Large Diameter Tubes Research Articles

Coke formation possibility during production of reducing gas in large scale direct reduction plant

For the production of synthesis gas utilised in Midrex direct reduction plants, catalytic steam/CO2 hydrocarbon reforming in tubular reformer is the major process. Owing to the high heat input through the Midrex reformer tube wall, the endothermic nature of reforming reactions, low mass velocity of feed gas and large tube diameter, the catalyst bed is exposed to considerable axial and radial temperature gradients. These radial concentration and temperature gradients may create local areas with potential for carbon formation. To investigate this phenomenon, a rigorous two-dimensional model is developed for simulating the operation of a Midrex reformer which applies a dual catalyst loading profile. Both process side and furnace side have been included in this integrated model. Simulation results are in good agreement with available data from an actual plant. Using this model, a thermodynamic approach is applied to recognise zones in which the risk of carbon formation is high inside the reformer tubes. The results show that the first half of tubes, both in centre and near the wall, is critical from carbon forming point of view. Furthermore, the model shows that how a certain catalyst loading profile will affect the operation of the reformer.

Finite element modeling of single-walled carbon nanotubes with introducing a new wall thickness

A three-dimensional finite element (FE) model for armchair, zigzag and chiral single-walled carbon nanotubes (SWCNTs) is proposed. By considering the covalent bonds as connecting elements between carbon atoms, a nanotube is simulated as a space frame-like structure. Here, the carbon atoms act as joints of the connecting elements. To create the FE models, nodes are placed at the locations of carbon atoms and the bonds between them are modeled using three-dimensional elastic beam elements. Using Morse atomic potential, the elastic moduli of beam elements are obtained via considering a linkage between molecular and continuum mechanics. Also, a new wall thickness ( = bond diameter) equal to 0.1296 nm is introduced. In order to demonstrate the applicability of FE model and new wall thickness, the influence of tube wall thickness, diameter and chirality on the Young's modulus of SWCNTs is investigated. It is found that the choice of wall thickness significantly affects the calculation of Young's modulus. For the values of wall thickness used in the literature, the Young's moduli are estimated which agree very well with the corresponding theoretical results and experimental measurements. We also investigate the dependence of elastic moduli on diameter and chirality of the nanotube. The larger tube diameter, the higher Young's modulus of SWCNT. The Young's modulus of chiral SWCNTs is found to be generally larger than that of armchair and zigzag SWCNTs. The presented results demonstrate that the proposed FE model and wall thickness may provide a valuable tool for studying the mechanical behavior of carbon nanotubes and their application in nano-composites.

Modernizing a TESA 102–377 for the production of 426-mm-diam. Tubes

An analysis is made of the technological capabilities of tube-production unit TESA 102–377 and the results obtained from a modernization that the unit underwent to enable it to produce 426-mm-diam. tubes with a wall thickness of up to 10 mm. The problem was solved by modernizing the guillotine shears, improving the technology used to weld wide plates, and developing and introducing an algorithm for determining the schedule to be followed in forming larger-diameter tubes. The schedule was determined with consideration of boundary conditions reflecting the dimensions of the existing mill. Also aiding in the production of larger-diameter tubes on the mill was a scheme that was developed for two-radius forming with a curvilinear axis. A series of experimental and theoretical studies was performed and the flying saw already in use on the unit was redesigned. All these measures have made it possible to cut measured tubes of larger diameter in the line of the existing mill. In addition, the life of the saw blades has been doubled.

The effects of ball milling process on the diameter dependent fracture of single walled carbon nanotubes

The effect of ball milling process on the fracture of single walled carbon nanotubes (SWNT) is investigated using gel electrophoresis and Raman spectroscopy. A comparative Raman spectroscopy analysis, performed on the migrated nanotubes in the gel, shows that large diameter tubes are cut shorter and move faster in the gel. The D peak of Raman spectrum increases after an extended operation of the ball milling process, while that of the ultrasonicated nanotubes does not show an apparent change.

Morphology of Multi-Walled Carbon Nanotubes Affected by the Thermal Stability of the Catalyst System

High-quality multi-walled carbon nanotubes (MWCNTs) were efficiently synthesized on the CaCO3 supported Fe−Co catalyst with catalytic chemical vapor deposition method using acetylene as carbon source. The relationship between the catalyst structure and the carbon nanotube growth was systematically studied by using multiple techniques including SEM, TEM, TGA, Raman spectroscopy, and XRD. It was found that the MWCNT product demonstrates two groups in the outer diameter distribution, which is associated with the partial decomposition of CaCO3. This thermal decomposition was found to have as a result the coexistence of two catalytic systems throughout the entire synthesis process: Fe−Co/CaCO3 and Fe−Co/CaO. It was also found that the smaller diameter nanotubes grow on the Fe−Co/CaO system, while Fe−Co/CaCO3 produces larger diameter tubes.

Tube diameter effect on deflagration-to-detonation transition of propane–oxygen mixtures

An experimental study was conducted to study the tube diameter effect on deflagration- to-detonation run-up distance. The tube diameter effect is associated with the amplification factor, flame acceleration and heat loss. A simplified correlation of the run-up distance and tube diameter is proposed for the fuel-lean, stoichiometric and slight fuel-rich mixtures. The amplification factor, which is evaluated from the initial conditions of the propane–oxygen mixtures, might also be used to get a quick estimation of the run-up distance in tubes of larger diameter.

Synthesis and Characterization of Polystyrene-Poly(arylene ether sulfone)-Polystyrene Triblock Copolymer for Proton Exchange Membrane Applications

This paper reports the study of the static and dynamic properties of single-walled boron nitride nanotubes by using the molecular structural mechanics approach. The stiffness parameters of the boron-nitride bond are based on the DREIDING force field. The effects of tube diameter and chirality are investigated. The computational results show that the Young's modulus and the shear modulus of boron nitride nanotubes increase monotonically with nanotube diameter and reach up to 0.9 TPa and 0.5 TPa, respectively, at large tube diameter. The nanotube chirality has only slight effects on the moduli only when tube diameters are small. The fundamental frequencies of boron nitride nanotubes are found to be dependent not only on the nanotube aspect ratio but also on the tube diameter.

Formation of off-centered double-walled carbon nanotubes exhibiting wide interlayer spacing from bi-cables

Novel off-centered coaxial carbon nanotubes have been fabricated by coalescing adjacent double-walled carbon nanotubes (DWNTs) using high temperature heat treatments. The eccentric nanotubes possess wide interlayer spacing, and arise by the merge of two narrow diameter (inner) tubes contained inside a large diameter tubes (also known as bi-cables). We propose a complete description of the coalescence process including the merge of two DWNTs, followed by the formation of bi-cables and the creation of off-centered DWNTs. We also performed molecular dynamics calculations that are in agreement with the experimental observations.

Diameter dependence of second-order Raman features of graphene tubes

The second-order Raman band ( ω G ′ ) for single wall carbon nanotubes (SWNTs) in the range ∼2600–2700 cm −1 depends on the excitation energy and the diameter. However, the diameter dependence is not well established and understood. Here we propose that the two different physical dependencies are separable and can be taken into account by a phenomenological here derived equation. Using this methodology we have been able to find consistency between data for double-WNTs and large diameter SWNTs. Further, the determined diameter dependence of G′ correctly predicts the frequency for an infinitely large tube diameter, which is equivalent to a planar graphene sheet.

Critical Heat Flux of Subcooled Water Flow Boiling for High L/d Region

The critical heat flux (CHF) of subcooled water flow boiling for a high length/diameter (L/d) region is systematically measured for the flow velocities (u = 6.93 to 13.32 m/s), the outlet subcoolings (ΔTsub,out = 12.5 to 113 K), the inlet subcoolings (ΔTsub,in = 45 to 148.7 K), the outlet pressure (Pout = 773.50 to 861.12 kPa), and the inlet pressure (Pin = 796.16 to 920.07 kPa). Type 304 stainless steel tubes of inner diameter (d = 2 mm) and heated lengths (L = 21.5, 79.45, and 149.7 mm) with L/d = 10.75, 39.73, and 74.85 are used. The CHF correlation against outlet subcooling including the effect of L/d already presented by the authors describes the CHF obtained in this work within a 15% difference. However, the correlation against inlet subcooling also presented by the authors in the same papers needs a small modification to describe the CHF obtained in this work for a high L/d range. The modified correlation describes not only the experimental data for L/d up to 75 on the 2-mm tube but also the CHF for the same range of L/d on larger diameter tubes predicted by the correlation against outlet subcooling within a 15% difference.

The formation of metallic plasmas in transient capillary discharges at high current

We report observations of the formation of a metallic plasma in a high aspect ratio z-pinch confined within a ceramic capillary. A series of experiments on different capillary geometries was undertaken in which titanium metal rings were used to promote the formation of a titanium plasma through preferential ablation. In an initial vacuum a titanium seed plasma is formed in the hollow cathode (HC) volume by a low energy laser spark. This pre-ionizing plasma is assisted in its expansion into the z-pinch volume by the electron beams generated by a pre-ionizing discharge in the capillary, due to the HC effect. Further intense e-beam activity occurs on applying the main driver current to the capillary electrodes before the discharge impedance abruptly drops to give rise to an ensuing high current z-pinch. A segmented titanium ring structure within the capillary promotes metal ablation. The discharges are performed in tubes of 60 to 110 mm length and 3 and 5 mm effective internal diameter. The main discharge current is provided from a small pulsed power switched coaxial line, at up to 150 kA. The generator may be configured to deliver two different rates of current rise and this is found to have a significant effect on the plasma dynamics. The plasma properties are obtained from observations of the axial x-ray emission. The diagnostics used are filtered Si diodes, filtered time-resolved multi-pinhole camera images and the time resolved soft x-ray spectrum from 3 to 20 nm. While a single species metal plasma is not obtained, a very significant proportion of Ti is achieved in the higher rate of current rise configuration. The fraction of Ti diminishes for the longest length discharges and for the larger diameter tube diameter, as does the observed z-pinch uniformity. There is a weak dependance of the electron temperature with tube geometry, but the plasma density falls substantially in the longer discharges. This coincides with diminished effectiveness of the transient HC.

Synthesis and characterization of long strands of nitrogen-doped single-walled carbon nanotubes

We describe the synthesis of N-doped single-walled carbon nanotubes (N-SWNTs), that agglomerate in bundles and form long strands (<10 cm), via the thermal decomposition of ferrocene/ethanol/benzylamine (FEB) solutions in an Ar atmosphere at 950 °C. The amount of benzylamine in the solution was varied from 0% to 26% by weight. Using Raman spectroscopy, we noted that as the N content is increased in the starting FEB solution, the growth of large diameter tubes is inhibited. We observed that the relative electrical conductivity of the strands increases with increasing nitrogen concentration. Thermogravimetric analysis (TGA) showed novel features for highly doped tubes that are related to chemical reactions on N sites.

Ab initio study of curvature effects on the physical properties ofCH4-doped nanotubes and nanoropes

We have performed an ab initio study of the energetics, structural, electronic, and optical propertiesof the CH4-doped ultrathin 4 Å and large diameter carbon nanotubes (CNTs). No adsorption of theCH4 molecule has been seen either on the groove or on the interstitial sites of the achiral4 Å nanoropes. In the case of weakly bonded systems such as the adsorption of theCH4 molecules on the nanotubes, a prominent role of the dispersion forces in the binding isobserved. The negative contribution of the zero point vibrational energy to thebinding energy is seen to be appreciable. The effects of the tube diameter and thedifferent chiralities of the carbon nanotubes on the adsorbate-induced physicalproperties have been investigated. In the large diameter tubes, the binding of theCH4 moleculein the endohedral adsorption is much stronger than that in the exohedral adsorption. The binding ofthe CH4 molecules depends upon the chirality of the nanotube and we find no adsorption on thechiral (4, 2) tube. We find that the local density approximation (LDA) over-binds theCH4 molecule and the generalized gradient approximation (GGA) under-binds it, andfor a reliable theoretical estimate one should take some weighted averageof the binding energies (BEs) determined in the LDA and the GGA. Thecurrently calculated BE and the adsorbate concentration are in reasonableagreement with the measured data available for the (10, 10) nanotubes. Theelectronic structure of the pristine tube is quite altered by the adsorption of theCH4 molecule on the surface of the tube because of the breaking of the symmetry of the hostlattice except the chiral (4, 2) tube, which has practically no symmetry. The adsorptionincurs splitting in the states in the whole energy range, especially in the large curvature4 Å tubes. The bandgap of the semiconducting achiral zigzag nanotube is reduced,whereas that of a chiral semiconducting tube is enhanced, by the adsorption of theCH4 molecules. Theadsorption of CH4 molecules does not alter significantly the peak structure in the optical absorption of thepristine tube, except for some changes in the energy locations and the relative intensities inthe achiral tubes. Most of the calculated peaks in the optical absorption of the pristinelarge diameter (10, 0) and (10, 10) nanotubes have been observed in the experimentalmeasurements.

Adsorption pathways of singletO2on4Åcarbon nanotubes

Using density functional calculations and the nudged elastic band method, we investigate the adsorption pathways of singlet ${\mathrm{O}}_{2}$ on $4\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ single-wall carbon nanotubes, which have been recently fabricated using a templating method. We find that the reaction barriers for these ultrasmall-radius tubes are lower than those previously reported for larger-diameter tubes. Moreover, another chemisorbed product is identified for the (3,3) tube, and the reaction path is very different from those reported for larger armchair tubes.

Initiation of heterogeneous detonation in tubes with coils and Shchelkin spiral

The possibilities of significant reduction of the critical energy of initiation of detonation in sprays of liquid hydrocarbon fuel in air are demonstrated experimentally. The structure developed for the purpose includes a tube of near-limiting diameter and a combination of Shchelkin spiral, tube coils, and reducing cones for transitioning a detonation wave to a tube of large diameter.

Determination of axial and circumferential stresses in the wall of a closed tube via an ultrasonic method using the acoustoelasticity effect

The possibilities of using the acoustoelasticity effect to study a stressed state of a large-diameter tube are considered. The results of measuring axial and circumferential stresses in the stalk of a tube with an gIH-5101A device, which implements the echo-pulse acoustic-testing method that uses shear and longitudinal waves propagating normally to the plane where stresses are applied, are presented.

Transition states and minimum energy pathways for the collapse of carbon nanotubes

Carbon nanotubes CNTs can undergo collapse from their customary cylindrical configurations to ribbons. The energy minima corresponding to these two states are identified using either atomistic molecular mechanics or the theory of finite crystal elasticity with reduced dimensionality. The minimum energy path between these two minima is found using the nudged elastic band reaction-pathway sampling scheme. The energetics of CNT collapse is explored for both single- and multi-walled CNTs as well as small bundles. The process has a strong diameter dependence, with collapse being more favorable for the larger diameter tubes, but is nearly independent of chirality. The saddle point always lies close to the collapsed state, and the absolute barrier energies— even for fairly short tube lengths—are sufficiently high, even when the reaction is highly exothermic, that thermal activation cannot initiate collapse via this pathway. The hydrostatic pressure required to buckle and collapse CNTs of various diameters is discussed.

Static and Dynamic Properties of Single-Walled Boron Nitride Nanotubes

This paper reports the study of the static and dynamic properties of single-walled boron nitride nanotubes by using the molecular structural mechanics approach. The stiffness parameters of the boron-nitride bond are based on the DREIDING force field. The effects of tube diameter and chirality are investigated. The computational results show that the Young's modulus and the shear modulus of boron nitride nanotubes increase monotonically with nanotube diameter and reach up to 0.9 TPa and 0.5 TPa, respectively, at large tube diameter. The nanotube chirality has only slight effects on the moduli only when tube diameters are small. The fundamental frequencies of boron nitride nanotubes are found to be dependent not only on the nanotube aspect ratio but also on the tube diameter.

A Preliminary Field Trial of Bait Stations for the Delivery of Oral Rabies Vaccine: Can Varying Diameter Exclude Non-Target Species?

Delivery of oral rabies vaccine can be an effective method for combating rabies, but broadcasting vaccine sachets over wide areas creates the potential for non-target species to ingest vaccine baits before the target species encounters them. An alternative is to present the vaccine at a bait station designed to allow access by target species, while excluding some non-target species. We tested whether bait stations constructed of PVC pipe of 3 different diameters (10, 15, and 20 cm) differed in their effectiveness in allowing access by striped skunks versus other, non-target, nocturnal mammals in the urban environment of Flagstaff, Arizona. We placed bait stations in sets of 3 at 13 locations during late February and early March 2005 and monitored their use for 5 nights using digital still cameras. We recorded visits by striped skunks, gray foxes, raccoons, and domestic cats and dogs. Large-diameter tubes were used by all species, though large dogs had limited access, and small-diameter tubes were entered only once (by a skunk). Medium-diameter stations were used by all species except dogs, but skunks entered these stations more readily (81% vs. 44% or less), indicating that baits in 15-cm tubes would be more readily accessed by skunks. If striped skunks are the primary target species, we recommend the use of medium-diameter bait stations, as these stations excluded all dogs and reduced bait uptake by cats, foxes, and raccoons. If foxes and raccoons are also targeted, large-diameter stations will be required, and these will be available to all cats and some small dogs but should exclude larger dogs. Given the cost of this type of bait station in both time and money, we recommend their use only in limited areas where potential interaction with non-target species is of special concern. However, when one considers the value of human life, the cost may be negligible.

Long-Lived Charged States in Single-Walled Carbon Nanotubes

We present continuous wave photoinduced absorption spectroscopy of single-walled carbon nanotubes dispersed in a polymer matrix. The spectrum is dominated by a modulation of the absorption line shape, predominantly of large diameter tubes, that we assigned to electroabsorption caused by local electric fields arising from trapped photoinduced charges. The lack of selectivity in the excitation points to an efficient migration of the photoexcited states, either the singlet excitons or the charges resulting from their dissociation.