2330
Energy & Fuels 2006, 20, 2330-2336
Diesel-Selective Hydrocracking of an Iron-Based Fischer-Tropsch Wax Fraction (C15-C45) Using a MoO3-Modified Noble Metal Catalyst Dieter Leckel* and Maurice Liwanga-Ehumbu Fischer-Tropsch Refinery Catalysis, Sasol Technology Research and DeVelopment, P.O. Box 1, Sasolburg 1947, South Africa ReceiVed July 13, 2006. ReVised Manuscript ReceiVed August 18, 2006
Iron-based low-temperature Fischer-Tropsch wax, in the carbon number range of C15-C45, was hydrocracked using an unsulfided Pt/SiO2-Al2O3 catalyst, modified with MoO3. The selective production of diesel having a low cloud point (-18 °C) combined with a high cetane number (71) was achieved. A bench-scale tricklebed reactor was used to study the effect of operating conditions on product selectivities and yield. The effect of pressure and hydrocarbon chain length on reactivity and diesel properties was studied in particular. Lowpressure operation (3.5 MPa) improved the isoparaffin to n-paraffin ratio for the carbon number range C15C22 but did not affect the degree of isomerization significantly after C22. The higher pressure of 7.0 MPa suppressed the isomerization and the C23+ percent conversion and improved the diesel selectivity by inhibiting secondary cracking. Increasing the pressure resulted in an overall paraffin reactivity decrease. A maximum in paraffin reactivity at C33 was observed at low-pressure operation, while high-pressure operation resulted in two maxima at C21 and C33. The observed unexpected paraffin reactivity pattern was the consequence of competing concentration and reactivity effects.
Introduction Low-temperature Fischer-Tropsch (LTFT) waxes contain essentially no sulfur or aromatics.1,2 As such, these waxes are a suitable source for the production of high-quality transportation fuels such as diesel after hydrocracking. In general, the major components in primary FT products are linear n-paraffins, which exhibit notoriously poor cold-flow properties. These products are therefore difficult to use where low cold-flow properties are needed, such as in jet fuel, diesel, and lube base oils. Coldflow properties can be improved by increasing the branching of the paraffins using catalysts with highly effective hydroisomerization and balanced hydrocracking activity. Catalysts based on noble metal and group VIII metals supported on zeolites and various other supports have been developed and extensively studied.3-8 These catalysts are metal/acid bifunctional catalysts that follow the carbenium ion mechanism by dehydrogenation of the alkanes on the metal sites, followed by * Author to whom correspondence should be addressed. Tel.: +27 16 960-3830. Fax: +27 11 522-3975. E-mail:
[email protected]. (1) Dry, M. E. Appl. Catal., A 1999, 189, 185-190. (2) Dry, M. E. Catal. ReV.sSci. Eng. 1981, 23 (1, 2), 265-278. (3) Hino, M.; Arata, K. J. Am. Chem. Soc. 1979, 101, 39. (4) Hino, M.; Arata, K. J. Chem. Soc., Chem. Commun. 1987, 1259. (5) Zhang, S.; Zhang, Y.; Tierney, J. W.; Wender, I. Appl. Catal., A 2000, 193, 155. (6) Welters, W. J. J.; Van der Waerden, O. H.; Zandbergen, H. W.; De Beer, V. H. J.; Van Santen, R. A. Ind. Eng. Chem. Res. 1995, 34, 1156. (7) Corma, A.; Martı´nez, A.; Pergher, S.; Peratello, S.; Perego, C.; Bellussi, G. Appl. Catal., A 1997, 152 (1), 107-125. (8) Rodrı´guez-Agundez, J.; Pe´rez-Pariente, J.; Chica, A.; Corma, A.; Cody, I. A.; Murphy, J. W.; Linek, S. J. WO 99/61558, 1999; assigned to Exxon Research Engineering Co. Rodrı´guez-Agundez, J.; Pe´rez-Pariente, J.; Chica, A.; Corma, A.; Chen, T. J.; Ruziska, P. A.; Henry, B. E.; Stuntz, G. F.; Davis, S. M. WO 99/61555, 1999; assigned to Exxon Research Engineering Co. Rodrı´guez-Agundez, J.; Pe´rez-Pariente, J.; Chica, A.; Corma, A.; Cody, I. A.; Murphy, W. J.; Linek, S. J. WO 99/61552, 1999; assigned to Exxon Research Engineering Co.
isomerization and cracking on the acid sites and finally hydrogenation.9,10 Studies on the hydroisomerization and hydrocracking of alkanes, the resulting product spectra, and the mechanism explaining the latter have been carried out in order to gain a more in-depth understanding of the catalytic process and thus assist in developing better catalysts for the hydrotreatment of heavy n-alkanes.11,12 Model compounds such as n-C16, n-C28, n-C36, and n-C44 were used by Calemma et al.13,14 to investigate the effect of chain length on the hydrocracking and hydroisomerization selectivity over an amorphous Pt/SiO2-Al2O3 catalyst. Research efforts on the hydroisomerization and selective hydrocracking of commercially produced FT waxes have intensified in recent years.15-17 Incentives, though, still exist to develop catalysts for the hydrocracking of FT waxes which maximize the production of diesel having good cold-flow properties combined with high cetane numbers. When diffuse reflectance FTIR spectroscopy of chemisorbed pyridine was used, it was shown by Rajagopal et al.18 that the ratio of Brønsted to Lewis acid sites concentration increased (9) Weitkamp, J. Erdo¨l Kohle, Erdgas, Petrochem. 1978, 31, 13. (10) Martens, J. A.; Jacobs, P. A.; Weitkamp. J. Appl. Catal. 1986, 20, 239-281. (11) Weitkamp, J. Ind. Eng. Chem. Prod. Res. DeV. 1982, 21, 550556. (12) Girgis, M. J.; Tsao, Y. P. Ind. Eng. Chem. Res. 1996, 35, 386. (13) Calemma, V.; Peratello, S.; Perego, C. Appl. Catal., A 2000, 190, 207. (14) Calemma, V.; Peratello, S.; Stroppa, F.; Giardano, R.; Perego, C. Ind. Eng. Chem. Res. 2004, 43 (4), 934. (15) Zhou, Z.; Zhang, Y.; Tierney, J. W.; Wender, I. Fuel Process. Technol. 2003, 83, 67-80. (16) Zhang, S.; Zhang, Y.; Tierney, J. W.; Wender, I. Fuel Process. Technol. 2001, 69, 59. (17) Leckel, D. Energy Fuels 2005, 19, 1795-1803. (18) Rajagopal, S.; Marzari, J. A.; Miranda, R. J. Catal. 1995, 151, 192-203.
10.1021/ef060319q CCC: $33.50 © 2006 American Chemical Society Published on Web 09/16/2006
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Table 1. Catalyst Characterization property Pt, wt % MoO3, wt % SiO2, wt % Al2O3, wt % BET, m2/g pore volume, cm3/g total acidity, mmol NH3/g catalyst average pore diameter, Å density, g/cm3
1.2 2.2 75 25 233 0.55 0.38 100 0.48
with silica content in a silica-alumina support, reaching a maximum at a SiO2-to-Al2O3 ratio of 75:25 by weight. It was further shown that, after doping the silica-rich supports with MoO3, the Brønsted/Lewis acid ratio reached a maximum at about 2 wt % MoO3 loading. On the basis of these findings, we prepared an amorphous, MoO3-modified silica-alumina acidic support on which platinum was loaded, and this catalyst was used in our present investigations. This paper reports on the selective hydrocracking of a commercially produced iron-based FT wax (Sasol M5 wax, C15-C45) into diesel. The effect of operating conditions such as temperature, pressure, and weight hourly space velocity (WHSV) on the product distribution and, specifically, the coldflow properties and ignition quality of the diesel produced are presented. In particular, we have studied the effect of hydrocarbon chain length and pressure on the reactivity and hydroisomerization selectivity. Experimental Section Catalyst. The catalyst used in the experiments was a noncommercial catalyst developed in our laboratories, and its method of preparation has already been reported.19 The catalyst was activated in situ by reduction in a fixed-bed reactor at 400 °C for 3 h using a hydrogen flow of 1.7 L per minute. The temperature-programmed desorption of ammonia was performed with a Micromeritics TPD 2900 NH3 analyzer for the determination of total acidity. The metal loading was determined by inductive coupled plasma. A Gemini Micromeritics device was used to measure Brunauer-EmmettTeller (BET) surface area. The physicochemical properties of the catalyst are given in Table 1. Reactor System. A bench-scale fixed-bed reactor, operating in down-flow mode, was used for the isothermal studies. The reactor consisted of a tube of 47.6 mm internal diameter and 1.8 meters in length. The catalyst (260 mL) was loaded in the middle section of the reactor tube, and sand (35-50 mesh) was used to fill the voids between the catalyst particles (extrudates) to avoid channeling. Inert packing (glass balls) above the catalyst bed was used to preheat the feed up to the reaction temperature. To support the catalyst, a fine mesh grid was welded onto the thermocouple well, which runs through the center of the reactor over its entire length. Six thermocouples were placed inside the well to monitor the temperature of the catalyst bed at different depths. The reactor was heated electrically by three heater elements placed along the reactor tube. Liquid feed and hydrogen entered concurrently from the top of the reactor. The gas and liquid products were separated in the last section of the reactor setup. The liquid product was collected in a catch pot, and the gaseous light hydrocarbons were passed through a cooling coil at 0 °C. This condensed liquid was collected in a second gas liquid separator. A gas sampling point was installed on the low-pressure side of the reactor system. Procedure. Catalyst activity was monitored by drawing product samples from the reactor after steady-state conditions were reached, typically after a period of 72 h after each change of reactor (19) Liwanga-Ehumbu, A.-M.; Visagie, J. L.; Leckel, D. O. GB2380953, April 23, 2003; US 2003/0173253 A1, September 18, 2003; WO 01/90280 A2, November 29, 2001.
Figure 1. Carbon number distribution of the Sasol LTFT M5 wax.
conditions. The following 8 h period was then used to collect a representative sample for product analysis. Mass balances of 96104% were achieved with a collection of the tail gas, condensed lighter hydrocarbons, and liquid products from the reactor stream. Condensed light hydrocarbons were kept refrigerated prior to the analysis. Operating Conditions. Hydrocracking was performed in the temperature range of 350-380 °C. Pressures of 3.5 and 7.0 MPa were applied. The WHSV was varied between 0.5 and 1.0 h-1, and the hydrogen-to-feed ratio was adjusted between 1000 and 1500:1 Nm3/m3. Experiments were performed in once-through mode. Feed. The products from the Sasol LTFT operations at Sasolburg are gas, condensates, and wax. The latter is predominantly paraffinic and, more specifically, has an n-paraffin content of about 94 wt %. Isoparaffins and olefins (mainly R-olefins in the range of C5C20) as well as a minor amount of oxygenates constitute the balance. The oxygenates present are alcohols, aldehydes, ketones, and esters. The waxes and condensates are distilled at Sasol Waxes into different fractions using vacuum and short-path distillation. The wax fractions are optionally worked up further in an unhydrogenated and a hydrogenated form. A hydrotreated M5 wax fraction, having a carbon number distribution between C15 and C45, was used in this hydrocracking study as the feed (see Figure 1). In-house Sasol analytical procedures, based on modified American Society for Testing and Materials (ASTM) methods, were used for the wax feed analyses. This included the ASTM methods D 1386, D 1387, and D 5622 using a Perkin-Elmer EA 1110 CHNSO analyzer. For the determination of carbon number distributions, high-temperature gas chromatography (GC-FID) analyses were performed using a Restek 15 m, 0.288 mm internal diameter, and 0.15 µm thickness MXT-1 metal capillary column. Product Analyses. The reactor tail gas was collected in a glass gas sample bomb that was connected to the reactor exit stream, and the gas was then analyzed by GC-FID. All liquid products were collected and fractionated into a naphtha (C5-C9) cut, diesel (C10C22, corresponding to 170-370 °C), and a residue product boiling above 370 °C (i.e., C23+). Simulated distillation (SIMDIS ASTM D-2887) was applied to obtain the boiling point distribution of the liquid products. An Agilent 6890N GC-FID was used to calculate conversion, selectivities, and yields. The products were separated on a HP 50 m paraffins, olefins, naphthalenes, and aromatics methyl siloxane column with a 200 µm internal diameter and a 0.5 µm film thickness. A Varian Unity Inova 400 MHz proton NMR was used to calculate the cetane number of the diesel fraction. Terminologies. Conversion, selectivities, and yields in this work were calculated as follows. Conversion is here understood to be true conversion because, according to the definitions used, naphtha and diesel already present in the feed are corrected for. The hydrocracking conversion (“true” conversion) is defined as described in eq 1.
2332 Energy & Fuels, Vol. 20, No. 6, 2006
Leckel and Liwanga-Ehumbu
%C23+ conversion ) wt% C23+ in feed - wt% C23+ in product 100 (1) wt% C23+ in feed
(
)
The diesel (C10-C22) selectivity was calculated according to eq 2. The naphtha (C5-C9) and gas (C1-C4) selectivities were calculated %(C10-C22) sel. )
(
wt% (C10-C22) in product - wt% (C10-C22) in feed
(wt% C23+ in feed - wt% C23+ in product)/wt% C23+ in feed
)
100 (2)
analogously. The yields were calculated by multiplying the fractional conversion by the selectivity. Cloud Point and Cetane Number. Determination of the cloud point was based on the ASTM D 2500 method. Reproducibility was within 2 °C. Cetane numbers above the value of 70, determined by the ASTM method D 613-95, have a reproducibility of only 6-7 cetane numbers, and about 2 L of diesel are required for the determination. A more convenient method requiring only small sample volumes and giving good relations for calculating cetane numbers on the basis of proton NMR analyses are described in Society of Automotive Engineers (SAE) papers SAE 861512 and SAE 892073. The latter publications give correlations including factors for aromatic as well as for olefinic protons. A third method, developed by O’Connor et al.,20 uses only the ratio of methylene to methyl protons to calculate the cetane number. As the number of methyl branches is equal to the number of branches plus two, this value is indicative of the degree of branching, whereas the amount of CH2 is indicative of the average degree of linearity of the sample. All three methods were compared to cetane numbers from the literature,21 and the method of O’Connor et al. gave the best correlations and was therefore subsequently used for the calculation of the cetane numbers in our investigations.
Results and Discussion General Comment. The authors are fully aware of the commonly used practice to compare selectivity and yield changes at a constant conversion level. However, to achieve similar conversions in wax hydrocracking, two reaction variables have to be varied at the same time. The result will be similar product selectivities, as was demonstrated in a previous study.17 Important differences in product distributions can only be observed when comparing experiments where only one variable is changed but keeping all other parameters constant, thus not comparing results at constant conversion. We intentionally deviated in this study from the standard practice to better illustrate the effect of variables on wax hydrocracking and the subsequent effect thereof on product selectivities, yields, and relevant diesel properties. Effect of Temperature. Temperature affects primarily the C23+ conversion, which increases with increasing temperature. Furthermore, the temperature has a significant influence on the distribution of components between the vapor and liquid phases. The vapor-liquid equilibria, however, are not expected to have a major effect over the entire temperature range of the hydrocracking reaction, in particular, with regards to the higher carbon numbers up to C45. Van Vuuren et al.22 reported that the solubility of hydrogen decreases with increasing chain length of the waxes. Thus, the solubility of hydrogen is higher in waxes with a lower average molecular weight. This effect should (20) O’Connor, C. T.; Forrester, R. D.; Scurrell, M. S. Fuel 1992, 71, 1323-1327. (21) Hardenberg, H.O. Mineralo¨technik 1984, 29 (5), 13-25.
Figure 2. Effect of temperature on C23+ conversion during hydrocracking of the Sasol M5 wax using a pressure of 3.5 MPa, WHSVs of 0.5 and 1.0 h-1, and a hydrogen-to-wax ratio of 1200:1 Nm3/m3.
enhance the hydrocracking rate of a light wax feed, such as the Sasol M5 wax used in this study. As shown in Figure 2, increasing the operating temperature during hydrocracking of the M5 wax and keeping all other reaction parameters constant increased the C23+ conversion. A 10 °C raise in operating temperature from 360 to 370 °C, at a WHSV of 1.0 h-1, improved the C23+ conversion from 35 to 81% (see Table 2). As reflected in Tables 2 and 3, high operating temperatures and subsequently high C23+ conversion improved the gas, naphtha, and diesel yields, but as expected, the higher operating temperatures led to more secondary cracking, which in turn resulted in higher naphtha and gas yields relative to the diesel. It can be noticed from Table 4 (WHSV ) 1.0 h-1) that increasing the temperature from 360 to 370 °C resulted in a decrease of the diesel-to-naphtha ratio from 4.6 to 3.1. The diesel cloud points improved from -7 to -17 °C. This is a consequence of the shorter-chain components (from the feed and products) being in the vapor phase at the higher temperature and the long-chain components in the liquid phase having relatively more time to be isomerized and hydrocracked. Lower space velocities enhanced these effects, as shown in Table 5. Low C23+ conversions (due to the lower temperature) resulted in an exceptionally high diesel-to-naphtha ratio (8.9), a low isomerization ratio (3.3), a high cloud point (-9 °C), and a high cetane number (75). Increasing the conversion to 86 wt % caused a significant decrease in the diesel-to-naphtha ratio (3.0), accompanied by a reduction in the cetane number to a value of 70 and a cloud point improvement from -9 to -17 °C. The lower cloud points are a consequence of the higher isoparaffinto-n-paraffin ratio in the diesel. The cloud points and cetane number achieved for the diesel fractions are of excellent quality compared to the German EURO IV specifications,23 which require a -20 °C cloud point in winter and 0 °C for summer. The cetane numbers obtained in this study (70-75) far exceed the required EURO IV minimum cetane number of 51. Effect of Pressure. The effect of pressure was investigated at 370 °C, 1.0 h-1 WHSV, and a 1200:1 Nm3/m3 hydrogento-wax ratio varying the pressure from 3.5 to 7.0 MPa. In Figure 3, the pressure effect on the C23+ conversion is presented. At low pressures, a high percent conversion was observed, while higher pressures resulted in a lower percent conversion. Table (22) Van Vuuren, D. S.; Hunter, J. R.; Heydenrych, M. D. The Solubility of Various Gases in Fischer-Tropsch Wax; Council for Scientific and Industrial Research Report CENG M-643; Council for Scientific and Industrial Research: South Africa, May 1987. (23) Motor Vehicle Emission Regulations and Fuel Specifications - Part 2, Detailed Information and Historic ReView (1996-2000); Report 2/01; CONCAWE: Brussels, Belgium, March 2001.
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Energy & Fuels, Vol. 20, No. 6, 2006 2333
Table 2. Product Selectivities and Yields Achieved during Hydrocracking of the Sasol M5 Wax at Different Temperatures Using a Hydrogen Pressure of 3.5 MPa, a WHSV of 1.0 h-1, and a Hydrogen-to-Wax Ratio of 1200:1 Nm3/m3 selectivities (wt %)
yields (wt %)
temperature (°C)
C23+ conversion (wt %)
C1-C4
C5-C9
C10-C22
C1-C4
C5-C9
C10-C22
360 365 370
35 51 81
1.8 1.8 2.5
17 20 24
81 78 75
0.6 0.9 2
6.0 10 19
28 40 68
Table 3. Product Selectivities and Yields Achieved during Hydrocracking of the Sasol M5 Wax at Different Temperatures, Using a Hydrogen Pressure of 3.5 MPa, a WHSV of 0.5 h-1, and a Hydrogen-to-Wax Ratio of 1200:1 Nm3/m3 selectivities (wt %)
yields (wt %)
temperature (°C)
C23+ conversion (wt %)
C1-C4
C5-C9
C10-C22
C1-C4
C5-C9
C10-C22
350 360 365
17 69 86
2.1 1.1 2.1
9.9 22 25
88 77 73
0.3 0.8 1.8
1.6 15 21
15 53 63
Table 4. Properties of the Diesel (C10-C22) Produced during Hydrocracking of the Sasol M5 Wax Using a Hydrogen Pressure of 3.5 MPa, a WHSV of 1.0 h-1, and a Hydrogen to Wax Ratio of 1200:1 Nm3/m3
360 °C C23+ conversion, wt % diesel/naphtha ratio iso-/n-paraffin ratio in the diesel fraction cloud point, °C cetane number
365 °C
EURO IV specification (Germany)
370 °C
35
51
81
-
4.6
3.9
3.1
-
4.0
4.1
5.1
-
-7
-11
-17
73
73
72
-20 (winter) 0 (summer) 51
Table 5. Properties of the Diesel (C10-C22) Produced during Hydrocracking of a Sasol M5 Wax Using a Hydrogen Pressure of 3.5 MPa, a WHSV of 0.5 h-1, and a Hydrogen-to-Wax Ratio of 1200:1 Nm3/m3 350 °C C23+ conversion, wt % diesel/naphtha ratio iso-/n-paraffin ratio in the diesel fraction cloud point, °C cetane number
360 °C
365 °C
17
69
86
8.9
3.5
3.0
3.3
4.6
5.3
-9 75
-18 71
-17 70
6 shows that increasing the pressure from 3.5 to 7.0 MPa decreased the C23+ conversion by about 40%. In terms of yields, the pressure increase resulted in a 35% lower diesel and 60% lower naphtha yield. Gas yields were found to decrease with increasing total pressure. Table 7 presents the effect of pressure on the diesel properties. Higher pressures generally resulted in lower C23+ conversion, higher diesel-to-naphtha ratios, lower isoparaffin-to-n-paraffin ratios, and higher cloud points and cetane numbers in the diesel fraction. The isoparaffin-to-n-paraffin ratio in the diesel fraction decreased from 4.1 to 3.0, after increasing the pressure from 3.5 to 7.0 MPa. The lower isomerization activity led to an increase in the cloud point of the diesel from -17 to -10 °C and resulted in higher cetane numbers. The vapor-liquid equilibria play a significant role at lower pressures and high temperatures and especially affect the species with lower carbon numbers, as was mentioned earlier. A low pressure (3.5 MPa) combined with a high temperature (370 °C) results in enriching the vapor phase with the lighter hydrocarbons and the liquid
Figure 3. Dependence of C23+ conversion on hydrogen pressure during hydrocracking of the Sasol M5 wax at a temperature of 370 °C, a WHSV of 1.0 h-1, and a 1200:1 Nm3/m3 H2-to-wax ratio.
phase with heavier components. This will increase the isomerization and hydrocracking rates of the heavier components and result in higher C23+ conversion and its concomitant effect on the product properties (see Table 7). High hydrogen-to-wax ratios will enhance this effect. As shown above, hydrocracking at the higher pressures suppressed the degree of isomerization and conversion. High pressures inhibit the dehydrogenation step, which is the initial step in the generally accepted mechanism for hydrocracking.24,25 This leads to lower conversions, because of the lower rate of olefin and carbenium ion formation, and the overall effect was, as observed, lower hydrocracking rates. Thybaut et al.26 called this behavior ideal hydrocracking. An increase in conversion at low pressures corresponds to nonideal hydrocracking, while decreasing conversions at higher total pressures is indicative of ideal hydrocracking. The effects of the total pressure on hydrocracking are related to the effect of total pressure on the alkene and, hence, carbenium ion formations. In nonideal hydrocracking [i.e., when the (de)hydrogenation reactions are not quasi-equilibrated], a kinetic effect of an increasing dehydrogenation rate leads to higher alkene and carbenium ion concentrations. The higher carbenium ion concentrations increase the rate of acid-catalyzed isomerization and cracking reactions, leading to higher hydrocracking rates at lower pressures. A thermodynamic effect rather than a kinetic effect (24) Schulz, H.; Weitkamp, J. Ind. Eng. Chem. Prod. Res. DeV. 1972, 11, 46. (25) Weitkamp, J.; Dauns, H. Erdo¨l Kohle, Erdgas, Petrochem. 1987, 40, 111. (26) Thybaut, J. W.; Narasimhan, C. S. L.; Denayer, J. F.; Baron, G. V.; Jacobs, P. A.; Martens, J. A.; Marin, G. B. Ind. Eng. Chem. Res. 2005, 44 (14) 5159-5169.
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Table 6. Product Selectivities and Yields Achieved during Hydrocracking of the Sasol M5 Wax Using the Pressure Range of 3.5-7.0 MPa, a Temperature of 370 °C, a WHSV of 1.0 h-1, and a Hydrogen-to-Wax Ratio of 1200:1 Nm3/m3 selectivities (wt %)
yields (wt %)
pressure (MPa)
C23+ conversion (wt %)
C1-C4
C5-C9
C10-C22
C1-C4
C5-C9
C10-C22
3.5 5.0 7.0
81 58 46
1.8 2.3 0.7
24 19 16
75 79 84
1.5 1.3 0.3
19 11 7.4
61 46 39
Table 7. Properties of Diesel (C10-C22) Produced during Hydrocracking of the M5 Wax Using a Temperature of 370 °C, a WHSV of 1.0 h-1, and a Hydrogen-to-Wax Ratio of 1200:1 Nm3/m3 3.5 MPa C23+ conversion, wt % diesel/naphtha ratio iso-/n-paraffin ratio in the diesel fraction cloud point, °C cetane number
5 MPa
7 MPa
81
58
46
3.1
4.3
5.3
4.1
4.0
3.4
-17 72
-12 72
-10 74
is observed in ideal hydrocracking. Lower alkene and carbenium ion concentrations lead to lower hydrocracking rates. Effect of Weight Hourly Space Velocity. Increasing the space velocity, at otherwise constant operating conditions, has a major effect on C23+ conversion and the product yields. Significant differences are also observed in the product properties, specifically the diesel properties investigated. The results in Tables 8 and 9 show the effect of space velocity on the product slate and diesel properties, from the hydrocracking of the Sasol M5 wax at 360 °C, 3.5 MPa, and a 1200:1 Nm3/m3 hydrogen-to-wax ratio. A 50% reduction in C23+ conversion occurred by increasing the weight hourly space velocity from 0.5 to 1.0 h-1 (see Table 8). The diesel selectivity improved by 5%, while the selectivity to naphtha was reduced by 20%. The diesel yield on the other hand decreased significantly by about 50%, while a 60% reduction in naphtha yield also occurred. The diesel properties which are presented in Table 9 indicate that the isomerization rate decreased after applying the higher weight hourly space velocity. The isoparaffin-to-n-paraffin ratio in the diesel fraction decreased from a ratio of 4.6 to 4.0, affecting thereby the cloud point, which increased from -18 to -7 °C. The cetane number, which is boosted by linear paraffins in the diesel, increased from a value of 71 to a value of 73. Effect of Paraffin Chain Length. The Sasol M5 wax includes paraffin molecules from C15 up to C45. The probability that the hydrocracked molecules are primarily converted into diesel products is therefore high. However, because the paraffin molecules in the wax investigated cover a broad carbon number range, several factors such as vapor-liquid equilibria, adsorption/desorption, and polarity could simultaneously affect the relative rates of paraffin hydrocracking. It must be emphasized that our observations and results are derived from an industrially produced FT wax, which is a mixture of alkanes of different chain lengths and not an individual Cn model alkane. Because lighter alkanes can be formed from heavier ones, the competitive adsorption of products, derived from the hydrocracking or hydroisomerization of long-chain reactants and uncracked reactants, is taking place in parallel, which is therefore embedded in our results on the chain-length effects. More particularly, it is very difficult to ascribe in such complex alkane mixtures the influence of the individual physicochemical parameters to the
overall reactivity effects observed. Tentatively, we try to explain our results and propose the following explanations. Figure 4 shows the product carbon number distribution at various C23+ conversions and pressures. In general, applying lower pressures (i.e., 3.5 MPa) improved markedly the percent conversion of the paraffins. In Figure 5, the disappearance rate ln(Co/C) of the n-paraffins is presented as a function of the individual paraffin carbon number, where Co represents the initial weight percentage of the paraffin carbon number in the feed and C is the weight percent of the same paraffin carbon number in the product at the given reaction condition. At 3.5 MPa of hydrogen pressure (81% C23+ conversion), the paraffin reactivity increased steadily from C16, reached a maximum at C33, and decreased thereafter again. Increasing the hydrogen partial pressures reduced the reactivity of all paraffins. However, the reduction observed for the C21-C24 paraffins was not as significant in relation to the higher carbon numbers. Although a maximum in reactivity is again noticed for the C33 paraffin, the reactivity of the C21 paraffin was higher than the reactivity of the C33 paraffin and substantially higher than that for paraffins above C33. At high pressures, the C21 paraffin showed the highest reactivity of all carbon numbers. Diffusional restrictions are, from our observations, not responsible for the observed lower reactivities because the shorter-chain paraffins in the range C21C30 are affected to the same extent as the C33+ alkanes. Rapaport27 and Stangeland28 reported an almost linear increase in the relative rates of hydrocracking for C5/C10/C15/C20 paraffins according to the sequence 1/32/72/120. Sie et al.29 hydrocracked paraffins up to C17 over a bifunctional catalyst and found that the paraffin reactivity increased considerably with increasing carbon number. The heat of adsorption on the catalyst surface was reported to be one of the factors contributing to the chainlength dependence for the observed cracking rate. According to the above observations, the heavier alkanes present in our C15-C45 LTFT wax fraction should therefore have been preferentially converted, which was not what we observed (see Figure 5). The differences in paraffin reactivity during hydrocracking of the M5 wax could result from the characteristic carbon number distribution of the M5 wax feed (see Figure 1). The distribution indicates lower concentrations of shorter (C15-C18) and longer carbon numbers (C36+) together with the bulk of C20-C35 paraffins. The long- and short-chain paraffins can be considered essentially dissolved in the bulk of these latter paraffins. The adsorption of the long- and short-chain paraffins is therefore influenced or is perhaps even inhibited by the bulk of the paraffins. The intrinsic higher adsorption of the longchain alkanes therefore is counteracted by the competitive concentration effect of the paraffins in the bulk of the material. At low hydrogen pressures (3.5 MPa), the dehydrogenation reaction is favored, and because of their higher concentrations, (27) Rapaport, I. B. Chemistry and Technology of Synthetic Fuels, 2nd ed.; Israel Program for Scientific Translation Ltd.: Jerusalem, 1962. (28) Stangeland, B. E. Ind. Eng. Chem. Proc. Des. DeV. 1974, 13 (1), 71-76. (29) Sie, T.; Senden, M. M.; van Wechem, H. M. H. Catal. Today 1991, 8, 371.
Diesel-SelectiVe Hydrocracking
Energy & Fuels, Vol. 20, No. 6, 2006 2335
Table 8. Product Selectivities and Yields Achieved during Hydrocracking of a Sasol M5 Wax at 360 °C, 3.5 MPa, a 1200:1 Nm3/m3 H2-to-Wax Ratio, and WHSVs of 0.5 and 1.0 h-1 selectivities (wt %)
yields (wt %)
WHSV (h-1)
C23+ conversion (wt %)
C1-C4
C5-C9
C10-C22
C1-C4
C5-C9
C10-C22
0.5 1.0
69 35
1.1 1.8
22 17
77 81
0.8 0.6
15 6.1
53 28
Table 9. Properties of the Diesel Fraction Produced at 360 °C, 3.5 MPa, a 1200:1 Nm3/m3 H2-to-Wax Ratio, and WHSVs of 0.5 and 1.0 h-1
iso-/n-paraffin ratio cloud point, °C cetane number
0.5 h-1 WHSV
1.0 h-1 WHSV
4.6 -18 71
4.0 -7 73
Figure 6. Effect of paraffin concentration and pressure on the reduction in reactivity of the individual paraffin in the M5 wax feed.
Figure 4. Change in carbon number distribution during hydrocracking of the Sasol M5 wax at various percent C23+ conversions using a temperature of 370°C, a WHSV of 1.0 h-1, and a 1200:1 Nm3/m3 hydrogen-to-wax ratio.
Figure 5. N-paraffin reactivity at different C23+ conversions and pressures expressed as rate of disappearance ln(Co/C) for the hydrocracking of the Sasol M5 wax at 370°C, a 1.0 h-1 WHSV, and a H2to-wax ratio of 1200:1 Nm3/m3.
the shorter-chain alkenes (i.e., C18-C30) are preferentially adsorbed on the acid sites. This in turn leads to the observed equivalent overall reactivity of the short-chain (around C18C30) and long-chain (C33+) paraffins. The result would be the reactivity pattern as indicated in Figure 5 for the low-pressure reaction. It does, however, still not explain the preferred conversion of the C33 alkane molecule. Increasing the hydrogen pressure led in general to a decrease in reactivity of all paraffins except for C21. The C21 paraffin has the highest concentration in the wax feed (see Figure 5), and its apparent reactivity seemed not to be effected by the higher pressure. The maximum reactivity in the higher carbon number range still persisted at carbon number C33, but the reactivity of the C21 paraffin still exceeded that of the C33
paraffin. At 7.0 MPa, the lowest reactivity was observed for the C27 and C36+ paraffins. The effect of higher pressure on the degree of reactivity reduction for the individual paraffins is shown in Figure 6. The plot shows that the highest reduction in reactivity is obtained with the C33 paraffin, and practically no pressure influence was observed for the C21 paraffin. The concentration of the individual paraffins in the feed is also plotted in Figure 6, and it appears as if the paraffin reactivity reduction can be correlated to the paraffin concentrations in the feed. The pressure increase did not affect the reactivity of the paraffins of higher concentrations (i.e., C21-C22). However, with increasing carbon number and decreasing concentrations, the higher pressure affected progressively the paraffin reactivity. As indicated above, the highest reduction in paraffin reactivity was noticed for C33; thereafter, the pressure effect diminished for the longer-chain paraffins. Three effects should therefore be considered in describing our observations: first, the effect of increasing paraffin reactivity with increasing chain length (the reactiVity effect), second, the low concentrations of the long-chain paraffins (the concentration effect), and third, the pressure effect. At low pressures and a high conversion, the reactivity effect prevailed over the concentration effect on the paraffin molecules up to carbon 33, whereafter the concentration effect dominated over the reactivity effect. At high pressures and a low conversion, on the other hand, the concentration effect prevailed over the reactivity effect up to C33, whereafter the reactivity effect appears to have a more marked influence but does not ultimately disguise the concentration effect. Previous hydrocracking studies30,31 have shown that n-paraffin reactivity increases with chain length; however, the studies were done with shorter-chain alkanes up to C17. The hydrocracking and hydroisomerization of longer-chain paraffins such as n-C16, n-C28, n-C36, and n-C44 for the production of base oils using a mesoporous 0.30% Pt/SiO2-Al2O3 catalyst was investigated (30) Weitkamp, J. Prepr. Pap.sAm. Chem. Soc., DiV. Pet. Chem. 1975, 20, 654. (31) Debrabandere, B.; Froment, G. F. In Hydrotreatment and Hydrocracking of Oil Fractions, Studies in Surface Science and Catalysis; Froment, G. F., Delmon, B.; Grange, P., Eds.; Elsevier: Amsterdam, 1997; Vol. 106, p 379.
2336 Energy & Fuels, Vol. 20, No. 6, 2006
Leckel and Liwanga-Ehumbu
Conclusion
Figure 7. Isoparaffin-to-n-paraffin ratio determined for the Sasol M5 wax hydrocracking product at 58% (5.0 MPa) and 81% (3.5 MPa) C23+ conversion.
recently by Calemma et al.32 It was reported that the conversion rate increased between C16 and C28 and decreased again between C28 and C44. Tentatively, it was suggested by these authors that the lower reactivities displayed by n-C36 and n-C44 might result from a decrease in the available surface area caused by the higher steric hindrance of the n-C36 and n-C44 molecules. The effect of paraffin chain length on the isomerization selectivity is shown in Figure 7, where the isoparaffin-to-nparaffin ratios of the hydrocracked M5 product versus the carbon number distribution is displayed. The isomerization selectivity was found to be highest in the diesel carbon number range (C10C22), peaking at C13. Thereafter, the selectivity for iso-Cn formation decreased noticeably. Beyond carbon number 22, the isoparaffin-to-n-paraffin ratio decreased with increasing chain length. With regards to the M5 wax carbon number distribution (C15-C45), the diesel molecules should mainly be primary cracking products, produced from paraffin molecules outside the diesel carbon number range. As such, they showed a higher degree of isomerization as compared to the C23-C40 carbon number range. Similar results were also obtained by Calemma et al.32 in their study on the hydrocracking and the hydroisomerization of the long-chain n-C16, n-C28, n-C36, and n-C44 paraffins, where the isomerization selectivity showed a decrease as a function of chain length. (32) Calemma, V.; Peratello, S.; Perego, C. Appl. Catal. A 2000, 190, 207-218.
Iron-based low-temperature Fischer-Tropsch wax in the carbon number range of C15-C45 was hydrocracked using a MoO3-modified amorphous Pt/SiO2-Al2O3 catalyst. The selective production of a highly isomerized diesel with low cloud points (-18 °C) combined with high cetane numbers (71) was achieved at a pressure of 3.5 MPa, a temperature of 360 °C, a WHSV of 0.5 h-1, and a hydrogen-to-wax ratio of 1200:1 Nm3/ m3. The excellent cloud points and cetane numbers obtained are in line with the German EURO IV diesel fuel specifications. The cetane number thereby far exceeds the minimum required cetane number specification of 51. Increasing the operating temperatures resulted in higher C23+ conversions and increased naphtha formation at the expense of diesel. A lower space velocity increased the C23+ conversion and the naphtha and diesel yields. The diesel cold-flow properties improved significantly because of an increase in isomerization. Higher conversions improved the isoparaffin-to-n-paraffin ratio in the carbon number range C15-C22. The degree of isomerization after C22, however, did not alter significantly. The highest isomerization reactivity was found for the carbon numbers 12-13. High pressures favored the hydrogenation reaction, suppressed the degree of isomerization, and improved the diesel selectivity by reducing secondary cracking. The overall paraffin reactivity decreased, however. A maximum in paraffin reactivity was observed for the carbon number 33, which was attributed to the intrinsic and distinctive alkane composition of the C15-C45 wax fraction studied. At low pressures and high conversions, the reactivity effect prevailed over the concentration effect on the paraffins up to carbon 33, whereafter the concentration effect overrode the reactivity effect. At high pressures and low conversions, on the other hand, the concentration effect prevailed over the reactivity effect up to C33. Acknowledgment. All work was done at the facilities of Sasol Technology Research and Development, and the permission to publish the results is appreciated. EF060319Q
