786 research outputs found
Production of hydrogen by unmixed steam reforming of methane
Unmixed steam reforming is an alternative method of catalytic steam reforming that uses separate air and fuelâsteam feeds, producing a reformate high in H2 content using a single reactor and a variety of fuels. It claims insensitivity to carbon formation and can operate autothermally. The high H2 content is achieved by in situ N2 separation from the air using an oxygen transfer material (OTM), and by CO2 capture using a solid sorbent. The OTM and CO2 sorbent are regenerated during the fuelâsteam feed and the air feed, respectively, within the same reactor. This paper describes the steps taken to choose a suitable CO2-sorbent material for this process when using methane fuel with the help of microreactor tests, and the study of the carbonation efficiency and regeneration ability of the materials tested. Elemental balances from bench scale experiments using the best OTM in the absence of the CO2 sorbent allow identifying the sequence of the chemical reaction mechanism. The effect of reactor temperature between 600 and on the process outputs is investigated. Temperatures of 600 and under the fuelâsteam feed were each found to offer a different set of desirable outputs. Two stages during the fuelâsteam feed were characterised by a different set of global reactions, an initial stage where the OTM is reduced directly by methane, and indirectly by hydrogen produced by methane thermal decomposition, in the second stage, steam reforming takes over once sufficient OTM has been reduced. The implications of these stages on the process desirable outputs such as efficiency of reactants conversion, reformate gas quality, and transient effects are discussed
On the gravity-driven draining of a rivulet of a viscoplastic material down a slowly varying substrate
We use the lubrication approximation to investigate the steady locally unidirectional gravity-driven draining of a thin rivulet of viscoplastic material, modeled as a biviscosity fluid (or, as a special case, as a Bingham material), down a slowly varying substrate. In contrast to the earlier work on viscoplastic rivulets we consider small-scale flows, such as those found in many industrial coating and printing processes, in which surface-tension effects play a significant role. We interpret our results as describing a slowly varying rivulet draining in the azimuthal direction from the top to the bottom of a large horizontal circular cylinder. Provided that the yield stress is nonzero we find that the flow is always unyielded near the top of the cylinder (where the rivulet becomes infinitely wide in the transverse direction), and, except in the special case when the viscosity ratio is zero, near the bottom of the cylinder (where it becomes infinitely deep). For sufficiently small values of the prescribed volume flux the flow is unyielded everywhere, but for larger values of the flux the flow near the substrate in the center of the rivulet is yielded. We obtain numerically calculated values of the semiwidth of the rivulet and of the yielded region as well as of the maximum height of the rivulet and of the yielded region for a range of parameter values, and describe the asymptotic behavior of the solution in the limits of large and small yield stress, large and small flux, and small viscosity ratio. In the special case of a Bingham material the flow near the top of the cylinder consists of an infinitely wide rigid and stationary plug, while elsewhere it consists of two rigid and stationary 'levÊ#169;es' at the edges of the rivulet and a central region in which the flow near the free surface is a 'pseudoplug' whose velocity does not vary normally to the substrate, separated from the 'fully plastic' flow near the substrate by a 'pseudoyield surface.' Š#169;2002 American Institute of Physics
Thin-film flow of a viscoplastic material round a large horizontal stationary or rotating cylinder
We consider the steady two-dimensional thin-film flow of a viscoplastic material, modelled as a biviscosity fluid with a yield stress, round the outside of a large horizontal stationary or rotating cylinder. In both cases we determine the leading- order solution both when the ratio of the viscosities in the 'yielded' and 'unyielded' regions is of order unity and when this ratio approaches zero in the appropriate distinguished limit. When the viscosity ratio is of order unity the flow consists, in general, of a region of yielded fluid adjacent to the cylinder and a region of unyielded fluid adjacent to the free surface, separated by the yield surface. In the distinguished limit the flow consists, in general, of a region of yielded fluid adjacent to the cylinder whose stress is significantly above the yield stress and a pseudo-plug region adjacent to the free surface, in which the leading-order azimuthal component of velocity varies azimuthally but not radially, separated by the pseudo-yield surface; the pseudo-plug is itself, in general, divided by the yield surface into a region of yielded fluid whose stress is only just above the yield stress and a region of unyielded fluid adjacent to the free surface whose stress is significantly below the yield stress. The solution for a stationary cylinder represents a curtain of fluid with prescribed volume flux falling onto the top of and off at the bottom of the cylinder. If the flux is sufficiently small then the flow is unyielded everywhere, but when it exceeds a critical value there is a yielded region. In the distinguished limit the yielded region always extends all the way round the cylinder, but the unyielded region does so only when the flux is sufficiently small. For a rotating cylinder a film with finite thickness everywhere is possible only when the flux is sufficiently small. Depending on the value of the flux and the speed of rotation the flow may be unyielded everywhere, have a yielded region on the right of the cylinder only, or have yielded regions on both the right and left of the cylinder. At the critical maximum flux the maximum supportable weight of fluid on the cylinder is attained and the pseudo-yield, yield and free surfaces all have a corner. In the distinguished limit there are rigid plugs (absent in the stationary case) near the top and bottom of the cylinder
The role of lithospheric flexure in the landscape evolution of the Wilkes Subglacial Basin and Transantarctic Mountains, East Antarctica
Reconstructions of the bedrock topography of Antarctica since the EoceneâOligocene Boundary (ca. 34 Ma) provide important constraints for modelling Antarctic ice sheet evolution. This is particularly important in regions where the bedrock lies below sea level, since in these sectors the overlying ice sheet is thought to be most susceptible to past and future change. Here we use 3D flexural modelling to reconstruct the evolution of the topography of the Wilkes Subglacial Basin (WSB) and Transantarctic Mountains (TAM) in East Antarctica. We estimate the spatial distribution of glacial erosion beneath the East Antarctic Ice Sheet, and restore this material to the topography, which is also adjusted for associated flexural isostatic responses. We independently constrain our postâ34 Ma erosion estimates using offshore sediment stratigraphy interpretations. Our reconstructions provide a betterâdefined topographic boundary condition for modelling early East Antarctic Ice Sheet history. We show that the majority of glacial erosion and landscape evolution occurred prior to 14 Ma, which we interpret to reflect more dynamic and erosive early ice sheet behaviour. In addition, we use closelyâspaced 2D flexural models to test previously proposed hypotheses for a flexural origin of the TAM and WSB. The preâ34 Ma topography shows lateral variations along the length of the TAM and WSB that cannot be explained by uniform flexure along the front of the TAM. We show that some of these variations may be explained by additional flexural uplift along the southâwestern flank of the WSB and the Rennick Graben in northern Victoria Land
Supersymmetric QCD corrections to the W-boson width
We calculate the one-loop supersymmetric QCD corrections to the width of the
-boson. We find that these are of order , where is the
supersymmetry breaking scale and the tree level hadronic
width for . Due to the appearance of the
suppression factor these are at least two orders of
magnitude smaller than the standard QCD corrections and hence of the order of the two-loop electroweak
effects. Therefore supersymmetric QCD corrections will only be of relevance
once experiments reach that level of accuracy.Comment: 7 pages, LaTeX file with 2 figures, uses epsf macro
Hydrogen production from the catalytic supercritical water gasification of process water generated from hydrothermal liquefaction of microalgae
The integration of hydrothermal liquefaction (HTL) and hydrothermal gasification (HTG) is an option for enhanced energy recovery and potential biocrude upgrading. The yields and product distribution obtained from the HTL of Chlorella vulgaris have been investigated. High conversion of algae to biocrude as well as near complete gasification of the remaining organic components in the aqueous phase was achieved. The aqueous phase from HTL was upgraded through catalytic HTG under supercritical water conditions to maximise hydrogen production for biocrude hydrotreating. High yields of hydrogen were produced (âŒ30 mol H2/kg algae) with near complete gasification of the organics (âŒ98%). The amount of hydrogen produced was compared to the amounts needed for complete hydrotreating of the biocrude. A maximum of 0.29 g H2 was produced through HTG per gram of biocrude produced by HTL. The nutrient content of the aqueous phase was analysed to determine suitability of nutrient recovery for algal growth. The results indicate the successful integration of HTL and HTG to produce excess hydrogen and maintain nutrient recovery for algal growth
Low Energy Thresholds and the Renormalization Group in the MSSM
We derive the 1-loop Renormalization Group Equations for the parameters of
the Minimal Supersymmetric Standard Model (MSSM) taking into account the
successive decoupling of each sparticle below its threshold. This is realized
by a step function at the level of each graph contributing to the
Renormalization Group Equations.Comment: 10 pages , Latex, no figure
Lepton Flavour Violation in a Class of Lopsided SO(10) Models
A class of predictive SO(10) grand unified theories with highly asymmetric
mass matrices, known as lopsided textures, has been developed to accommodate
the observed mixing in the neutrino sector. The model class effectively
determines the rate for charged lepton flavour violation, and in particular the
branching ratio for , assuming that the supersymmetric GUT
breaks directly to the constrained minimal supersymmetric standard model
(CMSSM). We find that in light of the combined constraints on the CMSSM
parameters from direct searches and from the WMAP satellite observations, the
resulting predicted rate for in this model class can be
within the current experimental bounds for low , but that the next
generation of experiments would effectively rule out this
model class if LFV is not detected.Comment: 23 page
Implications of the HERA Events for the R-Parity Breaking SUSY Signals at Tevatron
The favoured R-parity violating SUSY scenarios for the anomalous HERA events
correspond to top and charm squark production via the and
couplings. In both cases the corresponding electronic
branching fractions of the squarks are expected to be . Consequently the
canonical leptoquark signature is incapable of probing these scenarios at the
Tevatron collider over most of the MSSM parameter space. We suggest alternative
signatures for probing them at Tevatron, which seem to be viable over the
entire range of MSSM parameters.Comment: 20 pages Latex file with 4 ps files containing 4 figure
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