Viscosity and density of asymmetric hydrocarbon mixtures
Creators: name: Mohammed, Malami
Publisher: name: Imperial College London
Year: 2016

description: The main focus of this research has been on providing useful experimental viscosity and density data on some representative asymmetric hydrocarbon mixtures with particular interest on mixtures of higher-molecular-weight aliphatic hydrocarbons with dissolved methane. This thesis details incremental improvements in the design and operation of an existing vibrating-wire viscometer-densimeter (VWVD). These improvements were necessary in order to allow safe investigations of viscosity of highly flammable mixtures such as methane + n-hexadecane. The procedure for absolute viscosity measurement using the VWVD is described in detail. In the absolute mode of operation, the properties of the vibrating-wire (i.e. radius and length) and that of the suspended sinker (i.e. mass and volume) were determined via independent mechanical measurements. The diameter (and hence the radius) of a cetreless-ground tungsten wire was measured accurately using a laser micrometer whereas the length of the wire was carefully measured with an internal caliper. The mass and volume of the suspended sinker were determined via hydrodynamic weighing methods. Simultaneous viscosity and density measurements were made on binary mixtures of n-hexadecane, 2,6,10,15,19,23-hexamethyltetracosane (Squalane) or cyclohexane with dissolved methane each at four different compositions. Measurements were made in the temperature range from (298.15 to 473.15) K, pressure range (0.1 to 200) MPa and methane mole fractions x1 (0 to 0.4) with an overall combined expanded uncertainties within 0.3 % in density and 2 % in viscosity with a coverage factor k = 2. Additional measurements were conducted on perfluoropolyether (also known as krytox® GPL 102) over the same temperature and pressure range. The data generated were successfully correlated for density using the modified Tait equation and for viscosity, by means of the Tait-Andrade equation. These correlations described most of the data to within their expected uncertainties. In an attempt to model the viscosity of the binary mixtures, the extended hard sphere model was tested both in predictive and correlative approaches using different mixing rules. Surprisingly, in the predictive mode, the results indicated that, the conventional linear mole-fraction average rule provided a better prediction with a maximum absolute deviations (ΔMAD,ηs) in viscosity ranging from (9 to 64)% compared to the quadratic and the other more complicated combining rules such as the Van der Waals rules. This was observed consistently across all the three binary systems investigated in this work. In the correlative mode however, the maximum absolute deviations were in the range from (3 to 33) % when the molar core volumes of each mixture composition were treated as adjustable parameters with the higher ΔMAD,ηs mostly observed in the mixtures of methane + squalane. Furthermore, the performance of the extended hard sphere scheme and the Vesovic-Wakeham (VW) model in predicting the viscosity of asymmetric mixtures were tested. This was done by comparing the predicted results from both models against the experimental data obtained in this work. In this regard, three performance indicators namely: the maximum absolute deviations (ΔMAD,η); the average absolute deviations (ΔAAD,η) and the average bias (Δbias,η) of these models were compared. It was found that the VW model consistently produced lower values of all these quantities for the two binary systems investigated. For example, for the mixture of methane + n-hexadecane the ΔMAD,ηs obtained from the VW model range from (10 to 31)%, whereas those of the extended hard sphere model range from (10 to 47)%. On the basis of our analysis, the VW model seems to be more promising and therefore more likely to be recommended for predicting the viscosity of asymmetric mixtures. Finally, the results obtained from this project extend our knowledge and basic understanding of the viscosity characteristics of the so-called asymmetric mixtures especially to higher temperatures and pressures.