Role of n-Alkane Polydispersity on the Crystallization of n-Alkanes from Solution Michael Senra, † Ekarit Panacharoensawad, ‡ Kriangkrai Kraiwattanawong, ‡ Probjot Singh, § and H. Scott Fogler* ,† Department of Chemical Engineering, UniVersity of Michigan, 2300 Hayward Street, Ann Arbor, Michigan 48109, Petroleum and Petrochemical College, Chulalongkorn UniVersity, Bangkok 10330, Thailand, and ConocoPhillips Company, BartlesVille, Oklahoma 74005 ReceiVed August 10, 2007. ReVised Manuscript ReceiVed NoVember 10, 2007 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. Introduction In many industries, the ability to control crystallization is important to create crystals of a desired size, shape, and/or morphology. However, the petroleum industry is greatly im- pacted by unwanted crystallization because it begins a sequence of events that can cause significant problems in the production, transportation, and refining of crude oil. 1 The crystallization of select components of crude oil in pipelines can lead to the formation of wax deposits that can restrict the flow of oil and eventually plug the process equipment and/or pipelines. 2,3 These wax deposits have been described as lamellar, orthorhombic wax crystals contained in a random structure that form an organic gel. 4 Despite the fact that crude oils are extremely complex systems containing a multitude of components, it is generally accepted that the crystallizing materials that form the deposits are primarily n-alkanes. 5–7 However, this gel does not consist solely of n-paraffins but is a spanning network of solid paraffin crystals entrapping the liquid oil. 7,8 In general, wax deposition in oil pipelines occurs either when oil is being transported in colder environments or has a large fraction of n-alkanes present in it. 7,9 Crystallization does not occur in the reservoir because the temperature (70–150 °C) is high enough to ensure that the paraffins remain in solution. 7 However, as the crude oil passes through the colder pipelines (for example, the ocean floor is at 4 °C), the temperature of the crude oil drops, causing the higher molecular-weight paraffins to precipitate out of solution and crystallize. 10 The United States Department of Energy estimated that remediating pipeline blockages in subsea oil pipelines at depths of around 400 m can cost $1 million/mile. 10 This problem is only expected to get worse as time progresses because of the necessity to drill further and further offshore to find more oil because of the depletion of oil reserves near the shore. 3 This drilling further offshore exposes the warm oil to a cold environment for longer periods of time. 3 Greater insight on the formation of wax deposits can be gained with greater knowledge of how n-alkanes crystallize and deposit in solution. n-Alkanes (n-paraffins) are the major components of wax deposits because of their ability to crystallize * To whom correspondence should be addressed. Telephone: (734) 763- 1361. Fax: (734) 763-0459. E-mail: sfogler@umich.edu. † University of Michigan. ‡ Chulalongkorn University. § ConocoPhillips Company. (1) Misra, S.; Baruah, S.; Singh, K. SPE Prod. Facil. 1995, 50. (2) Pedersen, K. S.; Skovborg, P.; Ronnigsen, H. Energy Fuels 1991, 5, 924–932. (3) Wu, C.-H.; Wang, K.-S.; Shuler, P. J.; Tang, Y.; Creek, J. L.; Carlson, R. M.; Cheung, S. AIChE J. 2002, 48, 2107–2110. (4) Ashbaugh, H. S.; Radulescu, A.; Prud’homme, R. K.; Schwahn, D.; Richter, D; Fetters, L. J. Macromolecules 2002, 35, 7044–7053. (5) Hansen, A. B.; Larsen, E.; Pedersen, W. B.; Nielsen, A. B.; Roenningsen, H. P. Energy Fuels 1991, 5, 914–923. (6) Hennessy, A. J.; Neville, A.; Roberts, K. J. J. Cryst. Growth 1999, 198/199, 830–837. (7) Singh, P.; Fogler, H. S.; Nagarajan, N. R. J. Rheol. 1999, 43, 1437– 1459. (8) Singh, P.; Venkatesan, R.; Fogler, H. S.; Nagarajan, N. R. AIChE J. 2000, 46, 1059–1074. (9) Paso, K.; Senra, M.; Yi, Y.; Sastry, A. M.; Fogler, H. S. Ind. 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