V. Vesovic

Imperial College London, Londinium, England, United Kingdom

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Publications (97)179.91 Total impact

  • B. Balogun, N. Riesco, V. Vesovic
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    ABSTRACT: A new correlation for the viscosity of para-xylene (p-xylene) 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.
    Journal of Physical and Chemical Reference Data 03/2015; 44(1):013103. DOI:10.1063/1.4908048 · 3.11 Impact Factor
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    ABSTRACT: A five-dimensional potential energy surface (PES) for the interaction of a rigid methane molecule with a rigid nitrogen molecule was determined from quantum-chemical ab initio calculations. The counterpoise-corrected 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 quadruple-zeta quality with bond functions and were extrapolated to the complete basis set limit. An analytical site-site 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 dilute-gas 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.
    The Journal of Chemical Physics 12/2014; 141(22):224301. DOI:10.1063/1.4902807 · 3.12 Impact Factor
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    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.
    Journal of Physical and Chemical Reference Data 09/2014; 43(3):033101. DOI:10.1063/1.4891103 · 3.11 Impact Factor
  • Rudolf Umla, Velisa Vesovic
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    ABSTRACT: The recently developed Enskog-2 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, N-2, O-2, 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 Enskog-2 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 0.8-3.2%, depending on the way in which the length scaling parameters are estimated.
    Fluid Phase Equilibria 06/2014; 372:34-42. · 2.24 Impact Factor
  • Rudolf Umla, Velisa Vesovic
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    ABSTRACT: The recently developed Enskog-2σ 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 Enskog-2σ 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.
    Fluid Phase Equilibria 06/2014; 372:34–42. DOI:10.1016/j.fluid.2014.03.016 · 2.24 Impact Factor
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    ABSTRACT: An extended hard-sphere 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 hard-sphere 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 high-viscosity liquids not used in the development of the universal function and has shown to perform satisfactorily up to reduced densities of approximately 0.9.
    Fluid Phase Equilibria 02/2014; 363:239–247. DOI:10.1016/j.fluid.2013.11.032 · 2.24 Impact Factor
  • Robert Hellmann, Nicolas Riesco, Velisa Vesovic
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    ABSTRACT: The relaxation properties in the dilute-gas limit have been calculated by the classical trajectory (CT) method for a gas consisting of chain-like 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 self-diffusion 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.
    Chemical Physics Letters 06/2013; 574:37–41. DOI:10.1016/j.cplett.2013.04.067 · 1.99 Impact Factor
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    ABSTRACT: This paper contains new, representative reference equations for the thermal conductivity of n-heptane. 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 dilute-gas 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%.
    Journal of Physical and Chemical Reference Data 06/2013; 42(2):023101. DOI:10.1063/1.4794091 · 3.11 Impact Factor
  • Kazeem Akintayo Lawal, Velisa Vesovic
  • Robert Hellmann, Nicolas Riesco, Velisa Vesovic
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    ABSTRACT: The transport properties in the dilute gas limit have been calculated by the classical-trajectory method for a gas consisting of chain-like molecules. The molecules were modelled as rigid chains consisting of spherical segments that interact through a combination of site-site Lennard-Jones 12-6 potentials. Results are reported for shear viscosity, self-diffusion, 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.
    The Journal of Chemical Physics 02/2013; 138(8):084309. DOI:10.1063/1.4793221 · 3.12 Impact Factor
  • Rudolf Umla, Nicolas Riesco, Velisa Vesovic
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    ABSTRACT: In this work, we propose a new kinetic theory model (Enskog-2σ 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 Enskog-2σ 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 fluid-dependent 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 Enskog-2σ model.
    Fluid Phase Equilibria 11/2012; 334:89–96. DOI:10.1016/j.fluid.2012.08.006 · 2.24 Impact Factor
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    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 capillary-flow experiment was designed to ensure capillary-dominated flow, low inertial forces, negligible dispersion, and insignificant gravity effects, and the end-effect 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 (n-pentane, n-heptane, and n-octane). 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.
    Energy & Fuels 04/2012; 26(4):2145–2153. DOI:10.1021/ef201874m · 2.73 Impact Factor
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    ABSTRACT: New, simple, and practical correlations for shear viscosity, self-diffusion 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 quantum-mechanical, 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.
    Journal of Chemical & Engineering Data 03/2012; 57(4):1312–1317. DOI:10.1021/je3000926 · 2.05 Impact Factor
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    ABSTRACT: New expressions for the viscosity of liquid mixtures, consisting of chain-like molecules, are derived by means of Enskog-type 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 Vesovic-Wakeham (VW) method. We propose and implement a number of thermodynamically consistent mixing rules, taking advantage of SAFT-type analysis, in order to develop the VW method for chain molecules. The predictions of the VW-chain model have been compared in the first instance with experimental viscosity data for octane-dodecane and methane-decane mixtures, thus, illustrating that the resulting VW-chain model is capable of accurately representing the viscosity of real liquid mixtures.
    The Journal of Chemical Physics 02/2012; 136(7):074514. DOI:10.1063/1.3685605 · 3.12 Impact Factor
  • Kazeem A. Lawal, Velisa Vesovic, Edo S. Boek
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    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 deep-bed 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 porous-media 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 dynamic-filtration model is capable of rationalizing about 82% of the reference data sets within 7%.
    Energy & Fuels 11/2011; 25(12):5647–5659. DOI:10.1021/ef200764t · 2.73 Impact Factor
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    ABSTRACT: A six-dimensional potential energy hypersurface (PES) for two interacting rigid hydrogen sulfide molecules was determined from high-level quantum-mechanical ab initio computations. A total of 4016 points for 405 different angular orientations of two molecules were calculated utilizing the counterpoise-corrected supermolecular approach at the CCSD(T) level of theory and extrapolating the calculated interaction energies to the complete basis set limit. An analytical site-site potential function with eleven sites per hydrogen sulfide molecule was fitted to the interaction energies. The PES has been validated by computing the second pressure virial coefficient, shear viscosity, thermal conductivity and comparing with the available experimental data. The calculated values of volume viscosity were not used to validate the potential as the low accuracy of the available data precluded such an approach. The second pressure virial coefficient was evaluated by means of the Takahashi and Imada approach, while the transport properties, in the dilute limit, were evaluated by utilizing the classical trajectory method. In general, the agreement with the primary experimental data is within the experimental error for temperatures higher than 300 K. For lower temperatures the lack of reliable data indicates that the values of the second pressure virial coefficient and of the transport properties calculated in this work are currently the most accurate estimates for the thermophysical properties of hydrogen sulfide.
    Physical Chemistry Chemical Physics 06/2011; 13(30):13749-58. DOI:10.1039/c1cp20873j · 4.20 Impact Factor
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    ABSTRACT: This work reports the results of an investigation on industrial requirements for thermodynamic and transport properties carried out by the Working Party on Thermodynamic and Transport properties (http://www.wp-ttp.dk/) of the European Federation of Chemical Engineering, EFCE (http://www.efce.info/). A carefully designed questionnaire was sent to a number of key technical people in companies in the oil and gas, chemicals, and pharmaceutical/biotechnology sectors. Twenty-eight companies have provided answers which formed the basis for the analysis presented here. A number of previous reviews, specifically addressed to or written by industrial colleagues, are discussed initially. This provides the context of the survey and material with which the results of the survey can be compared. The results of the survey have been divided into the themes: data, models, systems, properties, education, and collaboration. The main results are as follows. There is (still) an acute need for accurate, reliable, and thermodynamically consistent experimental data. Quality is more important than quantity. Similarly, there is a great need for reliable predictive, rather than correlative, models covering a wide range of compositions, temperatures, and pressures and capable of predicting primary (phase equilibrium) and secondary (enthalpy, heat capacity, etc.) properties. It is clear that the ideal of a single model covering all requirements is not achievable, but there is a consensus that this ideal should still provide the direction for future development. The use of new methods, such as SAFT, is increasing, but they are not yet in position to replace traditional methods such as cubic equations of state (especially in oil and gas industry) and the UNIFAC group contribution approach. A common problem with novel methods is lack of standardization, reference data, and correct and transparent implementations, especially in commercially available simulation programs. The survey indicates a great variety of systems where further work is required. For instance, for electrolyte systems better models are needed, capable of describing all types of phase behavior and mixtures with other types of components. There is also a lack of data and methods for larger complex molecules. Compared with the previous reviews, complex mixtures containing carbon dioxide associated with a wide range of applications, such as capture, transport, and storage are becoming interesting to a number of survey participants. Despite the academic success of molecular simulation techniques, the survey does not indicate great interest in it or its future development. Algorithms appear to be a neglected area, but improvements are still needed especially for multiphase reactive systems (simultaneous chemical and physical equilibrium). Education in thermodynamics is perceived as key, for the future application of thermodynamics in the industry. A number of suggestions for improvement were made at all three levels (undergraduate, postgraduate, and professional development) indicating that the education is correctly perceived as an ongoing process.
    Industrial & Engineering Chemistry Research 10/2010; 49(22). DOI:10.1021/ie101231b · 2.24 Impact Factor
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    ABSTRACT: Fuel cell performance is determined by the complex interplay of mass transport, energy transfer and electrochemical processes. The convolution of these processes leads to spatial heterogeneity in the way that fuel cells perform, particularly due to reactant consumption, water management and the design of fluid-flow plates. It is therefore unlikely that any bulk measurement made on a fuel cell will accurately represent performance at all parts of the cell. The ability to make spatially resolved measurements in a fuel cell provides one of the most useful ways in which to monitor and optimise performance. This Minireview explores a range of in situ techniques being used to study fuel cells and describes the use of novel experimental techniques that the authors have used to develop an 'experimental functional map' of fuel cell performance. These techniques include the mapping of current density, electrochemical impedance, electrolyte conductivity, contact resistance and CO poisoning distribution within working PEFCs, as well as mapping the flow of reactant in gas channels using laser Doppler anemometry (LDA). For the high-temperature solid oxide fuel cell (SOFC), temperature mapping, reference electrode placement and the use of Raman spectroscopy are described along with methods to map the microstructural features of electrodes. The combination of these techniques, applied across a range of fuel cell operating conditions, allows a unique picture of the internal workings of fuel cells to be obtained and have been used to validate both numerical and analytical models.
    ChemPhysChem 09/2010; 11(13):2714-31. DOI:10.1002/cphc.201000487 · 3.36 Impact Factor
  • Kazeem Lawal, Velisa Vesovic
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    ABSTRACT: The viscosity and density of mixtures of carbon dioxide and 2,6,10,15,19,23-hexamethyltetracosane (squalane) are reported. The measurements were carried out using a vibrating wire instrument over a range of temperatures from (303.15 to 448.15) K and at pressures ranging from approximately the minimum miscibility pressure at a given composition to 170 MPa. Pure squalane and three different mixtures, with mole fractions of CO2 of 0.423, 0.604, and 0.788, were investigated. The estimated expanded relative uncertainties of the measurements were ± 2 % for viscosity and ± 0.2 % for density with a coverage factor of 2. The data for each composition were correlated by simple expressions with an absolute average relative deviation less than 2 % for viscosity and less than 0.2 % for density. The results show that the addition of CO2 to squalane at a given pressure and temperature reduces greatly the viscosity and increases slightly the density. A hard-sphere model, adjusted to fit the viscosity data of the pure substances but containing no adjustable binary parameters, was tested against the experimental data. Relative deviations bounded by approximately ± 60 % were found.
    Journal of Chemical & Engineering Data 09/2009; 54(9). DOI:10.1021/je800894y · 2.05 Impact Factor