Supporting Information: Can Fischer-Tropsch syncrude be refined to on-specification diesel fuel?

Transcription

1 Supporting Information: Can Fischer-Tropsch syncrude be refined to on-specification diesel fuel? Arno de Klerk Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 2V4, Canada Tel: , Modelling calculations and assumptions A number of distillate refining concepts are illustrated in the accompanying paper by making use of models. In refinery modelling, colloquially speaking, the devil is always in the details. There are many industrial tales where the incorrect selection of thermodynamics leads to the failure of a separation unit to meet desired performance in practice, or where scale-up in the development of a new technology missed key issues in a conversion unit. These cases do not necessarily reflect on the quality of the models, but may reflect on the validity of the assumptions on which they were based. This is especially true in Fischer-Tropsch refining, where crude oil based refinery modelling is often not valid, because of the inherent differences in the feed materials being processed. On a conceptual level it is important to understand the limitations of any model and in what ways the assumptions limit the interpretations that can be made from the model. This does not detract from the usefulness of simple models, but it is a reminder that they are limited in the level of information that can be provided. The confidence in a model is related to the validity of the assumptions for the specific case being modelled and it is not determined by the complexity of the model per se. In order to bridge the gap between modelling and reality, a more detailed discussion of the modelling assumptions for the distillate refinery designs and how they relate to reality are provided. Hydrocracking model (Figures 2 and 4). The first assumption inherent in the modelling is that the wax will be hydrocracked over an unsulphided Pt/SiO 2 -Al 2 O 3 catalyst with mild acidity, as S1

2 was reported by Calemma and co-workers (1) (Eni) and Leckel (2) (Sasol). This has been identified as one of the key catalyst types in Fischer-Tropsch refining (3) and it is likely that Shell is using an analogous type of catalyst for hydrocracking in the Shell Middle Distillate Synthesis (SMDS) process (4). Typical distillate selectivity values are 75-80% at 70% wax conversion. In industrial practice, the Sasol-Chevron gas-to-liquids designs (Oryx GTL and Escravos GTL) make use of Chevron s Isocracking technology. Unlike the hydrocracker in the SMDS process, Isocracking-technology employs a hydrocracking catalyst that is a sulphided base metal on acid support. Although both catalyst types have similar distillate selectivities at low wax conversion, at higher conversion the distillate selectivity for Isocracking is lower. (2) The second assumption relates to the recycling of unconverted wax. It is assumed that wax can be recycled to extinction. (5) This is a valid assumption and is borne out in industrial practice. However, the distillate selectivity may deteriorate when recycling unconverted wax, since the recycle is isomerized and more reactive than the fresh feed. This follows from the fundamentals of the wax hydrocracking mechanism and unless care is taken in the commercial design to compensate for this, the distillate selectivity will deteriorate compared to the oncethrough values. It has therefore been assumed that distillate selectivity during hydrocracking with recycle to extinction will be to the lower end of the selectivity range (75%). Cross-checking this with industrial practice indicates that it is a reasonable assumption. The reported yields from hydrocracking in the SMDS process range from 50% kerosene ( C) and 25% gas oil ( C) to 25% kerosene and 60% gas oil. (6) Since the kerosene boiling range includes C 10 material that were excluded from the definition of distillate in the paper, the assumption of 75% overall distillate yield used in the model falls within the range of commercial practice. The value is towards the upper end of the values reported for the Sasol-Chevron process (71-76%). (7) Although the residue fraction from Fe-HTFT synthesis is small, this material is very aromatic and it bears more resemblance to crude oil derived atmospheric residue than to wax. In the hydrocracking model no distinction is made to reflect this difference. Nevertheless, a distillate yield of 74% has been reported for the hydrocracking Fe-HTFT derived heavy vacuum gas oil over a Pt/SiO 2 -Al 2 O 3 catalyst. (8) The model assumption is therefore valid, despite the fundamental difference between LTFT wax and HTFT residue. S2

3 Oligomerization model (Figures 3 and 4). As with hydrocracking, the first assumption relates to the catalyst choice. Commercially available oligomerization technologies can be classified based on the type of catalyst they employ (Table S1). This is not a complete list and catalysts employed for non-fuels applications, such as lubrication oil production, have not been included. For modelling, H-ZSM-5 oligomerization has been selected, mainly because it is the only technology that has been applied industrially on commercial scale for the conversion of Fischer- Tropsch syncrude specifically to distillate. Distillate selectivity has been reported to be in the order of 65-70% at >90% olefin conversion. (9)(10) The second assumption is that the process will be operated at close to complete olefin conversion with no naphtha recycle. In practice some naphtha recycle is possible if the feed boiling range does not overlap with the recycle, for example, when feeding C 3 -C 4 gaseous feed, but recycling C 5 -C 10 naphtha. In the model recycling is not possible, because the feed is a full range C 3 -C 10 cut and build-up of paraffins in the recycle is unavoidable. (In a fuels refinery an expensive olefin-paraffin separation step will typically not be considered). Since the full range C 3 -C 10 cut is considered as feed, the feed contains not only olefins and paraffins, but also oxygenates and aromatics. The oxygenates are derived from the oil and aqueous fractions and the oxygenate-rich feed recovered overheads by primary separation from the aqueous product is not water-free. (11) Furthermore, the oxygenate composition of iron-based low temperature Fischer-Tropsch (Fe-LTFT) syncrude and iron-based high temperature Fischer- Tropsch (Fe-HTFT) syncrude differs. (12)(13) There is likewise a difference between cobalt and iron based Fischer-Tropsch syncrudes. (14) The third assumption is that the full-range C 3 -C 10 Fischer-Tropsch syncrude can be processed over H-ZSM-5. This has been verified in practice, but with the caveat that the metallurgy of the H-ZSM-5 process must be selected to cope with short chain corrosive carboxylic acids in the product. (10) The fourth set of assumptions relate to the conversion of oxygenates and aromatics. It is known that carbonyls and alcohols are converted over H-ZSM-5, (10)(15) but that aromatics are not alkylated appreciably (<5%) (16) by longer chain olefins in order to produce distillate range alkyl benzenes. For modelling purposes it has been assumed that only olefins and oxygenates are converted, with a distillate selectivity of 65% at 95% conversion. Cross-checking this with the industrial values reported by Minnie, (9) indicates that it is a realistic assumption. S3

4 Distillate fuel properties. Published values for straight run and refined distillates (Table 2 in main paper) were used as basis for the distillate properties in the model. (17)(8)(18)(19)(20)(21) The fuel properties were calculated by linear volumetric blending. The straight run material from Fischer-Tropsch synthesis is generally refined before characterizing its fuel properties. The fuel property data for German Co-LTFT derived straight run distillate (17) was used for both Co-LTFT and Fe-LTFT straight run distillates in the model. The quality of distillate produced by different wax hydrocracking processes differs mainly with respect to their cold flow properties. Hydrocracking is always accompanied by hydroisomerization. The extent of hydroisomerization is determined by the catalyst s acid-metal site balance and the strength of the acid sites. (22) Branched paraffins have much better cold flow properties, but a slightly worse cetane number than linear paraffins. The cold flow properties of a hydrocracked LTFT distillate is therefore determined by the isomerization propensity of the hydrocracking catalyst employed. Oligomerization over H-ZSM-5 is reasonably feed insensitive. (23) However, strictly speaking one should qualify this statement, since the fuel properties do to some extent depend on the nature of the feed. (10) This distinction has not been made in the model on the basis that published distillate fuel property data is very similar. (10) A word of caution: The distillation ranges for which distillate properties are quoted (Table 2 in main paper) differ and the numerical values calculated by the model should only be used as gross indicators. Diesel fuel refinery modelling (Figures 8 and 10). The model employed for the full refinery designs are more complex. Fischer-Tropsch refinery design is discussed as a separate review paper. (24) Literature cited (1) Calemma, V.; Peratello, S.; Pavoni, S.; Clerici, G.; Perego, C. Hydroconversion of a mixture of long chain n-paraffins to middle distillate: Effect of the operating parameters and products properties. Stud. Surf. Sci. Catal. 2001, 136, 307. S4

5 (2) Leckel, D. O. Noble metal wax hydrocracking catalysts supported on high-siliceous alumina. Ind. Eng. Chem. Res. 2007, 46, (3) De Klerk, A. Catalysts important in the refining of Fischer-Tropsch syncrude to fuels. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2009, 54 (1), 116. (4) Smith, R.; Asaro, M. Fuels of the future. Technology intelligence for gas to liquids strategies; SRI: Menlo Park, CA, (5) Dry, M. E. High quality diesel via the Fischer-Tropsch process - a review. J. Chem. Technol. Biotechnol. 2001, 77, 43. (6) Schrauwen, F. J. M. Shell middle distillate synthesis (SMDS) process. In Meyers, R.A. Ed. Handbook of Petroleum Refining Processes; McGraw-Hill: New York, 2004, pp (7) Dancuart, L. P.; De Haan, R.; De Klerk, A. Processing of primary Fischer-Tropsch products. Stud. Surf. Sci. Catal. 2004, 152, 482. (8) Leckel, D. O. Hydroprocessing Euro 4-type diesel from high-temperature Fischer-Tropch vacuum gas oils. Energy Fuels 2009, 23, 38. (9) Minnie, O. R. The effect of 1-hexene extraction on the COD process. M.Tech. Dissertation, University of South Africa, (10) De Klerk, A. Properties of synthetic fuels from H-ZSM-5 oligomerization of Fischer- Tropsch type feed material. Energy Fuels 2007, 21, (11) Hoogendoorn, J. C.; Salomon, J. M. Sasol: World's largest oil-from-coal plant. III British Chem. Eng. 1957, Jul, 368. (12) Hoogendoorn, J. C. Experience with Fischer-Tropsch synthesis at Sasol. In Clean Fuels Coal Symp.; Inst. Gas Technol.: Chicago, 1973, p.353. (13) Hoogendoorn, J. C. New applications of the Fischer-Tropsch process. In Clean Fuels Coal Symp., 2nd; Inst. Gas Technol.: Chicago, 1975, p.343. (14) Dry, M. E. Chemical concepts used for engineering purposes. Stud. Surf. Sci. Catal. 2004, 152, 196. (15) Chang, C. D.; Silvestri, A. J. The conversion of methanol and other O-compounds to hydrocarbons over zeolite catalysts. J. Catal. 1977, 47, 249. (16) De Klerk, A.; Nel, R. J. J. Benzene reduction in a fuels refinery: An unconventional approach. Energy Fuels 2008, 22, S5

6 (17) Ward, C. C.; Schwartz, F. G.; Adams, N. G. Composition of Fischer-Tropsch diesel fuel. Ind. Eng. Chem. 1951, 43, (18) Wu, T.; Huang, Z.; Zhang, W-g.; Fang, J-h.; Yin, Q. Physical and chemical properties of GTL-diesel fuel blends and their effects on performance and emissions of a multicylinder DI compression ignition engine. Energy Fuels 2007, 21, (19) Tijm, P. J. A. Shell middle distillate synthesis: the process, the plant, the products. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 1994, 39 (4), (20) Lamprecht, D.; Dancuart, L. P.; Harrilall, K. Performance synergies between lowtemperature and high-temperature Fischer-Tropsch diesel blends. Energy Fuels 2007, 21, (21) Köhler, E.; Schmidt, F.; Wernicke, H. J.; De Pontes, M.; Roberts, H. L. Converting olefins to diesel - the COD process. Hydrocarbon Technol. Int. 1995, Summer, 37. (22) Corma, A.; Martinez, A.; Pergher, S.; Peratello, S.; Perego, C.; Bellusi, G. Hydrocrackinghydroisomerization of n-decane on amorphous silica-alumina with uniform pore diameter. Appl. Catal. A 1997, 152, 107. (23) Garwood, W. E. Conversion of C 2 -C 10 olefins to higher olefins over synthetic zeolite ZSM- 5. Prepr. Am. Chem. Soc. Div. Petrol. Chem. 1982, 27 (2), 563. (24) De Klerk, A. Fischer-Tropsch fuels refinery design. Energy Environ. Sci. submitted. S6

7 Table S1. List of commercially available oligomerization technologies and catalyst types. Catalyst Technology Supplier Application Solid phosphoric acid CatPoly UOP Motor-gasoline, kerosene InAlk UOP Motor-gasoline Amorphous silica-alumina Polynaphtha (a) Axens Distillate Selectopol (a) Axens Motor-gasoline Montmorillonite Octol-A Hüls/UOP Motor-gasoline H-ZSM-5 zeolite COD MOGD PetroSA ExxonMobil Distillate Motor-gasoline, distillate H-ZSM-22 or -57 zeolite Emogas ExxonMobil Motor-gasoline, kerosene Acidic resin NExOCTANE Fortum Oy Motor-gasoline Homogeneous nickel Dimersol G Axens Motor-gasoline (a) This technology is also available with a zeolite-based catalyst. S7