Energy & Fuels 2007, 21, 2751-2762
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Fuel-Blending Stocks from the Hydrotreatment of a Distillate Formed by Direct Coal Liquefaction Andile B. Mzinyati* Fischer-Tropsch Refinery Catalysis, Sasol Technology Research and DeVelopment, Post Office Box 1, Sasolburg 1947, South Africa ReceiVed December 7, 2006. ReVised Manuscript ReceiVed May 22, 2007
The direct liquefaction of coal in the iron-catalyzed Suplex process was evaluated as a technology complementary to Fischer-Tropsch synthesis. A distinguishing feature of the Suplex process, from other direct liquefaction processes, is the use of a combination of light- and heavy-oil fractions as the slurrying solvent. This results in a product slate with a small residue fraction, a distillate/naphtha mass ratio of 6, and a 65.8 mass % yield of liquid fuel product on a dry, ash-free coal basis. The densities of the resulting naphtha (C5200 °C) and distillate (200-400 °C) fractions from the hydroprocessing of the straight-run Suplex distillate fraction were high (0.86 and 1.04 kg/L, respectively). The aromaticity of the distillate fraction was found to be typical of coal liquefaction liquids, at 60-65%, with a Ramsbottom carbon residue content of 0.38 mass %. Hydrotreatment of the distillate fraction under severe conditions (200 °C, 20.3 MPa, and 0.41 gfeed h-1 gcatalyst-1) with a NiMo/Al2O3 catalyst gave a product with a phenol content of 220 Å diameter
peak pore diameter (Å)
NiMo-1 NiMo-2 NiMo-3 NiMo-4 NiMo-5 NiMo-6 NiMo-7 NiMo-8 NiMo-9 NiMo-10
15.3 12.9 16.5 18.8 18.6 17.7 18.2 15.6 18.6 15.4
3.0 3.0 3.5 3.6 4.0 4.4 3.7 2.9 4.7 3.0
149 213 158 158 235 138 184 267 110 225
0.371 0.448 0.482 0.540 0.530 0.371 0.575 0.615 0.568 0.500
0.031 0.106 0.010 0.056 0.104 0.059 0.028 0.105 0.014 0.137
0.327 0.286 0.204 0.225 0.263 0.233 0.284 0.319 0.015 0.280
0.006 0.013 0.147 0.233 0.039 0.016 0.094 0.022 0.259 0.012
0.007 0.043 0.121 0.023 0.124 0.063 0.169 0.015 0.280 0.061
60 68 104 112 75 60 80 68 196 59 + 80
comparison of the Suplex product with that from other DCL processes (Table 1) shows that it has the advantage of a small heavy-oil fraction (2 mass %), resulting in less coal having to be liquefied to provide a good yield of the light and distillate fractions. The smaller throughput of coal required in the Suplex process also translates into a capital saving, in that the absolute amount of hydrogen required by the process is less, which lowers the cost of hydrogen production and gas cleanup units. Furthermore, the size of the reactor needed to handle the coal for the reaction would be smaller. Another distinguishing feature of the Suplex process is the use of Rheax16 sedimentation, a wet process for the separation of fine-particle mixtures, for the separation of solids from the Suplex oil. Hydrogen and fuel gas generation in the Suplex process can be achieved by gasification of some of the coal, rather than vacuum bottoms residue. Products Properties. The data in Table 1 show that a high yield of liquid fuels can be obtained from the Suplex process. The straight-run naphtha fraction from the Suplex process is recovered by atmospheric distillation, and the distillate fraction is recovered by vacuum distillation. The Suplex distillate fraction is, typical of DCL products,20 rich in polycyclic aromatics and heteroatoms. Consequently, upgrading is required to meet fuel specifications. This upgrading entails catalytic reforming and hydrotreatment in the case of the naphtha and severe hydrotreatment in the case of the middle oil. Such procedures are almost universal to coal liquefaction operations, and various oil fractions from DCL operations have been hydroprocessed to produce motor gasoline, jet fuel, and diesel fuel.21 The Suplex straight-run naphtha fraction [research octane number (RON) ) 90] can be routed to a naphtha hydrotreatment unit for hydrodeoxygenation (HDO), hydrodesulfurisation (HDS), and hydrodenitrogenation (HDN). The distillate fraction can be routed to a creosote-type hydrotreater, already described.22 Naphtha generated through cracking during
the hydroprocessing of the straight-run distillate fraction can be sent to the catalytic reformer for octane-number improvement. Aromatics from the Suplex product do not have to be hydrotreated and can be used as feedstocks for other chemical processes. The use of DCL for the production of aromatics has already been demonstrated in the literature.23 The data in Table 1 show that a high yield of the straightrun distillate fraction (200-405 °C) can be obtained from the liquefaction of coal with the Suplex process. Hydroprocessing of this fraction provides a means for the production of diesel fuel to meet increasing demands for diesel fuel in South Africa. The hydroprocessing of coal-derived liquids has the goal of hydrotreatment of heteroatomic and polyaromatic compounds and hydrocracking of the heavier constituents of the coal liquids, to upgrade them to liquid fuel specifications. The bulk of the hydrogenation that takes place involves the hydrogenation of aromatic rings. This may be the saturation of aromatic rings of hydrocarbon and heteroatomic compounds derived from the coal and HDN, HDO, and HDS reactions.22 Because hydrogenation is a reversible reaction with a maximum conversion at temperatures in the range of 350-400 °C, the influence of the temperature on this process is critical. Thus, the temperature influences the quality of the diesel fuel, as indicated in part by its hydrogen content and cetane number. Progressively higher temperatures and hydrogen partial pressures have to be employed to improve the cetane rating of coal-derived fuel and to approach complete heteroatom removal. Higher temperatures, however, lead to increased hydrocracking of the heavier constituents, which may not be desired in the hydroprocessing of the intermediate fractions. Too much hydrocracking of the intermediate-oil fractions under severe operating conditions has a detrimental effect on the yield of diesel and naphtha fractions and hydrogen consumption.
(16) Buchel, K. Chem. Ing. Tech. 1957, 29, 112 (17) Burke, F. P.; Brandes, S. D.; McCoy, D. C.; Winschel, R. A.; Gray, D.; Tomlinson, G. Summary Report of the DOE Direct Liquefaction Process Development Campaign of the Late Twentieth Century; DOE Contract number DE-AC22-94PC93054, 2001. (18) Cleaner Coal Technology Programme: Technology Status Report No. 10sCoal Liquefaction; Department of Trade and Industry: London, U.K., 1999. (19) Indonect.com Report 409 (Coal Resources). http://www.indonext.com/report/report409.html (accessed Nov 7, 2006). (20) (a) Farcasiu, M. Fuel 1977, 56, 10. (b) Jones, D. G.; Rottendorf, H.; Wilson, M. A.; Collin, P. J. Fuel 1980, 59, 59. (21) (a) General review: Zhou, P.-Z.; Marano, J. J.; Winschel, R. A. Prepr. Symp.sAm. Chem. Soc., DiV. Fuel Chem. 1992, 37, 1847. (b) SRCII: Katti, S. S.; Gates, B. C.; Grandy, D. W.; Youngless, T.; Petrakis, L. Ind. Eng. Chem. Res. 1988, 27, 1767 (and preceding papers in series). (c) NEDOL: Wilson, M. F. Prepr. Symp.sAm. Chem. Soc., DiV. Fuel Chem. 1993, 38, 1058. H-Coal: MacArthur, J. B.; Duddy, J. E.; Moomjy, A. V. Natl. Pet. Refin. Assoc., Tech. Pap. 1982, AM-82-43. IGOR: Strobel, B. O.; Loering, R. Prepr. Symp.sAm. Chem. Soc., DiV. Fuel Chem. 1992, 37, 448.
Reactor System. The hydrotreatment experiments were carried out with a continuous reactor of 28 mm internal diameter and 1.3 m length. The reactor was heated by electrical elements attached to three brass blocks, separated by asbestos spacers, spanning the length of the reactor and dividing the reactor into three zones. Temperature was recorded with six thermocouples running the entire length of the reactor in an axially inserted thermal well. The reactor was operated with downward co-current flow of the gaseous and liquid feeds. Catalysts. A series of commercially available and in-houseprepared alumina-supported NiO/MoO3 (NiMo) catalysts (1.5 mm extrudates or crushed samples formed from these) were used. Elemental analyses of the catalysts and their physical properties are given in Tables 2 and 6. Pore-size distributions were determined by using a Micromeritics mercury porosimeter.
Experimental Section
(22) Leckel, D. Energy Fuels 2006, 20, 1761. (23) Song, C.; Schobert, H. H. Fuel Process. Technol. 1993, 34, 157.
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Energy & Fuels, Vol. 21, No. 5, 2007 2755
Table 3. Characterization of the Catalyst Sample Used in Experiments To Evaluate the Mass-Transfer Influence pore-size distribution (cm3/g)
composition (wt %) catalyst
MoO3
NiMo-11 (extrudate) NiMo-11 (crushed) NiMo-12 (extrudate) NiMo-12 (crushed)
22.5
NiO
BET surface area (m2/g)
pore volume (cm3/g)
particle size (mm)
35-60 Å diameter
60-110 Å diameter
110-220 Å diameter
>220 Å diameter
175
0.544
0.068
0.095
0.341
0.035
263
0.278
1.5 0.4 ( 0.1 2.4 0.4 ( 0.1
0.190
0.002
0.002
0.004
3.0
21.9
3.5
Table 4. Suplex Middle Oil Feed Characterization Data elemental analysis (mass %)
NMR analysis (mass %)
feed
C
H
N
S
O
1 2 3 4 5 6 7 8 9
86.8 86.5 86.1 87.1 88.4 87.6 89.1 88.5 87.4
8.7 8.2 8.3 8.0 7.9 8.3 8.3 8.3 8.6
0.4 1.2 1.0 1.4 1.3 0.9 1.1 1.2 1.0
0.1 0.1 0.2 0.1 0.1 0.1 0.2 0.1 0.1
4.0 4.0 4.4 3.4 2.3 3.1 1.3 1.9 3.4
a
Haromatic
36.8 39.3 33.9 35.8 33.5 37.8 32.9
HR
27.2 28.7 33.1 29.0 28.1 28.6 29.0
Hβ
26.2 23.3 24.0 25.8 26.5 23.2 27.3
normal boiling point (°C) Hδ
4.9 8.7 9.0 9.4 10.5 8.1 9.4
xaroma (mass %)
ASTM D86 T95
final
RCR (mass %)
63.4 66.5 64.6 63.5 62.8 65.0 59.5
388 374 423 458 462 379 382 369 376
414 401 460 499 501 430 411 414 413
0.80 0.80 0.90 0.48 0.39
xarom ) {[(%C ÷ 12)/%H] - [(100 - %Haromatic)/200]}/[(%C ÷ 12)/%H] × 100. Table 5. Elemental Analysis of Suplex Middle Oil composition (mass %)
element
crude Suplex middle oil
redistilled Suplex middle oil
C H N S O
87.1 8.0 1.4 0.1 3.4
87.6 8.3 0.9 0.1 3.1
Catalyst particles (260 mL), supported on a fine mesh grid, were loaded into the middle section of the reactor tube, and the spaces between them were filled with sand (35-50 mesh) to flatten the velocity and temperature profiles. The space above the reactor was filled with glass beads. Catalyst Activation. Presulfiding of the catalyst was carried out by prewetting the catalyst bed with sulfur-containing light cycle oil (in-house) at 70 °C. The catalyst bed was then heated to 230 °C at a rate of 60 °C/h. Sulfiding was achieved by passing the light cycle oil with hydrogen at a typical space velocity (LHSV ) 0.33 h-1) and gas/liquid flow ratio (1250:1 H2/liquid) of the hydrotreatment reaction over the catalyst for 16 h. The breakthrough of dimethyl sulfide in the off gas was monitored by Dra¨ger tubes. When the concentration of dimethyl sulfide in the off gas was found to be 2000 ppm, the temperature was increased to 350 °C at a rate of 10 °C/h and maintained at that value for a further 6 h. No further sulfur was added during the run, with the 0.1 mass % of sulfur in the feed regarded as sufficient to maintain sulfur levels on the catalyst. Feedstock. Several Suplex oil feeds (Table 4) were used for the test work reported here. Feeds 3-5 were slightly heavier than the others, with boiling points up to 500 °C and Ramsbottom carbon residue (RCR) values of about 0.80 mass %. Sample Analysis. The following methods were used to analyze the hydrotreated Suplex middle-oil fractions: (a) American Society for Testing and Materials (ASTM) distillation (ASTM D86), (b) CHN analysis, (c) capillary column gas chromatography-flame ionization detector (GC-FID), (d) RON, (e) cetane number (ASTM D613), and (f) UV detection of aromatics.
Figure 2. Typical distillation profiles for Suplex distillate used as feed (Table 4) for catalytic hydroprocessing experiments.
detrimental effect on the hydrotreatment of these liquids.24 This effect generally takes the form of carbon deposition on the catalyst, which limits the catalyst lifetime. Consequently, the Suplex distillate fractions with high RCR values were redistilled to counter possible catalyst deactivation. This redistillation did not noticeably change the elemental composition of the fractions (Table 5), although there was a 13 volume % loss in the heavy end of the fraction because of distillation and a 20 °C decrease in the initial boiling point of the of the fraction. Redistillation did, however, lower the RCR levels of 0.80-0.90 mass % for the crude Suplex distillate fraction to 0.38 mass % for the redistilled distillate fraction, making the 200-400 °C fraction comparable to a 3:2:1 mixture of medium creosote (MC), heavy creosote (HC), and residue oil (RO) hydroprocessed at the ironbased high-temperature FT tar refinery of Sasol22 and other liquids from direct coal pyrolysis.25 Thus, only the Suplex distillate fractions that had been redistilled were used for the bulk of the test work. An undistilled Suplex distilled feed was used in one of the experiments to see the effect on diesel fraction selectivity. Figure 2 shows a typical distillation profile of a Suplex distillate fraction. Redistillation of the Suplex straight-
Results and Discussion A. Redistillation of the Suplex Distillate Fraction. Asphaltene and preasphaltene compounds found in the heavier fractions of coal-derived liquids have been reported to have a
(24) (a) Newson, E. Ind. Eng. Chem. Process. Des. DeV. 1975, 14, 27. (b) Corella, J.; Monzon, A. Ind. Eng. Chem. Res. 1988, 27, 369. (c) Rangwala, H. A.; George, Z. M.; Hardin, A. H. Energy Fuels 1991, 5, 835. (25) Teo, K. C.; Watkinson, A. P. Fuel 1990, 69, 1211.
2756 Energy & Fuels, Vol. 21, No. 5, 2007
Mzinyati
Figure 3. Effect of the temperature on the selectivity of Suplex oil hydroprocessing at P ) 20.3 MPa and WHSV ) 0.41 gfeed h-1 gcatalyst-1.
Figure 4. Effect of the temperature on the selectivity of Suplex oil hydroprocessing at P ) 17.7 MPa and WHSV ) 0.41 gfeed h-1 gcatalyst-1.
run distillate fraction on the industrial scale would adversely impact the economic viability of the process because of the addition of the extra process step. On the industrial scale, processing the entire straight-run distillate fraction would be the desired option. Experience at Sasol on the hydroprocessing of the creosote/residue oil mixtures22 shows that the correct choice of guard bedding before the reaction zone of the reactor and the use of severe hydroprocessing conditions can effectively mitigate against the fouling of catalyst heavy coal residues. Furthermore, the choice of a hydrotreating catalyst with a large peak pore size is important. In the current study, no attempt to remove the heavy components of the straight-run Suplex distillate fraction through guard bedding was made. The test work was, however, conducted on the redistilled fraction. Products from the Suplex Middle-Oil Fraction. Figures 3 and 4 were constructed from data obtained from the combined results of the hydroprocessing of the series of Suplex distillate feeds over the selection of catalysts described in Tables 2-6. Figures 3 and 4 show that, dependent upon the hydroprocessing conditions, substantial yields of diesel fuel can be
Table 6. Residue Content of Suplex Middle-Oil Fractions distillation cut (°C)
RCR (mass %)
200 °C fraction
yield (mass %) process
conditions
C1-C4
C5-200 °C
>200 ˚C
diesel/naphthaa
one stepb
P ) 20.3 MPa, T ) 418 °C, and WHSV ) 0.41 gfeed h-1 gcatalsyt-1
2
29
69
2.4
13.0
44
2
25
73
2.9
12.9
43
3
36
61
1.7
13.2
48
1 2 two stepb
1 2
P ) 20.3 MPa, T ) 395 °C, and WHSV ) 1.9 gfeed h-1 gcatalsyt-1 P ) 20.3 MPa, T ) 420 °C, and WHSV ) 1.0 gfeed h-1 gcatalsyt-1 P ) 17.7 MPa, T ) 400 °C, and WHSV ) 0.41 gfeed h-1 gcatalsyt-1 P ) 20.3 MPa, T ) 300 °C, and WHSV ) 0.43 gfeed h-1 gcatalsyt-1
mass % H
cetane number
a On the basis of mass % values. b The one-step process involves hydrotreating the entire Suplex distillate fraction under the stipulated conditions, whereas the two-step process involves the hydroprocessing of, initially, the entire distillate fraction and then only the diesel fuel obtained from it for cetane-number improvement.
Table 9. Comparison of the Composition of Hydroprocessed Suplex Distillates after the First Stage (P ) 20.3 MPa, T ) 390-450 °C, and WHSV ) 0.41 gfeed h-1 gcatalyst-1) and Second Stage (P ) 20.3 MPa, T ) 300 °C, and WHSV ) 0.41 gfeed h-1 gcatalyst-1) Hydrotreating Processes Naphtha processing mode first stage
RON
paraffins (mass %)
naphthenes (mass %)
aromatics (mass %)
S (ppm)
N (ppm)
phenol (ppm)
66
14
72
14
37
132
4
Diesel processing mode
cetane number
aromatics (mass %)
%H
first stage second stage
39 48
59 0.25
13.0 13.2
liquefaction products.37 Accordingly, the results of a comparative study of single- and two-stage hydroprocessing of Suplex middle oil are summarized in Table 8. The one-step process involves hydrotreating the entire Suplex distillate fraction under the stipulated conditions, whereas the two-step process involves the hydroprocessing of, initially, the entire distillate fraction and then only the diesel fuel obtained from it for cetane-number improvement. The data presented in Table 8 for the single-step process represent the optimized condition for the single-step process. Two-step hydrotreatments are necessary to improve the quality, specifically cetane number, of the diesel fuel produced. This hydrotreatment entails the further removal of aromatics to improve the cetane number of the diesel fraction. Higher space velocities are required in the two-stage process to obtain similar diesel/naphtha selectivity as the single-stage process under the similar reaction conditions. However, the data in Table 8 show the trade-off between diesel selectivity and the quality (%H and cetane) of diesel fuel produced, with a lower space velocity (WSHV ) 0.43 gfeed h-1 gcatalyst-1) and temperature (T ) 300 °C) second-stage hydroprocessing favoring higher cetane (48) diesel fuel at the expense of diesel selectivity. The results of this study are in line with the findings of the British Coal Corporation on the hydroprocessing of a straight-run coalderived middle distillate.38 Results on a test with a nickelmolybdenum catalyst show that higher diesel quality (cetane number) can be achieved by a less severe two-stage processing instead of a harsh single-step hydrotreatment, which ultimately results in better process economics for the two-stage hydroprocessing because of the milder operating conditions. A comparison of the heteroatom levels reported for the product in Table 9 with those reported for the feed in Table 4 shows the major reduction in heteroatom levels of the hydrotreated Suplex distillate. The data of Table 9 show that the (37) Yang, J.; Liu, Z.; Bi, J. Fuel 2003, 82, 1549. (38) British Coal Corporation. European Commission Report EUR 17503, 1998; pp 1-53.
S (ppm)
N (ppm)
3
118 67
secondary hydrogenation step is necessary for substantially greater removal of nitrogen from the diesel fraction. Analysis of the naphtha fraction after the first-stage hydrogenation shows very low concentrations of phenol, whereas those of nitrogen and sulfur are higher. The higher concentration of nitrogen, in comparison to those of phenol and sulfur, indicates that it is the most refractory heteroatom. This result implies an order of reactivity of HDO > HDS > HDN. This order of reactivity is in contrast to the findings of Murti et al.,39 whose study on the hydrotreatment of a coal-liquefaction-derived distillate gave a reaction order of HDS > HDO > HDN. Such an order of reactivity is to be expected because HDS does not necessarily require the complete hydrogenation of the aromatic ring,40 whereas HDO and HDN do.41 Leckel,22 however, notes that the catalyst pore size has an effect on the hydrotreatment reaction order such that an order of HDO > HDS > HDN may be expected for mesoporous NiMo. C. Properties of Suplex Fuels. The ultimate aim of the liquefaction process is the production of transportation fuels from coal. To bring meaning to the discussion of the Suplex process, the fuel properties of those produced from it have to be compared with standard fuel specifications. The selectivity of the Suplex process for the diesel fraction and the densities of the naphtha (0.86 kg/L) and middle oil (1.04 kg/L) fractions, as well as the aromatic content of the Suplex distillate fraction, show that the process presents valuable opportunities for the production of synthetic fuels or blending stocks with FT-derived fuels. The high density of the distillate fraction is especially suited toward blending with low-density FT diesels, and the density and low pour point of the fraction indicate that it is ideal for fractionating into jet fuel. (39) Sumbago Mutri, S. D.; Choi, K.-H.; Sakainshi, K.; Okuma, O.; Korai Y.; Mochida, I. Fuel 2005, 84, 135. (40) Schrepfer, M. W.; Arnold, R. J.; Stansky, C. A. Oil Gas J. 1984, 79. (41) Ritchie, J. J. Inst. Pet. 1995, 51, 296.
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Table 10. Summary of the Properties of Diesel Fuel Produced from the Hydroprocessing of a Combination of Suplex Middle Distillate Fractions by Various NiMo Catalysts (T ) 418 °C, P ) 20.3 MPa, and WHSV ) 0.41 gfeed h-1 gcatalyst-1)
property
diesel from hydroprocessed Suplex distillate
cetane number density15 °C (kg/L) aromatics (mass %) cold filter plug point (°C) flash point (°C) RCR (mass %) S content (ppm) N content (ppm)
48 0.90 27 -10 95 0.17 55
