Energy & Fuels 2001, 15, 1213-1219
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Effect of Crude Oil Properties on the Hydrodesulfurization of Middle Distillates over NiMo and CoMo Catalysts G. Marroquı´n-Sa´nchez,† J. Ancheyta-Jua´rez,*,†,‡ A. Ramı´rez-Zu´n˜iga,† and E. Farfa´n-Torres† Instituto Mexicano del Petro´ leo, Eje Central La´ zaro Ca´ rdenas 152, Me´ xico 07730 D.F., Mexico, and Instituto Polite´ cnico Nacional, ESIQIE, Me´ xico 07738 D.F., Mexico Received March 6, 2001. Revised Manuscript Received June 28, 2001
The effect of crude oil properties on hydrodesulfurized diesel quality was studied in a hydrotreating fixed-bed pilot plant. Three HDT feeds were prepared by blending the straightrun middle distillates obtained by TBP fractionation of crude oils in the following volumetric ratios: 60% straight-run gas oil, 20% kerosene, and 20% jet fuel. Pilot plant experiments were conducted at constant reaction pressure (54 kg/cm2) and hydrogen-to-oil ratio (2000 ft3/bbl). The effect of reaction temperature and liquid hourly space velocity were studied in the range of 340360 °C and 1.5-2.0 h-1, respectively, over two commercial catalysts (NiMo and CoMo). The effect of crude oil properties on product quality is analyzed in terms of hydrodesulfurization, hydrodenitrogenation, and aromatics removal.
1. Introduction The trends of specifications for diesel quality are clear. The well-known 0.05 wt % sulfur content was quickly adopted as a worldwide standard by year 1995, and further tightening of specifications is proposed for a number of markets throughout the world (years 20032005), which mainly includes changes in sulfur, cetane number, API gravity, aromatics, nitrogen, and ASTM distillation. The primary target in the near future will be diesel sulfur reduction. The production of this new product, referred to as ultralow sulfur diesel (ULSD), presents significantly new challenges. HDT technologies applied for reducing sulfur in diesel to 500 wppm will not adequately address future needs. The feasibility of revamping an existing unit to produce ULSD will depend on the original design and operating conditions.1 The ULSD specifications will emphasize the need for flexible technology solutions that can adapt with the changing regulations. There are essentially three basic approaches to produce ULSD in traditional HDT plants, either singularly or in combination: (1) increasing operating temperature, (2) expanding the unit to increase the catalyst volume, and (3) using the highest activity catalysts. These options were extensively discussed in some papers presented in the NPRA 2000 annual meeting.1,2 It should be mentioned that diesel is made up of various streams. Common streams include mainly * To whom correspondence should be addressed. Fax: (+52-5) 3338429. E-mail:
[email protected]. † Instituto Mexicano del Petro ´ leo. ‡ Instituto Polite ´ cnico Nacional. (1) Bjorklund, B. L.; Howard, N.; Heckel, T.; Lindsay, D.; Piasecki, D. Paper Presented at the NPRA 2000 Annual Meeting, March 2628, 2000, San Antonio, TX.
straight-run gas oil (SRGO) and light cycle oil (LCO). SRGO is an easy-to-desulfurize feed, while LCO is difficult to process.3 As the end point of these streams increases, the complexity of the heteroatoms also increases, because heavier fractions contain not only higher total sulfur levels, but also higher quantities of those molecules that are most difficult to desulfurize, i.e., 4-methyl DBT and 4,6-dimethyl DBT. Tailoring diesel feed boiling range has been reported to be an effective means of controlling the difficulty of diesel HDS.4 On the contrary, the blend in high proportion of high heteroatoms content feedstocks, such as LCO, with SRGO, will exhibit a poor HDS reactivity.3 Therefore, it is clear that improving feed quality can have a major effect on diesel product quality. In a previous work we have reported an experimental study about the hydrodesulfurization of different middle distillate blends.5 The streams used in that work for preparing HDT feeds were SRGO, kerosene, and jet fuel. It was shown that the optimization of the blend of these streams may be a good alternative for achieving highquality diesel production. However, the quality of these straight-run distillates to be incorporated in the blend to the HDT feedstocks depends mainly on the quality of the crude oil from which they were obtained. This means that improving (2) Shinflett, W.; DicCamillo, D.; Remans, T. Paper Presented at the NPRA 2000 Annual Meeting, March 26-28, 2000, San Antonio, TX. (3) Ancheyta, J.; Aguilar, E.; Salazar, D.; Betancourt, G.; Leiva, M. Appl. Catal. A 1999, 180, 195-205. (4) Hunter, M.; Gentry, A.; Lee, S.; Lucas, C.; Oliver, D. Pappal, Paper Presented at the NPRA 2000 Annual Meeting, March 26-28, 2000, San Antonio, TX. (5) Marroquı´n, G.; Ancheyta, J. App. Catal. A 2001, 207, 407-420.
10.1021/ef010054+ CCC: $20.00 © 2001 American Chemical Society Published on Web 08/18/2001
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Energy & Fuels, Vol. 15, No. 5, 2001
Marroquı´n-Sa´ nchez et al.
Figure 1. TBP Distillation curves for different crude oil types: (C-1) crude oil 1, (C-2) crude oil 2, (C-3) crude oil 3. Table 1. Crude Oil Properties properties
crude 1
crude 2
crude 3
API gravity pour point, °C kinematic viscosity at 15 °C, cST characterization factor, Kuop metal contents, wppm Ni V
28.69 -39 132.3 11.83
29.42 -39 109.2 11.85
32.27 -27 74.9 11.96
23.5 127.0
22.0 111.0
9.4 43.5
crude oil selection can also improve operation and allow a deeper hydrodesulfurization. On the basis of these reasons, and taking into account that there is not very much information reported in the literature about the effect of HDS feed properties on diesel product quality, in this work we use three crude oils to prepare different HDT feedstocks, which were hydrotreated in a fixed-bed pilot reactor over NiMo and CoMo catalyst using different operating conditions. 2. Experimental Section 2.1. Crude Oils. The properties of the three crude oils used in this study are given in Table 1. Crude oils 1 and 2 are similar in composition because they were prepared by blending Maya and Isthmus Mexican crude oils in volumetric ratios of 70/30 and 80/20, respectively, which are the typical blends used in Mexican refineries as feedstock. Figure 1 shows the true boiling point (TBP) distillation curves for the three crude oils. It is seen that crude 3 is lighter than the other two crude oils. Volume recovered in crude 3 was 82.5%, while it was 74.0% and 76.3% for crudes 1 and 2, respectively. Crude oil 3 is of paraffinic origin as can be noticed by means of API gravity and characterization factor. This crude exhibits less heteroatom content compared to crudes 1 and 2 as can be observed in Figure 2. Levels of asphaltenes, and hence levels of nickel and vanadium, are very small in crude oil 3. 2.2. Preparation of HDT Feeds. 2.2.1. Straight-Run Middle Distillates. SRGO, kerosene, and jet fuel were obtained by TBP fractionation with each of the three crude oils used in this study, following the D-2892 ASTM method. The main properties of the three middle distillates are given in Table 2. This ASTM method is used for the distillation of crude oils to a final cut temperature of 400 °C, and it employs a fractionating column having an efficiency of 14-18 theoretical plates operated at a reflux of 5:1. Kerosenes and jet fuels obtained for the three crude oils do not exhibit aromatics with three rings, and straight-run gas
Figure 2. Chemical and physical properties as a function of API gravity for different crude oil types: (b) sulfur, (O) nitrogen, (9) asphaltenes, (0) Ramsbottom carbon. oils only have very small content of these aromatics ( crude oil 3, for both catalysts. It means that HDT feedstock prepared with middle distillates obtained from crude oil 1 exhibits better levels of HDS, which agrees with experimental observations. Similar to the empirical approach described in the previous section, a comparison between actual and predicted sulfur content in product (full symbols) is shown in Figure 8. Predictions are not as good as in the case of the empirical approach, which is mainly due to the assumption that all feedstocks and catalyst have the same reaction order. It was not possible to estimate (9) Ancheyta, J.; Betancourt, G.; Marroquı´n, G.; Pe´rez, A.; Maity, S. K.; Cortes, T.; del Rı´o, R. Energy Fuels 2001, 15, 120-127. (10) 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.
Hydrodesulfurization of Middle Distillates
reaction orders for each feed and catalyst because the amount of experimental data was not enough. 3.4.3. Application of Both Approaches. Equations 1 and 2 and constants reported in Tables 5 and 6 were used for determining the reaction temperature required for obtaining 200 wppm in product diesel. For instance, for the most common LHSV used in Mexican and worldwide refineries (LHSV ) 2.5 h-1) and using the CoMo and NiMo catalysts assuming that all other operating conditions are unchanged, feedstock prepared from crude oil 1 requires reaction temperatures of 347 and 364 °C using the empirical approach for CoMo and NiMo catalysts, respectively. It is noticed that NiMo catalyst requires higher temperature to obtain the same sulfur content than CoMo catalyst. To obtain the same sulfur content (200 wppm) at 2.5 LHSV and using NiMo catalyst, the three feedstocks require the following reaction temperatures: 364, 370, and 372 °C using the empirical approach for crude oils 1, 2, and 3, respectively. Reaction temperatures predicted with the kinetic approach were about 8-10 °C higher than those calculated with the empirical approach. It means that the 0.87 wt % sulfur content of feedstock obtained from crude 3 has higher concentration of refractory compounds compared to the feed from crude 1, which has almost the same sulfur level (0.86 wt %), and an increase of about 8 °C is required to achieve the same sulfur concentration in hydrotreated product. It is important to mention that the temperature increase required for this product sulfur content would lead to extremely short cycle lengths.
Energy & Fuels, Vol. 15, No. 5, 2001 1219
With this analysis it is observed that in general for a given catalyst with feeds having the same sulfur concentration, those prepared with EBP higher than 354 °C, such as crude oil 3, require also a higher HDT reaction temperature at constant LHSV in order to obtain the same sulfur content in the product. 4. Conclusions The effect of crude oil properties on the hydrotreating of middle distillates has been studied in a fixed-bed pilot plant under typical operating conditions over two commercial catalysts (NiMo/γ-Al2O3 and CoMo/γ-Al2O3). The experimental results showed that sulfur content lower than 120 wppm in hydrotreated product can be achieved through single stage hydrotreating at the space velocity of 1.5 and the reaction temperature of 360 °C. When a paraffinic crude oil is used, it is possible to increase the amount of middle distillates for preparing the HDT feedstock keeping similar removal levels of heteroatoms compared to conventional crude oils. CoMo catalyst showed better hydrodesulfurization levels while NiMo catalyst presented better nitrogen removal and aromatics hydrogenation. The levels of conversion were different depending on the origin of the HDT feedstock. Acknowledgment. The authors thank Instituto Mexicano del Petro´leo for its financial support. EF010054+
