Energy & Fuels 2007, 21, 2509-2513
2509
Hydrogenation of Fischer-Tropsch Synthetic Crude Delanie Lamprecht* Fischer-Tropsch Refinery Catalysis, Sasol Technology Research and DeVelopment, P.O. Box 1, Sasolburg 1947, South Africa ReceiVed December 4, 2006. ReVised Manuscript ReceiVed June 29, 2007
Fischer-Tropsch Synthetic Crude is high in olefinicity and needs further hydroprocessing to conform to fuel specifications such as oxidation stability. Different catalysts, that is, sulfided NiMo/Al2O3, CoMo/Al2O3, and unsulfided Ni/Al2O3, have been used, and a kinetic model has been derived for the hydrogenation of Fischer-Tropsch Synthetic Crude. The kinetic model allows for predicting process conditions necessary to meet mandatory fuel specifications and market requirements as well as to compare the catalytic activity of these different hydrogenation catalysts on the basis of the kinetic parameters.
Introduction Fuel stability normally decreases in the order of paraffins > naphthenes > iso-paraffins > aromatics > olefins, with monoolefins being more stable than di-olefins.1,2 Polymerization of unsaturated hydrocarbons, for example, those present in Fischer-Tropsch (FT) Synthetic Crude (syncrude) and reactiveoxygen-containing compounds in fuels, are all possibly responsible for fuel instability, forming materials of higher molecular weight and boiling points than the original fuel. Further hydroprocessing of FT syncrude is therefore required. The rate of olefin saturation and oxygen removal during hydrogenation is greatly affected by the chemical composition of the catalyst used,3 the type of reactants, and the operating conditions. A generally accepted rate law for gas-phase hydrogenation reactions is given in eq 1:
υ)
kKHCPHC (1 + KHCPHC +
∑i KiPi)
f(PH2)
(1)
where i represent the inhibiting compounds, Pi the partial pressure, Ki the adsorption constant of the inhibiting compounds, k the rate constant, and f a complex function of the hydrogen partial pressure (PH2).4 Lee and Ollis5 reported a Langmuir-Hinshelwood first-order kinetic model for the hydrogenation and hydrodeoxygenation (HDO) of benzofuran and o-ethylphenol with activation energies (Ea) being 138 and 71 kJ mol-1, respectively. First-order rate * Tel: +27 16 960-4285. Fax: +27 11 522-1009. E-mail:
[email protected]. (1) Hilden, D. L. The Relationship of Gasoline Diolefin Content to Deposits in Multiport Fuel Injectors; SAE Technical Paper Series, No. 881642, 1988. (2) Kim, C.; Tseregounis, S. I.; Scruggs, B. E. Deposit Formation on a Metal Surface in Oxidised Gasolines; SAE Technical Paper Series, No. 872112, 1987. (3) Furimsky, E. Catalytic deoxygenation of heavy gas oil. Fuel 1978, 57, 494. (4) Leprince, P. Petroleum Refining, 3, ConVersion Processes; Institut Franc¸ ais du Pe´trole Publications: Technip, 2001. (5) Lee, C-L.; Ollis, D. F. Catalytic hydrodeoxygenation of benzofuran and o-ethylphenol. J. Catal. 1984, 87, 325-331.
equations were also used by Rollmann6 and others7,8 to calculate the HDO rate constants of model compounds over sulfided CoMo catalysts where the gas/liquid ratio was high enough to ensure complete vaporization. Several authors9-11 have found aromatic saturation to be first-order when experiments were carried out in the vapor phase. Galiasso9 found the reaction order of olefin saturation to be 11/2 order. With respect to oxygencontaining compounds, Gevert et al.12 concluded that the hydrogenation of dimethyl-substituted phenols occurs via two pathways: ( 1) the direct hydrogenolysis of the C-O bond at an electron-donating site in which the reactant is adsorbed “vertically” through the heteroatom, and (2) the hydrogenation of the phenol ring at an electron-withdrawing site in which the reactant is adsorbed “flatwise” through the π-system. Rollmann further demonstrated that the HDO rates are affected by H2 pressure, with an increase in rate constants with an increase in H2 pressure. La Vopa and Satterfield8 also demonstrated that the rate of HDO is directly proportional to the hydrogen partial pressure. The objective of this study was to derive a kinetic model for the hydrogenation of FT syncrude to predict process conditions necessary to meet mandatory fuel specifications and to compare the activity of the hydrogenation catalysts on the basis of the kinetic parameters. Experimental Materials and Feed. NiMo/Al2O3, CoMo/Al2O3, and Ni/Al2O3 catalysts used in the study are commercially available catalysts (6) Rollmann, L. D. Catalytic hydrogenation of model nitrogen, sulfur, and oxygen compounds. J. Catal. 1977, 46, 243. (7) Ternan, M.; Brown, J. R. Hydrotreating a distillate liquid derived from subbituminous coal using a sulphided CoO-MoO3-Al2O3 catalyst. Fuel 1982, 61, 11. (8) La Vopa, V.; Satterfield, C. N. Catalytic hydrodeoxygenation of dibenzofuran. Energy Fuels 1987, 1, 323-331. (9) Galiasso, R.; Garcia, W.; Ramirez de Agudelo, M. M.; Andreu, P. Hydrotreatment of cracked light gas oil. Catal. ReV.sSci. Eng. 1984, 26, 445-480. (10) Gu¨ltekin, G.; Ali, S. A.; Satterfield, C. N. Ind. Eng. Chem. Process Des. DeV. 1984, 23, 179-181. (11) Wilson, M. F.; Fischer, I. P.; Kriz, J. F. Hydrogenation of aromatic compounds in synthetic crude distillates catalysed by sulphided Ni-W/γAl2O3. J. Catal. 1985, 95, 155-166. (12) Gevert, S. B.; Eriksson, M.; Eriksson, P.; Massoth, F. E. Appl. Catal., A: Gen. 1994, 117, 151-162.
10.1021/ef060612q CCC: $37.00 © 2007 American Chemical Society Published on Web 08/31/2007
2510 Energy & Fuels, Vol. 21, No. 5, 2007
Lamprecht
Table 1. Catalyst Characteristics catalysts characteristics
NiMo/Al2O3
CoMo/Al2O3
Ni/Al2O3
nominal diameter (mm) extrudate shape surface area (m2/g) MoO3 (mass-%) NiO (mass-%) CoO (mass-%) Ni (mass-%)
1.1 quadralobe 138 19.5 4.0
1.3 cylindrical 265 16.0
3.5 spherical 58
5.0 10
supplied by Akzo Nobel (now Albemarle) and IFP. The characteristics of these catalysts are given in Table 1. The NiMo/Al2O3 used has a quadralobe shape with a diameter of 1.1 mm, whereas the CoMo/Al2O3 has a cylindrical shape. The Ni/Al2O3 catalyst is spherical with an average diameter of 3.5 mm. The NiMo/Al2O3 and CoMo/Al2O3 catalysts were presulfided in situ. A H2S level of 1300 ppm was maintained in the tailgas throughout the kinetic study. The reduction of the Ni/Al2O3 catalysts took place in situ at a pressure of 14 bar and a temperature of 400 °C. The feed materials were comprised of high-temperature FT (HTFT) Synthol Light Oil (SLO) as well as a blend of 60 mass-% HTFT SLO with 40 mass-% low-temperature Fischer-Tropsch (LTFT) Arge diesel feed. Selected fuel properties of the HTFT SLO feed and the blend with LTFT Arge diesel feed are shown in Table 2. The HTFT Synthol fluidized process operates at higher temperatures (320-340 °C) than the low-temperature Fischer-Tropsch Arge fixed bed process which operates at reaction temperatures of 220-240 °C. The Synthol products are more branched and olefinic and contain aromatic compounds, whereas the Arge fixed bed products are mostly long-chain paraffins containing no aromatics and thereby giving it excellent diesel fuel properties such as a high cetane number. Equipment and Reaction Procedure. The hydrogenation was carried out in a plug-flow reactor system with the feed and hydrogen flows from top to bottom. The internal diameter of the reactor was 28 mm, and the catalyst bed length was 500 mm. The reactor was loaded with 290 mL of catalyst. Since the comparative catalyst activity results were for commercial application use, the catalysts were not crushed. The voids between the catalysts were filled with 0.15-0.3 mm sand to increase the relative velocity and to improve feed distribution. A layer of inert packing above the catalyst bed was utilized to preheat the feed to the reaction temperature. Five thermocouples were spaced along the catalyst bed with another thermocouple in the preheating section. The hydrogenation reaction conditions of Synthol Distillate Hydrotreater (DHT) diesel on a commercial scale is shown in Table 3. A pressure of 25 bar was used in the once-through pilot plant operation during the hydrogenation of the HTFT SLO/LTFT Arge feed blend. The reaction process conditions for the HTFT SLO were similar to those shown in Table 3 for the commercial unit. To keep the reaction temperature in line with the commercial unit at the Sasol Synfuels refinery in Secunda, South Africa, a reaction temperature of 240 °C was chosen for the HTFT SLO/LTFT Arge blend. With a 30 °C rise over the catalyst bed in the Synthol DHT unit, a reaction temperature of 270 °C was also evaluated. The pilot plant was operated isothermally. The change in bromine number of the hydrogenated HTFT SLO/ LTFT Arge blend product with the use of CoMo/Al2O3, NiMo/ Al2O3, and Ni/Al2O3 catalysts was monitored daily. The product was fractionated into a petrol fraction ( Ni/Al2O3 > CoMo/Al2O3, whereas the order of activity for the acid removal is CoMo/Al2O3 ≈ NiMo/ Al2O3 . Ni/Al2O3. The oxidation stability of the Fischer-Tropsch diesel conformed to the required diesel fuel specification of 2 mg/L irrespective of what hydrogenation catalyst was used. Catalyst Deactivation. Deactivation of a sulfided CoMo/ Al2O3 catalyst was observed during hydrogenation of oxygencontaining feeds at low H2S content in tailgas (see Figure 4). Deactivation may be due to the replacement of the catalytic sulfur with oxygen.17 Furinsky18 suggested that in the absence of a sufficient amount of S-donating species, the replacement of S by O may occur on the catalyst surface with the hydroxide anion (OH-) being a less efficient donor than the SH- anion. The effect of increasing the H2S content with the addition of tert-butyl-disulfide in the tailgas on the activity and the rate of deactivation of the CoMo/Al2O3 catalyst was evaluated using the derived 11/2-order reaction rate equation. H2S levels of 320, 650, 970, and 1300 ppm in the tailgas were investigated. The change in bromine number with change in the H2S content in the tailgas is shown in Table 9 (run 1). The effect of increasing the H2S content in the tailgas on the deactivation of the CoMo/ Al2O3 catalyst again is shown in Figure 4. The operating temperatures required to attain a product with a bromine number of 7 g Br/100 g at a space velocity of 1 h-1 were calculated from the derived 11/2-order reaction rate equation (see eq 5) and is shown in Figure 5 where an increase in the H2S level from 320 to 650 ppm resulted in a 10 °C decrease in the required operating temperature. The effect of a further increase in the H2S level on the required operating temperature was marginal (see Figure 5).
{
LHSVr
1 Tnormalized
)
1 R - ln T Ea
(x (
1
-
1
}
) )
Cr xC0 1 1 LHSV xC xC0
(5)
where Tnormalized is defined as the reactor temperature at which the product would have had a bromine number of 7 g Br/100 g at a space velocity of 1 h-1 as simulated by using the kinetic model and parameters derived, Cr is the required bromine (17) Viljava, T. R.; Komulainen, R. S.; Krause, A. O. I. Effect of H2S on the stability of CoMo/Al2O3 catalyst during hydrodeoxygenation. Catal. Today 2000, 60, 83-92. (18) Furinsky, E. Chemistry of catalytic hydrodeoxygenation. Catal. ReV.sSci. Eng. 1983, 25, 421-458.
Hydrogenation of Fischer-Tropsch Synthetic Crude
Energy & Fuels, Vol. 21, No. 5, 2007 2513
Table 9. Selected Fuel Properties of the HTFT SLO Product Streams Obtained with Change in H2S Content in the Tailgas Using the CoMo/ Al2O3 Catalyst (run 1)a composite properties time on line (days) 1 2 3 4 5 6 7 8 9 10 11 12 13 14
T (°C)
normalized T (°C) for 11/2 order
LHSV (h-1)
H2S in tailgas (ppm)
daily Br no. (g Br/100 g)
297 298 302 305 303 306 306 305 310 315 316 316 316 316
275 278 284 297 296 301 291 288 291 291 291 290 289 289
1.18
320
1.2
650
1.2
970
1.2
1300
3.1 3.4 3.9 6.1 6.3 6.9 4.5 4.1 3.9 3.1 2.9 2.8 2.7 2.7
Br no. (g Br/100 g)
acidity (mg KOH/g)
5
0.02
4.2
0.02
2.9
0.02
2.7
0.02
a The normalized temperature is defined as the reactor temperature at which the product would have had a bromine number of 7 g Br/100 g at a space velocity of 1 h-1; P ) 58.4 bar.
Figure 5. Change in the normalized operating temperatures with change in tailgas H2S content observed during hydrogenation of HTFT SLO using the CoMo/Al2O3 catalyst. The normalized temperature is defined as the reactor temperature at which the product would have had a bromine number of 7 g Br/100 g at a space velocity of 1 h-1. P ) 58.4 bar.
Figure 6. Difference in the normalized operating temperatures observed during hydrogenation of HTFT SLO using the CoMo/Al2O3 catalyst with a change in the start of run H2S levels. The normalized temperature is defined as the reactor temperature at which the product would have had a bromine number of 7 g Br/100 g at a space velocity of 1 h-1. P ) 58.4 bar.
number of 7 g Br/100 g, and LHSVr is the required liquid hourly space velocity of 1 h-1. Results also showed that a H2S level of 1300 ppm during the start of the run (see run 3 in Figure 6) strongly increases the catalyst activity with respect to the run with an initial H2S content of 320 ppm in the tailgas (see run 2 in Figure 6). The CoMo/Al2O3 catalyst further does not attain the same activity during the start of run when the tailgas has a H2S content of 320 ppm compared to a tailgas H2S content of 1300 ppm (see Figure 6). Increase in the H2S content in the tailgas during hydrogenation therefore not only reduces the operating temperature but also improves the catalyst activity.
NiMo/Al2O3 showed the highest activity for hydrogenation of FT syncrude. While the apparent order of hydrogenation activity is NiMo/Al2O3 > Ni/Al2O3 > CoMo/Al2O3, the order of activity for the acid removal is CoMo/Al2O3 ≈ NiMo/Al2O3 . Ni/Al2O3. It was therefore concluded that the Ni/Al2O3 catalyst is not suited for the hydrogenation of HTFT SLO under the process conditions tested. Also concluded from the study was that an increase in the H2S content in the tailgas during hydrogenation not only increases catalytic activity for feeds containing oxygenated compounds, but also increases the catalyst life. The oxidation stability of the Fischer-Tropsch diesel conformed to the required diesel fuel specification of 2 mg/L, irrespective of what hydrogenation catalyst was used.
Conclusion While a first-order rate is normally found for the hydrogenation of light narrow cuts with respect to olefin saturation, we have shown that a reaction order greater than one is found for the heavier FT syncrude cuts.
Acknowledgment. The author acknowledges Z. A. Brodziak, H. Kok, and A. Pienaar for their contributions to the paper. EF060612Q
