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31.
A suite of density-differentiated macerals from several coals was analysed by Fourier transform infrared spectroscopy to obtain information on the nature and magnitude of the variations exhibited by the various maceral fractions. The most characteristic change between maceral groups was the variation in aliphatic hydrogen content, with exinite ? vitrinite ? inertinite. Since the separation technique (density gradient centrifugation) generally provided a number of fractions within a maceral group region, some of these were also analysed. In a series of density fractions from a low rank vitrinite, it was found that the aliphatic hydrogen content decreased as the density increased. The inertinites also exhibited a decrease in aliphatic hydrogen. The inertinite C—O bands had extinction coefficients different from those of vitrinites or exinites. The data suggest that quite profound variations in organic structure as determined by FT-i.r. spectroscopy can exist within a maceral group, so that for the most critical work on coals it is necessary to take this into account. 相似文献
32.
Two parasitic angiosperms,Agalinis purpurea (Scrophulariaceae) andStriga asiatica (Scrophulariaceae), are compared as to the chemical recognition events involved in host selection.Agalinis is a hemiparasite which can mature to seed-set without a host, whereasStriga is a holoparasite and survives for only a very limited time without a host. Both parasites, however, attach to a host through a specialized organ known as the haustorium and regulate the development of this organ through the recognition of chemical factors from host plants. We now describe the discovery of 2,6-dimethoxy-p-benzoquinone (2,6-DMBQ) as an haustoria-inducing principle fromSorghum root extracts. Our investigation of this compound has led us to suggest that one level of host recognition in these parasitic plants is mediated through their enzymatic digestion of the host root surface. Degradation of surface components liberates quinonoid compounds, such as 2,6-DMBQ, which in turn trigger haustorial development. 相似文献
33.
The main disadvantage of the Claus process is that by introducing air as oxidant a large volume of tail gas is produced. This must be treated to reduce atmospheric emissions of sulfur-containing gases. The costs of the tail-gas unit are a significant fraction of the total capital and operating costs for sulfur recovery. A new process uses thermal decomposition of hydrogen sulfide in the presence of carbon dioxide instead of air oxidation. The products of this reaction are hydrogen, carbon monoxide, elemental sulfur, water vapor and carbonyl sulfide. Carbonyl sulfide is easily converted to H2S and C02 by liquid- or vapor-phase hydrolysis. Unreacted H2S and C02 are recovered by absorption and recycled to the reactor. Since no air is introduced, there is no tail gas and the tail-gas unit is eliminated, giving a substantial reduction in capital investment. The concentrations of sulfur-containing gases in the product streams depend only on the operation of the absorber and stripper units and can be controlled to very low levels by increasing stripper boil-up. Process operating costs depend on the level of sulfur recovery required and can also be much lower than those of the modified Claus Process.
The process chemistry depends on a shift in the equilibrium of H2S decomposition caused by reaction of hydrogen with C02 by the reverse of the water-gas-shift reaction. Catalysts for this chemistry have been identified. Reactor conversion is further improved by rapid cooling of the reactor effluent gas. Other aspects of process design and operation confer further advantages with respect to the Claus process; however, the process equipment used is similar to that used in a Claus plant. Retrofit of existing plant to the new technology can therefore be considered. 相似文献
The process chemistry depends on a shift in the equilibrium of H2S decomposition caused by reaction of hydrogen with C02 by the reverse of the water-gas-shift reaction. Catalysts for this chemistry have been identified. Reactor conversion is further improved by rapid cooling of the reactor effluent gas. Other aspects of process design and operation confer further advantages with respect to the Claus process; however, the process equipment used is similar to that used in a Claus plant. Retrofit of existing plant to the new technology can therefore be considered. 相似文献
34.
Cameron Allan Gunn Jennifer L. DicksonChristopher G. Pretty Jane M. AlsweilerAdrienne Lynn Geoffrey M. ShawJ. Geoffrey Chase 《Computer methods and programs in biomedicine》2014
Introduction
Hyperglycaemia is a common complication of stress and prematurity in extremely low-birth-weight infants. Model-based insulin therapy protocols have the ability to safely improve glycaemic control for this group. Estimating non-insulin-mediated brain glucose uptake by the central nervous system in these models is typically done using population-based body weight models, which may not be ideal.Method
A head circumference-based model that separately treats small-for-gestational-age (SGA) and appropriate-for-gestational-age (AGA) infants is compared to a body weight model in a retrospective analysis of 48 patients with a median birth weight of 750 g and median gestational age of 25 weeks. Estimated brain mass, model-based insulin sensitivity (SI) profiles, and projected glycaemic control outcomes are investigated. SGA infants (5) are also analyzed as a separate cohort.Results
Across the entire cohort, estimated brain mass deviated by a median 10% between models, with a per-patient median difference in SI of 3.5%. For the SGA group, brain mass deviation was 42%, and per-patient SI deviation 13.7%. In virtual trials, 87–93% of recommended insulin rates were equal or slightly reduced (Δ < 0.16 mU/h) under the head circumference method, while glycaemic control outcomes showed little change.Conclusion
The results suggest that body weight methods are not as accurate as head circumference methods. Head circumference-based estimates may offer improved modelling accuracy and a small reduction in insulin administration, particularly for SGA infants. 相似文献35.
The effects of octylglucoside on the substrate specificity, kinetics and aggregation state of purified carnitine palmitoyltrasferase
(CPT) from beef heart mitochondria were investigated and compared to the effects of Triton X-100. Conditions in which CPT
can be assayed in the absence of micelles and albumin, thereby eliminating miceller effects on the kinetic parameters, are
described. When octylglucoside is substituted for Triton X-100, the specificity of CPT in the forward direction shifts towards
the long-chain acyl-CoAs, and large changes in the kinetic constants are observed. The K0.5 for L-carnitine varied as much as 50-fold, depending on the acyl-CoA and detergent used. At pH 8.0 and 200 μM palmitoyl-CoA,
the K0.5 for L-carnitine is 4.9 mM in 12 mM octylglucoside and 0.2 mM in 0.1% Triton X-100. Octylglucoside enhances the activity of
CPT with long-chain acyl-CoA and lowers the K0.5 for these substrates. At pH 6.0, the K0.5 for palmitoyl-CoA is 24.2 μM in 0.1% Triton X-100, in contrast to 3.1 μM in 12 mM octylglucoside. Octylglucoside is a competitive
inhibitor of CPT with octanoyl-CoA as substrate with a Ki of 15 mM. Nonlinear kinetics for both acyl-CoAs and L-carnitine are observed when the concentration of octylglucoside is
reduced to less than half of its critical micellar concentration (cmc). Gel filtration of CPT in octylglucoside below its
cmc gives a single protein peak with a molecular mass of ca. 660,000 daltons. These data indicate that the catalytically active
form of purified CPT is an aggregate that has quaternary structure and must have a very flexible catalytic site whose affinity
for substrate and catalytic efficiency can be altered greatly by changes in environment and experimental conditions. 相似文献
36.
Yukiko Hirano Ilde Guedes Marcus Grimsditch Chun-Keung Loong Nobuyoshi Wakabayashi Lynn A. Boatner 《Journal of the American Ceramic Society》2002,85(4):1001-1003
The room-temperature elastic constants of ErVO4 were considerably smaller than those of isostructural silicate and phosphate analogs. The generally "less-rigid" crystalline lattice and weaker metal-oxygen bond-strength in the RVO4 (R = rare earth elements) phases indicates that these materials are of interest for potential applications as an interphase component in toughened oxide ceramic composites. 相似文献
37.
The factors affecting the adsorption and desorption kinetics in a TEOM are reviewed in detail with particular attention given to the assumptions required to obtain physical transport parameters from the data. Two models are presented to simulate TEOM adsorption data in the case that concentration differences down the catalyst bed can be neglected, as is appropriate when the amount of catalyst used is small, the carrier gas flowrate is large, and/or the adsorbate partial pressure is low. In the first model, the effective diffusion coefficient, De, is taken to be constant. In the second model, the effective diffusion coefficient is assumed to obey the Darken equation, De=D0/(1−θ). The TEOM results obtained on n-hexane, n heptane, n-octane, toluene and p-xylene on a commercial FCC catalyst and on pure rare-earth exchanged zeolite Y under non-reacting conditions (373-) are analysed in detail. It is found that intracrystalline diffusion is not the limiting factor affecting the overall rates of adsorption and desorption for the systems studied. Instead, it is the transport of molecules between the adsorbed and vapour phases at the edge of zeolite crystallites that is the limiting transport step affecting the overall kinetics. For the FCC catalyst, the limiting step is the transport of molecules at the zeolite-matrix interface rather than, say, the matrix-vapour interface. Local rate constants for the desorption of the hydrocarbons at the rate-controlling interface have been obtained. 相似文献
38.
David C. Hitchcock Richard P. Rusin D. Lynn Johnson 《Journal of the American Ceramic Society》1991,74(9):2165-2169
Thermodynamic calculations predict, and experiments verify, that YBa2 Cu3 O7 -8 (123) powder is unstable in the presence of NOx -containing aerosol reactor exhaust gases at temperatures below about 600°C. Powders collected above the stability temperature are single-phase 123, while powders collected at lower temperature contain Ba(NO3 )2 formed by reaction of the powder with NOx , after exit from the hot zone. 相似文献
39.
Lynn F. Gladden 《Topics in Catalysis》2003,24(1-4):19-28
NMR has long been established as an in situ technique for studying the solid-state structure of catalysts and the chemical processes occurring during catalytic reactions. Increasingly, pulsed field gradient (PFG) NMR and magnetic resonance imaging (MRI) are being exploited in chemical reaction engineering to measure molecular diffusion, dispersion and flow hydrodynamics within reactors. By bringing together NMR spectroscopy, PFG NMR and MRI, we are now able to probe catalysts and catalytic processes from the angstrom-to-centimeter scale. This article briefly reviews current activities in the field of MRI studies applied to catalysts and catalytic reactors. State-of-the-art measurements, which can already be used in real reactor design studies, are illustrated with examples of single-phase flow with and without chemical reaction in a fixed-bed reactor. The ability to obtain high spatial resolution (< 200μm) in images of the internal structure and flow field within reactors is demonstrated, and the potential uses of these data in reactor design and understanding bed fouling phenomena are discussed. In particular, MRI has produced the first detailed measurements of the extent of heterogeneity in the flow field within fixed-bed reactors. The example of a fixed-bed esterification process is used to show how NMR spectroscopy and MRI techniques can be combined to provide spatially resolved information on both hydrodynamics and chemical conversion within a process unit. The emerging area of ultrafast MRI is then highlighted as an area of particular interest. Recent advances have demonstrated that it is possible to record 2D images over timescales of ~100ms in the magnetically heterogeneous environments typical of heterogeneous chemical reactors. These advances open up opportunities to image many unsteady state processes for the first time. Examples are given of real-time visualization of bubble-train flow in a ceramic monolith and exploring the stability of the gas–liquid distribution as a function of liquid flow rate in a trickle-bed reactor. 相似文献
40.
Melanie T. Schaal Ashley Y. Metcalf Joseph H. Montoya J. Paul Wilkinson Carol C. Stork Christopher T. Williams John R. Monnier 《Catalysis Today》2007,123(1-4):142-150
Electroless deposition has been used to prepare Cu–Pd/SiO2 bimetallic catalysts wherein initial Cu coverages are limited only to the pre-existing Pd surface. Cu loading on the Pd surface can be systematically varied by modification of deposition kinetic parameters. In this case deposition time was used as the kinetic variable for the preparation of a series of Cu–Pd catalysts. These materials have been characterized using atomic absorption, CO chemisorption, and FT-IR (adsorption of CO), and then evaluated for the hydrogenation of 3,4-epoxy-1-butene, a functionalized olefin having many potential reaction pathways. Catalyst performance and characterization results suggest that Cu is not distributed in a monodisperse manner on the Pd surface, indicating the existence of autocatalytic deposition of Cu on Cu sites. The FT-IR results suggest that although CO adsorption on all sites is suppressed by Cu addition, initial Cu deposition occurs more readily on certain sites. The bimetallic Cu–Pd sites that are formed exhibit unusually high activity for EpB conversion and formation of unsaturated alcohols and aldehydes. This bimetallic effect on catalyst activity and selectivity is best explained, not by the existence of either ligand or ensemble effects, but rather by the bifunctional nature of the Cu–Pd sites present on the surface of these catalysts. 相似文献