Time-Resolved X-Ray Tomography of a Fluidized Bed of Geldart A Particles

This paper discusses the influence of fines on the size of bubbles moving through a 23 cm ID fluidized bed of Geldart A particles imaged with an X-ray Tomographic Scanner. In earlier work [1], the bubble distribution in a fluidized bed of Geldart B particles was shown. The current study using Geldart A particles is more challenging to the reconstruction algorithm, since there are more bubbles, and they are smaller in size. We study the influence of adding fines (i.e. particles \u3c= 45 micron) to the system. When adding a mass fraction of fines of 24%, we find a decrease of the average bubble of 40% of the size for the original powder, in line with earlier results from pressure probes and optical probes [2]. We find that the entire distribution of the bubble sizes shifts to smaller values

Direct Minimization Generating Electronic States with Proper Occupation Numbers

We carry out the direct minimization of the energy functional proposed by Mauri, Galli and Car to derive the correct self-consistent ground state with fractional occupation numbers for a system degenerating at the Fermi level. As a consequence, this approach enables us to determine the electronic structure of metallic systems to a high degree of accuracy without the aid of level broadening of the Fermi-distribution function. The efficiency of the method is illustrated by calculating the ground-state energy of C$_2$ and Si$_2$ molecules and the W(110) surface to which a tungsten adatom is adsorbed.Comment: 4 pages, 4 figure

Carbon nanotubes : from molecular to macroscopic sensors

The components that contribute to Raman spectral shifts of single-wall carbon nanotubes (SWNT’s) embedded in polymer systems have been identified. The temperature dependence of the Raman shift can be separated into the temperature dependence of the nanotubes, the cohesive energy density of the polymer, and the buildup of thermal strain. Discounting all components apart from the thermal strain from the Raman shift-temperature data, it is shown that the mechanical response of single-wall carbon nanotubes in tension and compression are identical. The stress-strain response of SWNT’s can explain recent experimental data for carbon nanotube-composite systems

Comparison of optical probes and X-ray tomography for bubble characterization in fluidized bed methanation reactors

The performance of many fluidized bed reactors strongly depends on the bubble behavior since they influence the mass transfer to the dense phase where the catalyst is present. An example is the methanation in a fluidized bed that allows for conversion of unsaturated hydrocarbons in the gasification gas without catalyst deactivation [1]. The BFB reactor is a very challenging step in the process chain to produce SNG out of biomass as feedstock since next to the bubble behavior a lot of other parameters like temperature, pressure, particle size, attrition of the catalyst, internals, bed height and reactor diameter etc. affect the overall performance. The focus of this research work lies on the determination of the bubble properties which are an important factor to model a bubbling fluidized methanation reactor in order to predict and optimize its performance and to support its scale-up [2]. Tomographic methods such as X-ray measurements are often used to characterize bubbles in a fluidized bed. Compared to intrusive measurement, e.g. optical probing, this method possesses the advantage of measuring bubbles throughout the entire cross section. However, X-ray measurements cannot be applied to all installation, especially not in large scale plants. For these purpose, we have developed optical probes that can be employed to investigate the fluidization state in a hot pilot scale reactor. A main drawback of the optical measurements lies in their locally limited detection of the hydrodynamic pattern since they are only able to measure at one point in the reactor. Therefore, conclusions on the bubble behavior of the whole cross section based on optical measurements are not easy to derive. To compare the influence of the measurement method on the measured bubble properties, in the scope of this study, an artificial optical signal is created out of the existing X-ray measurement data set for a cold flow model of the pilot scale methanation reactor. The obtained bubble properties of both methods (i.e. evaluation of the derived artificial optical probe signal and image reconstruction based on the original X-ray tomographic data) are compared with regard to the hold-up, bubble rise velocity and the bubble size (for the X-ray method) or chord length (for the optical evaluation method), respectively. The process to obtain an artificial optical signal is depicted in Figure 1. The comparison shows that for the evaluation of optical probe data, statistical effects have to be considered carefully. The detected mean chord length of the optical method does not represent the mean bubble size determined by the X-ray method. Moreover, also a difference in the bubble rise velocity was detected for some fluidization states. This knowledge may be the basis for the derivation of a statistically sound method to calculate different hydrodynamic properties in fluidized bed reactors based on optical probe measurements. Please click Additional Files below to see the full abstract

Scattering of Polarized Protons from 6,7-Li at 200 MeV

This work was supported by the National Science Foundation Grant NSF PHY 81-14339 and by Indiana Universit

Combined FUS+ basophilic inclusion body disease and atypical tauopathy presenting with an ALS/MND-plus phenotype

AIMS: Amyotrophic lateral sclerosis / motor neurone disease (ALS/MND) is characterised by the presence of inclusions containing TDP-43 within motor neurones. In rare cases, ALS/MND may be associated with inclusions containing other proteins, such as fused in sarcoma (FUS), whilst motor system pathology may rarely be a feature of other neurodegenerative disorders. We here have investigated the association of FUS and tau pathology. METHODS: We report a case with an ALS/MND-plus clinical syndrome which pathologically demonstrated both FUS pathology and an atypical tauopathy. RESULTS: Clinical motor involvement was predominantly upper motor neurone, and was accompanied by extrapyramidal features and sensory involvement, but with only minimal cognitive impairment. The presentation was sporadic and gene mutation screening was negative. Post-mortem study demonstrated inclusions positive for FUS, including basophilic inclusion bodies. This was associated with 4R-tauopathy, largely as non-fibrillary diffuse phospho-tau in neurones, with granulovacuolar degeneration in a more restricted distribution. Double-staining revealed that neurones contained both types of protein pathology. CONCLUSION: FUS-positive basophilic inclusion body disease is a rare cause of ALS/MND, but in this case was associated with an unusual atypical tauopathy. The coexistence of two such rare neuropathologies raises the question of a pathogenic interaction

Vertical transport and electroluminescence in InAs/GaSb/InAs structures: GaSb thickness and hydrostatic pressure studies

We have measured the current-voltage (I-V) of type II InAs/GaSb/InAs double heterojunctions (DHETs) with 'GaAs like' interface bonding and GaSb thickness between 0-1200 \AA. A negative differential resistance (NDR) is observed for all DHETs with GaSb thickness $>$ 60 \AA below which a dramatic change in the shape of the I-V and a marked hysteresis is observed. The temperature dependence of the I-V is found to be very strong below this critical GaSb thickness. The I-V characteristics of selected DHETs are also presented under hydrostatic pressures up to 11 kbar. Finally, a mid infra-red electroluminescence is observed at 1 bar with a threshold at the NDR valley bias. The band profile calculations presented in the analysis are markedly different to those given in the literature, and arise due to the positive charge that it is argued will build up in the GaSb layer under bias. We conclude that the dominant conduction mechanism in DHETs is most likely to arise out of an inelastic electron-heavy-hole interaction similar to that observed in single heterojunctions (SHETs) with 'GaAs like' interface bonding, and not out of resonant electron-light-hole tunnelling as proposed by Yu et al. A Zener tunnelling mechanism is shown to contribute to the background current beyond NDR.Comment: 8 pages 12 fig

Variability in aerobic methane oxidation over the past 1.2 Myrs recorded in microbial biomarker signatures from Congo fan sediments

Methane (CH4) is a strong greenhouse gas known to have perturbed global climate in the past, especially when released in large quantities over short time periods from continental or marine sources. It is therefore crucial to understand and, if possible, quantify the individual and combined response of these variable methane sources to natural climate variability. However, past changes in the stability of greenhouse gas reservoirs remain uncertain and poorly constrained by geological evidence. Here, we present a record from the Congo fan of a highly specific bacteriohopanepolyol (BHP) biomarker for aerobic methane oxidation (AMO), 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol), that identifies discrete periods of increased AMO as far back as 1.2 Ma. Fluctuations in the concentration of aminopentol, and other 35-aminoBHPs, follow a pattern that correlates with late Quaternary glacial-interglacial climate cycles, with highest concentrations during warm periods. We discuss possible sources of aminopentol, and the methane consumed by the precursor methanotrophs, within the context of the Congo River setting, including supply of methane oxidation markers from terrestrial watersheds and/or marine sources (gas hydrate and/or deep subsurface gas reservoir). Compound-specific carbon isotope values of −30‰ to −40‰ for BHPs in ODP 1075 and strong similarities between the BHP signature of the core and surface sediments from the Congo estuary and floodplain wetlands from the interior of the Congo River Basin, support a methanotrophic and likely terrigenous origin of the 35-aminoBHPs found in the fan sediments. This new evidence supports a causal connection between marine sediment BHP records of tropical deep sea fans and wetland settings in the feeding river catchments, and thus tropical continental hydrology. Further research is needed to better constrain the different sources and pathways of methane emission. However, this study identifies the large potential of aminoBHPs, in particular aminopentol, to trace and, once better calibrated and understood, quantify past methane sources and fluxes from terrestrial and potentially also marine sources

Dark Matter, Light Stops and Electroweak Baryogenesis

We examine the neutralino relic density in the presence of a light top squark, such as the one required for the realization of the electroweak baryogenesis mechanism, within the minimal supersymmetric standard model. We show that there are three clearly distinguishable regions of parameter space, where the relic density is consistent with WMAP and other cosmological data. These regions are characterized by annihilation cross sections mediated by either light Higgs bosons, Z bosons, or by the co-annihilation with the lightest stop. Tevatron collider experiments can test the presence of the light stop in most of the parameter space. In the co-annihilation region, however, the mass difference between the light stop and the lightest neutralino varies between 15 and 30 GeV, presenting an interesting challenge for stop searches at hadron colliders. We present the prospects for direct detection of dark matter, which provides a complementary way of testing this scenario. We also derive the required structure of the high energy soft supersymmetry breaking mass parameters where the neutralino is a dark matter candidate and the stop spectrum is consistent with electroweak baryogenesis and the present bounds on the lightest Higgs mass.Comment: 24 pages, 8 figures; version published in Phys.Rev.