Ind. Eng. Chem. Res. 1997, 36, 2177-2183
2177
Reactivity of Asphalt Supercritical Fractions M. M. Liu,† J. M. Chaffin, R. R. Davison, C. J. Glover,* and J. A. Bullin Center for Asphalt and Materials Chemistry, Department of Chemical Engineering, and The Texas Transportation Institute, Texas A&M University, College Station, Texas 77843-3122
The reactivities of six supercritical fractions of asphalt SHRP AAF-1 were investigated in the context of their chemical composition. The supercritical fractions were obtained using pentane as the solvent. Each fraction was aged in a pressure oxygen vessel (POV) under seven conditions of temperature and pressure. As with whole asphalts, there is an initial rapid oxidation that slows to a constant rate. The oxidation rates for the constant rate region were measured at each of the conditions to determine the kinetic parameters of the fractions. Arrhenius plots were then constructed for the fractions to compare their reactivities. Variations in activation energy and reaction order were within experimental error. Results show that reactivity increases with fraction number for the supercritical fractions. The heavier fractions are more reactive not only because they contain more polar aromatics in terms of Corbett analysis fractions but also because the polar aromatics of the heavier fractions are larger in molecular size and are more reactive. Additionally, heavier fractions contain more asphaltenes that are larger in molecular size. Because for light fractions the reaction rates, and as previously shown the hardening rates, are very low and remain constant with oxidation, air blowing of some light fractions would be expected to produce an asphalt with a very low hardening rate. Data explicitly show that for petroleum fractions from the same crude source molecular size distribution is a good indicator of reactivity. This is not necessarily true for petroleum fractions from different crude sources, however. Introduction Supercritical fractionation is selective to both polarity and molecular size and is a powerful way to manipulate the composition of asphalt binders. Some preliminary studies on the properties of asphalt supercritical fractions and their blends have been reported (Stegeman et al., 1992; Jemison et al., 1995). Because the supercritical fractions and their blends had controlled chemical compositions, with respect to their Corbett analysis (Corbett, 1969), these studies contributed to understanding the compositional dependence of asphalt properties. Supercritical fractions also have been proposed as recycling agents for asphalt pavement rejuvenation (Chaffin et al., 1995; Chaffin et al., in press). Jemison et al. (1995) found that certain supercritical fractions and their blends have aging characteristics superior to the original asphalt in the sense that their hardening susceptibilities (HS) are lower than the HS of the original whole asphalt. Hardening susceptibility was defined as the slope of log viscosity vs carbonyl content (as measured by FTIR) relation for the asphaltic material resulting from oxidative aging. A lower HS (which is desirable) means a smaller increase in viscosity due to the same amount of carbonyl growth caused by oxidative aging. However, HS only represents the sensitivity of an asphalt’s viscosity to carbonyl formation. It does not represent the rate of carbonyl formation or the rate of hardening. No substantial data on the aging rates of the supercritical fractions or the blends were reported by Jemison et al. When studying the compositional dependence of physicochemical properties of asphaltic materials, Liu et al. (1995) found that all of the supercritical fractions
had lower HS values than the parent asphalt and that some of them also had better viscosity-temperature susceptibilities. One fraction was oxidized to the consistency of an asphalt binder (around 2000 poise) and still had a favorable viscosity-temperature susceptibility. If the reactivity of this fraction is also low, then a superior binder can be produced by air-blowing this fraction to a specified viscosity level. Thus a study of the reactivities of supercritical fractions is valuable. In their study of the application of size exclusion chromatography (SEC), or gel permeation chromatography (GPC), to asphalt technology, Davison et al. (1995) showed that the heavier supercritical fractions were of a larger molecular size. Furthermore, the polar aromatics grew progressively higher in molecular size in heavier fractions. Because of these observations, Corbett analyses were performed on the six supercritical fractions used in this study, which were derived from the Strategic Highway Research Program (SHRP) asphalt AAF-1. Molecular size distribution was measured using GPC for the six supercritical fractions, as well as their Corbett fractions and the parent asphalt. An attempt was made to identify any direct correlation between the reactivities and molecular size distributions of the supercritical fractions. To evaluate the reactivities of the fractions, a kinetic model for the carbonyl formation in asphaltic materials during oxidative aging was developed (Liu et al., 1996). During oxidative aging of an asphalt, the increase in carbonyl content approaches a constant rate after a more rapid nonlinear rate period. For isothermal and isobaric conditions, the long-term carbonyl formation during the constant rate region can be described by
CA ) CA0 + rCAt * Corresponding author. Telephone: (409) 845-3361. Fax: (409) 845-6446. E-mail: c
[email protected]. † Now affiliated with: Neste Trifinery Petroleum Services, P.O. Box 9606, Corpus Christi, TX 78469-9606. Telephone: (512) 289-6762. Fax: (512) 289-1351. S0888-5885(96)00564-7 CCC: $14.00
(1)
where CA is the carbonyl content measured by FTIR, rCA is the constant rate of carbonyl formation, t is the aging time, and CA0 is the intercept of the constant rate aging line. For each material, CA0 is pressure depend© 1997 American Chemical Society
2178 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 Table 1. Properties of the Unaged AAF-1 Supercritical Fractions fraction
CA
viscosity (P)
fraction
CA
F1F F2F F3F
0.167 0.211 0.364
rmea for the light fractions, but it cannot explain why rcal < rmea for the heavy fractions. It has also been reported that asphaltenes will increase the oxidation rate (Liu et al., 1996b). But since the asphaltene content is not very high even in the three heavy fractions and, as shown in Figure 6, the asphaltenes in F6F and F7F have smaller molecular sizes than those in the parent asphalt, the fact that rcal is smaller than rmea for the heavy fractions is not likely an effect of the asphaltenes. Conclusions The molecular size of the supercritical fractions increases with fraction number. While the saturates from the six supercritical fractions are very similar in molecular size distribution, both the polar aromatic fraction and the naphthene aromatic fraction from these fractions exhibit larger molecular sizes as the fraction number increases. The asphaltenes from F7F are larger in molecular size than those from F6F. Asphaltenes from both fractions are smaller in molecular size then those from the parent asphalt AAF-1. For the fractions from the same asphalt source, molecular size is a good indicator of the relative reactivity of the fractions. A heavier fraction oxidizes faster not only because the polar aromatics and possibly the asphaltenes have a higher content but also because the polar aromatics in the heavier fraction are larger in molecular size and are more reactive. For fractions from different asphalt sources, molecular size cannot be used as an indicator for relative reactivity. Fraction F3F ages more slowly than the parent asphalt AAF-1. This favorable aging property, in addition to its lower hardening susceptibility and lower viscosity-temperature susceptibility reported previously, indicates that this fraction is a good candidate for producing a superior asphalt binder by air-blowing this fraction to the desired viscosity level. Acknowledgment Support for this work was given by the Texas Department of Transportation (TxDOT), in cooperation with the U.S. Department of Transportation, and the Federal Highway Administration (FHWA). This work was also supported by the U.S. Department of Energy DOE,
Assistant Secretary for Energy Efficiency and Renewable Energy, under DOE Albuquerque Operation Office Cooperative Agreement DE-FC04-93AL94460. The contents of the paper reflect the views of the authors who are responsible for the facts and accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Federal Highway Administration, the Texas Department of Transportation, or the U.S. Department of Energy. This report does not constitute a standard, specification, or regulation. This report is not intended for construction, bidding, or permit purposes. Literature Cited Chaffin, J. M.; Davison, R. R.; Glover, C. J.; Bullin, J. A. Supercritical Refining of Asphalt to Produce Asphalt Recycling Agents. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1995, 40 (4), 799. Chaffin, J. M.; Liu, M. M.; Davison, R. R.; Glover, C. J.; Bullin, J. A. Supercritical Fractions as Asphalt Recycling Agents and Preliminary Aging Studies on Recycled Asphalts. Ind. Eng. Chem. Res., in press. Corbett, L. W. Composition of Asphalt Based on Generic Fractionation, Using Solvent Deasphaltening, Elution-Adsorption Chromatography, and Densimetric Characterization. Anal. Chem. 1969, 41, 576. Davison, R. R.; Bullin, J. A.; Glover, C. J.; Jemison, H. B.; Lau, C. K.; Lunsford, K. M., Bartnicki, P. L. Design and Use of Superior Asphalt Binders. Final Report, FHWA/TX-92/1249-1F; Texas Department of Transportation, 1992. Davison, R. R.; Glover, C. J.; Burr, B. L.; Bullin, J. A. Size Exclusion Chromatography of Asphalts. In Handbook of Size Exclusion Chromatograhy; Wu, Chi-San, Ed.; Marcel Dekker, Inc.: New York, 1995; p 211. Jemison, H. B.; Burr, B. L.; Davison, R. R.; Bullin, J. A.; Glover, C. J. Application and Use of the ATR, FT-IR Method to Asphalt Aging Studies. Fuel Sci. Technol. Int. 1992, 10, 795. Jemison, H. B.; Davison, R. R.; Glover, C. J.; Bullin, J. A. Fractionation of Asphalt Materials by Using Supercritical Cyclohexane and Pentane. Fuel Sci. Technol. Int. 1995, 13, 605. Lau, C. K.; Lunsford, K. M.; Glover, C. J.; Davison, R. R.; Bullin, J. A. Reaction Rates and Hardening Susceptibilities as Determined from POV Aging of Asphalts. Transp. Res. Rec. 1992, 1342, 50. Liu, M.; Lin, M. S.; Chaffin, J. M.; Davison, R. R.; Glover, C. J.; Bullin, J. A. Compositional Optimization for a Superior Asphalt Binder. Presented at the 26th Annual Meeting of the Fine Particle Society, Chicago, IL, Aug 22-25, 1995. Liu, M.; Lunsford, K. M.; Davison, R. R.; Glover C. J.; Bullin, J. A. The Kinetics of Carbonyl Formation in Asphalt. AIChE J. 1996, 42, 1069. Liu, M.; Lin, M. S.; Chaffin, J. M.; Davison, R. R.; Glover, C. J.; Bullin, J. A. Oxidation Kinetics of Asphalt Corbett Fractions and Compositional Dependence of Asphalt Oxidation. Manuscript in preparation, 1997. Petersen, J. C. Chemical Composition of Asphalt as Related to Asphalt Durability: State of the Art. Transp. Res. Rec. 1984, 999, 13. Stegeman, J. R.; Kyle, A. L.; Burr, B. L.; Jemison, H. B.; Davison, R. R.; Glover, C. J.; Bullin, J. A. Compositional and Physical Properties of Asphalt Fractions Obtained by Supercritical Extraction. Fuel Sci. Technol. Int. 1995, 10, 767.
Received for review September 11, 1996 Revised manuscript received March 11, 1997 Accepted March 19, 1997X IE9605645
X Abstract published in Advance ACS Abstracts, May 1, 1997.
