A simple and better method for the prediction of the molecular weight of petroleum fractions (C5−C120) is presented. It requires as input information the 50 wt % true boiling point (TBP) as derived from simulated distillation and the density only. It is demonstrated that the 50 wt % TBP of fractions is equivalent to the normal boiling point of pure compounds. This finding has facilitated the development of the pertaining data set, which is included. Excellent results are obtained with a standard deviation of about 2%, which is at least 3 times better than that of published methods, whereas direct measurements of molecular weights are time consuming and often less accurate. The new method is compared to the API procedure 2B2.1 (1987), which is the current industry standard but limited to molecular weights of 700, and encompasses now the full practical range of molecular weights of 75−1700.

A simple and new method for the accurate prediction of the hydrogen content of petroleum fractions is presented. The new method is inferred from the concept of the molar additivity of the structural contributions of the carbon types in the average hydrocarbon molecule. It requires only basic physical properties such as the density and refractive index at 20°C, and the 50 wt % true boiling point. The influence of heteroatoms and olefinic bonds was found to be largely implicit, except for oxygen. By its fundamental basis this new correlation features an unparalleled precision equivalent to that of the most accurate but time-consuming analytical methods. This is demonstrated by an included data set.