Publications (114)190 Total impact

Article: Influence of a magnetic field on the viscosity of a dilute gas consisting of linear molecules
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ABSTRACT: The viscomagnetic effect for two linear molecules, N2 and CO2, has been calculated in the dilutegas limit directly from the most accurate ab initio intermolecular potential energy surfaces presently available. The calculations were performed by means of the classical trajectory method in the temperature range from 70 K to 3000 K for N2 and 100 K to 2000 K for CO2, and agreement with the available experimental data is exceptionally good. Above room temperature, where no experimental data are available, the calculations provide the first quantitative information on the magnitude and the behavior of the viscomagnetic effect for these gases. In the presence of a magnetic field, the viscosities of nitrogen and carbon dioxide decrease by at most 0.3% and 0.7%, respectively. The results demonstrate that the viscomagnetic effect is dominated by the contribution of the jj ¯ polarization at all temperatures, which shows that the alignment of the rotational axes of the molecules in the presence of a magnetic field is primarily responsible for the viscomagnetic effect.  [Show abstract] [Hide abstract]
ABSTRACT: The compressed liquid viscosities of orthoxylene, metaxylene, paraxylene and ethylbenzene were measured using a vibratingwire viscometer at different temperatures and pressures. The measurements were performed over the temperature ranges of (273 to 373) K for oxylene and mxylene, (293 to 373) K for pxylene and (253 to 373) K for ethylbenzene, at pressures from (0.1 to 30) MPa, except for ethylbenzene for which the pressure range was up to 35 MPa. The combined expanded uncertainty of the reported viscosity is better than 2% with a confidence level of 0.95 (k = 2). The experimental data were correlated with the empirical Andrade–Tait equation which reproduced the results with the average absolute percentage deviations of (0.25, 0.15, 0.16 and 0.23)% for oxylene, mxylene, pxylene and ethylbenzene, respectively. The present results are in good agreement with most of the literature values. 
Article: Impact of Thermodiffusion on the Initial Vertical Distribution of Species in Hydrocarbon Reservoirs
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ABSTRACT: In this work we propose a methodology, based on molecular dynamics simulations, to quantify the influence of segregation and thermodiffusion on the initial state distribution of the fluid species in hydrocarbon reservoirs. This convectionfree approach has been applied to a synthetic oil composed of three normal alkanes and to a real acid gas. It has been found that the thermodiffusion effect induced by the geothermal gradient is similar (but opposite in sign) to that due to segregation for both mixtures. In addition, because of the combined effect of thermal expansion and thermodiffusion, it has been observed that the density gradient can be reversed, in the presence of a geothermal gradient. These numerical results emphasize the need of improving our quantification of thermodiffusion in multicomponent mixtures. The SCCOSJ10 experiments will be a crucial step towards this goal.  [Show abstract] [Hide abstract]
ABSTRACT: A model is proposed to predict the weathering of LNG stored in containment tanks, typically used in regasification terminals, due to the effects of heat ingress and BoiloffGas (BOG) release. The model integrates a rigorous thermodynamic model of LNG vapourliquid equilibrium and a realistic heat transfer model. It provides a number of advances on previously developed models, in so far as: (i) heat ingress is calculated based on the outside temperature and LNG composition, that allows for daily or seasonal variation; (ii) BoiloffRatio is not an input parameter, but is calculated as part of the simulations and (iii) the LNG density is estimated using an accurate experimentally based correlation. The model was validated using real industry data and the agreement obtained in predicting the overall composition of weathered LNG, its density and the amount vaporized was within current industry requirements. The model was run in the predictive mode to explore the sensitivity of BOG to different scenarios. In the initial stages of weathering the nitrogen content of LNG will have a marked effect on BOG generation. Even the presence of 0.5% of nitrogen will lead to nearly a 7% decrease in BOG, making the initial BOG unmarketable. The high sensitivity is a result of preferential evaporation of nitrogen and increase in the direct differential molar latent heat. In the final stages of weathering the heavier hydrocarbons govern the dynamics of BOG which becomes a strong function of the initial composition and the level of LNG remaining in the storage tank. The change in ambient temperature of 1°C will lead to a change in BOG of 0.2%, irrespective of the size of the tank and initial LNG composition.  [Show abstract] [Hide abstract]
ABSTRACT: A new correlation for the viscosity of paraxylene (pxylene) is presented. The correlation is based upon a body of experimental data that has been critically assessed for internal consistency and for agreement with theory. It is applicable in the temperature range from the triple point to 673 K at pressures up to 110 MPa. The overall uncertainty of the proposed correlation, estimated as the combined expanded uncertainty with a coverage factor of 2, varies from 0.5% for the viscosity of the dilute gas to 5% for the highest temperatures and pressures of interest. Tables of the viscosity generated by the relevant equations, at selected temperatures and pressures and along the saturation line, are provided.  [Show abstract] [Hide abstract]
ABSTRACT: A fivedimensional potential energy surface (PES) for the interaction of a rigid methane molecule with a rigid nitrogen molecule was determined from quantumchemical ab initio calculations. The counterpoisecorrected supermolecular approach at the CCSD(T) level of theory was utilized to compute a total of 743 points on the PES. The interaction energies were calculated using basis sets of up to quadruplezeta quality with bond functions and were extrapolated to the complete basis set limit. An analytical sitesite potential function with nine sites for methane and five sites for nitrogen was fitted to the interaction energies. The PES was validated by calculating the cross second virial coefficient as well as the shear viscosity and binary diffusion coefficient in the dilutegas limit for CH4–N2 mixtures. An improved PES was obtained by adjusting a single parameter of the analytical potential function in such a way that quantitative agreement with the most accurate experimental values of the cross second virial coefficient was achieved. The transport property values obtained with the adjusted PES are in good agreement with the best experimental data.  [Show abstract] [Hide abstract]
ABSTRACT: A new correlation for the viscosity of cyclohexane is presented. The correlation is based upon a body of experimental data that has been critically assessed for internal consistency and for agreement with theory. It is applicable in the temperature range from the triple point to 700 K at pressures up to 110 MPa. In the dilute gas region, at pressures below 0.3 MPa, the correlation is valid up to 873 K. The overall uncertainty of the proposed correlation, estimated as the combined expanded uncertainty with a coverage factor of 2, varies from 0.5% for the viscosity of the dilute gas and of liquid at ambient pressure to 5% for the viscosity at high pressures and temperatures. Tables of the viscosity generated by the relevant equations, at selected temperatures and pressures and along the saturation line, are provided. (C) 2014 AIP Publishing LLC.  [Show abstract] [Hide abstract]
ABSTRACT: The recently developed Enskog2 sigma model has been extended to correlate and predict the viscosity of simple liquids. The model is based on the Enskog formulation and makes use of two effective diameters to represent two aspects of molecular interactions in Enskog's treatment; namely, the effective diameter representing the excluded volume of a molecule and the effective diameter accounting for the increased probability of collision in comparison to the dilute gas. We have tested the model by analysing the viscosity of six simple fluids (Ar, N2, O2, CO2, CH4, and C2H6) in the liquid phase. The model is capable of correlating the liquid viscosity of the studied fluids with the AAD in the range of 0.10.7% and with maximum deviation not exceeding 4.6%. We made use of the observed universal behaviour of the effective diameters to show that the viscosities of the fluids studied are conformal to that of argon. Based on this finding the Enskog2 sigma model can be used in the predictive mode. Under these circumstances it reproduces the viscosity data obtained from the recommended correlations with AAD in the range 0.83.2%, depending on the way in which the length scaling parameters are estimated. 
Article: Viscosity of liquids—Enskog2σ model
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ABSTRACT: The recently developed Enskog2σ model has been extended to correlate and predict the viscosity of simple liquids. The model is based on the Enskog formulation and makes use of two effective diameters to represent two aspects of molecular interactions in Enskog's treatment; namely, the effective diameter representing the excluded volume of a molecule and the effective diameter accounting for the increased probability of collision in comparison to the dilute gas. We have tested the model by analysing the viscosity of six simple fluids (Ar, N2, O2, CO2, CH4, and C2H6) in the liquid phase. The model is capable of correlating the liquid viscosity of the studied fluids with the AAD in the range of 0.1–0.7% and with maximum deviation not exceeding 4.6%. We made use of the observed universal behaviour of the effective diameters to show that the viscosities of the fluids studied are conformal to that of argon. Based on this finding the Enskog2σ model can be used in the predictive mode. Under these circumstances it reproduces the viscosity data obtained from the recommended correlations with AAD in the range 0.8–3.2%, depending on the way in which the length scaling parameters are estimated.  [Show abstract] [Hide abstract]
ABSTRACT: An extended hardsphere model is reported that may be applied to correlate and predict the viscosity of gases, liquids and supercritical fluids. The method is based on the hardsphere model of Dymond and Assael and uses their roughness factors and molar core volumes to relate reduced viscosity to a universal function of reduced volume. The extended model behaves correctly in the limit of low densities and offers improved accuracy at high densities. The new universal reference function was determined from a large database of experimental viscosities for alkanes extending up to reduced densities of 0.84. It has been tested by correlating the viscosity of two highviscosity liquids not used in the development of the universal function and has shown to perform satisfactorily up to reduced densities of approximately 0.9. 
Article: Calculation of the relaxation properties of a dilute gas consisting of Lennard–Jones chains
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ABSTRACT: The relaxation properties in the dilutegas limit have been calculated by the classical trajectory (CT) method for a gas consisting of chainlike molecules that are rigid and interact through site–site Lennard–Jones 12–6 potentials. Results are reported for volume viscosity ηVηV, rotational collision number ζrotζrot and the ratio of the rotational to selfdiffusion coefficient Drot/DDrot/D. The results indicate that the volume viscosity increases with temperature and decreases with chain length. The rotational relaxation of chains is efficient, as it takes of the order of 1.75–2.7 collisions to attain equilibrium. The rotational collision number is only weakly temperature dependent.  [Show abstract] [Hide abstract]
ABSTRACT: This paper contains new, representative reference equations for the thermal conductivity of nheptane. The equations are based in part upon a body of experimental data that have been critically assessed for internal consistency and for agreement with theory whenever possible. In the case of the dilutegas thermal conductivity, a theoretically based correlation was adopted in order to extend the temperature range of the experimental data. Moreover, in the critical region, the experimentally observed enhancement of the thermal conductivity is well represented by theoretically based equations containing just one adjustable parameter. The correlations are applicable for the temperature range from the triple point to 600 K and pressures up to 250 MPa. The overall uncertainty (considered to be estimates of a combined expanded uncertainty with a coverage factor of 2) of the proposed correlation is estimated, for pressures less than 250 MPa and temperatures less than 600 K, to be less than 4%. 
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ABSTRACT: The transport properties in the dilute gas limit have been calculated by the classicaltrajectory method for a gas consisting of chainlike molecules. The molecules were modelled as rigid chains consisting of spherical segments that interact through a combination of sitesite LennardJones 126 potentials. Results are reported for shear viscosity, selfdiffusion, and thermal conductivity for chains consisting of 1, 2, 3, 4, 5, 6, 7, 8, 10, 13, and 16 segments in the reduced temperature range of 0.3  50. The results indicate that the transport properties increase with temperature and decrease with chain length. At high temperatures the dependence of the transport properties is governed effectively by the repulsive part of the potential. No simple scaling with chain length has been observed. The higher order correction factors are larger than observed for real molecules so far, reaching asymptotic values of 1.019  1.033 and 1.060  1.072 for viscosity and thermal conductivity, respectively. The dominant contribution comes from the angular momentum coupling. The agreement with molecular dynamics calculations for viscosity is within the estimated accuracy of the two methods for shorter chains. However, for longer chains differences of up to 7% are observed.  [Show abstract] [Hide abstract]
ABSTRACT: In this work, we propose a new kinetic theory model (Enskog2σ model) to analyze and predict the viscosity of simple fluids. The model is based on the Enskog formulation and makes use of two effective diameters to represent two aspects of molecular interactions in Enskog's treatment; namely, the effective diameter representing the excluded volume of a molecule and the effective diameter accounting for the increased probability of collision in comparison to the dilute gas. We have tested the model by analyzing the viscosity of five simple fluids (Ar, CH4, N2, CO2 and SF6) of increasing complexity at supercritical temperatures. The new Enskog2σ model outperforms previous approaches based on Enskog theory in terms of its correlative ability. We demonstrate that the effective diameter associated with the excluded volume exhibits a universal behaviour as a function of reduced temperature. The conformal behaviour can be achieved by means of a single fluiddependent length scaling parameter. We make use of this finding to develop a general model that allows the prediction of the viscosity of one fluid from the knowledge of the viscosity of a reference fluid, which in this work was chosen to be Ar. The accuracy of the predicted viscosity depends on the way in which the length scaling parameter was estimated. If the length scaling parameter is obtained from the knowledge of viscosity along a single isotherm, the accuracy of the predicted viscosity is similar to the uncertainty of the original correlation over its entire supercritical range. If a single viscosity value is used to estimate the length scaling parameter, the viscosities of supercritical CH4, N2, CO2 and SF6 are predicted with the maximum deviation of 3.3%, 4.2%, 9.9% and 10.5% respectively over the entire range of validity of the Enskog2σ model.  [Show abstract] [Hide abstract]
ABSTRACT: This paper reports a set of experiments carried out to examine asphaltene deposition in a glass microcapillary pipet, as a proxy to asphaltene deposition in reservoir pores. A new capillaryflow experiment was designed to ensure capillarydominated flow, low inertial forces, negligible dispersion, and insignificant gravity effects, and the endeffect was limited to 0.1% of capillary length. This was achieved by maintaining the flow rates in the range 5 ≤ QT ≤ 60 μL/min. The asphaltene precipitation was induced by bringing into contact a heavy oil sample, diluted with toluene, with a number of different precipitants (npentane, nheptane, and noctane). The deposition of asphaltene was monitored by imaging the capillary tube and by measuring the pressure drop across it. A new, simple model has been developed to relate the pressure drop to the change in the thickness of the deposited layer and subsequently to the change in permeability. The model indicates that the thickness varies as one over the fourth power of the pressure drop, while the change in permeability is proportional to the square root of the pressure drop. A series of experiments has been carried out to examine the effects of different flow rates, precipitant concentration, and the nature of the precipitant on asphaltene deposition. The deposit growth was generally monotonic and reasonably uniform, indicating the lack of erosion and entrainment. The results indicate the deposition rate increases with decreasing carbon number of the precipitant, while it is not unduly influenced by changes in a flow rate. No noticeable deposition was observed when the precipitant/solvent ratio was 1.22 despite the ratio exceeding the minimum threshold value of 1.08, obtained from gravimetry.  [Show abstract] [Hide abstract]
ABSTRACT: New, simple, and practical correlations for shear viscosity, selfdiffusion coefficient, bulk viscosity, and thermal conductivity of hydrogen sulfide in the limit of zero density are provided, together with a correlation for the second pressure virial coefficient. The correlations are based on the values of thermophysical properties generated from a highly accurate, fully quantummechanical, ab initio potential energy surface. The validation of the computed values of thermophysical properties against the rather scarce experimental data demonstrates an excellent agreement with the most accurate data sets. The analysis undertaken indicates that the correlated values provide the most reliable, accurate, and internally consistent representation of thermophysical properties of hydrogen sulfide. The correlations extend over the temperature range (180 to 2000) K. The behavior of each transport property is represented by an independent correlation of the appropriate effective collision cross section as a function of temperature, while the behavior of the second pressure virial coefficient is directly represented as a function of temperature. The uncertainty of the proposed transport property correlations varies from ± 0.4 % for the shear viscosity in the temperature range (300 to 700) K to ± 5.0 % for the bulk viscosity. The uncertainty of the second pressure virial coefficient correlation is estimated to be of the order of ± 1 cm3·mol–1 at temperatures above 400 K, decreasing to ± 30 cm3·mol–1 at 180 K.  [Show abstract] [Hide abstract]
ABSTRACT: New expressions for the viscosity of liquid mixtures, consisting of chainlike molecules, are derived by means of Enskogtype analysis. The molecules of the fluid are modelled as chains of equally sized, tangentially joined, and rigid spheres. It is assumed that the collision dynamics in such a fluid can be approximated by instantaneous collisions. We determine the molecular size parameters from the viscosity of each pure species and show how the different effective parameters can be evaluated by extending the VesovicWakeham (VW) method. We propose and implement a number of thermodynamically consistent mixing rules, taking advantage of SAFTtype analysis, in order to develop the VW method for chain molecules. The predictions of the VWchain model have been compared in the first instance with experimental viscosity data for octanedodecane and methanedecane mixtures, thus, illustrating that the resulting VWchain model is capable of accurately representing the viscosity of real liquid mixtures.  [Show abstract] [Hide abstract]
ABSTRACT: An investigation into the impairment of permeability in porous media as a result of the deposition of asphaltene particulates in a flowing stream is presented. By incorporating kinetics of asphaltene deposition under dynamic conditions into the deepbed filtration (DBF) theory, we have developed a new asphaltene deposition and impairment model, which has only three tuning parameters. The parameters, characterizing the key aspects of the deposition process and porousmedia hydraulics, are the stabilized filtration coefficient λ0, the filtration time constant τ, and the exponent n, relating porosity and permeability. The resulting models are tested against a large number of experimental data sets, representing common porous media and particulate streams. The validation exercise indicates that our dynamicfiltration model is capable of rationalizing about 82% of the reference data sets within 7%. 
Publication Stats
2k  Citations  
190.00  Total Impact Points  
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Institutions

19872015

Imperial College London
 • Department of Earth Science and Engineering
 • Department of Chemical Engineering
Londinium, England, United Kingdom


1986

Institute of Chemistry, Technology and Metallurgy
Beograd, Central Serbia, Serbia
