59,566 research outputs found
Analytical solutions for quasi-geostrophic inertial modes and onset of thermal convection in rapidly rotating spheroids
Organosolv pretreatment of Sitka spruce wood: conversion of hemicelluloses to ethyl glycosides
A range of organosolv pretreatments, using ethanol:water mixtures with dilute sulphuric acid, were applied to Sitka spruce sawdust with the aim of generating useful co-products as well as improving saccharification yield. The most efficient of the pretreatment conditions, resulting in subsequent saccharification yields of up to 86%, converted a large part of the hemicellulose sugars to their ethyl glycosides as identified by GC/MS. These conditions also reduced conversion of pentoses to furfural, the ethyl glycosides being more stable to dehydration than the parent pentoses. Through comparison with the behaviour of model compounds under the same reaction conditions it was shown that the anomeric composition of the products was consistent with a predominant transglycosylation reaction mechanism, rather than hydrolysis followed by glycosylation. The ethyl glycosides have potential as intermediates in the sustainable production of high-value chemicals
Collisions and expansion of an ultracold dilute Fermi gas
We discuss the effects of collisions on the expansion of a degenerate normal
Fermi gas, following the sudden removal of the confining trap. Using a
Boltzmann equation approach, we calculate the time dependence of the aspect
ratio and the entropy increase of the expanding atomic cloud taking into
account the collisional effects due to the deformation of the distribution
function in momentum space. We find that in dilute gases the aspect ratio does
not deviate significantly from the predictions of ballistic expansion.
Conversely, if the trap is sufficiently elongated the thermal broadening of the
density distribution due to the entropy increase can be sizeable, revealing
that even at zero temperature collisions are effective in a Fermi gas.Comment: 7 pages, 3 figures, revised after comments from referees and to
include correction
Isolation of high quality lignin as a by-product from ammonia percolation pretreatment of poplar wood
A two-step process combining percolation-mode ammonia pretreatment of poplar sawdust with mild organosolv purification of the extracted lignin produced high quality, high purity lignin in up to 31% yield and 50% recovery. The uncondensed fraction of the isolated lignin was up to 34%, close to that the native lignin (40%). Less lignin was recovered after pretreatment in batch mode, apparently due to condensation during the longer residence time of the solubilised lignin at elevated temperature. The lignin recovery was directly correlated with its molecular weight and its nitrogen content. Low nitrogen incorporation, observed at high ammonia concentration, may be explained by limited homolytic cleavage of -O-4 bonds. Ammonia concentrations from 15% to 25% (w/w) gave similar results in terms of lignin structure, yield and recovery
Ethane steam reforming over a platinum/alumina catalyst: effect of sulphur poisoning
In this study we have examined the adsorption of hydrogen sulfide and methanethiol over platinum catalysts and examined the effect of these poisons on the steam reforming of ethane. Adsorption of hydrogen sulfide was measured at 293 and 873 K. At 873 K the adsorbed state of hydrogen sulfide in the presence of hydrogen was SH rather than S, even though the Pt:S ratio was unity. The effect of 11.2 ppm hydrogen sulfide or methanethiol on the steam reforming of ethane was studied at 873 K and 20 barg. Both poisons deactivated the catalyst over a number of hours, but methanethiol was found to be more deleterious, reducing the conversion by almost an order of magnitude, possibly due to the co-deposition of sulfur and carbon. Changes in the selectivity revealed that the effect of sulfur was not uniform on the reactions occurring, with the production of methane reduced proportionally more than the other products, due to the surface sensitivity of the hydrogenolysis and methanation reactions. The water-gas shift reaction was affected to a lesser extent. No regeneration was observed when hydrogen sulfide was removed from the feedstream in agreement with adsorption studies. A slight regeneration was observed when methanethiol was removed from the feed, but this was believed to be due to the removal of carbon rather than sulfur. The overall effect of sulfur poisoning was to reduce activity and enhance hydrogen selectivity
SUSY QCD Corrections to Higgs-b Production : Is the \Delta_b Approximation Accurate?
The associated production of a Higgs boson with a b quark is a discovery
channel for the lightest MSSM neutral Higgs boson. We consider the SUSY QCD
contributions from squarks and gluinos and discuss the decoupling properties of
these effects. A detailed comparison of our exact order(alpha_s) results with
those of a widely used effective Lagrangian approach, the \Delta_b
approximation, is presented. The \Delta_b approximation is shown to accurately
reproduce the exact one-loop SQCD result to within a few percent over a wide
range of parameter space.Comment: figures added, version to be published in Phys Rev
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