| Wax deposition in subsea oil pipelines is a major challenge in the petroleum industry. The solid components of these wax-oil gel deposits are primarily n-alkanes. Wax-oil gel properties are a function of the composition of the crude oil, notably polydispersity and cocrystallization. n-Alkanes can cocrystallize with one another if their molecular sizes are sufficiently similar and if they crystallize in the same shape when independently crystallizing from solution, consistent with Kravchenko's rules for melts. The degree of cocrystallization is a function of the relative abundance of the n-alkanes in solution and the cooling rate. Cocrystallization allows for shorter, more soluble alkanes to precipitate out at higher temperatures, which is important for developing thermodynamic models to predict wax deposition. Even at small concentrations of the shorter n-alkane, cocrystallization also reduces the gelation temperature and pour point of a gel and the mass and wax fraction of a deposit. These quantities are affected because the larger nalkane in the cocrystal must bend to incorporate the shorter n-alkane, creating imperfections in the crystal structure that inhibit gel formation and deposition.;Although components that do not cocrystallize with the crystallizing n-alkane do not greatly affect the thermodynamics or the deposition of the crystallizing n-alkane, they can greatly affect gelation. Once a certain threshold concentration has been reached, the pour point and gelation temperature can be reduced by as much as 20° C. This decrease is caused by the shorter molecules present in sufficient concentration acting as a steric hindrance to prevent the larger molecules from associating with one another to form a volume spanning network. For this hindrance to occur, the molecule must be sufficiently sized and associate with the larger n-alkane molecule.;Stearic acid has solubility characteristics similar to n-alkanes, but does not cocrystallize with n-alkanes. It influences the nucleation kinetics of the crystallizing nalkane, but did not affect other thermodynamic crystallization parameters. At sufficient concentrations, stearic acid can increase the amount of deposit and the gelation temperature. However, the ability of stearic acid to influence the depositor, crystallization and gelation of n-alkanes is limited by strong self-association caused by strong hydrogen bonding. |
