An accelerated successive substitution method for single stage flash calculations

An acceleration technique, based on the Dominant Eigenvalue Method of Orbach and Crowe (1971), is proposed to accelerate a successive substitution method in which liquid and vapor compositions are used as the iteration variables. The performance of this scheme is compared with the accelerated successive substitution method of Mehra et al. (1983) and the General Dominant Eigenvalue Method (GDEM) of Crowe and Nishio (1975) as implemented by Michelsen (1982, 1987) which iterate upon the equilibrium ratios. The proposed acceleration scheme exhibits a less oscillatory behavior during convergence, and requires less storage than the accelerated methods using the K-factors as the iteration variables. However, the number of thermodynamic evaluations is comparable with that of Mehra et al. (1983) and is more than that of Michelsen (1982, 1987).     

Experimental investigation of hydrate formation behaviour of a natural gas bubble in a simulated deep sea environment

A vertically flowing, closed circuit, high pressure water tunnel was designed and constructed for holding individual gas bubbles stationary against an opposing flow for detailed observations. Hydrate formation behavior of natural gas bubbles was studied at constant pressure as well as under conditions of controlled decompression designed to simulate buoyant rise of the bubble.A bubble of simulated natural gas suspended in 3°C salt water formed hydrates when the pressure was 4826 kPa or higher. The simulated decompression accompanying buoyant rise had very little effect on hydrate formation behavior of a bubble starting from a pressure of 5516 kPa or above. At lower starting pressures, a slight increase in the reaction rate was detected in the initial stages of a run. The conversion of the simulated natural gas to hydrates was complete in runs starting from a pressure of 4826 kPa or above.     

Kinetics of carbon dioxide and methane hydrate formation

Experimental data on the kinetics of carbon dioxide hydrate formation and its solubility in distilled water are reported. The experiments were carried out in a semi-batch stirred tank reactor at nominal temperatures of 274, 276 and 278 K and at pressure ranging from 1.59 to 2.79 MPa for the kinetics experiments and at pressure ranging from 0.89 to 2.09 MPa for the solubility experiments. A minor inconsistency in the kinetic model developed by Englezos et al. (1987a) was removed and the model was modified to determine the intrinsic kinetic rate constant for carbon dioxide hydrate formation. The same model was also used to re-determine the intrinsic kinetic rate constant for methane hydrate formation. The model is based on the crystallization theory coupled with the two-film theory for gas absorption in the liquid phase. The Henry's constant (H) and apparent dissolution rate constant (K_La) required in the model were determined using the experimental solubility data. The kinetic model describes the experimental data very well. The kinetic rate constant obtained for the carbon dioxide hydrate formation was found to be higher than that for methane.     

Decomposition and multiphase reactions in the system UO2(NO3)2·6H2O-Ni(NO3)2·6H2O at elevated temperatures

Solid state reactions between uranyl nitrate hexahydrate and nickel nitrate hexahydrate in mixtures of various ratios have been studied at elevated temperatures. The binary system of uranyl nitrate hexahydrate and nickel nitrate hexahydrate was found to form a eutectic of composition 53 mol% uranyl nitrate hexahydrate and 47 mol% nickel nitrate hexahydrate at 40 °C. The overlap of evolution of nitric oxide (NO) and water vapour above 230 °C confirmed the presence of hydroxynitrates of uranium and nickel as intermediate products. These hydroxynitrates began to react above 280 °C to form nickel uranate, NiU₃O₁₀, in the case of mixtures containing 75 mol% uranyl nitrate hexahydrate. When the proportion of uranyl nitrate hexahydrate in the mixture was higher than 75 mol%, U₃O₈ formed along with nickel uranate. For the mixtures containing uranyl nitrate hexahydrate lower than 75 mol%, NiO was observed to form along with NiU₃O₁₀.     

Extraction of essential oil from geranium (Pelargonium graveolens) with supercritical carbon dioxide

This study investigated the supercritical fluid extraction (SFE) of geranium essential oil from geranium (Pelargonium graveolens) using supercritical carbon dioxide solvent. The extraction yield was measured as a function of pressure, temperature and carbon dioxide flow rate. At low pressure (10 MPa) and high temperature (343 K), waxes were co‐extracted with the essential oil, resulting in artificially elevated essential oil extraction yields as no method was available with the SFE apparatus used to separate co‐extracted waxes and oil. At high pressure (30 MPa) and low temperature (313 K), the amount of wax co‐extracted decreased. Under these ‘optimum’ conditions, the extraction yield increased with decrease in flow rate giving a maximum extraction yield of 2.53%. All samples were analyzed by gas chromatography–mass spectrometry and the effect of pressure and extraction time on oil composition was studied. The percentage compositions of terpene hydrocarbons, terpenols, geraniol and geranyl esters were significantly affected by pressure and extraction time. The oil samples obtained by SFE were also compared with commercially obtained steam distilled samples. All major components of the commercially obtained oils were present in the SFE‐obtained oils; however, the percentage composition of the major components differed greatly between steam distilled and SFE oils. Copyright © 2005 Society of Chemical Industry     

Spin transport in N-armchair-edge silicene nanoribbons

In this article, we have investigated spin polarized electronic transport in N-armchair-edge silicene nanoribbons (N-ASiNRs, where N is the number of atoms in armchair edge) using semi-classical Monte-Carlo approach. Monte Carlo simulations are used to model spin transport along with spin density matrix calculations in the semiconductor devices. The spin vector dephasing in the silicene nanoribbons are due to Elliott-Yafet (EY) and D’yakonov-Perel (DP) relaxation mechanisms. In this article, we theoretically studied spin polarized transport along the length of the N-armchair-edge silicene nanoribbons structure and the spin dephasing length is estimated to be in the range of 3 μm for N-ASiNRs. Next, we have investigated the ensemble averaged spin vector variation in N-ASiNRs along the length of the nanoribbons with varying temperature. We observe a negligible variation in the spin dephasing length in the temperature range of 4 K to 373 K. As the localized temperature increases from 373 K to 1000 K, we observe a decrease in the spin dephasing length by extrapolating the curve up to 1000 K and in these higher temperature ranges spin dephasing length decrease due to hot electron/phonon effects and higher scattering rates. We have also studied the ensemble averaged spin vector variation in N-ASiNRs along the length of the nanoribbons with varying width (N) of the nanoribbons and found negligible effect of width (N) on the spin dephasing length in N-ASiNRs. In our study, we finally find N-armchair-edge silicene nanoribbons to be the promising candidates for next generation spintronics devices.     

Treatment of oil contaminated waste waters by foam fractionation

A laboratory investigation was carried out to determine the applicability of a foam separation process to remove organic pollutants from waste water. The effect of the operating variables namely air flow rate, surfactant flow rate, pressure, pH and concentration of the solid adsorbent were studied for different feed streams. It was found that foam separation alone could achieve 98% removal of oil from a waste stream containing emulsified oil in water. The three phase fluidized bed adsorption process yielded 98% reduction of organics in the two industrial waste waters tested. The results of a cost study carried out to establish the feasibility of using a similar process on an industrial scale are also presented.