A general review is presented concerning jointly the crystallographic structures and the solid-solid and solid-liquid transition temperatures and enthalpies in the pure normal alkanes, as well as the structural and thermodynamic behavior of their synthetic binary, ternary, and multinary model mixtures and of real petroleum waxes. A major part of the structural and thermodynamic data of the literature, relative to the pure n-alkanes, are listed from methane up to the alkane with carbon atom number equal to 390. Variation relationships of the crystallographic long c-parameter that corresponds to the periodicity of the molecule layer packing along their axis, melting temperatures, and transition enthalpies are given as a function of the carbon atom number. The study of the literature on the n-alkane mixtures highlights the existence of isostructural binary, ternary, and multinary intermediate solid solutions which can be likened to a single pseudocomponent. This intermediate solid solution is equivalent to a hypothetical pure n-alkane whose carbon atom number is equal to the average carbon atom number of the mixtures. As a result, the solubility of these multinary solid solutions or real multiparaffinic waxes in linear, cyclic, or aromatic light solvents is identical to that of the pure n-alkanes: pseudobinary eutectic crystallization of the model or real wax as a single pseudocomponent on one hand and the light solvent on the other hand.

The crystallization of paraffinic materials from hydrocarbon mixtures subject to a cold environment is one of the problems faced by the petroleum industry. It can be found during oil production and in the transport and even in the use of refined materials such as diesels and fuels. A better understanding of the crystallization process and the capacity to model the wax formation would help to overcome this problem. This work is focused on the study of the two-phase region by developing a new experimental technique for measuring the composition of the phases in equilibrium and the amount of paraffins precipitating from a hydrocarbon mixture. The crystallization of several model systems, constituted by a mixture of normal alkanes between n-C19H40 and n-C28H58 dissolved in ethylbenzene in mass concentrations up to 25% weight, was promoted in an optical cell. The composition of the phases in equilibrium at several different temperatures below the cloud point was determined by chromatography. The amount of waxes formed and the cloud points were obtained by differential scanning calorimetry (DSC). The new technique is shown to be simple and accurate. Some particularities of paraffin crystallization were studied as well as the influence of aromatic compounds on the wax formation. A thermodynamic model was used to describe the measured data. It is based on the Flory-free-volume model for the liquid phase and on the predictive local composition model, a predictive form of the Wilson equation, for the nonideality of the solid solution. The model is shown to provide a good description of the SLE data for the studied systems.

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