44793-AC9
Understanding Wax Formation from a Molecular Perspective
This project is concerned with the study of wax formation in petroleum fuels from a molecular perspective. Petroleum fuels are modeled using n-alkanes as representative fuels and the conformation of individual molecules and their packing of clusters in crystal structures are studied in order to build an understanding of low temperature ordered (LO) solid phases, liquid-like rotator (R) phases and the transitions between them. Physically observed molecular conformations and crystal structures are assumed to correspond to stationary points (local and global minima) on an appropriate model of the potential energy surface while transition states are saddle points. To reliably determine important minima and saddle points, an in-house, multi-scale optimization method called the terrain/funneling method. At the small length scale, the terrain methodology of Lucia and Yang (2003) is used to gather average gradient and curvature information, which is communicated to the large length scale. The funneling method of Lucia, DiMaggio, and Depa (2004) is used at the large length scale to make large changes in the geometric structure and drive the optimization calculations to the global minimum.
Results in the second year of funding continue to build molecular understanding of wax conformation. Significant improvements in reliability and efficiency of the terrain/funneling method of global optimization have been achieved. These algorithmic improvements, which include the use of average and point-wise first and second derivative information and a novel probing strategy, provide unprecedented reliability and have enabled the solution of molecular conformation problems with many hundreds of unknowns. To date, the largest compound that has been successfully solved is normal triacontane (n-C30H62). Using an all-atom modeling approach, supporting numerical results show that the terrain/funneling approach
(1) is capable of finding the global minimum energy structure whereas other popular methods like basin hopping (Doye and Wales, 1997) cannot, and
(2) identifies major energy levels that influence transition dynamics. These energy plateaus represent important local minima and saddle points that define R à R and R à LO transitions in waxes.
These results are important because they provide reliable enabling tools for understanding some of the important transitions that govern wax formation. The fact that the terrain/funneling algorithm is able to uncover information relevant to transition dynamics is a very significant contribution to the molecular conformation literature. Additionally, the structure of clusters of n-alkane molecules (i.e., the ways in which individual molecular conformation influences the packing of several molecules into crystal structures) has been studied in order to build an understanding of nucleation and growth phenomena, which are clearly part of the wax formation process. This optimization approach to cluster conformation is the only true ab initio approach to crystal structure.
The impact of this research on the PI’s career has been significant. Our studies of wax formation and the surrounding results to date continue to provide strong evidence that the terrain/funneling methodology developed by the PI and his research group is a powerful enabling tool for molecular conformation and crystal structure determination in petroleum fuels as well as other materials. This work continues to lead to invited papers and presentations for the PI, the most recent of which is an invitation to deliver a plenary lecture at the 2009 Foundations of Computer Aided Process Engineering Design on multi-scale methods and complex processes. The PI also has been told that this work on molecular conformation and the terrain/funneling method directly led to the invitation to join the editorial board of the Journal of Global Optimization that he received in August 2007. The results to date also clearly suggest that this research has resulted in significant advances with regard to the main objective of this research – to build an understanding of wax formation of petroleum fuels from a molecular perspective. While there is still work to be done, this research project has provided the necessary time and support for laying the groundwork for future investigations of wax formation.
This project also had a significant positive impact on the training and development of the graduate student who participated in this research and who completed his Ph.D. degree requirements in May 2008 and is now employed at Universal Oil Products in Des Plaines, IL. The PI has been told by UOP that the students’ strong training in the energy and optimization areas was directly responsible for his being offered employment by UOP. This project also enabled the student to present his work at the November 2006 and 2007 American Institute of Chemical Engineers meetings, to co-author a presentation at 2007 Frontiers in Advances in Global Optimization conference, to co-author a paper that has been published in the Journal of Global Optimization, and to draft a second manuscript of this research that has subsequently been submitted for publication. A third publication from this work on clusters and crystal structure will also be submitted for publication.
