Published: March 25, 2011 r2011 American Chemical Society 1707 dx.doi.org/10.1021/ef200206h | Energy Fuels 2011, 25, 1707–1713 ARTICLE pubs.acs.org/EF A New DSC-Based Method to Determine the Wax Porosity of Mixtures Precipitated from Crude Oils Baudilio Coto,* ,† Carmen Martos, † Juan J. Espada, † María D. Robustillo, † Daniel Merino-García, ‡ and Jos  e Luis Pe~ na ‡ † Department of Chemical and Energy Technology, ESCET, Universidad Rey Juan Carlos, C/Tulip an s/n, 28933 M ostoles, Madrid, Spain ‡ Repsol Technology Centre, E-28931 M ostoles, Madrid, Spain ABSTRACT: Wax precipitation is one of the most important problems in flow assurance. The correct prediction of the wax appearance temperature (WAT) and the wax precipitation curve (WPC) is essential to anticipate potential solutions for this problem. The determination of both WAT and WPC requires reliable data on the amount of precipitated wax and the trapped crude oil remaining in these mixtures. The estimation of the latter is difficult due to the scarcity of experimental methodologies. In this work, a method based on differential scanning calorimetry (DSC) was developed to determine the trapped crude oil of precipitated mixtures. The WPCs were experimentally determined by fractional precipitation and DSC analysis for a variety of crude oils. The comparison of these curves requires the correction of the former by subtracting the trapped crude oil remaining in the precipitated mixtures. The trapped crude oil was calculated using two methods: one based on proton nuclear magnetic resonance ( 1 H NMR) previously developed and the other based on DSC, developed in this work. This method considers that the gel formed when decreasing the temperature contains not only solids, but also occluded crude oil. The values of trapped crude oil estimated by both methods were used to correct experimental WPC. The comparison between corrected WPC and those obtained by DSC analysis showed the superiority of DSC over 1 H NMR to calculate the amount of trapped crude oil. 1. INTRODUCTION Paraffinic waxes present in petroleum crude oils can precipi- tate when temperature decreases during production, transport through pipelines, or storage. The accumulation of solids on the walls can affect pipelines and equipment, increasing the cost of pumping, reducing, or even stopping the production. 1À5 For that, the knowledge of solid formation conditions is crucial to avoid economic consequences. Therefore, the prediction of wax appearance conditions is required to evaluate the risk of wax precipitation and to anticipate solutions for future problems in wells. The quantification of wax precipitation requires the wax appearance temperature (WAT) and the amount of wax formed as a function of the temperature (wax precipitation curve, WPC). The prediction of these magnitudes is frequently carried out by thermodynamic models, 6À13 but their application is limited due to the scarcity of reliable experimental information. 14 The WAT and the WPC can be experimentally determined by direct gravimetric precipitation techniques 15À17 and by indirect techniques such as differential scanning calorimetry (DSC), 18À21 nuclear magnetic resonance spectroscopy (NMR) 22 and Fourier transformed infrared spectroscopy (FT-IR). 23,24 DSC analysis allows quantification during phase transition from the measure- ment of heat change involved. This information can be converted into the amount of solid precipitated as a function of the temperature, thus yielding the WPC. However, most of the gravimetric methods use solvents during the precipitation process, leading to a decrease in the precipitation temperature. To overcome these limitations, a method based on fractional wax precipitation has been recently developed. 25 This method allows the WAT and the WPC to be determined from non diluted crude oils, thus avoiding the solvent effect. However, some considerations about the amount of precipitated wax should be made since the trapped crude oil (commonly described as wax porosity) remaining within the precipitated wax has to be considered. It is well-known that significant amounts of crude oil can have a noticeable effect on the total amount of the gel formed, and therefore it should be determined and included in the predictive models. Although the experimental determination of trapped crude oil is not well established, there are several techniques which potentially could be used. Thus, a method based on 1 H NMR has been recently developed and applied to different crude oils. 26,27 This method considers that aromatic hydrogen atoms are only present in the aromatic compounds of the trapped crude oil. By comparing the amount of aromatic hydrogen atoms in each precipitated fraction with that in the raw crude oil, it is possible to estimate the content of the entrapped crude oil. However, it has been shown 26 that this method presents important uncertainties, with errors in the aromatic hydrogen atom content that can reach around 30%, and final results should be considered with caution. The aim of this work is to check the capabilities of DSC analysis to determine the crude oil occluded in fractions pre- cipitated from different crude oils. The WPC for a variety of crude oils was obtained by both the fractional precipitation Received: February 7, 2011 Revised: March 16, 2011