Ind. Eng. Chem. Res. 2000, 39, 4897-4900
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Fractionation of Aromatic Heavy Oil by Dynamic Supercritical Fluid Extraction So-Jin Park,* Cheol-Joong Kim,† and Bo-Sung Rhee Daeduk Institute of Technology, SK Corporation, Taejon 305-370, Korea, and Department of Chemical Engineering, Chungnam National University, Taejon 305-764, Korea
A new separation process should be applied to improve the quality of pitch that will be used for the production of high-performance carbon materials. In this study, aromatic heavy oil has been fractionated with supercritical n-hexane. Dynamic supercritical fluid extraction was applied, and the dependence on the temperature and pressure of the extraction behavior and selectivity has been investigated. Aromatic heavy oil was fractionated into several fractions, which have a boiling point distribution that is clearly different from that of the feedstock, by extraction under vapor-liquid-like phase behavior conditions. The boiling point distribution of the extracts sampled under liquid-liquid-like phase behavior conditions, however, was almost the same as that of the aromatic heavy oil. As the extraction temperature increased, the extraction rate also increased without a compensation for the extraction selectivity of the boiling point distribution. The increasing of extraction pressure, however, decreased the extraction selectivity of the boiling point distribution although extraction rate was increased. Asphaltene was rejected by extraction under vapor-liquid-like phase behavior conditions, but there was no chemical composition extraction selectivity under liquid-liquid-like phase behavior conditions. 1. Introduction Aromatic heavy oils (AHOs) produced from refinery upgrading processes, ethylene plants, and coke plants have a high boiling point, a wide boiling point distribution, and high aromaticity. AHOs are usually used as a blending stock of heavy fuel oil; however, a few studies have focused on their applicability as a precursor for high-performance carbon materials to increase the profitability.1-3 To be used as the precursor of highperformance carbon materials, AHOs should first be converted to optically isotropic or anisotropic pitch. Pitch preparation processes can be categorized as follows: (a) selective separation of useful fractions from AHO, followed by a polycondensation reaction; (b) selective recovery of useful fractions from AHO during a polycondensation reaction or from pitch after the polycondensation reaction; and (c) selective removal of useless fractions from AHO during a polycondensation reaction and continued catalytic hydrogenation of pitch.4-7 The above-mentioned pitch processes are combinations of separation and reaction processes. Usually, solvent extraction and vacuum distillation techniques are used to separate useful or useless fractions from the substance; however, the low repeatability and selectivity of these processes induce the application of a new separation process that is superior to previous processes to improve the performance of the resulting carbon materials. The solubility and selectivity of a supercritical fluid (SCF) can be varied by changes in pressure and temperature, and the use of a cosolvent can expand the application fields of the SCF. Supercritical fluid extraction (SFE) has found application in solvent extraction, * Author to whom correspondence should be addressed. E-mail:
[email protected]. Fax: 82-42-823-6414. † Chungnam National University.
in separation processes, and in the upgrading of petroleum feedstocks. The SFE concept is also of interest in enhanced oil recovery processes that depend on the extraction of oil components by a solvent that is typically in a supercritical state.8,9 It is already well-known that supercritical propane can fractionate heavy oil into wax, resin, and asphaltene. Furthermore, solvent de-asphalting of the heavy oil process has already been successfully industrialized. However, the applicability of SFE to the pitch production process is not fully studied. In this study, AHO was separated into several fractions by dynamic SFE at various pressures and temperatures, and the effects of the extraction conditions on the extraction behavior and selectivity were investigated to determine the applicability of SFE for AHO fractionation. n-Hexane was selected as the extraction solvent after a consideration of the boiling point of AHO, and the boiling point distribution and chemical compositions of the extracts were analyzed to investigate the effects of the experimental conditions on the extraction selectivity. 2. Experimental Section 2.1. Material. The AHOs (F-AHO) used in this study were produced by the FCC (fluid catalytic cracking) process, and several important properties of the AHO are described on Table 1. It has a high density and a high carbon residue because of the high boiling point and high aromaticity. Technical-grade n-hexane (purity > 99.2%) was used without further purification, and HPLC-grade solvents were used for the TLC-FID analysis. 2.2. Apparatus and Method. The extraction system was custom-built by Autoclave Engineers, Inc. In the center of the apparatus was a temperature-controlled extractor with a capacity of 300 cm3. The extractor was equipped with a magnetic-drive packerless stirring device to decrease the time to reach an phase equilib-
10.1021/ie000325o CCC: $19.00 © 2000 American Chemical Society Published on Web 12/04/2000
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Table 1. Physical Properties and Chemical Compositions of F-AHO density, kg/m3 (at 288.15 K) sulfur, wt % Conradson carbon residue, wt % true boiling point, K initial boiling point 10% 50% 90% final boiling point compositions, wt % saturated aromatic resin asphaltene
1068.9 0.98 7.89 483.15 613.15 679.15 796.15 932.15 9.76 84.89 3.57 1.78
rium. n-Hexane was charged into the extractor, and the system was brought to the desired pressure using a high-pressure liquid pump (LDC); the pressure was measured with a Bourdon tube gauge (Ashcroft). The extract collector was cooled by a circulating cooling bath to ensure that the solvent was maintained in the liquid state. The temperature of the system was controlled to within (0.5 K with a PID auto temperature control system. To prevent condensation of the solvent, the line was heated above the boiling point of the solvent. The temperature indicator was calibrated with a reference thermometer (F250, ALS) to within (0.2 K, and the pressure gauge was referenced to the saturated vapor pressure of benzene and n-hexane to within (0.03 MPa accuracy. The extraction vessel was charged with 10 g of F-AHO in all of the experiments, and the extraction system was purged for 3 min with nitrogen gas in order to remove air from the system. The desired temperature of the extraction vessel was set and maintained by a temperature controller. The shut-off valve was closed, and n-hexane was pumped in (240 mL/hr) to pressurize the system to the desired extraction pressure. Once the system attained the desired pressure and temperature, the flow was started by opening the shut-off valve, and the flow rate was adjusted with the micrometering valve. After 30 g of n-hexane had passed through the extraction system, the extract accumulated in the receiver was weighed and stored for solvent drying. In each extraction sequence, nine extracts were collected, and each extract corresponded to the throughput of 30 g n-hexane. All of the samples were dried in a nitrogen atmosphere and balanced to calculate the extract composition. The temperature and pressure of the extraction cell were controlled to within (0.5 K and (0.05 MPa, respectively. 2.3. Analysis. The boiling point distributions of the extracts and the F-AHO were analyzed by gas chromatography simulated distillation equipped with a capillary column coated with cross-linked methylsilicone gum. The chemical compositions were analyzed with TLC-FID (Mark V, Iatron Laboratories), and n-hexane, toluene, and methanol/methylene chloride (5:95) were used as the developing solvents.10 3. Results and Discussion 3.1. Extraction Behavior. The effect of temperature and pressure on the extraction performance is presented in Figure 1. The experiments were conducted at a subcritical temperature (493.2 K), at a temperature close to the critical point of n-hexane (513.2 K), and at two supercritical temperatures (533.2 and 553.2 K). The
Figure 1. Extraction behavior at different temperatures and pressures.
critical temperature of n-hexane is 507.7 K, and the critical pressure is 3.01 MPa. Temperature and pressure exert a significant influence on n-hexane densities and are expected to influence the extraction process. The data indicate that, with subcritical pressure (2.0 MPa) in the vicinity of the critical temperature of n-hexane (493.2 and 513.2 K), the cumulative extraction yield increased linearly with the cumulative weight of solvent. Thus, it is expected that the phase composition of the binary n-hexane-F-AHO system remains invariant as a function of the system’s overall composition. Well above the critical temperature of n-hexane (533.2 and 553.2 K), however, there was a slight reduction in the slope with an increase in the cumulative weight of the solvent, and it is expected that the relatively heavier fraction of F-AHO is extracted at the later stage of extraction. The total amount of oil extracted increased with increasing temperature, even though a reduction in solvent density is expected. Thus, the density of the pure solvent is not the only governing parameter in these extractions, and the increase of the F-AHO vapor pressure is considered to be another major parameter for extraction. A complex interplay of the F-AHO volatility and the extracting solvent power of n-hexane determines the overall extraction performance.11,12 The phase behavior at this pressure (2.0 MPa) appeared to vapor-liquid-like. Well above the critical pressure of n-hexane (5.0 MPa), the amount of oil extracted increased dramatically compared to that extracted at subcritical pressure and decreased with increasing cumulative solvent weight at all temperatures. The increased extracted amounts compared to the subcritical pressure cases is consistent with a superior extraction performance based on the higher solvent density. Normally, the extraction behavior of complex hydrocarbon mixtures with a supercritical solvent is governed by the density of the pure solvent under the extraction conditions, and the solvent power of a supercritical fluid can be related to the solvent density in the critical region. The phase behavior at this pressure appeared to shift to liquid-liquid-like. In this case, however, the total amount of oil extracted also increased with increasing temperature just as in the previous subcritical pressure case (2.0 PMa). Compared with other investigations of extraction behavior that used solvents with lower critical temperatures, such as carbon dioxide and propane, these investigations used a solvent with a higher critical temperature, and the extraction temperature was high enough that the extraction is affected by increasing of F-AHO volatility. In the vicinity of the critical pressure of n-hexane (3.5 MPa), the extraction behavior shifted from liquidliquid-like (493.2 and 513.2 K) to vapor-liquid-like (533.2 and 553.2 K) depending on the temperature. Following the shift in phase behavior, the cumulative
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Figure 3. Comparison of the true boiling point distribution of the first extracts as a function of pressure (T ) 533.2 K). Figure 2. Comparison of the true boiling point distribution of the first extracts as a function of temperature (P ) 2.0 MPa).
extracted oil yield also increased. Through the decrease in the n-hexane density with the increase in temperature from 533.2 to 553.2 K, the extract performance of n-hexane decreased. 3.2. Extraction Selectivity. The compositional changes in the extracted phases that occurred during SFE were monitored in an attempt to establish the nature of the extraction and the partitioning process as a function of temperature, pressure, and the order of extraction steps. The F-AHO and selected extract samples were analyzed by gas chromatography simulated distillation. The boiling point distributions of the first extract as a function of temperature for the extraction at 2.0 MPa are presented in Figure 2 to investigate the effect of temperature change on extraction selectivity. It is observed from this figure that the boiling point distributions of the extracted oils are identical with respect to temperature variation. Therefore, it is very clear from the data that n-hexane does not exhibit selective extraction of the F-AHO with respect to temperature variation. The boiling point distributions of all extracts, however, are narrower than that of F-AHO, and this indicates the selective extraction of n-hexane for lighter compounds from F-AHO. This behavior makes it possible to fractionate F-AHO into two fractions that have boiling point distributions that differ from each other and from that of F-AHO. The boiling point distributions of the first extract as a function of pressure for the extraction at 533.2 K are plotted in Figure 3 to investigate the effect of pressure change on extraction selectivity. It is clearly observed from the data that the higher-pressure extract contains relatively heavier compounds, and the boiling point distribution of the 5.0 MPa extract is exactly the same as that of F-AHO because the phase behavior is liquidliquid-like. This, it is concluded from the data that extraction selectivity is decreased with increasing pressure. The boiling point distributions of the 2.0 and 3.5 MPa extracts, however, are still much narrower than that of F-AHO. This behavior also makes it possible to fractionate F-AHO into two fractions that have boiling
Figure 4. Comparison of the true boiling point distribution of extracts at various extraction steps (T ) 533.2 K, P ) 2.0 MPa).
Figure 5. Comparison of the true boiling point distribution of extracts at various extraction steps (T ) 533.2 K, P ) 3.5 MPa).
point distributions that differ from each other and from that of F-AHO. The boiling point distributions of the extracts with varying the order of extraction steps at the two different pressures 2.0 and 3.5 MPa are presented in Figures 4 and 5, respectively. More light compounds were extracted during earlier extraction steps at the same extraction pressure, and in all cases, increasing pressure resulted in increasing heavy compounds at the same step. These observations are consistent with continuous
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Table 2. Chemical Compositions of Extracts Sampled under Various Extraction Conditions temperature, K pressure, MPa fraction saturated, wt % aromatic, wt % resin, wt % asphaltene, wt %
493.15 2.0 1 10.15 87.50 2.35 0.00
513.15 2.0 1 10.00 87.60 2.40 0.00
1 10.25 87.44 2.31 0.00
2.0 3 11.15 86.80 2.05 0.00
5 11.34 86.07 2.59 0.00
extraction crude oil-CO2 experiment reported by Orr et al.13 With increasing extraction pressure, the density of the solvent increased, and consequently, the solubility of solute also increased. The chemical compositions of selected extracts and the F-AHO are presented in Table 2 to investigate the effects of the experimental conditions on the chemical composition extraction selectivity. The selective extraction of the saturated and asphaltene compounds will help to prepare a better pitch precursor, because saturated compounds easily crack to form gas and asphaltene compounds behave as impurities for highquality pitch. The chemical compositions of all extracts sampled under vapor-liquid-like phase behavior conditions were almost the same but were different from that of F-AHO in terms of asphaltene content. This means that asphaltene can be rejected by extraction under specific conditions, which would be very helpful in the production of a high-quality pitch precursor. The chemical compositions of the extracts sampled under liquidliquid-like conditions were almost the same as that of F-AHO, which means that there was no chemical composition extraction selectivity under liquid-liquidlike phase behavior conditions. 4. Conclusion Aromatic heavy oil was fractionated into several fractions that clearly have a different boiling point distribution from that of the feedstock by extraction under vapor-liquid-like phase behavior conditions. The boiling point distributions of the extracts sampled under liquid-liquid-like phase behavior conditions, however, were almost the same as that of the aromatic heavy oil. As the extraction temperature was increased, the extraction rate also increased without a compensation
533.15 3.5 1 3 11.89 12.25 84.75 85.31 3.36 2.44 0.00 0.00
5 13.51 83.70 2.79 0.00
1 8.90 86.80 3.10 1.20
5.0 3 9.50 85.17 3.53 1.80
5 9.24 85.7 3.61 1.45
553.15 2.0 1 10.00 87.50 2.50 0.00
F-AHO 9.76 84.89 3.57 1.78
in the extraction selectivity of the boiling point distribution. An increase in the extraction pressure, however, decreased the extraction selectivity of the boiling point distribution even though extraction rate was increased. Asphaltene was rejected by extraction under vaporliquid-like phase behavior conditions, but there was no chemical composition extraction selectivity under liquidliquid-like phase behavior conditions. Literature Cited (1) Chwastiak, S.; Barr, J. B.; Diechenko, R. Carbon 1979, 17, 49. (2) Singer, L. S. Fuel 1981, 60, 839. (3) Hwang, J. S.; Lee, C. H.; Cho, K. H.; Kim, M. S.; Kim, C. J.; Ryu, S. K.; Rhee, B.S. Hwahak Konghak 1995, 33, 551. (4) Singer, L. S. U.S. Patent 3,919,387, 1975. (5) Diefendorf, R. T.; Riggs, D. M. U.S. Patent 4,208,267, 1980. (6) Otani, S. Japanese Patent 57-100186, 1982. (7) Dauche´, F. M.; Bolan˜os, G.; Blasig, A.; Thies, M.C. Carbon 1998, 36, 953. (8) Nelson, S. R.; ad Corbett, R.W. In Proceedings of the 3rd International UNITAR Conference on Heavy Crude and Tar Sands, Edmonton, Alberta, Canada, 1985. (9) Nelson, S.R.; Goodman, R.G. Paper Presented at the AIChE Spring Meeting, Houston, TX, March 22-24, 1985. (10) Ray. J. E.; Oliver, K.M.; Wainwright, J.C. Proceedings of the Institute of Petroleum (London) Symposium on Petroanalysis, London, October 27-29, 1981. (11) Fly, J. F.; Baker, J. K. NBS Techical Note 1070; U.S. Department of Commerce, National Bureau of Standards: Government Printing Office: Washington, D.C., 1983. (12) Milind, D. D.; Jongsic, H.; Francis, V. H. Fuel 1992, 71, 1519. (13) Orr, F. M., Jr.; Silva, M. K.; Lien, C. SPEJ 1983, 23, 281.
Received for review March 16, 2000 Revised manuscript received October 9, 2000 Accepted October 9, 2000 IE000325O
