the riser. The obtained heat transfer coefficient in terms of Nusselt number has been correlated with the pertinent dimensionless groups     

Radiation tolerance of type IIa synthetic diamond detector for 14 MeV neutrons

Radiation tolerance of a type IIa synthetic diamond detector was examined from irradiation of mono-energetic 14 MeV neutrons. Measurements of I–V (current–voltage) characteristics and energy spectrum for 5.486 MeV alpha particles were performed after neutron irradiation. In the I–V characteristics measurement, enhancement of rectification was observed after neutron irradiation of up to 2.0 × 10¹² n/cm². Concurrently with the enhancement of rectification, significant decrease in signal amplitude was observed in energy spectrum measurement for alpha particles. It is considered that these changes were due to increase in the concentration of defects acting as shallow energy levels in the forbidden band. For neutron irradiation of higher than 1.6 × 10¹³ n/cm², weakening of the rectification characteristics and recovery of the signal amplitude were observed. These changes imply that deep energy levels, which were also considered to be introduced by defects, were dominant and weakened the effects of the shallow energy levels. Increase in the concentration of the deep trapping levels resulted in gradual decrease of the signal amplitude and degradation in the energy resolution. The peak for the alpha particles was obtained up to 5.5 × 10¹³ n/cm².     

Influence of some aromatic amino acids on the swelling behavior of acrylamide/maleic acid hydrogel

Summary Influence of some aromatic amino acids (histidine, phenylalanine and tryptophan) on the swelling behavior of acrylamide/maleic acid hydrogel (AAm/MA) prepared by γ-radiation was investigated. Swelling tests of AAm/MA hydrogel were made in buffer solutions and amino acid solutions at various pH at 37°C. The pH values are ionization of α-carboxyl groups (pK'₁), α-amino groups (pK'₂) and, isoelectric points (pI) of amino acids. The swelling of AAm/MA hydrogel increased when pH values of solutions were increased. The value of equilibrium swelling of AAm/MA hydrogel was 1035% at pH 10 buffer, while it was 880% at pH 2 buffer. The values of equilibrium swelling of AAm/MA hydrogel in phenylalanine, tryptophan and histidine solutions varied among 1130–1245% at pH 10, while they were among 790–975% at pH 2. The rate constant of swelling, diffusional exponent, network parameter and, diffusion and intrinsic diffusion coefficient were calculated by swelling kinetics. Diffusion of the penetrants into the hydrogel was found to be non-Fickian character. The diffusion coefficients of the hydrogel varied between 3.33×10⁻⁶– 7.71×10⁻⁶ cm ²s⁻¹, while the intrinsic diffusion coefficients waried between 4.03×10⁻⁶– 8.48×10⁻⁶ cm ²s⁻¹. Received: 22 December 1997/Revised version: 3 March 1998/Accepted: 5 March 1998     

Heat transfer in a high temperature fluidized bed

The heat transfer characteristics between the bed and immersed tube in a high temperature fluidized bed (7.5 cm I.D.×70 cm H) were investigated with sand and iron ore particles. The heat transfer coefficients were measured at operating temperatures of 200–600°C and gas velocities of 1–10 U_mf. The bed emissivity measured by the radiation probe was found to be 0.8–0.9. The experimentally obtained radiative heat transfer coefficient was in the range of 30–80 W/m²K for the operating temperature of 400–800°C and the contribution of radiation to total heat transfer was about 13% and 18% for the operating temperatures of 400°C and 600°C, respectively.     

Growth mechanism of monodispersed TiO2 fine particles by the hydrolysis of Ti(OC2H5)4

In order to investigate the growth mechanism of TiO₂, the monodispersed TiO₂ fine particles were prepared by hydrolysis of Ti(OC₂H₅)₄ using the seed preparation method. Although it was impossible to grow TiO₂ particles to more than 1 μm with conventional liquid phase reaction method, we obtained monodispersed T1O₂ fine particles of up to 2.5 urn. Nielsen’s chronomal analysis and Overbeek’s theory were applied to clarify the particle growth mechanism. The particle growth mechanism was found out as a first-order polynuclear layer growth mechanism and the growth rate constant, k_P was about 6.45X10^-6 cm/s.     

Spin-Probe Investigations of Head Group Behavior in Aqueous Dispersions of a Nonionic Amphiphilic Compound

Head group behavior of nonionic amphiphilic compound, (poly(oxyethylene) hydrogenated castor oil, HCO), in aqueous dispersions were investigated by EPR (electron paramagnetic resonance) in conjunction with a modern slow-tumbling simulation. The aliphatic spin probes, 5-doxylstearic acid (5-DSA) and 3β-doxyl-5α-cholestane (CHL), were used to obtain fluidity of the surface region of the membrane. The order parameter (S ₀) using the simulation for 5-DSA and CHL in the region were approximately 0.4 and 0.2, respectively. The ordering results suggest that the head group region of the membrane is somewhat fluid. The rotational diffusion coefficients (R _⊥ ≈ 1/(6τ_R)) for the probes were 3.4 × 10⁷ and 7.1 × 10⁷ s⁻¹, respectively. Activation energies, calculated using the temperature dependence of diffusion coefficients, were 18 and 17 kJ/mol for the probes. The EPR results imply that the CHL probe in the HCO membrane has quite different behavior in comparison with that of PC (phosphatidylcholine) from egg. Thus, the present EPR analyses have provided quantitative insight into the surface region of the amphiphilic membrane.     

Kinetic theory based computation of PSRI riser: Part II—Computation of mass transfer coefficient with chemical reaction

The design of circulating fluidized bed systems requires the knowledge of mass transfer coefficients or Sherwood numbers. A literature review shows that these parameters in fluidized beds differ up to seven orders of magnitude.To understand the phenomena, a kinetic theory based computation was used to simulate the PSRI challenge problem I data for flow of FCC particles in a riser, with an addition of an ozone decomposition reaction. The mass transfer coefficients and the Sherwood numbers were computed using the concept of additive resistances. The Sherwood number is of the order of 4 × 10⁻³ and the mass transfer coefficient is of the order of 2 × 10⁻³ m/s, in agreement with the measured data for fluidization of small particles and the estimated values from the particle cluster diameter in part one of this paper. The Sherwood number is high near the inlet section, then decreases to a constant value with the height of the riser. The Sherwood number also varies slightly with the reaction rate constant. The conventionally computed Sherwood number measures the radial distribution of concentration caused by the fluidized bed hydrodynamics, not the diffusional resistance between the bulk and the particle surface concentration. Hence, the extremely low literature Sherwood numbers for fluidization of fine particles do not necessarily imply very poor mass transfer.     

Measurement of the diffusion coefficient of free fatty acid in groundnut oil by the capillary-cell method

A capillary-cell method has been used for measuring the diffusion coefficients of oleic and stearic acids in groundnut oil at 130 C. The method demands careful fulfillment of the boundary conditions and, therefore, special attention has been paid to the establishment of the appropriate experimental conditions. Despite the suggestions of previous workers, a lot of effort is required to approximate the ideal conditions adequately. The experimental conditions proved to be optimal when capillaries with an inner diameter of 0.8 mm and a length of 20 mm were placed in a stationary solvent bath. The diffusion coefficients of oleic and stearic acid are (4.2±0.16)10⁻¹⁰ and (3.7±0.23)10⁻¹⁰ m²/s, respectively, at a confidence interval of 95%.     

Explosive decomposition of slightly compacted powders of lead azide over a wide range of laser pulse length

Abstact  Initiation of slightly compacted powders of lead azide of density not greater than 1.2 g/cm³ by laser radiation with a wavelength of 1.06 μm was studied experimentally over a wide range of pulse lengths (10⁻⁵–10⁻³ sec). It is shown that the ignition threshold of the slightly compacted samples does not change as the laser pulse length is increased. It is found that the explosion products contain molten lead particles whose sizes are an order of magnitude larger than the sizes of similar particles from explosion of samples of density 4.0 g/cm³. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 5, pp. 98–100, September–October, 2008.     

Possibility of electric-pulse sintering of powder nanostructural composites

The activities described in the paper are aimed at solving the problem of forming various articles from homogeneous metallic powders of copper and molybdenum, from a mixture of copper, titanium, and boron powders, and from a mixture of the copper powder with uniformly distributed particles of titanium diboride (up to 40 vol.%). The idea is to obtain compacts with the minimumchanges in the initial structure of the material and with high resistance to electric erosion under conditions of high currents. A method of electric-pulse sintering is used, where the compacting process is ensured with one capacitor discharge with a current density of ⩾10³ A/mm² and sintering time of ⩽10⁻⁴ sec. Conditions of sintering of powders pre-compressed by a pressure of several kilobars are studied. The density, microstructure, and phase composition of the sintered samples are examined. The erosion resistance of a Cu/TiB₂ porous nanostructural composite is tested on a model erosive plasma accelerator.     

Gold supported on well-ordered ceria films: nucleation, growth and morphology in CO oxidation reaction

Structure of gold nanoparticles formed by physical vapor deposition onto thin ceria films was studied by scanning tunneling microscopy (STM). Gold preferentially nucleates on point defects present on the terraces of the well-ordered, fully oxidized films to a low density. The nucleation expands to the terrace step edges, providing a large variety of low-coordinated sites. Only at high coverage, the Au particles grow homogeneously on the oxygen-terminated CeO₂(111) terraces. The morphology of Au particles was further examined by STM in situ and ex situ at elevated (up to 20 mbar) pressures of O₂, CO, and CO + O₂ at 300 K. The particles are found to be stable in O₂ ambient up to 10 mbar, meanwhile gold sintering emerges at CO pressures above ∼1 mbar. Sintering of the Au particles, which mainly proceeds along the step edges of the CeO₂(111) support, is observed in CO + O₂ (1:1) mixture at much lower pressure (∼10⁻³ mbar), thus indicating that the structural stability of the Au/ceria catalysts is intimately connected with its reactivity in the CO oxidation reaction.     

Synthesis and properties of novel polyurethanes containing 2,4-dioxybenzylidenecyanoacetate group as a NLO-chromophore for electro-optic applications

Summary Methyl 2,4-bis-(2'-hydroxyethoxy)benzylidenecyanoacetate (3) was prepared by hydrolysis of methyl 2,4-bis-(2'-vinyloxyethoxy)benzylidenecyanoacetate (2). Diol 3 was condensed with 2,4-toluenediisocyanate, 3,3'-dimethoxy-4,4'-biphenylene-diisocyanate, and 1,6-hexamethylenediisocyanate to yield polyurethanes 4–6 containing the NLO-chromophore 2,4-dioxybenzylidenecyanoacetate. The resulting polyurethanes 4–6 were soluble in common organic solvents such as acetone and DMF. Polymers 4–6 showed a thermal stability up to 280°C in TGA thermograms with T _g values obtained from DSC thermograms in the range of 66–114°C. The SHG coefficients (d₃₃) of poled polymer films were around 7.8 × 10⁻⁹ esu. Received: 25 February 2002/ Revised version: 25 March 2002/ Accepted: 29 March 2002     

Solutions to reduce release behavior of plasticizers out of PVC-made equipments: binary blends of plasticizers and thermal treatment

In order to reduce migration of plasticizers out of polyvinyl chloride (PVC), several techniques were attempted. First, binary blends of plasticizers were added to PVC, migration was decreased 100 or 1,000 times as compared to PVC samples containing only one plasticizer: the diffusion coefficient was of the order of 10⁻⁸ cm² s⁻¹ for the blend di-2-ethylhexylphthalate (DEHP)/di-2-ethylhexylterephthalate (TDHP) or 10⁻⁹ cm² s⁻¹ for the blend DEHP/DBP. A thermal treatment of PVC samples containing only one plasticizer leads to diffusion coefficients of the order of 10⁻¹⁰ cm² s⁻¹. This second method was also applied to PVC samples plasticized with binary blends. It lowers even more migration of both plasticizers out of PVC. But no particular phenomena were observed with ternary blends of plasticizers introduced in PVC.     

Experimental characterization of a short electrical mobility spectrometer for aerosol size classification

A prototype of a short column electrical mobility spectrometer (EMS) for size measurement of aerosol particle was design, constructed, and experimentally characterized. The short EMS consists of a particle charger, a size classifier column, and a multi-channel electrometer. Its particle size resolution is derived from a 10 channel electrometer detector. The short EMS is capable of size measurements in the range between 10 nm to 1,000 nm with a time response of about 50 s for full up and down scan. Particle number concentration in which the short EMS can measure ranges from 10¹¹ to 10¹³ particles/m³. The operating flow rate of the short EMS is set for the aerosol flow rate of 1.0–2.0 l/min and the sheath air flow rate fixed at 10.0 l/min. The inner electrode voltage of the classifier can be varied between 500–3,000 VDC. The short EMS operates at sub-atmospheric pressure, typically at 526 mbar. Validation of the short EMS performance was performed against a scanning electron microscope (SEM). Good agreements were obtained from comparison between sizes determined from the short EMS classifier and the SEM analysis. Signal current from the detector was also analyzed to give rise to number concentration of particles. Experimental results obtained appeared to agree well with the theoretical predictions.     

Formation and growth of dispersed carbon particles during pyrolysis of ethylene,benzene, and naphthalene in a reflected shock wave

The formation and growth rates of dispersed carbon particles were determined experimentally for pyrolysis of ethylene, benzene, and naphthalene in a reflected shock wave at temperatures of 1920–2560 K and hydrocarbon concentrations in argon of 1.8–20%. The diameter of the particles formed was estimated (30–600 Å). The maximum rate of particle formation at various temperatures [(0.7–96) · 10¹⁶ cm⁻³·sec⁻¹] and the particle growth rate (0.002–0.036 cm · sec⁻¹) were determined from results of measurements of reaction (residence) times. For pyrolysis of benzene, the activation energy of the overall process of particle formation is 410 kJ/mole, and for all hydrocarbons studied, the activation energy of the overall process of particle growth is 5–50 kJ/mole. The surface average particle diameter increases with increasing concentration of the initial hydrocarbon at a constant temperature. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 4, pp. 82–89, July–August, 2007.     

Equation of state for gaseous ethane determined from isotropic model potentials

A four-term virial equation of state was combined with isotropic model potentials to predict accurate volumetric and caloric thermodynamic properties of ethane in the gas phase. The parameters of the model potentials were determined from a fit to speed-of-sound data alone; no other data were used. The approximation used for the fourth virial coefficient included all interactions that contain up to two triplet potentials. Predicted ordinary second and third virial coefficients are in agreement with the data of Funke et al. [8]; we believe that predicted fourth virial coefficients are reliable and accurate. In the subcritical temperature region, the equation of state predicted compressibility factors that deviate by less than 0.04 percent at densities of up to 2.7 mol/dm³ (≈ 0.4ρ _c ). At supercritical temperatures, compressibility factors deviate by less than 0.02 percent at densities of up to 2.6 mol/dm³; also, in this region predicted isobaric heat capacity agrees with available data to within uncertainties of 0.4 percent at densities above 3 mol/dm³. We demonstrated that the four-term virial equation is more accurate than the three-term analogue.     

Deposition and characterization of nickel–niobium composite electrocoatings

Ni–Nb composite electrocoatings were obtained on carbon steel from Watts bath, containing suspended 20 μm size niobium powders. The effect of cathodic current density, electrolyte stirring rate and concentration of Nb particles in the bath on the deposit morphology and texture, volume fraction of co-deposited Nb particles and microhardness was investigated. The Ni–Nb composite layers presented a rough morphology with randomly oriented Ni grains, whereas pure Ni coatings obtained under the same experimental conditions were smooth and showed highly preferred orientation in the [110] or [100] direction. Stirring rate of the electrolyte and concentration of Nb particles in the bath are the main parameters affecting the incorporation of Nb particles. The Nb incorporated volume fraction was 11–14%, 17–19%, 27–32% and 34–37% for the 20 g L⁻¹ Nb/550 rpm, 20 g L⁻¹ Nb/400 rpm, 40 g L⁻¹ Nb/400 rpm and 40 g L⁻¹ Nb/550 rpm conditions, respectively. The microhardness of the Ni–Nb composite coatings obtained at 20 and 40 mA cm⁻² was higher than that of pure Ni layers, due to grain refining. Incorporation of Nb particles in Ni coatings improved the corrosion resistance of the deposits in NaCl and H₂SO₄ solutions.     

Effect of oxygen mobility in solid catalyst on transient regimes of catalytic reaction of methane partial oxidation at short contact times

Mathematical modeling of the effect of the oxygen mobility in a solid oxide catalyst on the dynamics of transients of fast catalytic reactions has been carried out. The analysis was based upon the redox mechanistic scheme with a due regard for diffusion of oxygen from the bulk of catalyst to its surface. Parameters of kinetic and mathematical models were selected via fitting of the experimental data for methane selective oxidation into syngas on 1.4%Pt/Gd_0.2Ce_0.4Zr_0.4O _x catalyst. The range of the Thiele parameter (φ) where the oxygen bulk diffusion affects the most strongly reaction transients corresponds to φε [0.3 ÷ 7]. For high-surface-area oxide catalysts, the bulk oxygen diffusion coefficients corresponding to this range of the Thiele parameter are in the range of 10^–18 ÷ 10^–13cm²/s.     

Separation of As(III) and As(V) by hollow fiber supported liquid membrane based on the mass transfer theory

Separation of As(III) and As(V) ions from sulphate media by hollow fiber supported liquid membrane has been examined. Cyanex 923 was diluted in toluene and used as an extractant. Water was used as a stripping solution. The extractability of As(V) was higher than As(III). When the concentration of sulphuric acid in feed solution and Cyanex 923 in liquid membrane increased, more arsenic ions were extracted into liquid membrane and recovered into the stripping solution. The mathematical model was focused on the extraction side of the liquid membrane system. The mass transfer coefficients of the aqueous phase (k _i ) and organic phase (k _m ) are 7.15×10⁻³ and 3.45×10⁻² cm/s for As(III), and 1.07×10⁻² and 1.79×10⁻² cm/s for As(V). Therefore, the rate-controlling step for As(III) and As(V) in liquid membrane process is the mass transfer in the aqueous film between the feed solution and liquid membrane. The calculated mass transfer coefficients agree with the experimental results.     

Novel device for in situ process characterization of oxide/oxide ceramic matrix composites fabricated by flexible injection

In this work, a new manufacturing process of CMC by liquid molding was studied. An instrumented device has been developed to characterize the through-thickness impregnation of ceramic fibers by a slurry charged with submicron ceramic particles. This instrument was used to characterize the permeability of the fibrous reinforcement and the formation of the ceramic cake by filtration of a ceramic particle suspension. Slurries containing different concentrations (10, 25, 33, and 40 vol%) of mono-dispersed alumina particles were filtered under different pressure conditions (345, 415, 485 kPa) to optimize the cake formation and filling of fibrous reinforcements while controlling the porosity level. Ceramic cakes exhibited an average permeability of 1.0 × 10⁻¹⁷ m² while the manufactured all-oxide composites resulted in a permeability of 0.6 × 10⁻¹⁷ m². Furthermore, a mathematical model based on Darcy's law was developed in this study to predict the rate of filtering and cake formation during injection using the permeability and filtration data measured with the experimental device. This mathematical model allows to determine the filtration time to produce a dense ceramic composite with an accuracy of ±15%, which corresponds to an error of less than 0.1 mm on the thickness of formed CMC.