Energy & Fuels 2002, 16, 189-193
189
Changes in Apparent Reaction Order and Activation Energy in the Hydrodesulfurization of Real Feedstocks J. Ancheyta,*,†,‡ M. J. Angeles,†,‡ M. J. Macı´as,‡ G. Marroquı´n,† and R. Morales‡ Instituto Mexicano del Petro´ leo, Eje Central La´ zaro Ca´ rdenas 152, 07730 Me´ xico, D.F., Mexico, and Instituto Polite´ cnico Nacional, ESIQIE, 07738 Me´ xico D.F., Mexico Received July 26, 2001. Revised Manuscript Received October 13, 2001
In this work we present experimental information about the effect of feed properties on apparent reaction order and activation energy in the hydrodesulfurization of middle distillates. Experiments were carried out in a pilot reactor at the following operating conditions: reaction temperature of 340, 350, and 360 °C; liquid hourly space-velocity of 1.5 and 2.0 h-1; reaction pressure of 54 kg/cm2 and hydrogen-to-oil ratio of 2000 ft3/bbl. Apparent reaction orders were in the range of 1.70-1.98, while apparent activation energies varied in the range of 14-42 kcal/mol. Reaction orders showed an increase as molecular weight of feed was increased. Activation energies were similar for feedstocks with same reaction order.
1. Introduction Sulfur has to be removed from oil fractions for both technical and environmental reasons. For instance, the specifications of diesel fuel are being lowered worldwide more and more, e.g., from 500 wppm to 350 and then to 50 wppm by year 2005.1-3 Catalytic hydrodesulfurization (HDS) has been extensively used to meet this regulation for environmental protection; however, due to changes in feedstock, catalyst, and operating conditions to achieve this ultralow sulfur diesel content, existing HDS plants have started revamping or optimizing. These changes are necessary because it has been found that the use of conventional catalysts for deep HDS in traditional gas oil hydrotreaters would require severe operating conditions such as high temperature, low space-velocity, and high hydrogen partial pressure. Such severe processing conditions generally lead to rapid catalyst deactivation and shorter cycle lengths.4 The feasibility of revamping an existing unit will depend on the original design and operating conditions. Another alternative for increasing HDS product quality is to improve the quality of HDS feedstock. We have shown in previous works that the optimization of middle distillate blend used as HDS feed can be a good option for achieving high-quality diesel production.2,3 If current feed properties to HDS process are changed, some modifications in catalyst composition are required, and hence, more research has to be done to * Author to whom correspondence should be addressed. Fax: (+525) 333 8429. E-mail:
[email protected]. † Instituto Mexicano del Petro ´ leo. ‡ Instituto Polite ´ cnico Nacional, ESIQIE. (1) Froment, G. F.; Depauw, G. A.; Vanrysselberghe, V. Ind. Eng. Chem. Res. 1994, 33, 2975-2988. (2) Marroquı´n, G.; Ancheyta, J.; Farfa´n, E.; Ramı´rez, A. Energy Fuels 2001, 15, 1213-1219. (3) Marroquı´n, G.; Ancheyta, J. Appl. Catal. A 2001, 207, 407-420. (4) Andari, M. K.; Abu-Seedo, F.; Stanislaus, A.; Qabazard, H. M. Fuel 1996, 75, 1664.
find the optimal properties of HDS catalyst. In this sense kinetic expressions are very helpful for catalyst screening. Detailed kinetics of hydrodesulfurization of some model sulfur compounds as thiophene, benzothiophene, dibenzothiophene (DBT), and alkyl-substituted DBTs have been extensively studied by various researches, and were summarized by Girgis and Gates.5 However, when selecting and evaluating new catalysts, these kinetic expressions are overinformative and catalyst activity can be simply quantified by a reaction order and an activation energy, which can eventually be related to physical characteristics of the catalyst.6 For HDS of model sulfur compounds first-order kinetics is commonly used; however, for real feedstocks an nth order kinetics with respect to total sulfur concentration is usually applied. The value of n depends on the type and distribution of sulfur compounds in the oil fraction as well as on the catalyst employed.7 Figure 1 shows different first-order kinetic constant values for most of the sulfur compounds included in diesel fractions.8 The differences in HDS reactivity of the sulfur components can be clearly distinguished. As it is well-known, DBTs with 4-, 6-, or 4-6 alkyl positions are the most refractory compounds. In petroleum fractions as the average boiling point is increased, the concentration of sulfur compounds also increases as can be seen in Figure 2, and HDS reactivity is reduced. Of course this rule is not always followed, mainly when fractions come from different crude oils, since HDS reactivity depends on the type of sulfur compounds, especially on the most refractory ones. (5) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30, 2021. (6) Letourneur, D.; Bacaud, R.; Vrinat, M.; Schweich, D.; Pitault, I. Ind. Eng. Chem. Res. 1998, 37, 2662. (7) Tsamatsoulis, D.; Papayannakos, N. Chem. Eng. Sci. 1998, 53, 3449-3458. (8) Ma, X.; Sakanishi, K.; Mochina, I. Ind. Eng. Chem. Res. 1994, 33, 218.
10.1021/ef0101917 CCC: $22.00 © 2002 American Chemical Society Published on Web 12/18/2001
190
Energy & Fuels, Vol. 16, No. 1, 2002
Ancheyta et al. Table 1. Reaction Orders and Activation Energies for Different HDT Feedstocksa,b density, at 15 °C
sulfur, wt %
distillation range, °C
n
EA, kcal/mol
SRGO SRGO HSRGO SRGO-LCO VGO SRGO-LCO residue oil residue oil residue oil AR
0.861 0.843 0.862 0.879 0.907 0.909 0.910 0.950 1.007 0.995
1.31 1.32 1.33 1.78 2.14 2.44 3.45 3.72 5.30 5.86
213-368 188-345 142-390 209-369 243-514 199-370 NA 281-538 NA NA
1.57 1.53 1.65 1.63 2.09 1.78 2.0 2.0 2.5 2.0
20.3 NA NA 16.5 33.1 16.37 68.6 29.0 36.1 29.0
used oil residue oil SRGO residue oil Coker Gas Oil
0.900 0.969 NA 0.964 0.984
0.70 1.45 1.47 2.90 4.27
NA NA NA NA 196-515
1 1 1.65 1 1.5
19.6 24.0 25.0 18.3 33.0
feed
Figure 1. Reactivities of different sulfur compounds in diesel fraction.
a Refs 7,9-18. b NA: not available; SRGO: straight-run gas oil; HSRGO: heavy straight-run gas oil; LCO: light cycle oil; VGO: vacuum gas oil; AR: atmospheric residue.
Figure 2. Relationship between feed boiling point and sulfur content.
Experimental information about reaction orders and activation energies of hydrodesulfurization of real feedstocks are scarce in the literature. Table 1 summarizes some of these reports. Data are ordered according to sulfur content. Some authors use their experimental information to calculate reaction orders7,9-14 (first part of Table 1), and others prefer to assume the value of “n”15-18 (second part of Table 1). In the first case, an increase in reaction order in the range of 1.5-2.5 is practically observed as the sulfur content is also increased. Some data do not follow this trend, which may be due to differences in experimental conditions. (9) Ancheyta, J.; Aguilar, E.; Salazar, D.; Betancourt, G.; Leiva, M. Appl. Catal. A 1999, 180, 195-205. (10) Vradman, L.; Landau, M. V.; Herskowitz, M. Catal. Today 1999, 48, 41-48. (11) Callejas, M.; Martı´nez, M. T. Energy Fuels 1999, 13, 629-636. (12) Chen, Y. W.; Hsu, W. C.; Lin, C. S.; Kang, B. C.; Wu, S. T.; Leu, L. J.; Wu, J. C. Ind. Eng. Chem. Res. 1990, 29, 1830-1840. (13) Papayannakos, N.; Marangozis, J. Chem. Eng. Sci. 1984, 39, 1051-1061. (14) Scamangas, A.; Papayannakos, N.; Marangozis, J. Chem. Eng. Sci. 1982, 37, 1810-1812. (15) Skala, D.; Saban, M. D.; Orlovic, A. M.; Meyn, Y. W.; Severin, D. K.; Rahimian, I. G. H.; Marjanovic, M. V. Ind. Eng. Chem. Res. 1991, 30, 2059-2065. (16) Chang, J.; Liu, J.; Li, D. Catal. Today 1998, 43, 233-239. (17) Bej, S. K.; Dabral, R. P.; Gupta, P. C.; Mittal, K. K.; Sen, G. S.; Kapoor, V. K.; Dalar, A. K. Energy Fuels 2000, 14, 701-705. (18) Yoi, S. M. Aostra J. Res. 1989, 5, 211-224.
On the contrary, this tendency with respect to sulfur content in feed is not observed for activation energies. For different sulfur contents (3.72 and 5.68 wt %), two feeds were reported to have same reaction order and activation energy (n ) 2 and EA ) 29 kcal/mol), or for almost the same sulfur content (3.45 and 3.72 wt %), the other two feeds presented very different activation energies (68.6 and 29 kcal/mol) for the same reaction order (n ) 2). Therefore, it is not totally clear the changes in reaction order and activation energy when sulfur content in feed is varied. The aim of this work is to study the effect of oil fraction properties on apparent reaction order and activation energy using experimental information obtained at pilot plant scale with a CoMo/γ-Al2O3 commercial catalyst and feedstocks prepared from different crude oils. 2. Experimental Section Three crude oils were used in this study for obtaining middle distillates by means of TBP fractionation following the D-2892 ASTM Method. (Crude oil 1: 28.79°API, 2.31 wt % sulfur; Crude oil 2: 29.42°API, 2.16 wt % sulfur; Crude oil 3: 32.27°API, 1.47 wt % sulfur). These middle distillates (straightrun gas oil, kerosene and jet fuel) were employed to obtain nine HDT feedstocks. The HDT feedstocks were prepared by blending the middle distillates in different volumetric ratios. The physical and chemical properties of the nine HDT feeds are presented in Table 2. Feeds 1, 4 and 7 were obtained by blending straight-run gas oil, kerosene and jet fuel in the following volumetric ratios: 20, 30 and 50 vol %. Feeds 2, 5 and 8 utilized the same amount of these three streams (33.3 vol %), and feeds 3, 6 and 9 were prepared with 40, 40 and 20 vol %, respectively. The HDS catalyst used in the present study was a CoMo/ γ-Al2O3 commercial available sample and its physical and chemical properties were reported in a previous work.2 The catalysts was in-situ presulfided with a desulfurized naphtha contaminated with 0.6 wt % carbon disulfide at the following operating conditions: pressure of 54 kg/cm2, hydrogento-oil ratio of 2000 ft3/bbl, 230 °C temperature, weight hourly space-velocity (WHSV) of 3 h-1, during 13 h, to ensure complete catalyst presulfiding. HDS experiments were conducted under steady-state operation in a fixed-bed pilot plant. A detailed description of the
Hydrodesulfurization of Real Feedstocks
Energy & Fuels, Vol. 16, No. 1, 2002 191
Table 2. Properties of Feedstocksa crude oil
feed
SG 20/4 °C
S, wt %
MW
MeABP, °C
IBP/EBP, °C
1
1 2 3
0.8130 0.8213 0.8271
0.55 0.68 0.77
179.0 187.4 196.0
224.4 237.1 248.9
160/334 156/344 169/350
2
4 5 6
0.8133 0.8233 0.8290
0.58 0.71 0.81
176.4 189.3 198.1
221.0 239.0 251.9
136/343 141/351 154/353
3
7 8 9
0.8134 0.8247 0.8310
0.51 0.64 0.76
181.7 193.2 200.8
228.1 245.0 255.6
169/364 176/374 176/376
a SG: specific gravity; S: sulfur; MW: molecular weight; MeABP: mean average boiling point; IBP: initial boiling point; EBP: end boiling point.
Table 3. Apparent Kinetic Constant at 350 °C for Different Feed and for n ) 1.65 crude oil
feed
Kap
1
1 2 3
97.65 61.98 42.92
2
4 5 6
65.61 44.63 39.10
3
7 8 9
70.66 46.40 33.05
HDS pilot plant, the isothermal reactor, and experimental procedure were presented elsewhere.3,19 In all cases pure hydrogen was used in a once-through mode, at the following operating conditions: reaction temperature of 340, 350, and 360 °C, liquid hourly space-velocity (LHSV) of 1.5 and 2.0 h-1, reaction pressure of 54 kg/cm2, and hydrogen-to-oil ratio of 2000 ft3/bbl. Product samples were collected at 4-8 h intervals after allowing a 2 h stabilization period. Density, total sulfur, and distillation curve in feed and hydrodesulfurized products were determined by ASTM D-1298, D-4294, and D-86 methods, respectively. Molecular weight (MW) was calculated as a function of specific gravity (sg) and Mean Average Boiling Point (MeABP) with the following equation:
MW ) 204.38e0.012MeABP e-3.07sg MeABP0.118 sg1.88 (1)
3. Results and Discussion 3.1. Verification of Isothermality and Plug-Flow Reactor Behavior. The temperature profile along the reactor length was measured in each experiment by a movable axial thermocouple located inside the reactor in order to check that the pilot reactor was kept isothermal. It was observed that the greatest deviation from the desired temperature value found in these tests was (2 °C. The low-temperature differences inside the pilot reactor were achieved because of the use of catalyst bed dilution and the glass wool plugs, which improve oil distribution at low liquid mass velocities and homogeneity of reactor temperature.4 The percentage of liquid feed vaporization of the liquid feed was calculated with a commercial process simulator. Computation of this value was done with the composition of the mixture (SRGO and hydrogen) at the (19) Marroquı´n, G. Reduction of aromatics in diesel fuel by catalytic hydrotreating. MSc Thesis, ESIQIE-IPN, Me´xico, 1999 (in Spanish).
reaction operating conditions just before the reactants enter the pilot reactor. Results indicate that the vaporized fraction of the liquid feedstock is greater than 12% and it increases as the reaction temperature is increased. This means that the flow regime in our pilot reactor is trickle. Our pilot-scale reactor is about 25-50 times shorter than typical commercial HDT reactors. For these reasons, low liquid velocities were used in order to match the LHSV of commercial plants. In addition to these differences in reactor size, we employed commercial catalyst samples and real feedstocks for carrying out experiments; it means that the pilot reactor length-to-catalyst particle diameter ratio (L/dp) is very low as compared to commercial reactors, and hence, axial dispersion may be present. To avoid this back-mixing effects we utilized a catalyst bed diluted with an inert material, which is well-known to drastically reduce axial dispersion. Therefore, fluidodynamics is largely dictated by the packing of the inert particles, and axial dispersion can be neglected for liquid and gas phases at the conditions used in the present study. Some additional experiments were conducted with different amounts of catalyst, space-velocities, and reaction temperatures according to common techniques reported in the literature to verify the presence of mass transfer gradients.20 Results showed that experimental studies can be carried out in the pilot plant without external mass transfer limitations (interphase), and because of catalyst size, only internal gradients (intraphase) are present. 3.2. Experimental Results. Sulfur contents in hydrodesulfurized products for each feedstock are presented in Figure 3 as a function of reaction temperature and space-velocity. It can be observed that product quality showed a decrease in sulfur content as the temperature is increased and as LHSV is decreased. It is clear that products obtained from feeds 1, 4, and 7 (void circles) exhibit the lowest sulfur concentration. These three streams were prepared with a high proportion of jet fuel. Feedstocks 3, 6, and 9 have lower sulfur concentration than the others (Table 2); however, they presented highest end boiling point and molecular weight, and hence, their contents of more refractory compounds are also high. This is the main reason for having highest sulfur content in products in the hydrodesulfurization of these feeds (squares symbols in Figure 3). This observed behavior means that reaction kinetics will be drastically affected by feed properties, which will be discussed in the next section. At the most common space-velocity and start-of-run reaction temperature in HDT commercial plants (LHSV ) 2.0 h-1 and T ) 340 °C), all feeds give products with sulfur content less that 350 wppm, which is the worldwide proposed diesel sulfur specification soon. Higher reaction severity is necessary to achieve 50 wppm sulfur content (diesel specification for year 2005): for crude oil 1, at 350 and 360 °C temperature for both space-velocities (1.0 and 2.5 h-1); for crude oils 2 and 3, at temperature of 360 °C and LHSV of 1.0 h-1. (20) Perego, C.; Peratello, S. Catal. Today 1999, 52, 133.
192
Energy & Fuels, Vol. 16, No. 1, 2002
Ancheyta et al.
Figure 4. Reaction order for different feeds as a function of molecular weight. (O) Crude oil 1, (b) Crude oil 2, (0) Crude oil 3.
best values of n and kap: m
SSE )
Figure 3. Variation of sulfur content in product. (O) Feeds 1, 4, and 7, (b) Feeds 2, 5, and 8, (0) Feeds 3, 6, and 9.
As can be seen with these results, it is possible to prepare diesel with very low sulfur contents (350 or 50 wppm) in traditional HDT units by adjusting operating conditions (reaction temperature and space-velocity) and optimizing the blend of middle distillates used as feedstock. 3.3. Apparent Reaction Order. When catalysts having commercially applied size and shape are used in trickle-bed reactors, experimental information obtained may be not completely useful for kinetic calculations, since intraparticle and external mass resistance are often significant in this type of reactors.20 In our case, we have verified the isothermal operation of the pilot reactor and its plug-flow behavior as well as the absence of external mass gradients; however, internal gradients are still present. For this reason, only apparent reaction kinetics can be obtained. On the basis of these considerations, the following rate equation can be used for estimating apparent reaction orders:
Sp )
[
(1 - n)kap + Sf1-n LHSV
]
1/1-n
(2)
where Sp and Sf are the total concentrations of sulfur in product and feed, respectively, kap and n are the apparent rate constant and reaction order, respectively, and LHSV is the liquid hourly space-velocity. The minimization of the following objective function, based on the sum or square errors (SSE) between experimental sulfur product content (Spexp) and those cpalculated with eq 2 (Spcalc), was applied to find the
exp calc 2 (Spi - Spi ) ∑ i)1
(3)
This objective function was solved using the least squares criterion with a nonlinear regression procedure based on Marquardt’s algorithm. Figure 4 shows that in general as molecular weight of feed is increased, the apparent reaction order also increases, and almost linear tendency is followed. Reaction orders were found in the range of 1.70-1.98, which agrees very well with those reported in the literature for this type of feeds. The slopes of lines shown in Figure 4 are very similar for feedstocks obtained from crude oils 1 and 3, but they are lesser than that obtained from crude oil 2. It should be mentioned that even feeds prepared with middle distillates from the later crude oil have low molecular weight and they exhibit higher sulfur content than the former. It can also be observed that feeds with the same molecular weight have different reaction order, which were higher in feeds with higher boiling range. Boiling range is very related to sulfur content (Figure 2), for this reason the same behavior was found when reaction order was plotted against sulfur content in feed (Figure 5). The differences in reaction order for feeds containing either same sulfur content or molecular weight may be due to the different type and concentration of more refractory compounds. It suggests that as these compounds’ content is increased, reaction order also increases. Of course, the increase in reaction order depends on the origin of feedstock. If we consider a constant reaction order value for all feedstocks, for instance 1.65 as many researchers do, we can evaluate rate constants for analyzing differences in reactivities. These results are shown in Table 3. It is seen that the heavier the feed the less kinetic constant values. Feedstocks obtained from crude oil 1 exhibited higher rate constant values. It means that sulfur in these feeds is more reactive, which is logical, since they have lower
Hydrodesulfurization of Real Feedstocks
Energy & Fuels, Vol. 16, No. 1, 2002 193
Figure 5. Reaction order for different feeds as a function of sulfur content in feed. (O) Crude oil 1, (b) Crude oil 2, (0) Crude oil 3. Table 4. Apparent Reaction Orders and Activation Energies crude oil
feed
n
EA (kcal/mol)
1
1 2 3
1.70 1.72 1.76
32.20 41.82 22.02
2
4 5 6
1.76 1.89 1.96
22.31 26.19 16.34
3
7 8 9
1.90 1.95 1.98
23.43 14.11 25.53
end boiling points compared to other feeds, and consequently their content of refractory compounds is also low. 3.4. Apparent Activation Energy. Apparent activation energies for all feedstocks were calculated by plotting the inverse of absolute temperature against the logarithm of apparent kinetic constant (1/T vs ln kap) according to the Arrhenius equation. Correlation coefficients higher than 0.985 were obtained in the regression analysis of these data. Final results are presented in Table 4. These values agree very well with those reported in the literature, which were shown in Table 1. It is very clear that there is not a relationship between apparent reaction order and activation energies. For feeds from crude oils 1 and 2, activation energies increase and then decrease while reaction order always is increased. However, for crude oil 3, activation energy decreases and then increases. For identical reaction order (n ) 1.76), very similar activation energies were obtained for feeds 3 and 4, 22.02 and 22.31 kcal/mol, respectively. Same results are observed with feeds 5 and 7, and with feeds 6 and 8. This similarity in activation energy values does not depend on the origin of feeds, since for instance, streams
3 and 4, which have the same reaction orders and activation energies, were prepared from crude oils 1 and 2, respectively. This contradicts other results summarized in Table 1. It should be mentioned that values of activation energies and reaction orders reported in the literature were obtained by using different reaction conditions, type of catalysts, experimental apparatus, and procedure, and our results were determined at the same experimental conditions. This is the main reason for having different behavior. 3.5. Utility of the Results. As we have mentioned in the Introduction, the main objective of our work is to contribute to the understanding of the changes in reaction order and activation energy when sulfur content in HDT feed is varied. To do this we have used experimental information employing both catalyst and feedstocks recovered from a commercial unit. The results obtained by means of this type of study can be used for making decisions in catalyst screening, since kinetic parameters (reaction order and activation energy) can be employed for quantifying catalyst activity. This information can also be used during common operation of commercial hydrotreating plants, where changes in feed quality are typically observed. If activation energy and reaction order for the new feed are known, the required reaction temperature or spacevelocity for achieving certain sulfur content in product can be easily estimated with eq 2 and an Arrhenius model. Conclusions Experimental information about the hydrodesulfurization of different feedstocks was used for determining apparent reaction orders and activation energies. Experiments were conducted in a trickle-bed pilot reactor at identical operating conditions. Apparent reaction orders were found in the range of 1.70-1.98, and they showed an increase as molecular weight of feed was also increased. Feeds with either the same molecular weight or sulfur content presented different reaction order, which was attributed to different type and concentration of more refractory compounds, according to end boiling point of feeds. Feedstocks with same apparent reaction order showed almost identical activation energies. This finding contradicted some literature reports, which may be due to differences in experimental conditions. Acknowledgment. The authors thank Instituto Mexicano del Petro´leo for its financial support. M. J. Angeles and M. J. Macı´as thank also CONACyT for financial support. EF0101917
