(44b) Gelation and Thermodynamic Analysis of Ternary Systems of Long-Chain n-Alkanes in a Short-Chain n-Alkane Solvent

The formation of wax deposits in offshore oil pipelines has remained an issue for the petroleum industry for several decades. These wax deposits lead to clogged pipelines that result in significant financial costs for lost production and required maintenance. Although a number of remediation methods exist, there is a lack of complete understanding of how the various components of crude oils affect the crystallization and gelation of the crudes. Of significant interest are long chain n-alkanes, which are typically the major component of waxes because of their low solubility and ease in forming an ordered crystal structure. In order to attempt to gain fundamental insight about what is occurring, model fuels have been utilized consisting of a short-chained n-alkane acting as the solvent and long-chained n-alkanes acting as the solutes. Previous work has shown that both polydispersity and cocrystallization can influence the thermodynamic and gelation characteristics of model oil systems where only two solutes were explored. This work explored systems with three solutes, systems where a mixture of polydispersity and cocrystallization would occur.

To study the influence of cocrystallization and polydispersity, systems containg C28, C32 and C36 were mixed with a dodecane solvent. It is known that C36 is capable of cocrystallizing with C32 and C32 is capable of cocrystallizing with C28. However, C36 is incapable of cocrystallizing with C28. Important parameters such as the cloud point, gel point, pour point and heat of crystallization were determined by the use of rheometry, differential scanning calorimetry and visual techniques. The mass ratios of C36 to C32 was fixed at 1:1, 1:2 and 2:1 while the amount of C28 was varied in the analysis. Work showed that these systems obeyed van't Hoff solubility theory. However, the DSC traces clearly showed that the way in which the materials were cocrystallizing was drastically different as the amount of C28 was increased. Adding C28 to C32-C36 solutions caused a decrease in the gel point of the system. This phenomenon can be explained by the fact that C28 acted as a dispersant and a crystal modifier, preventing the C32 and C36 molecules from interacting with one another and forming the necessary volume spanning network to form a gel. This result was also seen in prior work when C36-C28 systems were analyzed. However, the presence of C32 minimized the magnitude of the decrease of the gel point. These effects were minimized for the system containing 2% C36 and 4% C32. One explanation for why the presence of C28 does not depress the gel and pour points for this system is that there is too much C32 and C28 in the solution relative to the amount of C36, which overcomes the dispersant effect of the C28 on the longer n-alkanes. Another explanation is that since there is less C36 than C32, some C28 can cocrystallize with C32, resulting in more C28 crystallizing at higher temperatures and higher gel/pour points.