SOLID-LIQUID EQUILIBRIUM OF N-ALKANES USING THE CHAIN DELTA-LATTICE-PARAMETER MODEL

The formation of a solid phase in liquid mixtures with large paraffini c molecules is a phenomenon of interest in the petroleum, pharmaceutic al, and biotechnological industries among others. Efforts to model the solid-liquid equilibrium in these systems have been mainly empirical and with different degrees of success. An attempt to describe the equi librium between the high temperature form of a paraffinic solid soluti on, commonly known as rotator phase, and the liquid phase is performed . The Chain Delta Lattice Parameter model (CDLP) is developed allowing a successful description of the solid-liquid equilibrium of n-alkanes ranging from n-C-20 to n-C-40. The model is further modified to achie ve a more correct temperature dependence because it severely underesti mates the excess enthalpy. It is shown that the ratio of excess enthal py and entropy for n-alkane solid solutions, as happens for other soli d mixtures, is related with the values of the melting temperatures by a function common to the entire homologous series. When applied to sys tems with a symmetric behavior, this yields a correct description of b oth the enthalpic and entropic parts of the excess Gibbs free energy w ith the CDLP model. Further considerations about the pressure dependen ce of the model are also made showing that the model presented can be applied to pressures as high as 100 bar.