A modified hard-sphere theory for transport properties of fluids over the whole density range: I. Viscosity coefficients of low molecular weight hydrocarbons

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Abstract The rough hard-sphere model is used as a basis for the interpretation of viscosity coefficient data for liquid methane. It is found that effects of coupling of translational and rotational motions are absent and that the molecules behave like an assembly of smooth hard-spheres rather than rough hard-spheres. The effect of non-spherical molecular shape on the viscosity coefficient of C2 to C5 hydrocarbons at high density is investigated. Other work has shown that for a dilute assembly of spherocylinders the viscosty coefficient can have practically the same value as a hard-sphere gas with the same molecular volume. The hard-sphere theory is here applied to liquid hydrocarbon viscosity coefficient data and to the dense gas data and the results show that at these densities also the experimental data can be satisfactorily interpreted in terms of the smooth hard-sphere model. At lower densities the experimental results are lower than values calculated on the basis of the smooth hard-sphere theory, a consequence of the effect of intermolecular attractive forces in real fluids. These effects are taken into account by an empirical modification to the hard-sphere theory. The resulting expressions reproduce the experimental viscosity coefficient data for the C1-C5 hydrocarbons considered very satisfactorily over the whole density and temperature range.