Article pubs.acs.org/EF
Kinetics of Promotional Effects of Oil-Soluble Dispersed Metal (Mo, Co, and Fe) Catalysts on Slurry Phase Hydrocracking of Vacuum Gas Oil Emad A. S. Bdwi,† Syed A. Ali,‡ Mohammad R. Quddus,† Saad A. Al-Bogami,§ Shaikh A. Razzak,† and Mohammad M. Hossain*,†,‡ †
Department of Chemical Engineering, and ‡Center for Refining and Petrochemicals, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia § Research and Development Center, Saudi Aramco Oil Company, Dhahran 34466, Saudi Arabia ABSTRACT: This study deals with the promotional effects of dispersed cocatalysts on hydrocracking of vacuum gas oil (VGO). The influence of oil-soluble molybdenum-, iron-, and cobalt-based materials is investigated with and without the presence of a commercial first-stage W−Ni/Al2O3−SiO2 hydrocracking catalyst. The experiments are conducted in a batch autoclave reactor (at 8.5 MPa and 420 °C). The dispersed metal catalysts enhanced the hydrogenation activity and reduced coke formation. Cobalt- and molybdenum-based cocatalysts show lower coke formation than the Fe cocatalyst. An addition of 500 ppm of Co or Mo cocatalyst decreased the amount of coke to 0.9 wt % from 2.5 wt % observed during the thermal cracking. The dispersed catalyst together with the supported catalyst shows similar decrease in coke formation and enhanced the yield of naphtha. A 5lump kinetic model is developed based on the experimental data using dispersed and supported catalysts. The model incorporates coke formation and conversion of VGO to distillate, naphtha, and C1−C5 hydrocarbons. The VGO hydrocracking to distillate requires least activation energy (1.5 kcal/mol) as compared to the other competing reactions. On the basis of kinetic model results, it is concluded that VGO is most likely cracked to form distillate, followed by cracking of distillate, then distillate is cracked to form naphtha, and finally naphtha is cracked to gases.
1. INTRODUCTION The demand of lighter petroleum products is growing, and is expected to further increase in the coming years. On the contrary, the supplies of conventional light crude oils are depleting. In addition, the demand for low-value products like fuel oil, vacuum gas oil (VGO), and bitumen is decreasing due to environmental problems related to the combustion of these heavy oils.1 Total estimated reserve of bitumen and heavy oil is about 5.6 trillion barrels and this is significantly higher when compared with the remaining reserve of conventional crude oil of about 1 trillion barrels.2 Therefore, there is a growing need of efficient technologies that can process heavy oils and produce lighter products with minimum environmental impact. In this regard, hydrocracking is one of the most commonly used processes for heavy oil upgrading. Two well-known technologies are available for hydrocracking of heavy oil, i.e., (i) fixed-bed process and (ii) slurry process. The slurry process can handle heavy oils with high levels impurity.3,4 Another important advantage of slurry process is the intimate contact between the catalyst and feed due to the movement of smaller size catalysts inside the reactor. The available surface area of the slurry catalyst is also higher because of the well-mixed feed and catalyst, which minimizes the mass transfer limitations. The slurry process also provides the option of regeneration or refill of catalyst. This way, the process conditions can be controlled to achieve the desired products.5 Both the fixed and slurry hydrocracking processes employ dual-functional (cracking and hydrogenation) supported metal catalysts. The acidic function, provided by the support, is © 2017 American Chemical Society
responsible for cracking of the heavy hydrocarbon molecules, while the metal function facilitates the dehydrogenation/ hydrogenation reactions. In addition, there are some desirable side reactions such as hydroisomerization and dehydrocyclization enhanced by both the metallic and acidic functions.6,7 One of the important issues related to the hydrocracking is the catalyst deactivation by coke formation. Application of dispersed catalysts has been considered as an interesting alternative to minimize the formation of coke. The dispersed catalysts are effective in slowing down the deactivation rate of the supported catalyst because of their hydrogenation activity on the smaller crystal size.8,9 The dispersed catalysts do not pose any diffusional resistance and allow easy access of the large hydrocarbon molecules to the active sites, and thus reduce the coke formation by hydrogenating the cracked intermediate free radicals.5,10 Metal sulfides, which are formed in situ by the reaction between metal precursors and sulfur compounds in the feedstock or additional sulfur sources, have been proved to be the active species in hydrocracking processes.11 The dispersed metal catalysts can be classified as oil-soluble and water-soluble. Water-soluble dispersed catalyst has lower catalytic activity due to fast evaporation and agglomeration of the precursors which eventually form large particles and cause Received: December 14, 2016 Revised: February 2, 2017 Published: February 4, 2017 3132
DOI: 10.1021/acs.energyfuels.6b03322 Energy Fuels 2017, 31, 3132−3142
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Figure 1. (A) Experimental setup of the batch autoclave reactor (ref 18). (B) Experimental steps involved in hydrocracking of VGO using oil-soluble dispersed catalysts.
molybdenum, and iron, on heavy oil upgrading. The main goal is to produce more liquids yield (such as distillate and naphtha) and to reduce the amount of coke formation. The catalytic performances of the dispersed metal precursors are investigated by comparing the results of the hydrocracking reactions in: (i) thermal run, (ii) catalytic runs using only the dispersed metal precursors as standalone catalysts, and (iii) catalytic runs with the dispersed metal precursors as cocatalyst along with a commercial hydrocracking W−Ni/Al2O3−SiO2 catalyst. The kinetic modeling of the slurry phase hydrocracking of VGO is also carried out. A kinetic model with five lumped parameters was formulated based on the product distributions. The kinetic parameters are estimated by least-squares fittings of the model equations with experimental data in MATLAB. The estimated parameters discriminated using various statistical indicators and their physical significances.
difficulties in the dispersion of submicrometer particles in the feed.12−15 Precursors for oil-soluble dispersed catalysts include molybdenum naphthenate, cobalt naphthenate, nickel naphthenate, and molybdenum and nickel acetyl acetones. Nitride compounds such as cobalt nitride, nickel nitride, and ferric nitride can be used as water-soluble precursors. Other compounds such as phosphomolybdic and ammonium tugestenate can also be used as precursors.5 Tetrathiomolybdate has been investigated as a precursor of dispersed Mo catalyst for upgrading of Maya crude.8 In this regard, the use of ammonium heptamolybdate as a precursor has also been reported in the upgrading of cold lake vacuum residue.16 Cobalt nitrate was used as a Co catalyst precursor to process Athabasca oil sands bitumen.9 An investigation of Hamaca crude oil upgrading using Fe3(CO)12 as catalyst precursor was also reported.17 However, all the reported studies considered the dispersed metal precursors only as standalone catalyst. The concurrent application of supported catalysts and dispersed metal precursors is not explored thoroughly, despite the possibility of having an enhanced performance of such approach. Therefore, the present study investigates the promotional effects of oil-soluble dispersed catalyst, such as cobalt,
2. EXPERIMENTAL SECTION 2.1. Hydrocracking of VGO. The VGO used in this study is obtained from a Saudi Arabian refinery. It contains 2.8 wt % sulfur. VGO hydrocracking experiments were conducted in a batch autoclave reactor. A schematic diagram of the reactor and its components are shown in Figure 1A, while Figure 1B presents the details of experimental steps. 3133
DOI: 10.1021/acs.energyfuels.6b03322 Energy Fuels 2017, 31, 3132−3142
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Figure 2. FTIR spectra of (a) Co catalyst and (b) Mo catalyst. For each experiment, the desired amount of VGO was mixed with a specific amount of catalyst (dispersed and/or supported) and the mixture was transferred in the reactor. Following the leak test with nitrogen, the reactor was purged three times using hydrogen to make sure that there was no air inside the reactor. The reactor was then heated to the reaction temperature (∼420 °C) at low hydrogen pressure (4 MPa) to minimize the reaction during the heating period. When the internal temperature of the reactor reached 420 °C, the system was pressurized again with hydrogen to adjust the pressure to 8.5 MPa. At this time the stirrer was started (950 rpm) to ensure proper mixing of catalyst, feed, and hydrogen inside the reactor. Rigorous mixing of VGO and catalyst precursor inside the reactor ensured uniform dispersion of cocatalyst in the feed. The reaction was then allowed to continue for 1 h. Upon completion of the reaction, the reactor was allowed to cool down to 100 °C temperature and samples (products and spent catalysts) were collected. The liquid and solid mixture was separated by centrifugation (4000 rpm for 30 min) followed by filtration using a Millipore filter. The solid residue, which contained the catalyst and the coke, was further washed with toluene and centrifuged (3000 rpm for 20 min) to separate catalyst and solid coke. The separated solids were dried at 75 °C for 2 h and subsequently weighed. The difference between the fresh and used catalyst gives the amount of coke deposited on the catalyst. 2.2. Gas and Liquid Product Analysis. Gaseous products containing H2 and C1−C5 hydrocarbons were analyzed by a gas chromatograph (GC) equipped with flame ionization (FID) and thermal conductivity (TCD) detectors. The liquid products were analyzed by a thermogravimetric analyzer (TGA). The product distribution was defined as follows: naphtha, 90−221 °C; distillate, 221−343 °C; VGO, 343−565 °C.19 The VGO conversion was calculated using the following equation (eq 1):
conversion (%) =
WVGOf − WVGOp WVGOf
× 100
spent supported catalysts, which were used in the hydrocracking test with and without using dispersed metal catalyst. This analysis was performed to understand the effect of using the dispersed cocatalyst on the surface of the spent supported catalyst.
3. RESULTS AND DISCUSSION The slurry phase hydrocracking of VGO was conducted in a batch autoclave reactor using the oil-soluble dispersed catalyst in the following ways: (i) a standalone catalyst and (ii) an additive with a supported hydrocracking catalyst. The promotional effects of the oil-soluble metal precursors were analyzed based on the product distribution and coke formation during VGO hydrocracking. 3.1. Standalone Dispersed Catalysts. Cobalt 2-ethylhexanoate, molybdenum 2-ethylhexanoate, and iron naphthenate have been employed as oil-soluble dispersed catalysts. Experiments were conducted with three concentrations, i.e., 300, 500, and 1000 ppm of dispersed catalysts. It is important to mention here that the metal sulfides are the active phases for hydrogenation/dehydrogenation reactions. Therefore, it is essential to convert the metal species of both the supported and dispersed catalysts into their metal sulfides. In the present study, the catalyst sulfiding has been accomplished by in situ reaction between the catalyst’s metal species with the sulfur heteroatoms present in the VGO feed. The sulfidation of the metal species is confirmed by the FTIR analysis of the spent catalyst samples, as shown in Figure 2. The peaks which appeared between 400 and 450 cm−1 indicate that the metal precursors were successfully transferred to metal sulfide (active phase). The products of hydrocracking of VGO, which are presented in Figure 3, are classified as gas C1−C5 hydrocarbons, naphtha, distillate, coke, and unconverted VGO. The figure also shows the product analysis of the thermal hydrocracking run for comparison. Thermal hydrocracking run resulted in 35% conversion of VGO. With the addition of dispersed catalyst the conversion value decreased to about 31%. This reduction in VGO conversion was expected because the dispersed catalysts only contribute to the hydrogenation reactions. The cracking of the heavy hydrocarbon molecules depends mainly on the thermal energy despite the presence of dispersed catalyst. Therefore, the dispersed catalyst only increased the distillate fraction and decreased the yield of other fractions.20,21 This result was expected given the fact that the metal sulfides promote the hydrogenation/dehydrogenation reactions and do not participate in catalyzing the cracking reactions. Thus, the introduction of dispersed catalysts helped to increase the liquid product fractions and decrease the coke formation. The
(1)
where WVGOf and WVGOp are the weight of VGO in the feed and product, respectively. The product yield was defined as the weight percentage of the total product (eq 2).
yield (%) =
Wpi WP
× 100
(2)
where WP is the weight of total product and Wpi is the sum of the weight of naphtha, distillate, VGO, and gas. 2.3. Spent Catalyst Analysis. The spent dispersed catalysts were analyzed using Fourier transform infrared (FTIR) spectroscopy to confirm the in situ sulfiding of the metal precursor at reaction conditions. The FTIR spectroscopy analyses were recorded using a Nicolet 6700 Thermo Fisher Scientific instrument. For each experiment, 3 mg of sample was mixed thoroughly with 400 mg of KBr. Thereafter, the infrared spectrum of the sample was collected in the range of 400−4000 cm−1. The morphologies of the spent and the fresh catalysts were determined by a scanning electron microscope (SEM), (JEOL JSM-6460LV) operated at 5 kV equipped with energydispersive X-ray (EDX). SEM tests were conducted for two of the 3134
DOI: 10.1021/acs.energyfuels.6b03322 Energy Fuels 2017, 31, 3132−3142
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case of Mo and Co metal sulfides, the lowest coke formation was achieved at 500 ppm catalyst concentration. On the other hand the iron-based precursor showed the lowest amount of coke formation at 300 ppm. However, the total coke formation with Mo and Co at 300 ppm is still much lower than the Fe catalysts. Al-Marshed et al.22 also showed a lower activity of Fe dispersed catalyst as compared to molybdenum-based dispersed catalyst in decreasing the amount of coke formed and increasing the middle distillate. Under the studied reaction conditions, when the catalyst concentration was increased from 500 to 1000 ppm, all three types of dispersed catalysts showed significant increase in coke formation. This negative effect of dispersed catalyst could be explained by considering the crystal size and dispersion of the catalyst particles. The higher catalyst concentration can lead to larger crystal size and lower metal dispersion, resulting in a lower number of active sites. Consequently, the coke formation has been increased. Also, the dispersed metal crystals could have contributed to the coke formation by acting as a seed for the precipitation of solids. The increased dispersed metal concentration can also decrease the stability of asphaltene.23 As a result, the formation of coke increased.24 Therefore, the use of optimum concentration of dispersed catalyst is crucial to minimize the coke formation.24 Figure 5 shows the effect of dispersed metal concentrations on product distribution. For comparison, the results of thermal experiments are included in the figure. The yield of naphtha decreased with increasing amount of dispersed catalyst concentration. At 300 ppm catalyst concentration, both the naphtha and distillate fraction yields were increased as compared to the thermal run. The distillate yield decreased when the concentration increased from 300 to 500 ppm. However, increasing dispersed metal catalyst above 500 ppm has no significant effect on the yield of the distillate. Amount of unconverted VGO increased significantly when the concentration of dispersed catalyst increased. The use of 300 ppm dispersed catalyst concentration gave lower gas yields as compared to the thermal hydrocracking. It was observed that increased concentration of dispersed metal increases the yield of the gas. The higher catalyst concentration leads to larger crystal size and lower metal dispersion, resulting in a lower number of active sites. Consequently, overall activity of the catalyst significantly decreased, which is reflected with increased VGO and gas fractions. This observation suggests that excessive amount of dispersed catalyst is not helpful for increasing the yield of naphtha and distillate. 3.2. Dispersed Catalysts with Supported Catalyst. In cocatalyst experimental runs, a commercial first-stage hydrocracking catalyst (W−Ni/Al2O3−SiO2) was used to demonstrate the promotional effects of the dispersed catalysts. The properties of the supported commercial W−Ni/Al2O3−SiO2 catalyst are listed in Table 1. Two sets of experiments are conducted in the following manner: (i) using only the supported catalyst and (ii) using the supported and dispersed catalysts simultaneously. Two catalyst-to-oil ratios (1:10 and 1:40) of supported catalyst were investigated with the dispersed catalysts (500 ppm). Figure 6 shows the effect of dispersed metal catalyst on product distribution when experiments were performed in the presence of supported catalyst. Unlike the standalone dispersed catalyst runs, the supported catalyst affects the product distribution due to the cracking reactions over the acidic sites. In the thermal run, the VGO conversion was 35%, which is increased to 51% when 2.5 wt % supported catalyst was
Figure 3. Product distribution and VGO conversion (feed, VGO; standalone dispersed catalysts, 500 ppm; 420 °C; 8 MPa; reaction time, 1 h).
cracking reactions can be enhanced by introducing a supported catalyst, which has acidic functions. When dispersed metal catalyst was used, the amount of coke formed is expected to decrease compared to the thermal runs. The decreased coke formation can be explained by considering formation of more reactive hydrogen species and enhanced hydrogenation activity due to the dispersed catalysts. The extra reactive hydrogen facilitated the free radicals hydrogenation of the cracked intermediates and prevented coke formation.5,10 Overall, the enhanced hydrogenation activity due to the dispersed catalysts contributed to decrease the coke formation and increase the amount of distillate formation. Figure 4 presents the percentage of coke formed when cobalt, molybdenum, and iron metal precursors were used at different concentrations. It is clear that the amount of the catalyst significantly affects the coke formation. The type of metal precursors also have an influence on the coke formation. In the
Figure 4. Amount of coke formed over different concentrations of Co, Mo, and Fe standalone dispersed catalysts (feed, VGO; 420 °C; 8 MPa; reaction time, 1 h). 3135
DOI: 10.1021/acs.energyfuels.6b03322 Energy Fuels 2017, 31, 3132−3142
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Figure 5. Product yields obtained over standalone dispersed catalysts: (a) naphtha, (b) distillate, (c) VGO, and (d) gas at different catalyst concentrations (thermal *) (feed, VGO; T, 420 °C; P, 8.5 MPa; reaction time, 1 h).
the additional amount of heavy hydrocarbon molecules on the acidic sites of the supported catalysts. The cracked intermediates were subsequently hydrogenated on the active metal sites of both the supported and dispersed catalysts to give stable products. Further cracking followed by hydrogenation of the cracked intermediates is reflected by increased amounts of the liquid products in the naphtha range. A further increase of supported catalyst to 10 wt % resulted in a slight increase in the VGO conversion to 54%. The coke formation with 10 wt % supported catalysts was also higher than that of the 2.5 wt % supported catalyst runs, due to excessive cracking reactions. Therefore, under the studied reaction conditions, only 2.5 wt % supported catalyst is considered as a sufficient amount to achieve the desired level of VGO conversion. The product yields with both the cobalt- and molybdenum-based dispersed catalysts along with 2.5 wt % supported catalysts are comparable. At a lower amount of supported catalyst (2.5 wt %), the addition of dispersed metal catalyst (Co and Mo catalyst precursors) resulted in a lower amount of coke (less than 2 wt
Table 1. Properties of the Supported Catalyst Used for Hydrocracking property av length BET surface area compacted bulk density mean pore radius pore volume loss on ignition at 750 °C attrition loss side crushing strength chemical composition: SiO2 Al2O3 NiO WO3
unit
value
mm m2/g g/cm3 nm mL/g wt % % N/mm
1.90 288 0.71 4.0 0.56 9.6
