Role of n-alkane polydispersity on the crystallization of n-alkanes from solution

The process of crystallization is of great interest in a wide range of industries. In the oil industry, a major interest is the deposition of wax onto subsea oil pipelines, a costly phenomenon that hinders the production of crude oil. It is known that these deposits are a volume spanning network of orthorhombic, lamellar wax crystals consisting primarily of n-alkanes that entrap some of the crude oil to form a gel. The presence of other materials in a crystallizing system can have an impact on both thermodynamic and kinetic parameters. To analyze the effects of how n-alkanes impact the crystallization of one another,, three different types of apparatus (differential scanning calorimetry, densitometer, and a coldfinger apparatus) were used to explore a wide range of crystallization and deposition properties. The results of these experiments showed that longer chained n-alkanes greatly influence the crystallization properties of shorter n-alkanes, whereas shorter n-alkanes only slightly impact the crystallization properties of longer chained n-alkanes. This impact is directly related to the amount of cocrystallization that exists between the n-alkanes, which is dictated by the carbon number difference, solubility differences, and cooling rate. Cocrystallization shifts the temperature at which crystallization occurs and reduces the heat that is released by the system. Polydispersity and cocrystallization also reduce the mass and wax fraction of a deposit formed using a coldfinger apparatus.