Physical constants of mineral oil fractions

The development of reliable methods for structural analysis of mixtures is very laborious. Physical data of pure compounds may serve as a base for the investigations. It has, however, been proved that not in all cases can these data be simply correlated with those of the mixtures. Thus correlations of physical data of pure, individual hydrocarbons often prove not to be valid in the analysis of mineral oils. In this case physical constants of mineral oil fractions of widely different origin form a more reliable basis for the structural analysis, provided that their structure has been determined by absolute methods. 

The lines for equal values of o in the different cross-sections are parallel to the n axis. This means that the surface tension at a constant value of the viscosity and the density is independent of the refractive index and in fact of any other pl sical constant, i.e. the viscosity and the density are the only two physical constants of mineral oil fractions that are necessary to determine the value of the surface tension of these fractions. text continued on p. 48... 

A third example of the correlation of physical constants of mineral oil fractions is the determination of the surface tension from the ultrasonic sound velocity u and the density 22. 

In the n-d-M method use is made of three physical constants, namely the refractive index n (measured for the sodium D-line), the density d in g/ml and the molecular weight M, for the determination of the composition of mineral oil fractions. 

In this section it will be shown that it is not only possible to determine the chemical structure of mineral oil fractions from physical constants, but also to predict the values of other physical constants, That it is possible to correlate the physical constants of... 

The fact that only the two physical constants, viscosity and density, are necessary to determine the surface tension of mineral oil fractions, enabled Cornelissen, Harva and Waterman21 to construct a diagram with the coordinates log vand da4°. This diagram has been constructed by projecting the surfaces of equal values of the surface tension in the log v-n-d space model on the log v-d coordinate plane. In the diagram lines of equal values of the surface tension (at 20°C) were constructed (Fig. 43). 

The application of several methods for structural analysis of mineral oils is, in general, limited to those fractions in which no structural elements are present in larger quantities than normally occur in mineral oil fractions. In highly aromatic concentrates, for instance, the normal analytical methods (n-d-M v-n-d) may give inaccurate results, because different types of aromatics may influence the physical constants of the oil differently. 

Van Nes and Van Westen42 described the physical constants and elementary composition of intermediates in the complete hydrogenation of a large number of mineral oil fractions. These data were used by Geelen8 to study the behaviour of mono- and di-aromatics during catalytic hydrogenation. 

This was the first time that only two physical constants were used (the minimum number) to determine the ring-number of saturated mineral oil fractions. 

An example has been given here of the correlation existing between physical constants of saturated mineral oil fractions. Other examples will be given when the viscosity is discussed. 

In Fig. 14 the correlation is shown between some physical constants of saturated mineral oil fractions, as well as the relation between physical constants and Rn and %Cr. This diagram shows how the ring-number Rn and the percentage of carbon present in ring structure, %Cr, as well as the molecular weight M and the density df, can be determined for saturated mineral oil fractions by means of their log v and values. 

New methods of finding correlations between physical constants of straight-run as well as of saturated mineral oil fractions will be discussed below. 

The development of newer techniques (chromatography, thermodiffusion) for the separation of the different groups of hydrocarbons from mineral oil fractions allows a better characterization of such type-concentrates with the aid of physical constants. Combination of physical methods of separation with the statistical analysis of the products obtained, may lead to a more detailed and more complete knowledge of the composition of oils. 

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