Binary phase diagram (C14+C25) and isopleth (C14+wax): comparison of solubility of n-pentacosane and multiparaffinic wax in n-tetradecane

The crystallization of n-alkanes is known to be responsible for the solid deposit in flowlines of middle distillate fuels or petroleum cuts. To solve the problems of solid deposits in industrial equipment, it is essential to determine the thermodynamic and structural behaviour of the components of crude oils. The aim of the study was to determine the phase diagram of (n-tetradecane+n-pentacosane) was determined by simple and differential thermal analyses and X-ray diffraction; then it was compared with the pseudo-binary isopleth of the (n-tetradecane+multiparaffinic wax) multicomponent system. In both cases, the solid phases of compounds (γo(P1̄)-C14H30; βo(Pbcm)-C25H52; β′-wax multiparaffinic solid solution) were observed separately, but there was co-solubility in the liquid state. In the C25-rich side of the diagram, the binary mixtures (C14+C25) undergo structural changes corresponding to those of pure C25 when the temperature increases and those of β′ orthorhombic multiparaffinic solid solution of the wax are observed for the multicomponent system (C14+wax) in the wax-rich side. The binary diagram (C14+C25) displays an eutectic solidification (liquid↔γo(P1̄)-C14H30+βo(Pbcm)-C25H52) and the isopleth (C14+wax), a solidification that resembles an eutectic binary equilibrium (liquid↔γo(P1̄)-C14H30+β′-wax). The comparison of the two diagrams, particularly the curves of the crystallisation in the heavy n-alkane-rich side, shows that the solubility in C14 of the multiparaffinic solid solution, whose medium number of carbon atoms per molecule is equal to 25.6, is greater than that of C25.