Energy & Fuels 2006, 20, 1799-1805
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Oligomerization of Fischer-Tropsch Olefins to Distillates over Amorphous Silica-Alumina Arno de Klerk* Fischer-Tropsch Refinery Catalysis, Sasol Technology Research and DeVelopment, P.O. Box 1, Sasolburg 1947, South Africa ReceiVed April 21, 2006. ReVised Manuscript ReceiVed June 12, 2006
The oligomerization of Fischer-Tropsch-derived olefins over amorphous silica-alumina (ASA) has been investigated at pilot-plant scale in the region of 140-265 °C and 3.5-6.8 MPa. The distillate yield is thermodynamically limited, and yields of 65-70% were observed during once-through operation at 180 °C, 6 MPa, and a liquid hourly space velocity (LHSV) of 0.5 h-1. The ASA-derived distillate typically has a density of 810 kg/m3, a kinematic viscosity of 2.8-3.6 cSt, and good cold flow properties, but a low cetane number (28-30). Despite the high hydrogen transfer propensity of ASA, cycle lengths of more than 100 days were achieved and catalyst activity could be fully restored by controlled burnoff of the carbon. Deactivation occurred at a rate of 0.03-0.04 mg/h carbon per gram of catalyst.
Introduction Currently, there are three high-temperature Fischer-Tropsch (HTFT) refineries worldwide, and olefin oligomerization is a key refining technology in all of them.1,2 The Sasol Synfuels refineries in Secunda, South Africa are using a solid phosphoric acid (SPA)-based technology,3 and the PetroSA refinery in Mossel Bay, South Africa is using a zeolite (ZSM-5)-based technology.4 The purpose of olefin oligomerization in a Fischer-Tropsch refinery is 2-fold, namely (i) to convert normally gaseous olefins to liquid products and (ii) to produce a fuel component (motor-gasoline, jet fuel, or diesel) that can be hydrogenated to reduce the overall olefin content of the fuel pool. This is necessary because the product from HTFT synthesis is rich in short-chain olefins, with 70% of the C5C12 fraction being olefinic,5 which is well above the olefin content that is allowed in transportation fuel. For example, the Euro-4 motor-gasoline specification limits the olefin content to 18 vol %. The conversion of HTFT olefins, using different olefin oligomerization technologies, yields products that have very different properties.6 Oligomerization with SPA results in a product with good unhydrogenated and hydrogenated motorgasoline properties;7 however, it is not well-suited for the
production of distillates.3 Oligomerization with ZSM-5, by either the “Conversion of Olefins to Distillate” (COD)4,8 process, or the “Mobil Olefins to Gasoline and Distillates” (MOGD)9 process, yields a high-quality distillate; however, the motorgasoline is of a lower quality (with an unhydrogenated research octane number (RON) of 81-85 and a motor octane number (MON) of 74-75). Unfortunately, little is known about the oligomerization of HTFT olefins over amorphous silicaalumina (ASA).10 This paper will explore the use of ASA as a catalyst for distillate production from Fischer-Tropsch olefins, with the focus mainly on distillate yield and fuel properties. The study is of an applied nature and is limited to olefinic feed materials in the C3-C10 range that are low in oxygenates. FischerTropsch-derived material in the range studied normally contains percentage levels of oxygenates.5 The study used only feed materials low in oxygenates, because oxygenates were expected to produce water that could influence the oligomerization behavior of ASA.11 After initial screening that showed promising results, all tests were performed at pilot-plant scale. The performance of a commercial ASA catalyst was evaluated over the operating range of 140-265 °C, 3.5-6.8 MPa, and LHSV ) 0.1-2.0 h-1. Experimental Section
* Author to whom correpsondence should be addressed. Tel: +27 16 960-2549. Fax: +27 11 522-3517. E-mail:
[email protected]. (1) Marriott, J. N. Sasol process technologysthe challenge of synfuels from coal. ChemSA 1986, 12, 174. (2) Terblanche, K. The Mossgas challenge. Hydrocarbon Eng. 1997, (Mar.-Apr.), 2. (3) De Klerk, A. Distillate production by oligomerisation of FischerTropsch olefins over solid phosphoric acid. Energy Fuels 2006, 20, 439. (4) Ko¨hler, E.; Schmidt, F.; Wernicke, H. J.; De Pontes, M.; Roberts, H. L. Converting olefins to dieselsthe COD process. Hydrocarbon Technol. Int. 1995, (Summer), 37. (5) Steynberg, A. P., Dry, M. E., Eds. Fischer-Tropsch Technology; Elsevier: Amsterdam, 2004. (6) Leprince, P. Oligomerization. Leprince, P., Eds. Petroleum Refining, Vol. 3: ConVersion Processes; Editions Technip: Paris, 2001; p 321. (7) De Klerk, A.; Engelbrecht, D. J.; Boikanyo, H. Oligomerization of Fischer-Tropsch olefins: Effect of feed and operating conditions on hydrogenated motor-gasoline quality. Ind. Eng. Chem. Res. 2004, 43, 7449.
Materials. Two olefinic feed materials were used, namely a C3C6 HTFT condensate and a C7-C10 fraction from the oligomerization of HTFT condensate over an SPA catalyst (see Table 1). Both were obtained from the Sasol Synfuels refineries in Secunda, South Africa. The ASA catalyst used (spherical, average diameter (8) Knottenbelt, C. Mossgas “gas-to-liquids” diesel fuelssan environmentally friendly option. Catal. Today 2002, 71, 437. (9) Tabak, S. A.; Krambeck, F. J.; Garwood, W. E. Conversion of propylene and butylene over ZSM-5 catalyst. AIChE J. 1986, 32, 1526. (10) O’Connor, C. T. Oligomerization. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Weitkamp, J., Eds.; VCH: Weinheim, Germany, 1997; pp 2380-2387. (11) Finch, J. N.; Clark, A. The effect of water content of silica-alumina catalyst on 1-butene isomerization and polymerization. J. Phys. Chem. 1969, 73, 2234.
10.1021/ef060169j CCC: $33.50 © 2006 American Chemical Society Published on Web 07/20/2006
1800 Energy & Fuels, Vol. 20, No. 5, 2006
de Klerk
Table 1. Feed Composition of C3-C6 High-Temperature Fischer-Tropsch (HTFT) Condensate and C7-C10 Solid Phosphoric Acid (SPA) Oligomers Composition (mass %) C3-C6 compound propene propane butenes butanes pentenes pentanes hexenes hexanes heptenes heptanes octenes octanes nonenes nonanes decenes undecenes carbonyls (µg/g) alcohols (µg/g) acids (mg KOH/g) water (µg/g)
batch 1
batch 2
15.3 3.5 36.3 5.7 23.5 4.3 8.3 1.3 1.5 0.1 0.2 0
22.4 3.6 39.1 5.4 18.3 2.6 6.3 0.9 1.2 0.1 0.1 0
Oxygenate Content
