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Energy & Fuels 1999, 13, 694-701
Investigation of the Thermal Behavior and Interaction of Venezuelan Heavy Oil Fractions Obtained by Ion-Exchange Chromatography Parviz Rahimi* and Thomas Gentzis National Centre for Upgrading Technology, 1 Oil Patch Drive, Suite A202, Devon, Alberta T9G 1A8, Canada
Edgar Cotte´ PDVSA, Intevep, Caracas 1070A, Venezuela Received October 11, 1998. Revised Manuscript Received January 2, 1999
The coking propensity of Hamaca heavy oil (+510 °C) and its fractions separated by ion-exchange chromatography was investigated using hot-stage microscopy. The initial time of mesophase formation and its growth rate were measured for each fraction. The results showed that the amphoteric fraction was the most prone to coke formation, followed by the basic and acidic fractions. Mesophase formation for the neutral and aromatic fractions was delayed, and its growth rate was considerably slower. The relative order of coking propensity of the fractions is amphoteres > bases > Hamaca resid > acids > neutrals > aromatics. Although the Hamaca resid contained high concentrations of aromatic and neutral components with a relatively lower coking propensity, its coking propensity was much more similar to that of the acidic and basic fractions, which were less abundant in the resid. This propensity shows that the interactions among the individual components were not proportional to their concentrations in the feed and that the amphoteric fraction may have had a larger influence on coke formation relative to the other components. It was further observed that under the reaction conditions employed, the amphoteric fraction had high viscosity, did not develop a distinct mesophase stage, and formed fine-grained mosaic coke over a very short period of time. The results of this work may be used to assess the feasibility of selective removal of problematic components in the feedstocks prior to processing.
Introduction Predicting the product yields and quality during the hydrocracking of heavy oils is challenging because of the complexity of the cracking reactions and the variability in the chemical composition of the feedstocks. The use of analytical techniques that provide not only consistent and meaningful results, but also sufficient quantities of resid fractions for further evaluation and analyses is crucial. Petroleum fractions can be obtained using techniques such as distillation, SARA, liquid chromatography (LC), supercritical fluid extraction (SCFE), and ion-exchange chromatography (IEC). Furthermore, the relationships between the chemical composition of the feedstocks and the products are well established. For instance, the yields of gas, gasoline, light gas oil (LGO), and heavy gas oil (HGO) and amount of coke that result from fluid catalytic cracking (FCC) of the Californian Wilmington resid were shown to depend on feedstock composition.1 There is evidence that the majority of the light products, such as gas and gasoline, are derived primarily from the neutral com* To whom correspondence should be addressed. (1) Green, J. B.; Zagula, E. J.; Reynolds, J. W.; Wandke, H. H.; Young, L. L.; Chew, H. Energy Fuels 1994, 8, 856-867.
ponents of the feed. Large amounts of coke and small quantities of methane and C2+ hydrocarbons that form during the processing of petroleum heavy ends are attributed to the acidic and basic components in the feed. It has also been shown that the polar compounds present in acidic and basic fractions not only influence the storage stability and utilization of distillates but also play a key role in sediment formation in diesel fuels.2,3 Ovalles et al.4 studied the thermal stability and interfacial activity of acid, basic, and neutral fractions in the Cerro Negro Crude Oil from Venezuela, obtained by the IEC method. Following characterization of the fractions via spectroscopic techniques (FTIR, H and 13C NMR), they concluded that acid fractions behaved as natural surfactants in stabilizing oil/water emulsions and that it was the hydrophilic portion of the surfactants that showed higher acidity and high concentration of polar constituents. Acid fractions are known to be responsible for coke formation,5 to deactivate heterogeneous catalysts in day-to-day operations of refineries, (2) Pedley, J. F.; Hiley, R. W.; Hancock, R. A. Fuel 1988, 67, 11241130. (3) Pedley, J. F.; Hiley, R. W.; Hancock, R. A. Fuel 1989, 68, 2731. (4) Ovalles, C.; del Carmen Garcia, M.; Lujano, E.; Aular, W.; Bermudez, R.; Cotte´, E. Fuel 1998, 77, 121-126.
10.1021/ef980220m CCC: $18.00 © 1999 American Chemical Society Published on Web 03/04/1999
Venezuelan Heavy Oil Fractions
Energy & Fuels, Vol. 13, No. 3, 1999 695
Figure 1. Separation of the Hamaca +510 °C resid into its components using ion-exchange chromatography.
and to inhibit crude dehydration.6 Studying the relative coking propensity of these individual fractions and of the feed is extremely important because the obtained knowledge could lead to the ability to selectively remove components most detrimental to coking, which, in turn, could mean higher liquid yields while maintaining economic and environmental competitiveness. The objective of this study was to investigate the thermal cracking of the Venezuelan Hamaca heavy oil fractions using hot-stage microscopy (HSM). The fractions were separated by IEC from the Hamaca resid (+510 °C). Information gained will provide a greater insight into the coking propensity of all fractions, particularly the polar compound classes (i.e., acidic groups) in terms of mesophase induction period and rate of formation. It will also help to assess the feasibility of removing, by pretreatment (prior to hydrocracking), problematic fractions from any feed.
Table 1. Properties of Hamaca Resid (+510 °C) and Its Fractions amphoteres bases
acids
yield (wt %) 30.0 9.8 8.9 VPO MW 1832 1048 996 TGA 600 C 43.4 19.2 24.9 residue wt % C 83.0 84.5 83.6 wt % H 8.2 9.6 10.4 wt % N 2.1 1.7 1.8 wt % O 2.9 1.6 2.5 wt % S 2.8 2.8 2.3
arom- satur- Hamaca atics ates resid losses 41.1 600* 5.5
5.7 620* 0.0
4.5 NA NA
84.6 11.5 ND 1.6 2.5
85.2 15.0 ND Hamaca resid > acids > neutrals > aromatics. The amphoteric fraction was highly viscous, did not develop a distinct mesophase stage, and formed fine-grained mosaic coke rapidly. The basic and acidic fractions showed behavior similar to that of the amphoteric fraction. Neutral and aromatic fractions were less prone to coking and formed distinct mesophase spheres, which grew and coalesced slowly. Their influence on the coking of the Hamaca resid was minimal despite the high concentration of these components in the resid. Although present in small concentrations in the Hamaca resid, the acid and basic fractions had a greater impact on the coking propensity of the Hamaca feed. Removal of the acidic, basic, and amphoteric fractions from the feed prior to processing is desirable in order to diminish the coke yield. Acknowledgment. This research was conducted under the Consortium on “Hydrocracking Reactivity of Heavy Oils and Bitumens”. PDVSA Intevep, Syncrude Canada Ltd., and the National Centre for Upgrading Technology provided financial support for this work. The authors thank Dr. Janis Watkin for the editorial review of this manuscript. EF980220M
