Energy Fuels 2011, 25, 773–781 Published on Web 01/14/2011
: DOI:10.1021/ef101327d
Improvement of the Middle Distillate Yields during Crude Oil Hydrotreatment in a Trickle-Bed Reactor Aysar T. Jarullah, Iqbal M. Mujtaba,* and Alastair S. Wood School of Engineering, Design and Technology, University of Bradford, Bradford BD7 1DP, United Kingdom Received September 29, 2010. Revised Manuscript Received December 23, 2010
The growing demand for high-quality middle distillates is increasing worldwide, whereas the demand for low-value oil products, such as heavy oils and residues, is decreasing. Thus, maximizing the production of more liquid distillates of very high quality is of immediate interest to refiners. At the same time, environmental legislation has led to more strict specifications of petroleum derivatives. Hydrotreatment (HDT) of crude oil is one of the most challenging tasks in the petroleum refining industry, which has not been reported widely in the literature. In this work, crude oil was hydrotreated upon a commercial cobaltmolybdenum on alumina (Co-Mo/γ-Al2O3) catalyst presulfided at specified conditions. Detailed pilotplant experiments were conducted in a continuous-flow isothermal trickle-bed reactor (TBR), and the main hydrotreating reactions were hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodeasphaltenization (HDAs), and hydrodemetallization (HDM), which includes hydrodevanadization (HDV) and hydrodenickelation (HDNi). The reaction temperature (T), the hydrogen pressure (P), and the liquid hourly space velocity (LHSV) were varied with certain ranges, with constant hydrogen to oil (H2/Oil) ratio. The effects of T, P, and LHSV on the conversion of sulfur, nitrogen, vanadium, nickel, and asphaltene were studied. The results showed that high T and P and low LHSV in HDS, HDN, HDV, HDNi, and HDAs of crude oil improve the sulfur (S), nitrogen (N), metals [vanadium (V) and nickel (Ni)], and asphaltene (Asph) conversion. The hydrotreated crude oil has been distilled into the following fractions: light naphtha (LN), heavy naphtha (HN), heavy kerosene (HK), light gas oil (LGO), and reduced crude residue (RCR), to compare the yield of these fractions produced by distillation after the HDT process to those produced by conventional methods (i.e., HDT of each fraction separately after the distillation). The yield of the middle distillate showed greater yield compared to the middle distillate produced by conventional methods. The properties of RCR produced using both methods are also discussed.
contaminants, mainly S, N, V, Ni, and Asph.3,4 In other words, HDT increases the quality and quantity of distillate fractions. The HDT route includes contact of the oil feedstocks with hydrogen at high reaction temperatures and pressures. The compounds that have high molecular weight in the oil feedstock will be cracked and saturated with H2 to yield distillate fractions with increasing hydrogen/carbon (H/C) ratio and decreasing impurities. This process is carried out using a highactivity hydrotreating catalyst, which (a) enhances the elimination of undesirable impurities (S, N, V, Ni, and Asph) by promoting hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodevanadization (HDV), hydrodenickelation (HDNi), and hydrodeasphaltenization (HDAs) reactions, respectively, (b) increases the conversion of high-molecularweight compounds into light compounds, (c) reinforces hydrogenation of the cracked compounds that lead to an increase in the H/C ratio of the products, and (d) reduces the coke formation.5 Through the removal of the contaminants during HDT reactions, some conversion in the boiling range of the feedstock also takes place because of the impurities containing molecules, which are cracked into lighter products. In addition, some mild hydrocracking of the oil feedstock can occur through the HDT process depending upon the severity of the
1. Introduction During recent years, the worldwide need for transportation fuel or middle distillates (such as car fuel, jet fuel, and diesel fuel) with fuel quality (defined in terms of the amount of impurities, such as sulfur, nitrogen, etc.) that satisfies environmental legislations is growing.1 This demand dictates the necessity for conversion capacity, which will be able to selectively produce oil fractions, especially middle distillates. Therefore, petroleum refining industries have made efforts to find solutions for processing greater quantities of heavy oils for increasing the production of transportation fuels.2 To meet the challenges associated with petroleum refining, a number of technologies have been developed to refine and improve heavy oils into more valuable transportation fuels, in addition to reduce the content of the impurities, such as sulfur (S), nitrogen (N), vanadium (V), nickel (Ni), and asphaltene (Asph). Among these technologies, the catalytic hydrotreatment (HDT) process has the potential to increase the yield of distillate cuts while simultaneously reducing the concentration of *To whom correspondence should be addressed. Fax: þ44-(0)1274235700. E-mail:
[email protected]. (1) Ho, T. C. Appl. Catal., A 2010, 378, 52–58. (2) Alvarez, A.; Ancheyta, J. Ind. Eng. Chem. Res. 2009, 48, 1228– 1236. (3) Valavarasu, G.; Bhaskar, M.; Balaraman, K. S. Pet. Sci. Technol. 2003, 21, 1185–1205. (4) Hossain, M. M.; Al-Saleh, M. A.; Shalabi, M. A.; Kimura, T.; Inui, T. Appl. Catal., A 2004, 278, 65–71. r 2011 American Chemical Society
(5) Marafi, A.; Hauser, A.; Stanislaus, A. Energy Fuels 2006, 20, 1145–1149.
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Table 1. Problems Associated with Impurities impact of sulfur
nitrogen
metal
asphaltene
(1) environmental pollution, (2) soil pollution with acid materials, (3) corrosion of pipes, machines, and equipment, (4) catalyst poisoning, (5) reducing the activity of tetraethyl lead (TEL) added to gasoline, and (6) increasing the oxidization of hydrocarbons
(1) poisoning and severely limiting the catalyst activity and (2) having toxic effects on the storage stability of petroleum products
(1) very bad impact on the hydrotreating efficiency, (2) rapid deactivation for the downstream catalyst, and (3) severe deterioration and corrosion of machines
(1) responsible for high density and viscosity of crude oils or heavy oils, (2) catalyst deactivation, and (3) fouling and coking
Figure 1. Difference between (A) the conventional method and (B) this study.
operation. All of these reactions will bring some conversion for the heavy fractions into valuable derivative cuts.6 The presence of S, N, V, Ni, and Asph in crude oil or oil fractions has harmful effects upon the quality of oil products. The main impacts of these impurities is summarized in Table 1,2,7-11 which clearly shows that the removal of these contaminants is very important in refinery processes.12 The process of crude oil hydrotreating is a new challenge and a new technology that has not been widely reported. Mainly, all hydrotreating processes are implemented upon oil fractions and not upon the full crude oil (i.e., after the separation of crude oil into its fractions, such as gasoline, kerosene, and light and heavy gas oils). Then, the HDT processes are applied for each cut separately. This means that a large part of the contaminants, mainly, S, N, V, Ni, and Asph, will be deposited at the bottom of the atmospheric and vacuum distillation columns. Besides, the HDT processing of each section separately is fairly easy. However, the crude oil hydrotreating process is regarded as a large and difficult challenge because crude oil contains
several compounds and complex structures, in addition to multiple phases and the presence of the asphaltenes that contain a large amount of sulfur and metals (which close the active sites of the catalyst). In this study, HDS, HDN, HDV, HDNi, and HDAs reactions (see further details in the Appendix) were carried out in a continuous flow isothermal trickle-bed reactor (TBR) using crude oil as a feedstock and commercial cobalt-molybdenum on alumina (Co-Mo/γ-Al2O3) as a catalyst for a range of reactor temperatures, hydrogen pressures, and liquid hourly space velocities (LHSVs), with a constant hydrogen/oil ratio. The crude oil hydrotreated at the best operating conditions was distilled into light naphtha (LN), heavy naphtha (HN), heavy kerosene (HK), light gas oil (LGO), and reduced crude residue (RCR) to compare the yield and properties of these derivatives to the same fractions produced by conventional methods. Figure 1 shows the difference between the conventional method [HDT of oil fractions separately after distillation (panel A)] and this study [HDT of full crude oil and then distillation (panel B)].
(6) Bej, S. K.; Dalai, A. K.; Adjaye, J. Energy Fuels 2001, 15, 1103– 1109. (7) Ali, L. H.; Abdul-Karim, E. The Oil, Origin, Composition and Technology; Al-Mosul University: Mosul, Iraq, 1986. (8) Kim, L. K.; Choi, K. S. Int. Chem. Eng. 1987, 27, 340–357. (9) Pereira, C. J.; Cheng, J. W.; Suarez, W. C. Ind. Eng. Chem. Process Des. Dev. 1990, 29, 520–521. (10) Gilbert, F. F.; Guy, K. D.; Valerie, V. Ind. Eng. Chem. Res. 1994, 33, 2975–2988. (11) Al-Humaidan, F. S. Modelling hydrocracking of atmospheric residue by discrete and continuous lumping. M.Sc. Thesis, Kuwait University, Kuwait City, Kuwait, 2004. (12) Al-Adwani, H. A. H.; Lababidi, H. M. S.; Alatiqi, I. M.; Al-Dafferi, F. S. Can. J. Chem. Eng. 2005, 83, 281–290.
2. Experimental Section 2.1. Materials. Iraqi crude oil was used as a feed for the hydrotreating process. The main properties of the feedstock used in this work are shown in Table 2. The catalyst used for the HDT processes was commercial Co-Mo/γ-Al2O3. The catalyst used was an extrudate with a cylindrical shape and an equivalent diameter of 1.8 mm. The properties of the catalyst are listed in Table 3. 2.2. Equipment and Procedure. 2.2.1. HDT Pilot Plant. Pilot plants play an important role in petroleum refining industries. 774
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reactor is divided into three parts. The first part, with a length of 20 cm, was packed with inert particles (glass beads of 4 mm in diameter). This entrance section was used for heating the mixture to the required temperature, to ensure homogeneous flow distribution of gas and liquid and to avoid end effects. The second section with a length of 27.8 cm contained a packing of 60.3 g of catalyst. The bottom section was also packed with inert particles with a length of 17.2 cm to serve as a disengaging section. The reactor was operated in isothermal mode by an independent temperature control of five zone electric furnaces that provide an isothermal temperature along the active reactor section. The product section includes a low- and high-pressure gasliquid separator and product storage tank. The reactor outlet feeds the high-pressure separator, where the liquid and gas are separated. Finally, in the gas section, the gas exiting is passed through a gas flow meter before being released. 2.2.2. Experimental Runs. The main hydrotreating reactions considered in this work are HDS, HDN, HDAs, and HDM, which includes HDV and HDNi. These reactions have been carried out in a continuous-flow TBR using crude oil as a feedstock and cobalt-molybdenum as a catalyst. In this part, a number of experimental procedures will be covered, such as catalyst presulfiding and operating conditions. Before any run is started, a leak test must be carried out. This test is performed with nitrogen (N2) at 130 bar for 12 h. Once the leak test is passed, the run will start with catalyst presulfiding. 2.2.2.1. Catalyst Presulfiding. Catalyst presulfiding has been widely practiced in the petroleum refining industry. Its positive effect has been significantly observed in the HDT processes, where it creates the essential surface required for optimum activity by transforming the form of active sites from metal oxide to metal sulfide. The sulfiding pretreatment optimizes the catalyst performance in two different aspects. First, it improves the catalyst activity by permitting deeper diffusion of feed into the catalyst pellets. Better access of large molecules into the internal pores enhances the conversion rate and the distribution of metal deposits. The second effect of presulfiding is the reduction of the extent of early deactivation caused by coke deposition.17 It has been observed that passivation of the highly active acidic oxide site reduces the coke formation tendency by presulfiding practice. The coke deposition mechanism of the presulfided catalyst allowed it to have a substantially higher surface area than the unsulfided catalyst.18,19 The catalyst presulfiding process is carried out using either the dry or wet method. In the dry method, hydrogen sulfide (H2S) is injected with the hydrogen stream to sulfide the catalyst. On the contrary, the wet method is achieved by pumping gas oil that is spiked with carbon disulfide (CS2) or dimethyl disulfide (DMDS) and is frequently used in industries. Therefore, in this work, the wet presulfiding method was used. A total of 90 cm3 of the fresh catalyst was charged to the hydrotreating reactor after drying at 120 °C for 2 h, where the presulfiding process starts by increasing the reactor temperature to 120 °C. At this temperature, pumping of the presulfiding feed to the reactor starts. Catalyst presulfiding is accomplished by a solution of 0.6 vol % of CS2 in commercial gas oil. After 2 h, the reactor temperature gradually increases to 210 °C and remains there for 4 h at 100 bar with a liquid hourly space velocity (LHSV) of 2.0 h-1 and without hydrogen flow. Then, the reactor temperature is ramped to 300 °C. The next step in the catalyst activation takes 16 h with
Table 2. Feedstock Properties specific gravity at 15.6 °C American Petroleum Institute (API) gravity viscosity at 37.8 °C pour point sulfur content Conradson carbon residue (CCR) vanadium content nickel content nitrogen content asphaltene content ash content
deg cSt °C wt % wt % ppm ppm wt % wt % wt %
0.8558 33.842 37 5.7 -36 2.0 5.1 26.5 17 0.1 1.2 0.008
Table 3. Commercial Catalyst Properties (Co-Mo/γ-Al2O3) MoO3 NiO SiO2 Na2O Fe SO2 Al2O3
Chemical Specification wt % wt % wt % wt % wt % wt %
15 3 1.1 0.07 0.04 2 balance
Physical Specification form surface area pore volume bulk density mean particle diameter mean particle length
m2/g cm3/g g/cm3 mm mm
extrude 180 0.5 0.67 1.8 4
They are particularly used in the assessment of the performance of the catalytic process, providing valuable information, which can be employed as a predictive tool for industrial performance. The process flow diagram of the hydrotreating pilot plant is illustrated in Figure 2. TBRs have been widely used in petroleum refining industries for carrying out hydrotreating reactions, such as HDS, HDN, hydrodemetallization (HDM), and hydrocracking (HDC), of heavy oil feedstocks. TBRs have been the subject of many authors, and various investigators have discussed them in different reviews. They are frequently preferred because of their ease of control and flexibility of application to a wide range of feedstocks. There are three phases in these reactors: fixed bed of catalyst particles (solid phase), hydrogen (gas phase), and oil feedstock (liquid phase). The TBR process is marked by the simultaneous existence of gas and liquid through a bed of catalyst particles, where the reactions occur in a concurrent downflow mode.13-15 Also, see further descriptions on the TBR used in this work by Jarullah et al.16 Basically, the pilot plant consists of four sections: the feed section, the reactor section, the product section, and the gas section. The gas supply module provides the unit with high-pressure hydrogen required for hydrotreating reactions. The hydrogen is fed to the reactor through a heated high-pressure line. The unit was supplied with an electrical gas inlet flow sensor and hydrogen flow rate. The feed supply module primarily consists of a liquid feed tank and a feed pump. The feed tank is a cylindrical tank with a capacity of 2 L. The unit was supplied with a highpressure dosing pump to introduce the feed oil into the reactor. A calibrated micrometer was fitted in the pump to estimate the feedstock flow rate. The reactor section is the most critical zone in the unit. The feedstock and hydrogen pass through the reactor in a concurrent flow mode. The reactor tube was made of stainless steel with an inside diameter of 2 cm and a length of 65 cm. The length of the
(17) Absi-Halabi, M.; Stanislaus, A.; Qwaysi, F.; Khan, Z. H.; Diab, S. Stud. Surf. Sci. Catal. 1989, 53, 201–212. (18) Absi-Halabi, M.; Stanislaus, A.; Qamara, A.; Chopra, S. Stud. Surf. Sci. Catal. 1996, 100, 243–251. (19) Absi-Halabi, M.; Stanislaus A. Effect of process conditions and catalyst properties on catalyst deactivation in residue hydroprocessing. In Deactivation and Testing of Hydrocarbon-Processing Catalysts; American Chemical Society (ACS): Washington, D.C., 1996; ACS Symposium Series, Vol. 634, Chapter 17, pp 229-237.
(13) Kumar, V. R.; Balaraman, K. S.; Rao, V. S. R.; Ananth, M. S. Pet. Sci. Technol. 2001, 19, 1029–1038. (14) Bhaskar, M.; Valavarasu, G.; Sairam, B.; Balaraman, K. S.; Balu, K. Ind. Eng. Chem. Res. 2004, 43, 6654–6669. (15) Rodriguez, M. A.; Ancheyta, J. Energy Fuels 2004, 18, 789–794. (16) Jarullah, A. T.; Mujtaba, I. M.; Wood, A. S. Chem. Eng. Sci. 2011, DOI: 10.1016/j.ces.2010.11.016.
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Figure 2. General scheme of the hydrotreating pilot-plant unit.
Figure 3. Presulfiding procedure and conditions.
the following conditions: pressure of 100 bar, LHSV of 1 h-1, and hydrogen flow rate of 30 L/h. Finally, the feed is switched to crude oil. After 2 h, the reactor temperature is gradually increased to the reaction temperature, which is 335 °C. Figure 3 shows the presulfiding procedure and conditions. 2.2.2.2. Operating Conditions. The major effect of operational variables employed in HDT units and their influence on the reactor performance can be summarized as follows. To improve the S, N, V, Ni, and Asph conversion, three procedures can be chosen: (1) increase of the temperature, (2) decrease of the LHSV, and (3) increase of the pressure. However, there are many limitations that should be taken into account during the HDT process. An increase in the reactor temperature, especially above 410 °C, will lead to severe hydrocracking reactions and will give undesirable compounds in addition to a reduced catalyst life. A decrease in the LHSV will decrease the plant capacity; hence, new hydrotreating reactors will have to be added to keep the existing production capacity. Also, an increase in the hydrogen pressure above 10.13 MPa (100 atm) is not favorable; the partial
pressure does not increase, owing to existing physical constraints of maximum pressure.20-22 In this study, a number of experimental works for hydrotreating crude oil are carried out under the following operating conditions: reaction temperature of 335-400 °C, LHSV of 0.51.5 h-1, pressure of 4-10 MPa, and constant H2/oil ratio of 250. 2.2.3. Distillation Unit Pilot Plant. Figure 4 shows the laboratory distillation unit at atmospheric and vacuum pressure used for hydrotreated product fractionation. Note, the TBR used in this work is a pilot plant. Discrete operation is considered between the TBR and the distillation column; i.e., the distillation column comes into operation after the completion of the TBR operation and collection of feed for the distillation column. The hydrotreated feedstock is charged in a 5 L round-bottom flask, with an electric heating mantle at 1.2 kW. The heating mantle connected with a step-down transformer provides heat input adjustment. The temperature of the liquid crude oil in the flask was recorded using a thermocouple through a glass jacket (21) Jimenez, F.; Nunez, M.; Kafarov, V. Comput.-Aided Chem. Eng. 2005, 20, 619–624. (22) Jimenez, F.; Nunez, M.; Kafarov, V. Chem. Eng. J. 2007, 134, 200–208.
(20) Fogler, H. S. Elements of Chemical Reaction Engineering, 2nd ed.; Prentice Hall: Upper Saddle River, NJ, 1999.
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Figure 5. Experimental data variation of sulfur content versus LHSV at different T and P.
Figure 4. Laboratory distillation unit.
inside the distillation flask. A mercury thermometer located at the top of the distillation column records the boiling point of the distilled fraction. The 15 tray distillation column is 50 mm in diameter and 750 mm in length. The still head includes a highefficiency reflux condenser. The cooling medium was alcohol at a temperature as low as -29 °C to provide the necessary cooling in the early stage of distillation to reduce the loss of light hydrocarbons. The cooling medium was turned to ordinary tap water after the distillation temperature exceeded 20 °C. No distillates were collected before equilibrium was attained in the trays. A magnetic rod connected to the reflux timer was used to obtain the desired reflux ratio, where the reflux ratio was 3:5. The distillation unit was operated at atmospheric pressure until the distillation temperature reached 200 °C, and then the distillation unit system was connected to a vacuum system. The vacuum system consisted of a high-efficiency vacuum pump with highly tightened tube connections to provide a vacuum pressure as low as possible. The vacuum distillation unit was connected to the vacuum pump through a vapor trap, and the distillation continued using pressure at 1-0.1 mmHg.
Figure 6. Experimental data variation of sulfur conversion versus LHSV at different T and P.
parameters. Therefore, the impact of reaction parameters, which are process temperature, LHSV, and reactor pressure, on the quality of hydrotreated product will be discussed. It has been noticed from experimental results that an increase in the reaction temperature and pressure and a decrease in LHSV resulted in improved product quality by reducing the content of S, N, V, Ni, and Asph in all products and increasing the removal of these impurities (shown in Figures 5-14). A similar behavior has also been observed by many studies for HDS, HDN, HDV, HDNi, and HDAs processes using different oily feedstocks (but not on the full crude oil).23-29
3. Results and Discussion 3.1. Effect of Operating Conditions on Impurities Removal. A primary role of hydrotreating processes is to improve the quality of the oil feedstock by removing the impurities. Crude oils are characterized by having many impurities that can exert substantial effect upon the properties of the finished products and the efficiencies of refining operations. The efficiencies of desulfurization, denitrogenation, demetallization, and deasphaltenization are measured by the degree of sulfur, nitrogen, metal, and asphaltene removals, respectively. The effectiveness of these HDT reactions is influenced by process
(23) Abbas, A. S. Low sulfur feedstock from Basrah reduced crude oil for coke production. M.Sc. Thesis, University of Baghdad, Baghdad, Iraq, 1999. (24) Maria, A. C.; Martinez, M. T. Energy Fuels 1999, 13, 629–636. (25) Ancheyta, J.; Rivera, G. B.; Sanchez, G. M.; Arellano, A. M. P.; Maity, S. K.; Cortez, M. T.; Soto, R. R. Energy Fuels 2001, 15, 120–127. (26) Areff, H. A. The effect of operating conditions on vacuum gas oil hydrotreating on sulfur and aromatics content. M.Sc. Thesis, University of Tikrit, Tikrit, Iraq, 2001. (27) Bhaskar, M.; Valavarasu, G.; Meenakshisundaram, A.; Balaraman, K. S. Pet. Sci. Technol. 2002, 20, 251–268. (28) Trejo, F.; Ancheyta, J. Catal. Today 2005, 109, 99–103. (29) Alvarez, A.; Ancheyta, J. Appl. Catal., A 2008, 351, 148–158.
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Figure 9. Experimental data variation of vanadium content versus LHSV at different T and P.
Figure 7. Experimental data variation of nitrogen content versus LHSV at different T and P.
Figure 8. Experimental data variation of nitrogen conversion versus LHSV at different T and P.
Figure 10. Experimental data variation of vanadium conversion versus LHSV at different T and P.
The decrease in S, N, V, Ni, and Asph contents at high temperatures can be attributed to many reasons: at high reaction temperatures, the high effectiveness of thiophenic sulfur compounds found in the heavy fractions of crude oil7 and the unreactive nitrogen, asphaltene, and metal compounds or the compounds containing these impurities become activated enough to react with hydrogen. In addition, the large molecules are decomposed into smaller molecules that can diffuse more easily inside the catalyst pores and reach the inner active sites, where the HDT reactions occur. In addition, it increases oil diffusivity through the catalyst pores, owing to decreases in the oil viscosity. Also, the increase in the temperature raises the activation energy, leading to the increase in the number of molecules of the compounds interacted. As a result, the long compounds will cleave and diffuse within the catalyst. Furthermore, high temperatures increase the rate of proliferation and osmosis
in the pores of the catalyst on the active sites, where HDT reactions happen because of the low viscosity.23,30 As LHSV decreased, desulfurization, denitrogenation, demetallizaion, and deasphaltenization increased because of the contact time (residence time) increase between the molecules of reactants and catalyst, providing sufficient time for the reaction process.8,11,23,25 The reason for the increase in S, N, V, Ni, and Asph removals from the increase in hydrogen pressure is due to the contact between the hydrogen and hydrocarbons and the catalyst.25 3.2. Yield of Middle Distillates. As mentioned earlier, the market demand for middle distillates is increasing in comparison to that for heavy oils. Thus, it is important to increase the yield of middle distillates. Crude oil hydrotreating directly is a new technology and plays an important role in coping with these challenges. Figure 15 illustrates the comparison between the yield of middle fractions distilled from crude oil hydrotreated (full crude oil) and middle distillates produced by the conventional method (after the separation of crude oil into its
(30) Isoda, T.; Kusakabe, K.; Morooka, Sh.; Mochida, I. Energy Fuels 1998, 12, 493–502.
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Figure 14. Experimental data variation of asphaltene conversion versus LHSV at different T and P. Figure 11. Experimental data variation of nickel content versus LHSV at different T and P.
Figure 15. Comparison of the yield of oil fractions produced by the distillation process after the HDT process (present study) and conventional methods (HDT process of each fraction separately after the distillation process).
Table 4. Typical Process Conditions for Hydrotreating of Various Petroleum Fractions
Figure 12. Experimental data variation of nickel conversion versus LHSV at different T and P.
feedstock naphtha kerosene light gas oil vacuum gas oil reduced crude residue vacuum residue
temperature (K) pressure (atm) LHSV (h-1) 593 603 613 633 643-683 673-713
15-30 30-45 38-60 75-135 120-195 150-225
3-8 2-5 1.5-4 1-2 0.2-0.5 0.2-0.5
derivatives, such as naphtha, kerosene, and gas oil, the HDT processes are used for each fraction separately). The LN, HN, HK, and LGO fractions have been distilled from hydrotreated crude oil at the best operating conditions selected from experimental results that showed maximum conversion of S, N, V, Ni, and Asph, which are as follows: reaction temperature of 400 °C, LHSV of 0.5 h-1, and hydrogen pressure of 10 MPa. These fractions were distilled from the initial boiling point (IBP) to 90 °C, from 90 to 150 °C, from 150 to 230 °C, and from 230 to 350 °C for LN, HN, HK, and LGO, respectively. As can be seen from Figure 15, the yield of LN, HN, HK, and LGO increases via hydrotreating of full crude oil (before the distillation process), while the RCR percent is decreasing.
Figure 13. Experimental data variation of asphaltene content versus LHSV at different T and P.
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quantities of the impurities. Thus, the catalyst used will be deactivated rapidly because of plugging the active sites of the catalyst as a result of coke deposition, leading to the reduced efficiency of the HDT process.25
Table 5. Comparison of Properties of RCR Produced by Hydrotreated Crude Oil and Conventional Methods specifications
units
specific gravity at 15.6 °C API gravity viscosity at 50 °C pour point sulfur content vanadium content nickel content nitrogen content asphaltene content
deg cSt °C wt % ppm ppm wt % wt %
by conventional by crude oil method hydrotreated 0.9540 16.82 236.8 þ14 4.0 59.88 37.47 0.1763 5.9
0.9392 19.16 191.4 þ2 0.811 16.42 10.16 0.04122 0.91
4. Conclusions HDT of crude oil is one of the toughest and challenging tasks in the petroleum refining industry that has not been reported widely in the literature. Petroleum refining industries undergo continuous changes in the schemes of processing various crudes to achieve the market request for middle distillates with the desired properties, and the increase in middle distillate quantities can improve the refinery economics substantially as a result of transportation demand. HDT has the ability to increase the yield of distillate fractions and simultaneously reduce the contents of impurities (such as S, N, V, Ni, and Asph). The effect of different hydrotreating operation variables, such as reaction temperature, hydrogen pressure, and LHSV, on the quality of crude oil during HDT of crude oil has been investigated. The crude oil becomes more purified by removing the impurities (S, N, V, Ni, and Asph), with increases in the reaction temperature and pressure and decreases in the LHSV. To compare the yield of oil fractions produced by the distillation process after the HDT process and conventional methods, the hydrotreated crude oil has been distilled into LN, HN, HK, LGO, and RCR. The yield of middle distillate showed greater yield compared to the middle distillate produced by conventional methods and, consequently, a RCR yield decrease. It can also be noticed that the specifications of RCR produced by crude oil hydrotreating directly are better than the specifications of RCR produced by conventional processes. The contents of sulfur, nitrogen, metals, and asphaltene are much lower in comparison to the contents of RCR produced by conventional methods, which allows for the production of good fuel oils.
This increase in the percent of the yield fractions can be attributed to converting of heavy compounds and long molecules that are concentrated in heavy fractions (such as RCR) to light compounds as a result of hydrotreating of crude oil before the distillation process. In contrast, the conventional processes that are carried out for each fraction separately means that the heavy compounds and long molecules will be deposited at the bottom of the atmospheric and vacuum distillation column. Hydrotreating them using normal operations and conditions is difficult. Note, in conventional processes, the range of operating conditions for hydrotreating of different petroleum fractions is summarized in Table 4.31 3.3. Enhancement of the Properties of RCR during the Hydrotreating Process. After separation of crude oil hydrotreated to its fractions, the remaining part at the bottom of the distillation column is called RCR, which has the boiling range above 350 °C. RCR is mainly used as a feedstock to vacuum distillation units to produce base oils, as well as vacuum gas oil and lubricating oils under certain conditions. Also, it is used as a component of fuel oil used in power plants, fuel for furnaces, and a component of diesel oil.32 During hydrotreating of full crude oil directly with hydrogen at reaction temperature of 400 °C, LHSV of 0.5 h-1, and hydrogen pressure of 10 MPa, it can be observed that the properties of RCR produced via hydrotreating of crude oil directly are better than the properties of RCR produced by conventional processes. The sulfur, nitrogen, metal, and asphaltene contents are much lower compared to these contents of RCR produced by conventional methods, thus producing a good fuel oil, as shown in Table 5. It is also noted that the viscosity and density of RCR are less than those found in RCR produced by conventional methods, which makes flow easy at low temperatures and gives an indication to increase the light cuts. This decrease in the viscosity and density because of saturation of olefins and aromatics during the hydrotreating process and sufficient time for the saturation process convert a big part of them to saturated compounds, such as paraffin and cycloparaffin, that have low viscosity and density. In addition, a part of these compounds is converted to light compounds because of the cracking of the bonds for heavy compounds, which have a long chain and high density and viscosity.7,25,26,32 Furthermore, hydrotreating of RCR directly requires additional processes and hard operating conditions because they contain heavy and complex compounds, as well as large
The basic chemical concept of HDS and HDN processes is to convert sulfur and nitrogen compounds in the feedstock to desulfurized and denitrogenated hydrocarbon products and hydrogen sulfide (H2S) and ammonia (NH3), respectively.26,33 HDM reactions occur at the same time with HDS reactions. The HDM reaction is thermocatalytic and produces metallic sulfides, such as NiS and V3S4. The reactions lead to the deposition of sulfides on the active sites of the catalyst.34 The reactions of the HDAs process during hydrotreating are complex because of the presence of different physical and chemical processes. Most of the studies in HDAs have been focused on thermal decomposition, which includes many simultaneous reactions. Investigations on asphaltene conversion of several oil feedstocks have indicated that the large molecules during the hydrotreating process will be converted into smaller molecules and the hydrogenation of condensed polyaromatic rings that are found in asphaltene compounds are almost entirely suppressed.23,28 These reactions (HDS, HDN, HDM, and HDAs) are commonly performed in a
(31) Topsoe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating Catalysis Science and Technology; Springer-Verlag: New York, 1996. (32) Mahmood, Sh.; Abdul-Karim, R.; Hussein, E. M. Technology of Oil and Gas; Oil Training Institute: Baghdad, Iraq, 1990.
(33) Speight, J. G. The desulfurization of heavy oils and residues. Chemical Industries; Marcel Dekker, Inc.: New York, 2000. (34) Leprince, P. Petroleum Refining; Editions Technip: Paris, France, 2001; Vol. 3: Conversion Processes.
Appendix: Hydrotreating Reaction in General
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: DOI:10.1021/ef101327d
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three-phase reactor (gas-liquid-solid), which is called a TBR.15 The general hydrotreating reactions can be expressed as follows:26,34,35
HDN: R- NH2 þ H2 f RH þ NH3
HDS: R- S þ H2 f R þ H2 S
HDAs: R- Asph þ H2 f RH
HDM: R- M þ H2 f RH þ M
þ Asph- R ðsmaller hydrocarbonsÞ (35) Leyva, C.; Rana, M. S.; Trejo, F.; Ancheyta, J. Ind. Eng. Chem. Res. 2007, 46, 7448–7466.
where R represents the hydrocarbon molecule.
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