| 1. |
- Creaser, Derek, et al.
(författare)
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Cyclic operation of the oxidative dehydrogenation of propane
- 1999
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Ingår i: Chemical Engineering Science. - 0009-2509 .- 1873-4405. ; 54:20, s. 4437-4448
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Tidskriftsartikel (refereegranskat)abstract
- The cyclic operation of the oxidative dehydrogenation of propane over a V-Mg-O catalyst by the alternate feeding of propane and oxygen gas mixtures was investigated. Generally, the response of the reaction products following a step-change in composition determined the cyclic conditions that resulted in an improved yield of propene. By alternating propane and oxygen with a 1 : 1 cycle split, time-average propene yields higher than steady state could be obtained. The optimal cycle period was about 60 s. This same period provided high propane conversion, as well as high propene selectivity. The cycle split could be varied by shortening the oxygen-feed half of the cycle with little effect on the time-average results. The oxygen half-cycle needed only to be long enough to reoxidize the catalyst. The propane feed scheme was varied by feeding increasing amounts of propane in the oxygen half-cycle. The change in feed scheme caused an increase in the time-average propane conversion that resulted in some further improvement in the propene yield despite some loss in propene selectivity.
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| 2. |
- Creaser, D., et al.
(författare)
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Oxygen partial pressure effects on the oxidative dehydrogenation of propane
- 1999
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Ingår i: Chemical Engineering Science. - 0009-2509 .- 1873-4405. ; 54:20, s. 4365-4370
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Tidskriftsartikel (refereegranskat)abstract
- The effect of the gas-phase oxygen partial pressure for the oxidative dehydrogenation of propane over a V-Mg-O catalyst was investigated to explore reasons for improved propene yields when dehydrogenation was carried out under periodic operation. Steady-state experiments were performed in which the oxygen partial pressure was varied over a large range at constant propane partial pressures. In addition, the catalyst mass was varied to control the propane conversion so that selectivity and yield could be compared at a constant propane conversion. At the same propane conversion, propene selectivity increased as the partial pressure of oxygen decreased. Thus, propene yields can be improved at steady state by employing low oxygen partial pressures. It is likely that this is the reason for the higher propene yields observed under unsteady-state or periodic operation. It appears that the reaction mechanism must consist of more than one pathway for production of carbon oxides
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| 3. |
- Creaser, Derek, et al.
(författare)
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Transient kinetic analysis of the oxidative dehydrogenation of propane
- 1999
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 182:1, s. 264-269
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Tidskriftsartikel (refereegranskat)abstract
- Oxidative dehydrogenation of propane was studied using various transient techniques. Results support a redox mechanism in which propane reduces the catalyst, which is reoxidized by gas-phase oxygen. Only lattice oxygen participates in propene formation. Desorbable oxygen is a major source of poor selectivity, although lattice oxygen also causes total oxidation. Consequently, propene selectivity in the absence of gas-phase O2is superior to co-feed, steady-state selectivity at the same propane conversion. Propene selectivity is further improved by increasing the degree of catalyst reduction.
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| 4. |
- Creaser, Derek, et al.
(författare)
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Transient study of oxidative dehydrogenation of propane
- 1999
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Ingår i: Applied Catalysis A. - 0926-860X .- 1873-3875. ; 187:1, s. 147-160
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Tidskriftsartikel (refereegranskat)abstract
- Kinetics and the mechanism of the oxidative dehydrogenation of propane were investigated using various transient techniques. Results support a redox reaction mechanism in which propane and intermediate products react with lattice oxygen, reducing the catalyst surface, which is reoxidized by gas-phase O2. Partial reduction of the catalyst occurs during the start-up to a steady state. Successive pulsing with C3H8 reduced V5+ in the magnesium ortho-vanadate phase to V2+. Carbon-containing species were observed upon interruption of the reaction, although only minute amounts were formed. Cycling increases the amount of the carbon deposited, but this carbon is reactive and most of it is oxidized in the succeeding O2 pulse. Temperature-programmed oxidation (TPO) experiments on the catalyst used in steady-state operation revealed mainly strongly bound carbonaceous matter on the catalyst, but this carbon deposition did not affect catalyst activity. Thus, adsorbed oxygen is an important source of total combustion. Our experiments show, however, that lattice oxygen also produces total oxidation. Propene selectivity of the reaction in the absence of gas-phase O2 was superior to steady-state selectivity, at the same propane conversion. Propene selectivity could be further improved by increasing the degree of reduction of the catalyst
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| 5. |
- Steinby, K, et al.
(författare)
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The effect of temperature on the adsorption of a nonionic surfactant on a PMMA latex
- 1993
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Ingår i: Journal of Colloid and Interface Science. - 0021-9797 .- 1095-7103. ; 155, s. 70-78
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Tidskriftsartikel (refereegranskat)abstract
- Adsorption isotherms of a nonionic surfactant, viz. a nonylphenol-polyethylene oxide with 20 ethylene oxide units in the chain, NP-EO20, on poly(methyl methacrylate), PMMA, latex have been determined at 10, 25, 40, 60 and 80°C. At all temperatures the isotherms are of high affinity type, i.e. strong adsorption at low solution concentration reaching a limiting value at higher concentration. The adsorption is strongly temperature dependent and increasing with the temperature. Thermo--dynamic analysis reveals that the only factor significantly affecting the temperature dependence of the adsorption of nonionic surfactants of NP-EOn type is the polyethylene oxide - water interaction. Adsorption measurements can therefore be used as a sensitive probe for this interaction. Two explanations are given for the temperature dependence of the polyethylene oxide - water interaction. The first is that the temperature dependence of this interaction is dominated by the water structuring, normally occuring around hydrophobic molecules. The increase in adsorption with temperature is a result of the decreasing water structuring. The second explanation is that the polymer has a higher probability of occupying less favorable conformations as the temperature is increased, thus rendering the polymer less soluble in the water phase, which in turn increases the adsorption.
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| 6. |
- Kronberg, B, et al.
(författare)
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On the nature and role of water structuring in the adsorption of nonionic surfactants on hydrophobic surfaces
- 1990
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Ingår i: Progress in Colloid and Polymer Science. - 0340-255X .- 1437-8027. ; 83, s. 75-83
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Tidskriftsartikel (refereegranskat)abstract
- High pressure liquid chromatography (HPLC) results were used to calculate the temperature dependence of the hydrocarbon-water interaction parameter. The temperaturte dependence has been interpreted in terms of water structuring around the hydrocarbon. The role of water structuring is to enhance the solubility of hydrocarbons in water, which is equivalent to a decrease of the hydrocarbon-water interaction parameter. The poor solubility of hydrocarbons in water is not due to the water structuring, but to other factors such as the large energy required to form a cavity in the water for the hydrocarbon. This is a reflection of the very large cohesive forces in liquid water. These results, together with the temperature dependence of the polyethylene oxide-water interaction parameter, have been used to calculate the temperature dependence of the adsorption of a nonionic surfactant, viz. an ethoxylated nonyl phenol with 20 ethylene oxide units (NP-EO20). It is shown, that water structuring causes, an increased adsorption with temperature. Without water structuring a decreased adsorption is predicted.
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| 7. |
- Kronberg, B, et al.
(författare)
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Thermodynamics of the hydrophobic effect in surfactant solutions – micellization and adsorption
- 1995
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Ingår i: Pure and Applied Chemistry. - 0033-4545 .- 1365-3075. ; 67, s. 897-902
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Tidskriftsartikel (refereegranskat)abstract
- The thermodynamics of surfactant micellization and adsorption onto hydrophobic surfaces are examined. The ideal and the Flory-Huggins approximations for the combinatorial entropy of transfer are contrasted. Two main contributions for the Gibbs free energy of transfer from the aqueous solution to the micelle or the surface are considered. The first is the collapse of the water cavity left behind when the solute is transferred and the second is the destruction of the water structuring around the solute. It is the balance between these two contributions which causes the minimum in critical micelle concentration and the maximum in adsorption of ionic surfactants with temperature. Separation of these two contributions is achieved using a two-state model recently reported for water molecules around non-polar solutes.
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| 8. |
- Kronberg, B, et al.
(författare)
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Understanding the hydrophobic effect
- 1994
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Ingår i: Journal of Dispersion Science and Technology. - 0193-2691 .- 1532-2351. ; 15, s. 333-351
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Tidskriftsartikel (refereegranskat)abstract
- The hydrophobic effect is the common expression for processes where nonpolar groups in molecules are spontaneously removed from water. Thermodynamic analysis of hydrocarbon solubility in water, micellization and adsorption of surfactants show that the hydrophobic effect can be understood in terms of two contributions. The first contribution is attributed to the structuring, or rearrangement, of water molecules in the vicinity of a hydrophobe. This contribution is favorable, and hence increases the solubility of hydrocarbons in water, increases the cmc, and decreases the adsorption of surfactants. The second contribution is attributed to the formation of a cavity in the water in order to accommodate the hydrophobe. This contribution dominates over the first one and is unfavorable, i.e. it decreases the solubility of hydrocarbons in water, decreases the cmc, and increases the adsorption of surfactants. Thus, the cause of the hydrophobic effect is to be found in the large energy required to form a cavity in the water. On the other hand the temperature dependence of the hydrophobic effect is entirely determined by the water structuring, or rearrangement, in the vicinity of a hydrophobe.
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| 9. |
- Mancera, RL, et al.
(författare)
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Thermodynamics of the hydration of non-polar substances
- 1998
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Ingår i: Biophysical Chemistry. - 0301-4622 .- 1873-4200. ; 70, s. 57-63
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Tidskriftsartikel (refereegranskat)abstract
- We re-examine the numerical value and physical significance of TS , the temperature where the entropy of transfer Δ WLCP fitting function. It is concluded that the interpretation of TS as the temperature where hydration ceases cannot be sustained. As previously reported [R.L. Baldwin, N. Muller, Proc. Natl. Acad. Sci. USA, 89 (1992) 7110], hydration must vanish at a temperature T1 >TS, where its experimental manifestation, i.e., ΔWL CP, is zero. We discuss the concept of water relaxation around a non-polar solute molecule and its relation to the hydration process.
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| 10. |
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