The Viscosity of Liquid Hydrocarbons and their Mixtures

Center for Molecular and Engineering Thermodynamics Department of Chemical Engineering University of Delaware Newark, DE 19716 

VISCOSITY-TEMPERATURE RELATIONS AT LOW PRESSURES FOR PURE LIQUID, 7 Empirical Andrade-Type Relations, 7 Corresponding States Methods for Pure Hydrocarbons, 9 Other Prediction and Correlation Methods for the Viscosity of Pure Hydrocarbon Liquids, 11 

VISCOSITY OF LIQUID HYDROCARBON MIXTURES AT AMBIENT PRESSURE, 13 

Extension of Andrade-Type Correlations to Mixtures, 14 Extension of Corresponding States Methods for Viscosity of Mixtures, 15 Extension of the Theoretically Based Methods to Mixtures, 15 Viscosity Models for Undefined Mixtures, 16 

VISCOSITY OF LIQUID HYDROCARBONS AND THEIR MIXTURES AS A FUNCTION OF PRESSURE, 17 Models that Correct Ambient Pressure Viscosity for Pressure, 18 Models that Incorporate Pressure Implicitly, 18 

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This chapter deals with correlation and prediction methods for the viscosity of liquid hydrocarbons and their mixtures. In particular, the change of viscosity of such fluids with temperature, pressure, and composition is considered. We begin with a brief introduction of terms and definitions, and then discuss the experimentally observed behavior of the viscosity of liquid hydrocarbons as a function of temperature, pressure, and composition. Next, the main types of viscosity models applicable to liquid hydrocarbons and their mixtures are reviewed. We also Indicate the accuracy of several recent viscosity correlation and prediction methods that represent the general types of models in current use. The emphasis in this review is on the recent viscosity models, especially those after 1987, as reviews exist of the earlier methods [1,2], and because the recent methods are usually more accurate. 

There are a large number of models used for the correlation and/or prediction of the viscosity of liquid hydrocarbons and their mixtures. Since there is no exact statistical mechanical or molecular-level theory for liquid viscosity, all of the models available contain some degree of empiricism. Also, there is considerable variation in the structure of these models in that most have been formulated to address only a speeific viscosity estimation problem. For example, some liquid hydrocarbon viscosity models have been proposed only for predicting the viscosity of an undefined petroleum mixture, and their input parameters have been selected accordingly. There are models that use some experimental viscosity data, while others are completely predictive, at least within a class of substances. Some viscosity models are suitable for incompletely defined petroleum cuts, whereas others can be used only for well-defined hydrocarbons and their mixtures. Further, some models include the effects of pressure and dissolved gases on liquid hydrocarbon viscosity, while others are for use only at atmospheric pressure. 

Several models that include the effect of pressure on viscosity are outlined herein. For applications at high pressures, one may also require estimates of the viscosity of liquid hydrocarbons and their mixtures with dissolved gases (such as with CO2, Nj, H2S, etc.) because, due to the high solubility of such gases in hydrocarbon mixtures at elevated pressures, there is a very large reduction in the mixture viscosity. Indeed, such behavior is part of the basis for enhanced oil recovery by miscible gas injection. Even though the effect of dissolved gases is beyond the scope of this chapter, some comments about this are included due to the importance of this subject. 

The relaxation of gaseous methane, ethane and propane is by the spin-rotation mechanism and each pure component can be correlated with density and temperature [15]. However, the relaxation rate is also a function of the collision cross section of each component and this must be taken into account for mixtures [16]. This is in contrast to the liquid hydrocarbons and their mixtures that relax by dipole-dipole interactions and thus correlate with the viscosity/temperature ratio. 

Mineral oil—a mixture of liquid hydrocarbons obtained from petroleum. These are useful as levigating agents to wet and incorporate solid substances (e.g., salicylic acid, zinc oxide) into the preparation of ointments that consist of oleaginous bases as their vehicle. There are two types of mineral oils listed in the US. Pharmacopeia/National Formulary (USP/NF). Mineral oil USP is also called heavy mineral oil with a specific gravity between 0.845 and 0.905 and a viscosity of not less 34.5 cSt (cSt = mm /s) at 40°C. Light mineral oil, NF has a specific gravity between 0.818 and 0.880 and a viscosity of not more than 33.5 cSt. Table 2 lists the commercially available mineral oil fractions. 

A theory which would tie together the phenomenology of the viscosity behavior of liquids with their molecular structure would be desirable not only as an intellectual accomplishment but also as a useful aid in predicting the behavior of liquids as lubricants over a wide range of conditions. However, this goal is far from being attained because of basic deficiencies in present-day theories of liquids and because of the complex constitution of the hydrocarbon mixtures present in lubricating oils. 

For the prediction of viscosity of well-defined mixtures of liquids, the method of Cao et al. is very good in general [27,32]. However, since hydrocarbon mixtures form almost thermodynamically ideal solutions and their viscosities are simple functions of composition, simpler methods such as those of Allan and Teja [12] or of Orbey and Sandler [13] result in almost equal accuracy without the need for binary interaction parameters. These last two, simpler models can only be used for hydrocarbons, while the model of Cao et al. is of more general applicability. 

Water-insoluble liquid soils are commonly known as oily soils. Naturally occurring oily soils include hydrocarbons, saturated or unsaturated fatty acids, esters of fatty acids, and alcohols. Natural oily soils found on textiles are mixtures of oily components. Frequently, oil soils contain dispersed solid particulate matter (e.g., used motor oil). The most important properties of oily soils are their viscosity [1,2], polarity [3,4], and solubility in detergent solutions or dry-cleaning solvents. The removal of oily soil by detergency is facilitated by a low viscosity at the wash temperature. The polarity of soil affects adhesion of the soil on fibers, interaction with... 

After mixing of some imidazolium ILs with excess of aromatic hydrocarbons, biphasic mixtures are obtained. The lower layer is IL-rich phase, which exhibits the characteristics of typical liquid clathrates, i.e., low viscosity (relative to the initial neat ILs), immiscibility with excess aromatic solvents, and non-stoichiometric (but reproducible) compositions, while the concentration of IL is too low to be detected in the upper layer. Recently, it was found that the immiscibility gap became smaller when the polarity of substituted benzenes increased and their molecular size decreased (Shiflett Yokozeki, 2008 Shiflett et al., 2009 Shiflett Niehaus, 2010). However, when both dipole and quadrupole moments are present in an aromatic compound, they have an antagonistic effect, reducing the solubility in the IL (Shimizu et al., 2009). Up to now, such liquid clathrates or IL-biphases have been used in the fields of organic syntheses (Boxwell et al., 2002 Clavier et al., 2008 DeCastro et al., 2000 Surette et al., 1996), polymerization (Csihony et al., 2002), sep>arations of aromatics from hydrocarbons (Arce et al., 2007 Selvan et al., 2000) and so on. 

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