3988
Ind. Eng. Chem. Res. 1997, 36, 3988-3992
Characterizing Petroleum Vacuum Residue by Supercritical Fluid Extraction and Fractionation Tie-Pan Shi,* Yun-Xiang Hu, Zhi-Ming Xu, Tong Su, and Ren-An Wang State Key Laboratory of Heavy Oil Processing, University of Petroleum, Beijing 102200, People’s Republic of China
The goal of this work is to provide a novel separation method for petroleum vacuum residua instead of the traditional SARA (saturates, aromatics, resins, and asphaltenes composition analysis) or the high-vacuum distillation method. Supercritical fluid extraction and fractionation was employed to separate four Chinese vacuum residua into 15-17 narrow fractions with total extraction yields of 71.7-87.8 wt %. Physicochemical properties such as density, average molecular weight, kinematic viscosity, and carbon residue of each narrow fraction were obtained. Elemental analyses (C, H, N, S), heavy metals (Ni, V) concentrations, SARA analyses, and 1HNMR tests provide insights in their chemical composition. Introduction With the increasing demand for commercial light oil and the declining quality of crude oil (Swam, 1995), it is necessary to fully utilize the heavy residue. The problem is more serious for Chinese refineries because the yields of vacuum residua are often 40-60 wt % (Cheng, 1994). Due to the complexity of heavy oil, current research on heavy oil is still incomparable to that of crude oil and cannot satisfy the appeal of the refining industry. First of all, an eligible separation method is in need. Existing methods, such as highvacuum, short-path distillation (Nilsson et al., 1986; Moore et al., 1991), allow the cut point to be extended to about 700 °C, thus limiting the total yield of the distillates. Composition analysis by liquid chromatography is frequently used to separate and quantify the hydrocarbon families (saturates, aromatics, resins, asphaltenes, etc.) making up heavy residuals; however, the amounts of the subfractions are inadequate for reactivity studies. In search of the separation methods for heavy oils, methods that have industrial backgrounds are always of interest. Over half a century ago, Wilson, Keith, and Haylett (1936) devised a process that became the basis of the propane deasphalting process still in use today for refining lube oils. In the 1970s, the ROSE (residuum oil supercritical extraction) (Gearhart and Garwin, 1976) process was developed. The common characteristic of propane deasphalting and ROSE is that their extraction step is carried out at compressed liquid conditions, not at supercritical conditions. As a potential solvent with tunable density simply by changing pressure and temperature, supercritical fluid (SCF) is more competitive than compressed liquid; therefore, extraction with supercritical fluids has been applied to separations of various compounds. Taking advantage of the studies on the phase equilibria of residuum-light hydrocarbon systems at high pressures in this laboratory (Peng et al., 1989), a residuum supercritical fluid extraction pilot plant with a capacity of 15000 ton/yr was built and has been running for years in Shenghua Refinery, University of Petroleum (Yang et al., 1991). It is well-known that, with a fractionating packed column, selectivity of supercritical fluid extraction can be improved. For instance, Zosel (1980) used super* Author to whom correspondence should be addressed. Tel: 86-10-69745566-3743. E-mail:
[email protected]. S0888-5885(97)00152-8 CCC: $14.00
critical ethane to separate cod-liver oil into 50 fractions. A recent study by Sato et al. (1996) provided an experimental proof. If suitable solvent and operating parameters are selected, supercritical fluid extraction and fractionation (SFEF) will provide high extraction yields and enough amounts of samples for reactivity and conversions studies. Hence, a supercritical fluid extraction and fractionation apparatus was established as a characterization tool for heavy oils. Trial separations of vacuum residua by Peng et al (1988) using supercritical butane (with isobutane of 85.43 wt %) in our research group revealed the good repeatability of the apparatus. Past efforts in our laboratory showed that supercritical fluid extraction and fractionation which resembles the true-boiling-point (TBP) distillation for crude oils may serve as a powerful characterization tool for vacuum residua. This paper will present the comprehensive results of our work on vacuum residua characterization by using SFEF. Experimental Section Figure 1 shows the schematic diagram of the SFEF apparatus. Compared to carbon dioxide, which is the most frequently used supercritical fluid, light hydrocarbons (C3-C5 alkanes) can provide higher yields in SFEF of vacuum residua. When propane was used as the supercritical fluid in our experiments, the total extraction yield of Daqing vacuum residuum was about 32%, while for the butane fraction (with isobutane of 81.76%) the yield was about 70%, and the n-pentane, about 87%. Hence n-pentane (with purity of 99.9%) was chosen as the supercritical fluid in this work. The critical temperature and critical pressure of n-pentane are 196.6 °C and 33.3 atm, respectively. It is widely accepted that the density of a supercritical fluid is a key parameter in supercritical fluid extraction. Although density does not have an explicit relation to temperature and pressure, it can be directly changed by varying temperature and pressure. In SFEF, keeping temperature or the temperature gradient is always adopted because it eliminates perturbations in the extractor and the fractionation column. A linear pressure program was adopted in our work which can reproduce well. Since there is a limited current understanding on residuum-light hydrocarbon systems at elevated pressures, it is insufficient to predict all the phase behaviors of different vacuum residua in supercritical fluids. Thus, selection of operating tempera© 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3989 Table 1. Basic Properties of Daqing, Dagang, Shengli, and Gudao Vacuum Residua density, d420 average molecular weight H/C atomic ratio sulfur, % nitrogen, % CCR, % Ni, ppm saturates, % aromatics, % resins, % asphaltenes, % aromaticity, fA
Daqing
Dagang
Shengli
Gudao
0.9392 1051
0.9790 1083
0.9724 967
0.9945 969
1.79 0.14 0.40 8.2 10.0 41.9 32.7 25.4 0 0.14
1.63 0.24 1.00 16.3 66.9 30.0 32.2 37.3 0.5 0.23
1.63 3.01 0.85 16.0 55.7 16.1 30.6 51.1 2.2 0.23
1.58 2.52 1.42 15.6 40.7 17.3 31.0 48.4 3.3 0.25
Figure 1. Schematic diagram of SFEF apparatus. 1. Extractor. 2. Packed bed. 3. Inlet of residuum. 4. SCF solvent distributor. 5. Solvent heater. 6. SCF cooler. 7. Pressure regulator. 8. Heater. 9. Sample receiver. 10. Computer. 11. Solvent pump. 12. Solvent condenser. 13. Solvent storage. 14. Solvent storage.
tures and pressures depends much on operation experiences. In the separations of vacuum residua, the temperature in the extractor was set at 240 °C, the rectification column had a temperature gradient from 240 °C at the bottom to 250 °C at the top. The temperatures were a bit far from the critical temperature of n-pentane to avoid flooding of oil in the fractionation column. Pressure was set to rise linearly from 4.0 to 12.0 MPa over 8 h and was realized by a computer which helped in assuring the repeatability of the experiment. The circulation rate of the solvent was kept at 100 mL/min (liquid state) at room temperature. At the start of the experiment, the feedstock was charged into the extractor. The residuum were brought into contact with the supercritical fluid, and a certain percentage of the residuum will be dissolved into the SCF. The light phase exiting from the top of the packed column was expanded through a pressure regulator. The solvent then evaporated and was recycled. Gradually raising the pressure increases the solubility of residuum in SCF. In this work, about every 5 wt % extracts of the residuum was collected as a “narrow fraction” or “narrow cut”. Finally, the raffinate was released from the extractor. Routine tests such as ultimate analyses, molecular weight, density, kinematic viscosity, refractive index (for narrow fractions only), Conradson carbon residue (CCR), and SARA composition were determined for the vacuum residua and their narrow fractions. 1HNMR analyses were used to calculate the structural parameters of the average molecule of the samples. Metals analyses, such as Ni, V, Fe, Na, and Ca were also made as a reference for their performance in a catalytic cracking reaction. Results and Discussion Physicochemical properties of Daqing, Dagang, Gudao, and Shengli vacuum residua are summarized in Table 1. Figure 2 shows the extraction pressure versus the cumulative DAO (deasphalted oil) yield. The four curves have relatively plain regions which always lie in the middle and indicate that narrow fractions with such yields might have properties changing less significantly. Similar trends can be seen in the average molecular weight vs the cumulative midpercent yield curve in Figure 3. Hydrogen to carbon atomic ratios, which are generally taken as notable parameters of heavy oil, of the narrow fractions against their cumulative midpercent DAO yields, were depicted in Figure 4.
Figure 2. Pressure vs the cumulative extraction yield in SFEF separation.
Figure 3. Average molecular weight of SFEF narrow fractions of Daqing, Dagang, Gudao, and Shengli vacuum residua vs midpercent cumulative yield.
H/C atomic ratios decrease with increasing extraction yield. The differences in H/C atomic ratios of four series of narrow fractions concur with those of the vacuum residua. H/C ratios have a systematic variation, and the front fractions have H/C ratios relatively higher than those of the whole vacuum residua. The H/C atomic ratios of Daqing no. 3, no. 17, and the reffinate are respectively 1.86, 1.67, and 1.38, which shows the considerable variation between the narrow fractions derived from a residuum. The cuts in the rear and the raffinate have quite low hydrogen saturation compared to that of the front fractions.
3990 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
Figure 7. Kinematic viscosity of SFEF narrow fractions of Daqing, Dagang, Gudao, and Shengli vacuum residua. Figure 4. H/C atomic ratios of SFEF narrow fractions of Daqing, Dagang, Gudao, and Shengli vacuum residua vs cumulative midpercent DAO yield.
Figure 8. CCR of SFEF narrow fractions of Daqing, Dagang, Gudao, and Shengli vacuum residua.
Figure 5. Densities of SFEF narrow fractions of Daqing, Dagang, Gudao, and Shengli vacuum residua.
Figure 6. Refractive indices of SFEF narrow fractions of Daqing, Dagang, Gudao, and Shengli vacuum residua.
Densities and refractive indices of the four series of narrow cuts are shown in Figures 5 and 6. Both densities and refractive indices of those narrow fractions increase against their SFEF yield consistently. Sur-
prisingly, the two familes of curves show conformity in their shape which probably indicates that there may be a similar mechanism of how structure affects density and the refractive index. Viscosity of fractions from TBP distillation increase with their boiling point. It is quite similar in Figure 7 where kinematic viscosity of the narrow fractions show exponential increase vs cumulative DAO yield. Generally, higher value of carbon residue indicates higher coking propensity in the conversion. CCR values of the series of narrow fractions vs the cumulative extraction midpercent yields are illustrated in Figure 8. Of the four series, narrow fractions of Daqing always have lower CCR values. Although Dagang, Shengli, and Gudao vacuum residua have close CCR values which are 16.3, 16.0, and 15.6%, respectively, their narrow fractions with close cumulative yields have different CCR values, especially when those of Dagang are compared with those of Shengli and Gudao. Raffinate of Dagang has a CCR values of 45.3%, while for Gudao it is 35.8% and for Shengli 22.0%. This shows the uneven distribution of heavy, highly-condensed aromatic compounds. SARA analyses were performed, and percentages of saturates, aromatics, and resins are illustrated in Figure 9. All narrow cuts are free of asphaltenes. That is to say, all the asphaltenes in vacuum residuum are enriched in the raffinate after the SFEF separation. For the three vacuum residua concerned, there are the same trends of how contents of saturates, aromatics, and
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3991 Table 2. Nickel Concentrations (ppm) of 30, 40, 50, and 60 wt % (approximately) DAO Fractions of Daqing, Dagang, Shengli, and Gudao Vacuum Residua vacuum residuum 30% 40% 50% 60%
Daqing
Dagang
Shengli
Gudao
10.0
66.9 20.3 21.5 24.9 27.0
55.7 10.9 11.9 14.8 17.7
40.7 11.8 13.3 13.6 16.2
1.16 1.43
Figure 9. Saturates, aromatics, and resins percentages of SFEF narrow fractions of Daqing, Dagang, and Gudao vacuum residua.
Figure 11. Aromaticity of SFEF narrow fractions of Daqing, Dagang, and Gudao vacuum residua.
Figure 10. Nickel and vanadium concentrations of SFEF narrow fractions of Daqing, Dagang, Gudao, and Shengli vacuum residua.
resins vary with cumulative DAO yields. There is decreasing percentages of saturates and the opposite for resin. The percentages of aromatics in narrow fractions increase to a maximum and then decrease as the fractions become heavier. The reason may be due to the high resins content of Chinese vacuum residua. Among the three series of narrow fractions, fractions of Daqing residua have the highest percentages of saturates and the lowest percentages of aromatics and resins while fractions of Gudao are the opposite and Dagang’s lie in between. Nickel and vanadium are the primary metal hazards to catalyst deterioration in a fluidized catalytic cracking unit. Figure 10 shows exponential increase of nickel and vanadium concentrations against the cumulative yields. Opposite to the oils from other areas in the world, nickel contents in Chinese oils are much higher than those of vanadium. Nickel and vanadium contents of SFEF fractions (with DAO yields about 30, 40, 50, and 60 wt %) of Daqing, Dagang, Gudao, and Shengli are listed in Table 2, from which we can see the efficiency of Ni and V removal by the SFEF separation. Structural parameters reflect the average chemical nature of the petroleum residua. 1H-NMR analyses were given, and the structural parameters of the average molecules were calculated using the modified BrownLadner method (Cheng, 1994). Aromaticity (fraction of aromatic carbon) of the narrow cuts are plotted in Figure 11 where one can perceive the opposite trend
Figure 12. Total ring numbers and aromatic ring numbers of the SFEF narrow fractions of Daqing, Dagang, and Gudao vacuum residua.
with comparison to H/C atomic ratio curves. In Figure 12 we can see that both aromatic ring numbers (RA) and total ring numbers (RT) of the average molecules rise against the cumulative yields which may partly explain the declining percentages of saturates in Figure 9. The properties of the raffinates of the vacuum residua summarized in Table 3 show that the raffinates are the most difficult to handle in the upgrading process. The low hydrogen contents, high heteroatom contents, CCR values, and metals concentrations of the raffinates are the causes of the high coke yields and the catalyst deactivation. It would be less effective to process the residua without removal of such species. The analyses of the narrow cuts proved the validity of SFEF as a characterization tool for heavy oil. The properties have systematic changes against the increase of the cumulative yields. In the light of the results obtained in this work, we can conclude that SFEF of residua is parallel to the TBP distillation of crudes. Coupling of SFEF with the traditional characterization
3992 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 Table 3. Raffinates of Daqing, Dagang, Gudao, and Shengli Vacuum Residua yield, wt % density (g/cm3, 20 °C) carbon residue, wt % molecular weight H/C atomic ratio nitrogen, % sulfur, % nickel, ppm vanadium, ppm saturates, % aromatics, % resins, % asphaltenes, %
Daqing
Dagang
Shengli
Gudao
12.2 >0.9654 40.3 2458 1.38 0.98 0.30 111.6 1.18 0.2 7.9 92.1 0.3
24.5 >1.0009 45.3 7636 1.26 1.18 0.30 199.3 2.1 1.6 11.3 84.0 3.1
28.2 >1.0002 22.0 5515 1.53 1.70 5.14 122.3 8.75 0 17.6 48.7 33.7
26.1 >1.0185 35.8 5549 1.36 1.64 4.08 114.5 10.7 0.2 3.2 62.4 34.2
methods will further our understanding of heavy oil and thereby will help to initiate innovative processing strategies. Conclusion Petroleum vacuum residua were separated with the supercritical fluid extraction and fractionation (SFEF) method into 15-17 narrow fractions much in the same way of TBP distillation of crude oils. Compared with traditional high-vacuum distillation, this separation method was featured by its low operating temperature and high extraction yield. The systematic variation in the properties of the narrow cuts from the residua give insights to the vacuum residua and provide suggestions for their upgrading process.
Gearhart, J. A.; Garwin, L. ROSE process improves resid feed. Hydrocarbon Process. 1976, 55, 125. Moore, M. F.; Mayo, S. L.; Goolsby, T. L. Catalytic cracking of residual petroleum fractions. Fuel Sci. Technol. Int. 1991, 9 (3), 283. Nilsson, P.; Massoth, F. E.; Otterstedt, J. E. Catalytic cracking of heavy vacuum gas oil. Appl. Catal. 1986, 26, 175. Peng, C.-L.; Wang, R.-A.; Fan, Y.-H.; Yang, G.-H. Phase equilibria of the mixture containing petroleum residue and supercritical solvent. In Proceedings of 1st International Symposium on Supercritical Fluids, Nice, France; Perrut, M., Ed.; Societe Francaise de Chimie: Nice, France, 1988; p 829. Sato, M.; Goto, M.; Hirose, T. Supercritical fluid extraction on semibatch mode for the removal of terpene in citrus oil. Ind. Eng. Chem. Res. 1996, 35, 1906. Swam, E. J. Sulfur, coke and crude quality-conclusions: US crude slate continues to get heavier, higher in sulfur. Oil Gas J. 1995, 93 (2), 37. Wilson, R. E.; Keith, P. C.; Haylett, R. E. Liquid propane: use in dewaxing, deasphalting, and refining heavy oils. Ind. Eng. Chem. 1936, 28, 1065. Yang, G.-H.; Fan, Y.-H.; Wang, R.-A.; Jia, S.-S. The extraction of petroleum residua with light hydrocarbon fractions under supercritical conditions. In Proceedings of the International Conference on Petroleum Refining and Petrochemical Processing, Beijing, China; Hou, X.-L., Ed.; International Academic Publishers: Beijing, 1991; p 201. Zosel, K. Separation with supercritical gases: practical applications. In Extraction with supercritical gases. Schneider, G. M., Stahl, E., Wilke, G., Eds.; Verlag Chemie: Basel, 1980; p 1.
Received for review February 17, 1997 Revised manuscript received June 11, 1997 Accepted June 13, 1997X
Acknowledgment China National Natural Science Foundation and SINOPEC provided financial support for this work.
IE970152B
Literature Cited Cheng, Z.-H., Ed. Heavy oil processing technology; China PetroChemical Press: Beijing, 1994 (in Chinese).
Abstract published in Advance ACS Abstracts, July 15, 1997. X
