China refinery tests asphaltenes extraction process

April 5, 2010
As global reserves of conventional crude oil continue to decline, increased reliance on heavy and ultraheavy feedstocks will result in higher upgrading costs and more importantly a comparatively larger carbon footprint.

As global reserves of conventional crude oil continue to decline, increased reliance on heavy and ultraheavy feedstocks will result in higher upgrading costs and more importantly a comparatively larger carbon footprint.

A demonstration plant in China has shown that selective extraction of asphaltenes technology1 2 can be built and operated with low technical and commercial risk. SELEX-Asp brings heavy and ultraheavy feedstocks in line with conventional oil processing costs and CO2 emissions. The process can be added to existing refineries as a separate operating unit and can reduce upgrading costs while delivering high reliability with low capital outlay.

It not only selectively removes asphaltenes as dry granulates, it removes large amounts of other impurities such as sulfur and metals. It delivers a clean feedstock similar to conventional crude oil, which allows a nearly seamless use of existing refinery processing units.

Current practice

Heavy crudes and residua contain high concentrations of contaminants that interfere with typical refining processes. If light crude is readily available, the easiest and most convenient way to process heavy crude is to blend it with light crude.

Depletion of conventional light crude, however, has limited options for processing heavy crude. Heavy oil upgraders remove most of the contaminants in heavy crudes and produce pipeline-transportable refinery blending feedstocks.

Commercial heavy oil upgraders use existing refinery processes such as coking and hydroprocessing. These processes, however, are not capable of upgrading the heavy fraction of the feedstock. That fraction is either converted into coke by thermal cracking or coke precursor by hydroprocessing.3

Coker product yield is relatively low and thermally cracked products are difficult to process in refinery units to meet specifications for clean transportation fuels, compared with virgin feedstocks.4

In residue hydroprocessing, a costly ebullated-bed reactor system must be used. Because hydroprocessing catalysts are deactivated quickly by residua that contain high concentrations of carbon residue and metals,5 the economics of residue hydroprocessing are less attractive.

Also, the operability of these upgrading processes is highly unreliable. Frequent unscheduled shutdowns and extended turnarounds still occur, even after 40 years of commercial operation. Moreover, upgrading is energy intensive and a redundant processing step that overlaps many of the downstream refining operations.

The availability of energy resources and the cost for waste disposal from the upgrader add to the economic burden.

Applying a separation process to clean up the contaminants in residua is not a new idea.6 7 Recent residue chemistry research showed that most of the molecules in the residua have fingerprints similar to those found in transportation fuels, i.e., less than four aromatic rings.8

The undesirable components are concentrated in the asphaltenes. If the asphaltenes are removed from the bulk residuum, the remaining resid fraction can be processed in conventional refinery processes such as resid fluid catalytic cracking and packed-bed hydroprocessing.

No viable commercial process existed capable of selective asphaltene removal, however. For example, solvent deasphalting, a mature separation technology for cutting deep into the resid fraction in preparing feedstock for RFCC and lube oil units,9 has been considered as a potential heavy oil upgrading option in the past.10 It was rejected because of low deasphalted oil yield. The SDA asphalt bottoms contain substantial amounts of desirable oil components, which require further processing such as coking for recovery.10

Selective extraction

SELEX-Asp is a supercritical solvent extraction process11 first commercialized in the 1940s to produce decaffeinated coffee. It is widely used in the pharmaceutical, fine chemical, and food industries. In the early 1980s, the China State Key Laboratory of Heavy Oil Processing started developing supercritical solvent extraction technology for petroleum applications.12

The principle of supercritical solvent extraction for a petroleum system is based on the combination of an anti-solvent with multicomponent phase equilibria. More information on the principle of supercritical solvent extraction can be found in reference texts.

SELEX-Asp has been enhanced to selectively remove the solid asphaltenes from residuum and to recover the desirable oil components from residuum.

A supercritical or near-critical solvent has key characteristics that make it favorable for separating asphaltenes from residuum:

• Unique from other solvents, it exhibits vapor density and diffusivity, which enhance asphalt phase separation.

• It facilitates turbulent mixing of petroleum feedstock and solvent, which enhances mass transfer.

SELEX-Asp has several advantages over conventional SDA. These include smaller and simpler extractor design, higher energy efficiency, fewer solvent requirements as well as deeper and cleaner separation. The selection process provides the highest theoretical liquid yield of desirable oil components in the form of asphaltenes-free resid.13 14

It is equipped with a solid-vapor separation system that allows the asphaltenes to be discharged as dry granulates, which can be easily handled.1 2 If it is economically attractive, the asphaltene granulates can be combusted to produce heat or used as feedstock for gasification to produce syngas.15

The technical basis of SELEX-Asp has been investigated and validated in various scales of experimental setups. A comprehensive feedstock processability database has been developed for the world's major heavy crudes such as Canadian bitumens, Venezuelan extra heavy oils, Arabian Heavy, and Chinese heavy oils.16-18

China refinery

PetroChina's Panjin refinery, near Liaohe oil field, has an 80,000 bbl/year processing capacity, an RFCC unit for gasoline and diesel production, and a delayed coker for metallurgical coke production. The feedstock consists of mostly Liaohe heavy oil, which is low in sulfur but has a very high total acid number (average 7, high 11).

The refinery also supplies fuel oil to Liaohe oil field for steam generation in the in situ thermal recovery operation. The fuel oil consists of vacuum resid, RFCC slurry oil, and refinery slop streams.

With depletion of available conventional crude supply to the refinery, PetroChina conducted a feasibility study on using of SELEX-Asp to optimize the value of vacuum resid.

Pilot

A vacuum resid sample was obtained from the Panjin refinery and characterized. Extensive bench-scale laboratory tests were conducted to determine the optimum SELEX-Asp operating conditions for the Panjin refinery's vacuum resid. The process was carried out in a 1-b/d continuous-flow pilot unit for 2 days.

Fig. 1 shows the process flow diagram of the SELEX-Asp pilot unit. The n-pentane solvent used was obtained from a refinery fractionator. A heated mixture of vacuum resid feed and n-pentane solvent was fed to the extractor in which asphaltenes were separated from the vacuum resid. An auxiliary solvent stream was introduced at the lower section of extractor. The total solvent-to-oil ratio was 4 by weight.

The asphaltenes and some of the n-pentane solvent were withdrawn from the bottom of the extractor and routed to a solid-vapor separator. In the separator, asphaltenes were produced in the form of dry granulates and the n-pentane solvent was recovered and then cooled in a solvent vessel. A key advantage of SELEX-Asp compared with conventional SDA is that it requires no heating to recover solvent from the asphaltenes-solvent mixture.

The deasphalted oil and majority of n-pentane solvent were withdrawn from the top of the extractor and fed to a heater. The heated DAO/n-pentane mixture was routed to the supercritical solvent-recovery column, in which n-pentane solvent was recovered and recycled for reuse and the DAO was withdrawn from the bottom of column.

Performance

Table 1 provides a summary of the properties of vacuum resid feed, SELEX-Asp DAO, and asphaltenes. The vacuum resid contained 19 wt % Conradson carbon residual and 242 ppm (wt) metals. SELEX-Asp removed 21.55 wt % of vacuum resid as asphaltenes which contained 47% CCR and 1,000 ppm (wt) metals, which accounted for 56% and 90% of total CCR and metals, respectively, in the vacuum resid.

The SELEX-Asp DAO was asphaltenes-free and contained 11 wt % CCR and 56 ppm (wt) metals. The quality of the DAO met the specifications for RFCC blending stock. The amount of sulfur and nitrogen in the DAO was lower than for vacuum resid. Moreover, the viscosity of DAO was considerably lower (by 90%) compared with that of vacuum resid.

Field demo unit

The success of SELEX-Asp in recovering a substantial amount (78.45 wt %) of high quality DAO from vacuum resid prompted PetroChina to construct a 500-b/d field demonstration unit at the Panjin refinery (Fig. 2).

PetroChina constructed a 500-b/d field demonstration unit at the Panjin refinery (Fig. 2).

The vacuum resid feed was a split stream from a commercial vacuum distillation unit. The process operating conditions were similar to those of the pilot plant.

In the field demonstration unit (Fig. 3), an additional column was added to allow splitting of DAO into light and heavy DAO. The temperature of the heavy DAO extraction column was adjusted to obtain a set yield of heavy DAO.

The large-scale field demonstration unit also allowed better heat integration among the process units. The solvent recovered from the light DAO extraction column was routed to a heat exchanger, which heated up the DAO-solvent mixture withdrawn from the top of the asphaltene extractor.

Table 2 shows the properties of the vacuum resid feed and SELEX-Asp products from the field demonstration run. The stream samples were obtained within 30 min after the process was at steady state.

The results show that the quality of vacuum resid feed used in the field demonstration run was slightly different from that used in the pilot plant test. The sulfur, nitrogen, and CCR content of the commercial feed were slight lower than that of the feed used in pilot test, while the concentration of metals was 45% higher. Feed quality variability is very common in commercial operations.

The amount of asphaltenes (20.58 wt %) removed from the SELEX-Asp field demonstration unit agreed with that produced from the pilot plant. The CCR content (48 wt %) and H/C ratio (1.18) of asphaltenes from the field demonstration run were similar to those from the pilot plant. This shows that the chemical characteristics of asphaltenes in the vacuum resid feeds used in field demonstration and pilot plant runs were both highly carbonized.

SELEX-Asp asphaltene granulates are free-flowing dry particles (Fig. 4).

Fig. 4 shows SELEX-Asp asphaltene granulates, which are free-flowing dry particles. Since the vacuum resid was low-sulfur feedstock, the asphaltenes obtained were blended with the RFCC slurry oil and refinery slop, which was sold as fuel oil. Some of the asphaltenes were bagged and burned in a manner similar to pulverized coal granulates.

The yields of light DAO and heavy DAO were 66 wt % and 13 wt %, respectively. The light DAO, which was routed to the RFCC unit, contained 10 wt % CCR and 70 ppm (wt) metals. The heavy DAO, which contained 14.5 wt % CCR and 216 ppm (wt) metals, was routed to the delayed coker to produce metallurgical coke.

Acknowledgments

The authors acknowledge financial support from PetroChina, National Basic Research Program of China (2010CB226901 and 2010CB226902), International Science and Technology Cooperation Program of China (2008DFA41160), and operational support from PetroChina Panjin refinery. ✦

References

1. Zhao, S., Xu, C., Wang, R., Xu, Z., Sun, X., and Chung, K.H., "Deep Separation Method and Processing System for the Separation of Heavy Oil through Granulation of Coupled Post-Extraction Asphalt Residue," US Patent 7597794 B2, Oct. 6, 2009.

2. Ibid., Chinese Patent ZL200510080799.0, July 15, 2009.

3. Chung, K.H., and Xu, C., "Narrow-cut characterization reveals resid processing chemistry," FUEL, 2001, Vol. 80, No. 8, p. 1165.

4. Scinta, J., Bares, J., Ewert, W., Randolph, B., and Vetters, E., "Challenges in Processing Oil Sands Derived Feeds," Oil Sands and Heavy Oil Technologies Conference & Exhibition, Calgary, July 18-20, 2007.

5. Gray, M.R., Zhao, Y., McKnight, C.M., Komar, D.A., and Carruthers, J.D., "Coking of hydroprocessing catalysts by resdiue fractions of bitumen," Energy & Fuel, 1999, Vol. 13, p. 1037.

6. Houde, E.J., and McGrath, M.J., "Residue Upgrading," PTQ, Second Quarter 2006, p. 81.

7. Savastano, C.A., "The Solvent Extraction Approach to Petroleum Demetallation," Fuel Sci. Tech. Int. 1991, Vol. 9, No. 7, pp. 855-871.

8. Zhao, S., Kotlyar, L.S., Woods, J.R., Sparks, B.D., and Chung, K.H., "Molecular Nature of Athabasca Bitumen, Petroleum Science and Technology," 2000, Vol. 18, Nos. 5 and 6, pp. 587-606.

9. Wilson, R.E., Keith, P.C., and Haylett, R.E., "Liquid propane use in dewaxing, deasphalting, and refining heavy oils," Ind. Eng. Chem.,1936, Vol. 28, pp. 1065-1078.

10. Iqbal, R., Khan, A., Eng, O., and Floyd, R., "Unlocking current refinery constraints," PTQ, Second Quarter 2008, p. 31.

11. Chung, K.H., Xu, Z., Sun, X., Zhao, S., and Xu, C., "Selective asphaltene removal from heavy oil," PTQ, Fourth Quarter 2006, p. 99.

12. Chung, K.H., Xu, C., Hu, Y., and Wang, R., "Supercritical fluid extraction reveals resid properties," OGJ, Jan. 20, 1977, p. 66.

13. Zhao, S., Wang, R., and Lin, S., "High pressure phase behaviour and equilibria for Chinese petroleum residua and light hydrocarbon systems, Part I," Petroleum Sci. Tech., Vol. 24, pp. 285-295, 2006.

14. Zhao, S., Wang, R., and Lin, S., "High pressure phase behaviour and equilibria for Chinese petroleum residua and light hydrocarbon systems. Part II," Petroleum Sci. Tech., Vol. 24, pp. 297-318, 2006.

15. Sadhukhan, J., and Zhu, X., "Integration strategy of gasification technology: A gateway to future refining," Ind. Eng. Chem. Res., 2002, Vol. 41, pp. 1528-1544.

16. Zhao, S., Kotlyar, L.S., Woods, J.R., Sparks, B.D., Gao, J., Kung, J., and Chung, K.H., "A benchmark assessment of residue: comparison of athabasca bitumen with conventional and heavy crudes," Fuel, 2002, Vol. 81, No. 6, pp. 737-746.

17. Zhao, S., Xu, Z., Xu, C., Chung, K.H., and Wang, R., "Systematic characterization of petroleum residua based on SFEF," Fuel, 2005, Vol. 84, No. 6, pp. 635-645.

18. Zhao, S., Kotlyar, L.S., Sparks, B.D., Woods, J.R., Gao, J., and Chung, K.H., "Solids contents, properties and molecular structures of asphaltenes from different oilsands," Fuel, 2001, Vol. 80, pp. 1907-1914.

The authors

Suoqi Zhao ([email protected]) is deputy director and professor, heavy oil chemistry and supercritical fluid technology, for China State Key Laboratory of Heavy Oil Processing, Beijing.
Chunming Xu ([email protected]) is deputy advisory chair and professor, heavy oil chemistry and processing technology, for China State Key Laboratory of Heavy Oil Processing, Beijing. He is also vice-president of academics for China University of Petroleum.
Xuewen Sun ([email protected]) is associate professor and senior scientist, heavy oil processing technology, petrochemicals, and supercritical fluid extraction, for China State Key Laboratory of Heavy Oil Processing, Beijing.
Zhiming Xu ([email protected]) is senior scientist, heavy oil chemistry and processing technology, and supercritical fluid extraction, for China State Key Laboratory of Heavy Oil Processing, Beijing.
Keng H. Chung ([email protected]) is advisor and distinguished professor, heavy oil processing and technologies, for China State Key Laboratory of Heavy Oil Processing, Beijing. He is also a process troubleshooting expert for Canadian oil sands operations and energy consultant for major investment bankers.
Yangdong Xiang ([email protected]) is chief engineer and general manager assistant, heavy oil processing and refining technology, for PetroChina Liaohe Petrochemical Co., Panjin, China.

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